Mitral Stenosis

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Overview of Mitral Stenosis

Etiology

Almost always due to rheumatic disease, normally becomes symptomatic ~ 20 years after the infectious insult, although acute increases in DO2 requirements (ex. pregnancy, sepsis) can result in earlier onset of symptoms.

> 50% of patients will have another, coexisting valvular pathology (most commonly mitral regurgitation, but 25% will have aortic involvement as well)

Valve Dimensions / Measurements

Normal mitral valve area is 4-6 cm2, symptoms usually commence at 2 cm2 or less. “Critical stenosis” is defined as 1 cm2 or less, and is usually symptomatic at rest. 1-1.5 cm2 usually causes mild symptoms on exertion, and 1.5 cm2 -2.0 cm2 is usually asymptomatic unless significantly stressed. Valve area can be calculated using the Gorlin equation: area = flow/[k x sqrt(pressure gradient)] where flow is cardiac output/diastolic filling time. Assuming a constant area, this can be arranged to pressure gradient = (CO/diastolic time)2, which suggests that small changes in CO or diastolic time (with tachycardia) can lead to large changes in the pressure gradient.

LA pressures > 25 mm Hg will lead to acute pulmonary edema, and if present in the long term will lead to increased PVR.

Effect on cardiac anatomy

Decreased filling and cardiac output, relies on atrial kick in order to acquire sufficient LVEDV, pulmonary venous pressures increase, working their way back to the PA and potentially resulting in pulmonary artery hypertension and PVR (when LA pressure is higher than 25 mm Hg).

25% of patients will have LV dysfunction (secondary to another valve lesion, myocarditis from the original lesion, or coexisting hypertension and/or coronary artery disease)

Increased PVR can ultimately lead to right ventricular failure. The left atrium is often markedly dilated, predisposing to arrhythmias as well as atrial clots.

Mitral Stenosis in Non-Cardiac Surgery

Pre-Medication

According to Stoelting, these patients may be more susceptible to preoperative medications [Stoelting RK. Basics of Anesthesia, 5th ed. Elsevier (China) p. 373, 2007]. Many of them will be on digitalis – if so, HR should be less than 80 bpm (otherwise they are subtherapeutic), and [K] should be checked preoperatively.

Monitoring

Full monitoring (systemic and pulmonary artery pressures) is normally indicated for major surgery, in particular to measure volume status as well as pulmonary vascular resistance. Note that wedge pressures are an indicator of transvalvular gradients and not LVEDP in patients with mitral stenosis.

Hemodynamics

Increases in heart rate (tachycardia, arrhythmias) are to be avoided, thus avoid pancuronium, ketamine, consider not reversing paralytics or reducing the dose of neostigmine/glycoyrrolate. In particular, sinus rhythm is critical. Because increased pulmonary artery pressures are common in these patients, there are two theoretical reasons to avoid nitrous oxide – first, nitrous oxide can increase PVR, and second, nitrous oxide reduces the FiO2 (oxygen decreases PVR). That said, a controlled study of 11 mitral stenosis patients (elective MVR) showed an increase in PVR from 159 to 213 dynes.sec.cm-5 following 10 minutes of 50% N2O, but no significant changes in heart rate, systemic blood pressure, cardiac index, or SVR [Hilgenberg JC et al. Anesth Analg 59: 323, 1980]

Hypotension can be difficult to treat. Despite the fact that many take diuretics, hypovolemia is rarely the cause and the response to volume is often disappointing. Some authors advocate phenylephrine over ephedrine as it does not stimulate B-receptors, increase heart rate, etc., however Barash recommends ephedrine or epinephrine.

Mitral Stenosis in Cardiac Surgery

Post-Repair / Bypass

Despite the increased risk of RV failure, when struggling to come off of bypass the LV is most often at fault, either due to surgical injury or sudden distension in a chronically underloaded ventricle.

Note that after a surgical repair, V waves are almost always seen and represent LA filling from the right (because cardiac output is increased post-repair), and not mitral regurgitation.

Echocardiographic Assessment of Mitral Stenosis

The ASE recommends the following quantitative assessments – valve area (by both planimetry and pressure half-time), mean pressure gradient, and pulmonary systolic pressure. When additional information is needed, calculation of valve area by either continuity equation and/or PISA, as well as measurement of mean pressure gradient and/or pulmonary systolic pressure during exercise may be helpful.

Mitral Inflow Pattern ASE Recommendations for MS Quantification 1. Valve Area A. Planimetry (< 1.0 cm2 is severe) B. Pressure Half-Time (< 1.0 cm2 is severe) 2. Mean Pressure Gradient (> 10 mm Hg is severe) 3. Pulmonary Artery Systolic Pressure (> 50 mm Hg is severe) For complete recommendations and commentary, see the ASE Guidelines.