Manual Pressure Augmentation During Defibrillation Does Not Improve Survival

Optimizing Defibrillation Efficacy in Cardiac Arrest

Out-of-hospital cardiac arrest remains a significant clinical challenge, necessitating constant refinement of resuscitation techniques to improve survival rates [1]. Beyond high-quality chest compressions, clinicians have investigated various mechanical adjuncts and positioning maneuvers, such as impedance threshold devices and head-up CPR, to enhance hemodynamics and coronary perfusion [2, 3]. In the realm of electrical therapy, active chest compression has been explored during elective cardioversion to reduce the energy required for successful rhythm conversion by decreasing the distance between the electrodes and the myocardium [4]. However, many interventions that successfully optimize physiological parameters often fail to demonstrate a clear benefit in survival to hospital discharge [5]. A recent randomized controlled trial now evaluates whether manual pressure applied to defibrillation pads during shock delivery can bridge this gap between a measurable physiological effect and meaningful clinical recovery.

Trial Design and Patient Population

To evaluate the efficacy of manual pressure augmentation, researchers conducted an investigator-initiated, open-label, two-arm trial across 216 ambulance stations in Victoria, Australia. The study employed a cluster-randomized design, a method where entire clinical teams or locations are randomized rather than individual patients. This approach is often used to assess procedural interventions, as it helps ensure the technique is integrated into team protocols and minimizes cross-contamination between study arms. The enrollment period ran from April 1, 2022, to January 31, 2023, and included adults aged 18 or older who experienced an out-of-hospital cardiac arrest with an initially shockable rhythm.

The final intention-to-treat population, which analyzes all participants based on their original group assignment to preserve the integrity of randomization, comprised 560 patients. Of these, 279 were assigned to the intervention group and 281 to the control group receiving standard defibrillation. The trial was ultimately terminated prematurely due to external safety reviews and operational delays. Importantly, this decision was made without unblinding the outcome data, which preserved the ability to conduct an unbiased analysis of the collected results. Despite the shortened timeline, the sample size was sufficient to assess the primary clinical endpoints.

The Manual Pressure Augmentation Protocol

The intervention group received manual pressure augmentation (MPA), a technique in which a paramedic applies firm, direct pressure to the defibrillation pads during the electrical discharge. This was performed using a choreographed sequence with specific safety protocols to standardize the application of force and protect providers. In contrast, the control group received standard defibrillation without any additional manual pressure. To isolate the effect of the pressure itself, other critical variables were held constant across both arms of the study. All shocks were biphasic at 200 joules, and all patients had an anterior-lateral pad position. This standardization ensured that any observed differences could be attributed to the manual pressure rather than variations in energy delivery or pad placement.

The primary goal of MPA is to decrease transthoracic impedance, which is the body's natural resistance to the flow of electrical current. By physically compressing the chest and improving pad contact, the technique aims to allow more energy to reach the myocardium. However, implementing this protocol in a real-world emergency setting proved challenging, as compliance with the MPA technique was low at just 23.6%. This low adherence rate is a critical factor in interpreting the study's overall findings.

Primary and Secondary Clinical Outcomes

The study's primary endpoint was survival to hospital discharge, a key indicator of a resuscitation intervention's real-world effectiveness. The findings showed no clinical benefit associated with manual pressure augmentation. In the intention-to-treat analysis, survival to hospital discharge was nearly identical between the groups: 39.8% (111 of 279 patients) in the intervention group versus 39.9% (112 of 281 patients) in the control group. This lack of a discernible effect suggests that the physiological changes induced by MPA did not translate into improved patient survival.

Statistical analysis confirmed this null finding. The absolute risk difference for survival was -0.1% (95% CI -8.2% to 8.0%), and the adjusted odds ratio was 1.00 (95% CI 0.71 to 1.40; p=0.99), indicating that patients receiving MPA had the same odds of survival as those receiving standard care. This conclusion was further supported by the secondary outcomes. The researchers found that 12-month survival, favorable neurologic outcome, and quality of life were also similar between the two groups, reinforcing that the intervention did not alter either short-term or long-term recovery trajectories for survivors of out-of-hospital cardiac arrest.

Physiological Effects and Safety Profile

While manual pressure augmentation failed to improve clinical outcomes, it did succeed in its primary physiological objective. The study demonstrated that the technique significantly lowered transthoracic impedance. In the intention-to-treat analysis, MPA reduced impedance by an average of 8.5 ohms (95% CI -12.9 to -4.1; p<0.001) compared to standard defibrillation. The effect was even more pronounced in the per-protocol analysis, which examined only the subset of patients who received the intervention exactly as designed, showing a reduction of 15.0 ohms (95% CI -22.8 to -7.2; p<0.001). The divergence between this clear physiological effect and the neutral clinical outcome suggests that reducing impedance by this magnitude may not be a limiting factor for successful defibrillation with modern biphasic devices, which are already highly effective at delivering sufficient current.

From a practical standpoint, the low compliance rate of 23.6% highlights the difficulty of adding a complex, choreographed step into the time-critical environment of a cardiac arrest resuscitation. Regarding provider safety, a key concern with this technique, the risk of accidental shock was minimal and comparable between groups. Perceptible shocks to providers occurred at a rate of 0.75 per 1,000 shocks in the intervention group versus 0.71 per 1,000 in the control group. Crucially for any team considering such a procedure, no serious injuries to providers were reported, indicating the safety protocols were effective.

References

1. Zhu N, Chen Q, Jiang Z, et al. A meta-analysis of the resuscitative effects of mechanical and manual chest compression in out-of-hospital cardiac arrest patients. Critical Care. 2019. doi:10.1186/s13054-019-2389-6

2. Aufderheide TP, Nichol G, Rea TD, et al. A Trial of an Impedance Threshold Device in Out-of-Hospital Cardiac Arrest. New England Journal of Medicine. 2011. doi:10.1056/nejmoa1010821

3. Huang C, Chen K, Lin Z, et al. The effect of the head-up position on cardiopulmonary resuscitation: a systematic review and meta-analysis. Critical Care. 2021. doi:10.1186/s13054-021-03797-x

4. Taha HI, Nazir A, Ibrahim AA, et al. Active Compression During External Cardioversion of Atrial Fibrillation: A Meta‐Analysis of Randomized Controlled Trials. Annals of Noninvasive Electrocardiology. 2025. doi:10.1111/anec.70074

5. Li H, Wang D, Yu Y, Zhao X, Jing X. Mechanical versus manual chest compressions for cardiac arrest: a systematic review and meta-analysis. Scandinavian Journal of Trauma Resuscitation and Emergency Medicine. 2016. doi:10.1186/s13049-016-0202-y