Essay 2

profileGod
MayerParamedicResponseTimeandSurvivalfromCardicArrest.pdf

SW SC,, 6 Med.. Vol. 13D. pp. 267 to 271

c PerFamon Press Lid 1979 Prmled m Great lhlu~n

PARAMEDIC RESPONSE TIME AND SURVIVAL FROM CARDIAC ARREST*

JONATHAN D. MAYER Department of Geography and Affiliate, Center for Health Services Research, University of Washington

Seattle, WA 98195, U.S.A.

Abstract-11 has been assumed in the past that minimizing response time is an important goal in emergency medical services (EMS) systems planning. However, this assumption has yet to be proven. In this analysis, 525 cardiac arrest cases are examined and the relationship between response time and survival determined. Paramedic response time is confirmed to be related statistically to long and short term survival from ventricular fibrillation.

INTRODUCTION

It has been axiomatic in the planning and develop ment of emergency medical service (EMS) systems that vehicle response times must be minimized to pro- mote patient survival. Planners, geographers, and operations researchers have accepted this notion with minimal questioning. The reasons for such acceptance are clear. If short response times maximize patient survival, then an appreciable impact may be made on mortality simply by an efficient spatial allocation of emergency units, through the use of location-alloca- tion and set-covering procedures. Thus, the assump- tion is made that if response time is reduced in a region, this will be reflected through an increase in patient survival.

Vehicle placement, the efficiency of which underlies response time, is one of the simplest variables to manipulate in EMS planning. Indeed, in comparison to problems of patient delay in deciding to seek aid for emergencies such as acute myocardial infarction (“heart attack”), response time minimization, while computationally complex, has proven to be rather simple to implement. Recent research on prehospital delays in obtaining coronary care indicates that many patients do not recognize the symptoms of acute heart problems, and when they do, they will frequently deny the existence of critical symptoms. Thus, the delay on the part of the patient in deciding to seek care has proven to be the lengthiest component of prehospital delay.

It has been difficult to reduce delays resulting from denial because of its psychological roots [l-3]. Prag- matically, therefore, it has been much simpler to manipulate response times through changing ambu- lance locations than to reduce appreciably patient delays. Response time has been seen as being especially important following the diffusion of para- medic programs which emphasize patient stabilization at the scene of an emergency through advanced life support measures such as cardiac defibrillation and the administration of appropriate medication.

* Following completion of the research, it was learned that Drs Leonard Cobb and Alfred Hallstrom of the Uni- versity of Washington completed similar unpublished research, with their goal being the development of a predic- tive model of survival using logistic regression.

Because of the emphasis on response time minimi- zation, it is surprising that there has been no attempt to document the clinical importance of rapid response time and to test empirically the assumption that rapid response times are reflected in patient survival. Recent indications are that total elapsed time between the onset of cardiac arrest and the administration of defi- nitive care bears a linear relationship to probability of survival [4], yet the importance of response time per se is unknown. As Cretin writes:

Whether or not response time is a good measure on which to evaluate an ambulance system depends on how response time relates to the patient’s risk of dying. It is intuitively clear that.. . a quicker response time is at least as good as.. . a slower response. The details of the relationship between response time and survival are important, how- ever, and far less intuitive. In the absence of data, we can only speculate [S].

In this study, the relationship between response time and survival is tested for an epidemiologically appro- priate set of critical emergencies.

THE NATURE OF CARDIAC ARREm

It is generally acknowledged that cardiac arrests are the most critically life threatening of medical emergencies. Unless resuscitative measures are insti- tuted within four to six minutes following the onset of the arrest, irreversible brain damage due to anoxia occurs, and the probability of resuscitating the patient also decreases. Thus, early medical intervention may influence outcome in cardiac arrests more clearly than in any other type of medical emergency. It is with cardiac arrests that response time may be expected to have the greatest influence.

There are four forms of cardiac arrest, all character- ized by pulselessness and ab&ce of respiration. By definition, the heart ceases its function of perfusing blood t,o the rest of the body, although electrical action may nonetheless be present. The most common form of cardiac arrest is ventricular fibrillation (VF), which consists of totally disorganized electrical ac- tivity in the heart which precludes blood circulation. Definitive treatment for VF consists of electrical countershock, and the administration of intravenous lidocaine, epinephrine, and other cardiac medications. If VF remains untreated, osysrole-the second type of

267

268 JONATHAN D. MAYER

cardiac arrest-occurs, and electrical activity in the heart completely ceases. Treatment consists solely of intravenous and intracardiac medication. The third form of cardiac arrest is idioventricular rhythm and consists of the failure of the normal cardiac pace- maker-the sinoatrial node-and the development of an ectopic or abnormal focus of electrical activity in the ventricle. Finally, electromechani~l (EM) dissoci- ation consists of fairly normal cardiac electrical ac- tivity, accompanied by an absence of cardiac pumping activity.

The different forms of cardiac arrest can be detected only by electro-cardiographic monitoring. In the absence of such monitoring, pulselessness and breathlessness accompanied by coma are the main diagnostic features. Until definitive medical care is provided by a paramedic unit or in a hospital, artifi- cial circulation and respiration must be initiated using cardio-pulmonary resuscitation (CPR). This consists of rhythmically compressing the patient’s sternum, thereby compressing the heart and maintaining suffi- cient circulation, and coordinating the compressions with artificial respiration. Previous studies have demonstrated that trained bystanders may improve the patient’s survival chances by commencing CPR as a stopgap measure [fi, 73. It has also been found that patients in VF are most easily resuscitated, while there is littie chance of patients with other forms of cardiac arrest living long enough to be discharged from a hospital [S].

DATA. METHODS AND DRSCRlPTlON

In this study, data have been collected for all out- of-hospital cardiac arrests occurring in Seattle between September i, 1977, and August 31, 1978. Data are from patients records maintained by the Seattle Fire Department which provides the sole re- sponse to all medical emergencies in Seattle. Thus, through its Medic-I program, described in a previous issue of this journal, the Fire Department responds to virtually all cardiac arrests within the city [9]. A total of approximately 21,000 records were examined, and the diagnosis of cardiac arrest was made on the basis of the paramedics’ diagnoses, as well as an examin- ation of treatment rendered and of the clinical narra- tive, since it was discovered that the paramedics erred in a small percentage of cases in the correct diagnostic coding of cardiac arrests.

Of the cardiac arrests which were identified, a number were eliminated from further consideration. Some patients had been dead for so long that the paramedics provided no treatment; these patients were considered to be dead on arrival (DOA), and were eliminated from the sample. In addition, cardiac arrest may occur as a result of drowning, electrocu- tion, drug overdose, or blood loss due to major trauma and injury.These cases which clearly were not of cardiogenic etiology were also eliminated from further consideration.

For the remaining 525 cases, information was col- lected on the type of cardiac arrest, the location of the incident, the patient’s age, sex, and race, and the patient’s condition on release from the Fire Depart- ment to a hospital or to the morgue. Information on whether CPR was initiated by a bystander prior to

the arrival of the Fire Department was also collected. Response times were also included on the patient records. Their accuracy was verified by timing the interval between the dispatch of the units and their arrival on the scene for a sample of the arrests using voice tapes which are maintained by the Fire Depart- ment. Response times are recorded to the nearest minute, with the expected error therefore being 30 seconds. The actual error was 27 seconds. In addi- tion, actual response times were verified by riding the units on four occasions and ascertaining the response time using a watch. Response times were almost uni- versally accurate.

Finally, for VF cases, a list of long-term survivors was made available by the Seattle Fire Department. Thus, it was possible to specify whether the patient was a “short-term” survivor based on information on “condition on release’* and it was possible to specify whether a patient was a “long-term” survivor based on their successful discharge from a hospital. It should be noted that long-term survival should ideally be a matter of living well past the hospital discharge, since a patient who dies the day after dis- charge could hardly be considered to be a long-term survivor. Unfortunately, follow-up data were not available.

Of the 525 cases, 323 (61.5%) patients were in VF, 163 (31%) were in asystole, 33 (6.3%) were in idioven- tricular rhythm, and 6 (1.2”/,) were in EM dissoci- ation. Short-term case fatality rates (CFR), defined as the number of deaths divided by the number of cases in each diagnostic category are shown in Table 1. The percentage distribution of arrest types and CFR’s is consistent with previous epidemiological studies [lo], while case fatality rates confirm a relatively high pro~bi~ty of short-term survival from VF and low probabilities for the other arrests. Because survival data are available for VF patients, it is to these which we now turn.

SHORT-TERM SURVIVAL AND RESPONSE TIME

The first obstacle to be overcome in successfully r~u~i~ting a patient in VF is the successful conver- sion of VF to a normal or life-sustaining rhythm, so that the patient may be admitted to a hospital. Obviously, this is a prerequisite of long-term survival. It is therefore important to analyze the relationship between response time and short-term survival.

The Seattle Fire Department has instituted a “tiered” or “layered” response system. In life-threaten- ing emergencies, the nearest fire engine company is dispatched s~ult~~usly with and in addition to one of the city’s four paramedic units. Fire engine companies always carry personnel who are trained as

Table 1. Case Fatality Rates (CFR), short-term

Type N % CFR (%)

Ventricular fibrillation 523 61.5 36.5 Asystole 163 31.0 79.8 Idioventricuku rhythm 33 6.3 69.7 EM dissociation 6 1.2 83.3

Paramedic response time and survival from cardiac arrest 269

Table 2. Initial response time and CFR Table 4. Paramedic response time and CFR

VF Asystole

Response time CFR CFR (mins) N (%) N (%)

<I 16 31.3 12 58.3 I 42 31.0 18 77.8 2 104 30.8 58 79.3 3 97 37.1 50 80.0 4 42 45.2 18 94.4

>5 22 59.1 7 85.7

Response time (mins)

Cl l-2 34 5-6 7-8 9-10 >I0

VF Asystole

CFR CFR N (%) N (%)

15 33.3 12 58.3 13 38.5 18 100.0 82 19.5 37 70.3 80 35.0 41 85.4 70 44.3 35 82.9 35 54.3 14 78.6 28 50.0 16 87.5

Table 3. Life status and response times, VF, short-term Actual value (expected value)*

Response time (mins) Alive Dead Total

<I (I, (b

16

I (:;, ct:,

42

2 (Z, (Z, ‘04

3 (Z, (::,

97

4 (::, (E,

42

>5 (11)

13 22 (8)

Total 205 118 323

x2 = 8.46; df = 5; P < 0.14. * Rounded to nearest integer.

times equal or exceed three minutes. Thus, for re- sponse times of less than three minutes, approxi- mately 30”/, of the patients are not resuscitated, while for response times of over five minutes, nearly 60% of the patients are not resuscitated. Fatality rates vary more irregularly for asystolic patients.

The hypothesis that initial response time is related to CFR may be tested more formally by crosstabulat- ing response time intervals with patient life status. As Table 3 suggests, there is minima1 divergence between expected and actual cell values for VF patients except in the interval in excess of five minutes. Moreover, the resulting x2 statistic is not significant, suggesting minimal distributional differences in patient survival between response time intervals.

Emergency Medical Technicians, and are able to ini- tiate basic life support.. Thus, the analysis will con- sider the response times of the first arriving unit as well as of the paramedic unit.

The same sort of analysis may be conducted strati- fying cases by paramedic response time. As Table 4 suggests, there is a general, but non-monotonic trend towards increasing CFR with increasing response time. Crosstabulating paramedic response time by life status confirms this impression, indicating significant disparity between actual and predicted values. These results are shown in Table 5 for both VF and asystole.

If response time is related to survival, then a gen- eral trend should exist such that increasing response times will be associated with increasing case fatality rates. The results of stratifying CFR’s by response times are shown in Table 2. A similar analysis, for comparison, is included for asystolic patients. For VF cases. the CFR appears to be stable until response

What the preceding analysis suggests is that the relationship between short-term survival and response time is not strong for the first-arriving or “initial” responder which is normally not a paramedic vehicle. For VF cases, however, the relationship between sur- vival and paramedic unit response time is quite strong. This is consistent with the previously cited finding that the magnitude of the delay to definitive care is a crucial determinant of survival. Moreover, there is not a strong relationship between response

Table 5. Life status and paramedic response time

Response time (mins) Alive

VF

Dead Total Alive

Asystole

Dead Total

O-3

4-6 (E,

7-9 (::,

>I0 (::,

Total 205

66 (96, 22 31 (25)

124 (ii, 54 67 (53)

91 7 35 42 (9) (33)

42 d, 19 23 (18)

323 33 130 163

VF: z2 = 15.2; df = 3; P < 0.005. Asystole: x2 = 1.96; df = 3: P < 0.58.

270 JONATHAN D. MAYER

Table 6. Response times and CFR. long-term VF

Initial response time CFR Paramedic response time CFR

<l 62.5 <l 60.0 I 72.1 l-2 69.2 2 62.5 3-4 53.7 3 69. I 5-6 70.0 4 81.0 7-8 76. I

>5 86.4 9-10 88.6 > 10 82.1

time and survival for asystolic patients. In the end, however, long-term survival is far more important than short-term survival. If the patient survives to be admitted to a hospital, only to die in the cardiac care unit, the resuscitation is hardly successful, even though the cardiac arrhythmia may have been con- verted successfully.

LONGTERM SURVIVAL AND RESPONSE

TIME

As mentioned previously, hospital discharge data were available only for VF patients. Thus, it is pos- sible to ascertain long-term survival for VF patients only until the time of hospital discharge. The data, while not ideal, should yield a more appropriate por- trait of the success of resuscitation than do the short- term data.

Using the same methodology as for short-term sur- vival, it is clear that paramedic response times are related to survival. Table 6 lists initial and paramedic response times and VF case fatality rates. Overall, the long-term case fatality rate was 69.8”/ therefore sug- gesting that nearly 30% of the VF patients are resusci- tated and live to be discharged from the hospital.

Table 7. Life status and response times. long-term VF

Response time Alive Dead

Initial

Paramedic t:, & 7-9

t::, 3

(64) > 10 7

(13) (:z)

Initial response: x2 = 8.50: df = 5; P < 0.14. Paramedic response: 1 2= 18.3;df=3;P<0.004.* l Results are similar using different response time strati-

fications.

There also exists an obvious trend towards higher fatality rates being associated with longer response times; thus, for paramedic response times of less than one minute, the CFR is only 60%, while for parame- dic responses of over ten minutes, the CFR exceeds 82%. Table 7 is a crosstabulation of long-term life status and response time. The analysis suggests that the distributional differences of life status, when stratifying by response time, are quite pronounced for paramedic response but are not pronounced for initial response time.

SUMMARY

The previous results imply that paramedic response times are crucial determinants of long-term survival, and thus that a significant number of lives may be saved by minimizing the response time of paramedic vehicles. Since response times are determined largely by the relative locations of an emergency and of the paramedic units, the clear implication is that the effi- cient location of paramedic vehicles is necessary to maximize a patient’s surviving ventricular fibrillation. The lack of statistical significance between initial re- sponse time and life status should not suggest that minimizing initial response time. is unimportant. Indeed, all available literature emphasizes the need for initiating CPR as soon as possible after the onset of cardiac arrest. It should be expected that in rural areas, where response times are necessarily longer than in urban areas, initial response time will be more clearly related to survival probabilities.

The relationship between paramedic response time and life status in VF cases cannot necessarily be extended to other types of cases. For example, it is not clear that response time is an important determin- ant of survival for burn victims, trauma patients, or drug overdose patients. Further research will be necessary to assess the effects of response time, if any, on morbidity and mortality in a wide variety of diag- nostic categories. What has been suggested in this analysis, however, is that minimizing response time per se can clearly influence survival rates in cardiac arrest cases. This possibly naive assumptiori has there- fore been verified.

Acknowledgemenrs-The author wishes to thank the Seattle Fire Department, and particularly Lieutenant Ralph Maughan, for continuing cooperation in providing the data necessary for this analysis. The author has no affilia- tion with the Seattle Fire Department. The author also wishes to acknowledge the financial support of the Center for Health Services Research of the University of Washing- ton’s School of Public Health and Community Medicine.

1.

2.

3.

4.

5. 6.

Paramedic response time and survival from cardiac arrest 271

REFERENCES

Mayer J. D. Emergency medical service delays, re- sponse time, and survival. Med. Cure, forthcoming, 1979.

i .

Cretin S. A mode1 of the risk of death from cardial infarction. Massachusetts Institute of Technology, Op erations Research Center, Cambridge, 1974. Olin H. S. and Hackett T. P. Denial of chest pain in 32 patients with acute myocardial infarction. JAMA 190. 9. 977, 1964. Eisenberg M. et al. Paramedic programs and out-of- hospital cardiac arrest: I. Factors associated with suc- IO. cessful resuscitation. Am. J. pub/. Hlth 69, 39, 1979. Cretin, Op. cit. Baum R. S. et al. Survival after resuscitation from out-

of-hospital ventricular fibrillation. Circulation SO, 123 1, 1974. Eisenberg, Op. cit. Pozen M. W. er al. Studies of ambulance patients with ischemic heart disease. I. The outcome of pre-hospital life-threatening arrhythmias in patients receiving elec- trocardiographic telemetry and therapeutic interven- tions. II. Selection of patients for ambulance telemetry. Am. J. publ. Hlth 67, 527. 1977. Mayer J. D. Seattle’s paramedic program: Geographi- cal distribution, response times, and mortality. Sot. Sci. Med. 13D. 45, 1979. Eisenberg M. er (I/. Epidemiology of cardiac arrest and resuscitation in a suburban community. J. Am. Co//. emer. Phys. 8, I, 1979.