O2 supply dependence of respiration in patients with mitral stenosis undergoing valvuloplasty

Although systemic oxygen consumption (V O2) is independent of O2 delivery (Q O2) in normal subjects, studies have suggested that supply dependence of V O2 may occur in patients with chronic diseases associated with reduced Q O2. In this regard, we previously found that Q O2 and V O2 increased when cardiac output was improved after balloon valvuloplasty in patients with aortic stenosis. However, their increases in Q O2 were relatively small, and it was not known whether the increase in V O2 was caused by the increase in delivery or was merely a response to the transient hypotension induced by valvuloplasty. Because patients with mitral stenosis frequently exhibit greater improvements in cardiac output after valvuloplasty than do patients with aortic stenosis, the present study sought to determine (1) whether V O2 is increased after valvuloplasty in patients with mitral stenosis, and (2) whether the magnitude of the increase in V O2 correlates with the magnitude of the improvement in cardiac output and Q O2. Oxygen delivery, V O2, and hemodynamics were measured in 57 patients with mitral stenosis before and 20 to 30 min after undergoing balloon valvuloplasty. After valvuloplasty, Fick-derived oxygen delivery increased by 13.0% (95% confidence interval: 10.8 to 15.2%), whereas V O2 (expired gas) increased by 8.3% (95% confidence interval: 9.5 to 12.3%). A correlation between Fick-derived Q O2 and V O2 was found (p<0.005) with a slope of 0.66 (95% confidence interval: 0.07 to 1.24), but the O2 extraction ratio did not change (-1.0%; 95% confidence interval: -2.7 to 0.5%). A significant correlation between the change in Q O2 and the change in V O2 was also seen (p<0.02). These findings suggest that the increase in V O2 may have been a consequence of the increase in Q O2 rather than a response to the procedure itself.