Differentiating between arrhythmia and noise in an arrhythmia control system

Abstract: This invention provides an apparatus and method for differentiating between arrhythmia and noise in an antiarrhythmia device (15-19). A high gain channel (29, 30) is provided for noise detection. A low gain channel (27, 28) is provided for noise detection and for detecting the presence of an arrhythmia when no noise is detected on the high gain channel (29, 30). Arrhythmia therapy circuitry delivers antiarrhythmia therapy. A noise detection criterion is applied to each of the high and low gain channels (27-30).

Description: This invention relates to implantable medical devices which monitor the cardiac state of a patient by sensing the patient's intrinsic rhythm for the presence of arrhythmias and which deliver therapy in the form of electrical energy to cardiac tissue in an attempt to revert detected arrhythmias and restore a normal sinus rhythm to the patient. As used herein, the term arrhythmia refers to any abnormal rhythm of the heart which may be amenable to treatment by electrical discharges and specifically includes tachyarrhythmias, fibrillation, tachycardias, supraventricular tachycardia (SVT), ventricular tachycardia (VT), ventricular flutter and ventricular fibrillation (VF), and bradycardia. The term therapy as used herein includes the processes used between the detection and reversion of an arrhythmia'and includes the actions of antitachycardia or bradycardia pacing and cardioversion. The term cardioversion refers to the discharge of electrical energy onto the cardiac tissue in an attempt to terminate or revert a tachycardia and may range from a high (40 Joules or more) to a low (less than 1 Joule) energy discharge. The discharge may be monophasic or biphasic but is not restricted to these waveforms. Cardioversion shocks may or may not be synchronized to the rhythm of the heart. Defibrillation is a particular example of cardioversion. This invention applies equally to devices which deliver energy synchronised to an R-wave and to those that do not, and it applies to devices which use lower energy pulses (up to 1 Joule) as well as to devices which deliver cardioversion shocks alone or in combination with antitachycardia and bradycardia pacing pulses. Noise protection circuits have been incorporated into demand pacemakers in order to prevent noise signals from being detected as natural heart beats. When such false detection occurs, the artificial stimulation is suppressed thus leading to a potentially dangerous situation for the patient. These noise protection circuits include filters to attenuate noise signals of particular frequencies and refractory periods during which time detected signals are ignored as described in U.S. Patent No. 4,173,230 to Digby entitled "Noise Elimination and Refractory Period Control in Demand Pacers". U.S. Patent No. 4,649,931 to Beck entitled "Sampled Data Sense Amplifier" describes a sense amplifier for a cardiac pacemaker which generates a detect signal in response to a depolarization of cardiac tissue. In operation, this sensing system searches for a sampling frequency which permits the detection of the physiological signal in the presence of continuous wave noise. This is a time-consuming procedure which is appropriate only when the interference present is periodic. Noise protection is also important in antitachyarrhythmia devices where the false detection of noise signals may lead to the delivery of unnecessary therapy to the patient. Known antitachyarrhythmia devices use a single channel for sensing and require a predetermined number of intervals to lie within a specified window for a tachyarrhythmia to be detected. The intermittent sensing of low level noise can, however, lead to the false detection of tachycardia and the subsequent delivery of unnecessary and possibly fatal therapy to the patient. The switching of gains can be used in single channel devices to alter the sensitivity required at the sensing electrode for a sense to be registered. The adaptation of this technique for noise detection would result in undesirable time delays in the detection procedure and in the delivery of any subsequent therapy. There is a need, therefore, for a device which is capable of reliably differentiating between an arrhythmia and noise and having a minimal time delay for the detection procedure. EP-A-253 505 discloses an antiarrhythmia device comprising an apparatus for differentiating between arrhythmia and noise in a signal from a patient's heart. The signal from the heart is led through two channels. The gain of one channel is set by a computer but, when set to high gain, it is not used for noise detection US Patent specification 4,091,818 describes a cardiac pacing apparatus with electromagnetic interference protection. When noise is detected in a second one of two signal processing channels, the cardiac pacer is switched to a safe mode. It is an object of the invention to increase patient safety by reliably detecting noise in an implantable arrhythmia control device. It is a further object of the invention to reliably differentiate between an arrhythmia and noise by means of a detection procedure having a minimal time delay. According to the invention, there is provided an antiarrhythmia device as defined in claim 1 below. Further objects, features, and advantages of the invention will become apparent upon consideration of the following detailed description in conjunction with the drawings, and given by way of example only, in which :- FIG. 1 is a block diagram of an arrhythmia control system; FIG. 2 is a block diagram of the pacemaker of FIG. 1; FIG. 3 is a block diagram of the sensing circuitry of FIG. 2; FIG. 4 is a block diagram of the microprocessor of FIG. 1; FIG. 5 is a table illustrating the detection of a tachycardia using the sensing circuitry of FIG. 3; and FIGS. 6-9 illustrate the detection of noise using the sensing circuitry of FIG. 3. Referring to FIG. 1, there is depicted a block diagram of an arrhythmia control system (ACS) 10 which comprises: a cardiac lead 12 connected to the patient's heart 11; a pacemaker 16 for the detection of analog signals representing cardiac electrical activity and for the delivery of pacing pulses to the heart; a microprocessor 19 which, in response to various inputs received from the pacemaker 16 as well as from the defibrillator 15, performs various operations so as to generate different control and data outputs to both the pacemaker 16 and the defibrillator 15 which produces a high voltage to charge its capacitors and then discharges them in response to control signals from the microprocessor 19; and a defibrillator electrode lead 13 for transferring the energy of a defibrillator shock 14 from the ACS 10 to the surface of the heart. ACS 10 is preferably an implanted device. Referring to FIG. 2, there is depicted a block diagram of the pacemaker 16 of FIG. 1. As seen therein, pacemaker 16 comprises circuitry for pacing 24, sensing 25, and telemetry 31. In addition, there is a control block 32. In operation, the sensing circuitry 25 detects analog signals 23 from the heart 20 and converts the detected signals to digital signals. Furthermore, the sensing circuitry 25 receives input sense controls 28 and 30 from the control block 32 which determine the sensitivities applied to the detection circuits. A change in these sensitivities will affect the voltage deviation required at the sensing electrode for senses to be registered. There are various types of logic arrangements which can be used to change the sensitivity corresponding to the detection of cardiac activity. The pacing circuitry 24 also receives inputs from the control block 32 including a pace control and a pacing energy control. The pace control determines the type of pacing to occur while the magnitude of the pulse energy is determined by the pacing energy control. The operation of the logic which changes the pulse energy is described in more detail in the co-pending European Patent Application No. 89.300232.9 filed 11th January 1989 entitled "Apparatus and Method for Controlling Pulse Energy in Antitachyarrhythmia and Bradycardia Pacing Device". The pacing circuitry 24 generates the pacing pulse 22 which is delivered to the patient's heart 20 by means of the cardiac lead 21. The telemetry circuit 31 provides a bi-directional link between the control block 32 and an external device such as a programmer. It allows data such as the operating parameters to be read from or altered in the ACS implant. Referring to FIG. 3, there is shown a block diagram of the sensing circuitry 25 of FIG. 2. As seen therein, the sensing circuitry comprises two amplifiers 41 and 42 and two comparators 43 and 44. The detected analog signal 23 is input to both the low gain amplifier 41 and the high gain amplifier 42 where, in a preferred embodiment, there is a 3-6 dB difference in the gain of amplifier 41 to amplifier 42. The output of the low gain amplifier is compared with the low gain control signal 28 by comparator 43. The level of this low gain control signal is selected so that cardiac activity is reliably sensed. Similarly, the output of the high gain amplifier is compared with the high gain control signal 30 by comparator 44 where the level of this high gain control signal 48 is set so that noise activity is reliably sensed. Referring to FIG. 4, there is shown a block diagram of the microprocessor 19 of FIG. 1. It comprises two 16-bit timers 65 and 66, CPU 67, vectored interrupt block 68, ROM 62, RAM 63, external memory 64, ports 61 and an internal communication bus 60. The microprocessor 19 receives various status and/or control inputs from pacemaker 16 and defibrillator 15 such as the low gain sense 27 and the high gain sense 29, and performs operations such as arrhythmia detection and noise detection. Tachycardia detection may be performed using any known tachycardia detection algorithm. In a preferred embodiment, an x out of y tachycardia detection criterion is used. This requires at least x intervals out of the previous y intervals to be less than the tachycardia detection interval (TDI). Similarly, noise detection may be performed using any known noise detection algorithm. In a preferred embodiment, an n out of z noise detection criterion is used. This requires at least n intervals in the previous z intervals to be less than the noise detection interval (NDI). The control outputs generated by the microprocessor 19 include the pace control 77 which determines the type of pacing to take place, the pacing energy control 74 which determines the magnitude of the pulse energy, the shock control 71 which signals that a shock is to be delivered to the patient, the dump control 70 which indicates that a shock is to be dumped at an internal load within the defibrillator, the charge control 72 which determines the voltage level of the shock to be delivered, and the sensitivity controls, low gain control 75 and high gain control 76, which determine the sensitivity settings of the sensing circuits. FIGS. 5-9 refer to the four conditions which can result when the noise detection criterion is simultaneously applied to the low gain and the high gain channels. Noise is detected when the noise detection criterion is satisfied on the high gain channel. When noise is detected, the low gain signal can still be used for arrhythmia detection except when the noise detection criterion is also satisfied on the low gain channel. In a preferred embodiment, the noise detection criterion is satisfied on the low gain channel when at least n intervals in the previous z intervals are less than the NDI where n = 2, z = 10 and NDI = 100 milliseconds and on the high gain channel when at least n' intervals in the previous z' intervals are less than the NDI where n' = 9, z' = 10 and NDI = 100 milliseconds. FIG. 5 depicts the four possible conditions of the noise detection algorithm. They are (i) the noise detection criterion is not satisfied on either the low gain channel or the high gain channel and therefore noise is not detected and the low gain channel can be used for arrhythmia detection; (ii) the noise detection criterion is not satisfied on the low gain channel but it is satisfied on the high gain channel and therefore noise is detected but the low gain channel can still be used for arrhythmia detection; (iii) the noise detection criterion is satisfied on the low gain channel but not on the high gain channel and therefore noise is not detected and the low gain channel can be used for arrhythmia detection; and (iv) the noise detection criterion is satisfied on both the low gain channel and the high gain channel and therefore noise is detected and the low gain channel cannot be used for arrhythmia detection. FIG. 6 depicts the operation of the sensing circuitry of FIG. 3 when noise is not detected. The detected analog signal 90 is shown to be sinus rhythm. The noise detection criterion is not satisfied on either the low gain channel 92 or the high gain channel 94. The low gain channel is therefore used for arrhythmia detection which establishes that an arrhythmia does not exit. FIG. 7 depicts the operation of the sensing circuitry of FIG. 3 when noise is detected. Low amplitude noise is shown to be present on the detected analog signal 100. The noise detection criterion is not satisfied on the low gain channel 102 but at least n' = 9 out of the previous z' = 10 intervals on the high gain channel 104 are less than NDI = 100 milliseconds and hence noise is detected. However, because the amplitude of the noise is such that it has not affected the low gain channel, this signal is still used for arrhythmia detection which establishes that an arrhythmia does not exist. FIG. 8 depicts the operation of the sensing circuitry of FIG. 3 when noise is not detected. The detected analog signal 110 is shown to be sinus rhythm. Double sensing of the signal 110 is seen to occur, resulting in at least n = 2 out of the previous z = 10 intervals on the low gain channel 112 being less than NDI = 100 milliseconds thus satisfying the noise detection criterion on the low gain channel. However, at least n' = 9 out of the previous z' = 10 intervals are not less than NDI' = 100 milliseconds on the high gain channel 114. Therefore, noise is not detected and the low gain channel is used for arrhythmia detection which establishes that an arrhythmia does not exist. FIG. 9 depicts the detection of noise using the sensing circuitry of FIG. 3. As shown at 120, noise is present in the signal. The low gain channel intermittently senses the noise activity and as a consequence at least x out of y sensed intervals are less than the TDI where x = 8, y = 10, and TDI = 400 milliseconds in a preferred embodiment. The tachycardia detection criterion is therefore satisfied on the low gain channel. However, the noise detection criterion is also satisfied on both the low gain channel and the high gain channel. The noise present in the signal has therefore been correctly detected and the unnecessary delivery of antitachyarrhythmia therapy to the patient has been avoided. Although the invention has been described with reference to a particular embodiment, it is to be understood that this embodiment is merely illustrative of the application of the principles of the invention. Furthermore, the noise detection intervals could include frequency histograms or statistical measures, such as means, medians or variance which may be implemented with suitable programming of the microprocessor.

Claims: An antiarrhythmia device (1) comprising an apparatus for differentiating between arrhythmia and noise in a signal (40) from a patient's heart including: a high gain channel detecting means (42,44) for receiving the above signal (40) for noise detection; a low gain channel detecting means (41,43) for noise detection and for detecting the presence of an arrhythmia in the above signal (40) when no noise is detected by said high gain channel detecting means; arrhythmia therapy means (16,19) responsive to said low gain channel detecting means (41,43) for delivering arrhythmia theraphy; and means (19, 32) for applying a noise detection criterion to each of said high and low gain channel detecting means (42,44); whereby, when: (i) the noise detection criterion is not satisfied on either the low gain channel or the high gain channel, noise is not detected and the low gain channel can be used for arrhythmia detection; (ii) the noise detection criterion is not satisfied on the low gain channel but it is satisfied on the high gain channel, noise is detected but the low gain channel can still be used for arrhythmia detection; (iii) the noise detection criterion is satisfied on the low gain channel but not on the high gain channel, noise is not detected and the low gain channel can be used for arrhythmia detection; and (iv) the noise detection criterion is satisfied on both the low gain channel and the high gain channel, noise is detected and the low gain channel cannot be used for arrhythmia detection. An apparatus according to claim 1, further comprising arrhythmia detection means (19) responsive to a tachyarrhythmia detected by said low gain channel detecting means (41,43), and for controlling said arrhythmia therapy means (16,19). An apparatus according to claim 2, wherein said arrhythmia detection means (19) is responsive to a fibrillation. An apparatus according to claim 2, wherein said arrhythmia detection means (19) is responsive to a tachycardia. An apparatus according to claim 1, further comprising arrhythmia detection means (19) responsive to a bradycardia detected by said low gain channel detecting means (41,43), and for controlling said arrhythmia therapy means. An apparatus according to claim 1, wherein said high gain channel detecting means determines that said noise detection criterion is met when x out of y intervals are less than a specified value. An apparatus according to claim 1, wherein said high gain channel detecting means includes a calculating means for calculating frequency histograms to determine whether said noise detection criterion is met. An apparatus according to claim 1, wherein said high gain channel detecting means includes a calculating means for calculating statistical measures such as mean, median, or variance to determine whether said noise detection criterion is met. An apparatus according to claim 1, wherein at the time of delivering said therapy, said high gain channel is used for detecting the presence of a tachyarrythmia. An apparatus according to claim 9, wherein said tachyarrhythmia is either a tachycardia or a fibrillation.

Inventors: MAKER PHILIP JOHN, MAKER, PHILIP JOHN
Applicants: TELECTRONICS NV, TELECTRONICS N.V.
Application year: 1989
Main classification: A61N1
IPC codes: A61N1