Prehospital cardiac arrest treated by urban first-responders: Profile of patient response and prediction of outcome by ventricular fibrillation waveform

ORIGINAL CONTRIBUTION advanced life support emergency medical services resuscitation

Prehospital Cardiac Arrest Treated by Urban First-Responders: Profile of Patient Response and Prediction of Outcome by Ventricular Fibrillation Waveform From the Division of Emergency Medicine,* the Center for Prehospital Research and Training,* and Department of Pediatrics, University of California, San Francisco.

Michael Callaham, MD, FACEP* Odelia Braun, MD* Wesley Valentine, RN* Douglas M Clarkt Claudia Zegans, MD*

Receivedfor publication June 15, I992. Revisions received October 13 and December 7, 1992. Accepted for publication December 14, 1992. Presented at the Society for Academic EmeNency Medicine Annual Meeting in Washington, DC, May 1991.

Study objectives: To determine the speed and characteristics of patient response to urban first-responder defibrillation and to determine whether amplitude of ventricular fibrillation (VF) can predict outcome in these patients.

Type of participants: All adult patients in prehospital VF treated by fire department first-responders (265).

Design and interventions: A prospective observational study occurring between February 1, 1989, and January 1, 1991. Patients were defibrillated according to advanced cardiac life support and first-responder protocols. ECG and time data were recorded digitally. Main results: Sixty-five percent of patients converted from VF to a more stable rhythm at least once during first-responder monitoring. Fifty-four percent of converted patients refibrillated at least once, and 42% of all stable conversions occurred after at least one episode of refibrillation., Seventy percent of all refibrillations occurred less than six minutes after the defibrillator was turned on, and 23% occurred after more than ten minutes. The proportion of stable conversions decreased from 30% on first conversion to 2% on fourth conversion. With each successive conversion the interval to refibrillation grew shorter, and development of a pulse or blood pressure became less likely. Presence of blood pressure or pulse after conversion had a sensitivity for hospital discharge of 54% and a specificity of 98%. Maximum VF amplitude before countershock was highly predictive of postshock rhythm, stable conversion in the field, time interval before refibrillation, inpatient admission, and hospital discharge. VF amplitude was unrelated to response interval or interval to defibrillation but was positively related to bystander CPR. Logistic regression identified VF amplitude as the most important predictor of hospital discharge; traditional variables such as response interval and bystander CPR were not predictive once amplitude had been accounted for. Changes in VF amplitude during the course of resuscitation efforts were frequent and also predictive of outcome.

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Conclusion: Patients in VF who were treated by early counter-

shock refibrillated much more frequently than previously reported. Refibrillations occur both early and late. Initial VF maximum amplitude is strongly predictive of outcome. Future reports of VF cardiac arrest should control for this previously neglected variable. Increased amplitude of VF during repeated refibrillation episodes is associated with increased hospital discharge, so future studies of advanced cardiac life support interventions should explore changes in VF amplitude as an outcome variable. [Callaham M, Braun O, Valentine W, Clark BM, Zegans C: Prehospital cardiac arrest treated by urban first-responders: Profile of patient response and prediction of outcome by ventricular fibrillation waveform. Ann EmergMed November 1993;22:1664-1677.] INTRODUCTION Virtually all survivors of prehospital cardiac arrest are patients who are in ventricular fibrillation (VF) on arrival of health care personnel. >3 Although early defibrillation by first-responders is becoming a standard of care, little has been reported about early defibrillation patients other than simple survival rates. 4 Few prognostic variables are available immediately to the emergency care provider at the patient's side. None of these variables reflect the individual patient's physiologic state or can be used to judge the effectiveness of interventions during CPR.5, 6 The amplitude of the VF waveform on the ECG or cardiac monitor is available immediately to the clinician, and a previous study suggests that it may predict the outcome of patients with VE 7 Improvement in VF amplitude might be a useful outcome measurement for prehospital interventions. However, VF amplitude has never been studied in patients receiving early defibrillation and has been studied only in a limited fashion in other patients. We therefore conducted an observational study of prehospital patients in VF treated by first-responders to profile these patients' response to initial treatment and determine how quickly other advanced life support (ALS) modalities are needed if countershock fails. We also sought to determine whether the amplitude of VF waveform could predict conversion to a stable rhythm, hospital admission, and hospital discharge.

MATERIALS AND METHODS All patients more than 18 years old who were in nontraumatic normothermio cardiac arrest and were treated by fire department first-responders in San Francisco,

22/1665

California, between February 1, 1989, and December 31, 1990, were evaluated. Patients in VF at the time of first ECG analysis by the fire department semiautomatic defibrillator were the subjects of this study ~ San Francisco has a hilly urban environment of 49 square miles with a population of 723,900;'the median age is 35.8 years, and 50% are male. Forty percent of the population is 25 to 44 years old, 5.8% are 45 to 49 years old, 4.9% are 50 to 54 years old, 4.2% are 55 to 59 years old, 4.5% are 60 to 64 years old, 8% are 65 to 75 years old, and 7% are more than 75 years old (1990 US Bureau of the Census data). The city's ethnic mix is 54% white, 29% Asian, 11% black, and 6% "other." (Twenty-one percent are of Hispanic origin but are not classified separately as to race.) There are 6,700 housing units per square mile. Nine percent of the population is foreignborn and has arrived in the United States since 1985; 22% speak English poorly or not at all. Nine percent have not completed the ninth grade, and 28% have at least a bachelor's degree. San Francisco has both a basic life support (BLS) and a paramedic ALS system. Citizen calls are received first at a 911 dispatch center; those that are medical emergencies then are relayed to a paramedic dispatch center. If the call appears to involve a cardiac arrest or major resuscitation, it is relayed simultaneously to the fire department dispatch center and to city paramedic ambulance units. The San Francisco Fire Department provides first response to three-fourths of all cardiac arrests in the city and is equipped with Laerdal semiautomatic defibrillators (HeartStart 2000% Laerdal, San Diego, California). Fire department protocols call for the immediate connection of the patient to the defibrillator with uniform use of anterior and posterior defibrillation pads, with airway management and CPR initiated simultaneously. (Airway management includes oral airway, oxygen administration, bag-valve-mask ventilation, and suction if needed.) As soon as the machine is connected, the rhythm is analyzed. If the patient is in VE as many as six shocks are administered in sets of three as rapidly as possible, with one minute of CPR (and reanalysis) between the sets if fibrillation continues. Subsequent response (including further countershocks) and transport to hospital are provided by ALS units staffed with two paramedics, using standard advanced cardiac life support (ACLS) protocols, s Paramedics consult by radio with a mobile intensive care nurse and an emergency medical services (EMS)certified and trained emergency physician at the single base hospital for the city. Paramedic supervisors selected for clinical skills also respond on-scene to the majority of

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VFWAVEFORM CaI[aham et a1

cardiac arrests. EMS guidelines allow paramedics to withhold resuscitation in specific situations of obvious irreversible death (eg, lividity, rigor mortis) and after consultation with the base hospital; during the study period, resuscitation was attempted on 40% of all paramedic resuscitation calls. The average paramedic has seven years of experience, and the average ambulance responds to 5,500 calls per year and transports 3,416 patients. The intubation success rate is 83%. Detailed demographic and outcome data were collected from fire department, paramedic, receiving hospital emergency department, and inpatient records. The defibrillator incorporates a microcassette recorder that simultaneously

records the patient's ECG on one channel and an audio recording of the resuscitation on the Other. An ECG recording similarly is made on the unit's solid state memory module, whichalso records defibrillator activity throughout the duration of the call. These recording units are active as long as the fire department defibrillator is turned on, which is at least until paramedics arrive on the scene and frequently until transport to the hospital. The accuracy of time measurements in cardiac arrest studies is a major methodologic obstacle, 9 so no estimates or times recorded by personnel directly providing health care were used in this study. Times of all events were obtained from computerized fire department or paramedic

Table 1.

Response times for all cardiac arrests during the study period

Patient Group

Fire Department Response Interval (mean, SD)

90% Interval

P

Paramedic Response Interval (mean, SD)

90% Interval

P

All cardiac arrests 3.8 (2.1) 5.3 9.7 {3.7) 14 Non-VF arrests (asystole plus electromechanical dissocation) 3.8 (2.2) 5.2 9.6 (3.8) 14 Survivors of all arrests 3.5 (1.2) 5.2 .36 8.6 (4,2) 14 Nonsurvivors of all arrests 3.8 (2.1) 5.3 .36 9.8 (3.6) 14 The90% interval is the 90th percentile (the response interval during which g0% of calls occurred).The Pvalues refer to a comparison between values for survivors and those for nonsurvivars by unpairedtwo-tailed t-test Similar values for patients with VF are listed in subsequent tables.

,20 .20

Table 2.

Characteristics of VF study group patients (continuous variables)

Variable

95% Lower CI

Fire department response interval (rain) System BLS response interval (min)* Paramedic response interval (min}

3,8 (2) 5.t (2.4) 9.9 (3.6)

3.5 4.8 9.5

No. of shocks per patient No. of conversions per patient No. of refibrillations per patient

3.5 (2.9) 1.1 (1.2) 1.7 (0.9)

3:2 0.96 1.5

3,88 1.2 1.8

259 265 106

Maximum amplitude of first VF (mV) Maximum amplitude of second VF (mV) Maximum amplitude of third VF (mV) Maximum amplitude of fourth VF (mV) Maximum amplitude of fifth VF (mV) No. of shocks to first conversion No. of shocks to second conversion No. of shocks to third conversion No. of shocks to fourth conversion

0.88 (0.49) 0.84 (0.46) 0.73 (0.4} 0.87 (0.34} 0.8 (014) 1.7 (1.6) 1.4 (0.9) 1.1 (0.4) 1.1 (0.3)

0.82 0.74 0.59 0.65

0.94 0.94 0.86 1.1

1.5 1.2 0.9 0.9

2.0 1.6 1.3 1.3

258 92 39 11 3 180 74 26 10

1.1 2.2 4.4 11

1.4 3.6 6.7 13.2

255 174 94 242

Defibrillator on tO first shock (min) Defibrillator on to first conversion (min} Defibrillator on to first refibrillatien (min) Total first-responder record time (min) *Seetext. Maximumamplitude is of measuredVF.

1.2 (1.6) 2.9 (4.8) 5.5 (5.7) 12.1 (8,3)

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95% Upper CI

No. for This Variable

Mean (SD)

4.0 5.5 10.5

242 200 216

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division dispatch data or from the internal solid state module with a clock contained in the defibrillator. (The only exception to this was a small subset of patients in whom time estimates similar to those of Weaver et al were used for comparison with that study; these estimates are identified as such in the text and were not used in any other portion of this study.) Paramedic and fire dispatch use synchronized clocks, and all dispatch and response times were obtained from computer records. Fire department response interval was measured from receipt of call by the fire department (from the paramedic dispatch center) to the time of engine arrival on-scene (but not at the patient's side). System BLS response interval was measured from the receipt of the call by paramedic dispatch to the time of engine arrival on-scene; this interval equals the fire department response interval plus any delay that is incurred in relaying the call to the fire department but does not include any delay in relaying the call from 911 tO the paramedic dispatch center. Paramedic response interval was measured from time of receipt of call by the paramedic dispatch center to the time of paramedic ambulance arrival on-scene (but not at the patient's side). Fire department and paramedic response times are reported not only as mean values, but also as 90th percentile values (response interval that includes 90% of all calls) because this is a common management tool in EMS systems. Total measured interval to countershock was defined as the time from receipt of the call at paramedic dispatch to the time of fire department arrival on-scene plus the time from when the defibrillator was turned on to delivery of the first shock. (There is an additional delay of several minutes between fire department arrival on-scene and arrival at the patient's side when the Table 3.

Characteristics of VF studygroup patients (categorical variables) Variable

No. (%)

No. of conversions (total) No. of refibrillations (total) Patients with stable conversions Patients with stable conversion on first conversion Patients with blood pressure or pulse with conversion Patients who died in field Patients whe died in ED Patients admitted to hospital Patients discharged alive Data were available for all 265 patients for a][ variables.

296 177 139 (53)

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80 (30) 26 (35) 62 (24) 137 (52) 53 (20) 24 (9)

defibrillator is turned on; this interval is not measured in our system or in any other and is not included in the Calculations. The time at which the defibrillator is turned on represents the first and best documentation of firstresponder arrival at the patient's side.) The maximum amplitude of VF was defined as the maximum peak-to-trough amplitude of VF in milfivoks during the initial three to seven seconds when the machine is analyzing the rhythm and before it begins to charge, when it is certain that no one is touching the patient or otherwise creating interference (and when health care personnel, coincidentally, often have no other conflicting tasks). This amplitude was identified and measured to the nearest 0.1 mV manually with ECG calipers by One of the authors. To confirm the reproducibility of these interpretations, all values also were measured independentlyby computer software directly from the digital waveform data (Igor version 1.25, WaveMetrics, Lake Oswego, Oregon). Conversion was defined as transformation of the VF waveform after a countershock to an organized rhythm of nonspecific rate and configuration (not asystole), with or without pulse or blood pressure. Such a conversion always involved a continuing rhythm lasting at least one minute. Stable conversion was defined as an electrical conversion to a rhythm other than asystole that did not revert to VF during the prehospital monitoring by the fire department. Thus, stable conversion refers to being electrically stable; the hemodynamic effectiveness and stability of the resulting electrical rhythm were variable. Return of pulse was defined as a palpable pulse or measurable blood pressure, as determined by first-responders or paramedics. Time of conversion was defined as the time at which the first complex of the new rhythm was recorded in the defibrillator digital module. VF was diagnosed by the electronic module in the defibrillator (whose accuracy and criteria have been reported previouslylO, 11) and confirmed by paramedics and one of the authors. Results were analyzed by Z 2 test and contingency table, unpaired two-tailed t-test, analysis of variance, correlation coefficient, and linear and logistic regression as appropriate. All results are reported as mean (_+standard deviation [SD]) unless otherwise specified. P < .05 was considered significant. RESULTS Demographics of Patients With Cardiac Arrest During the study period, there were 1,121 calls of possible cardiac etiology to which the fire department responded; of these,

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VF WAVEFORM Call&am et al

835 met the criteria for initiation of resuscitation The remainder of patients were declared dead in the field by ?aramedics after base hospital consultation, according

Figure 1. Time interval from when the defibrillator is turned on and the first conversion No. of patients

to EMS protocols. Two h u n d r e d seventy-four of the 835 patients in cardiac arrest (33%) were in VF on firstresponder arrival; 9% survived to hospital discharge. An

Figure 3. Time interval from the first conversion to th,efirst refibrillation

30liNo. of patients

60

25

50

..........................

20,

40.[ 15~ 30' 10

20 10

2

4 6 8 10 12 14 16 18 20 22 Defibrillator on to first conversion (min)

'

~

0 2 4 6 8 10 12 14 16 18 20 22 Time from first conversion to first refibrillation (rain)

Figure 4. Time interval from when the defibrillator is turned on to the last episode of refibrillation

Figure 2. Time interval from when the defibrillator is turned on to thefirst refibriIlation in patients with successful initial conversions

No. of cases

Count

0

0

2.5

5

7.5

10

12.5

15

17.5

Defibrillator on to first conversion (min)

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20

22.5

0 5 10 15 2o 25 30 Time from defibrillator on to last refibriilation (rain)

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additional 320 patients (38%) were in asystole, and 241 (29%) were in electromechanical dissociation; there were 12 survivors (2%) from these two categories combined. The Overall cardiac arrest survival rate was 4%. Fifty-three percent of all cardiac arrests were witnessed, and 16% received bystander CPR. Seventy-five percent of survivors had witnessed arrests. Thirty-nine percent of survivors received bystander CPR versus 15% of those who died (P = .0001). Figure 5. Elapsed time intervals between defibrillation, conversion, and refibrillation episodes (in minutes). With each successive episode, refibrillation occurs more quickly, but so does conversion. Minutes between conversion and refibrillation evenis

[ [ ] Minutes from prior conversion to refibrillation [ • Minutes from prior fibrillation to conversion

NN

The fire department and paramedic response times are given (Table 1). There was no significant difference it/paramedic response interval between VF patients who survived to discharge versus those who did not (11.1 versus 9.9 minutes, P =. 16). Complete data were available for 265 of the patients in VE who formed the basis for this repor t. The average age of these patients in VF was 65 years (SD, 15); 75% were male; 79% were witnessed, and 28% received bystander CPR: The demographics of the entire VF study group and their responses to countershock are given (Tables 2 and 3). Response of Patients to Early Countershock The response to countershock is shown (Table 2). Eighty percent of first conversions were achieved with fewer than three shocks. Ofie hundred seventy-three patients (65%) converted at least once. One hundred thirty-four of all patients with conversions (77%) were electrically stable (not necessarily on the first conversion), but 94 patients (54%) refibrillated a total of 162 times. No cases of failure by the semiautomatic defibrillators to diagnose VF were identified, nor Table 4. Correlation and regression between VF amplitude (in mV) and response times (simple regression)

Variable

N First

episode

Second episode

Third episode

Fourth episode

Figure 6. Stability and refibrillation rates of each successive conversion of VE. With each episode, a smaller percentage of conversions is stable, and a higher proportion refibrillate. % of conversions

Fire department response interval System BLS response interval Total measured interval to countershock

t'ercent renorlliatea

26/1669

No. of Patients

P

.11

4.17, - .45

235

.09

.03

5.23, -.14

193

.70

.02

6.01, -.12

190

.76

Amplitude {mV) Variable

Secondconye)'sion Third conversion

Intercept, Slope

Table 5. Amplitude of VF in relation to other variables

[ ] Percent stable conversion

First conversion

R

Fourthconversion

Present ( S D )

Sex 0.85 (0,44) M Bystander CPR 0.98 (0,51) Witnessed arrest 0.89 (0.58) P waves present 1 and 4 minutes after shock 1.2 Stable conversion 1,02 (0.5) Blood pressure and pulse with first conversion 1.19(0.43) Admitted as inpatient 1.0 (0.44) Hospital discharge 1.15 (0.43) Datawere analyzedby unpairedtwo-tailed t-test.

Absent (SD)

Patients

P

0.96 (0.59) F 0.84 (0.47) 0.83 (0.57)

265 258 256

.iO .03 .43

0.95 0.71 (0.4)

254 247

.006 .0001

1.01 (0.44) 0,84 (0.49) 0.85 (0.48)

74 252 258

.13 .04 .006

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did they erroneously shock patients not in VE However, in one case the machine recorded VF but did not indicate whether to shock, resulting in a delay in treatment until paramedics arrived; this patient was resuscitated and survived to hospital discharge. Patients refibrillated both early and late after conversion. The first refibrillation occurred an average of 2.6 minutes (SD, 3.3 minutes) after conversion and an average of Table 6.

Probability of selected outcomes based on amplitude of VF Witnessed Patients

Variable

Stable Conversion, Stable Conversion, Bystander CPR No Bystander CPR

Hospital Discharge (all)

Amplitude(mY) 0.2 1.0 2.0

.4333 .6853 .8895

.3139 .5657 ,8281

.OO0008 .001323 .457766

Relative risk I versus0.2 mV 2 versus 1 mV

1.58 1.29

1.80 1.46

165 346

Figure 7.

The relative risk describes the relative probability of the outcome at different amplitudes. BystanderCPRdid not affect hospital discharge. Datawere analyzed by linear and logistic regression.

Table 7.

Predictive value of VF amplitude for electrically stable conversion, return of pulse infield, hospital admission, and hospital discharge Threshold (mV)

0.2

0.8

0.9

1

1.2

Stable conversion Sensitivity 100 Specificity 1 PPV* 54 i/PV 100

68 69 72 65

60 74 73 61

~ 50 77 72 57

35 86 74 53

89 33 30 90

83 42 31 89

78 49 33 88

39 65 26 77

100 0 20 100

67 54 27 87

62 61 29 86

56 68 31 86

31 77 25 81

1O0 0 9 100

83 53 15 97

83 60 17 97

78 64 19 97

39 76 14 93

Pulse Sensitivity Specificity PPV NPV

5.5 minutes (SD, 5.7 minutes) after the defibrillator was turned on and connected to the patient (Figures 1, 2, and 3). Seventy percent of all refibrillations occurred less than six minutes after the defibrillator was turned on, and 55% occurred in less than four minutes (Figure 2). However, refibrillations also occurred over an extended period after conversion (Figure 4). Thirty-six percent of first refibrillations occurred more than ten minutes after the first conversion, and 32% of second refibrillations occurred more than ten minutes after the second conversion. The average time between when the defibrillator was turned on and the last episode of refibrtllation was 7.7 minutes (SD, 6.3 minutes; 95% confidence interval [CI], 6.4, 9.1 minutes; range, 1.2 to 31 minutes), and 23% of all refibrillations occurred after more than ten minutes (Figure 4). The time intervals between conversions and refibrillations are given (Figure 5). Conversion did not require more shocks with successive refibrillations (Table 2), but the proportion of stable conversions decreased from 30%

Receiver operating characteristic curves for ability of VF amplitude to predict electrically stable conversion, return of pulse infield, inpatient admission, and hospital discharge. The curves represent the combined sensitivi F (truepositive) and 1-specificity (false-positive) rates of this variable at differing thresholds. The first point in the upper right represents a threshold of O.1 mV, and the lowest left point represents a threshold of 1.8 mE The test producing a curve in the upper most left corner is the most accurate test; this graph demonstrates the similarity of prediction for all three variables and allows the reader to pick a threshold value in mV that best combines the desired sensitivity and false-positive rate desired. Sensitivity (%) 9O. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1O0 0 24 100

Inpatientadmittance Sensitivity Specificity PPV NPV

Hospitaldischarge Sensitivity Specificity PPV NPV

,

30

*PPV,positive predictive value; NPV, negative predictive value. (All values are given in percent). Severaldifferent thresholds are reported; all thresholds are given in mV.

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+

Elccmcally stable conve~ion

~

Inpatient admission

- B - - Hospltal discharge 20

t

~o~ ~ - ~ " .....................................................................................

ol

.

0

10

.

. 20

. 30

.

. 4O

. 5O

.

. ~

70

80

90

I

l~

1670/27

VF WAVEFORM

CaIlahamet al

on first conversion to 2% on fourth conversion (Table 3 and Figure 6). However, 42°£ of stable conversions were achieved after at least one initial episode of refibrillation. With each successive conversion, the time interval to refibrillatton grew shorter. Patients with greater amplitude (1.0 mV or more) refibrillated an average of 3.4 minutes (SD, 3.7 minutes) after conversion versus 1.9 minutes (SD, 2.8 minutes) for patients with lower amplitude (less than 1 mV, P = .03). One minute after the first series of shocks, 34% of patients were still in VE 7% were in asystole, and 59% were in other rhythms (including electromechanical dissociation). At four minutes after shock, 56% had refibrillated, 2% were in asystole, and 42% were in other rhythms. Postshock rhythm was not associated with fire department

response interval (P = .58) or with total measured interval to countershock (P = .66). However, it was associated with VF amplitude. Patients who had no conversion or refibriIlation one minute after shock had a VF amplitude of 0.95 mV versus 0.45 mV for those in asystole and 1.1 mV for those in other rhythms (P = .07). Patients who had re fibrillated at four minutes after shock had a VF amplitude of 0.64 mV versus 0.92 mV for those in asystole and 1.0 mV for those in other rhythms (P = .0001). Thirty-five percent of patients with electrically stable conversion had a pulse or blood pressure in the field. Table 10.

Relationship of hospital discharge to categorical variables Patients Who Were Discharged

Table 8.

Comparison of patients admitted as inpatients with those who died before hospital admission (continuous variables)

Variable

Patients Who Were Admitted (mean, SD)

Patients Who Died (mean, SD)

Fire department response interval (min) 3.8 (1.8) Delay in paramedic dispatch receipt to notification of fire department {min) 1.3 (0.9) Total measured interval to countershock (min) 5.7 (2.1) Maximum amplitude of first VF (mV) 1.0 (0.44) Change in maximum amplitude during refibrillation (%) 10 (65) Datawere analyzedby unpairedtwo-tailedt-test.

No. of Patients

P

3.8 (2)

239

.85

1.7 (1.3)

182

.10

5.9 (2.7)

193

.53

0.84 (0.5)

256

.03

89

.06

-13 (42)

Variable

No.

%

No.

%

P

Witnessed arrest 23/24 Bystander CPR 13/24 Stable conversion 18/24 Blood pressure and pulse with first conversion 11/15 Patient ever refibrillated 10/24 Stable first conversion 15/18 Stable second conversion 0/18 Stable third conversion 1/18

96 54 75

184/239 62/179 121/240

77 26 50

.06 .006 .04

73 42 83 0 6

7/52 96/145 63/120 33/120 14/120

12 39 53 27 12

.001 ,96 .08

Data were analyzed by X 2 and contingency analysis.

Table 11.

Relationship of hospital discharge to continuous variables

Variable Table 9.

Comparison of patients admitted as inpatients with those who died before hospital admission (categorical variables) Patients Who Were Admitted

Patients Who Died

Variable

No.

%

No.

%

Male Witnessed arrest Bystander CPR Stable conversion Blood pressure and pulse with first conversion Patient ever refibril]ated

35/53 49/53 21/53 38/52

18 92 40 73

161/207 156/208 53/210 9£/210

82 75 25 47

.11 .01 .06 .001

13/26 26/53

50 49

5/46 78/132

11 37

.0007 .15

Data were analyzed by Z z test and coritfngancy analysis.

P

Patients Who Died

Patients Who Were Discharged (mean, SD)

Fire department response interval (rain) System BLS response interval (min) Delay in paramedic dispatch receipt to notification of fire department (rain) Total measured interval to ceuntershock (rain) Paramedic response interval (rain) No. of shocks to first conversion (rain) Maximum amplitude of first VF (mV) Change in amplitude during refibrillations (%)

Patients Who Died (mean, SD)

No. of Patients

P

3.4 (1.3)

3.8 (2)

241

.36

4.4 (1.8)

5.2 (2.5)

199

.18

1.1 (.8)

1.7 (13)

184

.08

5.1 (1.8)

6 (2.6)

195

.15

9.9 (3.6)

215

.16

1.2 (0.7)

1,8 (1.6)

80

.09

1.1 (4.3)

0.85 (6.48)

258

.006

90

.03

11,1 (4)

32 (73)

-11 (46)

Data were analyzed by unpaired two-tailed t-test.

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Of patients who regained a pulse or blood pressure after conversion, 73% regained it after the first, 4% after the second, 8% after the third, 12% after the fourth, and 4% after the fifth conversion. Blood pressure or pulse in the field occurred in 75% of those who survived to inpatient admission versus 21% of those dying earlier (P = .0001) and in 100% of patients surviving to discharge versus only 24% of those who died in hospital (P = .0001). All patients with refibrillation who had a pulse or blood pressure on first conversion also had it on subsequent conversions. Blood pressure or pulse on any conversion had a sensitivity for hospital discharge of 54%, a specificity of 98%, a positive predictive value of 93%, and a negative predictive value of 80%. Thirty-seven percent of patients with an electrically stable conversion remained stable with no subsequent pharmacologic therapy in the field; their survival rate was 50%. The remainder of these patients deteriorated and required administration of IV drugs (usually epinephrine) by paramedics, as well as other ALS measures; their survival rate was 10%. In the entire population of patients who received early countershock, 34% required no subsequent catechol therapy by paramedics, and 29% of these patients survived; 66% did require subsequent catechol therapy, and 6% of these patients survived. Ventricular Fibrillation Amplitude The mean amplitude was 0.88 mV (SD, 0.48 mV; 95% CI, 0.82, 0.94; range, 0.1 to 3.0 mV), and the distribution approached normal. (The number of cases for each 0.1-mV step in amplitude = t8.551 - [3.984 (amplitude)] - 1.198 (amplitude)<) There was no significant relationship between VF amplitude and fire department response interval, total measured interval to countershock, or defibrillator on to first conversion interval (Table 4). This was true regardless of whether the arrest was witnessed and whether there was bystander CPR. In a subset of the first 50 patients, we also obtained bystander estimates of time from collapse to 911 call similar to the estimated interval data used by Weaver and colleagues, r There was no association between the amplitude and these estimated response times (R =. 13, R2 = .02, P = .40). Ninety-four patients had repeated episodes of fibrillation; the changes in amplitude and the time intervals between these episodes were measured. The correlation between changes in amplitude and elapsed time was very low (R2 = .07), evenwhen patients with measurement times of more than five minutes were analyzed separately In 28% of patients, the VF amplitude increased with the first refibrillation (mean, 0.32 mV; SD, 0.4 mV); in 62%, it decreased (mean, -0.4 mV; SD, 0.4 mV); and in 10%,

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there was no change. Twenty-five patients (27%) had increases in refibrfilation amplitude averaging 0.3 mV (95% CI, 0.2, 0.45 mV; range, 0.1 to 1.5 mV), or 53% of baseline. The mean change in amplitude from initial VF to final refibrillation was -8% (SD, 50%; 95% CI1-18%, 200%; range, -73% to 200%). Patients who developed a pulse had a 17% VF amplitude increase with refibrillation (SD, 59%) versus a 23% decrease (SD, 29%) for those who did not (P = .009). However, there were no significant differences in this variable between those who developed a stable conversion and those who did not. Admitted patients had an increase of 10% in amplitude versus a decrease of 13% in those who were not admitted (P= .06). Patients who survived to discharge had an average 32% increase in VF amplitude with refibrillation versus an 11% decrease in those who died (P = .03). Only six survivors had refibrillation episodes; three had an increase in amplitude, and three had a decrease. Manual VF amplitude measurement was compared with direct computer measurement of the waveform in 88 randomly selected patients in whom digital data were available that met the criteria of a presenting rhythm of at least 5.12 seconds in duration (as required by the computer algorithm). 12 In this subset, computer and manual interpretation of amplitude showed very high agreement (r = .986). Predictive Aspects of Ventricular Fibrillation Amplitude The maximum amplitude of the initial episode of VF was highly associated with bystander CPR, postshock rhythm, electrically stable conversion, inpatient admission, and hospital discharge (Table 5). The probability of stable conversion and hospital discharge was increased by factors of as much as 1.80 with increased VF amplitude (Table 6). The sensitivity, specificity, positive predictive value, and negative predictive value of the amplitude of VF at several thresholds to predict key outcome variables are shown (Table 7). The receiver operating characteristic curve of this variable is delineated further in Figure 7, which demonstrates the similarity of prediction for all four variables and allows the reader to pick a threshold value in millivolts that best combines the desired sensitivity and false-positive rate. The same values were calculated for the maximum amplitude of the first (and subsequent) refibrillation episodes; although showing similar trends, all of these were inferior in accuracy of prediction to the amplitude of the initial VE The characteristics of patients who were resuscitated and survived long enough to be admitted as inpatients were examined individually (Tables 8 and 9). Most vari-

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ables, including response interval, age, number of shocks, whether the patient ever refibrillated, and number of refibrillations, did not differentiate between patients who survived to hospital admission and those who did not. Only the most clinically relevant variables are included in the tables. Similarly, we examined the characteristics of patients who ultimately survived to hospital discharge (Tables 10 and 11). Age, response interval, whether the patient ever refibrillated, number of refibrillations, and interval to first conversion did not differ between patients who survived to discharge and those who did not. An increase in VF amplitude during repeated refibrfllations, however, was associated with an increased likelihood of survival to discharge. Logistic regression then was used to determine which independent variables best predicted outcome. We suspect that time to the 911 call in the unwitnessed group may have been longer, because different patterns of associations emerged between the witnessed and unwitnessed groups. These two groups therefore were analyzed separately. The only variable predictive of amplitude was bystander CPR (P = .02). No other variables, including sex, age, or any response interval, were predictive of amplitude once bystander CPR was taken into account. This was true for both witnessed and unwitnessed arrests. In the unwitnessed group, no variable predicted stable conversion except amplitude. In the witnessed group, logistic regression identified two variables that were predictive: amplitude (g-coefficient,. 11; SEM, .05; P = .02) and bystander CPR (g-coefficient, .79; SEM, .45; P = .08). For example, a VF amplitude of 2.0 mV was 1.29 times more likely to result in stable conversion than one of 1.0 mV in a patient with bystander CPR, and 1.46 times more likely in a patient without bystander CPR (Table 6). Response times were not predictive once amplitude and bystander CPR had been taken into account. When we analyzed the impact on hospital admission, we examined only those patients who had had a stable conversion because this is a necessary condition for hospital admission. (Including in the analysis the large numbers of patients who died in the field would obscure the effect of treatment.) In unwitnessed arrests, no variable predicted admission. In witnessed arrests, only amplitude predicted admissions; age, bystander CPR, and response interval were not p.redictive. Among patients with witnessed arrest admitted to hospital, logistic regression identified the only predictor of discharge as maximum amplitude (g-coefficient,. 66; SEM, .31; P = .03). An amplitude of 2 mV increased the

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likelihood of hospital discharge 346-fold compared with 1 mV (Table 6). More traditional variables such as response interval and bystander CPR were not predictive once amplitude had been accounted for. Only one unwitnessed patient was hospitalized, so analysis could not be performed on this group. DISCUSSION

It is well established that early defibrillation, preferably by first-responders, is highly effective in prehospital cardiac arrest. 13-1s Accordingly, early defibrillation has been recommended as a standard of care by the American Heart Association.< ~9 It also has been emphasized that firstresponder early defibrillation does not obviate the need for prompt ALS treatment, which independently contributes to survival.+,2o,21 However, the literature reveals little about early defibrillation patients other than survival rates. Specifically, the exact time course of events and predictors of outcome in these patients have not been reported in detail. This was the first goal of our study. Whether patients defibrillated by first-responders maintain stable cardiac rhythms without ALS is an important practical issue. Only one previous study addressed this question. The authors reported that their re fibrillation rate was low and refibrillations occurred very quickly, so patients subsequently were stable for transport. 22 The results of our study are quite different. Sixty-five percent of our patients converted initially, a rate that is very similar to that reported by others.S< 23 However, our patients suffered an average of 1.7 refibrillations each, with the first occurring within 2.6 minutes of conversion. Our refibrillation rate of 54% contrasts sharply with the rate of 17% reported by Stults and Brown. 22 Refibrillations in our patients occurred both early and late. Most were early; 55% of all refibrillations occurred with four minutes after the defibrillator was turned on, 64% within five minutes, and 87% within ten minutes (Figure 2). Stults and Brown reported that all re fibrillations occurred within four minutes of conversion and that only one patient required more than three shocks. In contrast, our patients continued to refibrillate throughout the prehospital care period. They received an average of 3.5 shocks each, and 20% of first conversions required three or more shocks. Twenty-three percent of all re fibrillations occurred after more than ten minutes, and about onethird of first and subsequent refibrillations occurred ten or more minutes after the preceding conversion (Figure 4). Although refibrillation did not affect the admission or survival rate (Tables 1 and 2), the long period over which

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patients refbrillated suggests that if ALS is available, it might have a significant role to play. Stults and Brown reported that refibrillation responded well to countershock alone, but we found that each successive refibrillation was less likely to convert to an electrically stable rhythm, was more likely to refibrillate, and occurred at increasingly shorter time intervals (Figures 5 and 6). Conversions did not require more shocks with each successive refibrillation, but the proportion of stable conversions decreased from 30% on the first conversion to 2% on the fourth conversion, and the time interval to each refibrillation grew Successively shorter. Several informal publications claim that ALs needs to begin within 12 to 15 minutes of emergency medical technician-defibrillation arrival, but these claims are not based on published studies. 24,25 Our data suggest that even in an urban system with short frst-responder response times, ALS treatment might be useful within a few minutes after the defibrillator is turned on. Two-thirds of refibrillations will occur within five minutes of firstresponder arrival, and paramedics must be present several minutes before refibrillation to be fully prepared to treat immediately. Refbrillation is less likely to be converted by first-responder countershock with every episode and can recur as long as 30 minutes after first-responder arrival, thus complicating transport. However, continued efforts are worthwhile because conversions after even three or four refbrillation episodes may be stable and result in hospital discharge. Many clinicians probably would argue that treatments such as better airway control and IV medications would be of benefit in cases refractory to first-responder countershock. However, the relative benefits and efficacy of these ALS measures versus continued attempts at countershock alone are unknown. Our study did not examine the effectiveness of ALS interventions in this setting but may provide some insight. Before ALS personnel arrival, each refibrillation in our patients was less likely to respond successfully to shock by first-responders. However, ALS arrival was no guarantee that this deterioration would be reversed. Most patients surviving to hospital discharge in our study were defibrillated successfully without ALS treatment. When first-responders achieved stable conversion and no ALS catechol drug treatment was required, survival to discharge was 50%. Among those VF patients who also required ALS catechol treatment, survival was only 6%. Further study is needed to specifically address and resolve the issue of the effectiveness of ALS intervention in patients receiving early defibrillation.

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The second goal of our study was to determine if VF amplitude could predict outcome. It would be extremely useful to identify a variable that reflects the physiologic state of the individual in cardiac arrest, predicts outcome, and is readily available in the field. No such variable has been reported except for end-tidal carbon dioxide, which requires additional equipment and expertise not presently possessed by most first-responders or paramedics. 26 Factors such as witnessed arrest, presenting cardiac rhythm, bystander CPR, and paramedic response interval predict outcome,5, 6 but these parameters reflect only the statistical profle of a population, not the physiologic response of an individual. Even the best of them is too imprecise a measure to apply to the individual patient. 27 Other predictors of individual awakening and survival are measured one or more hours after resuscitation and thus are of no use during CPR. 2s The predictive value of VF amplitude has been studied previously only once. This study categorized VF binomially (either less than or more than 0.2 mV), and the patients were treated by paramedics, not by first-responders With early defibrillation.7 Another study has shown "coarse" VF to be a favorable fnding but did not examine this variable in detail. 21 So little attention has been paid to VF amplitude that large studies focusing on coarse VF never even define "coarse. ''29 Our data suggest that the previous definition of '~coarse" as more than 0.2 mV is useful chiefly for its negative predictive value for stable conversion or survival (Table 7). VF of such low amplitude represents only 3% of our urban population treated by firstresponders Compared with 17% of the patients of Weaver and colleagues who were treated by paramedics. There is no standardized measure of VF electrical energy; some have averaged the peak amplitudes over a period of six seconds, 1t whereas animal studies have used sophisticated Fourier analysis requiring equipment not available in the field.3O,31 It currently is not known which of these measures best reflects myocardial physiology o r outcome. We chose a methodology very similar to that of Weaver and colleagues 7 because we sought a simple, rapidly obtainable measure that required minimal technology and could be carried out on the spot by health care providers. Although the maximal VF amplitude may appear to be an arbitrary choice (compared with average values or the more elaborate Fourier transformation methods that require computer analysis), in our experience it appeared to be consistent and reproducible between manual and digital measurement. However, optimal training for manual interpretation and interobserver reliability are issues that need further study.

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Logistic regression analysis of our data identified VF amplitude as a better predictor of outcome than previously reported variables such as response interval and bystander CPR. For example, a VF amplitude of 1 mV predicts hospital discharge with a sensitivity of 78%, specificity of 64%, positive predictive value of 19%, and negative predictive value of 97% (Table 7). This value predicted stable conversion in the field and inpatient admission to the hospital with similar levels of confidence. A patient with an amplitude of 2 mV was 346 times more likely to survive than one with 1 mV,, who in turn was 165 times more likely to survive than one with 0.2 mV (Table 6). The predictive power of the VF amplitude is further explored in the receiver operating characteristic curve (Figure 7). Receiver operating characteristic curves represent the combined sensitivity (true-positive) and 1-specificity (false-positive) rates of the amplitude at differing thresholds and are the best way to compare the accuracy of different tests or the prediction of different variables)Z, 33 The test (or variable) producing a curve that is located at the uppermost left corner is the most useful one. Clinicians may desire different levels of sensitivity versus specificity in varying clinical situations; Figure 7 also allows the reader to pick a threshold value in millivolts that best combines the desired sensitivity and false-positive rate. The only variable that predicted the amplitude of VF by logistic regression was bystander CPR (Table 5). This is consistent with other studies showing improved survival with bystander CPR 3e and suggests that bystander CPR may directly contribute to a better cardiac physiologic state. For subsequent outcomes such as stable conversion, admission, and hospital discharge, only amplitude was a powerful predictor by logistic regression. A minimal benefit to bystander CPR was seen in stable conversion but not in later outcomes. Response interval was not significant at this stage, once amplitude had been taken into account. This suggests that future studies of VF and ACLS interventions should report and control for the amplitude, which has not been done. The maximum amplitude of subsequent episodes of refibrillation was not strongly predictive of outcome and was markedly inferior in predictive power to the initial VF episode. It could be argued that despite these statistics, VF amplitude has no clinical implications and will not affect patient management. Although currently true, this may not remain so for long. Animal studies demonstrate that prolonged VF (whose duration may be suggested by amplitude) responds better to initial catecholamines and CPR than to immediate countershock) 5 Thus, it1 the near future clinical treatment may indeed by influenced 15ymeasures of VF energy

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In addition, VF amplitude should not be regarded as a fixed variable over which the cIinician has no control in the individual patient. Instead, it may be a useful measure of the success of ALS interventions. Animal studies document that on-going interventions can restore a high VF amplitude and result in at least four-hour sdrvival after even 30 minutes of completely untreated continuous VE 36 Our patients showed variable changes in VF amplitude during episodes of refibrillation. In 37%, amplitude increased over time, presumably due to ongoing BLS and intervening episodes of perfusion. Increases in amplitude during refibrillation episodes were significantly associated with developing a pulse or blood pressure in the field, with hospital admission, and with hospital discharge. These results suggest the possibility that changes in VF amplitude may be a useful outcome measure for ALS interventions. Although logistic regression demonstrated that VF amplitude was the best predictor of outcome, a number of other variables were associated with hospital admission or discharge (Tables 8 to 11). These variables included witnessed arrest, bystander CPR, stable conversion, and blood pressure or pulse with conversion. Although only 35% of patients with an electrically stable conversion had a pulse or blood pressure in the field, all patients who were discharged alive had this sign. Blood pressure or pulse on any conversion had a sensitivity for hospital discharge of 54%, specificity of 98%, positive predictive value of 93%, and negative predictive value of 80%. Factors such as number of shocks required to defibrillate or number of refibrillations were not associated with poor outcome. We found no relationship between any time interval (including response interval) and VF amplitude. This was true regardless of whether arrest was witnessed and whether there was bystander CPR. This finding contradicts the only other study to examine this relationship, r Weaver and colleagues reported that VF amplitude depended on the length of time from estimated collapse until BLS and ALS. Their study reported amplitude only as a binomial classification of less than or more than 0.2 mV, and the treatment and response times were dependent on estimates by bystanders. The difference between our results and their results may also be explained by the fact that their data were recorded by paramedics, who had a response interval approximately 50% longer than that of our first-responders with defibrillators (and whose patients accordingly underwent a longer period of treatment limited to CPR only). In addition, our time data were recorded electronically and

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I

included no estimates. However, even when we examined a small subset of patients on whom we had similar bystander estimates of the interval between collapse and the call to 911, we found no relationship between time and VF amplitude. Thus, VF amplitude should be regarded as an uncertain and unproved indicator of amount of time since collapse in arrests of relatively short duration. Our failure to find any relationship between various response times and outcome in this study is in marked contrast to much of the cardiac arrest literature.5,<2~s There are several possible explanations. First, there was a trend toward better outcome with shorter response times in our study, although this trend did not achieve statistical significance. Second, the type II error for a number of these variables was quite substantial. For example, for the outcome of hospital discharge, g-coefficient was .52, .51, and .62 for fire department response interval, total measured interval to countershock, and paramedic response interval, respectively (Table 11). Third, our flrst-responder response times were short, possibly below the level at which response interval makes a large difference in outcome. Fourth, there are a number of unmeasured time intervals in both our study and previous reports. These include the time necessary for bystanders to access 911 in a multicultural city where only 53% of all arrests were witnessed and only 16% received bystander CPR, the time for citizens speaking a variety of languages to get their message across to 911 operators, the time required to transfer the call from the 911 dispatch center to the paramedic dispatch Center, and the time from arrival on-scene to arrival at the patient's side, which may be considerably longer in an urban environment than in a suburban one. 3s Response interval was associated strongly with outcome in our study only when we included bystander estimates of the interval between patient collapse and call to 911; the reliability of such estimates is unknown. We tried to minimize inaccuracy by using no interval estimates at all (in contrast to some prior studies) and b y analyzing total measured interval to countershock, a parameter not available in precise form in many prior studies. Nonetheless, it is not completely clear why our response interval results differ from those of previous studies, and the relationship of VF amplitude and measured response interval to outcome needs further investigation. Intriguing concerns that were not addressed in this study are exactly what VF amplitude represents and whether it is an inclependent or a dependent variable. Our data show that bystander CPR influences amplitude, although amplitude remains a stronger predictor of

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outcome. Other studies previously cited have found a relationship with duration of cardiac arrest; we did not. However, VF amplitude probably als0 represents the crucial factor of the physiologic ~'vigor" of the myocardium, an interpretation supported by animal studies in which improved coronary perfusion increased'VF amplitude. 36 The strong association of greater amplitude with survival in our study also supports this hypothesis. However, other studies cast doubt on this interpretation. In dogs, coarse VF may not represent increased synchronization of myocardial activation and may in fact represent no more than orientation and placement of ECG leads. 39 This possibility is reinforce d by the fact that patients with higher transthoracic impedance are more likely to have "fine" fibrillation and that amplitude decreases with increased transthoracic impedance. *o Transthoracic impedance is related to tissue conductivity (and probably to tissue blood flow and metabolic alterations during cardiac arrest) but is not related to duration of VE ~o However, the powerful ability of VF amplitude to predict outcome in our patients independent of other variables suggests that amplitude may represent some true physiologic factor, and this may be of more clinical importance than our current ignorance as to whether amplitude is primarily a dependent or an independent variable. The current state of understanding of this issue is incomplete and unsatisfactory, and further study is needed. Our study had a number of limitations. Our study population and first-responder performance may differ from those in other cities, and these results might not be applicable to other populations. The semiautomatic defibrillator was not left on for the entire prehospital treatment interval in all cases, so monitoring ceased, and our data may underrepresent the actual number of late refibrillations. Our hospital discharge rate among VF patients was lower than that of a number of other cities with firstresponder defibrillation,÷s which also may point to differences between cities. The success of subsequent ALS treatment by paramedics after failed countershock by first-responders is dependent on the promptness, training, and skill of local paramedics and also may differ among cities. We did not evaluate the effectiveness of specific ALS measures after failed countershock. Our method of evaluating VF amplitude is a manual one, and we did not attempt to evaluate inter-rater reliability in assessing this measurement. We also did not attempt to determine the optimal method of measurement or of training to take the measurement. Future studies should examine these issues to confirm that the methodology of measurement is reproducible by others.

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CONCLUS ON Although 65% of patients in VF who are defibrillated by urban first-responders converted at least once. 54% refibrillated_ many more than one time. Refibrillation tended to occur both early and late. which might suggest that prompt AL5 backup may be needed for this group of patients. However. whether such backup is effective was not examined m our study. Initial maximum VF amplitude is strongly associated with outcome, including stability of conversion, return of pulse in the field, admission to hospital, and overall survival. It is the only easily available predictive variable that in some way reflects the physiologic state of the individual patient. Bystander CPR increases VI: amplitude. but amplitude alone predicts subsequent outcomes such as postshock rhythm, electrically stable conversion. hospital admission, and hospital discharge, whereas traditionally used measures such as response interval_ witnessed arrest, and bystander CPR do not. Future reports of VF arrest should report and control for the magnitude of VF amplitude, and future studies of ACLS interventions should report and examine it as an outcome variable REFERENCES 1. Hargarten KM, Stueven HA, Waite EM, et al: Prehospital experience with defibrillation of coarse ventr}cuiar f]brigation: A ten-year review. Ann EmergMarl 1990;19:157-162. 2. EisenbergMS, Herwood BT, Cummins 80, et el: Cardiac arrest and resuscitation: A tale of 29 cities. Ann Erner#Mad 1990;19:179-186. 3. Hunt RC, McCabeJB, Hamilton GC, et al: Influence of emergencymedical services systems and prehespital defibrillation on survival of sudden cardiac death victims. Am J EmergMad 1989;7:68-82. 4. Cummins RO, Ornato JP, Thies WH, et al: Improving survival from sudden cardiac arrest: The "chain of survival" concept. A statement for health professionals from the Advanced Cardiac Life Support Subcommittee and the EmergencyCaidiac Care Committee, American Heart Association. Circulation1991;83:1832-1847 5. EisenbergM, Hallstrom A, 13ergnerL: The ACtS score: Predicting survival from out-of hospital cardiac arrest. JAMA 1981;246:50-52. 6. Aprahamian C, Thompson BM, Gruchow HW, et al: Decision making in prehospital sudden cardiac arrest. Ann EmergMad 1986;15:445. 7. Weaver WD, CobbLA, Dennis D, et el: Amplitude of ventdcuJarfibrillation waveform and outcome after cardiac arrest, Ann InternMed1985;102:53-55. 8. American Heart Association: Standards and guidelines for cardiopulmonary resuscitation (CPR) and emergencycardiac care (ECC).JAMA 1986;255:2905-2954. 9. Cummins R, ChambeBain[3, Abramson N, et al: Recommendedguidelines for uniform reporting of data from out-of-hospital cardiac arrest: The Utstein style. Ann EmergMad 1991;20:861-874. 10. Cummins RO, EisenbergM, Bergner L, et al: Sensitivity. accuracy,and safety of an automatic external defibriJlator. Lancet1984;2:318-320. 11. Cummins R, Stults K, Haggar B, et ah A new rhythm library for testing autematic external defibrillators: Performanceef three devices. J Am Coil Curdle/1988;11:597-602. 12. LoberW: Prediction of cardiac arrest outcomes using digital signal processing of initial electrocardiograms(thesis). University of California, Berkeley, 1992. 13. EisenbergMS, Cummins R0: Defibrillation performed by the emergencymedical technician. Circulotion 1986;74(suppl4):Iwg-IV-12. 14. 01sonDW, LaRochetleJ, Fark D, et ah EMT-defibrillation: The Wisconsin experience.Ann Finery7Mad 1989;18:806-811. 15. Ornate JP, Craren EJ, 6onzaiez ER, et al: Cost-effectivenessof defibrillation by emergency medical technicians. Am J EmergMad 1988;6:108-112.

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16. Paris PM: EMT-defibrillation: A recipe for saving lives, Am J EmergMarl 1988;6:282-287. i7. Vukov LF, White RD, BachmanJW, et al: New perspectives on rural EMT defibri[lation. An EmergMad 1988;17:318-321. 18. Cummins RO: EMT-defibrillation: National guidelines fo[ implementation. Am J EmergMe, 1987;5:254-257. 19. Cummins R, Thies W: Encouragingearly defibrilratian: The American Heart Association ant automated external defibrillators. Ann EmergMeal1990;19:1245-1248. 20. EisenbergM, Cummins R, Hallstrem A: Defibrillation by emergencymedical technicians. Ci CareMad 1985;13:921-922. 21. Weaver WD, Hill D, FahrenbruchCE, et ah Use of the automatic external defibrillator in the management of OLJt-df-hospitalcardiac arrest. N EnglJ Mad 1988;319:661-666. 22. Stults K, Brown D: Refibrillation managed by EMT-Ds: Incidence and outcome without paramedic back-up. Am J EmergMad t986;4:491495. 23. Cummins R, EisenbergM, Litwio P, et el: Automatic external defibriNators used by emerger medical technicians: A controlled clinical trial. JAMA 1987;257:1805-1810. 24. Cummins R: Is EMT-D right for you? in Making EMT-D Work: Proceedingsfrom the Universi of Iowa workshop. J EmergMad Serv1986;11:27-56. 25. EisenbergM: EMT defibrillation (letter). Ann EmergMad 1£85;14:487. 26. Callaham M, Barton C: Prediction of outcome of cardiac arrest by end-tidal capnometry. Cri CareMad 1990;18:358-362. 27. Hallstrom AP, Cobb LA, Swain M, et al: Predictorsof hospital mortality after out-of-hospital cardiopulmonary resuscitation. Crit CoreMad 1985;13:927. 28. Longstreth VVT, Diehr P, Inui TS: Prediction of awakening after out-of-hospital cardiac erres N EWI J Mad 1983;308:1378. 29. K'eweJskiR, ThompsonB, Horwitz L, et ah BystanderCPR in prehospital coarse ventricular fibril ration. Ann EmargMad 1£84;13:1016-1020. 30. Carlisle EJ, Allen JD, KernohanWG, et al: Fourieranalysis of ventrlcular fibrillation of varie, aetiology. EurHeortJ 1990;11:173-181. 31. Brown CG, Griffith RF,Van LP, et el: Median frequency--A new parameter for predicting defibrillation success rate. Ann EmergMad1991;20:787-789 32. Beck J, Shultz B: The use of relative operating characteristic (ROC)curves in test performan( evaluation. ArchPatho/Lab Mad 1986;110:13-20. 33. Metz C: Basic principles of ROCanalysis. SerninNucl Med1978;8:283-298. 34. Cummins R, EisenbergM: Prehospital cardiopulmenary resuscitation: Is it effective? JAMA 1985;253:2408-2412. 35. Niemenn J, Cairns C, Sharma J, eta[: Treatment of prolonged ventricular fibrillation: Immediate countershockversus high-dose epinephrine and CPRpreceding countershock. Circulation 1992;85:281-287. 36. Reich H, Angelos M, Safar P, et al: Card!acresuecitability with cardiopulmonery bypass afteJ increasing ventricular fibrilration times in dogs. Ann EmergMad 1990;19:887-890. 37. Roth R, Stewar~ RD, RogersK, et al: Out-of-hospital cardiac arrest: Factorsassociated with survival. Ann EmergMad 1984;13:237~243.38.BeakerLB, Ostrander MP, Barrett J, et ah Outcome of CPR in a large metropolitan area--Where are the survivors?Ann Emery7Mod 1991;20:355-361. 39. Jones D, Klein G: Ventricular fibrillation: The importance of being coarse?J E/ectrocardiol 1984;4:393-40& 40. Dalzell GW, Adgey AA: Determinants of successful transthoracie defibd[lation and outcome in ventricular fibrillation. Br HeartJ 1991;65:311-316. 41, Haynes BE, Mendoza A, McNeil M, et ah A statewide early defibrillation initiative including laypersonsand outcome reporting. JAMA 1991;266:545-547. The authors thank Richard Juster, PhD, for his invaluable assistance with statistical analysis; Charles Saunders, MD, and John Applegarth of the Public Health Department's Paramedic Division for providing dispatch data; William B Lobar, BSEE, MS, for his comparison of computer digital and manual amplitude readings; Chris Barton, MD, for his review of the manuscript; and the San Francisco Fire Department for its quality assuranCe and data collection programs that made this research possible.

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