11.7.2 Pathophysiology

As the mitral surface area decreases, the P_LA progressively increases, dilating the atrium; this dilation slows the rise in pressure by increasing atrial compliance. The Gorlin formula used in catheterisation shows that flow and pressure gradient across an orifice are geometrically related [4,6]:

 

MSA (cm²) = K · CO / √ΔPm

where:    CO = cardiac output (L/min)

ΔPm = mean pressure gradient (mmHg)

K = constant

 

From this it can be deduced that the transvalvular pressure gradient is a function of the square of the flow velocity (ΔP ≈ V² ), regardless of the surface area of the valve. This is also demonstrated by Bernoulli's equation: ΔP = 4 (V_max )² . A doubling of cardiac output therefore quadruples the pressure gradient and increases P by the same amount. A mitral stenosis that is asymptomatic at rest will therefore quickly become limiting during exercise. Exercise, stress, hypervolemia, pregnancy or hyperthyroidism are all responsible for a sudden increase in P_LA, causing pulmonary interstitial fluid leak, leading to acute dyspnoea and the risk of pulmonary oedema. 

 

The resistance across a valve stenosis depends on the degree of valve opening and the blood flow through the valve. It is given by equation [1]:

 

RES = K · CO / AS²

 

where: RES = total resistance

CO = cardiac output (L/min)

AS = area surface (cm)²

K = constant (1'333)

 

Resistance, and therefore pressure gradient and upstream pressure (in this case P_LA), increases linearly with flow but exponentially with decreasing orifice area. This explains the decompensation induced by pregnancy in a previously well-tolerated mitral stenosis and the large benefit obtained by widening with commissurotomy, even if the orifice area remains modest.

 

As the tachycardia shortens diastole more than systole, the acceleration of the heart rate reduces the time available for the volume of each cardiac cycle to pass through the mitral valve, which has a very restricted flow; the flow must accelerate, so the gradient increases geometrically. The result is hypertension in the left atrium, which affects the pulmonary veins and causes pulmonary congestion. In addition, the low filling capacity and small size of the LV limit stroke volume, which can hardly increase when the heart rate slows down. Cardiac output is therefore doubly limited in relation to the heart rate.

 LV filling is less dependent on atrial contraction than when ventricular compliance is impaired [8]. It is the acceleration of the total ventricular rate that causes haemodynamic decompensation during the transition to AF. Although sinus rhythm has an electrical advantage, it is not necessarily effective. This is because the huge atrium of mitral stenosis, with its thin dilated wall, is unable to generate a pressure to overcome the valve obstruction. This electro-mechanical dissociation can be seen on Doppler echo: despite sinus rhythm, there is no atrial contraction A flow through the mitral valve in telediastole. In the LA cavity, which can be up to 350 mL, the flow is extremely slow. On echocardiography, we usually see a slow swirling of the blood mass, resembling wisps of smoke (spontaneous contrast). The risk of mural thrombus is high, particularly in the atrial appendage (LAA) where congestion is marked (Figure 11.89); anticoagulation is recommended (anti-vitamin K agent for INR 2-3) as the risk of arterial embolism is high.

 

 

 

 The LV is generally small in pure mitral stenosis, but systolic function is preserved; EF is normal in 75% of patients [2]. Ventricular mass is normal or reduced. Alterations in segmental kinetics often occur at the base of the LV because the stiffness of the mitral valve prevents normal contraction of the underlying segments [10]. Function may be reduced by concomitant myocarditis specific to ARF, which occurs in 20% of cases.

Pulmonary hypertension (PHT) is common in mitral stenosis because mean P_LA remains elevated throughout the cardiac cycle, whereas in MI it is elevated only during systole. This chronic increase in P_LA affects the pulmonary veins and induces pulmonary venous hypertension by stasis (postcapillary hypertension). The accumulation of interstitial fluid impairs gas exchange, leading to dyspnoea. Work of breathing is increased. This is followed by a reactive increase in PAR, which becomes fixed in the long term (precapillary hypertension). This is mainly responsible for the increase in LV afterload, as it is out of proportion to the left atrial pressure [3]. It is an adaptive phenomenon that restricts flow to the left heart, reduces blood accumulation in the LA and limits the risk of pulmonary oedema [9,11]. This leads to a cascade of right heart dysfunction: RV hypertrophy and dilatation, tricuspid regurgitation, RA dilatation and systemic venous stasis (see Figure 11.86). When resting systolic PAP exceeds 50 mmHg (mean PAP ≥ 35 mmHg), the functional limits of the LV no longer allow exercise, as PAP increases disproportionately. This combination of pre- and post-capillary PHT  triples mortality [7]. The prevalence of PHT is 75% in mitral stenosis but approximately 30% in mitral regurgitation [5]. In general, mitral stenosis is characterised by LV dysfunction but preserved LV function.

Although the left ventricle retains its contractile function, it is unable to vary its output because its filling capacity is limited. It operates at a low and fixed diastolic volume and is unable to increase its rate: when it accelerates, diastolic filling becomes inadequate. It has no preload reserve to modify its position on the Starling curve and improve its systolic performance during exercise: the systolic volume also remains low and fixed. Nor can the LV compensate for arterial vasodilatation or hypovolemia by tachycardia. The P/V loop (Figure 11.90) illustrates the typical haemodynamic situation: normal intraventricular pressures, limited stroke volume (< 50 mL), normal ejection fraction (EF 0.5 - 0.6); ejection work is reduced but pressure work is normal. Stroke volume is limited by the flow through the stenosis and the duration of diastole. Systemic arterial pressure, which is the product of cardiac output and systemic arterial resistance (SAP ≈ CO · SAR), is essentially dependent on SAR, which must remain high.

 Figure 11.90: Pressure-volume loop in mitral stenosis. The restriction to ventricular filling results in a very small ventricular volume at values of Emax and compliance that are still normal. However, this is not always the case: in rheumatic myocarditis, for example, the LV is dysfunctional, the slope of Emax is reduced and the compliance curve is straightened. Stroke volume (SV) and ejection work are reduced, but pressure work is normal.

 

Hemodynamic features of mitral stenosis

LV filling failure: low, fixed stroke volume Slow diastolic LV filling: intolerance to tachycardia Intolerance to hypovolemia Massive increase in PLA on exercise (tachycardia) or hypervolaemia Dilatation of LA: risk of AF and thrombus Preserved LV systolic function Stasis and pulmonary hypertension: venous (postcapillary) and arterial (precapillary) PHT Pressure overload of RV: dilatation, HRV, dysfunction, tricuspid insufficiency

 

© CHASSOT PG, BETTEX D, August 2011, last update November 2019