Respiratory deadspace measurements in neonates during extracorporeal membrane oxygenation

J H Arnold, J E Thompson, P K Benjamin
Critical Care Medicine 1993, 21 (12): 1895-900

OBJECTIVES: To evaluate the bias and precision of a simple, bedside method of quantification of minute CO2 production (study 1) and then apply the technique to measure physiologic deadspace in a group of neonates undergoing extracorporeal membrane oxygenation (ECMO) (study 2).

DESIGN: A prospective, clinical study comparing a simple method of quantifying minute CO2 production with a criterion standard (study 1); a cohort study evaluating the utility of deadspace measurements in neonates undergoing extracorporeal membrane oxygenation (study 2).

SETTING: Tertiary care pediatric intensive care unit (ICU) in a university hospital.

PATIENTS: Study 1: Thirteen neonates (weighing from 2.5 to 6.2 kg) were studied during mechanical ventilation being administered for a variety of respiratory diseases. Study 2: Fifteen neonates with respiratory failure were examined during the course of ECMO. INTERVENTIONS (METHODS): Study 1: The CO2 concentration of expired gas sampled at the exhaust port of the test ventilator was continuously measured and transformed to minute CO2 production by the following formula: (calculated minute CO2 production) = (tension of CO2 in exhaust gas) x 0.0013 x (ventilator pneumotachometer minute volume), where 0.0013 is the conversion factor to express gas tension as fractional volume at standard conditions. Minute CO2 production was measured independently with a previously validated bedside calorimeter and the calculated and measured values were compared. Study 2: The CO2 concentration of expired gas sampled at the exhaust port of the test ventilator was continuously measured and transformed to mixed expired CO2 by the following formula, which corrects for compressible volume measured by the ventilator pneumotachometer: mixed expired CO2 = (tension of CO2 in exhaust gas) x (ventilator pneumotachometer minute volume)/(minute volume at proximal airway). We then utilized the Bohr-Enghoff method to calculate the deadspace/tidal volume ratio: deadspace/tidal volume ratio = (PaCO2--mixed expired CO2 tension)/PaCO2.

MEASUREMENTS AND MAIN RESULTS: Study 1: Calculated minute CO2 production ranged between 11.8 and 38.9 mL/min. When compared with measured minute CO2 production, the bias and precision were -0.8 and 1.7 mL/min, respectively. The mean percent difference for calculated minute CO2 production was 3.7%. Study 2: Deadspace/tidal volume ratio was calculated during temporary separation from ECMO support as the patients demonstrated improvements in gas exchange and lung compliance. Fifty-two measurements were made in 15 patients, and 13 of 15 patients demonstrated a decrease in deadspace/tidal volume ratio during the course of ECMO. The mean decrease was 21% and the decrease was statistically significant.

CONCLUSIONS: Minute CO2 production can be measured simply and accurately, using equipment readily available in most ICU settings. The same method can be utilized to calculate the deadspace/tidal volume ratio, which provides valuable information regarding the gas exchanging efficiency of the neonatal lung during ECMO.

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