Chest Compressions First May Make Defibrillation More Effective in Some Patients
September 17, 2010 (Ann Arbor, Michigan) — The outcomes of out-of-hospital cardiac-arrest patients treated with chest compressions before defibrillation are generally about the same as those treated with defibrillation first, a new meta-analysis shows . But a subgroup analysis suggests that chest compressions may improve the effectiveness of subsequent defibrillation in patients who have already been in arrest for several minutes.
"The interval from compression to defibrillation is highly critical, as impaired myocardial oxygenation distinctively decreases defibrillation success rates while myocardial preoxygenation may improve outcome," Dr Pascal Meier (University of Michigan, Ann Arbor) and colleagues explain in an article published online September 9, 2010 in BMC Medicine. "There is, however, clinical equipoise whether professional chest compression only, promptly followed by defibrillation, could increase myocardial 'readiness' for defibrillation." Unfortunately, randomized clinical trials comparing the two strategies have shown conflicting results, and most studies are too small to allow definite conclusions, but a recent observational study by Dr Alex Garza (Georgetown University, Washington, DC) and colleagues with 143 patients indicated potential benefit for predefibrillation chest compressions, Meier et al explain.
In the first meta-analysis to systematically review the research comparing chest compression-first with defibrillation-first strategies for treating out-of-hospital cardiac arrest, Meier and colleagues pooled results of four trials enrolling a total of 1503 subjects [2,3,4,5]. These studies were selected because they randomized treatment assignment to chest compression first vs defibrillation first and included outcome data on one of the four following clinical outcomes: return of spontaneous circulation, survival to hospital discharge, neurological outcome at discharge, or survival at one year.
The meta-analysis found no difference between patients treated with the chest-compression-first vs a defibrillation-first approaches in the rate of return of spontaneous circulation (odds ratio 1.01; p=0.979), survival to hospital discharge (OR 1.10, p=0.686), or favorable neurologic outcomes (OR 1.02, p=0.979).
However, one-year survival rates were slightly better in patients treated with chest compression first (OR 1.38, p= 0.092), but the confidence interval ranged from 0.95 to 2.02, indicating the estimate is insufficiently precise to conclusively show this strategy is superior. Importantly, for cases with prolonged response times, defined as at least five minutes after the patient went into arrest or if the initial arrest was not witnessed, point estimates suggest the chest-compression-first strategy may be superior (OR 1.45, p=0.353), but the confidence interval ranged from 0.66 to 3.20, so this hypothesis needs to be tested further, the authors explain.
Against the experimental and observational studies showing better outcomes with chest-compressions first, the meta-analysis of randomized clinical trials in humans showed the defibrillation-first and compressions-first approaches appear to be equivocal. Meier et al suggest that the discrepancy with the animal models could simply be due to differences in animal and human physiology, especially because most of the animal studies focused on ventricular fibrillation, which may not reflect the majority of cardiac arrests in humans.
The discrepancy with the observational studies may be due to confounding factors unaccounted for in those studies that are not present in the randomized controlled trials in the meta-analysis. Also "it may be that the treatment effect of chest compression first may be dependent on the response interval from the time of call to [emergency medical services] response. Further research, with patient-level data, will need to be conducted to assess whether this finding is consistent," Meier et al conclude.
Urgent Need to Improve Out-of-Hospital Outcomes
According to Meier et al, there are about 295 000 EMS-assessed out-of-hospital cardiac arrests in the US each year, with ventricular tachycardia and ventricular fibrillation the most common arrhythmias. Despite all the effort to improve the "chain of survival," out-of-hospital-arrest survival rates have stayed nearly flat at around 7.6% for over 30 years, and the average rates of survival to hospital discharge are as low as 0.3% in some communities. Survival rates depend heavily on the time to initiation of chest compressions, the time until defibrillation, and the underlying rhythm. Unfortunately, defibrillation and chest compressions cannot be performed simultaneously, and controversy about which treatment should have priority remains.
The current European Resuscitation Council (ERC) and the American Heart Association (AHA) guidelines, last updated in 2005, emphasize the importance of early defibrillation, but the AHA guidelines state that in cases of nonwitnessed events, one cycle of cardiopulmonary resuscitation (CPR)/chest compressions may be considered before defibrillation.
Commenting on the implications of the meta-analysis results, Meier told heartwire , "The most important [lesson] is to do defibrillation as soon as possible. I wouldn't change that. But what I would do, is that if I knew the patient had had a long duration of cardiac arrest or if there is no witness--cardiac arrest for nobody to see and the patient lost consciousness, [indicating there has been] a long period of time when they haven't had good circulation--I would start with chest compression for one to two minutes before I would do defibrillation."
In the paper, the authors cite animal studies showing that compression empties the right ventricle, thereby avoiding RV distension during ventricular fibrillation, which helps to reduce the risk of nonperfusing postdefibrillation rhythms. "Currently, the focus is much more on early defibrillation--we've put [automated external defibrillators] AEDs everywhere--and the focus is so much on defibrillation that we almost forget about the importance of chest compression.
"We've shown that chest compressions can really matter and should be done as soon as possible" in patients who have been in arrest for a while, he said. "Defibrillation is still the final therapy [to get the patient back] to normal rhythm. But the chances of defibrillation being successful are higher if you do chest compressions first to prepare the heart to improve circulation."
Meier added that future studies should try to better define a threshold for this response interval that determines whether chest compressions will help or not. The five-minute interval used for this study is a fairly arbitrary estimate.