Method and apparatus for modifying delivery of a therapy in response to onset of sleep

A method and apparatus for providing a therapy to the patient that includes a therapy component configured to provide the therapy to the patient, sensing circuitry sensing a parameter of the patient, and a microprocessor coupled to the therapy component and the sensing circuitry to determine onset of a first state of the patient in response to the sensed physiologic parameter, and to determine whether the onset of the first state is detected for a predetermined time period.

FIELD OF THE INVENTION

This invention relates generally to medical devices, and, more particularly to implantable medical devices for providing various types of therapies to patients.

BACKGROUND OF THE INVENTION

Implantable cardioverter defibrillators (ICDs) are capable of detecting cardiac arrhythmias and delivering electrical stimulation therapies to terminate arrhythmias. Tachycardia may be terminated by anti-tachycardia pacing therapies or high-voltage cardioversion shocks. Fibrillation may be terminated by high-voltage defibrillation shocks. These high-voltage shocks, which are referred to inclusively herein as “cardioversion/defibrillation shocks,” can be life-saving to a patient but can be very painful.

Atrial arrhythmias, such as atrial tachycardia (AT) and atrial fibrillation (AF), may not be directly life-threatening and may occur repeatedly in some patients. Therefore, in order to avoid delivering frequent, painful shock therapies, atrial cardioversion/defibrillation therapies employing high-voltage shocks may be programmed to be disabled in an ICD, or programmed to be delivered after the AT/AF episode has been detected for a sustained period of time, for example 2 hours or longer. Atrial arrhythmia detection algorithms may remain enabled because a physician may want to monitor for the presence of AT and AF for the purposes of managing medical therapies, such as anti-coagulation therapy and anti-arrhythmic drugs. Furthermore, non-painful, anti-tachycardia pacing therapies may be delivered in an attempt to terminate a detected atrial arrhythmia. If these less aggressive therapies fail, however, or if all atrial arrhythmia therapies are disabled, the atrial arrhythmia may be sustained for long periods of time.

During sustained AT/AF episodes, blood stasis in the atria can result in the formation of clots or thrombus. If AT/AF is suddenly terminated, either spontaneously or through medical intervention, coordinated atrial contraction may dislodge the clot, producing thromboembolism and leading to a high risk of stroke. Furthermore, retrospective analysis of arrhythmia incidence in patients implanted with the Medtronic Model 7250 dual chamber ICD revealed that atrial fibrillation (AF) is a co-existent arrhythmia with ventricular tachycardia (VT) or ventricular fibrillation (VF) in a significant patient population. Approximately 18% of all VF episodes and 3% of all VT episodes were accompanied by recent onset AF or AT. Stein KM et al., J Am Coll Cardiol Proc., 1999. The termination of AT/AF using treatment modalities that are acceptable and tolerable to the patient is therefore desirable over sustained periods of untreated AT/AF. Atrial arrhythmia treatments that are tolerable to the patient are needed in order to increase patient acceptance and physician use of such therapies.

In U.S. Pat. No. 5,630,834, issued to Bardy, an automatic atrial defibrillator having the ability to determine whether the patient is likely to be asleep senses the occurrence of atrial fibrillation and delivers defibrillation pulses in response thereto. Defibrillation pulses which are at energy levels which would normally be painful to the patient are delivered only in response to occurrences of atrial fibrillation while the patient is determined to be asleep. Defibrillation pulses at lower, non-painful levels may be delivered while the patient is determined not to be asleep. Detection of the fact that the patient is sleeping can be accomplished using a real time clock, which may be used in conjunction with a physical activity sensor, and/or a posture sensor. A timekeeping method for sleep detection, however, is limited when the patient changes his/her bed time and/or wake time, travels to a different time zone, etc.

Detection of sleep based on time of day, even when combined with activity and/or posture, may not discriminate night time inactivity from a period of deep sleep, when a patient's perception of a normally painful defibrillation shock is most likely to be suppressed. In currently available ICD's, atrial defibrillation shocks may be scheduled to occur in the middle of the night, e.g., 2:00 A.M., in a patient having persistent AF. The patient may be aware that a defibrillation shock is scheduled to occur at a particular time during the night and, in anticipation of the impending shock, remain awake.

A need remains, therefore, for a method and apparatus for delivering therapies that are uncomfortable or painful to a patient during a period of deep sleep such that the discomfort perceived by the patient is suppressed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a diagram of one embodiment of an implantable medical device (IMD) system100including a cardiac pacemaker102, an atrial lead104, and a ventricular lead106implanted in a patient108. The pacemaker102produces electrical pulses (i.e., pacing pulses) that stimulate a heart110of the patient108. One end of the atrial lead104is electrically coupled to the pacemaker102, the other end of the atrial lead104extends through a vein112into a right atrium of the heart110. One end of the ventricular lead106is electrically coupled to the pacemaker102, the other end of the ventricular lead106extends through the vein112and into a right ventricle of the heart110. Electrically conductive electrodes attached to the ends of the atrial lead104and the ventricular lead106located within the heart110are used to deliver pacing pulses to the heart110, and to receive intrinsic electrical signals present within the heart110.

The pacemaker102may be housed within a hermetically sealed, biologically inert outer canister or housing. At least a portion of the housing may be electrically conductive, and may serve as an electrode in pacing and/or sensing circuits of the pacemaker102.

The IMD system100ofFIG. 1also includes a programming unit114for programming the pacemaker102. A programming head116is connected to the programming unit114, and enables two-way communication between the programming unit114and the pacemaker102as indicated inFIG. 1. For example, the programming head116may include a radio frequency (RF) antenna, and may send RF signals to, and receive RF signals from, the pacemaker102.

FIG. 2is a diagram of one embodiment of the cardiac pacemaker102ofFIG. 1. As described above, the pacemaker102produces pacing pulses delivered to the heart110of the patient108(FIG. 1) via the atrial lead104and the ventricular lead106. In the embodiment ofFIG. 2, the pacemaker102includes lead interface circuitry200, pacing output circuitry202, a central processing unit (CPU)204, a memory206, timing/pacing control circuitry208, a minute ventilation (MV) sensing circuit210, an activity sensing circuit212, a telemetry unit214, and an antenna216.

The atrial lead104and the ventricular lead106conduct pacing pulses produced by the pacemaker102to the heart110of the patient108(FIG. 1), and also conduct intrinsic electrical signals present within the heart110to the pacemaker102. The lead interface circuitry200forms an electrical interface between the atrial lead104and the ventricular lead106and other components of the pacemaker102. As will be described in detail below, the pacing output circuitry202produces atrial and ventricular pacing pulses for stimulating the heart110. The CPU204executes instructions stored in the memory206, and controls the operations of other components of the pacemaker102.

Adapted for connecting to the atrial lead104and the ventricular lead106and capable of delivering pacing pulses to the right atrium and the right ventricle of the heart110(FIG. 1), the pacemaker102ofFIGS. 1 and 2may be termed a “dual-chamber” pacemaker. The pacemaker102may be programmable to operate in one or more of several different predefined operating modes, including a “demand” mode. In the “demand mode,” the pacemaker102senses intrinsic electrical signals present within the heart110of the patient108(FIG. 1), and produces pacing pulses only when the pacing pulses are needed. For example, the pacemaker102may be programmed with a value indicating whether or not the “demand” mode is enabled, a “low rate limit” value indicating a low limit of an intrinsic beat rate of the heart110of the patient108(FIG. 1), and an “atrioventricular (AV) interval” value indicating a maximum length of time between an atrial contraction or “atrial beat” and a subsequent ventricular contraction or “ventricular beat.”

The timing/pacing control circuitry208may include various registers for storing values indicative of programmed parameters of the pacemaker102, and various counters for performing timing functions. For example, the CPU204may store programmed “demand” mode, “low rate limit,” and “AV interval” values in one or more registers of the timing/pacing control circuitry208.

The timing/pacing control circuitry208includes sensing circuitry that receives and detects intrinsic electrical signals present within the heart110of the patient108(FIG. 1). Specifically, the sensing circuitry of the timing/pacing control circuitry208receives a first electrical signal indicative of an intrinsic contraction of the right atrium via the atrial lead104. In response the first electrical signal, the sensing circuitry may generate an “atrial beat” signal within the timing/pacing control circuitry208.

If the “demand” mode of the pacemaker102is enabled, the timing/pacing control circuitry208may provide an “atrial trigger” signal to the pacing output circuitry202if a frequency at which the “atrial beat” signals are generated is below the programmed “low rate limit.” In other words, the timing/pacing control circuitry208may provide an “atrial trigger” signal to the pacing output circuitry202if the intrinsic beat rate of the heart110(FIG. 1) falls below the programmed “low rate limit.” In response to the atrial trigger signal, the pacing output circuitry202may produce an atrial pacing pulse, and provide the atrial pacing pulse to the right atrium of the heart110(FIG. 1) via the atrial lead104. The atrial pacing pulse typically causes the right and left atria of the heart110to contract in unison.

The sensing circuitry of the timing/pacing control circuitry208also receives a second electrical signal indicative of an intrinsic contraction of the right ventricle via the ventricular lead106. In response the second electrical signal, the sensing circuitry may generate a “ventricular beat” signal within the timing/pacing control circuitry208. If the “demand” mode of the pacemaker102is enabled and the “ventricular beat” signal is not generated within the programmed “AV interval” following an “atrial beat” signal, the timing/pacing control circuitry208may provide a “ventricular trigger” signal to the pacing output circuitry202. In response to the “ventricular trigger” signal, the pacing output circuitry202may produce a ventricular pacing pulse, and provide the ventricular pacing pulse to the right ventricle of the heart110(FIG. 1) via the ventricular lead106. The ventricular pacing pulse typically causes the right and left ventricles of the heart110to contract in unison.

The minute ventilation sensing circuit210produces a minute ventilation output signal indicative of the minute ventilation of the patient108(FIG. 1). In one embodiment, the minute ventilation sensing circuit210produces the minute ventilation output signal dependent upon changes of electrical impedance in a thoracic cavity of the patient108, and the minute ventilation output signal constitutes digital values indicative of the minute ventilation of the patient108produced at regular time intervals. In other embodiments, the minute ventilation output signal may be a continuous analog signal.

As described above, electrically conductive electrodes are attached to the ends of the atrial lead104and the ventricular lead106(FIG. 1), and at least a portion of the outer canister or housing of the pacemaker102(FIGS. 1–2) may be electrically conductive. The minute ventilation sensing circuit210may deliver an electrical current excitation signal between a first electrode, at the end of either the atrial lead104or the ventricular lead106, and the outer canister or housing of the pacemaker102. The current excitation signal may include, for example, current pulses delivered at a predetermined rate (e.g., 16 pulses per second, or 16 Hertz). An electrical voltage signal may be measured between a second electrodes, at the end of the atrial lead104or the ventricular lead106, and the outer canister or housing of the pacemaker102. A thoracic impedance signal may be generated by dividing a magnitude of the electrical voltage signal by a magnitude of the electrical current excitation signal.

The thoracic impedance signal is a voltage signal having three main components: a direct current (d.c.) offset voltage, a cardiac component resulting from the function of the heart110of the patient108(FIG. 1), and a respiratory component. The minute ventilation sensing circuit210may include, for example, a bandpass filter (e.g., having a passband of, for example, 0.05 Hz to 0.8 Hz), and the thoracic impedance signal may be passed through the bandpass filter to substantially remove the d.c. offset voltage and the cardiac component. The resulting “filtered” thoracic impedance signal, emerging at an output of the bandpass filter, substantially comprises the respiratory component.

The minute ventilation sensing circuit210may also include sample-and-hold circuitry and comparison circuitry (not shown). As described above, the minute ventilation sensing circuit210may deliver current pulses at a predetermined rate (e.g., 16 Hz). The predetermined rate defines a time interval between pulses, referred to herein as a “cycle time.” At the beginning of each cycle time, the minute ventilation sensing circuit210delivers a current pulse. The sample-and-hold circuitry may sample the filtered thoracic impedance signal at the beginning of each cycle time, thereby acquiring a “current” value of the filtered thoracic impedance signal. The comparison circuitry may compare the “current” value of the filtered thoracic impedance signal to a “previous” value of the filtered thoracic impedance signal, acquired by the sample-and-hold circuitry at the beginning of the preceding cycle time. The comparison circuitry may produce an analog “difference” voltage equal to a difference between the “current” value of the filtered thoracic impedance signal and the “previous” value of the thoracic impedance signal.

The minute ventilation sensing circuit210may also include analog-to-digital conversion circuitry, summing circuitry, and a register (not shown). The analog-to-digital conversion circuitry may convert the analog difference voltage produced by the comparison circuitry to a digital “count” value representing the difference between the “current” value of the filtered thoracic impedance signal and the “previous” value of the thoracic impedance signal at the beginning of the preceding cycle time. The summing circuitry may sum the digital “count” values produced by the analog-to-digital conversion circuitry over a predetermined number of the cycle times (i.e., over a predetermined time interval). The resulting sum of the digital “count” values, acquired over the predetermined time interval, is referred to herein as a “minute ventilation count value.” A digital “minute ventilation count value” is thus present in the register at the end of each predetermined time interval, wherein the digital “minute ventilation count value” is indicative of the minute ventilation of the patient108(FIG. 1). At the end of each predetermined time interval, the digital “minute ventilation count value” (i.e., the contents of the register) may be provided to the CPU204(e.g., via an interrupt or programmed input/output mechanism), and the register may be cleared.

For example, the minute ventilation sensing circuit210may deliver current pulses at a rate of 16 Hz as described above. The summing circuitry may sum 32 of the digital “count” values produced by the analog-to-digital conversion circuitry over a predetermined 2-second time interval. At the end of each 2-second time interval, the digital “minute ventilation count value” (i.e., the contents of the register) may be provided to the CPU204(e.g., via an interrupt or programmed input/output mechanism), and the register may be cleared.

It is noted that there are several known methods for producing measures of minute ventilation of the patient108(FIG. 1), any one of which may be employed by the minute ventilation sensing circuit210to produce the minute ventilation output. For example, in other contemplated embodiments, the minute ventilation output may be a continuous analog waveform indicative of the minute ventilation of the patient108(FIG. 1). The continuous analog waveform may be sampled at regular intervals, and the analog samples may be converted to corresponding digital values.

The activity sensing circuit212senses movement or physical activity of the patient108(FIG. 1), and produces an “activity output” indicative of a magnitude of the movement or physical activity of the patient108. In one embodiment, the “activity output” constitutes digital “activity values” produced at regular time intervals. In other embodiments, the “activity output” may be a continuous analog signal.

The activity sensing circuit212may include, for example, an element producing an electrical signal when subjected to mechanical stress (e.g., a piezoelectric crystal), and a mechanical apparatus for subjecting the element to mechanical stress when the patient108moves or is physically active. The element and the mechanical apparatus for subjecting the element to mechanical stress when the patient108moves or is physically active may form, for example, an accelerometer (not shown). The accelerometer may produce an output signal. Alternately, the activity sensing circuit212may include a piezoelectric sensor bonded to an inner surface of the outer canister or housing of the pacemaker102(FIGS. 1–2), and the piezoelectric sensor may produce the output signal.

The activity sensing circuit212may include a bandpass filter, and the output signal of the accelerometer or piezoelectric sensor may be coupled to an input of the bandpass filter. An output signal produced by the bandpass filter may be compared to a threshold value (e.g., a programmable threshold value). Peaks in the output signal of the bandpass filter which exceed the threshold value, referred to herein as “activity counts,” may indicate movement or physical activity of the patient108(FIG. 1) of sufficient magnitude that an increase in pacing rate may be warranted.

The activity sensing circuit212may include circuitry for summing “activity counts” occurring within predetermined time intervals (e.g., two second time intervals), and a register for storing the sum of the “activity counts.” At the end of each regular time interval, the corresponding sum of the “activity counts,” contained within the register, constitutes a digital “activity value.” The contents of the register may be provided to the CPU204at the end of each regular time interval (e.g., via an interrupt or programmed input/output mechanism), and the register may be cleared.

It is noted that there are several known methods for producing measures of movement or physical activity of the patient108(FIG. 1), any one of which may be employed by the activity sensing circuit212to produce the “activity output.”

The pacemaker102is typically programmed with a “high rate limit” value indicating a high limit of an intrinsic beat rate of the heart110of the patient108. If a “rate response” mode of the pacemaker102is enabled (e.g., via a programmable parameter), the CPU204may execute software instructions stored in the memory206that implement the “rate response” mode.

In this situation, the CPU204may vary the “low rate limit” value and/or the “AV interval” value stored in the timing/pacing control circuitry208, dependent upon the minute ventilation output produced by the MV sensing circuit210and/or the activity output produced by the activity sensing circuit212. the CPU204may vary the “low rate limit” value and/or the “AV interval” value according to a transfer function (e.g., a programmable transfer function) to achieve a rate response defined by the “low rate limit” value, the “high rate limit” value, and the transfer function. As a result, the rate at which the pacing output circuitry202produces the atrial pacing pulses is varied between the “low rate limit” and the “high rate limit” dependent upon the minute ventilation output produced by the MV sensing circuit210and/or the activity output produced by the activity sensing circuit212. For example, a “target” pacing rate at which pacing output circuitry202produces the atrial pacing pulses may be expressed as:
“target”pacing rate=“low rate limit”+f(sensing circuit output)
where f is a linear or monotonic function of the minute ventilation output produced by the MV sensing circuit210and/or the activity output produced by the activity sensing circuit212.

For example, when the activity output produced by the activity sensing circuit212indicates that an activity level of the patient108(FIG. 1) has increased, the “target” pacing rate may be increased from the “low rate limit” by incremental amounts determined by the activity output produced by the activity sensing circuit212. As long as the activity output produced by the activity sensing circuit212indicates activity of the patient108, the “target” pacing rate may be periodically increased by incremental amounts until the “high rate limit” is reached. When the activity output produced by the activity sensing circuit212indicates activity of the patient108has ceased, the “target” pacing rate may be gradually reduced by incremental amounts until the “low rate limit” is reached.

The rate response function f is preferably selected such that the “target” pacing rate is based on a combination of the outputs of the activity sensing circuit212and the minute ventilation sensing circuit210. For example, the rate response function f may be selected such that the “target” pacing rate is based substantially on the activity output produced by the activity sensing circuit212when the patient is relatively inactive, and based substantially on the minute ventilation output produced by the minute ventilation sensing circuit210when the patient is relatively active. Any one of several known methods for combining or “blending” outputs of activity sensors and minute ventilation sensors may be employed in generating the “target” pacing rate.

The telemetry unit214is coupled to the antenna216, and communicates with the programming head116(FIG. 1) via antenna216. For example, the antenna216may be a radio frequency (RF) antenna, and the telemetry unit214may send RF signals to, and receive RF signals from, the programming head116(FIG. 1). In the embodiment ofFIGS. 1 and 2, CPU204communicates with the programming unit114(FIG. 1) via the telemetry unit214, the antenna216, and the programming head116. CPU204receives values to be stored in memory locations of the memory206from the programming unit114via the telemetry unit214. The received values may be, for example, the values of programmable parameters, which determine the operation of the pacemaker102. CPU204may also use the telemetry unit214to transmit values residing in memory locations of the memory206to the programming unit114. The transmitted values may be, for example, the values of programmable parameters, which determine the operation of the pacemaker102, and/or data indicative of sensed parameters of the patient108(FIG. 1).

FIGS. 3A–3Din combination form a flow chart of one embodiment of a method300for determining an onset of sleep in a patient (e.g., patient108ofFIG. 1) having an implantable medical device (e.g., pacemaker102ofFIGS. 1–2) implanted therein. The method300may be embodied within software residing in the memory206(FIG. 2) of the pacemaker102. The CPU204(FIG. 2) may carry out the method300when executing the software embodying the method300.

The method300includes a “preliminary” portion302and a “recurrent” portion304. During the preliminary portion302, two minute ventilation threshold values are determined. At least some of the steps of the recurrent portion304are carried out at predetermined time intervals. The minute ventilation threshold values determined during the preliminary portion302are used during the recurrent portion304to determine the onset of sleep in a patient having the implantable medical device implanted therein.

During a step306of the preliminary portion302, “minute ventilation values” are received at predetermined time intervals over a predetermined period of time. The minute ventilation values are indicative of the minute ventilation of the patient having the implantable medical device implanted therein. For example, in one embodiment of the minute ventilation sensing circuit210(FIG. 2) described above, the minute ventilation sensing circuit210delivers current pulses at a rate of 16 Hz, thereby defining pulse “cycles” having “cycle times” of 1/16 or 0.0625 seconds. The minute ventilation sensing circuit210converts an analog difference voltage between a “current” value of a thoracic impedance signal, obtained during a “current” pulse cycle, and a “previous” value of the thoracic impedance signal, obtained during a pulse cycle preceding the current pulse cycle, to a digital “count” value. The minute ventilation sensing circuit210sums 32 of the digital “count” values produced by the analog-to-digital conversion circuitry in a register over a predefined 2-second time interval. The minute ventilation sensing circuit210provides a “minute ventilation value” contained in the register at the end of each 2-second time interval, then clears the register.

During a second step308of the preliminary portion302, the minute ventilation values received during the step306are used to determine a first minute ventilation threshold value and a second minute ventilation threshold value. The first and second minute ventilation threshold values are used to determine a transition from an “awake” state of the patient to a “sleep” state of the patient. Due to the diurnal nature of the human wake-sleep cycle, the “predetermined period of time” in the step306is preferably at least 24 hours, and preferably a multiple of 24 hours, such that: (i) a first portion of the minute ventilation values received during the predetermined period of time are obtained when the patient is awake, (ii) a second portion of the minute ventilation values received during the predetermined period of time are obtained when the patient is asleep, and (iii) a ratio between the first portion and the second portion is representative of a wake-sleep cycle of the patient108.

The first minute ventilation threshold value is greater than the second minute ventilation threshold value, and is used to screen the received minute ventilation value to determine if the carrying out of the remaining steps of the recurrent portion304is warranted. In calculating the first minute ventilation threshold value, a median value of the minute ventilation values received during the predetermined period of time is determined. The first minute ventilation threshold value is set to half the median value. The median value is substantially the “middle” minute ventilation value. That is, a number of the minute ventilation values received during the predetermined period of time are greater than the median value, and a substantially equal number of the minute ventilation values are less than the median value.

For example, during the step306, the CPU204(FIG. 2) of the pacemaker102(FIGS. 1 and 2) may receive minute ventilation values from the minute ventilation sensing circuit210(FIG. 2) at 2-second intervals over a 24-hour period of time. Each time the CPU204receives a minute ventilation value, the CPU204may store the minute ventilation value in the memory206(FIG. 2). At the end of the 24-hour period of time, the CPU204may access the minute ventilation values stored in the memory206, determine a median value of the minute ventilation values, and set the first minute ventilation threshold value to half the median value.

Alternately, the CPU204(FIG. 2) of the pacemaker102(FIGS. 1–2) may form a histogram of received minute ventilation values within the memory206(FIG. 2), and use the histogram to estimate the median value of the minute ventilation values. A range of expected minute ventilation values may be divided into equally-sized sub-ranges or “bins,” and different memory locations of the memory206may be allocated for each of the bins. Each time the CPU204receives a minute ventilation value, the CPU204may determine which bin the minute ventilation value corresponds to, and add ‘1’ to a running count maintained in the memory location allocated for that bin. At the end of the predetermined period of time (e.g., 24 hours), the CPU204may access the memory locations allocated for the bins, locate a bin wherein a number counts in bins above and below the bin are substantially equal, and select the median value within the sub-range of minute ventilation values represented by the bin. The CPU204may then set the first minute ventilation threshold value to half the selected median value.

Regarding the determining of the second minute ventilation value, the CPU204(FIG. 2) of the pacemaker102(FIGS. 1–2) may keep a running estimate of a mean value (i.e., an average value) of the received minute ventilation values. The mean value represents a “central tendency” of the received minute ventilation values. At the end of a predetermined time interval (i.e., after receiving a predetermined number of minute ventilation values), the CPU204may calculate a measure of deviation of the minute ventilation values received during the time interval from a “current” estimate of the mean value. The CPU204may form a histogram of the deviations of the minute ventilation values from the mean value.

As further described below, a histogram of deviations of minute ventilation values from a mean value, formed over a “sleep-wake” cycle of the patient, has a first peak representing deviations of minute ventilation values from the mean value when the patient is asleep, a second peak representing deviations of minute ventilation values from the mean value when the patient is awake, and a “trough” between the first and second peaks representing deviations of minute ventilation values from the mean value when the patient is transitioning between the “awake” state and the “sleep” state. The CPU204may select a value for the second minute ventilation threshold value between the first and second peaks of the histogram.

For example, the CPU204(FIG. 2) may calculate a standard deviation of minute ventilation values received during predetermined time intervals (i.e., time “windows”), and may form a histogram of resulting minute ventilation standard deviation values within the memory206(FIG. 2). A range of expected minute ventilation standard deviation values may be divided into equally-sized sub-ranges or “bins,” and different memory locations of the memory206may be allocated for each of the bins. At the end of each time window, the CPU204may calculate the minute ventilation standard deviation value, determine which bin the minute ventilation standard deviation value corresponds to, and add ‘1’ to a running count maintained in the memory location allocated for that bin. At the end of the predetermined period of time (e.g., 24 hours), the CPU204may access the memory locations allocated for the bins. The CPU204may locate a bin having a lowest count between two other bins having the highest counts. The two bins having the highest counts include a bin of the first peak and a bin of the second peak, and the bin having the lowest count in between the first and second peaks is a bin of the trough of the histogram.

As further described below, the CPU204may select the second minute ventilation threshold value as a value (e.g., a minimum value) within the sub-range of minute ventilation values represented by the bin having the lowest count. Alternately, the CPU204may select the second minute ventilation threshold value as a value (e.g., a minimum value) within a sub-range of minute ventilation values represented by a bin between the bin having the lowest count and the bin of the first peak having one of the two highest count. Further, The CPU204may select the second minute ventilation threshold value as a value (e.g., a minimum value) within a sub-range of minute ventilation values represented by a bin midway between the bins of the first and second peaks and having the two highest counts.

As described above, at least some of the steps of the recurrent portion304are carried out at predetermined time intervals. During a step310of the recurrent portion304, a minute ventilation value is received during one of the predetermined time intervals. The minute ventilation value is used to calculate an “MV Stdev Short” value and an “MV Stdev Long” value. The “MV Stdev Short” value is a standard deviation of minute ventilation values received during m time intervals including the current time interval and an immediately preceding m−1 time intervals. The “MV Stdev Long” value is a standard deviation of minute ventilation values received during n time intervals including the current time interval and the immediately preceding n−1 time intervals. In general, n≧m; however, for improved performance, n is preferably greater than m. For example, the value of m may be selected such that the “MV Stdev Short” value is calculated over a 2–5 minute period of time, and the value of n may be selected such that the “MV Stdev Long” value is calculated over a 10–15 minute period of time.

As described above, the minute ventilation sensing circuit210(FIG. 2) may produce a new minute ventilation value at the end of predetermined time intervals (e.g., 2-second time intervals). The CPU204(FIG. 2) may keep a running estimates of mean values (i.e., average values) of minute ventilation values received during various predetermined periods of time or time “windows.” The CPU204may update the running estimates of the mean values each time a new minute ventilation value is produced by the minute ventilation sensing circuit210using:
Mean(i)=MV(i)/p+Mean(i−1)−Mean(i−1)/p
where Mean(i) is the mean value estimate during an ith time interval, MV(i) is the minute ventilation value produced the minute ventilation sensing circuit210during the ith time interval, p is the total number of elapsed time intervals, and Mean(i−1) is the mean value estimate during the time interval immediately preceding the ith time interval.

Regarding calculation of the “MV Stdev Short” value during a “current” time interval k, a mean value estimate Mean(k) value may be calculated using the minute ventilation values received during the current time interval and an immediately preceding m−1 time intervals (i.e., p=m), and the “MV Stdev Short” value may be calculated using:

Regarding calculation of the “MV Stdev Long” value during a “current” time interval k, a mean value estimate Mean(k) value may be calculated using the minute ventilation values received during the current time interval and an immediately preceding n−1 time intervals (i.e., p=n), and the “MV Stdev Long” value may be calculated using:

For example, the CPU204(FIG. 2) of the pacemaker102(FIGS. 1–2) may receive minute ventilation values at 2-second time intervals, and memory locations of the memory206(FIG. 2) may be allocated for minute ventilation values obtained during the immediately preceding n−1 2-second time intervals. During the step312, the CPU204(FIG. 2) of the pacemaker102(FIGS. 1–2) may receive a “current” minute ventilation value, and access the memory locations allocated for the minute ventilation values obtained during the immediately preceding n−1 2-second time intervals. The CPU204may use the “current” minute ventilation value and the minute ventilation values obtained over the immediately preceding m−1 time intervals to compute the “MV Stdev Short” value. The CPU204may also use the “current” minute ventilation value and the minute ventilation values obtained over the immediately preceding n−1 time intervals to compute the “MV Stdev Long” value.

During a decision step314, the “MV Stdev Long” value is compared to the first minute ventilation threshold value determined during the step308. If the “MV Stdev Long” value is less than the first minute ventilation threshold value, an optional step316may be accomplished. On the other hand, if the “MV Stdev Long” value is greater than or equal to the first minute ventilation threshold value, a step336is accomplished. During the step336, an “elapsed time” value is set to zero, and the recurrent portion304of the method300is exited.

Steps316–320represent an optional “activity cross-check” section of the recurrent portion304of the method300. Steps316–320are believed to enhance performance of the method300, but need not be accomplished for method300to work. During the optional step316, an “activity value” is received during the predetermined time interval, wherein the activity value is indicative of a degree of movement of the patient during the predetermined time interval.

For example, the CPU204(FIG. 2) of the pacemaker102(FIGS. 1 and 2) may receive activity values from the activity sensing circuit212(FIG. 2) at 2-second intervals. The activity sensing circuit212may include and accelerometer, a bandpass filter, comparison circuitry, summing circuitry, and a register. An output signal of the accelerometer may be passed through the bandpass filter, and the resultant filtered output signal provided to the comparison circuitry. The comparison circuitry may compare the filtered output signal to a threshold value (e.g., a programmable threshold value). Peaks in the filtered output signal, which exceed the threshold value, are referred to herein as “activity counts.” The summing circuitry may sum the “activity counts” occurring within a 2-second time interval in the register. At the end of each 2-second time interval, the activity sensing circuit212may provide the digital sum of the “activity counts” contained in the register, constituting the “activity value,” and the register may be cleared.

During the optional step318, the activity value is used to calculate an “ActThreshold” value, wherein the “ActThreshold” value is a sum of all “activity values” obtained during q time intervals including the current time interval and an immediately preceding q−1 time intervals. The value of q may be, for example, 20. The “ActThreshold” value during a “current” time interval k may be expressed as:
ActThreshold=Σj=1qMV(k−q−j)
where MV(i) is the minute ventilation value produced the minute ventilation sensing circuit210during the ith time interval.

During the optional decision step320, the activity value and the “ActThreshold” value calculated during the step318are compared. If the activity value is less than the “ActThreshold” value, an optional step322may be accomplished. On the other hand, if the activity value is greater than or equal to the “ActThreshold” value, the step336is accomplished. As described above, during the step336, the “elapsed time” value is set to zero, and the recurrent portion304of the method300is exited.

Steps322–324represent an optional “time-of-day cross-check” section of the recurrent portion304of the method300. Steps322–324are believed to enhance performance of the method300, but need not be accomplished for method300to work. During the optional step322, a “TimeofDay” value is obtained, wherein the “TimeofDay” value is indicative of a current time of day. During the optional decision step324, the “TimeofDay” value is compared to a predetermined “ExpectedSleepTime” value, wherein the “ExpectedSleepTime” value is indicative of a time of day the patient is expected to go to sleep each day. The “ExpectedSleepTime” value may be, for example, a programmable value. If the “TimeofDay” value is greater than or equal to the “ExpectedSleepTime” value, an optional step322may be accomplished accomplished. On the other hand, if the “TimeofDay” value is less than the “ExpectedSleepTime” value, the step336is accomplished. As described above, during the step336, the “elapsed time” value is set to zero, and the recurrent portion304of the method300is exited.

During a step326, a length of the predetermined time interval is added to the “Elapsed Time” value. The “Elapsed Time” value is compared to an “ExpectedSleepTransitionTime” value during a decision step328, wherein the “ExpectedSleepTransitionTime” value is a period of time allotted for the patient to transition from the “awake” state to the “sleep” state. The “ExpectedSleepTime” value may be, for example, a programmable value. If the “Elapsed Time” value is greater than or equal to the “ExpectedSleepTransitionTime” value, a decision step330is accomplished. On the other hand, if the “Elapsed Time” value is less than the “ExpectedSleepTransitionTime” value, the step336is accomplished. As described above, during the step336, the “elapsed time” value is set to zero, and the recurrent portion304of the method300is exited.

During the decision step330, the “MV Stdev Long” value is compared to the “MV Stdev Short” value. If the “MV Stdev Long” value is greater than or equal to the “MV Stdev Short” value, a decision step332is accomplished. On the other hand, if the “MV Stdev Long” value is less than the “MV Stdev Short” value, the step336is accomplished.

The decision step330enhances the method300by detecting abrupt transitions from the “sleep” state to the “awake” state. Typically, as the patient transitions from the “awake” state to the “sleep” state, the patient's minute ventilation decreases monotonically over time. Thus while the patient is sleeping, the “MV Stdev Long” value is typically greater than or equal to the “MV Stdev Short” value. However, when the patient wakes up abruptly and becomes active, the “MV Stdev Short” value will become greater than the “MV Stdev Long” value, indicating the patient has transitioned from the “sleep” state to the “awake” state.

During the decision step332, the “MV Stdev Long” value and the second minute ventilation threshold value, calculated during the step308, are compared. If the “MV Stdev Long” value is less than the second minute ventilation threshold value, a step334is accomplished. On the other hand, if the “MV Stdev Long” value is greater than or equal to the second minute ventilation threshold value, the step336is accomplished. As described above, during the step336, the “elapsed time” value is set to zero, and the recurrent portion304of the method300is exited. During the step334, the patient is determined to have transitioned from the “awake” state to the “sleep” state.

FIG. 4is a flow chart of one embodiment of a method400for providing a therapy to a patient (e.g., the patient108ofFIG. 1). The method400may be embodied within software residing in the memory206(FIG. 2) of the pacemaker102(FIGS. 1–2). The CPU204(FIG. 2) may carry out the method400when executing the software embodying the method400. During a first step402of the method400, an onset of sleep is determined in the patient. The step402of the method400may be accomplished by carrying out the steps of the method300ofFIGS. 3A–3D. During a step404, the therapy provided to the patient is modified.

For example, in the embodiment ofFIG. 1, the patient108has the pacemaker102implanted therein, and the atrial lead104and the ventricular lead106extend from the pacemaker102and into the heart110of the patient108. In the embodiment ofFIG. 2, the pacemaker102includes the pacing output circuitry202, the CPU204, and the timing/pacing control circuitry208. The pacing output circuitry202produces atrial and ventricular pacing pulses for stimulating the heart110. The CPU204may store programmable “demand” mode, “low rate limit,” and “AV interval” values in one or more registers of the timing/pacing control circuitry208.

The timing/pacing control circuitry208includes sensing circuitry that receives and detects intrinsic electrical signals present within the heart110of the patient108. Specifically, the sensing circuitry of the timing/pacing control circuitry208receives a first electrical signal indicative of an intrinsic contraction of the right atrium via the atrial lead104. In response the first electrical signal, the sensing circuitry may generate an “atrial beat” signal within the timing/pacing control circuitry208.

If the “demand” mode of the pacemaker102is enabled, the timing/pacing control circuitry208may provide an “atrial trigger” signal to the pacing output circuitry202if a frequency at which the “atrial beat” signals are generated is below the programmed “low rate limit.” In other words, the timing/pacing control circuitry208may provide an “atrial trigger” signal to the pacing output circuitry202if the intrinsic beat rate of the heart110falls below the programmed “low rate limit.” In response to the atrial trigger signal, the pacing output circuitry202may produce an atrial pacing pulse, and provide the atrial pacing pulse to the right atrium of the heart110via the atrial lead104.

The CPU204may embody the above described method300for detecting onsets of sleep in the patient108, and/or the method400for providing a therapy to a patient. For example, having detected an onset of sleep in the patient108(e.g., during the step334of the method300), the CPU204may reduce the “low rate limit” value stored in the timing/pacing control circuitry208from a normal “resting rate” value (e.g., 60 beats per minute) to a “sleep rate” value, wherein the “sleep rate” value is less than or equal to the “resting rate.” The “sleep rate” value may be, for example, a programmable value. The “sleep rate” value may be, for example, between 50 beats per minute and 60 beats per minute.

The above described methods300and400may also be useful for other purposes than reducing “low rate limit” values from normal “resting rate” values to “sleep rate” values in pacemakers. For example, the method300may be used to detect onsets of sleep for monitoring sleep-related events (i.e. sleep apnea, etc.), and the method400may be used in providing other medical therapies (e.g., electrical shocks for treating atrial fibrillation, administration of medications, etc.).

FIGS. 5A and 5Bwill now be used to further describe the calculation of the second minute ventilation threshold value.FIG. 5Ais a histogram of minute ventilation values of a patient obtained via minute ventilation sensing circuitry over a 24-hour period. In obtaining data for the histogram ofFIG. 5A, minute ventilation sensing circuitry delivered current pulses at a rate of 16 Hz, thereby defining pulse “cycles” having “cycle times” of 1/16 or 0.0625 seconds. The minute ventilation sensing circuitry converted an analog difference voltage between a “current” value of a thoracic impedance signal, obtained during a “current” pulse cycle, and a “previous” value of the thoracic impedance signal, obtained during a pulse cycle immediately preceding the current pulse cycle, to a digital “count” value. The minute ventilation sensing circuitry summed32of the digital “count” values produced by the analog-to-digital conversion circuitry in a register over predefined 2-second time intervals. At the end of each 2-second time interval, the minute ventilation sensing circuitry produced a “minute ventilation value” contained in the register, and the register is cleared.

A range of expected minute ventilation values was divided into equally-sized sub-ranges or “bins,” and different memory locations of a memory were allocated for each of the bins. Each time a minute ventilation value was produced by the minute ventilation sensing circuitry, a determination was made as to which bin the minute ventilation value corresponds to, and a ‘1’ was added to a running count maintained in the memory location allocated for that bin. At the end of the 24-hour period, the running counts maintained in the memory locations allocated for the bins were read out.

FIG. 5Bis a histogram of standard deviations of the minute ventilation values used to form the histogram ofFIG. 5Aand received within 1-minute time windows. As described above, the minute ventilation sensing circuitry produced a new minute ventilation value at the end of each 2-second time interval. A running estimate of a mean of the minute ventilation values was updated each time a new minute ventilation value was produced by the minute ventilation sensing circuitry as described above. At the end of each 1-minute time window, ending with a 2-second time interval k, a mean value estimate Mean(k) was calculated using the minute ventilation values received during the current time interval and an immediately preceding 29 time intervals (i.e., p=30) as described above, and the standard deviation of the 30 minute ventilation values received during the time window was calculated using:

The histogram ofFIG. 5Bwas formed within a memory. A range of expected minute ventilation standard deviation values was divided into equally-sized sub-ranges or “bins,” and different memory locations of the memory were allocated for each of the bins. At the end of each 1-minute time window, the corresponding minute ventilation standard deviation value was calculated. A determination was made as to which bin the minute ventilation standard deviation value corresponded to, and ‘1’ was added to a running count maintained in the memory location allocated for that bin. At the end of the 24-hour time period, the contents of the memory locations allocated for the bins were read out.

The histogram ofFIG. 5Bhas a first peak500, a second peak502, and “trough”504located between the first peak502and the second peak504. The first peak500represents a portion of the minute ventilation values produced by the minute ventilation sensing circuit210when the patient is asleep. The second peak502represents a different portion of the minute ventilation values produced by the minute ventilation sensing circuit210when the patient is awake.

Regarding use of the histogram ofFIG. 5Bto determine the second minute ventilation threshold value, the second minute ventilation threshold value may be selected from among the minute ventilation values located in the trough504. For example, the second minute ventilation threshold value may be selected as a value (e.g., a minimum value) within the sub-range of minute ventilation values represented by the bin having the lowest count (i.e., a bin having the lowest count within the trough504.). Alternately, the second minute ventilation threshold value may be selected as a value (e.g., a minimum value) within a sub-range of minute ventilation values represented by a bin between the bin having the lowest count within the trough504and a bin of the first peak500having the highest count. Further, the second minute ventilation threshold value may be selected as a value (e.g., a minimum value) within a sub-range of minute ventilation values represented by a bin midway between a bin of the first peak500having a highest count, and a bin of the second peak502having a highest count.

FIG. 6is a graph of “MV Stdev Long” and “MV Stdev Short” values described above, wherein the “MV Stdev Long” and “MV Stdev Short” values were calculated using minute ventilation values produced during 2-second time intervals and indicative of the minute ventilation of a patient. InFIG. 6, the first minute ventilation threshold value described above defines an “MV Threshold 1” level, and the second minute ventilation threshold value described above defines an “MV Threshold 2” level. As illustrated inFIG. 6, the “MV Threshold 1” level is greater than the “MV Threshold 2” level. As described above, the first minute ventilation threshold value is used to screen a received minute ventilation value to determine if the received minute ventilation value is sufficiently low as to warrant further analysis to detect an onset of sleep.

A time of day labeled “Sleep Onset” inFIG. 6is a time the method300ofFIGS. 3A–3Ddetermine an onset of sleep in the patient. Prior to the “Sleep Onset” time, the “MV Stdev Short” and “MV Stdev Long” values drop below the “MV Threshold 1” level several times, and occasionally drop below the “MV Threshold 2” level, indicating a decrease in patient activity and an impending transition from an “awake” state to a “sleep” state. At a time of day labeled “Time 1” inFIG. 6, prior to the “Sleep Onset” time, the “MV Stdev Long” value drops below the “MV Threshold 1” level, thus indicating received minute ventilation values are sufficiently low as to warrant further analysis to detect an onset of sleep. (See the step314of the method300,FIG. 3A.) At a time of day labeled “Time 2” inFIG. 6, between the “Time 1” and “Sleep Onset” times, the “MV Stdev Long” value drops below the “MV Threshold 2” level, and remains below the “MV Threshold 2” level for all subsequent times of day. The “Sleep Onset” time occurs a period of time after “Time 2” equal to the “ExpectedSleepTransitionTime” described above. (See the step328of the method300,FIG. 3D.)

FIG. 7is a graph of “MV Stdev Long” values described above, wherein the “MV Stdev Long” values were calculated using minute ventilation values produced during 2-second time intervals and indicative of the minute ventilation of a patient. As inFIG. 6, the first minute ventilation threshold value described above defines a level labeled “MV Threshold 1,” and the second minute ventilation threshold value described above defines a level labeled “MV Threshold 2.”

A time of day labeled “Sleep Onset” inFIG. 7is a time the method300ofFIGS. 3A–3Ddetermine an onset of sleep in the patient. Prior to the “Sleep Onset” time, the “MV Stdev Long” value substantially remains above the “MV Threshold 1” and “MV Threshold 2” levels, indicating a relatively high level of patient activity characteristic of an “awake” state of the patient. Just prior to the “Sleep Onset” time, the “MV Stdev Long” value drops below the “MV Threshold 1” level several times, and occasionally drops below the “MV Threshold 2” level, indicating a decrease in patient activity and an impending transition from the “awake” state to a “sleep” state. At the “Sleep Onset” time, the “MV Stdev Long” value has dropped below the “MV Threshold 2” level for a period of time equal to the “ExpectedSleepTransitionTime” described above. (See the step328of the method300,FIG. 3D.)

The patient woke up at a time of day labeled “Patient Wakes Up” inFIG. 7. At various times between the “Sleep Onset” time and the time labeled “Patient Wakes Up” inFIG. 7, the “MV Stdev Long” value rises above the “MV Threshold 2” level, but does not rise above the “MV Threshold 1” level. A peak700in the “MV Stdev Long” value, exceeding the “MV Threshold 1” level, occurs around a time the patient got out of bed briefly. Between a time of day corresponding to the peak700and the time labeled “Patient Wakes Up” inFIG. 7, the “MV Stdev Long” value is above the “MV Threshold 2” level, and occasionally rises briefly above the “MV Threshold 1” level, indicating an increase in patient activity and an impending transition from the “sleep” state to the “awake” state. Subsequent to the time labeled “Patient Wakes Up” inFIG. 7, the “MV Stdev Long” value remains above the “MV Threshold 1” and “MV Threshold 2” levels, indicating a relatively high level of patient activity characteristic of the “awake” state of the patient.

FIG. 8is an illustration of an alternative embodiment of an implantable medical device system including an implantable cardioverter defibrillator, referred to hereafter as “ICD”,10coupled to a patient's heart by way of three leads6,15, and16. A connector block12receives the proximal end of a right ventricular lead16, a right atrial lead15and a coronary sinus lead6, used for positioning electrodes for sensing and stimulation in three or four heart chambers. InFIG. 8, the right ventricular lead16is positioned such that its distal end is in the right ventricle (RV) for sensing right ventricular cardiac signals and delivering pacing or shocking pulses in the right ventricle. For these purposes, right ventricular lead16is equipped with a ring electrode24, a tip electrode26, optionally mounted retractably within an electrode head28, and RV coil electrode20, each of which are connected to an insulated conductor contained within the body of lead16. The proximal end of the insulated conductors are coupled to corresponding connectors carried by a connector14at the proximal end of lead16for providing electrical connection to the ICD10.

The right atrial lead15is positioned such that its distal end is in the vicinity of the right atrium and the superior vena cava (SVC). Lead15is equipped with a ring electrode21and a tip electrode17, optionally mounted retractably within electrode head19, for sensing and pacing in the right atrium. Lead15is further equipped with an SVC coil electrode23for delivering high-energy shock therapy. The ring electrode21, the helix electrode17and the SVC coil electrode23are each connected to an insulated conductor with the body of the right atrial lead15. Each insulated conductor is coupled at its proximal end to a connector terminal carried by connector13.

The coronary sinus lead6is advanced within the vasculature of the left side of the heart via the coronary sinus and great cardiac vein. The coronary sinus lead6is shown in the embodiment ofFIG. 8as having a defibrillation coil electrode8that may be used in combination with either the RV coil electrode20or the SVC coil electrode23for delivering electrical shocks for cardioversion and defibrillation therapies. In other embodiments, coronary sinus lead6may also be equipped with a distal tip electrode and ring electrode for pacing and sensing functions in the left chambers of the heart. The coil electrode8is coupled to an insulated conductor within the body of lead6, which provides connection to the proximal connector4.

For sensing and pacing functions, the electrodes17and21or24and26may be used as bipolar pairs, commonly referred to as a “tip-to-ring” configuration, or individually in a unipolar configuration with the device housing11serving as the indifferent electrode, commonly referred to as the “can” or “case” electrode. The device housing11may also serve as a subcutaneous defibrillation electrode in combination with one or more of the defibrillation coil electrodes8,20or23for defibrillation of the atria or ventricles.

It is recognized that alternate lead systems may be substituted for the three lead system illustrated inFIG. 8. While a particular multi-chamber ICD and lead system is illustrated inFIG. 8, methodologies included in the present invention may be adapted for use with a single chamber atrial cardioverter/defibrillator, dual chamber ICDs, or other multichamber ICDs.

FIG. 9is a diagram of one embodiment of the ICD ofFIG. 8. ICD10produces high-energy shock pulses delivered to the heart via coil electrodes20,23and/or8in response to detecting fibrillation. In the embodiment ofFIG. 9, the ICD10includes lead interface circuitry200, pacing output circuitry202, a central processing unit (CPU)204, a memory206, timing/pacing control circuitry208, a MV sensing circuit210, an activity sensing circuitry212, a telemetry unit214, and an antenna216, all of which may correspond generally to the identically-labeled components included in pacemaker102described above in conjunction withFIG. 9. ICD10additionally includes cardioversion/defibrillation output unit218for delivering high-voltage shock pulses.

The lead interface circuitry200is adapted for receiving the high-voltage right atrial lead15, high-voltage right ventricular lead16, and high-voltage coronary sinus lead6. The electrodes carried by the leads15,16, and6may be selected via lead interface circuitry200for the various sensing, pacing, and cardioversion/defibrillation functions of ICD10.

The timing/pacing control circuitry208includes sensing circuitry that receives and detects intrinsic electrical signals from sensing electrodes included on right atrial lead15and right ventricular lead16. Timing/pacing control circuitry208includes various registers for storing values indicative of programmed parameters of the ICD10, and various counters for performing timing functions. For example, the durations of escape intervals used in controlling the timing of pacing pulses delivered by ICD10are stored by timing/pacing control circuitry208. The value of a count present in an escape interval counter when reset by sensed R-waves or P-waves can be used to measure R-R intervals, P-P intervals, P-R intervals, and R-P intervals, which measures are stored in memory206and used to diagnose the occurrence of a variety of arrhythmias by CPU204.

Timing/pacing and control208may further include an analog-to-digital converter to digitize electrical signals received from the heart from sensing electrodes on leads15and16. CPU204may employ digital signal analysis techniques to characterize the digitized signals received from timing pacing/control208and stored in memory206to recognize and classify the patient's heart rhythm employing any of the numerous signal processing methods known in the art.

In response to the detection of atrial or ventricular tachycardia, an anti-tachycardia pacing therapy may be delivered to the patient's heart by loading a regimen from CPU204into the pacer timing/control circuitry208according to the type of tachycardia detected. In the event that higher voltage cardioversion or defibrillation shock pulses are required, CPU204activates the cardioversion/defibrillation output unit218to initiate charging of high voltage capacitors via a charging circuit included therein. Timing of the delivery of the defibrillation or cardioversion shock pulse is controlled by pacer timing/control circuitry208.

In accordance with the present invention, activation of cardioversion/defibrillation output unit218by CPU204may be delayed or cancelled pending the detection of sleep according to minute ventilation values provided by MV sensing circuit210to CPU204as described above. In modern ICDs, the particular arrhythmia therapies are programmed into the device ahead of time by the physician, and a menu of therapies is typically provided. The menu of therapies is often referred to as “tiered” therapies in that the therapies tend to progress from relatively lower-energy, less aggressive therapies, to higher-energy, more aggressive therapies. For example, on initial detection of tachycardia, an anti-tachycardia pacing therapy may be selected. On redetection of tachycardia, a more aggressive anti-tachycardia pacing therapy may be scheduled. If repeated attempts at anti-tachycardia pacing therapies fail, a higher-level cardioversion shock pulse therapy may be selected thereafter. The amplitude of a cardioversion/defibrillation shock may be incremented in response to failure of an initial shock or shocks to terminate tachycardia/fibrillation.

When such menus of therapies are available, a tiered therapy sequence may be initiated by CPU204in response to a detected atrial arrhythmia. On redetection, the therapy sequence may proceed up to a programmed high-voltage shock therapy. A programmed shock therapy may be postponed until a predetermined time after sleep onset is detected by CPU204according to the method300ofFIGS. 3A through 3D.

Alternatively, when atrial arrhythmias requiring a high-voltage shock therapy are detected simultaneously with the detection of a sustained sleep detection, the programmed shock therapy may be delivered without delay. However, if an atrial arrhythmia requiring a high-voltage shock therapy is detected without concurrent sleep detection by CPU204, any programmed high-voltage shock therapies may be canceled. Programmed pacing therapies or lower-voltage cardioversion therapies may proceed according to a menu of therapies.

FIG. 10is a flow chart summarizing the steps performed in one embodiment of a method for controlling the delivery of high-energy shock pulses for treating atrial arrhythmias pending the detection of sleep. Method800is initiated at step802upon detection of AT or AF. At step804, a determination is made whether sleep onset was detected within a predetermined interval of time prior to the AT/AF detection. Sleep onset is preferably detected based on the deviation of MV values from a MV statistical parameter as described previously in conjunction withFIGS. 3A through 3D. However, sleep onset may also be detected using other known detection schemes, such as described, for example, in U.S. Pat. No. 6,128,534 issued to Park et al., U.S. Pat. No. 5,814,087 issued to Renrie, and U.S. Pat. No. 5,476,483 issued to Bornzin et al., all of which are incorporated herein by reference in their entireties.

If sleep onset was previously detected within the predetermined time interval such that the patient is currently believed to be asleep, a programmed cardioversion/defibrillation (CV/DF) shock is delivered at step806. If sleep onset was not previously detected within a predetermined time interval, as determined at decision step804, a programmed CV/DF shock is cancelled at step808. It is to be understood that other arrhythmia therapies programmed to be delivered in a menu of therapies may be delivered upon detection of AT/AF at step802. If a high-energy cardioversion or defibrillation shock is included in the menu of therapies, however, prior to preparing for shock delivery, i.e., prior to initiating capacitor charging, a verification that sleep onset was previously detected is made (step804). The sleep onset detection is preferably made within a predetermined window of time, for example within the last 15 to 30 minutes, such that the patient is still expected to be asleep.

FIG. 11is a flow chart summarizing the steps performed in an alternative embodiment of a method for controlling the delivery of high-energy shock pulses for treating atrial arrhythmias pending the detection of sleep. Rather than canceling a programmed shock therapy if sleep onset has not been detected, a programmed shock therapy may be delayed until a time after sleep onset is detected. Method900begins at step902upon AT/AF detection, after which monitoring for the onset of sleep is performed at step904. Sleep onset is detected based on long-term and relatively shorter-term MV parameter evaluation as described previously.

If sleep onset is detected, as determined at decision step906, method900proceeds to step910to monitor for arousal. Otherwise, method900continues monitoring for sleep onset by returning to step904. At step904, MV parameter values are determined at the end of each MV sensor time interval, e.g., at the end of 2 seconds. At step906, a comparative analysis of the MV parameter values, e.g., “MV Stdev Long” and “MV Stdev Short” as described previously, for determining if the onset of sleep has occurred.

Monitoring for arousal at step910involves a similar process in that at the end of each MV sensor time interval, MV parameter values are determined. These MV parameter values are evaluated in a comparative analysis at step912to determine if sleep is no longer indicated, i.e., arousal has occurred. As described previously, while the patient is sleeping, the “MV Stdev Long” value is typically greater than or equal to the “MV Stdev Short” value. However, when the patient wakes up abruptly and becomes active, the “MV Stdev Short” value will become greater than the “MV Stdev Long” value, indicating the patient has transitioned from the “sleep” state to the “awake” state. Thus, arousal may be detected at step912based on a “MV Stdev Short value” becoming greater than the “MV Stdev Long” value subsequent to the sleep onset detection.

If arousal is detected, method900returns to step904to resume monitoring for sleep onset. Any elapsed time measured since the onset of sleep detection is reset to zero at step914. If arousal is not detected at decision step912, the elapsed time since sleep onset detection is measured at step916. The MV sensor time interval, e.g., 2 seconds, is added to the “current” elapsed time value at step916.

At step918, the elapsed time is compared to a “shock wait time.” The “shock wait time” is a predetermined, programmable amount of time after which a delayed CV/DF shock will be delivered. The “shock wait time” preferably corresponds to the time normally required for a patient to reach a deep sleep stage after sleep onset. Typically, a person enters deep sleep approximately 20 minutes after sleep onset occurs if not aroused or disturbed. A “shock wait time” may appropriately be set, therefore, between 20 and 60 minutes, for example.

If the elapsed time has reached the “shock wait time” as determined at decision step918, the ICD10may verify that the AT/AF is still being detected at decision step920and, if so, deliver the delayed CV/DF shock thereafter at step922. If the elapsed time since sleep onset detection has not reached the “shock wait time,” the patient is presumably not yet in a deep sleep stage. Method900returns to step910to continue monitoring for arousal and measuring the elapsed time since sleep onset if arousal is not detected.

If AT/AF is no longer being detected after the elapsed time equals the “shock wait time”, as determined at decision step918, delivery of the delayed shock is no longer appropriate. Method900is then terminated at step924, and the delayed shock is canceled.

FIG. 12is a flow chart providing additional details included in one embodiment of the step for detecting arousal included in method900ofFIG. 11. Steps for monitoring for arousal that may be included in step910of method900are grouped within dashed line inFIG. 12. After detecting sleep onset at step906of method900(FIG. 11) as described previously, monitoring for arousal commences at step926wherein computational circuitry receives the next MV value determined during a predetermined time interval, e.g., 2 seconds, from MV sensing circuitry. The received MV value is used to calculate a current “MV Stdev Short” value and a current “MV Stdev Long” value as indicated at step928and as described previously. At decision step930, the current “MV Stdev Short” value is compared to the current “MV Stdev Long” value. If the “MV Stdev Short” value has become greater than the “MV Stdev Long” value, the patient may have become active indicating arousal.

If the current “MV Stdev Short” value has also exceeded the second “MV threshold” value described previously, arousal is detected at step934. On the other hand, if the “MV Stdev Short” value is not greater than the “MV Stdev Long” value (step930), or if the “MV Stdev Short” value is greater than the “MV Stdev Long” value but is still less than the second “MV Threshold” value (step932), a “sleep” state detection is sustained as indicated at step936. After completing the subroutine represented by steps926through936, method900proceeds to decision step912and thereafter to step914or916(FIG. 11) depending on whether arousal has been detected.

FIG. 13is a flow chart summarizing the steps performed in yet another embodiment of a method for controlling the delivery of high-energy shock pulses for treating atrial arrhythmias pending the detection of sleep. Steps902through924included in method950shown inFIG. 13correspond to identically-labeled steps included in method900ofFIG. 11, described above. During execution of the previously-described method900, a pending shock therapy may be withheld indefinitely if the programmed “shock wait time” is never reached (at step918) after a sleep onset detection is made (step906). The “shock wait time” may not be reached during a period of sleep, for example, when a patient is restless or experiences frequent arousals during the night. Thus, a pending shock therapy will not be delivered. It may be undesirable, however, to withhold a pending AT/AF shock therapy for an extended period of time, for example for more than 24 to 48 hours. In such cases, a pending shock therapy may be scheduled to occur at a particular time of day, e.g., 4 a.m., or after a shortened “shock wait time,” whichever occurs first.

Thus, method950ofFIG. 13includes additional steps for ensuring that a pending shock therapy is not withheld indefinitely. If arousal is detected at step912ofFIG. 13, after a sleep onset detection is made at step906, an elapsed time counter is reset at step914as described previously. However, before returning to step904to monitor for sleep onset again, method950determines if a “maximum pending shock time” has expired at step940. If not, method950proceeds to step904. However, if a “maximum pending shock time” has expired, the “shock wait time” is shortened at step942. By shortening the “shock wait time,” the pending shock therapy is more likely to be delivered following the next sleep onset detection.

A “maximum pending shock time” may be defined as an interval of time measured from the time of AT/AF detection. A maximum pending shock time may be programmed as a number of minutes or hours following an AT/AF detection, e.g., 12, 18 or 24 hours. Alternatively, a “maximum pending shock time” may be programmed to correspond to a time of day. For example, the maximum pending shock time may expire at a scheduled time of day, such as 7:00 a.m., indicating the end of “night time.” If the “shock wait time” is not reached during the first night following AT/AF detection, the “maximum pending shock time” will be reached. Upon reaching the “maximum pending shock time,” at step940the “shock wait time” is shortened at step942.

After the “shock wait time” is shortened, method950continues to step904to monitor for the onset of sleep. If the elapsed time measured following a sleep detection (step916) reaches the shortened “shock wait time” (step918), and AT/AF is still being detected (step920), the shock therapy is delivered at step922. However, if the shortened “shock wait time” is not reached (step918), and the “maximum pending shock time” has been exceeded, a shock therapy may be delivered at a scheduled, “default” shock delivery time. Thus, if the elapsed time does not reach the shortened “shock wait time” at decision step918, method950determines if the “maximum pending shock time” has been exceeded and a scheduled “default” shock delivery time has been reached at decision step944. If these conditions are satisfied, the shock therapy is delivered at step922after verifying the sustained AT/AF detection at step920. A “default” shock delivery time may be programmed to a time of day, for example 4:00 a.m., at which the patient is expected to be resting.

If the scheduled “default” shock time has not been reached at decision step944, method950returns to step910to continue to monitor for arousal. The pending shock therapy will thus be delivered, at step922: after the originally programmed “shock wait time” following a sleep onset detection; after a shortened “shock wait time” if a “maximum pending shock time” has expired; or at a scheduled “default” shock delivery time if a “maximum pending shock time” has expired, which ever occurs first.

Some of the techniques described above may be embodied as a computer-readable medium comprising instructions for a programmable processor such as microprocessor204or timing/control circuitry208shown inFIGS. 2 and 9. The programmable processor may include one or more individual processors, which may act independently or in concert. A “computer-readable medium” includes but is not limited to any type of computer memory such as floppy disks, conventional hard disks, CR-ROMS, Flash ROMS, nonvolatile ROMS, RAM and a magnetic or optical storage medium. The medium may include instructions for causing a processor to perform any of the features described above for initiating a session of the escape rate variation according to the present invention.