Life was simple for the pre-Socratic Greek philosopher, Heraclitus, who in 483 BC pronounced “The way up and the way down are one and the same.” The world was generally proceeding according to his simple philosophy until contradictory studies of the compression and relaxation phases of external cardiac massage suggested that defibrillation efficacy was not the same on the way up as the way down.

Two animal studies initially suggested a relationship between the compression phase and defibrillation efficacy. Li et al. assessed the effects of timing of defibrillation during the chest compression cycle, concluding that the defibrillation success rate was significantly higher when shocks were delivered in the upstroke phase of both manual and mechanical chest compression.

Whether this phase-relationship is the same during clinical resuscitation has taken further elucidation. Olsen et al. recently reported that termination of fibrillation (TOF) was lower with defibrillation during continuous load-distributing band (LDB) compressions for the first shock, based on retrospective data from the CIRC trial.However, in an accompanying editorial, Deakin pointed out that without an understanding of the compression phase at the delivery of each shock, it was difficult to attribute these findings solely to the effect of the LDBsince Olsen et al. had not differentiated between the compression cycle phase. The same was pointed out by Carron and Yersin in the LINC study which, as with the CIRC trial, had not reported shock success in relation to the compression phase.

In this volume of Resuscitation, a further study using data from the CIRC trial has analysed defibrillation shock success according to the compression phase in which it was delivered. The authors report that shocks delivered in the compression phase of LDB chest compressions had lower TOF rates than shocks delivered while pausing the LDB device. The TOF success rate was significantly lower for the first shock when delivered in the compression phase of the LDB compression cycle than for control shocks delivered in a LDB compression pause (14% reduction, 72% vs 86%). The same results were found for the first up-to-three shocks group (11% reduction, 71% vs 82%). These results remained unchanged after adjusting for witnessed arrest and bystander CPR (first shock group only), defibrillator shock energy and transthoracic impedance. There were no differences in TOF success rates between the decompression or relaxation phase and the controls in either first shock or up-to-three shocks analysis.

An important aspect when considering these results is the composition of what the authors term a ‘control’ group. In order to mimic the methods used by Li et al.,,  the authors defined a control group of shocks (rather than patients) from the same patient population with initial VF/VT and LDB CPR prior to shock delivery. The control data derived from shocks delivered when patients were defibrillated in a compression pause due to protocol violation (LDB device stopped by the rescuer in order to defibrillate). Controls represented shocks and not patients and therefore one patient could contribute with shocks to both control group and different chest compression cycle phases, depending on the timing of the shock delivery in relation to chest compressions. A retrospective study, not powered for secondary endpoint analysis using a control group constructed from shocks that were protocol violations, significantly limits that validity of any conclusions that can be drawn from this data. Unintended pauses in LDB CPR occurred significantly more often when the shock was delivered in the relaxation phase of the LDB compression cycle (compression 17%, decompression 16%, relaxation 49%,p?<?0.001), biasing the control group further.

Additionally, there are large difference in pre-shock pauses between the animal studies,  (2?s) and Steinberg’s study (median of 12?s [5–20?s] for the first shock control group and a median of 13?s [4–20?s] for the ‘up to three shock’ control group). The authors acknowledge that the results of their analyses should be considered a combined result of both zero pre-shock pause and defibrillation in a specific phase of the LDB chest compression cycle and that they have subsequently not been able to separate these results in a meaningful way because of the retrospective nature of the study. However, they still conclude “The present clinical data do not lend support to findings in pigs of higher success rate for shocks delivered in the decompression phase of the chest compression cycle than with a pre-shock pause.”

Furthermore, the control group comprised of shocks delivered when the LDB was ‘paused’ during shock delivery. The LDB has three phases; compression, decompression and relaxation but it is not clear in which phase this pause occurred. Without understanding of the LDB phase in which the pause occurred, the ability to draw valid conclusions from this data is further limited.

The authors discuss the possible physiological basis for their findings, suggesting the involvement of mechanisms involving effects on current pathways from changes in pad and/or cardiac orientation that affect transmyocardial current pathways or physical strain on the heart from simultaneous compressions and defibrillation. This may be premature in view of the uncertainties of their conclusions and is limited by our poor understanding of defibrillation mechanisms.

Resuscitation research presents many challenges unique to the specialty and the pathophysiology of resuscitation is reluctant to give up many of its secrets. The authors are to be commended in their further efforts to elucidate the effect of compression phase on the efficacy of fibrillation. Heraclitus also observed that “Nature loves to hide” and further clinical work investigating both manual and mechanical compression is clearly required, to understand these effects more fully.

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