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 du ring variable ventilation and oxygenation. Design. Prospective study. Setting. Research laboratory in a hospital. Subjects. Seven lambs (5.5 +/- 0.6 kg, mean +/- so). Interventions. The lambs initially were anesthetized by 50 mg/kg ketamine i ntravenously. After tracheostomy, the lambs were mechanically ventilated an d paralyzed by using I mg/kg vecuronium bromide followed by 0.1 mg.kg(-1).h r(-1). One femoral vein was cannulated with a pulmonary artery flotation ca theter and used for cardiac output and pulmonary artery pressure measuremen ts. A femoral artery was cannulated for measuring mean arterial blood press ure, measuring heart rate, and blood sampling for gas exchange analyses. Fi nally, the right internal jugular vein and carotid artery were cannulated a nd used for the AV-ECMO. Normothermia (38 +/- 0.5 degreesC), fluid balance (5 mL.kg(-1).hr(-1) normal saline), and anesthesia (5 mg.kg(-1).hr(-1), int ravenous ketamine) were maintained. Ventilator settings were adjusted to es tablish a baseline PaC0(2) (25-35 mm Hg) at an FIO2 of 0.4. The AV-ECMO cir cuit was established by using a hollow fiber oxygenator, primed with matern al 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 minu te volume was maintained during stepwise increases In arteriovenous shunt. A significant increase in endogenous cardiac output occurred at arterioveno us shunt of 25 and 40 mL.kg(-1).hr(-1) (analysis of variance followed by Tu key-Kramer multiple comparisons test), which was attributed to a significan t increase of 30% in the heart rate. Effective cardiac output (difference b etween the thermodilution value and the AV-ECMO flow rate) and mean arteria l blood pressure were not significantly changed. CO2 removal, measured at 1 5% arteriovenous shunt, was significantly increased with decreasing ventila tion to 25% and 50% of the baseline (analysis of variance and Tukey-Kramer test). Oxygenation through the membrane was measured after reducing inspire d FIO2 from 0.4 to 0.21, 0.15, and 0.10 with 15% arteriovenous shunt and ba seline minute ventilation. Oxygen delivery by the oxygenator was significan tly increased at FIO2 Of 0.10, providing a maximum of 19.5% of the total ox ygen consumption at an arterial hemoglobin-oxygen saturation of 60%. Conclusions. Healthy lambs are capable of maintaining effective cardiac out put in the presence of moderate arteriovenous shunts (15%). AV-ECMO may pro vide efficient ventilatory support in the neonatal population with hypercap nia. The amount of oxygen delivery with AV-ECMO depends on arterial desatur ation.