Abstract 17924: Difference of Heart Rate and Oxygen Uptake Recovery From Exercise Indicates Severity of Idiopathic Pulmonary Arterial Hypertension
Background: During incremental exercise in healthy normal subjects the near-linear heart rate (HR) vs O2 uptake (VO2) slope is dependent on increases in arteriovenous O2 difference (C(a-v)O2) and stroke volume (SV). In normal subjects recovering from peak exercise, VO2 declines faster than HR because the C(a-v)O2 rapidly decreases towards resting levels. This creates a non-linear relationship between HR and VO2 during recovery with a significant hysteresis when viewed graphically. Because idiopathic pulmonary arterial hypertension (IPAH) patients are primarily limited by their low cardiac output (CO) and SV response to exercise, we postulated that their exercise-induced increase in C(a-v)O2 would decline slower than normal during recovery, thereby decreasing the normal HR vs VO2 hysteresis.
Methods: We analyzed data from standard ramp incremental cycle cardiopulmonary exercise tests in 95 NYHA class II-IV PPH patients and in 22 normal control subjects. The protocols required 3 min rest, 3 min unloaded cycling at 60 rpm, 5-10 W/min ramp in PPH or 15-30 W/min ramp in controls to peak tolerance, followed by 2-3 min recovery. IN all tests the degree of hysteresis was measured as the difference in HR between recovery and exercise at the half-way point between peak and resting (ΔHR@midVO2). We correlated the values of ΔHR@midVO2 vs % predicted peak VO2 (%pkVO2) and % predicted O2 pulse (%pkO2P). The latter measures SV x C(a-v)O2. Data are reported as mean±SD.
Results: The 95 IPAH patients’ demographics were: 45±14 yr, F/M=86/9, NYHA = 2.7±0.6, CO = 4.2±1.5 L/min, SV = 49±21 ml/beat, %pkVO2 = 44±14% and %pkO2P = 60±18%. The ΔHR@midVO2 was 38.9±10.3 bpm in normal subjects and 8.4±8.6 bpm in PPH patients (p<0.0001). The %pkVO2 and %pkO2P, which are indices of disease severity, correlated well with ΔHR@midVO2 (r=0.81 and r=0.79, p<0.0001).
Conclusion: The reduced recovery hysteresis of HR vs VO2 in IPAH patients likely reflects a slower return of the C(a-v)O2 towards normal due to the low cardiac output response to exercise in these patients.
- © 2013 by American Heart Association, Inc.