Mechanisms of Effort Intolerance in Patients With Rheumatic Mitral Stenosis: Combined Echocardiography and Cardiopulmonary Stress Protocol

Abstract

Objectives: This study sought to evaluate mechanisms of effort intolerance in patients with rheumatic mitral stenosis (MS).

Background: Combined stress echocardiography and cardiopulmonary testing allows assessment of cardiac function, hemodynamics, and oxygen extraction (A-Vo₂ difference).

Methods: Using semirecumbent bicycle exercise, 20 patients with rheumatic MS (valve area 1.36 ± 0.4 cm²) were compared to 20 control subjects at 4 pre-defined activity stages (rest, unloaded, anaerobic threshold, and peak). Various echocardiographic parameters (left ventricular volumes, ejection fraction, stroke volume, mitral valve gradient, mitral valve area, tissue s' and e') and ventilatory parameters (peak oxygen consumption [Vo₂] and A-Vo₂ difference) were measured during 8 to 12 min of graded exercise.

Results: Comparing patients with MS to control subjects, significant differences (both between groups and for group by time interaction) were seen in multiple parameters (heart rate, stroke volume, end-diastolic volume, ejection fraction, s', e', Vo₂, and tidal volume). Exercise responses were all attenuated compared to control subjects. Comparing patients with MS and poor exercise tolerance (<80% of expected) to other subjects with MS, we found attenuated increases in tidal volume (p = 0.0003), heart rate (p = 0.0009), and mitral area (p = 0.04) in the poor exercise tolerance group. These patients also displayed different end-diastolic volume behavior over time (group by time interaction p = 0.05). In multivariable analysis, peak heart rate response (p = 0.01), tidal volume response (p = 0.0001), and peak A-Vo₂ difference (p = 0.03) were the only independent predictors of exercise capacity in patients with MS; systolic pulmonary pressure, mitral valve gradient, and mitral valve area were not.

Conclusions: In patients with rheumatic MS, exercise intolerance is predominantly the result of restrictive lung function, chronotropic incompetence, limited stroke volume reserve, and peripheral factors, and not simply impaired valvular function. Combined stress echocardiography and cardiopulmonary testing can be helpful in determining mechanisms of exercise intolerance in patients with MS.

Keywords: congestive heart failure; echocardiography; exercise testing.