Effects of arteriovenous extracorporeal therapy on hemodynamic stability, ventilation, and oxygenation in normal lambs.

OBJECTIVE: To evaluate hemodynamic stability and gas exchange in a neonatal animal model of pumpless arteriovenous extracorporeal membrane oxygenation (AV-ECMO) with extracorporeal shunt flow of up to 15% of cardiac output during variable ventilation and oxygenation.

DESIGN: Prospective study.

SETTING: Research laboratory in a hospital.

SUBJECTS: Seven lambs (5.5 +/- 0.6 kg, mean +/- sd).

INTERVENTIONS: The lambs initially were anesthetized by 50 mg/kg ketamine intravenously. After tracheostomy, the lambs were mechanically ventilated and paralyzed by using 1 mg/kg vecuronium bromide followed by 0.1 mg.kg(-1).hr(-1). One femoral vein was cannulated with a pulmonary artery flotation catheter and used for cardiac output and pulmonary artery pressure measurements. A femoral artery was cannulated for measuring mean arterial blood pressure, measuring heart rate, and blood sampling for gas exchange analyses. Finally, the right internal jugular vein and carotid artery were cannulated and used for the AV-ECMO. Normothermia (38 +/- 0.5 degrees C), fluid balance (5 mL.kg(-1).hr(-1) normal saline), and anesthesia (5 mg.kg(-1).hr(-1), intravenous ketamine) were maintained. Ventilator settings were adjusted to establish a baseline Paco2 (25-35 mm Hg) at an Fio2 of 0.4. The AV-ECMO circuit was established by using a hollow fiber oxygenator, primed with maternal sheep blood (150-200 mL).

MEASUREMENTS AND MAIN RESULTS: The physiologic effects of the AV-ECMO shunt were evaluated at 15, 25, and 40 mL.kg(-1).hr(-1) ECMO flow, corresponding roughly to 4%, 8%, and 15% of the cardiac output values. The baseline minute volume was maintained during stepwise increases in arteriovenous shunt. A significant increase in endogenous cardiac output occurred at arteriovenous shunt of 25 and 40 mL.kg(-1).hr(-1) (analysis of variance followed by Tukey-Kramer multiple comparisons test), which was attributed to a significant increase of 30% in the heart rate. Effective cardiac output (difference between the thermodilution value and the AV-ECMO flow rate) and mean arterial blood pressure were not significantly changed. CO2 removal, measured at 15% arteriovenous shunt, was significantly increased with decreasing ventilation to 25% and 50% of the baseline (analysis of variance and Tukey-Kramer test). Oxygenation through the membrane was measured after reducing inspired Fio2 from 0.4 to 0.21, 0.15, and 0.10 with 15% arteriovenous shunt and baseline minute ventilation. Oxygen delivery by the oxygenator was significantly increased at Fio2 of 0.10, providing a maximum of 19.5% of the total oxygen consumption at an arterial hemoglobin-oxygen saturation of 60%.

CONCLUSIONS: Healthy lambs are capable of maintaining effective cardiac output in the presence of moderate arteriovenous shunts (15%). AV-ECMO may provide efficient ventilatory support in the neonatal population with hypercapnia. The amount of oxygen delivery with AV-ECMO depends on arterial desaturation.