A new trial investigated whether defibrillation with manual pressure augmentation (MPA) improves outcomes in adults with initially shockable out-of-hospital cardiac arrest (OHCA). Conducted across 216 ambulance stations in Victoria, Australia, between April 2022 and January 2023, the study compared standard defibrillation with defibrillation augmented by manual downward pressure applied to defibrillation pads. The rationale for MPA is that applying pressure reduces transthoracic impedance, potentially increasing electrical current delivery to the heart and improving cardioversion success.
Ambulance stations were randomised 1:1 to either intervention (MPA) or control (standard defibrillation), with treatment determined by the first paramedic team on scene. Eligible patients were adults (≥18 years) with OHCA due to a shockable rhythm (ventricular fibrillation or pulseless ventricular tachycardia) who received attempted resuscitation. All patients received standard advanced life support care, including biphasic defibrillation at 200 J with anterior–lateral pad placement.
In the intervention group, paramedics applied MPA during shock delivery using a structured protocol designed to ensure safety. This included wearing double nitrile gloves, avoiding contact with other surfaces, and applying firm pressure (approximately 10–15 kg) directly over the pads. The primary outcome was survival to hospital discharge. Secondary outcomes included 12-month survival, neurological recovery (measured using the Glasgow Outcome Scale Extended), quality of life (EQ-5D-5L), return of spontaneous circulation (ROSC), and various prehospital metrics such as shock success and transthoracic impedance.
A total of 560 patients were included in the intention-to-treat analysis (279 intervention, 281 control). Baseline characteristics were well balanced between groups: the median age was 64 years, most patients were male, and the majority of arrests were witnessed with high rates of bystander CPR. Prehospital care, including response times and medication use, was similar across groups.
The primary outcome showed no difference between groups. Survival to hospital discharge was 39.8% in the MPA group and 39.9% in the control group. There were no significant differences in secondary outcomes. Twelve-month survival and favourable neurological outcomes were nearly identical between groups, and quality-of-life scores were comparable. Short-term outcomes such as ROSC, event survival, and termination of ventricular arrhythmias also showed no meaningful differences.
Despite the lack of clinical benefit, MPA significantly reduced transthoracic impedance. In the intention-to-treat analysis, impedance was reduced by an average of 8.5 ohms, with even greater reductions (15 ohms) observed in the per-protocol population. However, this physiological improvement did not translate into better clinical outcomes. Exploratory analyses suggested that current delivery was paradoxically lower in the MPA group, likely due to impedance-compensating mechanisms in modern defibrillators, which may have offset any theoretical benefit.
Safety outcomes were reassuring. Perceptible shocks to paramedics were rare and occurred at similar rates in both groups, with no serious injuries reported. Survey responses indicated high acceptability among paramedics, with most reporting no discomfort and willingness to use MPA again. However, one safety incident occurred in a high-moisture environment, highlighting the importance of environmental precautions.
The findings suggest that while MPA effectively reduces transthoracic impedance, this does not improve survival or other clinically important outcomes in OHCA. Several explanations are proposed. Modern biphasic defibrillators may already deliver sufficient current for effective cardioversion, reducing the marginal benefit of impedance reduction. Additionally, cardiac arrest outcomes are influenced by multiple factors beyond electrical delivery, including CPR quality, timing of interventions, and underlying physiology. It is also possible that MPA may benefit specific subgroups, such as patients with high body mass index or refractory arrhythmias, which this trial was not powered to assess.
Source: Resuscitation
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