Relationship between blood pressure and cardiac output in preterm infants requiring mechanical ventilation

Objective: To assess the contribution of cardiac output in determining the blood pressure of preterm infants and to identify other factors that may be important. Methods: Sixty-seven preterm infants requiring mechanical ventilation (median birth weight, 1015 gm; median gestational age, 28 weeks) underwent an echocardiographic study at an average age of 19 hours (range, 7 to 31 hours). Measurements taken included left ventricular ejection fraction, left and right ventricular outputs by means of pulsed Doppler and the diameter of both the ductal and atrial shunt jets with the use of color Doppler as a measure of the size of shunt, Simultaneous measurements of intraarterial blood pressures, mean airway pressure, and inspired fraction of oxygen were recorded. Results: After we allowed for the influence of ductal shunting, the correlation between the left ventricular output and mean arterial blood pressure was significant but weak (r = 0.38), There were infants with low blood pressures and normal cardiac outputs, and conversely there were infants with low cardiac outputs and normal blood pressure. The infants with a mean arterial blood pressure of less than 30 mm Hg had a significantly lower gestational age (27 vs 28 weeks), higher mean airway pressure (9.0 vs 7.0 cm H2O), larger ductal diameter (1.6 mm vs 0.7 mm) and a lower systemic vascular resistance (163 vs 184 mm Hg/L per minute per kilogram of body weight). Multilinear regression identified higher mean airway pressure and larger ductal diameter as significant negative influences on mean arterial blood pressure, with higher gestational age and higher left ventricular output as significant positive influences. Conclusions: Normal blood pressure cannot necessarily be equated with normal systemic flow. These data emphasize the importance of other influences, and in particular that of varying systemic vascular resistance, in the determination of blood pressure in preterm infants.