The action of theophylline on the neonatal heart was studied in 11 clinically stable preterm infants. Mean (±SE) birth weight was 1230±390 g and the gestational age 30.7±2.8 wks. Median postnatal age was 12 days. Indications for theophylline therapy were BPD in 3, extubation in 7, and apnea in 1 neonate. Theophylline was given as IV aminophylline using a loading dose of 6.8 mg/kg and a maintenance dose of 2 mg/kg q 8 h. Cardiac output, stroke volume; and heart rate were measured using a combination of pulsed Doppler ultrasound and M-mode echocardiography before the start, on days 1, 2, 3 and 7, and after discontinuation of the drug. Arterial blood pressure was obtained by Dinamap.
An increase in cardiac output (p<0.01) was found on days 1, 2, 3, and 7. Stroke volume was significantly higher (p<0.02) on days 1, 2, 3, but comparable to pretreatment values by day 7. Heart rate was augmented significantly (p<0.01) throughout the treatment period. Mean arterial blood pressure did not change. All but one of the neonates had theophylline levels between 6 and 13 mg/1.
We conclude that during the first days of theophylline therapy both inotropic and chronotropic effects prevail. At 7 days the inotropic effect has disappeared, whereas the chronotropic effect persists. The metabolic cost of the increased cardiac output during theophylline therapy in the preterm infant deserves further attention.