Longitudinal co-variation of radiographic chest-wall thickness and transcutaneous–arterial CO₂ discrepancy in congenital diaphragmatic hernia: a single-patient case study

Background Transcutaneous carbon dioxide (tcPCO₂) monitoring enables continuous non-invasive assessment of ventilation in neonates, but local tissue conditions may contribute to disagreement between tcPCO₂ and arterial PaCO₂ measurements. We explored within-patient temporal co-variation among radiographic chest-wall thickness, sensor heating power, and the discrepancy between tcPCO₂ and arterial PaCO₂ in a critically ill neonate. Methods We conducted an exploratory single-patient longitudinal case study over 28 days. Serial measurements of tcPCO₂, arterial PaCO₂, radiographic chest-wall thickness ratio (%skin), an unvalidated radiographic measure explored as a potential surrogate of tissue edema, sensor heating power, and daily fluid balance were evaluated. Both the signed difference (tcPCO₂−PaCO₂) and absolute difference |tcPCO₂−PaCO₂| were summarized descriptively. Temporal trajectories and within-patient associations were explored with attention to serial dependence and shared time trends. Results Increasing radiographic chest-wall thickness coincided with greater tcPCO₂–PaCO₂ discrepancy and higher sensor heating power. The three variables showed broadly similar rise-and-fall temporal trajectories. Unadjusted within-patient correlations were strong ( r = 0.766–0.813), and residual correlations persisted but were attenuated after removal of the quadratic Day/Day 2 trend (residual r = 0.545–0.625). These residual correlations were interpreted descriptively and not as evidence of a time-independent relationship. PCA identified a dominant principal component explaining 85.9% of total variance, descriptively summarizing shared temporal covariance among radiographic chest-wall thickness, sensor heating power, and tcPCO₂–PaCO₂ discrepancy. Conclusion In this single-patient longitudinal case study, radiographic chest-wall thickness, sensor heating power, and tcPCO₂–PaCO₂ discrepancy showed parallel temporal changes. These findings generate the hypothesis that changes in local tissue conditions associated with increased radiographic chest-wall thickness may contribute to tcPCO₂–PaCO₂ disagreement, but causal inference is not possible and validation in multi-patient studies is required.