ORBIT: Out-of-Hospital Rectilinear Biphasic Investigational Trial

Luis Gruberg, MD, FACC


December 30, 2003

Editorial Collaboration

Medscape &

Presenter: Laurie J. Morrison, MD, Sunnybrook and Women's College Health Sciences Centre (Toronto, Ontario, Canada)

Out-of-hospital cardiac arrest (OOH-CA) is the main cause of cardiac death, and it affects approximately 1000 Americans every day. When cardiac arrest occurs, the difference between life and death depends on how rapidly normal heart rhythm can be restored using an automated external defibrillator (AED).

Monophasic defibrillators deliver a positive charge and require 200-360 joules of energy to restore cardiac rhythm. By contrast, rectilinear biphasic defibrillators deliver a positive charge for half of the shock and a negative charge for the rest, requiring less energy (150-200 joules) to restore cardiac rhythm. Beyond the fact that biphasic defibrillators deliver a smaller charge to the patient, they are smaller in size, less costly, and have long-life batteries.


The Out-of-hospital Rectilinear Biphasic Investigational Trial (ORBIT) trial was designed to compare the safety and efficacy of a rectilinear biphasic defibrillator with a damped sine monophasic defibrillator for the termination of ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT) in patients with OOH-CA treated by emergency medical personnel in the community setting.

Primary Endpoint

The primary endpoint of the study was conversion, defined as organized electrical activity within 5 seconds of the shock after 1-3 escalating shocks (biphasic, 120 J, 150 J, 200 J; monophasic, 200 J, 300 J, 360 J)

Study Design

The Toronto Emergency Medical Services (EMS) is the sole provider of prehospital care in the metropolitan Toronto area. This system responds to approximately > 400,000 annual calls, with > 2500 annual cardiac arrests. The EMS consists of 900 paramedics with 48 ambulances based in 22 stations. This was a randomized controlled trial where stations were randomized in a 1:1 fashion to equip ambulances with either biphasic or monophasic defibrillators.


Patient enrollment was faster than predicted, with 436 patients enrolled in the stipulated time period. There were 4532 cases of OOH-CA during that period, 540 of which were attributed to VT/VF. A total of 95 patients were excluded from the analysis (54 due to the fact that they had a nonrandomized device and 41 due to incomplete documentation), and the central validation committee removed 9 additional patients from the study; final analysis included 436 patients. With the exception of prior shock, the clinical characteristics were similar between the 2 groups (Table 1).

Table 1. ORBIT: Clinical Characteristics
Characteristic Monophasic
(n = 218)
(n = 218)
Age (yrs) 67 67
Male (%) 72 71
VF (%) 88 87
Amiodarone (%) 44 39
Response time (min) 7.7 7.7
Down time (min) 10.6 10.9
Bystander CPR (%) 33 38
Prior shock (%)* 44 39
CPR, cardiopulmonary resuscitation; VF, ventricular fibrillation
*P < .05

The study, which had a 0.86 power to detect a 15% significant difference, found that there was no difference between the monophasic or biphasic devices with respect to conversion of organized electrical activity after the first shock or between 1 and 3 shocks. However, the biphasic defibrillator was associated with a significantly higher rate of conversion for first highest-energy shock (Table 2).

Table 2. ORBIT: Prehospital Endpoints
Prehospital Endpoints Monophasic
(n = 218)
(n = 218)
First shock efficacy 55 67
First shock high* 30 49
Single episode 92 77
Two episodes 76 63
Three episodes 21 36
Four episodes 14 21
Return of spontaneous circulation 88 90
*P < .05

There were no differences in survival rates between the 2 groups. A total of 123 patients (56%) who were treated with the monophasic defibrillator arrived to the emergency department alive compared with 125 patients (57%) in the biphasic defibrillator group. Only 14 patients (6.4%) were alive at discharge in the former group and 15 patients (6.9) in the latter group. One-year outcomes are still pending.


ORBIT, the largest study ever conducted comparing the safety and efficacy of these 2 types of defibrillators, failed to show any superiority of the use of biphasic over monophasic defibrillators.

Discussant: Richard E. Kerber, MD, University of Iowa (Iowa City)

Defibrillation was introduced by Paul Zoll, MD, in 1956 using an alternating current form. Previous published studies have shown an advantage in restoration of normal cardiac rhythm with biphasic defibrillators, although they failed to prove a survival advantage. Furthermore, previous studies that compared these 2 types of defibrillators in patients with atrial fibrillation who underwent electrical cardioversion have also shown a superiority of the biphasic defibrillators in restoration of normal sinus rhythm. The current study precludes us from drawing specific conclusions over the actual energy required or what the optimal waveform is for OOH-CA. As recognized by the presenter, the timing of CPR, its quality, and low bystander CPR rates may have also limited the conclusions.

What is clear is that the present study failed to show an advantage of either of these defibrillators, and even worse, all of these trials have shown dismal outcomes for these patients. Such findings emphasize a strong need to improve community-based programs to decrease response times and improve bystander CPR rates to increase survival rates of patients who suffer OOH-CA in the community setting.