Consequences of Mitral Stenosis

Pulmonary Edema

Symptoms of MS typically begin when the normal mitral valve (MV) area (4.0–5.0 cm ² ) is reduced to less than 2.5 cm ² . When the valve area reaches less than 1.5 cm ² , patients can develop symptoms at rest (see Fig. 94.2 ). ^, However, symptoms may occur in valves with larger areas when patients exercise or experience emotional stress, infection, pregnancy, or atrial fibrillation (AF). Under these circumstances, there is an increase in transmitral flow or a decrease in the diastolic filling period, which results in a rise in left atrial (LA) pressure and the development of dyspnea and eventually pulmonary edema. This pathway also explains why patients’ exercise tolerance decreases as MS progresses.

The mechanism for dyspnea in patients with MS results from a decrease in blood flow from the left atrium into the left ventricle during diastole ( Figure 94.3 A, B ). To compensate, there is an increase in LA pressures to drive blood forward into the left ventricle. This augmentation in LA pressures also results in increased pulmonary venous pressure and distention of the pulmonary veins and capillaries. When the pulmonary venous pressure exceeds the plasma oncotic pressure, pulmonary edema results. However, in patients with chronic MS, pulmonary edema may not occur even when the stenosis is severe and pulmonary venous pressures are very high. This is because there may be an associated decrease in pulmonary microvascular permeability.

Pulmonary Hypertension

In addition to pulmonary edema, patients with MS may develop PH. The degree of pulmonary vascular disease is an important determinant of PH in patients with MS. Initially, PH associated with MS is known as passive or obligatory when it is caused by the elevated LA pressures needed to drive blood across the stenotic MV. There are no abnormalities in the pulmonary arterial bed at this stage. Moderate PH occurs when the right ventricle needs to generate high systolic pressure to ensure adequate cardiac output. When this occurs, elevated pulmonary capillary wedge pressure (PCWP) and mean pulmonary arterial pressure with minimal elevation in transpulmonary gradient ( < 12 mm Hg) and normal pulmonary vascular resistance are found on right heart catheterization.

Chronically elevated PCWP results in functional and structural abnormalities in the pulmonary vascular bed, leading to the development of pulmonary vascular disease and eventually severe PH. This form of PH is known as reactive PH. Initially this begins with increased capillary endothelial basement membrane thickness due to excessive deposition of type IV collagen. This increase in interstitial connective tissue as well as increased production of extracellular matrix components increases extravascular fluid storage capacity as an adaptive reaction to protect the lungs from pulmonary edema. In conjunction with the elevated venous pressure, disruption of the endothelium leads to activation of vascular serine elastase and matrix metalloproteinases and growth factors that result in migration of smooth muscle and hypertrophy and fibrosis from elastin synthesis. The consequences of these changes are enlarged, thickened pulmonary veins; pulmonary capillary dilatation; capillary dilatation; alveolar hemorrhage; and lymphatic vessel and lymph node enlargement. However, plexiform lesions are not present in these patients. It must be noted that not all these changes are seen in patients with elevated pulmonary venous pressures, and there may be a genetic predisposition. In addition, reversible obstruction may develop at the level of the pulmonary veins. ^,

Patients with reactive PH have increased mean pulmonary arterial pressure, transpulmonary gradient, and pulmonary vascular resistance. As well, treatments to lower the PCWP may not normalize the pulmonary arterial pressure, as seen in passive PH.

Right Heart Failure

Right heart failure occurs in MS from stress on the right ventricle to generate the forces necessary to drive blood across the stenotic MV. Initially, this stress results in right ventricular pressure overload. However, the development of PH from MS-related pulmonary vasoconstriction and LA hypertension compounds the stress and leads to the development of right heart failure. Right heart failure is associated with right ventricular dilatation and tricuspid regurgitation (TR), which is discussed later. Right heart failure also results in elevated jugular venous pressure, liver congestion, and peripheral edema. Note that left ventricular (LV) function can be normal in isolated MS.