Development of a decision algorithm for a semiautomatic defibrillator

SPECIAL CONTRIBUTION defibrillator, decision algorithm

Development of a Decision Algorithm for a S e m i a u t o m a t i c Defibrillator A decision algorithm was developed for a semiautomatic defibrillator. The function of the algorithm is to evaluate the ECG of a patient and determine whether a defibrillation shock should be delivered. The development process included establishment of defibrillation criteria, creation of ECG databases, algorithm design, development of test protocols, and clinical testing. The result was an algorithm with sensitivity and specificity sufficiently accurate to allow a defibrillation shock to be delivered safely outside the hospital. [Edwards DC: Development of a decision algorithm for a s e m i a u t o m a t i c defibrillator. A n n Emerg Med D e c e m b e r 1989;18: 1276-1279.] INTRODUCTION Many US states now have early defibrillation programs as a result of research showing that such programs improve the survival of cardiac arrest victims. 1-4 These programs require many factors to be successful, including rapid response time, personnel training, and medical control, s-s Recent developments in automatic external defibrillators (AEDs) have resulted in equipment that simplifies the implementation of an early defibrillation program. A key element of a safe and effective defibrillator for out-of-hospital use is an accurate decision algorithm that allows the rapid delivery of a defibrillation shock. The development of such an algorithm for use in defibrillators suitable for early defibrillation programs is described.

D Craig Edwards Bellevue, Washington From the First Medical Devices Corporation, Bellevue, Washington. Received for publication April 24, 1989. Accepted for publication August 28, 1989. Presented at the SAEM-IRIEM Research Symposium in Fort Lauderdale, Florida, February 1989. Address for reprints: D Craig Edwards, First Medical Devices Corporation, 2445 140th Avenue, NE, Suite 150, Bellevue, Washington 98005.

METHODS The development process includes establishment of defibrillation criteria, development of test ECG databases, design of the device operating functions and decision algorithm, testing of the algorithm in the laboratory, and testing of the algorithm in the field.

Defibrillation Criteria The design of an algorithm for automatic defibrillation depends on the development of medically appropriate decision criteria and accuracy specifications. Recommendations for potential defibrillation criteria were reviewed, gAo and the result was the decision to defibrillate patients who are in cardiac arrest as a result of ventricular fibrillation (VF) or rapid pulseless ventricular tachycardia (VT). Patients with any other rhythm were not to be defibrillated. VT of more than 180 beats per minute was judged sufficiently rapid to need defibrillation if this was nonperfusing. Asystole was one of the rhythms that would not be defibrillated and was defined as any ECG with an amplitude of less than 0.15 inV. A review of recommendations lo-12 and discussions with medical personnel resulted in the recommendation of a goal of more than 90% sensitivity and more than 99% specificity.

Test Databases An accepted method for testing the accuracy of a decision algorithm is to measure the sensitivity and specificity for a set of ECG rhythms. However, no set of ECG rhythms has been developed and accepted as a national standard for such testing. 13 Three independent ECG databases were developed to support the design activities and laboratory testing of the algorithm; one of these databases was for algorithm development, and the

18:12 December 1989

Annals of Emergency Medicine

1276/39

DECISION ALGORITHM Edwards

other two were for algorithm testing. The largest database was created to support the algorithm design process and included many types of rhythms from a variety of sources that were collected and stored on FM magnetic tape. Then, these sequences of ECGs were digitally sampled 100 times per second, and each value was stored in a c o m p u t e r as an e i g h t - b i t word. Next, the data were edited into 8,200 samples of three-second intervals. Each segment was a n n o t a t e d w i t h the assistance of medical personnel as "shock," "no shock," or "noise." There were 1,800 s h o c k s e g m e n t s and 6,400 no shock segments. Those segments with significant artifact from CPR or tape splices were not included. The segments identified as VF or VT (more than 180 beats per minute) were labeled as shock and required defibrillation. The segments identified as no shock or noise did not have a defibrillation shock delivered. All other rhythms, including asystole and VT with less than 180 beats per m i n u t e , were defined as no shock segments. Noise was defined as a segm e n t w i t h no r e c o g n i z a b l e ECG rhythm present. The two other databases were developed to evaluate the algorithm's performance before field use; each was completely independent of the development database. Independence was required to ensure that the algorithm accuracy tests used unique and separate r h y t h m s in its interpretation. The first of these databases contained 104 rhythms gathered from patients who were found in cardiac arrest in the field. The rhythms were subsequently annotated by the emergency medical services (EMS) consultants to have 68 shock rhythms and 36 no shock r h y t h m s . The second d a t a b a s e c o n t a i n e d 132 r h y t h m s gathered from a separate set of pat i e n t s who w e r e found in cardiac a r r e s t in t h e f i e l d . S e v e n t y - o n e r h y t h m s were a n n o t a t e d as shock, and 61 rhythms were annotated as no shock. The shock rhythms in both databases included VF and VT, and the no shock rhythms included normal sinus, sinus tachycardia, pacemaker, asystole, and bradycardia.

Device and Algorithm Operation The algorithm is a computer program that acts as the brain of the AED and controls the operation of 40/1277

the defibrillator to initiate a specific sequence of actions based on its int e r p r e t a t i o n of the p a t i e n t ' s condition. Because a device cannot substitute for a trained human being completely, this computer algorithm r e l i e s on i n t e r a c t i o n w i t h an emergency medical technician (EMT) to achieve the best possible overall results. Based on the planned application by an EMT, the design of the device includes two features to increase safety and effectiveness. In the design of an AED, it is possible to deliver the shock a u t o m a t i c a l l y or s e m i a u t o m a t i c a l l y . Semiautomatic defibrillation requires that the operator press a control to deliver the shock, whereas with a u t o m a t i c defibrillation, the shock is delivered as soon as the algorithm determines that a shock should be delivered. The device we describe used the semia u t o m a t i c approach, w h i c h allows the EMT to review environment and patient conditions before delivery of the shock to improve safety. The second feature i n c o r p o r a t e d into the device is the c o n t i n u o u s measurement of the patient's impedance, which is a measure of the electrical resistance of the patient's trunk b e t w e e n the two e l e c t r o d e s . The large d e f i b r i l l a t i o n electrodes (typically, 50 cm 2) are applied to the pat i e n t and c o n n e c t e d to the device with cables. Because the electrodes could be applied incorrectly or become dislodged during the rescue attempt, the device continuously measures the patient impedance. The imp e d a n c e is l o w if the e l e c t r o d e s touch and very high if one electrode falls off. If an abnormal impedance i n d i c a t i n g lead failure or i n c o r r e c t p o s i t i o n i n g is detected, the device stops analyzing the ECG and alerts the operator. For the a p p l i c a t i o n of field defibrillation by trained EMTs or paramedic personnel, the recommendations of the American Heart Assoc i a t i o n , 9 t h e US D e p a r t m e n t of Transportation, 14 and others i s were r e v i e w e d to d e t e r m i n e the i n i t i a l treatment of patients who are in cardiac arrest. All protocols require the EMT to confirm that a cardiac arrest has occurred before attaching the device to the patient. The decision a l g o r i t h m uses the ECG that is collected from the two d e f i b r i l l a t i o n e l e c t r o d e s t h a t are placed in a modified lead II configuAnnals of Emergency Medicine

ration. The ECG is digitally sampled at 100 sampleS per second, and these values are grouped into three-second ECG intervals. Each three-second interval of ECG is evaluated by the algorithm with a series of 13 digital filters, w h i c h results in 13 variables. Each digital filter evaluates many characteristics of the ECG, including ECG amplitude, order, and rate; frequency content, regularity, and order ratios; rate of change index; maxi m u m - t o - m e d i a n a m p l i t u d e ratio; noise content; and number of peaks detected. The algorithm then computes a value from a proprietary m a t h e m a t i c a l f o r m u l a t h a t uses these variables to make a decision of shock or no shock for each three-second i n t e r v a l . Next, the a l g o r i t h m evaluates three c o n s e c u t i v e threesecond intervals of ECG and requires two of the three i n t e r v a l s to be a shock decision to allow the delivery of a shock. Once a shock decision is made by the algorithm, the device a l e r t s the o p e r a t o r who t h e n dep r e s s e s a c o n t r o l to d e l i v e r t h e shock.

Laboratory Testing The a l g o r i t h m was designed and t e s t e d on the 8,200 ECG s a m p l e database, which consists of almost s e v e n h o u r s of c o n t i n u o u s ECG rhythms; each three-second interval w i t h its t r e a t m e n t a n n o t a t i o n was stored on a computer. Special computer programs then were developed to compare the a n n o t a t i o n recommended by medical personnel with the algorithm decision and to provide sensitivity and specificity statistics. T h e a l g o r i t h m was a l s o t e s t e d using the two independent databases. Because both of these databases were on tape, the output of the tape recorder was connected directly to the device, and each rhythm was tested. The medical personnel's recommendation was the "gold standard" used to compare the algorithm's decision for each of the ECG segments for the sensitivity and specificity results.

Field Testing The goal of the field test was to validate the accuracy of the algor i t h m under field conditions. The field testing was a c c o m p l i s h e d by placing some AEDs in EMS services under medical supervision and after in-service training of the EMS personnel who would use the equip18:12 December 1989

rnent. Each EMT was trained to follow the AHA protocol 9 for cardiac arrest. This protocol requires the EMT to initially check the patient for the presence of a pulse, breathing, and consciousness. If n o n e are p r e s e n t , the AED cables are a t t a c h e d to the defibrillation electrodes and the electrodes are a t t a c h e d to t h e p a t i e n t . The AED is then turned on, and the ECG is evaluated by the a l g o r i t h m by pressing t h e " a n a l y z e " c o n t r o l button. If the algorithm r e c o m m e n d s a shock, the EMT is trained to press the " s h o c k " c o n t r o l b u t t o n , w h i c h results in a 200-J defibrillation pulse delivered through the electrodes. The EMT then p e r f o r m s one m i n u t e of CPR and reassesses the patient. If the patient is still in cardiac arrest, the EMT checks the AED and if it indicates a shock, the " s h o c k " c o n t r o l button is pressed again. After a n o t h e r m i n u t e of CPR, if the AED d e t e r m i n e s VF or VT, another defibrillation, this t i m e at 360 J, is delivered. After the third shock, CPR alone was c o n t i n u e d u n t i l m o r e advanced care was available. If t h e patient developed a pulse but subsequently r e t u r n e d to c a r d i a c arrest, the E M T c o u l d r e p e a t t h e t h r e e shock protocol as described above. All A E D s w e r e e q u i p p e d w i t h a tape recorder t h a t c o n t i n u o u s l y recorded both the voice and ECG data. In addition, the units incorporated a novel s o l i d - s t a t e r e c o r d i n g s y s t e m that recorded selected ECG and other information. T h e i n f o r m a t i o n stored using the solid-state s y s t e m was used to a u t o m a t i c a l l y create a printed report t h a t i n c l u d e d an e v e n t log for each rescue a t t e m p t w i t h samples of the ECG included. T h i s s o l i d - s t a t e recording provided a w r i t t e n report of time the device was turned on, t i m e electrodes were first a t t a c h e d w i t h initial r h y t h m , t i m e of any lead disconnect, t i m e of each d e f i b r i l l a t i o n before and after r h y t h m , and t i m e and r h y t h m w h e n the device was disconnected. The AED was m o n i t o r e d for m e c h a n i c a l r e l i a b i l i t y , e a s e of use, and accuracy of the decision algorithm. Both clinical test report forms and personal i n t e r v i e w s w i t h the EMTs who u s e d t h e A E D s w e r e u s e d to gather this information. A questionnaire concerning the operation of the liquid crystal display as well as the ,use of t h e e l e c t r o d e s , b a t t e r y , and controls was filled out after each pa18:12 December 1989

tient encounter. Five units each were placed in the Dallas Fire D e p a r t m e n t and the Hennepin C o u n t y Medical Center A m b u lance Service in Minneapolis. The dev i c e was c o n n e c t e d to a n y p a t i e n t w h o was found by the EMT to m e e t the cardiac arrest criteria of pulseless, nonbreathing, and unconscious. T h e data from each rescue a t t e m p t were evaluated by the medical director and a c l i n i c a l engineer for algor i t h m performance and medical reco m m e n d a t i o n of t r e a t m e n t for each recorded r h y t h m . T h e m e d i c a l reco m m e n d a t i o n of t r e a t m e n t for each r h y t h m w a s t h e s t a n d a r d u s e d to c o m p a r e t h e device p e r f o r m a n c e in the field.

RESULTS The algorithm results of specificity and s e n s i t i v i t y used the d e f i n i t i o n s from reference 12. The algorithm was tested using the development database of 8,200 segments and had a r e s u l t i n g s e n s i t i v i t y of 94% a n d specificity of 99%. T h e first of the t w o i n d e p e n d e n t d a t a b a s e t r i a l s of 104 r h y t h m s res u l t e d in a s e n s i t i v i t y of 95.6% to 97.1% and a specificity of 100%. T h e second i n d e p e n d e n t database trial of 132 r h y t h m s showed a sensitivity of 89% to 93% and a specificity of 100%. The device m e t the p r e d e t e r m i n e d s e n s i t i v i t y and specificity values during b o t h laboratory and clinical field testing. In t h e f i e l d t e s t i n g , 76 p a t i e n t s were entered into the study, and their ECG segments were evaluated and compared w i t h the algorithm res p o n s e to t h e s e s e g m e n t s . T h e res u l t s of t h e 382 t h r e e - s e c o n d segm e n t s collected were correct shock, 82; incorrect no shock, 10; correct no s h o c k , 285; i n c o r r e c t s h o c k , five; s e n s i t i v i t y , 89%; a n d s p e c i f i c i t y , 98%. The u n i t did not have any significant m e c h a n i c a l reliability issues in t h e field tests, b u t s e v e r a l suggestions were m a d e concerning the visual messages and their sequences.

DISCUSSION A l l p a t i e n t s w h o were c o n n e c t e d to the device were i n c l u d e d in the data. Two patients accounted for the five i n c o r r e c t s h o c k d e c i s i o n s because of a protocol error. The device was connected to two patients who Annals of Emergency Medicine

were conscious, breathing, and had a pulse to "see w h a t w o u l d h a p p e n . " T h e e l i m i n a t i o n of the data associated w i t h this p r o t o c o l error w o u l d result in a specificity of 100%. It is p o s s i b l e t h a t t h e a l g o r i t h m could be further improved w i t h the d e v e l o p m e n t of an e x p a n d e d E C G database that includes representative E C G c a r d i a c a r r e s t d a t a s u c h as noise, artifact, and a wide variety of rhythms. With more baseline data and current m a t h e m a t i c a l optimization techniques, a more accurate E C G a n a l y s i s a l g o r i t h m w o u l d result. A n o t h e r potential for accuracy i m p r o v e m e n t c o n c e r n s the trade-off b e t w e e n sensitivity and specificity. It is p o s s i b l e to d e s i g n an a l g o r i t h m w i t h m o r e sensitivity and less specificity if such a change is m e d i c a l l y acceptable. CONCLUSION A d e c i s i o n a l g o r i t h m was developed for use in an AED that m e t the d e s i g n goals and c r i t e r i a of sensit i v i t y and specificity as tested w i t h three separate ECG databases and in prehospital trials.

REFERENCES 1. Eisenberg MS, Copass MK, Hallstrom AP, et ah Treatment of out-of-hospital cardiac arrests with rapid defibrillation by emergency medical technicians. N Engl J Med 1980;302:1379-1383. 2. Stults KR, Brown DD, Schug VL, et ah Prehospital defibrillation performed by emergency medical technicians in rural communities. N Engl J Med 1984;310:219-223. 3. Weaver WE), Hill D, Fahrenbruch CE, et ah Use of the automatic external defibrillator in the management of out-of-hospital cardiac arrest. N EngI J Med 1988;319:661-666. 4. Eital DR, Walton SL, Guerci AD, et ah Outof-hospital cardiac arrest: A six-year experience in a suburban-rural system. Ann Emerg Med 1988;17:808-812. 5. Cummins RO, Eisenberg MS, Graves JR, et ah "EMT-defibrillation, Is it right for you?" JEM$ 1985;60-64. 6. Cummins RO, Eisenberg MS, Graves JR, et ah Implementing EMT-D. JEMS 1985;44-47. 7. Cummins RO, Eisenberg MS, Litwin PE, et ah Automatic external defibrillators used by emergency technicians. JAMA 1987;257: 1605-1610. 8. Cummins RO, Austin D, Graves JR, et ah An innovative approach to medical control: Semiautomatic defibrillators with solid-state memory modules for recording cardiac arrest events. Ann Emerg Med 1988;17:818-824. 9. American Heart Association: Standards and guidelines for cardiopulmonary resuscitation and emergency cardiac care. JAMA 1986;255: 2945-2951. 10. Association for the Advancement of Medi-

1278/41

DECISION ALGORITHM Edwards

cal Instrumentation: Design, testing, and reporting performance results of automatic external defibrillators. AAMI technical information report No 3-P-5/87. AAMI, Arlington, Virginia, 1987. 11. Cummins RO, Eisenberg MS, Bergner L, et ah Sensitivity, accuracy, and safety of an autom a t i c external defibrillator. Lancet 1984;

42/1279

318-320.

nal defibrillators: Performance of three devices. [ Am Coll Cardiol 1988;735-1097.

12. Cummins RO, Eisenberg MS, Moore JE, et ah Automatic external defibrillators: Clinical, training, psychological, and public health issues. Ann Emerg Med 1985;14:755-760.

14. United States Department of Transportation: National Standard Curriculum, EMT-Ambulance, ed 3. Washington, DC, DOT, 1984.

13. Cummins RO, Stults KR, Haggar B, et ah A new rhythm library for testing automatic exter-

15. Proceedings, EMT-Defibrillation Draft Standard Setting Seminar, San Francisco, April 1986.

Annals of Emergency Medicine

18:12 December 1989