JACC Vol I, No.3September 1984:487-92

-----------------------

Basic Emergency Medical Care of Patients With Acute MyocardialInfarction: Initial Prehospital Characteristics andIn-Hospital Complications

JOYCE C. PRESSLEY, MPH, B. HADLEY WILSON, MD, HARRY W. SEVERANCE, JR., MD,MARY P. RANEY, MED, RAY A. McKINNIS, PHD, MICHAEL W. SMITH, EMT-P,MICHAEL C. HINDMAN, MD, GALEN S. WAGNER, MDDurham. North Carolina

487

This prospective study documents the natural history ofthe prehospital phase of 110 patients with acute myo-cardial infarction transported by a basic emergencymedical system during a 22 month period. Ambulancesin a mixed urban-rural county were staffed by basicemergencymedical technicianscertifiedin basic lifesup-port and the administration of intravenous fluids. Sys-tolicbloodpressure, pulse rate and cardiac rhythm werenoted for all patients at the time of ambulance arrivaland intermittently during transport. Analyses of patientdata were performed to determine the relation betweenthe occurrence of subsequent in-hospital urgent com-plications and death and 1) patient delay time, 2) initialpulserate, 3) initial systolic bloodpressure, and 4) initialcardiac rhythm.

Several studies (1-7) have retrospectively analyzed theoutcome of patients with acute myocardial infarction whenmanaged with mobile intensive care. These studies generallyhave suggested a decrease in hospital mortality and sup-ported the widespread acceptance and implementation ofmobile intensive care for patients with acute myocardialinfarction. Mobile intensive care offers multiple possibleinterventions for patients with a variety of presenting char-acteristics. It is, therefore, necessary to determine whichpatients would benefit from which aspects of mobile inten-sive care. The answer requires the observation of patientsduring the prehospital phase of acute myocardial infarctionwho are transported by emergency medical technicians, but

From the Division of Cardiology, Department of Medicine, Duke Uni-versity Medical Center, Durham, North Carolina. This work was supportedby Research Grant HL-17670 from the National Heart, Lung, and BloodInstitute, Bethesda, Maryland, and by Grant HS 04356 from the NationalCenter for Health Services Research, Hyattsville, Maryland. Manuscriptreceived November 7, 1983; revised manuscript received April 10, 1984,accepted April 19, 1984.

Address for reprints: Galen S. Wagner, MD, Box 31211, Duke Uni-versitv Medical Center, Durham, North Carolina 27710.

198; by the American College of Cardiology

Twenty-three (21%) of the 110 patients died and 66(60%) experienced at least one in-hospital urgent com-plication. When initial rhythm, pulse rate and bloodpressure were considered, patients with hypotension hada higher mortality rate than did those who were eithernormotensive or hypertensive. The 10 patients with ini-tial sinus bradycardia but no hypotension constituted asubgroup with zero mortality.

These results identify high and low risk patient sub-groups that may benefit from either providing or with-holdinginterventions directed toward hemodynamic sta-bilization during the prehospital phase of acute myo-cardial infarction.

not managed with mobile intensive care. This would elu-cidate the relation between the patients' initial character-istics and their subsequent clinical course.

This prospective study reports the outcome of patientswith acute myocardial infarction managed by prehospitalbasic emergency medical care in a community that had notyet implemented a mobile intensive care system. Patientswere grouped according to initial prehospital characteristicsto determine the relation of these factors to in-hospital urgentcomplications in the absence of prehospital interventions.The results from this study can be used as a basis for 1)identifying those who mayor may not benefit from mobileintensive care, and 2) evaluating the effectiveness of mobileintensive care systems in reducing the morbidity and mor-tality of patients with acute myocardial infarction.

MethodsPatient population. Durham County Hospital Corpo-

ration provides ambulance service for the city and countyof Durham, North Carolina. The city has a population of

0735-1097/84/$3.00

Downloaded From: http://content.onlinejacc.org/ on 05/01/2015

488 PRESSLEY ET AL.PREHOSPITAL CARE FOR MYOCARDIAL INFARCTION

lACC Vol. 4, No.3September 1984:487-92

approximately 100,000 in a 40 square mile area centrallylocated within the county. An additional 53,000 people re-side in the remaining 260 square miles of the county. Duringthe study period, ambulances were staffed by basic emer-gency medical technicians certified in basic life support andadministration of intravenous fluids (5% dextrose in wateror Ringer's lactate). When cardiac arrest was encountered,cardiopulmonary resuscitation was performed without con-current administration of drugs or electrical defibrillation.Three area hospitals, Durham County General, Durham Vet-erans Administration and Duke University Medical Center,serve the county population. The 110 patients in the studywere transported to one of these three hospitals, nearly all(101 patients) to either Durham County General or DukeUniversity Medical Center.

All patients transported by Durham County Hospital Cor-poration ambulances for apparent acute cardiac eventsduring the 22 months from November 1, 1978 until Sep-tember 1, 1980 were evaluated for inclusion in this study.Patients in cardiac arrest on ambulance arrival at the sceneincluding those with subsequently documented acute myo-cardial infarction have been previously reported (8). Thisreport includes all other patients transported in whom acutemyocardial infarction was subsequently documented duringhospitalization.

Protocol. Electrocardiographic recording was institutedby the basic emergency medical technicians as soon as pos-sible after ambulance arrival and was continued until arrivalin the hospital emergency department. All recordings wereevaluated by one or more of the investigators, and all cardiacrhythms were defined by type. Systolic blood pressure andpulse rate were noted for all patients at the time of ambulancearrival and intermittently during transport.

Initial Killip class was determined as soon as possibleafter the patient's arrival at the emergency department. In-farct location was determined by electrocardiographicchanges on admission and subsequent recordings.

Definitions. The' 'prehospital phase" was the time fromonset of acute symptoms until the time of ambulance arrivalat the emergency department. "Patient delay time" was thetime from the onset of acute symptoms until the time aphone request for ambulance assistance was received by thecommunications center. Information concerning the "timeof onset of symptoms" was obtained by the medical tech-nicians from the patient or bystanders or by hospital per-sonnel after the patient's arrival in the emergency depart-ment. "Response time" was the time from receipt of thephone request for assistance until ambulance arrival at thepatient site. "Transport time" was the time from the arrivalof the ambulance at the patient site until arrival at the emer-gency department. A time card stamped at the communi-cations center recorded the time of receipt of the call, am-bulance arrival at the patient site and arrival at the emer-gency department.

Documentation ofan "acute myocardial infarction" wasdetermined during the hospital stay by 1) the presence oftransiently elevated serum levels of total creatine kinase andthe presence of the cardiac-specific isoenzyme, creatine ki-nase-MB, and 2) either the presence of new 0.03 second orgreater Q waves (9) or the characteristic lactic dehydrogen-ase isoenzyme reversal. Patients with creatine kinase-MBbut without either significant Q waves or the confirmatoryenzyme data were considered to have had only "suspectedacute myocardial infarction" (10) and are not included inthis study.

"Cardiac arrest" was determined by the medical tech-nicians during the prehospital period and was defined as anyperiod of absolute absence of pulse and blood pressure.Confirmation by electrocardiogram was required for docu-mentation of ventricular tachycardia, ventricular fibrillationor asystole. "Hypotension" was a systolic blood pressureof less than 90 mm Hg, "normotension" was a systolicblood pressure of 90 mm Hg or more but less than 160 mmHg and "hypertension" was defined as a systolic bloodpressure of 160 mm Hg or greater. "Bradycardia" was aheart rate of less than 60 beats/min; "tachycardia" includedrates of 100 or more beats/min.

, 'Hospital morbidity" was the presence ofanyone ofsix"urgent complications" including: I) ventricular tachy-cardia or fibrillation, 2) second or third degree atrioven-tricular (AV) block, 3) pulmonary edema, 4) cardiogenicshock, 5) persistent hypotension, or 6) extension of infarc-tion (1 I). In-hospital urgent complications included bothmorbidity and mortality.

Data analysis. Analysis of the patient data includedcomparisons of prehospital experience with morbidity andmortality during hospitalization. Because delay times werehighly skewed, the Wilcoxon rank sum test was used foranalyzing the delay times of various groups. Patient delaytime was not used as a variable in the 21 patients for whomit could not be determined precisely. All univariable anal-yses utilized Fisher's exact test to determine the significanceof difference between values.

ResultsClassification of ambulance calls. Approximately

21,540 calls for ambulance assistance were received duringthe period of this study. Of these, 8,390 were canceled and6,630 were for nonemergency transport. Of the 6,520 emer-gency trips, 850 were for patients with chest pain, 190 forpatients in cardiac arrest and 340 for patients who were deadon arrival of the ambulance at the scene.

There were 116 patients with acute myocardial infarction(Table 1). Their mean age was 66 years with a predom-inantly male sex distribution (63%) and a predominantlywhite race distribution (85%). Thirty-four (29%) of the 116study patients had anterior or lateral infarction, 33 (28%)

Downloaded From: http://content.onlinejacc.org/ on 05/01/2015

JACCVol 4, No.3September 1984:487-92

PRESSLEY ET AL.PREHOSPlTAL CAREFORMYOCARDIAL INFARCTION

489

----------------------

Table 1. Patient CharacteristicsFactor No.

bradyasystole in two. Two patients, one in each rhythmcategory, survived to hospital discharge.

----------------------

*Standard deviation; trange; *six of these 116 patients were in cardiacarrest on arrival.

had inferior or posterior infarction, 14 (12%) had infarctioninvolving more than one location and the location of theinfarction was indeterminate in 35 patients (30%). Fifty-two patients (45%) were initially judged to be in Killip classI; 22 09%) in class II, 19 (16%) in class III and 13 (11%)in class IV. This designation was not obtained at the timeof hospital admission for 10 patients (9%). The mean patientdelay time was 159 minutes, mean response time was 7minutes and mean transport time was 26 minutes.

In addition to the 116 study patients transported by thebasic emergency medical technicians, 365 other patients(excluding interhospita1 transfers) were received through theemergency departments and treated for acute myocardialinfarction during the study period at the two principal acutecare hospitals. The mortality rate for patients transportedby ambulance (21%) was not significantly different fromthat of those arriving by other means (17%).

Six of the 116 patients with acute myocardial infarctionwere in cardiac arrest when initially seen by the basic emer-gency medical technicians. Ventricular tachycardia waspresent in one patient, ventricular fibrillation in four andasystole in one. Three of these patients died during hospi-talization and three survived and were subsequently dis-charged from the hospital. These six patients who were incardiac arrest at the time of ambulance arrival are reportedelsewhere (8) and will not be considered further in thisstudy.

Four of the 110 patients not in cardiac arrest at the timeof ambulance arrival did experience a cardiac arrest beforearrival at the hospital. The initial rhythm of these four pa-tient- was sinus bradycardia in two and normal sinus rhythmin two. One with sinus bradycardia and one with normalsinus rhythm had accompanying hypotension. The rhythmat the time of arrest was ventricular fibrillation in two and

Total no

Demographic factorsMean age (yr)MaleWhite

Historical factorsHypertensionDiabetesCongestive heart failureMyocardial infarctionAngira

TimesMean patient delay time (min)Mean response time (min)Mean transport time (min)

116*

65.7 12.2* (38 to 92)t73 (63%)99 (85.3%)

56 (48.3%)18 (15.5%)10 (8.6%)26 (22.4%)40 (34.5%)

159 (0 to 1,174)t7 (0 to 35)t

26 (2 to 88)t

Prehospital Characteristics andIn-Hospital Complications

Patient delay times. A documentable time of onset ofacute symptoms was available in 89 patients (81%). Themean patient delay time was 159 minutes. No significantdifferences were observed when patient delay times wereanalyzed by age, race and sex. The mean patient delay timeswere longer 1) for those younger than 60 years of age (192minutes) than for those 60 years of age and older (140minutes) (p = 0.20), 2) for white (167 minutes) than forblack (117 minutes) patients (p = 0.55), and 3) for men(194 minutes) than for women (92 minutes) (p = 0.30).

Forty-four patients (49%) requested an ambulance lessthan 1 hour after the onset of symptoms; 45 (51%) delayed1 hour or more. Those patients who delayed less than 1hour had a higher hospital mortality rate (23%) than thosewho delayed 1 hour or longer (13%); however, this differ-ence was not statistically significant.

Rate,rhythm andpressure. Among these 110patients,66 (60%) had at least one in-hospital urgent complicationand 23 (21%) died (Table 2). At the time of initial prehos-pital evaluation, an abnormal heart rate or systolic bloodpressure, or both, was present in 75 (68%) of the 110 pa-tients. Of the 13 patients with hypotension, 7 (54%) alsohad bradycardia and 1 (8%) had tachycardia. Of the 31patients with hypertension, 3 (10%) had bradycardia and 9(29%) had tachycardia.

The following three groups were considered separately:group 1 included those patients with both normal heart rateand normotension, group 2 included those with hypotensionand group 3 included those with hypertension. Of the 35patients (32%) in group 1, 19 (54%) experienced urgentcomplications and 2 (6%) died during hospitalization. Group2 (hypotension) consisted of 13 patients (12%); 10 (77%)experienced urgent complications and 9 (69%) died. Group3 (hypertension) contained 31 patients (28%); 14 (45%) hadurgent complications and 5 (16%) died. Hospital morbiditydid not differ significantly among these groups, but therewas a higher hospital mortality rate in group 2. The mortalityrate in hypotensive patients (group 2) was significantlyhigher than that of patients with either normotension andnormal heart rate (group 1) (p = 0.00002) or hypertension(group 3) (p = 0.001).

The mortality rate of 16% for hypertensive patients(group 3) was similar to that of 14% for all normotensivepatients. Within group 3, however, all five deaths occurredin the group of 18 patients (28%) with a systolic pressureof 160 to 180 mm Hg. None of the 13 patients with aninitial systolic blood pressure of more than 180 mm Hg diedduring hospitalization.

Downloaded From: http://content.onlinejacc.org/ on 05/01/2015

490 PRESSLEYET AL.PREHOSPITAL CARE FOR MYOCARDIAL INFARCTION

lACC Vol. 4. No.3September 1984:487-92

Patients with bradycardia (due to either sinus brady-cardia or high degree AV block) were compared using theirinitial accompanying systolic blood pressure recordings.Those patients without hypotension had a significantly lowermortality rate (p = 0.007) than did those with hypotension.None of the 10 patients with sinus bradycardia but withouthypotension died. There were six patients who were initiallyin high degree AV block. Three (50%) were also hypoten-sive. Two of these, along with the one patient with AVblock but without hypotension, died during hospitalization.

Patients without hypotension or high degree AV blockwere compared according to the presence or absence oftachycardia. Among these 94 patients, the mortality ratewas 23% (7 of 30) in those with an initial heart rate of 100beats/min or more versus 9% (6 of 64) in those with aninitial heart rate of less than 100 beats/min (p = 0.11).

DiscussionThe mean age of 65.7 years for patients with acute myo-

cardial infarction in this study was slightly but not signifi-cantly higher than 1) that reported in other studies (12,13),or 2) that of patients with acute myocardial infarction (63.4years) in Durham County during the same time period whowere transported by other means. The distributions of thelocation of the infarction and KiIlip classes were unre-markable for the study group. The mortality rate for patientswith myocardial infarction transported by ambulance wasnot significantly different from that for those who arrivedby other means.

Patient delay time. At least 40% of cardiac mortalityhas been reported to occur within I hour of the onset ofsymptoms (1,14). Prolonged patient delay time beforecalling for ambulance assistance is regarded as a significantcause of death and has been considered to be the rate-

Table 2. Initial Patient Rhythm and Blood Pressure

limiting step in the effectiveness of mobile intensive careprograms (15-17). The mean patient delay time of 22/3 hoursin this study is slightly longer than that reported by Simonet aI. (18) and less than the 3 to 6 hour delay times reportedin other studies (1,5,19). The delay time of less than 1 hourin 49% of patients in this study compares favorably withthat of mobile intensive care systems. Hospital mortalityrate for 48 patients transported within their first hour ofsymptoms was higher in this study of a basic emergencymedical care system (23%) than the rate of7% (3) and 9%(14) reported in studies of mobile intensive care.

Hypotension. This study demonstrates a significantlyhigh risk in patients with initial systolic hypotension. Pre-vious experimental studies (20) showed that an abrupt de-crease in coronary blood flow occurs when perfusion pres-sures decline below 60 to 70 mm Hg. This decline is pro-portionately greater than the accompanying reduction inmyocardial oxygen consumption and could explain the ob-served extension of the area of ischemic injury associatedwith hypotension (21).

Hypertension. A previous study (22) of patients withinitial hypertension during acute myocardial infarction ran-domly managed with and without antihypertensive therapyconcluded that treatment limited infarct size. Larger infarctshave previously been correlated with increased mortality(23,24). However, none of the 13 patients in our study withmarked hypertension (initial blood pressure of more than180 mm Hg) died during hospitalization. In support of thislast finding is the work of Braunwald et al. (25), whichshowed that elevation of arterial pressure in dogs increasescoronary blood flow proportionately more than myocardialoxygen consumption. Because these results are confusingand fail to indicate guidelines for intervening in patientswith initial hypertension during acute myocardial infarction,further observations of the relation between early hyperten-sion and subsequent mortality are indicated.

Hypotension Normotension Hypertension(Sys < 90 mm Hg) (Sys > 90, < 160 mm Hg) (Sys > 160 mm Hg) Total

Sinus rhythm 5 (5%) 35 (32%) 19 (17%) 59 (54%)Comp = 3 (60%) Comp = 19 (54%) Comp = 8 (42%) Comp 0= 30 (51%)Death = 3 (60%) Death = 2 (6%) Death = 4 (21%) Death 0= 9 (15%)

Sinus bradycardia or 7 (6%) 10 (9%) 3 (3%) 20 (18%)atrioventricular block Comp = 6 (86%) Comp = 7 (70%) Comp = I (33%) Comp 0= 14 (70%)

Death = 5 (71%) Death = I (10%) Death = 0 Death 0= 6 (30%)Sinus or supraventricular 1(0.9%) 21 (19%) 9 (8%) 31 (28%)

tachycardia Comp = I (100%) Comp = 16 (76%) Comp = 5 (56%) Comp 0= 22 (71%)Death = I (100%) Death = 6 (29%) Death = 1(11%) Death 0= 8 (26%)

Total 13 (12%) 66 (60%) 31 (28%) 110Comp = 10(77%) Comp = 42 (64%) Comp = 14 (45%) Comp 0= 66 (60%)Death = 9 (69%) Death = 9 (14%) Death = 5 (16%) Death 0= 23(21%)

Comp = complication; Sys = systolic blood pressure.

Downloaded From: http://content.onlinejacc.org/ on 05/01/2015

JACC Vol. 4. No.3Septernbe 1984:487-92

PRESSLEY ET AL.PREHOSPITAL CAREFOR MYOCARDIAL INFARCTION

491

Bradycardia, with or without hypotension. This studyshowed an extremely high mortality rate for patients withbradyc ardia and systolic hypotension managed by basicemergency care alone. Warren and Lewis (26) previouslyemphasized the risk associated with early hypotension ac-companied by bradycardia. In contrast, patients with initialsinus hradycardia but without systolic hypotension experi-enced no subsequent in-hospital mortality. This is consistentwith previous studies (27) that have indicated that bradyar-rhythnuas in some patients with acute infarction should notbe treated and with experimental animal studies (21,28,29)that demonstrated that sinus bradycardia without hypoten-sion has a beneficial effect on myocardial ischemia by re-ducing myocardial demand for oxygen.

Tachycardia. Patients in this study with an initial heartrate of 100 beats/min or more showed a tendency toward ahigher mortality. Several studies (30,31) suggest that cor-onary blood flow may be reduced as much as 35% duringsupraventricular tachycardias. Both hypotension and tachy-cardia decrease coronary perfusion and may extend myo-cardial ischemic damage (21). It will be important in sub-sequent studies of mobile intensive care to determine ifmanagement of either hypotension or tachycardia results inimproved survival in these higher risk groups.

Conclusion. The observation of simple variables suchas pulse rate, systolic blood pressure and cardiac rhythm atthe time of the initial prehospita1 evaluation permits thestratification of risk of death during acute myocardial in-farction. However, these observations do not permit suchstratification regarding hospital morbidity. The high mor-tality rate in patients with initial hypotension indicates ahigh risk group for whom prehospital intervention might bedesigned. These data can be used as a basis for evaluationof interventions directed toward the stabilization of the con-dition of patients with acute myocardial infarction duringthe prehospital phase.

We thank Mickey Tezai and the personnel of the Durham County HospitalCorporation Ambulance Division for obtaining the patient information, andJackson L. Smith III of Merck, Sharp and Dohme for providing referencesof current literature. We also thank Gail McKinnis and Amanda Stallingsfor preparation of the manuscript.

References1. Pantridge JF, Geddes JS. A mobile intensive-care unit in the man-

agement of myocardial infarction. Lancet 1967;2:271-3.2. Crampton RS, Aldrich RF, Gascho JA, Miles JR, Stillerman R. Re-

duction of prehospital, ambulance and community coronary death ratesby the community-wide emergency cardiac care system. Am J Med1975;58:151-65.

3. Grace WJ, Chadbourn JA. Mobile coronary care unit. In: AHA/RedCross National Conference on Standards for CPR and ECC. Wash-ington, DC, May 16-18, 1973. 1973:209-10.

4. Lewis RP, Frazier JT, Warren JV. Mobile coronary care: an approach

to the early mortality of myocardial infarction (abstr). Am J Cardiol1970;26:664.

5. Schroeder JS, Lamb IH, Hu M. The prehospital course of patientswith chest pain. Am J Med 1978;64:742-8.

6. Hampton JR. Importance of patient selection in evaluating a cardiacambulance service. Br Med J 1976;1:201-3.

7. Sherman MA. Mobile intensive care units: an evaluation of effec-tiveness. JAMA 1979;241:1899-901.

8. Wilson BH, Severance HW, Raney MP, et al. Out-of-hospital man-agement of patients with cardiac arrest by basic emergency medicaltechnicians. Am J Cardiol 1984;53:68-70.

9. Wagner GS, Freye CJ, Palmeri ST, et al. Evaluation of a QRS scoringsystem for estimating myocardial infarct size. I. Specificity and ob-server agreement. Circulation 1982;65:342-7.

10. White RD, Grande P, Califf L, Palmeri ST, Califf RM, Wagner GS.Diagnostic and prognostic significance of minimally elevated creatinekinase MB in patients with suspected acute myocardial infarction. AmJ Cardiol (in press).

11. Severance HW, Morris KG, Wagner GS. Criteria for early dischargeafter acute myocardial infarction. Arch Intern Med 1982;142:39-41.

12. Lewis RP, Stang JM, Fulkerson PK, Sampson KL, Scoles A, WarrenJV. Effectiveness of advanced paramedics in a mobile coronary caresystem. JAMA 1979;241: 1092-4.

13. Eisenberg MS, Copass MK, Hallstrom A, Cobb LA, Bergner L.Management of out-of-hospital cardiac arrest: failure of basic emer-gency medical technician services. JAMA 1980;243:1049-51.

14. Adgey AAJ, Allen JD, Geddes JS, et al. Acute phase of myocardialinfarction. Lancet 1971;2:501-4.

15. Sidel VW, Acton L, Lown B. Models for the evaluation of prehospitalcoronary care. Am J Cardiol 1969;24:674-88.

16. The Effect of Emergency Medical Systems on Prehospital Cardio-vascular Care. An Evaluation of Studies and an Inventory of DataBases. Washington, D.C.: National Highway Traffic Safety Admin-istration, and Bethesda, Maryland: National Heart, Lung, and BloodInstitute, National Institutes of Health, 1981; DOT publication no.HS-805-204.

17. Simon AB, Simon 10. Utilization of ambulance services by patientswith acute cardiac emergencies. Md State Med J 1974;23:32-4.

18. Simon AB, Feinleib M, Thompson HK Jr. Components of delay inthe prehospital phase of acute myocardial infarction. Am J Cardiol1972;30:476-82.

19. Moss AS, Wynar B, Goldstein S. Delay in hospitalization during theacute coronary period. Am J Cardiol 1969;24:659-65.

20. Mosher P, Ross J, McFate PA, Shaw RF. Control of coronary bloodflow by an autoregulatory mechanism. Circ Res 1964;14:250--9.

21. Maroko PR, Braunwald E. Modification of myocardial infarction sizeafter coronary occlusion. Ann Intern Med 1973;79:720--33.

22. Shell WE, Sobel BE. Protection of jeopardized ischemic myocardiumby reduction of ventricular afterload. N Engl J Med 1974;291:481-6.

23. Sobel BE, Bresnahan GF, Shell WE, Yoder RD. Estimation of infarctsize in man and its relation to prognosis. Circulation 1972;46:640--8.

24. Boor PJ, Reynolds ES. Myocardial infarct size: clinicopathologicalagreement and discordance. Hum Pathol 1977;8:685-95.

25. Braunwald E, Sarnoff SJ, Case RB, Stainsby WN, Welch GH Jr.Hemodynamic determinants of coronary flow: effect of changes inaortic pressure and cardiac output on the relationship between myo-cardial oxygen consumption and coronary flow. Am J Physiol1958;192:157-63.

26. Warren JV, Lewis RP. Beneficial effects of atropine in the pre-hospitalphase of coronary care. Am J Cardiol 1976;37:68-72.

27. Rotman M, Wagner GS, Wallace AG. Bradyarrhythmias in acutemyocardial infarction. Circulation 1972;45:703-22.

Downloaded From: http://content.onlinejacc.org/ on 05/01/2015

492 PRESSLEY ET AL.PREHOSPITAL CARE FOR MYOCARDIAL INFARCTION

lACC Vol. 4. No.3September 1984:487-92

28. Redwood DR, Smith ER, Epstein SE. Coronary artery occlusion inthe conscious dog: effects of alterations in heart rate and arterialpressure on the degree of myocardial ischemia. Circulation1972;46:323-32.

29. Maroko PR, Kjekshus JK, Sobel BE, et al. Factors influencing infarctsize following experimental coronary artery occlusions. Circulation1971;43:67-82.

30. Corday E, Irving OW, Disturbances of Heart Rate, Rhythm and Con-duction. Philadelphia: WB Saunders, 1961:207-24.

31. Corday E, Gold H, DeVera LB, Williams JH, Fields J. Effect of thecardiac arrhythmias on the coronary circulation. Ann Intern Med1959;50:535-53.

Downloaded From: http://content.onlinejacc.org/ on 05/01/2015