Background—A medical treatment that decreases the likelihood of left ventricular (LV) dysfunction or symptoms would benefit patients with moderate to severe degenerative mitral regurgitation. The aim of this pilot study was to determine the short-term effects of a -blocker on mitral regurgitant volume and LV work in these patients.

Methods and Results—Twenty-five patients with moderate or severe degenerative mitral regurgitation were randomized in a double-blind crossover study to the ₁-selective adrenergic blocker metoprolol (mean daily dose, 119 mg; range 23.75 to 190 mg) and placebo for 14±3 days. At the end of each treatment period, ascending aortic flow and LV stroke volume were measured by cardiac magnetic resonance imaging, and mitral regurgitant volume was calculated. On -blocker, heart rate decreased from 65±10 by 10±7 bpm (mean±SD) and systolic blood pressure decreased from 138±18 by 16±12 mm Hg (P<0.0001 for both). No significant change occurred in LV ejection fraction (from 65±5%; change, -0.6±2.7%; P=0.3) or mitral regurgitant volume (from 59±36 mL; change, 3±13 mL; P=0.3), but forward stroke volume increased from 89±21 by 5±11 mL (P=0.03). Because heart rate was lower on metoprolol, cardiac output decreased from 5.68±1.04 by 0.56±0.78 L/min (P=0.001), but a greater decrease occurred in LV output, from 9.51±2.22 by 1.30±1.08 L/min (P<0.0001). Mitral regurgitant volume per minute decreased from 3.83±2.41 by 0.74±1.00 L/min (P=0.001). The decrease in LV work on -blocker (mean, 21%; 95% confidence interval, 15 to 27) was greater (P=0.001) than the decrease in cardiac output (mean, 9%; 95% confidence interval, 3 to 15).

Conclusions—In this pilot study, short-term treatment with a -blocker did not change mitral regurgitant volume per beat but decreased LV work in patients with moderate to severe degenerative mitral regurgitation. Further research is needed to determine whether longer-term treatment with -blockers will decrease progressive LV dysfunction and symptomatic deterioration.