Purpose Oxygen pulse (O2-P), defined as VO2 divided by heart rate, has been used to evaluate cardiac function during exercise in adults. The utility of O2-P in children during exercise has not been well studied. Our objective was to determine if O2-P in children during exercise is affected by ventricular function and hemodynamic factors.
Methods Exercise test results of children referred to Mattel Children's Hospital at UCLA between July 2003 and June 2004 were analyzed. These tests were conducted on a cycle ergometer using a 5-, 10-, or 15-watt increment ramp protocol. There were 171 exercise tests analyzed, mean age of 15.3 years. O2-P predicted for height and sex was calculated using published normalized data. We analyzed the absolute O2-P corrected for weight and O2-P% against left ventricular function (defined by ejection fraction [EF]) and against hemodynamic factors: aortic stenosis (AS), aortic regurgitation (AR), pulmonary regurgitation (PR), and pulmonary stenosis (PS).
Results We identified five patterns of O2-P change, in relation to work rate increase, during exercise: linear increase (A), curvilinear increase (B), linear increase then plateau after anaerobic threshold (C), linear increase before plateau then linear decrease after plateau (D), and linear increase before and after plateau (E). Children with lower EFs were more likely to have early O2-P plateau or O2-P decrease with increasing exercise (Patterns C and D). For patients with normal hemodynamics (no more than mild AS/AR), the absolute O2-P (p=0.02) and O2-P% (p=0.03) was significantly lower in patients with abnormal EF than patients with normal EF. Likewise, for patients with no more than mild AS/AR/PS/PR, the absolute O2-P (p=0.02) and O2-P% (p=0.02) was significantly lower with abnormal EF than with normal EF. Hemodynamic factors, such as AS, AR, PS, PR, did not have a significant effect on O2-P.
Conclusion O2-P is useful in monitoring children during exercise testing, especially patients with abnormal left ventricular function. Hemodynamic factors, such as valve stenosis or regurgitation, did not affect O2-P dynamics significantly.