Management of recurrent ventricular tachycardia: Economic impact of therapeutic alternatives

Management of Recurrent Ventricular Tachycardia: Economic Impact of Therapeutic Alternatives DIANE FERGUSON, RN, SANJEEV SAKSENA,

MD, ESTELLE GREENBERG,

and WILLIAM CRAELIUS, PhD

The clinical and economic results of antiarrhythmic therapy selected on the basis of electrophysiologic (EP) studies in patients with recurrent ventricular tachycardia (VT) were examined and compared with previously administered empiric therapy. Twenty-nine patients with recurrent VT and organic heart disease, aged 39 to 78 years (mean 59 f 11) were evaluated. All patients had empiric therapy before EP studies and EP-based therapy after EP evaluation. Hospital records were analyzed from arrhythmia diagnosis 1 to 39 months (mean 7.5 f 10.4) before EP evaluation until completion of follow-up 1 to 20 months (mean 13.3 f 7.4) after EP studies. Clinical efficacy was assessed by comparing actual arrhythmic deaths or recurrences during EP-based therapy with predicted values on empiric therapy. Charges based on diagnosisrelated groupings for empiric and EP-based therapy were compared. Charges for EP evaluation were included in the calculation for EP-based therapy.

During empiric therapy, 1 to 7 unsuccessful drug trials (mean 3.7 f 1.8) were performed, with arrhythmia recurrences noted in all patients during a mean 7.5month VT duration. Twenty-seven of 29 patients required 1 to 70 electrical terminations. There were 84 hospitalizations (mean 2.1 f 1.7) with a total length of hospital stay of 913 days (mean 31.0 f 19.9). EP evaluation required 90 EP procedures (mean 3.0 f 1.5) with a length of stay of 890 days (mean 23.8 f 12.0). During a follow-up period of 1 to 28.5 months (13.3 f 7.4) on EP-based therapy, 1 patient died suddenly. There were 5 repeat hospitalizations for adverse effects with a total length of stay of 48 days (mean 1.7 f 5.1). There is a significant reduction in arrhythmia recurrences and sudden death (p KO.05) and hospital charges (p
Studies on the utilization of health care facilities have emphasized that a major proportion of available medical resources are consumed by a small number of patients who require recurrent hospita1izations.Q The most common diagnosis associated with recurrent admissions is cardiovascular disease, of which life-threatening cardiac arrhythmias are a specific subgr0up.l Recently, antiarrhythmic therapy for recurrent sustained ventricular tachycardia (VT) or ventricular fibrillation (VF) based upon clinical electrophysiologic (EP) evaluation has been reported to provide effective long-term control.s-a While there is a published report dealing with European costs of EP procedures, there is no available

analysis of the overall medical and economic impact of EP evaluation for cardiac arrhythmias, particularly as related to the U.S. health care system.7 One attempt at cost containment within the U.S. health care system is New Jersey’s experiment with prospective reimbursement. This approach seeks to categorize patients by diagnosis-related groups to standardize health care expenditures for similar disorders. Within this system, we examined the economic impact and long-term clinical efficacy of antiarrhythmic therapy selected on the basis of EP evaluation in patients with recurrent sustained VT or VF. Methods Patient selection: The study group consisted of patients consecutively referred to the Cardiac Electrophysiology Service of Newark Beth Israel Medical Center for the management of recurrent, sustained VT or VF during the period from December 1, 1980, to March 30,1982. Selection criteria included availability of complete medical information from

From the Section of Cardiac Electrophysiology, Department of Medicine and Department of Health Care Evaluation, Newark Beth Israel Medical Center, Newark, New Jersey. Manuscript received July 5, 1983; revised manuscript received October 3, 1983; accepted October 7, 1983. Address for reprints: Sanjeev Saksena, MD, Newark Beth Israel Medical Center, 201 Lyons Avenue, Newark, New Jersey 07112. 531

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CLINICAL AND ECONOMIC IMPACT OF VENTRICULAR TACHYCARDIA

initial diagnosis to the completion of follow-up, completion of a full EP evaluation, and availability of hospital bills. In this study, “recurrent” was defined as 3 or more episodes of VT, and “sustained” as an arrhythmia that lasted more than 30 seconds or was associated with hemodynamic compromise that required immediate pharmacologic or electrical termination. Study design: We examined and compared the clinical and economic sequelae of empiric therapy before EP evaluation, the EP hospitalization, and the follow-up period on EP-based therapy. The variables examined were primarily related to clinical efficacy, average lengths of stay and hospital charges in each treatment period. The study design included a retrospective review of patient case histories from the initial diagnosis of the arrhythmia until completion of follow-up. Hospital charts for the arrhythmia-related admissions were reviewed. Referring physicians’ and Newark Beth Israel Medical Center EP Service records were analyzed. Surveillance during the follow-up period was performed using periodic patient survey questionnaires, primary physician information and arrhythmia clinic visits. Hospital bills were reviewed for total charges, individual procedure charges and length of stay. Study parameters: Clinical efficacy: The end points used to evaluate the clinical efficacy of therapy were recurrence of presenting symptoms, documented spontaneous arrhythmia recurrence and sudden death. Therapeutic failure was defined as the occurrence of any 1 end point. Arrhythmic deaths during EP-based therapy were compared with the predicted death rate on empiric therapy. This was done by comparing arrhythmia recurrences and arrhythmic deaths in our study population with available historical control data for uncontrolled, empirically treated VT or VF.8*g Economic impact: Hospital charges were the fiscal data base for this study. Hospital bills were evaluated to determine an average cost per patient day. EP charges reflect expenses for labor, disposable and reusable supplies, and equipment. Charges, exclusive of physician charges, were determined for each type of EP study and increased annually by the economic factor. The prospective reimbursement system, based on DRGs, is the governing element in establishing rates for reimbursement for New Jersey hospitals. The economic factor is determined by the New Jersey Department of Health and is intended to measure pure marketbasket price increases as reflected in the cost of the hospital marketbasket.‘O With the exception of EP studies themselves, EP admissions largely mirror prior admissions where empiric therapy was used. During this study, the average cost per patient day was $3OO>i and the average charge per patient day was $495. The mean charge per patient day is determined by the total charge for the EP admission, less the EP procedure charge, divided by the total number of patient days for the EP admission. Gross

TABLE I

rates are calculated by adding the total reimbursable cost of direct patient care and the total reasonable fixed cost, then dividing by the total direct cost to obtain a “gross-up” factor. The rate is a product of cost times factor.12 The major components of direct and indirect patient-care costs reflected in DRG rates are outlined in Table I. Statistical analysis: Clinical efficacy for the study group was compared to an historical control.8,g Since duration of foilow-up for individual patients was unequal and study dropouts occurred due to unrelated medical illnesses, appropriate statistical corrections were applied for changes in the study population. Significance of results was determined using a chi-square analysis. Economic impact was assessed during each treatment period by comparing lengths of stay and expenditures in health care dollars. Electrophysiologic evaluation: The techniques and programmed electrical stimulation protocol used in our laboratory have been described. l3 Figure 1 is a schematic of the serial EP procedures used. Initially, patients undergo serial drug testing with conventional antiarrhythmic agents. However, when specific intolerance to or failure of the highest tolerated dose of an agent had been previously documented, this was not done. Patients refractory to or intolerant of conventional drugs are then tested with 2 investigational type I antiarrhythmic agents. If these are unsuccessful, angiocardiographic and EP studies are reviewed before selection of the next therapeutic option. These options include antitachycardia devices with or without conventional drug therapy,

*

D/C

ANTIARRHYTHMICS

+ 1 CONTROL EPS 1

+ SERIAL

DRUG TESTING CONVENTIONALAGENT3 -

I.V. PROCAINAYIDE

.

I.V. PHENYTOfN

C PO. .

1

I LIDOCAINE

QUINIDINE

P.O. DISOPYRAMIDE

EXPERIMENTAL AGENTS *

YEXILETINE

.

APRINDtNE

Major Components of Patlent Care Costs Reflected in Diagnosls-Related Group Rates

Direct Patient Care (Variable)

Indirect Patient Care (Fixed) EOP MAPPING

Nursing care Diagnostic and therapeutic ancillary services Patient care general services Hospital-based physician services

General administration Fiscal administration Maintenance Education Research Patient care coordination Malpractice insurance

Reprinted from A Prospective Reimbursement System Based on Patient Case-Mix for New Jersey Hospitals. New Jersey State Department of Health.

_...._

-

INTRAOP MAPPING a RESECTION

.

POSTOP

EP STUDY

AMIODARONE DISCHARGE

SERIAL DRUG TESTING

FIGURE 1. Electrophysiologic (EP) evaluation procedures during EP hospitalization. EPS = EP study; F/U = follow-up.

February 1, 1984

TABLE II

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Clinical Characteristics of Study Population During Empiric Therapy

Duration of VT (before EP) VT duration <1 year Presenting symptoms Sudden death Palpitations Syncope Drug therapy No. of empirical drug trials per patient Therapeutic drug concentrations Subtherapeutic Uncertain Not performed Recurrent cardiac arrests Arrhvthmia termination Electrical termination of arrhythmia No. of electrical terminations

l-39 months (mean 7.5 f 10.4) 21 patients

~ PREDICTED DEATHS

9 patients 8 patients 16 patients l-7 patients (mean 3.7 f 1.6) 17 patients 1 petient 4 patients 7 patients 20 patients

.

O

;f;;tients

, 2

/I 4

6

I

,

,

,

,

,

,

8

10

12

14

16

18

20

FOLLOWUP

PERIOD

I MONTHS

,

22

ACTUAL ARRHYTHMIC DEATHS

,

,

,

]

24

26

28

30

1

EP = electrophysiologic study; VT = ventricular tachycardia.

cardiac electrosurgery for arrhythmia ablation and chronic amiodarone therapy. Patients who require cardiac surgery for indications other than VT are also simultaneously considered for arrhythmia ablation. Serial drug testing is either deferred or not completed and mapping studies and surgical resection are performed instead. A follow-up EP study is performed at the time of antitachycardia device implantation. Surgical resection is followed by postoperative EP studies to determine surgical efficacy. The amiodarone protocol at our institution at the time of the study required 3 to 4 weeks of in-hospital drug therapy.14 This is based on our clinical experience that some patients require prolonged therapy for arrhythmia control and that these particular patients cannot be prospectively identified. Results Patients: Twenty-nine patients (22 men and 7 women), aged 39 to 78 years (mean 59 f ll), with recurrent sustained VT or VF satisfied the inclusion criteria. Coronary artery disease was present. in 19 patients, cardiomyopathy in 7 and valvular heart disease in 3. Twenty-two patients were in New York Heart Association (NYHA) functional class III and 7 were in class IV. Clinical characteristics of arrhythmia: Empiric treatment: Table II is a summary of the major clinical characteristics for the study group after arrhythmia diagnosis during empiric treatment. Presenting symptoms included the sudden death syndrome, palpitations and syncope. The mean duration of empiric therapy was 7.5 f 10.4 months. Each patient underwent a mean of 3.7 f 1.6 empiric drug trials supervised by a cardiologist

TABLE Ill

Antiarrhythmic Therapy Selected on Basis of Electrophysiologic Evaluation in Study Population No. of Rs

Conventional drugs Experimental drugs Amiodarone Aprindine Mexiletine Antitachycardia pacemaker Endocardial resection

9 19 15 1 : 2

FIGURE 2. Comparison of actual arrhythmic deaths on electrophysiologic-based therapy in study population with predicted arrhythmic deaths for the uncontrolled arrhythmia on empiric therapy from historical data.8

during this period. All patients had arrhythmia recurrences during empiric therapy that required repeat hospitalization. Twenty-seven patients required frequent electrical arrhythmia termination (1 to 70 DC countershocks) for hemodynamic compromise during VT and VF in this period. Electrophysiologic evaluation: Twenty-nine patients underwent 90 EP studies (mean 3.0 f 1.5) for selection of antiarrhjrthmic therapy. Table III is a list of selected antiarrhythmic therapy for these patients. Three patients required a combination of 2 therapeutic modes. During electrophysiologic-based therapy: Follow-up on EP-based therapy ranged from 1 to 26.5 months (mean 13.3 f 7.4). During follow-up, 23 patients were NYHA functional class I and II, 4 were class III and 1 remained class IV. There was 1 sudden death after 1 year of therapy. This patient had an additional (clinically unobserved) VT morphology induced during his EP studies. This was not suppressed during initial drug testing and the patient refused further drug trials. There were no arrhythmia recurrences in 28 patients continued on EP-based therapy. Two patients discontinued treatment after 8 and 20 months, respectively, and noted arrhythmia recurrences. Reinstitution of therapy reestablished arrhythmia control. Figure 2 is a comparison of the predicted arrhythmic deaths and recurrences for the study group if continued on empiric treatment with actual arrhythmic deaths and recurrences on EP-based therapy. There is a significant reduction in arrhythmic deaths and recurrences on EPbased therapy. Economic analysis: Empiric treatment: During empiric treatment, there were 64 hospitalizations for VT recurrences (mean 2.1 f 1.7 hospitalizations/patient). The total length of hospital stay was 913 days (mean 31.0 f 19.9). The length of hospital stay during empiric therapy was somewhat longer for patients eventually treated with amiodarone (34 f 16 days) compared with those treated with other forms of ther-

CLINICAL AND ECONOMIC IMPACT OF VENTRICULAR TACHYCARDIA

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AMIODARONE

than in those who received other forms of treatment (3.5 f 1.6 vs 2.7 f 1.1, p <0.05). The total calculated hospital charges were $403,000 (mean $13,897 per patient).

NON-AMIODARONE

50 si zi S 40 F ln g

30

F P y

20

1

Follow-up

P’2

z i? =

IO

P<.ool

0

1

:MP

EP BASED

EMP

EP BASED

k

f$

SI

f%

FIGURE 3. Length of hospital stay during empiric (EMP) and electrophysiologic (EP)-based therapy in patients receiving amiodarone and other forms of antiarrhythmic therapy.

EP EVALUATION

EP

9(

therapy:

Clinical studies have documented the value of arrhythmia induction techniques in selecting effective long-term therapy in patients with recurrent VT.4-6 Similar observations are being made in patients with supraventricular tachycardia.15-l7 The present study extends previous clinical observations and demonstrates the clinical efficacy of all types of EP-based therapy for VT. We noted a high level of arrhythmia control during a moderate follow-up period. VT recurrences after EP evaluation were most often due to noncompliance or incomplete arrhythmia control. Recurrences due to progression of the underlying heart disease were not encountered in this follow-up period. There was a small but significant incidence of late adverse drug reactions requiring repeat EP evaluation. The clinical efficacy of EP-based therapy is confirmed by a significant decrease in actual arrhythmia recurrence rates and sudden death compared with predicted values from historical control

-

MONTHS

EMPIRICAL %

electrophysiologic-based

Discussion

apy (21 f 16 days), but this was not statistically significant (Fig. 3). The total calculated hospital cost was $452,000 (mean $15,586 per patient). Electrophysiologic evaluation: There were 29 hospitalizations (mean 1.0) for EP evaluation. The total length of hospital stay was 690 days (mean 23.8 f 12.0). There was a significantly longer hospital stay for patients receiving amiodarone therapy compared with those who received other forms of treatment (33 f 10 vs 15 f 8 days, p <0.05). Ninety EP studies (mean 3.0 f 1.5) were performed. More EP procedures were performed in patients discharged on amiodarone therapy

700

on

There were a total of 5 hospitalizations for treatment side effects in this group, with a total length of stay of 48 days (mean 1.7 f 5.1). During this limited follow-up period there was a significantly lower hospital stay in patients who received amiodarone compared with other forms of therapy (p
*,24-19

-

18-13

V-0,

12-7

EMPIRICAL

%-

0

\

ADM:SSSION -

O-6

7-12 EP

13-m BASED

5

IV-24/ -

FIGURE 4. Lefl, total hospital charges in dollars for study patients during empiric therapy, electrophysiologic (EP) evaluation and EP-based therapy during follow-up. Right, temporal sequence of hospital stay for ventricular tachycardia during empiric treatment, evaluation and EP-based therapy.

February 1, 1984

data.8 While an exact control group is not available, the control data certainly approximate, if they do not underestimate, arrhythmia recurrences.8P The safety of EP procedures has also been established.18 Most important, effective arrhythmia control on EP-based therapy leads to a significant improvement in the quality of life. This is seen as improvement in NYHA functional class status. During follow-up, 10 previously disabled patients in this study returned to work and productive lifestyles. The economic impact of different therapeutic approaches to recurrent VT can be defined by our investigation. Cardiovascular disease is the primary consumer of health care resources. These patients experience a pattern of repeated hospitalizations with the same diagnosis.ly2 Our observations during the empiric treatment of recurrent VT confirmed this pattern. Although a process of patient selection is inherent in the definition of our study group, this high degree of hospital resource consumption during empiric therapy occurred in a short mean treatment period (7.5 months). The EP hospitalization was associated with significant costs. These were primarily related to length of stay and, to a lesser extent, to EP procedure charges. While the average length of stay for our patients was long, this was due to a prolonged in-hospital stay for 15 patients who were discharged on amiodarone due to late recurrences of VT in the third and fourth week of therapy.14 Patients not t.reated with amiodarone had a significantly shorter length of stay (15 days). Our data compare favorably with reports on the empiric testing of antiarrhythmic drugs with exercise and ambulatory electrocardiographic monitoring in which the mean length of hospital stay was 17 days? Use of the latter approach requires repeated drug washout periods, which may be undesirable for our study population. Recent modifications in the scheme of EP drug testing in which response to 1 drug, namely, procainamide, may predict that response to other drugs may further shorten the EP hospitalization.lg The absence of a dedicated EP laboratory during the study period also had a negative economic impact. Ross et al7 analyzed the higher costs associated with EP procedures. These procedures are longer in duration and more personnel-intensive than angiocardiographic studies. Although costs in their study were based on European health care costs ($550 per procedure, excluding capital costs), our study shows somewhat higher charges per procedure ($695, excluding capital costs). Fewer personnel were involved in our procedures and data analysis (7 vs 4). The time required for the procedures and analysis of data in these 2 centers, however, was similar.7,20 A mean of 3.0 EP procedures was required to find 1 effective therapeutic modality. This differs from earlier approaches to EP-based therapy, in which all patients underwent a minimum number of drug tests, and may be more cost effective.21 After selection of EP-based therapy, limited expenditures are incurred due to treatment side effects. In analyzing the cost benefit ratio, the length of stay data demonstrate a reduction in hospital days with a pre-EP

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to post-EP ratio of l&l, expressed in hospital days. The expenditure in real dollars demonstrates a reduction in the ratio of pre-EP to post-EP of $452,000:$24,000, or l&8:1. This reduction occurs even though the EP therapy follow-up period is longer than the empiric therapy period. In projections using comparable treatment periods, EP-based therapy is even more cost effective. The cost of an EP hospitalization is significant, but the investment can be recovered within 1 year. While the use of investigational agents like amiodarone in EP therapy alone can theoretically influence our overall results, similar results were seen in patients treated with other forms of EP-based therapy. Important health care cost containment issues are raised by our observations. While recognizing the limitations of a retrospective analysis, we believe that our data do provide health care agencies with a basis for evaluating EP centers. Prospective reimbursement methods are based on case mix data from a base year 3 or 4 years before the reimbursement year. Hospitals accept that the system does not address state-of-the-art changes in clinical practice. 22This factor is an obstacle in the establishment of EP centers. Rate appeals are designed to evaluate and justify expenditures related to additional staffing, supplies and equipment. Certificate of Need applications are required where new facilities must be constructed to accommodate these changes. At Newark Beth Israel Medical Center, the cost of establishing an EP laboratory was $518,740 in 1983.23 The need for adequate ancillary support facilities also must be stated. Support from cardiac surgical, pacemaker and angiocardiographic services is crucial. This implies the need for establishing EP centers in facilities able to provide support services. The need for specially trained personnel has also been recently emphasized.24 These needs should restrict the availability of EP services to tertiary health care facilities. In the appropriate environment and with adequately trained personnel, we believe that selection of antiarrhythmic therapy based on EP studies represents a clinically and economically desirable approach to the management of patients with life-threatening ventricular arrhythmias. Acknowledgment: The authors are grateful for the assistance of Ruth Harrison in the preparation of this manuscript. References 1. ZOOk CH, Moore FD. High cost users of medlcal care. N Engl J Med 1977;302:996-1002. 2. Schroeder SA, Showstack JA, Roberts HE. Frequency and clinical description of high-cost patients in 17 acute-care hospitals. N Engl J Med 1979;300:1307-1309. 3. Horowitz LN, Josephson ME, Farshidi A, Spielman SR, Michelson EL, Greenspan AN. Recurrent sustained ventricular tachycardia. 3. Role of the electrophysiologic study in selection of antiarrhythmic regimens. Circulation 1978;58:986-997, 4. Mason JW, Winkle RA. Electrode-catheter arrhythmia induction in the selection and assessment of antiarrhythmic drug therapy for recurrent VT. Circulation 1978;58:972-985. 6. Fisher JD, Kim SG, Furman S, Mates JA. Role of implantable pacemarkers in refractory ventricular tachycardia. Am J Cardiol 1982;49: 194-206. 6. Horowitz LN, Harken AN, Kastor JA, Josephson ME. Ventricular resection guided by epicardial and endocardial mapping for treatment of recurrent ventricular tachycardia. N Engl J Med 1980;302:589-593. 7. Rose DL, FarreJ, Bar F, Vanagt E, Dassen W, Wiener I, Wellens HJJ. Comprehensive clinical electrophysiologic studies in the investigation of

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CLINICAL AND ECONOMIC IMPACT OF VENTRICULAR TACHYCARDIA

documented or suspected tachycardias. Time, staff, problems and costs. Circulation 1980;61:1010-1016. Cobb LA, Baum RS, Alvarez H, Schaffer WA. Resuscitation from out-ofhospital ventricular fibrilfation: 4 year followup. Circulation 1975;52:suppl lll:223-228. Graboys TA, Lown B! PodrId PJ, DeSllva R. Long-term survival of patients with malignant ventrrcular arrhythmias treated with antiarrhythmic drugs. Am J Cardiol 1982;50:437-443. Grlmaldl PL, Mlchelettl J. DRG’s: A Practitioner’s Guide. Chicago: Pluribus Press, 1983:94. Grlmaldl PL. Physicians and the DRG model. J Med Sot NJ 1980;77: 279-281. Grlmaldl PL, Mkheleffi J. DRG’s: A Practitioner’s Guide. Chicago: Pluribus Press, 1983: 122. Saksena ST Rothbarf ST, Cappello G. Chronic effects of amiodarone in patients wrth refractory ventricular tachycardia. Int J Cardiol 1983;3: 339-346. Saksena S. Roihbarf ST, Lipiak K. Optimising amiodarone therapy for refractory ventricular tachycardia. Clin Res 1983;31(3): 672 A. Bauernfeind RA, Wyndham CR, Dhlngra RC, Swlryn SP, Pallleo E, Strasberg B, Rosen KM. Serial electrophysiologic testing of muftiple drugs in patients with atrioventricular nodal reentrant paroxysmal tachycardias. Circulation 1980;62:1341-1348. Bauernfeind RA, Sweryn SP, Slrasberg B, Palileo E, Scagllottl D, Rosen

17.

18. 19. 20.

21.

KM. Electrophysiologic drug testing in prophylaxis of sporadic paroxysmai atrial fibrillation. Technique, application and efficacy in severely symptomatic preexcitation patients. Am Heart J 1982;103:941-949. Saksena S, Jacobs J, Rdhbart ST. Role of electrophysiologic studies with acute drug testing in refractory supraventricular tachycardia. In: Steinbach K, ed. Cardiac Pacing. Darmstadt: Verlag. 1983, in press. DiMarco JP, Garan H, Ruskin JN. Complications in patients undergoing cardiac EP procedures. Ann Intern Med 1982;97:490-493. Waxman HL, Buxton E, Sadowskl LN, Josephson ME. The response to procainamide during EP study for sustained ventricular tachyarrhythmias predicts the response to other medications. Circulation 1983;67:30-37. Saksena S, Parsonnet V, Hussafn SM, Levfseur-Mendonca M, Kaufman WE, Rathyen W, Ferguson D. Management of recurrent ventricular tachycardia: a reappraisal. J Med Sot NJ 1982;79:991-995. Kastor JA, Horowitz LN, Harken AR, Josephson ME. Clinical electrol$ys%ology of ventricular tachycardra. N Engl J Med 1981;304:1004-

22. Grlmaldl PL. Mkheleffl J. DRG’s: A Practftioner’s Guide. Chicaoo: Pluribus Press, 1983:lOO. 23. New Jersey State Dept. of Health-Newark Beth Israel Med Ctr. Certificate of Need Aoolication for Cardiac Electroohvsioloov and Pacemaker Labo.~. ., ratory 1981. 24. Surawicz 8. Intracardiac extrastimulation studies: How to? Where? By whom? Circulation 1982;65:428-431. WI