Influence of inotropy and chronotropy on the mitral valve sphincter mechanism.

Background: This study was designed to isolate and quantify the effects of ventricular inotropic and chronotropic state on the normal mitral valve annular sphincter mechanism.
Methods: Sonomicrometry tansducers were placed around the mitral annulus in six sheep; atrial pacing wires were also placed. One week later, esmolol was titrated to produce a baseline hemodynamic state with a heart rate of 90 bpm; hemodynamic and sonomicrometry data were recorded. Then animals were paced at 120 bpm and 150 bpm; data were recorded at each heart rate. Isoproterenol infusion was titrated to achieve a heart rate, without pacing, of 120 and 150 bpm; again, data were recorded. Annular area was calculated at end diastole (ED) and end systole (ES) for all experiments using sonomicrometry array localization. Analysis of variance was used to assess the independent effects of heart rate and inotropic state on annular area.
Results: Atrial pacing at 120 bpm produced ES and ED annular areas of 777 +/- 150 mm(2) and 748.8 +/- 140.1 mm(2), respectively. At the same heart rate, isoproterenol-treatment resulted in significantly smaller ES and ED areas: 699 +/- 160 mm(2) and 641.9 +/- 156.5 mm(2), respectively. Atrial pacing at 150 bpm produced ES and ED annular areas of 745.2 +/- 131.3 mm(2) and 723.7 +/- 141.3 mm(2), respectively. At the same heart rate, isoproterenol-treatment resulted in significantly smaller ES and ED areas: 652.8 +/- 146.4 mm(2) and 569.7 +/- 155.9 mm(2), respectively.
Conclusions: The inotropic state of the left ventricle directly affects the mitral valve annular orifice area, independent of heart rate. This inotropic effect on valve size is more pronounced at ED than at ES in the sheep.