In the setting of severe mitral regurgitation due to a flail leaflet, what occurs first?

In the context of severe mitral regurgitation caused by a flail leaflet, the initial hemodynamic changes primarily affect the left atrium due to the retrograde flow of blood from the left ventricle during systole. This backward flow leads to volume overload of the left atrium, causing it to dilate.

While a dilated right ventricle may develop over time due to increases in pulmonary pressures from chronic left atrial volume overload and eventual pulmonary hypertension, this occurs after the left atrium has already started to dilate. Increased left ventricular (LV) pressure and left ventricular hypertrophy are typically secondary effects resulting from the volume overload experienced by the heart. Initially, the volume overload impacts the left atrium as it receives more blood than it can handle because of regurgitant flow, leading to its dilation.

Therefore, recognizing that the left atrium is the first chamber to reflect changes in response to severe mitral regurgitation helps clarify why this occurs before observable changes like right ventricular dilation or increased LV pressure and hypertrophy.