Abstract:
A circuit includes a sensor coupled to a processor. The sensor senses an electrical signal that is representative of a patient parameter, and the processor determines a condition of the patient by analyzing first and second overlapping portions of the sensed electrical signal. For example, a portable AED can include such a circuit to sense first and second overlapping sections of an ECG. By utilizing this overlapping-window technique, the AED can obtain and analyze multiple sections of ECG data, and thus can make a shock/no-shock decision, more quickly than an AED using contiguous-window analysis. Thus, the overlapping-window technique allows one to use both longer ECG sections (better accuracy per window) and more of these longer sections (better voting accuracy) over a given analysis time. Furthermore, this overlapping-window technique significantly reduces or eliminates boundary problems because the boundary of one ECG section is within the interior of either the preceding or the following overlapping ECG section.

Description:
TECHNICAL FIELD 
   The invention relates generally to electronic circuits and systems, and more particularly to a circuit and method for analyzing a patient&#39;s heart function using overlapping analysis windows. For example, a portable automatic external defibrillator (AED) can analyze overlapping portions of an electrocardiogram (ECG) to determine if a patient&#39;s heart would benefit from a defibrillating shock. By analyzing overlapping portions of the patient&#39;s ECG, the AED often makes a shock/no-shock decision more quickly and more accurately than AEDs using other analysis techniques. 
   BACKGROUND OF THE INVENTION 
   Portable AEDs have saved many lives in non-hospital settings and, as a result of advances in AED technology, the number of lives saved per year is rising. Typically, a portable AED analyzes a patient&#39;s heart function and instructs an operator to administer an electrical shock if appropriate. For example, a shock can often revive a patient who is experiencing ventricular fibrillation (VF). Because older models of portable AEDs include only basic diagnostic and safety features, they are often difficult to operate. Therefore, only specially trained persons such as emergency medical technicians (EMTs) can use these older models to administer shocks. Newer models, however, often include advanced diagnostic and safety features that allow minimally trained persons to administer shocks. Consequently, more people are using portable AEDs to save lives. 
   Because a heart condition that responds to an electrical shock can cause permanent damage or death within a short time if left untreated, a portable AED should be able to diagnose a shockable heart condition and be ready to shock a patient within seconds. Without cardiopulmonary resuscitation (CPR), a person in cardiac arrest will typically suffer permanent anoxia-induced brain damage within 4-6 minutes from the onset. Unfortunately, many people do not know how to administer CPR. And, even in the best of circumstances, it can take 1-4 minutes to retrieve the AED and 1-2 additional minutes to attach the pads to the patient, connect the pads to the AED, and activate the AED. Therefore, even if the patient is discovered immediately, the AED often has less than a minute to diagnose and shock the patient before he/she is in danger of suffering permanent brain damage. Clearly, the faster the AED can diagnose and shock the patient, the better the chances that the patient will survive with no permanent brain damage. 
   Unfortunately, many portable AEDs implement heart-analysis techniques that require a relatively long time to analyze the patient&#39;s ECG and to make a shock/no-shock decision based on the analysis. 
     FIGS. 1 and 2  illustrate contiguous windowing, which is a heart-analysis technique used by many portable AEDs. For example, referring to  FIG. 1 , a portable AED (not shown in  FIG. 1 ) samples and analyzes contiguous “windows”, i.e., sections  10   a - 10   f , of a patient&#39;s ECG. Typically, the AED individually analyzes multiple ECG sections  10 , compares the respective analysis results to one another or to predetermined comparison values, and makes a shock/no-shock decision based on this comparison. 
   Referring to  FIG. 1 , an AED (not shown in  FIG. 1 ) using contiguous windowing often requires a relatively long time to make a shock/no-shock decision. For example purposes, assume that the AED is programmed to analyze at least ten ECG sections  10  before making a decision, and that each section  10  is two seconds long. Therefore, the AED requires a minimum of twenty seconds to make a shock/no shock decision. Even though twenty seconds may not seem like a long time, every second required to make a shock/no-shock decision decreases the chances that a patient will survive with no permanent damage. 
   In addition, changes in the patient&#39;s heart function may increase the time that the AED requires to make a shock/no-shock decision. For example purposes, assume that before the AED can make a shock/no-shock decision, it is programmed to analyze ECG sections  10  until at least a predetermined number of ten sequential sections give consistent analysis results. The AED then bases its shock/no-shock decision on one or more of these consistent analysis results. This decision-making process is often called “voting”. The theory behind voting is that if a predetermined percentage of analyzed ECG sections yield consistent, i.e., similar results, then these results are more likely to be accurate than inconsistent results yielded by other ECG sections. For example, an AED may be programmed to accept the result yielded by the majority of analyzed ECG sections and ignore different results from the minority of analyzed ECG sections. In the illustrated example, the ECG section  10   a  indicates that the patient&#39;s heart is beating with a normal sinus rhythm, but the sections  10   b - 10   f  indicate that the patient is in VF. Therefore, because the analysis results obtained from the ECG section  10   a  will clearly be inconsistent with the results obtained from the sections  10   b - 10   f , the AED must analyze at least seven ECG sections—the inconsistent section  10   a  plus at least six (a majority of ten) consistent sections starting with the section  10   b —before making a shock/no-shock decision. If the six ECG sections starting with the section  10   b  are inconsistent, however, then the AED must analyze more ECG sections  10 . Thus, the AED requires a minimum of fourteen seconds to make a shock/no-shock decision in this situation. Furthermore, although in this example the transition from normal sinus rhythm to VF occurs near the boundary between the ECG sections  10   a  and  10   b , the same problem often arises when the transition occurs within a section  10 . 
   Still referring to  FIG. 1 , one way to reduce the time that an AED requires to make a shock/no-shock decision is to shorten each of the ECG sections  10 . For example, assuming that the AED is programmed to analyze at least ten sections  10  as discussed above, reducing the length of each section  10  from two seconds to one second reduces the minimum decision time from twenty to ten seconds. As the lengths of the ECG sections  10  decrease, the chances of an AED making an incorrect shock/no-shock decision increases. Specifically, as their lengths decrease, each of the sections  10  represents a smaller portion of the ECG. If a section  10  is too small, it does not contain enough ECG information to support an accurate analysis of the section. If all the sections  10  are too small, the AED makes a series of inaccurate analyses that may cause the AED to make an inaccurate shock/no-shock decision. 
   Another way to view this problem is as a tradeoff between section length and the number of sections. For example, for a given analysis time, e.g., 20 seconds, one can use longer sections (better accuracy per section) with fewer results to vote from (less voting accuracy) or shorter sections (less accuracy per section) with more results to vote from (more voting accuracy). 
   In addition, referring to  FIG. 2 , even when the ECG sections are not too short, an AED (not shown in  FIG. 2 ) using contiguous windowing may incorrectly diagnose a patient&#39;s heart condition, and thus may determine that a defibrillating shock will benefit a patient when in actuality the shock may harm the patient. In the illustrated example, the patient is experiencing bradycardia, which is characterized by abnormalities in the patient&#39;s QRS wave and by an abnormally low heart rate. Unfortunately, shocking a patient experiencing bradycardia is at best useless and at worst can send the patient into VF or cause other cardiac damage. Therefore, it is important that the AED recognize bradycardia and other unshockable heart conditions and generate a no-shock decision if it determines that a patient is experiencing any of these conditions. 
   More specifically, if a boundary, i.e., the beginning or end, of an ECG section  12  intersects an important part of the ECG, then the AED&#39;s analysis of that section may yield an incorrect diagnosis, and the AED may make an incorrect shock/no-shock decision based on this incorrect diagnosis. In the illustrated example, the AED analyzes contiguous ECG sections  12   a ,  12   b ,  12   c , which are each one and a half seconds long. Unfortunately, the beginning of the section  12   a  intersects a QRS complex, and thus the section  12   a  contains only part of the complex. Because there are no other full complexes within the section  12   a , the AED&#39;s analysis of the section  12   a  may yield an incorrect result. But if the ECG sections  12   b  and  12   c  and a predetermined number of following sections  12  respectively include full QRS complexes, the AED can use voting to ignore the result from the section  12   a  and correctly make a no-shock decision as discussed above. Although as discussed above this may increase the time that the AED requires to make a shock/no-shock decision, the AED makes a correct decision. Conversely, if the ECG sections are shortened, e.g., to 0.5 seconds in order to obtain a quicker response, a majority of the sections will be lacking a QRS complex. These sections may be incorrectly interpreted as benefiting from a shock, resulting in an inappropriate shock diagnosis. 
   Still referring to  FIG. 2 , there are currently no analysis techniques for overcoming the intersecting-boundary problem other than to vote among multiple contiguous ECG sections, thereby delaying diagnosis, or to have a skilled operator (not shown in  FIG. 2 ) study the ECG and determine if the AED&#39;s shock/no-shock decision is correct. 
   Therefore, the need has arisen for a heart-condition analysis technique that is faster and more accurate than the contiguous-window analysis technique. 
   SUMMARY OF THE INVENTION 
   In one aspect of the invention, a circuit includes a sensor coupled to a processor. The sensor senses an electrical signal that is representative of activity in a patient&#39;s heart, and the processor determines a condition of the patient&#39;s heart by analyzing first and second overlapping portions of the sensed electrical signal. 
   For example, a portable AED can include such a circuit to sense a first section of an ECG during a first time period and to sense a second section of the ECG during a second time period that overlaps the first time period. By utilizing this overlapping-window technique, the AED can obtain and analyze multiple sections of ECG data, and thus can make a shock/no-shock decision, more quickly than an AED using contiguous-window analysis. Thus, the overlapping-window technique allows one to use both longer ECG sections (better accuracy per window) and more of these longer sections (better voting accuracy) over a given analysis time. Furthermore, this overlapping-window technique significantly reduces or eliminates boundary problems because the boundary of one ECG section is within the interior of one or more of the either the preceding or the following overlapping ECG sections. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a conventional contiguous-window analysis of the ECG of a patient who suddenly enters VF. 
       FIG. 2  illustrates a conventional contiguous-window analysis of the ECG of a patient experiencing bradycardia. 
       FIG. 3  illustrates an overlapping-window analysis of a portion of the ECG of  FIG. 1  according to an embodiment of the invention. 
       FIG. 4  illustrates an overlapping-window analysis of the ECG of  FIG. 2  according to an embodiment of the invention. 
       FIG. 5  is a block diagram of a memory circuit for storing overlapping sections of an ECG according to an embodiment of the invention. 
       FIG. 6  is a timing diagram of some of the signals shown in  FIG. 5 . 
       FIG. 7  is a block diagram of a memory circuit for storing overlapping sections of an ECG according to another embodiment of the invention. 
       FIG. 8  is a block diagram of an AED circuit that implements an overlapping-window analysis according to an embodiment of the invention. 
       FIG. 9  is a perspective view of a portable AED that incorporates the AED circuit of  FIG. 8  according to an embodiment of the invention. 
   

   DESCRIPTION OF THE INVENTION 
     FIGS. 3 and 4  illustrate overlapping-window analysis of an ECG according to respective embodiments of the invention. As discussed below, an AED using overlapping-window analysis often can diagnose a patient&#39;s heart condition more quickly and more accurately than an AED using contiguous-window analysis. Furthermore, an AED using overlapping-window analysis is often more immune to boundary problems than an AED using contiguous-window analysis. Moreover, although overlapping-window analysis is described below in terms of a portable AED analyzing an ECG, other types of medical equipment can use this technique to analyze other types of signals, such as an electrogram that represent a patient&#39;s heart activity, or an electroencephalogram that represents a patient&#39;s brain activity. 
     FIG. 3  illustrates an overlapping-window analysis of a portion of the ECG of  FIG. 1  according to an embodiment of the invention. Like the contiguous ECG sections  10  of  FIG. 1 , each section  14  is two seconds long, although in other embodiments the sections  14  may be longer or shorter than two seconds or may not all have the same length. But unlike the sections  10 , the sections  14  overlap one another. For example, the beginning of the second section  14   b  coincides with the midpoint of the first section  14   a , and the end of the section  14   b  coincides with the midpoint of the third section  14   b . Thus, the first half of each ECG section  14  overlaps the last half of the respective preceding section, and the last half of each ECG section  14  overlaps the first half of the respective following section. This is referred to as 50% overlap, although in other embodiments the overlap can be greater or less than 50%. Therefore, overlapping-window analysis allows an AED circuit (not shown in  FIG. 3 ) to analyze overlapping sections of an ECG or other heart signal. Because the AED can use conventional algorithms to analyze each of the overlapping ECG sections, a detailed discussion of these algorithms is omitted. 
   By analyzing overlapping sections of a patient&#39;s ECG, an AED can often make a shock/no-shock decision more quickly than it can by analyzing contiguous ECG sections. Specifically, because the ECG sections  14  overlap one another, the AED can analyze more sections  14  of the ECG within a given time period than it can contiguous sections  10  ( FIG. 1 ). For example, the AED can analyze ten overlapping sections  14  in eleven seconds as compared to analyzing ten contiguous sections  10  in twenty seconds. Thus, 50% overlapping cuts the analysis time almost in half! 
   Furthermore, analyzing overlapping sections of a patient&#39;s ECG is often more accurate than analyzing contiguous sections of the ECG. As discussed above in conjunction with  FIGS. 1 and 2 , if an ECG section is too small, it often contains too little information to yield an accurate indication of the patient&#39;s heart condition. Therefore, analyzing a number of longer, overlapping ECG sections of an ECG segment is often more accurate than analyzing a similar number of shorter, contiguous ECG sections of the same segment. For example, analyzing an eleven-second ECG segment with ten overlapping two-second sections  14  is often more accurate than analyzing the ECG segment with eleven contiguous one-second ECG sections. Because a section  14  is twice as long as a one-second section, it contains approximately twice as much information as the one-second section. Therefore, the longer sections  14  each provide a “bigger picture” of the patient&#39;s ECG than do the shorter contiguous sections, and thus tend to yield a more accurate indication of the patient&#39;s heart condition. 
   Moreover, by analyzing overlapping sections of a patient&#39;s ECG, an AED can often detect changes in a patient&#39;s heart condition more quickly than by analyzing contiguous sections. For example, assume that before an AED can make a shock/no-shock decision, it is programmed to analyze ECG sections until a majority of five sequential sections gives consistent analysis results. Referring to  FIG. 1 , because the ECG does not indicate VF until the beginning of the section  10   b , an AED using contiguous windowing must analyze at least four ECG sections  10   a - 10   d , and thus requires at least eight seconds to determine that the patient is in VF. Conversely, referring to  FIG. 3 , an AED using the illustrated overlapping-windowing technique may be able to diagnose VF in as few as six seconds. Specifically, because the ECG does not indicate VF until the middle of the section  14   b , an AED using the illustrated overlapping-windowing technique analyzes at least five ECG sections  14   a - 14   e . But because the sections  14  overlap one another by 50%, five ECG sections  14  occupy a shorter period of time (six seconds) than four of the contiguous ECG sections  10  (eight seconds) of  FIG. 1 . Of course, increasing the overlap or decreasing the length of the sections  14  may further reduce the minimum analysis time. 
     FIG. 4  illustrates an overlapping-window analysis of the ECG of  FIG. 2  according to an embodiment of the invention. Like the ECG sections  12  of  FIG. 2 , the ECG sections  16   a - 16   f  are each 1.5 seconds long, although in other embodiments the sections  16  may be longer or shorter. But unlike the sections  12 , the sections  16  overlap one another by 50%, although in other embodiments the sections  16  may overlap one another by more or less than 50%. Therefore, the beginning of a section  16  is within the preceding section and the end of the section  16  is within the following section. For example, the beginning of the second section  16   b  at time W B  coincides with the midpoint of the first section  16   a , and the end of the section  16   b  at time W D  coincides with the midpoint of the third section  16   c . Thus, if an important part of the ECG intersects the boundary of a section  16 , this ECG part is most often wholly within another section  16 . Therefore, an AED can analyze ECG sections  16  that wholly contain important parts of the ECG. For example, if the QRS complexes of the ECG were to intersect with the boundaries of the sections  16   a ,  16   c , and  16   e  at the respective times W A , W C , and W E , then these same QRS complexes also intersect the midpoints of the alternate sections  16   b ,  16   d , and  16   f . Therefore, by analyzing the alternate sections  16   b ,  16   d ,  16   f , and so on, the AED can analyze whole QRS complexes and thus correctly diagnose bradycardia and make a no-shock decision. 
     FIG. 5  is a block diagram of a memory circuit  20  that can store overlapping sections of an ECG according to an embodiment of the invention. The memory circuit  20  includes three memories  22   a ,  22   b , and  22   c , which may be disposed within a common memory array or within respective memory arrays. Each of the memories  22   a ,  22   b , and  22   c  stores data representing a respective overlapping ECG section in response to a common signal CLOCK and respective memory-enable signals CE 1 , CE 2 , and CE 3 . For example, referring to  FIG. 4  and as discussed below, at various points during the ECG analysis, the memory  22   a  stores data representing the ECG section  16   a , the memory  22   b  stores data representing the section  16   b , and the memory  22   c  stores data representing the section  16   c . In one embodiment, the stored data are conventional analog or digital samples—typically voltage samples—of the ECG. Once the data representing an ECG section is stored in a memory  22 , the AED (not shown in  FIG. 5 ) can analyze the overlapping ECG section stored within that memory  22 . As discussed below, once the AED analyzes the stored data, the memory  22  begins to store another ECG section. Therefore, for 50% overlap, the three memories  22   a ,  22   b , and  22   c  can sequentially store data for all of the overlapping ECG sections regardless of how many sections the AED analyzes. But more or fewer memories  22  may be needed for different amounts of overlap. 
   Referring to  FIGS. 4-6 , the operation of the memory circuit  20  is discussed according to an embodiment of the invention.  FIG. 6  is a timing diagram of the signals CE 1 , CE 2 , and CE 3  of  FIG. 5 , where the times W A -W E  respectively correspond to the same times W A -W E  in  FIG. 4 , and where CE 1 , CE 2 , and CE 3  are active logic 1 and inactive logic 0. 
   Before time W A , CE 1 , CE 2 , and CE 3  are inactive logic 0 such that the memories  22   a - 22   c  are disabled from storing samples of the ECG. 
   Next, between times W A  and W B , the memory  22   a  stores data representing the first half of the ECG section  16   a . Specifically, a sample circuit (not shown in  FIGS. 4-6 ) generates a stream of ECG samples, which are coupled to the memories  22   a - 22   c . At time W A , CE 1  transitions to an active logic 1, and thus enables the memory  22   a  to begin storing the ECG samples that represent the ECG section  16   a . Therefore, at time W B , the memory has stored samples that represent the first half of the section  16   a . To clearly illustrate this,  FIG. 5  shows that half the memory  22   a  is filled between the times W A  and W B , thus indicating that the memory  22   a  has just enough capacity to store the ECG samples representing the section  16   a . In other embodiments, however, the memories  22   a - 22   c  may have larger capacities. 
   Then, between the times W B  and W C , the memory  22   a  stores data representing the second half of the ECG section  16   a , and the memory  22   c  stores data representing the first half of the ECG section  16   b . Specifically, at time W B , the signal CE 2  transitions to an active logic 1, and thus enables the memory  22   b  to begin storing the ECG samples that represent the second ECG section  16   b . Furthermore, the memory  22   a  begins storing the same samples, which also represent the second half of the ECG section  16   a . Thus, by storing the same portion of the ECG—the overlapping portion between times W B  and W C —in two memories  22   a  and  22   b , the memory circuit  20  stores overlapping ECG sections  16   a  and  16   b.    
   Next, between the times W C  and W D , the AED analyzes the data stored in the memory  22   a , the memory  22   b  stores data representing the second half of the ECG section  16   b , and the memory  22   c  stores data representing the first half of the ECG section  16   c . Specifically, at time W C , CE 3  transitions to active logic 1 and CE 1  transitions to inactive logic 0. Furthermore, the memory  22   a  contains ECG samples that represent the entire ECG section  16   a , and the memory  22   b  contains ECG samples that represent the first half of the ECG section  16   b . Between W C  and W D , the AED analyzes the data in the memory  22   a , and thus analyzes the first ECG section  16   a , while the memory  22   b  stores the second half of the ECG section  16   b  and the memory  22   c  stores the first half of the ECG section  16   c . Therefore, while the AED analyzes data in one memory  22 , the other two memories  22  continue to store ECG samples. 
   Then, between the times W D  and W E , the AED analyzes the data stored in the memory  22   b , the memory  22   a  stores data representing the first half of the ECG section  16   d , and the memory  22   c  stores data representing the second half of the ECG section  16   c . Specifically, at time W D , CE 1  transitions to active logic 1 and CE 2  transitions to inactive logic 0. Furthermore, the memory  22   b  contains ECG samples that represent the entire ECG section  16   b , and the memory  22   c  contains ECG samples that represent the first half of the ECG section  16   c . Between W D  and W E , the AED analyzes the data in the memory  22   b , and thus analyzes the second ECG section  16   b , while the memory  22   c  stores the second half of the section  16   c  and the memory  22   a  stores the first half of the section  16   d.    
   This cycle of storing and analyzing data continues until the AED analyzes the desired number of overlapping ECG sections  16 . 
   Referring to  FIG. 5 , other embodiments of the memory circuit  20  are discussed. For example, although described as storing ECG sections having a 50% overlap, the memory  20  for storing ECG sections can be modified to have a smaller or larger overlap. Furthermore, although they are described as storing the same ECG samples for the overlapping portion of two ECG sections, the memories  22   a - 22   c  may store different samples for the same overlapping portion. For example, as discussed above, between the times W B  and W C  the memories  22   a  and  22   b  store the same ECG samples for the second half of the ECG section  16   a  and the first half of the ECG section  16   b , respectively. 
   Moreover, as discussed below in conjunction with  FIG. 7 , other circuits can be used or designed, such as a linear register to store overlapping portions of a patient&#39;s ECG. 
     FIG. 7  is a block diagram of a linear memory circuit  24  that can store overlapping sections of an ECG according to another embodiment of the invention. Specifically, the memory circuit  24  is more efficient than the memory circuit  20  of  FIG. 5  because it can store the same number of contiguous ECG sections as the memory circuit  20  with fewer storage locations. 
   The memory circuit  24  includes a number of storage locations  26 , which each store a sample of the patient&#39;s ECG. Assuming for example purposes that each window  16  ( FIG. 4 ) is sixteen samples long, initially the circuit  24  stores the ECG section  16   a  in locations  26   a - 26   p , section  16   b  in locations  26   i - 26   x , and section  16   c  in locations  26   q - 26   ff  Therefore, in this example, the memory circuit  20  ( FIG. 5 ) requires forty eight storage locations to store three windows  16 , but the memory circuit  24  requires only thirty two storage locations  26  to store three windows  16 . 
   Referring to  FIGS. 4 and 7 , in operation, between times W A -W E , the memory circuit  24  sequentially stores the ECG samples for the ECG sections  16   a - 16   c  starting at the location  26   a  and ending at the location  26   ff . Once an entire section  16  stored, the AED analyzes it while the circuit  24  finishes storing the remaining samples of the next section  16 . Similarly, between times W E -W I  (W H  and W I  omitted from  FIG. 4  for clarity), the circuit  24  stores the ECG samples for the next three windows  16   d - 16   f  by sequentially overwriting the locations  26   a - 26   ff . The circuit  24  repeats this process until AED stores and analyzes the desired number of ECG sections  16 . 
   Still referring to  FIG. 7 , although the memory circuit  24  is described as being large enough to store three overlapping ECG sections  16 , in other embodiments the circuit  24  may be able to store more or fewer sections  16 . 
     FIG. 8  is a block diagram of an AED circuit  30 , which analyzes overlapping sections of a patient&#39;s ECG (not shown in  FIG. 8 ) according to an embodiment of the invention. In the described embodiment, the circuit  30  includes the memory circuit  20  of  FIG. 5 , although in other embodiments the circuit  30  may analyze overlapping ECG sections using the memory circuit  24  of  FIG. 7  or another ECG-storage circuit. 
   Referring to  FIG. 8 , conventional defibrillator pads  32   a  and  32   b  are coupled to the circuit  30  via a conventional connector  34  and are operable to sense a patient&#39;s ECG and to apply an electrical shock to the patient. A shock-delivery-and-ECG front-end circuit  36  samples the patient&#39;s ECG during an analysis mode of operation and provides a shock to the patient via the connector  34  and pads  32   a  and  32   b  during a shock-delivery mode of operation. A gate array  38  receives the ECG samples from the circuit  36  and provides them to a processor unit (PU)  40 , which stores the samples in the memory circuit  20  and analyzes the overlapping ECG sections that the stored samples represent as discussed above in conjunction with  FIGS. 4-6 . Although the memory circuit  20  is shown coupled directly to the processor unit  40 , the circuit  20  may actually be part of the processor unit  40  or be coupled to the processor unit  40  through other circuits such as the gate array  38 . If the analysis of the overlapping ECG sections indicates that the patient is suffering from a shockable heart condition, then the processor unit  40  instructs the circuit  36  via the gate array  38  to enable delivery of a shock when an operator (not shown in  FIG. 8 ) presses a shock button  42 . Conversely, if the analysis of the overlapping ECG sections indicates that the patient is not suffering from a shockable heart condition, then the processor unit  40  disables the shock delivery circuitry  36  from delivering a shock to the patient. 
   Still referring to  FIG. 8 , the circuit  30  includes a power-management circuit  46  for distributing power from a battery  44  to the subcircuits of the circuit  30 . An on/off switch  48  turns the circuit  30  on and off, a status circuit  50  indicates the status of the circuit  30 , and a gate array  52  interfaces the power-management circuit  46 , the on/off circuit  48 , and the status circuit  50  to the circuit  36 , the processor unit  40 , and the gate array  38 . A display  54  displays information to an operator (not shown in  FIG. 7 ), a speaker  56  provides audio instructions to the operator, and a microphone  58  records the operator&#39;s voice and other audible sounds. A data card  60  is connected to the gate array  38  via a port  62 . The card  60  stores the operator&#39;s voice and other audible sounds along with the patient&#39;s ECG and a record of AED events for later study. A status-measurement circuit  64  provides the status of the circuit  30  subcircuits to the processor unit  40 , and LEDs  66  provide information to the operator such as whether the processor unit  40  has enabled the circuit  36  to deliver a shock to the patient. A contrast button  68  allows the operator to control the contrast of the display screen  54 , and a memory such as a read only memory (ROM)  70  stores programming information for the processor unit  40  and the gate arrays  38  and  52 . 
   The AED circuit  30  and other AED circuits are further discussed in the following references, which are incorporated by reference: U.S. Pat. No. 5,836,993; U.S. Pat. Nos. 5,735,879, ELECTROTHERAPY METHOD AND APPARATUS, filed Aug. 6, 1993; 5,607,454, ELECTROTHERAPY METHOD AND APPARATUS, filed Apr. 14, 1994; and, 5,879,374, DEFIBRILLATOR WITH SELF-TEST FEATURES, filed May 10, 1994. 
     FIG. 9  is a perspective view of a portable AED  80 , which incorporates the circuit  30  of  FIG. 8  according to an embodiment of the invention. For clarity, common elements in  FIGS. 8 and 9  are referenced with like numerals. 
   During an emergency where it is determined that a patient (not shown in  FIG. 9 ) may need a shock, an operator (hands shown in  FIG. 9 ) retrieves the AED  80  and installs the battery  44  if it is not already installed. Next, the operator removes the pads  32   a  and  32   b  from a protective package (not shown in  FIG. 9 ) and inserts a pad connector  82  into the connector  32 . Then, the operator turns the on/off switch  48 , which is a key switch in this embodiment, to the “on” position to activate the AED  80 . Following the instructions displayed on the display  54  or “spoken” via the speaker  56 , the operator places the pads  32   a  and  32   b  on the patient in the respective positions shown in the pictures on the pads and on the AED  80 . After the operator places the pads  32   a  and  32   b  on the patient, the processor unit  40  ( FIG. 8 ) analyzes the patient&#39;s ECG to determine whether the patient is suffering from a shockable heart condition. If the processor unit  40  determines that the patient is suffering from a shockable heart condition, then the display  54  or the speaker  56  instructs the operator to depress the shock button  42  to deliver a shock to the patient. Conversely, if the processor unit  40  determines that the patient is not suffering from a shockable heart condition, the display  54  or the speaker  56  informs the operator to seek appropriate non-shock treatment for the patient. Furthermore, the processor unit  40  disables the shock-delivery circuit  36  such that even if the operator presses the shock button  42 , the circuit AED  80  does not shock the patient. 
   As discussed above in conjunction with  FIG. 8 , the microphone  58  may record the voice of the operator and of other rescuers and, the data card  60  may store these voices and the patient&#39;s ECG for later study. Such study may be for the purposes of instructing others in rescue techniques, for evaluating the performances of the operator or other rescuers, or for improving the AED  80 . 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.