Flow-to-Compliance Ratio as a Bedside Marker of Strain Rate in Pediatric Acute Respiratory Distress Syndrome: Analysis of Observational Cohort Data From 2022

OBJECTIVES: To determine the relationship between the ratio of peak inspiratory flow and respiratory system compliance (Flow/CRS) and strain rate in cases of pediatric acute respiratory distress syndrome (PARDS). DESIGN: Secondary analysis of observational cohort data collected in 2022. SETTING: Single-center tertiary PICU in Chile. PATIENTS: Thirty-one mechanically ventilated patients with early PARDS, 15 years or younger. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: All patients were supported with volume-controlled ventilation during measurement of pulmonary mechanics, including tidal volume (Vt), inspiratory time (Ti), inspiratory flow, and respiratory system compliance (CRS). End-expiratory lung volume (EELV) was assessed using the nitrogen multiple-breath wash-in/washout method. Lung strain (Vt/EELV), strain rate (lung strain/Ti, s-1), and the Flow/CRS ratio ([L·min-1]/[mL·cm H2O-1]) were calculated. The median (interquartile range [IQR]) strain rate was 0.56 s-1 (IQR, 0.40-0.73 s-1) and the median Flow/CRS ratio was 0.94 (IQR, 0.77-1.20). In the complete dataset, Flow/CRS ratio demonstrated a moderate positive correlation with measured strain rate (Spearman, ρ = 0.42; 95% CI, 0.08-0.67; p = 0.009). Linear regression confirmed a proportional relationship (R2 = 0.17; p = 0.022), but the Bland-Altman analysis showed systematic bias (mean difference, -0.43 ± 0.29 s-1) with 95% limits of agreement from -0.99 to 0.14 s-1. Sensitivity analysis excluding three outliers (leaving data from 28 patients) demonstrated a stronger association (ρ = 0.63; p = 0.0018) and improved model fit (R2 = 0.41; p < 0.001), with the following equation: strain rate = 0.61·(Flow/CRS) + 0.01. CONCLUSIONS: In PARDS, the Flow/CRS ratio is associated with measured strain rate but exhibits systematic bias, precluding direct interchangeability. Flow/CRS may serve as a pragmatic bedside marker reflecting deformation-velocity behavior. Prospective validation in larger and more heterogeneous cohorts is required.