Capnography in Pediatric Critical Care Unit and Correlation of End-Tidal and Arterial Carbon Dioxide in Ventilated Children

Abstract

Recording of end-tidal carbon dioxide (EtCO₂) noninvasively reflects a real-time estimation of arterial carbon dioxide (PaCO₂ [partial pressure of CO₂]). However, as the EtCO₂ is dependent on metabolism, perfusion, and ventilation, predicting PaCO₂ from EtCO₂ is not linear. The objective of the study was to find out the predictability of PaCO₂ from EtCO₂ in PICU and to evaluate the factors affecting the correlation of EtCO₂ and PaCO₂ in critically ill ventilated children. The design involved was prospective observational study. The setting discussed over here is that of pediatric intensive care unit (PICU) of tertiary care hospital. A total of 160 children between 1 month and 14 years received mechanical ventilation. EtCO₂, PaCO₂, PaO₂/FiO₂ (PF) ratio, oxygenation index (OI), and ventilation index (VI) are the factors involved in main outcome measures. A total of 535 pairs of EtCO₂ and PaCO₂ were recorded in 160 ventilated children during the stable hemodynamic state. Mean age and weight (Z-score) of patients were 31.15 ± 40.46 months and −2.10 ± 1.58, respectively. EtCO₂ and PaCO₂ differences were normal (2–5 mm of Hg) in 393 (73.5%) pairs. High gradient (>5 mm of Hg) was mostly found with children with pneumonia, prolonged ventilation, and pressure mode of ventilation (p < 0.05). EtCO₂ had a strong positive correlation with PaCO₂ (r = 0.723, 95% confidence interval [CI] = 0.68 and 0.76) and not significantly affected by PF ratio or OI. However, presence of pneumonia and high ventilation index (VI > 20) adversely affected the relationship with poor correlation coefficient (r = 0.449, 95% CI = 0.30, 0.58 and r = 0.227, 95% CI = 0.03, 0.41, respectively). EtCO₂ reading showed good validity to predict PaCO₂ and not affected by oxygenation parameters. The correlation was affected by the presence of pneumonia and high ventilation index; hence it is recommended to monitor PaCO₂ invasively in these patients till a good correlation is established.

Keywords

capnography - end-tidal carbon dioxide - PaCO₂ - pediatric intensive care unit - arterial blood gas

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