Effect of stepwise reduction in minute ventilation on PaCO2 in ventilated newborns

To study the effect of step reduction of expired minute ventilation (MV) on PaCO2 in ventilated newborns and to determine whether MV within a defined range can predict PaCO2.

Prospective descriptive.

Referral neonatal unit of a teaching hospital.

Forty neonates stable on mechanical ventilation receiving minute ventilation in the range of 150-210 ml/kg/min. were studied. The spectrum of disorders for which the babies were ventilated included apnea of prematurity in 16, pneumonia in 14, meconium aspiration syndrome in 6 and hyaline membrane disease in 4. Median age at study was 6 days and median weight at study was 2.1 kgs. The MV was reduced from 210 to 150 mL/kg/min in three steps and concomitant PaCO2 was measured. Reductions were not done if PaCO2 was more than 50 mmHg. MVs were plotted against PaCO2 and a regression equation to predict PaCO2 from MV was calculated.

A stepwise increase was seen in CO2 with reduction of MV over the range studied. The median MV and median PaCO2 achieved in the three steps were 201 mL/kg/min and 36.7 mm of Hg, 180 mL/kg/min and 41.7 mm of Hg, 160 mL/kg/min, and 44.3 mm of Hg. The regression equation to predict PaCO2 was PaCO2 = 70 - 0.17 x MV in mL/kg/min, r = -0.45, r2 = 0.20, residual variance (s2) = 39.37; gave a predicted PaCO2 within 12.5 mmHg. for a given MV.

Reducing minute ventilation led to an increase in the levels of PaCO2. Minute volumes of 160 ml/kg/min correlated with PaCO2 value of 44.3 mm of Hg. MV as low as 160 mL/kg/min are well tolerated by newborns.