Abstract:
An auditory pulse monitor for noninvasively detecting the amplitude of arterial pulses on a beat-by-beat basis. A light-weight optical sensor including a light source and photodetector is adapted for application to the skin surface of a subject over a tissue bed containing an arterial supply. The photodetector generates an output signal proportional to the amplitude of an arterial pulse, and an electronic circuit connected to the photodetector generates a signal having a frequency proportional to the photodetector output signal level. A speaker or other audio indicator connected to the electronic circuit generates an audible tone indicating the amplitude of the arterial pulse. Another aspect of the invention is an improvement in automated or automatic external defibrillators (AEDs). An AED is disclosed which optically detects arterial pulses after delivering a defibrillation shock and signals the need for CPR if it detects inadequate cardiac pumping following successful defibrillation.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to devices and methods for monitoring cardiac pumping, and more particularly to a new device and method for noninvasively detecting the quality of cardiac pumping resulting from cardiopulmonary resuscitation (CPR) or from automatic external defibrillation. 
   CPR is a well known and valuable emergency procedure for reviving a person suffering cardiac arrest. When the heart stops pumping blood, the resulting lack of fresh oxygen to the brain can cause brain damage within minutes and death can soon follow. CPR involves repetitive chest compression coordinated with mouth-to-mouth breathing, and its effectiveness depends on coordinated delivery of adequate chest compressions and rescue breaths and, to a large extent, on the quality and timing of the chest compressions. Much attention has been devoted to the subject and particularly to techniques for training emergency medical personnel as well as ordinary citizens how to perform CPR properly even under stress associated with treating a life-threatening condition and even if fatigued from a sustained effort. 
   A pressure-sensitive chest pad has been proposed as a feedback tool for a rescuer administering chest compressions during CPR. For example, an automatic external defibrillator (AED) recently introduced by Zoll Medical Corporation, the Zoll AED PLUS, is available with a chest pad with which it is said to monitor rate and depth of chest compressions when the rescuer presses on the pad. Voice and visual prompts encourage a compression depth of 1½-2 inches. However, the AED gives no indication of the effectiveness of pumping of blood. One of the rescuers is prompted to check the victim for the presence of a pulse or other signs of circulation such as normal breathing, movement, coughing or color of the lips or skin. Such methods do not give the typical human rescuer feedback fast enough to enable the rescuer to vary the style of chest compression in order to optimize blood pumping. 
   Perfusion monitors designed to measure blood gases, such as the monitor proposed in U.S. Pat. No. 5,579,763 to Weil et al., can take minutes to respond and thus also fail to provide sufficiently rapid feedback, e.g., beat-by-beat feedback, for a typical human rescuer. 
   A need exists for a simple and practical device which can give a positive indication of the effectiveness of blood pumping in response to chest compression during CPR. There is also a need for a simple and practical way to noninvasively detect cardiac pumping following defibrillation with an AED. 
   Electromechanical dissociaton (EMD) is a condition in which the R waves of the ECG either do not produce ventricular contractions or produce extremely weak contractions, resulting in little or no blood pumping. The condition is sometimes known as pulseless electrical activity (PEA). It occurs frequently because ventricular fibrillation is often not treated with CPR for a period of time or the CPR is inadequately performed. In such situations, the heart muscle, being deprived of oxygenated blood because there is no blood pumping, becomes injured and consequently beats weakly or not at all after successful defibrillation. Thus, even though an AED may indicate that the criterion for successful defibrillation has been met, namely abolishing the high-frequency fibrillation waves in the ECG, the heart muscle cannot respond adequately to the resulting R waves and the victim is likely to die as a result if other intervention is not promptly provided. 
   Another postdefibrillation situation for failure of the ventricles to contract is atrioventricular (A-V) block. A-V block is not uncommon immediately after successful ventricular defibrillation and no ventricular pulses are produced. 
   The appropriate therapy for EMD and A-V block is the prompt application of effective CPR. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, a beat-by-beat auditory pulse monitor allows a rescuer who is administering CPR to evaluate the effectiveness of each chest compression to pump blood. The inventive device detects the arterial pulse resulting from each chest compression and generates an audible tone indicating the amplitude of each pulse. An optical sensor including a light source and photodetector is applied to a skin surface of a subject over a tissue bed containing an artery, preferably where there is a substantially planar underlying bone surface, such as the forehead, which reflects incident radiation. In a preferred embodiment, the frequency of the audible tone is proportional to the instantaneous amplitude of the pulse generated by that chest compression, varying continuously over a cardiac cycle. Thus, the rescuer has an immediate feedback signal that informs him/her of the effectiveness of each chest compression and he/she can modify the compression style to obtain the largest amplitude pulse, identified by the tone with the highest pitch. 
   Another aspect of this invention is an improvement in automated or automatic external defibrillators (AEDs). An improved AED according to the present invention detects arterial pulses after delivering a defibrillation shock and signals the need for CPR if it detects inadequate cardiac pumping following successful defibrillation. 
   Other objects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of one embodiment of the optical sensor portion of a pulse monitor according to the present invention, shown from the rear in position on the forehead of a human subject. 
       FIG. 2  is a block diagram of one embodiment of a circuit to which the optical sensor of  FIG. 1  is connected in an auditory pulse monitor according to the present invention. 
       FIG. 3  is an exploded, front perspective view of one embodiment of a self-contained pulse monitor according to the present invention. 
       FIGS. 4 and 5  are records of ECG, blood pressure and optical sensor output waveforms obtained prior to and during induced ventricular fibrillation in an animal experiment. 
       FIG. 6  is a block diagram of an AED equipped with the optical pulse sensor to detect electromechanical dissociation according to the present invention. 
       FIG. 7  is a record of waveforms obtained following defibrillation after a period of untreated fibrillation, and shows the presence of EMD and the effect of rhythmic chest compressions revealed in the blood pressure and optical pulse sensor records. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
   Referring to  FIGS. 1 and 2 , one embodiment of an auditory pulse monitor according to the present invention includes an optical sensor  10  connected to a circuit  12 . The small optical sensor is preferably adapted for quick placement on the forehead of a subject  14  and includes one or more light-emitting diodes (LEDs)  16  that surround a photodetector  18  as shown in  FIG. 1 . The LEDs are preferably infrared devices, e.g., PDI-E801 or PDI-E804 880 nm LEDs available from Photonic Detectors, Inc. The LEDs and photodetector are preferably matched to operate at a desired wavelength. One example of a suitable photodetector is a Fairchild Semiconductor QSD723 phototransistor, with a peak sensitivity at 880 nm. Another suitable operating wavelength is 805 nm. At 805 nm oxygenated blood (HbO 2 ) and blood without oxygen (Hb) transmit equally well. Therefore the pickup has no oxygen-saturation error. An advantage of either of the example wavelengths is that there are virtually no environmental light sources in this infrared region. 
   Although the specific placement of the optical sensor on the forehead of a human subject is not critical, the maximum amplitude for pulse detection has been found to occur on or near the center of the forehead, slightly above the eyebrows. With the LEDs equally spaced about a photodetector as in  FIG. 1 , the sensor has rotational symmetry and is therefore substantially insensitive to orientation on the skin surface. It is presently preferred to have such an arrangement with three LEDs. It is particularly desirable to have a light-weight sensor in order to minimize movement artifacts; the preferred optical sensor preferably weighs only about 2 grams. The optical sensor may be held on the subject&#39;s forehead with an elastic band, or by suction applied to an annular chamber around the LEDs and photodetector, or with double-sided tape such as Stomaseal from 3M. 
   Circuit  12  includes a first amplifier  20  connected to a voltage-controlled oscillator (VCO)  22  the output of which is connected to a second amplifier  24  and a loudspeaker  26  as shown in  FIG. 2 . The circuit also contains appropriate circuitry for driving the LEDs simultaneously, and may be contained in a small case together with a battery power source connected by a cable to the optical sensor such that the auditory pulse monitor is a self-contained device for rapid application in the field. Alternatively, as shown in  FIG. 3 , a self-contained unit may be constructed with circuit  12 , including a battery power source, on a printed circuit board affixed to the rear surface of sensor  10 . The LEDs and photodetector may be retained within a molded plastic disc as shown in  FIG. 3 , with cylindrical through-holes formed in the plastic disc to receive and align the LEDs and photodetector, which may also be directly electrically connected to conductive traces on the circuit board. 
   Arterial pulses are detected by reflectance from the tissues in the tissue bed under the optical pickup. The photodetector may be AC-coupled via capacitor  28  and amplifier  20  to the VCO, which produces a frequency proportional to the voltage applied to it, and may also be DC-coupled as shown. The circuit is designed such that, with no arterial pulse, it produces a low-frequency tone when the pickup is on the forehead to indicate that the pickup is in place. The circuit also provides a characteristic tone that indicates that the sensor is not on the body, i.e., a sensor-off tone. 
   The LEDs may be pulsed on and off to reduce power consumption. For example, in one embodiment having two PDI-E801 LEDs connected in series, the LEDs are pulsed on at a pulse repetition rate of 100 Hz with a pulse width of 200 μS, resulting in a 2% duty cycle, and at a current level of approximately 400 mA. A high speed op-amp, e.g., Analog Devices AD823, may be connected as a buffer between the phototransistor, e.g., Fairchild QSD723, and a sample-and-hold, e.g., Maxim MAX394. The phototransistor output signal is preferably sampled near the end of the LED ON time, e.g., beginning at 190 μS after the start of the LED ON pulse, with a sampling interval of 2.5 μS as one example. The sampling delay compensates for the relatively slow response time of the phototransistor, and, by sampling only near the end of the LED ON time, output ripple is minimized. The sampled signal is filtered with first order reconstruction filters, e.g., Analog Devices AD823, with a bandwidth from 0.5 Hz to 30 Hz, and the resulting analog signal, representing the pulsatile changes in blood volume adjacent to the sensor&#39;s optics, may be supplied to a VCO and audible device as described above. 
   In operation, each time the subject&#39;s chest is compressed and blood is pumped, the frequency of the tone increases in proportion to the instantaneous amplitude of the arterial pulse produced by that chest compression. The tone frequency varies continuously over a cardiac cycle. Thus, with each chest compression, the higher the tone frequency attained, the larger the arterial pulse produced. In this way a rescuer can vary his/her style of chest compression to obtain the highest pitch tone for each chest compression and thereby maximize the effectiveness of blood pumping. The rescuer hears a variable-frequency whistling sound due to the varying pulse amplitude during chest compression. 
     FIGS. 4 and 5  are records illustrating the use of the optical sensor to detect the arterial pulse produced by rhythmic chest compression during ventricular fibrillation. In these examples, an optical sensor of the type described above was placed on the head of a 16 kg anesthetized pig.  FIG. 4  illustrates the ECG, photodetector output pulses and blood pressure with the heart beating normally until point X. The ECG and blood pressure are obtained in a conventional manner. At point X, ventricular fibrillation was induced. Note the immediate changes in the ECG and disappearance of the pulsatile optical pulses and disappearance of pulsatile blood pressure, characteristic of ventricular fibrillation. Observe also, during fibrillation, the small atrial (A) pulses in blood pressure; these are present because the atria are still beating. Note that the optical pickup also detects them. Soon however, the atrial pulses disappear because the atria cease beating with increasing hypoxia. 
   The record of  FIG. 5 , made several minutes after that of  FIG. 4 , shows fibrillation in the ECG, no optical pulses and no pulsatile blood pressure. At the first point C in the graph, chest compressions were commenced which were clearly detected by the optical sensor and are shown in the blood-pressure record. Cessation of chest compression (after 5 compressions) resulted in disappearance of the optical pulses and blood-pressure pulses. 
   Turning now to  FIG. 6 , an AED  30  according to another embodiment of the present invention includes an energy storage circuit  32 , two defibrillation electrodes  34 , a control circuit  36  and an optical blood pulse detector  38 . The energy storage circuit includes a conventional capacitor network as well as electronically controllable switches for capacitor charging and discharging. The control circuit includes a microprocessor suitably programmed to control the delivery of defibrillation pulses and to analyze the ECG signal obtained from the electrodes. The AED analyzes the frequency of the electrogram and prompts the rescuer to deliver a defibrillation shock if it detects fibrillation in the ECG. Alternatively, the AED may be configured to automatically deliver a shock upon detection of fibrillation. 
   The optical blood pulse detector, which may be a sensor such as optical sensor  10  placed on the victim&#39;s forehead as described above, supplies pulses corresponding to detected arterial pulses to the control circuit, and the control circuit analyzes the input pulses after delivery of a defibrillation shock. The control circuit may be programmed to analyze the pulses from the optical sensor after first analyzing the ECG and determining that the frequency of the electrogram is within a predetermined range indicative of successful defibrillation. The optical blood pulse detector may be connected to the AED control circuit by a cable or, with appropriate modulation/demodulation circuitry, may be connected by a wireless link, e.g., an RF, infrared or ultrasonic link. The AED preferably has a voice chip, i.e., a voice synthesizer integrated circuit, which may be used to generate a tone with varying pitch proportional to arterial pulse amplitude. Alternatively, the AED may be provided with a microphone and audio circuitry for directly responding to the frequency-modulated audible tone from a stand-alone auditory pulse monitor such as described above. If the defibrillation shock achieves the desired result of eliminating the fibrillation but the detected arterial pulse amplitude indicates weak cardiac pumping or no pumping, the control circuit signals the need for CPR. The control circuit may prompt the rescuer with a voice command such as the following: “Defibrillation achieved, resume CPR.” The control circuit may be further programmed to detect the absence of QRS waves in the post-defibrillation ECG, indicative of A-V block, or the presence of QRS waves in the post-defibrillation ECG, indicative of electromechanical dissociation (EMD) if there is little or no cardiac pumping, and may issue a corresponding voice command, e.g., “Defibrillation achieved, A-V block present, apply CPR” or “Defibrillation achieved, EMD present, apply CPR.” As an alternative to an audible signal, the AED may visually signal the need for CPR, e.g., with an indicator light, preferably a flashing light. Thus, the AED is capable of detecting and appropriately responding to post-defibrillation A-V block and EMD, and could save many lives as a result. 
   The victim&#39;s vulnerability during EMD is illustrated in  FIG. 7 , which is a record of waveforms obtained following defibrillation after four minutes of untreated (no CPR) ventricular fibrillation. R waves are present in the ECG but the blood-pressure pulses are very weak (or absent), indicating EMD. At the point labeled “Start” in the drawing, rhythmic chest compressions were applied which produced large-amplitude pulses in the blood-pressure record and corresponding optical blood pulses from the photodetector in the optical sensor. The defibrillation shock caused the ECG to return to near normal, and CPR produces a more normal ECG. Very weak pumping after cessation of chest compression at the point labeled “Stop” indicates the need for further CPR. 
   While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. For example, while discussed in relation to a forehead application, an optical sensor such as disclosed above also performs well over the manubrium (top of the sternum), along the sternum to the xiphoid (bottom end of the sternum), and on other body sites where there is a flat bone underlying a tissue bed.