In vitro validation of a metabolic monitor for gas exchange measurements in ventilated neonates

Objective: To evaluate the Datex Deltatrac II for measurements in neonates requiring mechanical ventilation. Design: Prospective laboratory evaluation, using a ventilated lung model and gas injection. During simulation of 79 neonatal respiratory settings, assessment of oxygen consumption (V(overdot)O-2), carbon dioxide production (V(overdot)CO2) and respiratory quotient (RQ) was compared to a reference method (mass spectrometry, wet gas spirometry) using the statistical method of Bland and Altman. Interventions: Respiratory variables, which may influence the accuracy and precision of gas exchange measurements, were varied within the following ranges: inspired oxygen fraction (FIO2): 0.21-0.8, expired carbon dioxide fraction ((FECO2) over bar) and inspiratory-expiratory oxygen fraction (DFO2): 0.0032-0.0256, expiratory flow rate: 1.0-2.5 l/min, inspiratory pressure: 10-55 mbar, respiratory rate 25-60/min, constant RQ of 1. This resulted in 79 tests with V(overdot)CO2 and V(overdot)O-2 ranging from 8-64 ml/min. Measurements and results. The coefficient of repeatability for ten single subsequent Deltatrac measurements was 8.09 ml/min for V(overdot)O-2 and 9.17 ml/min for V(overdot)CO2 compared to 2.02 ml/min and 0.90 ml/min for V(ovedot)O-2, and V(overdot)CO2 with repeated reference measurements. The coefficient of repeatability of the Deltatrac measurements improved considerably when means of subsequent 5 min intervals were compared: 0.68 ml/min for V(overdot)O-2 and 0.28 ml/min for V(overdot)CO2. The difference between the two methods (Deltatrac-reference) was -3.8% (2 s: 11.4%) for V(overdot)O-2, 13.2 % (2s: 7.9%) for V(overdot)CO2 and 17.6% (2 s: 16.7%) for RQ. The agreement between methods deteriorated with smaller ((FECO2) over bar or DFO2 and increasing FIO2. Conclusions: Considering limits of agreement of less than +/- 20% as clinically acceptable, results for V(overdot)O-2 assessment indicate acceptable accuracy and precision whereas V(overdot)CO2 and RQ assessments exceed this limit. Limited accuracy and precision result from detection of CO2 following dilution of expiratory gases and increased sensitivity to error propagation by Haldane equations due to the small differences between inspiratory and expiratory gas fractions.