Supine Exercise in Hepatopulmonary Syndrome Patients With Orthodeoxia

This is a 1 year randomized crossover controlled trial study of the effect of supine exercise position (intervention arm) compared to the upright exercise position (control arm) within 4 weeks. This is a single-center study conducted at St. Michael's Hospital, Toronto, Ontario. The exercise will be performed at a constant work rate, individualized for each participant. Peak work rate will be calculated using results from the most recent room air 6-minute walk test (6MWT), within the past 6 months. The equation used to estimate peak work rate is: Peak Work Rate = 0.168 x 6MWD (m) - 4.085 (ref Kozu Respirology 2010). The individualized constant work rate will be set at 70-80% of this estimated peak work rate. The main stopping criterion will be the point at which, after standardized encouragement, the subject is unable to continue because of symptoms (i.e. patient does not wish to continue or cannot maintain a minimum peddling frequency of 40 rpm for ≥ 10 seconds). This is defined as the "tolerable limit" (tLIM). Additional safety-related stopping criteria will include: the appearance of life-threatening arrhythmias, a drop in systolic blood pressure by ≥ 10 mm Hg from baseline, or a desaturation below a set point for ≥ 30 s. The set saturation point will be chosen individually for each patient, as the lower of: 80% or the lowest saturation seen on room air 6MWT. Exercise tests in each position, for each subject, will be standardized with respect to the proper seat adjustment relative to leg length and pedaling cadence (50-60 rpm). Inspiratory capacity will be measured before and after the exercise maneuver. The cycle ergometer resistance will be set to the pre-determined constant work rate, as described above. There will be continuous monitoring of saturation, ECG, gas exchange, blood pressure, and subjective dyspnea/leg fatigue (Borg scale), with standardized verbal encouragement throughout. Participants will be asked to bring running shoes and comfortable exercise clothes, ensure that they have eaten before the test, to take all usual medications, and to avoid major exercise for 24 hours before the test.