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Path-specific effects for pulse-oximetry guided decisions in critical care
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Applies a path-specific causal framework (V-specific direct effect) with a doubly robust and self-normalized estimator to measure racial bias mediated by pulse-oximetry discrepancies on invasive ventilation in eICU and MIMIC-IV.
Problem
Existing work shows statistical associations between pulse oximeter bias and ICU outcomes but does not isolate causal pathways of racial disparity mediated only through oximeter discrepancies, and path-specific estimators lack finite-sample guarantees.
Approach
Defines the V-specific Direct Effect (VDE) as a nested-counterfactual path-specific effect with multiple mediators, derives a doubly robust estimator with a self-normalized importance-weighting variant, and provides finite-sample error bounds and asymptotic normality.
Key results
- On eICU, VDE for invasive ventilation rate is -0.21 percentage points (95% CI -0.23 to -0.19); on MIMIC-IV it is -0.46 percentage points (95% CI -0.56 to -0.36), indicating minimal mediated disparity.
- VDE for ventilation duration is 2.3 hours in eICU (95% CI 2.2 to 2.5) and 7.4 hours in MIMIC-IV (95% CI 6.9 to 7.9) for Black vs White patients.
- The self-normalized estimator shows significantly lower variance than the canonical doubly robust estimator in synthetic and semi-synthetic experiments with small samples.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Mediate the race effect on ventilation decisions through pulse-oximetry variables | Invasive ventilation rate for Black versus White patients · eICU and MIMIC-IV | VDE (percentage points): eICU: $-0.21$ percentage points, 95\% CI [$-0.23$ to $-0.19$]; MIMIC-IV: $-0.46$ percentage points, 95\% CI [$-0.56$ to | Mixed |
| Mediate the race effect on ventilation decisions through pulse-oximetry variables | Invasive ventilation duration for Black versus White patients · eICU and MIMIC-IV | VDE (hrs): eICU: $2.3$ hrs, 95\% CI [$2.2$ to $2.5$]; MIMIC-IV: $7.4$ hrs, 95\% CI [$6.9$ to $7.9$] | Mixed |
| Self-normalize the estimated importance-sampling nuisance parameters | Doubly robust estimator · Synthetic and semi-synthetic eICU data | Estimator variance | Helped |
| Ignore the other post-admission mediators W when estimating the discrepancy-mediated effect | Ventilation-rate fairness estimate · eICU and MIMIC-IV | Estimated unfairness | Hurt |
Surprising findings
- Ignoring other post-admission mediators (NIE*) can flip the direction of the estimated disparity, in contrast to prior work relying on single-mediator analyses.
- Racial discrepancies in oximetry have minimal impact on invasive ventilation rates, contrary to prior work, though they are more pronounced for ventilation duration.
Limitations
- Identifiability relies on a specified causal graph; latent common causes between W and V would make the VDE unidentifiable.
- Sparse SaO2 measurements limit matched discrepancy samples and effective sample size.
- Only the first invasive ventilation event is considered and temporality is not explicitly modeled.
Experimental setup
Datasets: eICU Collaborative Research Database MIMIC-IV
Benchmarks: synthetic linear SCM semi-synthetic eICU eICU real-world MIMIC-IV real-world
Metrics: relative error mean and 95% confidence interval over bootstraps VDE, NDE, NIE, TE effect estimates (percentage points, hours
Baselines: non-self-normalized doubly robust estimator NIE* (mediation ignoring other mediators)
Builds on: doubly robust estimation for path-specific effects natural direct and indirect effects structural causal models
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