More studies continue to be published on the outcome of mechanical chest compression. Not only do these devices deliver uninterrupted high quality chest compressions, but they also enable chest compressions to be performed continuously, even during defibrillation, enabling the delivery of a high chest compression fraction. Additionally, the load-distributing band (LDB) can, with some defibrillators, synchronise defibrillation to the relaxation phase, when it is thought to be more successful. Our understanding of the pathophysiology of cardiac arrest, animal studies and some clinical data would suggest that these attributes would combine to enhance outcome from mechanical CPR, but studies to date have failed to demonstrate any difference in overall survival to hospital discharge between the mechanical and manual CPR groups., ,  The reasons for this are unclear and further analysis of some of these studies may help understand why this is the case.

In this edition of Resuscitation, Olsen et al. present further analysis of the original Circulation Improving Resuscitation Care (CIRC) trial data, which compared high quality manual cardiopulmonary resuscitation (M-CPR) with Autopulse integrated load-distributing band CPR (LDB-CPR) in out-of-hospital cardiac arrest. Based on previous studies which demonstrated that pre-shock pauses adversely affect the success of the ensuing defibrillation, the authors specifically investigated the effect of pre-shock chest compression pauses on the termination of fibrillation (TOF) and return of organised rhythm (ROOR) for CIRC patients in both the mechanical and manual CPR groups. Both LINC, PARAMEDICand the present CIRC–trial protocols specified that defibrillation should be attempted without stopping chest compressions. The authors proposed that shorter pre-shock pauses in both LDB-CPR and M-CPR patients would be associated with higher rates of termination of fibrillation (TOF) and a return of an organised rhythm (ROOR), and specifically that patients within the LDB-CPR group with no pre-shock pause would have the highest TOF of all. This secondary analysis concluded that for first shocks with LDB-CPR, TOF was associated with pre-shock pause duration, but there was no association with the rate of ROOR. However, for M-CPR, where chest compressions were interrupted to give defibrillation shocks, there was no association between pre-shock pause duration and TOF or ROOR, contrary to other studies of M-CPR demonstrating a convincing relation between pre-shock pause with TOF and ROSC.

Since 2010, both European and American the resuscitation guidelines have emphasised the need to minimise peri-shock interruptions to chest compressions.,  These recommendations were based on a number of studies showing that peri-shock interruption of CPR is associated with a decreased probability of conversion of VF to another rhythm, defibrillation failure, reduced ROSC and survival, and immediate resumption of chest compressions after defibrillation is associated with better survival rates and/or survival with favourable neurological outcome.,  However, results have not been consistent across all studies and a previous AED study, which included CPR during charging, and immediate resumption of chest compressions after shock delivery, did not show significantly improved survival to admission or to discharge.The results of Olsen’s study have also failed to demonstrate a relationship between pre-shock pause and TOF or ROOR during M-CPR, although the relationship with TOF when using LDB-CPR was consistent with studies on which recommendations to minimise peri-shock pauses were based.

Understanding these inconsistent results is challenging and adds to the difficulties in interpreting the results of studies of mechanical chest compression during CPR. Uninterrupted chest compressions delivered by mechanical devices may improve shock success, but perhaps adversely affect overall outcome by making the myocardium more prone to recurrent VF. Although not demonstrated by all studies, there is some evidence that immediate resumption of chest compressions after defibrillation is associated with earlier VF recurrence, which may be a mechanism to at least consider when interpreting these results. Olsen et al. did not analyse post-shock pauses in their post hoc analysis, which might be helpful in interpreting these results further. Recent studies have also demonstrated that defibrillation may be more successful during the relaxation phase of each chest compression, and it is unclear from these results whether shocks delivered to the LDB-CPR group were synchronised to the upstroke. Both the pre-shock and post-shock components of the resuscitation sequence may contribute to overall outcome and in order to better understand the complexities of mechanical CPR outcomes, perhaps both need examining as independent variables rather than the assumption that they both contribute adversely to outcome.

However, one swallow does not make a summer and it is important that the emphasis on minimising peri-shock pauses remains. Despite current guidelines recommendations, interruptions to chest compressions during CPR remain significant, even in well-trained teams. A common cause of these interruptions is defibrillation, as the team stands back while the shock is delivered. Attempts to perform hands-on defibrillation, thereby mimicking that achieved with M-CPR are unsafe when using clinical examination gloves,, ,  but may be possible when using appropriately rated safety gloves.,  Maximising chest compression fraction is, and will remain, a priority in the delivery of high quality CPR and mechanical devices can certainly achieve this, even if not an improved outcome.

Conflict of interest statement

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