Capnometry after an inspiratory breath hold, PLAT CO2 , as a surrogate for P aCO 2 ${P}_{{\mathrm{aCO}}_{2}}$ in mild to moderate pediatric acute respiratory distress syndrome: A feasibility study.

OBJECTIVE: Accurate and reliable noninvasive methods to estimate gas exchange are necessary to guide clinical decisions to avoid frequent blood samples in children with pediatric acute respiratory distress syndrome (PARDS). We aimed to investigate the correlation and agreement between end-tidal P CO 2 ${P}_{{\mathrm{CO}}_{2}}$ measured immediately after a 3-s inspiratory-hold (PLAT CO2 ) by capnometry and P aCO 2 ${P}_{{\mathrm{aCO}}_{2}}$ measured by arterial blood gases (ABG) in PARDS. DESIGN: Prospective cohort study. SETTING: Seven-bed Pediatric Intensive Care Unit, Hospital El Carmen de Maipú, Chile. PATIENTS: Thirteen mechanically ventilated patients aged ≤15 years old undergoing neuromuscular blockade as part of management for PARDS. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: All patients were in volume-controlled ventilation mode. The regular end-tidal P CO 2 ( P ETCO 2 ) ${P}_{{\mathrm{CO}}_{2}}({P}_{{\mathrm{ETCO}}_{2}})$ (without the inspiratory hold) was registered immediately after the ABG sample. An inspiratory-hold of 3 s was performed for lung mechanics measurements, recording P ETCO 2 ${P}_{{\mathrm{ETCO}}_{2}}$ in the breath following the inspiratory-hold. (PLAT CO2 ). End-tidal alveolar dead space fraction (AVDSf) was calculated as [ ( P aCO 2 - P ETCO 2 ) / P aCO 2 ] $[({P}_{{\mathrm{aCO}}_{2}}\mbox{--}{P}_{{\mathrm{ETCO}}_{2}})/{P}_{{\mathrm{aCO}}_{2}}]$ and its surrogate (S)AVDSf as [ ( PLAT CO 2 - P ETCO 2 ) / PLAT CO 2 ] $[{(}_{\mathrm{PLAT}}{\mathrm{CO}}_{2}\mbox{--}{P}_{{\mathrm{ETCO}}_{2}}){/}_{\mathrm{PLAT}}{\mathrm{CO}}_{2}]$ . Measurements of P aCO 2 ${P}_{{\mathrm{aCO}}_{2}}$ were considered the gold standard. We performed concordance correlation coefficient (ρc), Spearman's correlation (rho), and Bland-Altmann's analysis (mean difference ± SD [limits of agreement, LoA]). Eleven patients were included, with a median (interquartile range) age of 5 (2-11) months. Tidal volume was 5.8 (5.7-6.3) mL/kg, PEEP 8 (6-8), driving pressure 10 (8-11), and plateau pressure 17 (17-19) cm H2 O. Forty-one paired measurements were analyzed. P aCO 2 ${P}_{{\mathrm{aCO}}_{2}}$ was higher than P ETCO 2 ${P}_{{\mathrm{ETCO}}_{2}}$ (52 mmHg [48-54] vs. 42 mmHg [38-45], p < 0.01), and there were no significant differences with PLAT CO2 (50 mmHg [46-55], p > 0.99). The concordance correlation coefficient and Spearman's correlation between P aCO 2 ${P}_{{\mathrm{aCO}}_{2}}$ and PLAT CO2 were robust (ρc = 0.80 [95% confidence interval [CI]: 0.67-0.90]; and rho = 0.80, p < 0.001.), and for P ETCO 2 ${P}_{{\mathrm{ETCO}}_{2}}$ were weak and strong (ρc = 0.27 [95% CI: 0.15-0.38]; and rho = 0.63, p < 0.01). The bias between PLAT CO2 and P aCO 2 ${P}_{{\mathrm{aCO}}_{2}}$ was -0.4 ± 3.5 mmHg (LoA -7.2 to 6.4), and between P ETCO 2 ${P}_{{\mathrm{ETCO}}_{2}}$ and P aCO 2 ${P}_{{\mathrm{aCO}}_{2}}$ was -8.5 ± 4.1 mmHg (LoA -16.6 to -0.5). The correlation between AVDSf and (S)AVDSf was moderate (rho = 0.55, p < 0.01), and the mean difference was -0.5 ± 5.6% (LoA -11.5 to 10.5). CONCLUSION: This pilot study showed the feasibility of measuring end-tidal CO2 after a 3-s end-inspiratory breath hole in pediatric patients undergoing controlled ventilation for ARDS. Encouraging preliminary results warrant further study of this technique.