AP or AL pad position for first choice shock in VF?

Background

There have been many interesting studies around defibrillation in recent years, and I think that’s fantastic. For most of my career, defibrillation has just been presumed to be as good as it can be with antero-lateral (AL) pads and pauses for pulse checks. These days, it seems that all assumptions are up for a bit of #dogmalysis, and I think that’s great. We’ve also seen some amazing papers and data on strategies such as Dual Sequence Defibrillation that are changing practice worldwide.

Despite this, out-of-hospital cardiac arrest (OHCA) remains a leading cause of death globally, with ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT) as the most treatable causes of OHCA. Defibrillation is the mainstay of treatment, but there are many ways in which this can be done in terms of energy, sequence, waveforms and pad positions. The bottom line is that defibrillation is complex, and it’s unlikely that we have optimised it yet.

Although numerous studies have investigated defibrillator pad placement for cardioversion in atrial fibrillation (and we have reviewed them here on St Emlyn’s recently), few have explored the optimal pad positioning for OHCA. Current guidelines in the UK (JRCALC) recommend the anterior-lateral (AL) or anterior-posterior (AP) pad placement based on indirect evidence, primarily extrapolated from non-cardiac arrest studies. I was happy with this, but then we saw the DOSE-VF trial, which demonstrated that vector change (going AL to AP) worked better. Following the publication of that trial, I remember thinking whether if we started with AP, then that would indeed be a better option? This week we have a paper that helps inform that question. This paper by Lupton et al. (2024) (abstract below, but please read the whole thing yourself) addresses this gap by evaluating the association between initial defibrillator pad position (AP versus AL) and outcomes in patients presenting with VF or pVT during OHCA.

Abstract

Importance: Ventricular fibrillation (VF) or pulseless ventricular tachycardia (PVT) are the most treatable causes of out-of-hospital cardiac arrest (OHCA). Yet, it remains unknown if defibrillator pad position, placement in the anterior-posterior (AP) or anterior-lateral (AL) locations, impacts patient outcomes in VF or pVT OHCA.
Objective: To determine the association between initial defibrillator pad placement position and OHCA outcomes for patients presenting with VF or PVT.
Design, setting, and participants: This prospective cohort study included patients with OHCA and VF or pVT treated by a single North American emergency medical services (EMS) agency from July 1, 2019, through June 30, 2023. The study included patients with OHCA treated by a large suburban fire-based EMS agency that covers a population of 550 000. Consecutive patients with an initial EMS-assessed rhythm of VF or pVT receiving EMS defibrillation were included. Pediatric patients (younger than 18 years), interfacility transfers, arrests of obvious traumatic etiology, and patients with preexisting do-not-resuscitate status were excluded.
Exposure: AP or AL pad placement.
Main outcomes and measures: Return of spontaneous circulation (ROSC) at any time with secondary outcomes of pulses present at emergency department (ED) arrival, survival to hospital admission, survival to hospital discharge, and functional survival at hospital discharge (cerebral performance category score of 2 or less). Measures included adjusted odds ratios (aOR), multivariable logistic regressions, and Fine-Gray competing risks regression. Results: A total of 255 patients with OHCA were included (median [IQR] age, 66 [55-74] years; 63 females [24.7%]), with initial pad positioning documented as either AP (158 patients [62.0%]; median [IQR] age, 65 [54-74] years; 37 females [23.4% ] ) or AL (97 patients [38.0%]; median [IQR] age, 66 [57-74] years; 26 females [26.8% ] ). Patients with AP placement had higher adjusted odds ratio (aOR) of ROSC at any time (aOR, 2.64 [95% CI, 1.50 -4.65]), but not significantly different odds of pulses present at ED arrival (1.34 [95% CI, 0.78-2.30]), survival to hospital admission (1.41 [0.82-2.43]), survival to hospital discharge (1.55 [95% CI, 0.83- 2.90]), or functional survival at hospital discharge (1.86 [95% CI, 0.98-3.51]). Competing risk analysis found significantly greater cumulative incidence of ROSC among those at risk with initial AP placement compared with AL (subdistribution hazard ratio, 1.81 [95% CI, 1.23-2.67]; P = .003).
Conclusions and relevance: In this cohort study of patients with OHCA and VF or pVT, AP defibrillator pad placement was associated with higher ROSC compared with AL placement.

What kind of study is this?

This study is a prospective cohort analysis conducted in a single large suburban fire-based emergency medical services (EMS) agency covering a population of 550,000. The study period spanned four years, from July 1, 2019, to June 30, 2023, and included consecutive patients with OHCA treated by EMS for an initial rhythm of VF or pVT. The study adhered to the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) guidelines and was approved by the institutional review board of Oregon Health and Science University.

The research question sought to determine whether the initial pad position (AP or AL) was associated with key clinical outcomes such as return of spontaneous circulation (ROSC) and survival to hospital discharge. The authors used adjusted odds ratios (aOR) and competing risk regression models to control for confounders and account for time-dependent outcomes.

It’s important to remember that this is an observational study. In the EMS protocol, they placed AP pads as the first option to enable easier vector change or DSD later. That’s an interesting approach and not one that we follow here. It makes sense if using mCPR, but it’s not in the guidelines, and as there is a choice element here about which pad position to go for first, there is potential for a lot of bias being introduced. It’s very difficult to find out the ‘why’ some people did not get the advised AP technique as opposed to the second option (AL). However, although we don’t know the reasons in detail, they are likely to be different in some way.

Tell me about the patients

The study included 255 adult patients with OHCA presenting with an initial shockable rhythm of VF or pVT. The median age of the cohort was 66 years, with a range of 55 to 74 years. Approximately 25% of the cohort were females. The inclusion criteria were strictly limited to adults (18 years and older) who had a shockable initial rhythm treated with defibrillation by EMS. The study excluded pediatric patients, interfacility transfers, traumatic cardiac arrests, and patients with pre-existing do-not-resuscitate (DNR) orders.

Baseline characteristics such as age, sex, arrest location (public versus non-public), bystander CPR, and time to EMS arrival were recorded and compared between the AP and AL pad positioning groups. 158 patients (62%) had initial AP pad placement, while 97 (38%) had AL placement. There were no significant baseline differences between groups, except for a slightly higher mean weight in the AL group.

What were the measured outcomes?

The primary outcome was ROSC at any time during the resuscitation effort.

Secondary outcomes included:

• Presence of pulses at emergency department (ED) arrival
• Survival to hospital admission
• Survival to hospital discharge
• Survival to hospital discharge with good neurological function, defined as a Cerebral Performance Category (CPC) score of 2 or less

Process outcomes included the number of EMS shocks delivered, proportion of patients achieving ROSC within 20 minutes, time to initial ROSC, time to sustained ROSC, and the need for vector change (altering pad placement during resuscitation). Additionally, the study explored exploratory outcomes such as interactions between pad position and estimated patient weight, defibrillator energy level, and manufacturer type.

What are the main results?

Overall the authors suggest that the AP pad position had better results.

• ROSC at Any Time (the primary outcome of this study): Initial AP pad placement was associated with a significantly higher likelihood of achieving ROSC (adjusted odds ratio [aOR], 2.64; 95% confidence interval [CI], 1.50–4.65) compared with AL placement.
  • In patient numbers this was 117/158 (74%) in the AP group vs. 49/97 (50.5%) in the AL group. That’s a pretty substantial difference and isn statistically significant.
• Pulses Present at ED Arrival: There was no significant difference in the odds of pulses present at ED arrival between AP and AL positions (aOR, 1.34; 95% CI, 0.78–2.30).
• Survival to Hospital Admission: No significant difference was observed for survival to hospital admission between the two pad placements (aOR, 1.41; 95% CI, 0.82–2.43).
• Survival to Hospital Discharge and Neurologic Outcomes: The odds of survival to hospital discharge (aOR, 1.55; 95% CI, 0.83–2.90) and favourable neurological outcomes (aOR, 1.86; 95% CI, 0.98–3.51) were higher in the AP group, but these did not reach statistical significance.
• Process Outcomes: Fewer shocks were needed on average in the AP group, and AP placement was associated with a lower need for subsequent vector changes.

Are there any methodology issues?

Well, yes, there are. The study’s observational nature is likely (inevitable) to lead to confounding. Pad placement was at the discretion of the EMS crews, potentially introducing selection bias. The single-agency setting limits the generalisability of the findings, and the small sample size may have limited the statistical power to detect differences in some outcomes. Of all these, the observational nature of the study and the choice of paramedics are most likely to cause problems.

Additionally, the study relies on EMS-estimated weights rather than direct measurements, which could have introduced measurement bias. Finally, the outcome of ROSC, while clinically relevant to us, is not as important to patients and families who will almost certainly value survival and function more.

Should we change practice based on this study?

In a word, no. Although the results of this study suggest that AP pad placement may be superior to AL placement for initial defibrillation, this is not an RCT, and there are far too many potential biases to make me change my practice. What this means is that I would love to see this study lead to a multicentre RCT where we can truly find an answer (because I think it’s a question that deserves an answer).

A randomised controlled trial (RCT) could more definitively establish causality and assess the impact on longer-term outcomes such as neurologically intact survival.

Summary

This prospective cohort study by Lupton et al. examined whether the initial defibrillator pad placement (AP versus AL) changed outcomes for patients with OHCA presenting with VF or pVT. The results indicate that initial AP pad placement is associated with higher rates of ROSC compared with AL placement, although no significant differences were observed in longer-term survival outcomes. Although interesting, the study design and potential confounders mean that we should not consider changing practice until more evidence comes along.

References

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2. Defibrillation pads and paddles https://litfl.com/defibrillation-pads-and-paddles/
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