Left-digit bias in outcomes and invasive treatments among out-of-hospital cardiac arrest patients with shockable rhythm: a nationwide multicenter observational study with regression discontinuity design analysis

Left-digit bias (LDB) occurs when the leftmost digit of a number disproportionately influences decision-making, creating discontinuities at round-number thresholds. In healthcare setting, patients aged 79 are perceived as “in their 70s” while those aged 80 are seen as “in their 80s,” despite minimal actual differences. Previous studies documented LDB across medical specialties, such as myocardial infarction treatment decisions [1]. However, studies in cardiac arrest populations showed negative results, with no significant LDB found in either in-hospital or witnessed out-of-hospital cardiac arrest (OHCA) [2, 3]. These negative findings might reflect the heterogeneity of cardiac arrest populations and the protocolized nature of pre-hospital care, which can mask biases. Additionally, in-hospital decisions about resource-intensive interventions like extracorporeal cardiopulmonary membrane oxygenation (ECMO) involve greater physician discretion under time pressure—conditions that can reveal LDB effect. This study investigated whether LDB affects clinical outcomes and invasive treatment implementation in OHCA patients with shockable rhythms (ventricular fibrillation or pulseless ventricular tachycardia) using the OHCA registry containing in-hospital data in Japan.

We conducted a secondary analysis of the JAAM-OHCA Registry, a prospective multicenter observational cohort involving approximately 140 hospitals across Japan [4]. Further details of the registry and the present analysis were described in the Supplemental Materials. We included adult patients (≥ 18 years) enrolled between June 2014 and December 2020 with shockable initial rhythm documented by emergency medical services. Patients with transport time exceeding 121 min were excluded. Primary outcome was favorable neurological outcome at 1 month (Cerebral Performance Category 1–2). Secondary outcomes included 1-month survival and implementation of extracorporeal membrane oxygenation (ECMO), targeted temperature management (TTM), and coronary angiography (CAG).

Regression discontinuity design (RDD) identified potential discontinuities at ages 60, 70, and 80. RDD exploits quasi-random assignment of patients just below and above thresholds—patients aged 69 and 70 should be similar except for age perception. We implemented local linear regression with triangular kernel weighting, adjusting for sex, witness status, bystander cardiopulmonary resuscitation provision, bystander automated external defibrillator use, physician-staffed pre-hospital care involvement, transport to a tertiary care center, interval from emergency call to hospital arrival, advanced airway management, initial rhythm upon hospital arrival, weekend admission, and nighttime admission.

Among 72,041 OHCA patients, 5,943 met inclusion criteria (eFigure 1 in the Supplemental Materials). The mean age was 64.4 ± 15.5 years (18–59 years: 35.1%; 60–69 years: 23.7%; 70–79 years: 24.2%; ≥80 years: 16.9%). The majority were male (79.5%) with witnessed arrests (74.6%). Overall favorable neurological outcomes occurred in 23.6%, declining from 33.2% (ages 18–59) to 9.8% (≥ 80 years). One-month survival showed similar age-related decline (42.5–17.5%). ECMO utilization decreased from 39.4% (ages 18–59) to 7.7% (≥ 80 years) (eTable 1 in the Supplemental Materials). RDD revealed no significant discontinuities in neurological outcomes at any threshold: adjusted differences were − 5.0% (95%CI: -11.5% to + 1.6%, P = 0.14) at age 60; -2.8% (95%CI: -7.9% to + 2.4%, P = 0.29) at age 70; +8.6% (95%CI: -10.6% to + 18.3%, P = 0.08) at age 80. One-month survival showed no discontinuities (Table 1). On the other hand, ECMO implementation demonstrated significant discontinuity at age 70, dropping from 34.2 to 24.5% (adjusted difference − 9.7%, 95%CI: -18.2% to -1.2%, P = 0.03) (Fig. 1). No significant discontinuities appeared at ages 60 or 80. TTM and CAG showed no LDB at any threshold.

Table 1 Adjusted regression discontinuity analyses of outcomes at each age threshold

Full size table

Fig. 1

Threshold at age 70 and Implementation of Extracorporeal Membrane Oxygenation After Shockable Out of Hospital Cardiac Arrest: Regression Discontinuity Plots. Illustrating the proportion of patients who received extracorporeal membrane oxygenationagainst their exact age. Each blue dot represents oneyear age bin; red lines are separatelocal linear regressions estimated on either side of the cutoff with triangular kernelweighting, and the vertical black line marks the threshold

Full size image

This discontinuity likely reflects ECMO’s unique decision-making context—extreme resource intensity, specialized requirements, and time pressure promote cognitive heuristics. The age-70 threshold may anchor to Extracorporeal Life Support Organization consensus listing age < 70 as inclusion criteria [5]. Paradoxically, despite ECMO implementation bias, clinical outcomes showed no corresponding discontinuity at age 70. This suggests differential selection: ECMO was liberally deployed in younger patients, including those with limited recovery potential, while patients over 70 underwent stricter selection. This imbalance could equalize outcomes—younger groups’ results diluted by futile cases, older groups outcomes reflecting only the fittest candidates. Our findings have important implications. First, chronological age alone can poorly discriminate appropriate ECMO candidates. Second, while current practice may achieve reasonable patient selection through different mechanisms across age groups, it potentially excludes elderly patients who could benefit. To address these issues, development and implementation of rapid, multifactorial assessment tools incorporating arrest characteristics, physiological reserve, and early response indicators could help overcome age-related cognitive biases. Future research should explore whether such structured decision protocols can effectively mitigate LDB while maintaining appropriate patient selection, particularly as ECMO technology becomes more widely available. Study limitations include potential unmeasured confounding inherent to observational design and limited generalizability to non-shockable rhythms. The registry’s timeframe (2014–2020) may not reflect current practice patterns, though cognitive biases likely persist.

In conclusion, we identified significant LDB affecting ECMO implementation at age 70 without corresponding neurological outcome differences in shockable OHCA. While cognitive bias influences resource allocation, paradoxically, outcomes remain similar across the age threshold—possibly through differential selection stringency.

Illustrating the proportion of patients who received extracorporeal membrane oxygenation against their exact age. Each blue dot represents oneyear age bin; red lines are separate local linear regressions estimated on either side of the cutoff with triangular kernel weighting, and the vertical black line marks the threshold.

The data that support the findings of this study are available from the JAAM-OHCA registry committee, but restrictions apply to the availability of these data, which were used under license for the current study and so are not publicly available. Data are, however, available from the authors upon reasonable request and with permission from the JAAM-OHCA registry committee.

CAG:

Coronary angiography

CI:

Confidence interval

ECMO:

Extracorporeal membrane oxygenation

LDB:

Left-digit bias

OHCA:

Out-of-hospital cardiac arrest

RDD:

Regression discontinuity design

TTM:

Targeted temperature management

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We appreciate all members and institutions of the JAAM-OHCA Registry for their contribution.

This work was supported by Grants-in-Aid for Scientific Research from The Ministry of Education, Culture, Sports, Science and Technology (grant number 23KK0309 and 24K19500).

Authors and Affiliations

1. Department of Emergency Medicine, Kyoto Prefectural University of Medicine, Kaji-cho 465, Kamigyo-ku, Kyoto, 6028566, Japan

   Yuki Miyamoto, Sho Oka, Tadaharu Shiozumi, Koki Nakada & Tasuku Matsuyama

2. Department of Social Medicine, Division of Environmental Medicine and Population Sciences, Graduate School of Medicine, Osaka University, Suita, Japan

   Tetsuhisa Kitamura & Ling Zha

3. Department of Health Data Science, Tokyo Medical University, Tokyo, Japan

   Sho Komukai

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1. Yuki MiyamotoView author publications

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Contributions

Conceptualization: YM, TS; Methodology: YM, LZ, SK; Data collection: TK, SO, KN, and TM; Formal analysis: YM, SK; Writing - original draft: YM; Writing - review & editing: all authors; Supervision: TK, TM; Funding acquisition: TM. All authors have read and approved the final manuscript.

Corresponding author

Correspondence to Tasuku Matsuyama.

Ethics approval and consent to participate

The protocol was approved by the Ethics Committee of Kyoto University as the corresponding institution (R-1045), and each hospital also approved the JAAM-OHCA Registry protocol, as necessary. The institutional review board at Kyoto Prefectural University of Medicine approved the secondary analysis of de-identified data (Approval ID: ERB-C-650-1).

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Miyamoto, Y., Kitamura, T., Zha, L. et al. Left-digit bias in outcomes and invasive treatments among out-of-hospital cardiac arrest patients with shockable rhythm: a nationwide multicenter observational study with regression discontinuity design analysis. Crit Care 29, 389 (2025). https://doi.org/10.1186/s13054-025-05629-8

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• Received: 18 August 2025

• Accepted: 21 August 2025

• Published: 29 August 2025

• DOI: https://doi.org/10.1186/s13054-025-05629-8

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