Monitoring Dead Space in Mechanically Ventilated Children: Volumetric Capnography Versus Time-Based Capnography

BACKGROUND: Volumetric capnography dead-space measurements (physiologic dead-space-to-tidal-volume ratio [V_D/V_T] and alveolar V_D/V_T) are considered more accurate than the more readily available time-based capnography dead-space measurement (end-tidal alveolar dead-space fraction [AVDSF]). We sought to investigate the correlation between volumetric capnography and time-based capnography dead-space measurements.

METHODS: This was a single-center prospective cohort study of 65 mechanically ventilated children with arterial lines. Physiologic V_D/V_T, alveolar V_D/V_T, and AVDSF were calculated with each arterial blood gas using capnography data.

RESULTS: We analyzed 534 arterial blood gases from 65 children (median age 4.9 y, interquartile range 1.7–12.8). The correlation between physiologic V_D/V_T and AVDSF (r = 0.66, 95% CI 0.59–0.72) was weaker than the correlation between alveolar V_D/V_T and AVDSF (r = 0.8, 95% CI 0.76–0.85). The correlation between physiologic V_D/V_T and AVDSF was weaker in children with low P_aO2/F_IO2 (<200 mm Hg), low exhaled V_T (<100 mL), a pulmonary reason for mechanical ventilation, or large airway V_D (>3 mL/kg). All 3 dead-space measurements were highly correlated (r > 0.7) in children without hypoxemia (P_aO2/F_IO2 >300 mm Hg), mechanically ventilated for a neurologic or cardiac reason, or on significant inotropes or vasopressors.

CONCLUSIONS: In mechanically ventilated children without significant hypoxemia or with cardiac output-related dead-space changes, physiologic V_D/V_T was highly correlated with AVDSF and alveolar V_D/V_T. In children with significant hypoxemia, physiologic V_D/V_T was poorly correlated with AVDSF. Alveolar V_D/V_T and AVDSF correlated well in most tested circumstances. Therefore, AVDSF may be useful in most children for alveolar dead-space monitoring.