Beyond the Presenting Rhythm: Estimating Shockable Rhythm at the Time of Cardiac Arrest in Out-of-Hospital Cardiac Arrest

Q: What are the key findings of this study?

Generalised additive modelling estimated shockable rhythm probability at collapse as 61% in males and 41% in females, vs 43% and 24% observed at EMS arrival.

Q: What is the clinical evidence from this study?

Study design: Observational. Population: Out-of-hospital cardiac arrest (OHCA) (n=5277). Intervention: Age, sex, and elapsed time from emergency call to EMS arrival (modeled at time of collapse, t=0) vs. Observed rhythm at first EMS assessment (and male vs female sex). Primary outcome: Estimated probability of a shockable rhythm (VF/pVT) at the time of collapse (t=0) vs observed presenting shockable rhythm at EMS arrival (95% CI 56-67% (males); 27-55% (females), p=<0.001).

Q: What does this research mean for the field?

The probability of an initial shockable rhythm at the time of collapse in out-of-hospital cardiac arrest is substantially higher than observed at first EMS arrival (modeled at 61% vs observed 43% in males, and 41% vs 24% in females). Novelty: ClaimNovelty.NOVEL_FINDING. Consensus alignment: ConsensusAlignment.NEUTRAL.

Q: What question did this study set out to answer?

To estimate the probability that an out-of-hospital cardiac arrest initially presented with a shockable rhythm at the time of collapse and characterize how this probability varies by age and sex.

Brian Burns; Rajat Kalra; Demetris Yannopoulos; Natalie Kruit; MW Dennis; Tony Keech; Nicole Packham; Sandra Ware; Geoff Healy

doi:10.21203/rs.3.rs-11069706/v1

October 29, 2026Open Access

Beyond the Presenting Rhythm: Estimating Shockable Rhythm at the Time of Cardiac Arrest in Out-of-Hospital Cardiac Arrest

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Key result

Initial shockable rhythms at collapse are ~18 percentage points higher than observed at EMS arrival.

95% CI 56-67% (males); 27-55% (females)
P<0.001
n=5,277

Why the study?

Survival in out-of-hospital cardiac arrest strongly depends on a shockable rhythm at first rhythm analysis, making it important to characterize the determinants of an initially shockable rhythm and how its probability varies by age and sex.

Key Points

To estimate the probability that an out-of-hospital cardiac arrest initially presented with a shockable rhythm at the time of collapse and characterize how this probability varies by age and sex.
Analyzed records of adult non-traumatic witnessed out-of-hospital cardiac arrests with bystander CPR attended by New South Wales Ambulance from January 2017 through March 2022 (N=5,277 eligible out of 45,607 attended).
Applied generalized additive modeling (GAM) to model the dynamic relationship between documented rhythm, elapsed response time, age, and sex to estimate rhythm state at collapse (t=0).
At the estimated time of collapse (t=0), the modeled probability of a shockable rhythm was 41% (95% CI 27–55%) among females versus an observed presenting rate of 24%, and 61% (95% CI 56–67%) among males versus an observed presenting rate of 43%.
The relationship between age and shockable rhythm was non-linear in males (edf 4.14, χ² = 167.2, p < 0.001) but approximately linear in females (edf 1.00, χ² = 51.8, p < 0.001).
Elapsed time was significantly associated with a shockable rhythm in both males (edf 1.00, χ² = 89.3, p < 0.001) and females (edf 1.88, χ² = 23.9, p < 0.001), showing greater non-linearity in females.

Study Design

Type

Observational (n=5,277)

Multicenter

No

Structured PICO

P

Population

5,277 adults (median age 70 years, 31% female) in New South Wales, Australia with witnessed non-traumatic out-of-hospital cardiac arrest and bystander CPR between January 2017 and March 2022.

O

Outcome

Presenting rhythm dichotomised as shockable (ventricular fibrillation [VF] or pulseless ventricular tachycardia [pVT]) versus non-shockable, determined from the monitor defibrillator when first applied to the chest by EMS, and model-estimated probability of an initial shockable rhythm at the time of collapse (t = 0).surrogate

Main Result

Absolute Event Rate: 61% vs 41%

p-value: p=<0.001

Generalised additive modelling indicates that the true incidence of initial shockable rhythms at the moment of out-of-hospital cardiac arrest collapse is likely 17–18 percentage points higher in both sexes than what is subsequently recorded upon EMS arrival, and varies non-linearly by age and sex.

Limitations

Initial cardiac rhythm at collapse was not directly observed and was estimated from the relationship between elapsed time (000 call to EMS arrival) and EMS-documented rhythms.
Observations close to response time zero were sparse.
Residual confounding may exist as transitions from VF/pVT to non-shockable rhythms may be influenced by unmeasured factors such as bystander CPR quality and underlying patient characteristics.
Estimated probabilities reflect population-level patterns within a single EMS system and may not represent individual-level risk or generalize to other cohorts without external validation.
Initial cardiac rhythm was not directly observed at collapse, but estimated from the relationship between elapsed time and rhythms documented after EMS arrival
Observations close to response time zero were sparse
Transition from VF/pVT to a non-shockable rhythm may be influenced by unaccounted factors such as bystander CPR effectiveness, resuscitation interventions, and underlying patient characteristics
Estimated probabilities describe population-level patterns rather than individual-level clinical predictions and require validation in independent cohorts

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Implication

Shockable rhythm probability at collapse exceeds observed EMS rates; leaves open whether faster defibrillation improves survival in witnessed OHCA.

Abstract

Background Out of hospital cardiac arrest (OHCA) remains a leading cause of death worldwide with survival strongly dependent on the presence of a shockable rhythm at the time of first rhythm analysis. Thus, determinants of a shockable rhythm in cardiac arrest is important. Methods We analysed adult non-traumatic witnessed OHCA in New South Wales from 2017–2022 with bystander CPR. We used generalised additive modelling (GAM) to estimate the probability that an OHCA in this cohort was initially shockable and to characterise how this probability varied according to age and sex. Results NSW Ambulance attended 45,607 during the period January 2017 through March 2022 of which 5,277 met eligibility criteria. The GAM identified significant sex-specific relationships between age, elapsed time and the probability of a shockable rhythm. The relationship between age and shockable rhythm was non-linear in males (edf 4.14, χ² = 167.2, p < 0.001), whereas the relationship was approximately linear in females (edf 1.00, χ² = 51.8, p < 0.001). Elapsed time was significantly associated with shockable rhythm in both males (edf 1.00, χ² = 89.3, p < 0.001) and females (edf 1.88, χ² = 23.9, p < 0.001), with greater evidence of non-linearity among females. At the estimated time of collapse (t = 0), the GAM estimated a probability of a shockable rhythm of 41% (95% CI 27–55%) among females, representing a 17% increase compared with the observed presenting shockable rhythm rate of 24%. Among males, the corresponding estimated probability was 61% (95% CI 56–67%), representing an 18% increase compared with the observed presenting rate of 43%. Conclusions Our modelling suggests the proportion of arrests initially presenting as shockable may be substantially higher than that observed at first EMS rhythm analysis. We conclude that presenting rhythm in OHCA is a dynamic, time-dependent phenomenon and demonstrates the potential value of GAM-based modelling for estimating the probability of a shockable rhythm at or near the time of collapse. These findings provide a more nuanced framework for understanding age and sex differences in OHCA and highlight the importance of considering the timing of rhythm assessment when interpreting epidemiological differences in shockable rhythm.

Discussion

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Cite This Study

Burns et al. (2026) conducted an observational in Out-of-hospital cardiac arrest (OHCA) (n=5,277). Age, sex, and elapsed time from emergency call to EMS arrival (modeled at time of collapse, t=0) vs. Observed rhythm at first EMS assessment (and male vs female sex) was evaluated on Estimated probability of a shockable rhythm (VF/pVT) at the time of collapse (t=0) vs observed presenting shockable rhythm at EMS arrival (95% CI 56-67% (males); 27-55% (females), p=<0.001). Generalised additive modelling estimated shockable rhythm probability at collapse as 61% in males and 41% in females, vs 43% and 24% observed at EMS arrival.

synapsesocial.com/papers/6ae682a3e432e7a5c4ef3670 https://doi.org/10.21203/rs.3.rs-11069706/v1

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Beyond the Presenting Rhythm: Estimating Shockable Rhythm at the Time of Cardiac Arrest in Out-of-Hospital Cardiac Arrest

Key Points

Study Design

Structured PICO

Main Result

Limitations

Ask about this paper

Implication

Abstract

Discussion

Cite This Study

Beyond the Presenting Rhythm: Estimating Shockable Rhythm at the Time of Cardiac Arrest in Out-of-Hospital Cardiac Arrest

Key Points

Study Design

Structured PICO

Main Result

Limitations

Ask about this paper

Implication

Abstract

Discussion

Cite This Study

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