A65-12 Supplemental Oxygen Decreases Cardiac Effort in Pulmonary Arterial Hypertension

Background Evidence for oxygen therapy in pulmonary arterial hypertension (PAH) is limited and extrapolated from chronic obstructive pulmonary disease. Small studies suggest functional improvement with oxygen in precapillary pulmonary hypertension, but the immediate physiologic effect of oxygen during the six-minute walk test (6MWT) has not been characterized. Cardiac Effort is the number of heart beats used during 6MWT/ 6-minute walk distance (6MWD). We hypothesized that Cardiac Effort would be sensitive to the benefits of oxygen (decrease as oxygen delivery improves), while 6MWD would remain unchanged in PAH patients without resting hypoxemia. Methods In this single-center, prospective, blinded, crossover pilot study, PAH participants on stable PAH therapy and controls completed two 6MWTs during a single visit, one with 2 L/min oxygen and one with ambient air, pulling identical, blinded tanks in random order. Continuous electrocardiogram monitoring was performed with Fourth Frontier single lead electrocardiogram chest strap. Heart rate was averaged every 4 seconds and Cardiac Effort was calculated (total heart beats/6MWD). Additional parameters included 6MWD, heart rate reserve (peak-rest), heart rate recovery (heart rate at 6 minutes – 7 minutes), Borg Dyspnea Index, and oxygen saturation. Nonparametric paired testing was used. Results Nineteen PAH participants and nine controls were enrolled (Table 1). None had resting hypoxemia. In PAH, supplemental oxygen improved post-walk oxygen saturation (91% (84, 94) vs 88% (81, 90), p = 0.004). Cardiac Effort (1.67 beats/m (1.47, 2.07) vs 1.73 (1.52, 2.19), p = 0.0001), heart rate expenditure (620 beats (605, 677) vs 643 beats (607, 684), p = 0.0004), peak heart rate (109 beats/min (107, 127) vs 116 beats/min (105, 122), p = 0.003), and heart rate at 6 minutes (108 beats/min (100, 125) vs 111 beats/min (105, 122), p = 0.009) all improved with 2 L supplemental oxygen in PAH patients (Table 1). 6MWD changed by 2 m (-7, 9) and there was no change in Borg score (Table 1). The treatment effect for percent change in Cardiac Effort between PAH and controls was -4% (-8, -2) p = 0.0002, there was no treatment effect with 6MWD, p = 0.6. In controls, no changes occurred. Conclusions In PAH participants not requiring supplemental oxygen at rest, 2 L supplemental oxygen decreased Cardiac Effort (improved physiology) during equivalent workload achieved during 6MWT. Fixed oxygen flow rate may have blunted some of the benefit. Physiologic monitoring during 6MWT provides greater insight beyond 6MWD alone. Further evaluation of oxygen therapy in PAH is warranted, including longer-term use and titrated flow rates during 6MWT.