Changes in stroke volume cause change in cardiac output in neonates and infants when mean airway pressure is altered

Background: Based on early studies in the lamb, and in spite of more recent studies in humans, it has been the received opinion that neonates and infants can not change their stroke volume significantly, but are mainly dependent on changes in heart rate, to change cardiac output. To further evaluate the relationship between cardiac output and stroke volume during mechanical ventilation of neonates and infants, we have studied the effects on cardiac output and stroke volume by two different ways of changing mean airway pressure. Methods: In one group, mean airway pressure was decreased by using a patient triggered mode: pressure support ventilation; in the other, mean airway pressure was increased by increasing positive end-expiratory pressure (PEEP). Changes in cardiac output, heart rate and stroke volume were assessed with the Doppler technique, measuring blood flow velocity in the ascending aorta. Results: Without a significant change in heart rate, we found a significant increase in cardiac output of +16 +/- 2% (P<0.01) with a decrease in mean airway pressure and a decrease in cardiac output of -13 +/- 4%, (P<0.02) with an increase in mean airway pressure, depicting a change in stroke volume of +17 +/- 2% (P<0.02) and -14 +/- 5%, (P<0.01) respectively. Conclusions: We conclude that neonates and infants are able to regulate cardiac output by changing the stroke volume to a greater extent than presumed, at least when cardiac output is influenced by changes in the mean airway pressure.