Pauses for shock delivery in chest compressions are detrimental to the success of resuscitation and may be eliminated with the use of mechanical chest compressors. However, it is important to deliver the shock at just the most optimal time. Researchers found defibrillation efficacy is maximal when electrical shock is delivered in the upstroke phase of mechanical chest compression. The study was published in the June issue of Resuscitation.

Yongqin Li, PhD, and colleagues from the Weil Institute of Critical Care Medicine in Rancho Mirage, Calif., undertook a study to assess the effects of timing of defibrillation in the mechanical chest compression cycle on the defibrillation threshold (DFT) using a porcine model of cardiac arrest.

Ventricular fibrillation was electrically induced and untreated for 10 seconds in eight animals. Mechanical chest compression was then continuously performed for 25 seconds, followed by a biphasic electrical shock, which was delivered to the animal at six randomized coupling phases, including a control phase, with a pre-determined energy setting. The control phase was chosen at a constant two seconds following discontinued chest compression.

Researchers used a novel grouped up-and-down DFT testing protocol to compare the success rate at different coupling phases. After a recovery interval of four minutes, they repeated the testing sequence, resulting in a total of 60 test shocks delivered to each animal.

Li and colleagues found no difference between the delivered shock energy, voltage and current among the six study phases. The defibrillation success rate, however, was significantly higher when shocks were delivered in the upstroke phase of mechanical chest compression.

They determined that the optimal phasic relationship between synchronized shock and mechanical chest compressions was during the upstroke of  compressions.