Effect of hemoglobin on the oxygen uptake of patients with coronary artery disease

Oxygen uptake (VO2) is frequently measured or predicted in patients referred to cardiac rehabilitation programs to quantify functional capacity and prescribe exercise. When entering Phase II programs, some patients are in an anemic state that normalizes over the course of the program. The purpose of this study was to evaluate the relationship between the change in hemoglobin levels and the change in VO2 in a group of Phase II cardiac rehabilitation patients.

Sixty-six cardiac patients (45 men and 21 women) underwent a graded exercise test with the collection of expired air before and following participation in a Phase II cardiac rehabilitation program. Blood was sampled before each test for hemoglobin-hematocrit analysis. The change in absolute VO2 over the course of the rehabilitation program was chosen as the response variable in a simple regression model designed to measure the effect of normalization of hemoglobin levels during that period.

A simple regression of the change in absolute VO2 on patient age and the changes in hemoglobin concentration and maximal work rate produced coefficient estimates that are statistically significant and have the signs one would expect. A bootstrap re-estimation of the regression model gives essentially the same coefficients and supports the results of the simple model.

These results indicate that low levels of hemoglobin concentration adversely affect the VO2 of patients with cardiac disease and should be accounted for when assessing functional capacity. This may be even more critical when estimating rather than measuring VO2. Finally, absolute VO2 may not be the best measure of functional capacity. A submaximal marker, such as the VO2 at the ventilatory threshold, may be a more useful response variable.