Body mass index and diaphragm dysfunction in critically ill adults: a prospective cohort study

Diaphragm dysfunction (DD) may be present at ICU admission or be detected during subsequent critical care. Body mass index (BMI) influences respiratory loading and diaphragm morphology, yet it has usually been treated as a descriptive characteristic or adjustment variable rather than as the exposure of primary interest. We therefore examined the association between BMI and protocol-defined DD in critically ill adults. This prospective cohort study consecutively enrolled 221 adult ICU patients between January and June 2023. BMI was analyzed primarily as a continuous exposure; World Health Organization BMI categories were examined as exploratory analyses. The primary endpoint was DD detected at one or more scheduled ICU assessments, including day 1, defined as quiet-breathing diaphragm excursion ≤ 10 mm on bedside ultrasound. Logistic regression estimated odds ratios (ORs) and 95% confidence intervals (CIs), with adjustment for age, sex, Charlson comorbidity index, SOFA score, mechanical ventilation, and surgery. Overall, 126 of 221 patients (57.0%; 95% CI 50.4%–63.4%) had DD detected at least once. The proportions with DD were 0.0% (95% CI 0.0%–43.4%) among 5 underweight patients, 48.0% (95% CI 38.3%–57.7%) among 98 normal-weight patients, 68.1% (95% CI 58.1%–76.6%) among 94 overweight patients, and 62.5% (95% CI 42.7%–78.8%) among 24 patients with obesity. Patients with DD had higher BMI than those without DD [25.40 (23.70–28.30) vs. 24.50 (21.50–26.40) kg/m²; P = 0.0018]. Each 1-kg/m² increase in BMI was associated with higher odds of DD in univariable analysis (OR = 1.116, 95% CI 1.034–1.204) and after prespecified adjustment (adjusted OR = 1.126, 95% CI 1.040–1.218). Higher BMI was associated with greater odds of DD being detected during scheduled ICU assessments. Because the endpoint included DD present on day 1 and the study was observational, these findings should not be interpreted as showing that BMI causes incident DD. BMI may nevertheless identify patients in whom closer, context-aware respiratory-muscle assessment is warranted.