Accuracy of an indirect calorimeter for mechanically ventilated infants and children: The influence of low rates of gas exchange and varying Fio2

OBJECTIVETo test the accuracy and validity of the Deltatrac II MBM-200 metabolic monitor for use in mechanically ventilated infants and children in the pediatric intensive care unit. DESIGNLaboratory validation of an indirect calorimeter with a ventilated lung model. The influence of low tidal volumes and low levels of oxygen consumption (o2) and carbon dioxide production (co2) in combination with different levels of inspired oxygen concentrations (Fio2) was investigated. SETTINGUniversity research laboratory. SUBJECTSLow tidal volumes were provided with two intermittent flow types of ventilators, a Servo 300 and a Servo 900C. INTERVENTIONSA butane flame with a o2 approximating 20 mL/min and 40 mL/min was ventilated. To investigate the effect of different levels of Fio2 on the accuracy of o2, co2, and respiratory quotient (RQ), measurements were performed at Fio2 target values of 0.25, 0.40, and 0.60. MEASUREMENTS AND MAIN RESULTSNo significant differences were found between the ventilators regarding o2, co2, and RQ measurements. The mean deviation of o2 increased significantly with increasing Fio2 to −7.98% with a o2 of 21.0 mL/min and to −8.46% with a o2 of 38.9 mL/min (Fio2, 0.558) with a variability (2 sd) of ± 4.86% and ± 6.82%, respectively. The mean deviation and variability of co2 in all tests remained within 8%. The mean deviation of RQ increased significantly with increasing Fio2 to 5.5% with a o2 of 21.0 mL/min and to 5.69% with a o2 of 38.9 mL/min (Fio2, 0.558) with a variability (2 sd) of ± 5.62% and ± 5.76%, respectively. The minute to minute delivered Fio2 fluctuated significantly when increasing the level of Fio2. CONCLUSIONSThe Deltatrac II MBM-200 metabolic monitor appears accurate for low levels of o2 and co2 during mechanical ventilation with Fio2 levels up to 0.390. With increasing Fio2 to 0.558, the increase in deviation of o2 for single measurements can be of clinical relevance for mechanically ventilated infants and children. The increased fluctuation of delivered Fio2 on higher levels of Fio2 is likely the cause of the inaccuracy.