Statistical Note: Confounding and Causality in Observational Studies

Editor's Choice Articles for September

The September 2023 issue and this year has already proven to be important for improving our understanding of pediatric acute respiratory distress syndrome (PARDS); Pediatric Critical Care Medicine (PCCM) has published 16 articles so far. Therefore, my three Editor's Choice articles this month highlight yet more PCCM material about PARDS by covering the use of noninvasive ventilation (NIV), the trajectory in cytokine profile during illness, and a new look at lung mechanics. The PCCM Connections for Readers give us the opportunity to focus on some clinical biomarkers of severity and mortality risk during critical illness.

Early combination therapy with immunoglobulin and steroids is associated with shorter ICU length of stay in Multisystem Inflammatory Syndrome in Children (MIS-C) associated with COVID-19: A retrospective cohort analysis from 28 U.S. Hospitals

Objectives

Suggested therapeutic options for Multisystem Inflammatory Syndrome in Children (MIS‐C) include intravenous immunoglobulins (IVIG) and steroids. Prior studies have shown the benefit of combination therapy with both agents on fever control or the resolution of organ dysfunction. The primary objective of this study was to analyze the impact of IVIG and steroids on hospital and ICU length of stay (LOS) in patients with MIS‐C associated with Coronavirus Disease 2019 (COVID‐19).

Study Design

This was a retrospective study on 356 hospitalized patients with MIS‐C from March 2020 to September 2021 (28 sites in the United States) in the Society of Critical Care Medicine (SCCM) Discovery Viral Infection and Respiratory Illness Universal Study (VIRUS) COVID‐19 Registry. The effect of IVIG and steroids initiated in the first 2 days of admission, alone or in combination, on LOS was analyzed. Adjustment for confounders was made by multivariable mixed regression with a random intercept for the site.

Results

The median age of the study population was 8.8 (Interquartile range (IQR) 4.0, 13) years. 247/356 (69%) patients required intensive care unit (ICU) admission during hospitalization. Overall hospital mortality was 2% (7/356). Of the total patients, 153 (43%) received IVIG and steroids, 33 (9%) received IVIG only, 43 (12%) received steroids only, and 127 (36%) received neither within 2 days of admission. After adjustment of confounders, only combination therapy showed a significant decrease of ICU LOS by 1.6 days compared to no therapy (exponentiated coefficient 0.71 [95% confidence interval 0.51, 0.97, p = 0.03]). No significant difference was observed in hospital LOS or the secondary outcome variable of the normalization of inflammatory mediators by Day 3.

Conclusions

Combination therapy with IVIG and steroids initiated in the first 2 days of admission favorably impacts ICU but not the overall hospital LOS in children with MIS‐C.

Association of Arterial Hyperoxia With Outcomes in Critically Ill Children: A Systematic Review and Meta-analysis

IMPORTANCE

Oxygen supplementation is a cornerstone treatment in pediatric critical care. Accumulating evidence suggests that overzealous use of oxygen, leading to hyperoxia, is associated with worse outcomes compared with patients with normoxia.

OBJECTIVES

To evaluate the association of arterial hyperoxia with clinical outcome in critically ill children among studies using varied definitions of hyperoxia.

DATA SOURCES

A systematic search of EMBASE, MEDLINE, Cochrane Library, and ClinicalTrials.gov from inception to February 1, 2021, was conducted.

STUDY SELECTION

Clinical trials or observational studies of children admitted to the pediatric intensive care unit that examined hyperoxia, by any definition, and described at least 1 outcome of interest. No language restrictions were applied.

DATA EXTRACTION AND SYNTHESIS

The Meta-analysis of Observational Studies in Epidemiology guideline and Newcastle-Ottawa Scale for study quality assessment were used. The review process was performed independently by 2 reviewers. Data were pooled with a random-effects model.

MAIN OUTCOMES AND MEASURES

The primary outcome was 28-day mortality; this time was converted to mortality at the longest follow-up owing to insufficient studies reporting the initial primary outcome. Secondary outcomes included length of stay, ventilator-related outcomes, extracorporeal organ support, and functional performance.

RESULTS

In this systematic review, 16 studies (27 555 patients) were included. All, except 1 randomized clinical pilot trial, were observational cohort studies. Study populations included were post-cardiac arrest (n = 6), traumatic brain injury (n = 1), extracorporeal membrane oxygenation (n = 2), and general critical care (n = 7). Definitions and assessment of hyperoxia differed among included studies. Partial pressure of arterial oxygen was most frequently used to define hyperoxia and mainly by categorical cutoff. In total, 11 studies (23 204 patients) were pooled for meta-analysis. Hyperoxia, by any definition, showed an odds ratio of 1.59 (95% CI, 1.00-2.51; after Hartung-Knapp adjustment, 95% CI, 1.05-2.38) for mortality with substantial between-study heterogeneity (I2 = 92%). This association was also found in less heterogeneous subsets. A signal of harm was observed at higher thresholds of arterial oxygen levels when grouped by definition of hyperoxia. Secondary outcomes were inadequate for meta-analysis.

CONCLUSIONS AND RELEVANCE

These results suggest that, despite methodologic limitations of the studies, hyperoxia is associated with mortality in critically ill children. This finding identifies the further need for prospective observational studies and importance to address the clinical implications of hyperoxia in critically ill children.