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Prove It: Dual defibrillation of refractory ventricular fibrillation

What's the evidence that delivery of dual defibrillation improves survival outcomes for patients who suffer out-of-hospital cardiac arrest and fail to respond to conventional therapy?


Rescue 23 and Engine 12 respond to a report of an unconscious person at a neighborhood baseball park. On arrival, the crew finds a 36-year-old male lying on the field near the pitcher’s mound with two bystanders performing CPR. A police officer is also present and reports he was passing by the park when bystanders flagged him down. The officer had an AED in the vehicle and delivered two shocks before EMS arrival.

Bystanders report the patient was pitching a softball when the batter hit a line-drive toward the pitcher. The patient attempted to turn away from the ball, but was struck in the chest. After the impact, the patient walked around for a few seconds and then collapsed to the ground.

The firefighters take over CPR and the medics transfer the patient to a manual defibrillator. The patient remains in coarse ventricular fibrillation. Medic Sanchez establishes an intraosseous line in the patient’s right humerus and administers 1 mg of epinephrine. Over the next few minutes, the patient receives continuous high-quality CPR, three additional defibrillation attempts, 300 mg of amiodarone, and one additional milligram of epinephrine. Despite these therapies, the patient remains in refractory ventricular fibrillation.

Medic Lassiter applies a second set of defibrillation pads to the patient’s chest in an anterior-posterior position. Once in place, he attaches the second set of pads to the police officer’s AED and presses the analyze button. While the AED is charging, Lassiter charges the manual defibrillator to the maximum output. When both devices are armed, Lassiter asks everyone to clear and depresses the SHOCK buttons on each machine at the same time. After delivery of the two shocks, the firefighters quickly resume CPR.

After two-minutes of high quality chest compressions, the patient achieves ROSC. The total time of cardiac arrest was estimated to be about 35 minutes. Since the patient showed no signs of regaining consciousness, Medic Sanchez performs endotracheal intubation and verifies correct placement with end-tidal capnography. The medics acquire a 12-lead ECG, which shows non-specific ST segment abnormalities. Regardless, they transport the patient to the local STEMI center where a chest CT scan reveals evidence of a myocardial contusion. Cardiac catheterization finds no evidence of significant coronary artery disease. The patient eventually makes a full recovery with only mild short-term memory loss.

Study review of double defibrillation

Researchers reviewed two-year’s worth of case files from an existing cardiac arrest quality assurance/quality improvement database of the San Antonio Fire Department  to examine the relationship between dual defibrillation in the field and neurologically intact survival [1]. In this EMS system, the standard response to a possible cardiac arrest included two ambulances, each staffed with two paramedics, and an engine company staffed with four firefighters. To be included in the study, patients had to receive a single dose of both a vasopressor and an antiarrhythmic and have recurrent or refractory VF after three shocks delivered in the field. For the fourth shock, the lead paramedic had the discretion to deliver a single shock or to provide a dual defibrillation attempt from the two monitor/defibrillators available on the scene. Each manual defibrillator used in the system delivered a maximum output of 200 joules.

If the lead paramedic chose dual defibrillation, the team placed one set of defibrillation pads in the anterior-posterior position and another set in the standard anterolateral position. Each pad set was attached to a separate monitor/defibrillator and one paramedic pushed the shock delivery buttons simultaneously when the machines were fully charged. The primary outcome variable was neurologically intact survival, defined as discharge from the hospital with a Cerebral Performance Category score of 1 or 2.

Results of SAFD double defibrillation study

During the study period, the SAFD treated 3,470 cases of out-of-hospital cardiac arrest. Only 302 of those cases met the inclusion criteria of recurrent or refractory VF. Paramedics delivered standard defibrillation (SD) with a single fourth shock in 229 cases (control group), dual defibrillation (DD) in 50 cases (intervention group), and 23 cases had incomplete data resulting in exclusion from the analysis.

There were no statistically significant differences between the two groups in most variables known to influence the outcome following out-of-hospital cardiac arrest, such as age, gender, rate of bystander CPR or rate of AED use by bystanders. However, patients in the single defibrillation group were significantly more likely to have a bystander-witnessed arrest than those in the dual defibrillation group (54.6 percent vs. 38 percent, respectively). Since having a bystander-witnessed cardiac arrest increases the odds of survival [2], group selection inadvertently favored higher survival for the SD group rather than for the DD group.

Despite this selection bias, there were no significant differences between the two groups in any of the secondary outcome variables, including ROSC rates, survival to hospital admission or survival to hospital discharge. In addition, there was no significant difference between the two groups for the primary outcome variable of neurologically intact survival. Dual defibrillation appeared no better or no worse than continued single shock defibrillation.

What this means for you

Patients with recurrent or refractory VF represent a challenge for paramedics in the field. On one hand, the mere presence of VF suggests the arrested heart has energy reserves sufficient to support normal beating if only the arrhythmia would stop and allow the normal pacemaker to take over. However, this energy is finite and without ROSC and restoration of adequate coronary perfusion pressures, fibrillation will deplete the energy reserves in a matter of minutes. As long as fibrillation is present, termination of resuscitation efforts in the field is inappropriate. While transporting these patients to a hospital seems reasonable and consistent with historical practice, the quality of manual CPR in the back of a moving ambulance is dismal [3,4]. Without achieving ROSC in the field, these patients have little hope for survival [5].

Dual defibrillation, which is sometimes referred to a double-sequential defibrillation, may offer a therapeutic solution to terminating VF and achieving ROSC when standard defibrillation techniques fail [6]. Although dual defibrillation usually involves the simultaneous delivery of shock from two different machines, it is almost impossible in the field to deliver the shocks at exactly at the same moment. Without electrically connecting the two machines, there will always be slight delays between the two shocks regardless of how precisely a single rescuer attempts to depress the two shock delivery buttons at the same time.

More than 25 years ago, physicians at Yale-New Haven Hospital reported using dual defibrillation for five patients who developed refractory ventricular fibrillation during electrophysiology studies over a three-year period [6]. Physicians delivered the two shocks between 0.5 and 4.5 seconds apart. Body mass index measurements showed one patient in the normal weight category, one in the overweight category and three in the obese category. Each patient converted on the first dual defibrillation attempt following unsuccessful single shocks that ranged from a low of 7 shocks to a high of 20 shocks per patient.

The first published case report of delivering dual defibrillation shocks in the field involved a 51+year-old obese male who suffered an out-of-hospital cardiac arrest and failed to respond to standard ACLS therapy for 25 minutes [7]. Paramedics were treating the patient for an anterolateral STEMI when he suddenly developed ventricular fibrillation in the back of the ambulance. The patient received several doses of epinephrine, 300 mg amiodarone, a supraglottic airway and three shocks before arrival in the emergency department. There, the patient received additional resuscitation medications and two additional shocks. For the sixth shock, the ED physician used two defibrillators to deliver a 400 joule dual shock and the patient achieved ROSC. After stabilization, the cardiologist found 100 percent occlusion of the left anterior descending artery and provided a drug-eluding stent and intra-aortic balloon pump. The patient eventually made a full recovery with no neurological deficit.

Over a three-year period, paramedics in Wake County, North Carolina treated ten patients with dual defibrillation [8]. In seven of the cases, ventricular fibrillation stopped following delivery of the dual shock. Three of the ten patients who received dual shocks achieved ROSC in the field, although no patient survived to hospital discharge. One possible reason for the lack of survivors is that dual defibrillation occurred late in the resuscitation effort after the patient failed to respond to five single shocks. In six of the cases, the first dual shock did not occur until more than 35 minutes after cardiac arrest occurred.

Researchers in New Jersey attempted to test whether earlier administration of the dual shock might improve outcomes [9]. Instead of delivering the dual shock after five unsuccessful single shocks, paramedics delivered a dual shock after only three failed standard shocks. Over a four-month period, seven patients received dual shocks, which successfully terminated VF in five of the cases although most required administration of multiple sets of dual shocks before dysrhythmia termination. Four patients achieved ROSC in the field, and three survived to hospital discharge with no neurological deficits, although one patient died 21 days after discharge. All three survivors were witnessed arrests.

The underlying mechanism that allows sequential defibrillation to be effective in some cases remains unknown. However, one proposed explanation is that the first shock may lower the defibrillation threshold so the second shock becomes more effective when delivered almost simultaneously. Animal studies indicate the energy necessary to achieve defibrillation is significantly lower when a second shock occurs within 100 milliseconds or less after the first shock [10-13]. Subsequent studies indicate sequential shocks also lower the defibrillation threshold in humans [14-16].

Others argue that heavier patients simply require more joules to achieve defibrillation success [17,18]. However, the American Heart Association’s "Get With The Guidelines Resuscitation Investigators" recently completed a review of a large in-hospital cardiac arrest registry and could find no evidence of an association between body mass index and first shock termination of cardiac arrest or survival measures [19].

Finally, a leading explanation is that providing two simultaneous shocks may change the vector of therapy and increase shock duration, which together increase the chances of producing a successful defibrillation attempt [6]. Under normal conditions, the depolarization wave passes through heart muscle in an organized fashion. During depolarization, each myocyte (heart muscle cell) conducts energy in a certain direction, which is called a vector. Since there are about 2 to 3 billion myocytes in the adult human heart [20], there are about 2 to 3 billion individual electrical vectors. Mathematically, one could add the magnitude and direction of each electrical vector and determine the net direction or vector of the electrical impulse creating the heartbeat.

In contrast, myocyte vectors are not organized during ventricular fibrillation and depolarization is chaotic, thereby creating a constantly changing net vector. Using animal models, researchers found it easier to depolarize myocardial cells when the cell is parallel rather than perpendicular to the vector of the electrical stimulus [21-23]. Providing a single shock may simultaneously depolarize all myocytes oriented parallel to the defibrillation pathway, however the myocytes oriented perpendicular to the defibrillation pathway may remain unaffected and can sustain ventricular fibrillation [24]. Providing a simultaneous second shock along a different defibrillation pathway may depolarize the group of myocytes that continue to fibrillate.

In addition, in some cases, the first and second shocks may occasionally overlap, i.e., the second shock begins before the first shock ends. Overlapping energy follows a separate path between the first and second vector resulting in three discrete directional current phases [25]. This third defibrillation vector could cause simultaneous depolarization of an additional subset of myocytes that were not depolarized by the other two vectors, which can further help to terminate ventricular fibrillation unresponsive to standard resuscitation measures.

Before attempting to provide dual defibrillation in the field, it is important to consider a couple of additional points. Using two defibrillators to simultaneously deliver shocks is not an FDA approved use for these machines [17]. Doing so constitutes an off-label use of the defibrillators. In addition, delivering double sequential defibrillation may void the defibrillator warranty. During defibrillation, a capacitor within each defibrillator opens for a few milliseconds to deliver the energy. It is possible that during dual defibrillation, energy from one of the defibrillators enters the opened capacitor and damages the defibrillator circuitry.

Limitations of the present study

As with all studies, this one has several important limitations that influences how widespread one can apply the results. First of course is the fact this was an observational study and therefore the authors had no way of controlling any of the variables that could have influenced the outcome. Paramedics decided who would receive dual defibrillation and who would continue to receive single shocks. The dataset used in this analysis did not include information about how many single shocks paramedics delivered before deciding to use dual defibrillation. It is reasonable to assume that some medics followed the treatment protocols exactly and delivered the dual shock as early as the fourth shock. However, it is also possible that some of the cases included in the dual shock group received those shocks much later in the resuscitation attempt. This increases the chances of selection bias whereby medics provide early administration of dual defibrillation only in situations where they think it will be effective.

This analysis also included patients with recurring VF. Commonly, recurrent VF describes a patient who was in VF, converted to something else following a single shock, and converted back to VF again at some point during the EMS encounter.  These patients may respond favorably to additional shocks at the same energy level and thus not need the increased energy of a dual shock. If the greatest benefit of dual shocks occurs in refractory VF, inclusion of recurrent VF into the group could skew the results to make dual shocks appear more effective than they really were.

It is also important to know the meaning of the term successful defibrillation. In many studies involving defibrillation efficacy, the term simply means the shock converted the shockable rhythm into something that was not shockable. It often does not mean the patient had ROSC or survived. Thus, the term "successful defibrillation" still applies when a patient in ventricular fibrillation develops asystole after the shock and resuscitation attempts are subsequently terminated in the field.

Summary

This study could find no evidence that delivery of dual defibrillation improves survival outcomes for patients who suffer out-of-hospital cardiac arrest and fail to respond to conventional therapy. Despite this lack of evidence, many EMS agencies across the country have already added or are considering adding dual defibrillation to their EMS system’s treatment guidelines for a certain subset of patients. Without the results from a well-designed prospective study, the true benefit of dual defibrillation remains unknown.

Although many prehospital providers have historically operated on the principal that "If some is good, more must be better", simply delivering twice as much defibrillation energy is not necessarily a good idea [26]. As EMS learned with countless other therapies, more is not necessarily better.

The author has no financial interest, arrangement, or direct affiliation with any corporation that has a direct interest in the subject matter of this presentation, including manufacturer(s) of any products or provider(s) of services mentioned.

References
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