Purpose: An oxygenator with low resistance may be perfused by the native cardiac output (CO) and serve as a Total Artificial Lung (TAL). However, the absence of compliance may increase pump load in a pulsatile system. Impedance is the resistance to pulsatile flow, and is expressed as a function of harmonic. While the zero harmonic impedance (Z9) represents the resistance to steady flow, the first harmonic (Z1) dominates the load to pulsatile flow. The tension-time index (TTI) is a correlate of myocardial oxygen consumption. We sought to characterize right ventricular (RV) load and adaptation in the setting of pulmonary replacement with a TAL. Methods: Five adult male sheep were instumented with micromanometers in the RV, proximal pulmonary artery (PA), and left atrium (LA), and an ultrasonic flow probe was placed around the proximal PA. Conduits were anastamosed to the PA and LA, and connected to a TAL. Data were obtained under two conditions: 1) conduits clamped, allowing native pulmonary circulation, and 2) distal PA occluded, diverting the total RV output through the TAL. Epicardial and intracardiac echocardiographs were obtained. The pulsatility index (PI) was calculated by dividing the amplitude of flow in the first harmonic by the CO. Results: Although CO, mean PA pressure (mPAP) and resistance were unchanged (CO: 6.2 ± 1.5 vs 5.6 ± 0.9 L/min, p=NS; mPAP: 18.5 ± 3.6 vs 18.1 ± 3.2 mmHg, p=NS; Z0: 3.1 ± 0.6 vs 3.3 ± 0.5 Woods Units, p=NS), there was a substantial increase in Z1 (0.4 ± 0.1 vs 5.8 ± 1.4 Woods Units, p<0.01). This elevated impedance was associated with an increase in the TTI from 843.5 ± 113.3 to 1032.1 ± 224.3 mmHg-s/min, p=0.04. Blood flow ejected from the right ventricle became less pulsatile, reflected by a reduction in the PI from 0.64 ± 0.12 to 0.23 ± 0.09, p<0.01. Echocardiographs suggested incomplete closure of the pulmonic valve with forward diastolic flow. In the setting of pulmonary replacement with a TAL, high impedance with unchanged resistance alters cardiac function, and may have long-term effects upon the RV. Addition of a compliance chamber can reduce Z1, and may restore normal ventricular function.