Aortic stenosis echocardiography

Resident Survival Guide

Aortic Stenosis Microchapters
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-In-Chief: Mohammed A. Sbeih, M.D. [2]; Lakshmi Gopalakrishnan, M.B.B.S. [3]; Rim Halaby; Usama Talib, BSc, MD [4]

Overview

Echocardiography is the best non-invasive test to evaluate the aortic valve anatomy and function. It is indicated in the case of presence of symptoms suggestive of valvular problems or in the case of detection of a systolic murmur with a grade greater than 3/6. Doppler echocardiography allows the measurement of the maximum jet velocity and can be used to estimate the effective orifice area of the aortic valve as well as the gradient across the aortic valve using the modified Bernoulli equation (gradient = 4 x velocity²). The flow must be constant, so as the velocity increases, the valve area decreases proportionally. Attention to technical details is important as they may lead to underestimation of the severity of the aortic stenosis. Echocardiography can also be used to assess the severity of left ventricular hypertrophy.

Optimal Echocardiography Views

• Parasternal Long Axis View:

The parasternal long axis view visualizes the right and non coronary leaflets.

• Parasternal Short Axis View:

In the parasternal short axis view, the aortic leaflets open equally and form a circular orifice during systole. During diastole, the normal leaflets form a three pointed star with prominence at the closing point (nodules of Arentius).

Assessment of the Aortic Valve Area and Resistance

The assessment of Aortic Valve Area and Resistance are helpful in diagnosing Aortic Valve Stenosis.

• Based on Bernoulli's equation, using doppler echocardiography the pressure gradient across the valve can be measured using the formula:

  Pressure Gradient (mmHg) = 4 x (Maximum Stenotic Jet Velocity)² - 4 x (Left Ventricular Outflow Tract Velocity)²

• However, for all practical purposes, since the left ventricular outflow tract velocity is usually less than 1 m/sec; the above equation can be simplified into:

  Maximum Pressure Gradient (mmHg) = 4 x (Maximum stenotic Jet Velocity)²

• The above two equations derived, aid in calculating the maximum pressure gradient that is obtained using the instantaneous aortic jet velocity that is assessed with doppler echocardiography; however, cardiac catheterization is required to calculate peak pressure gradient across the valve.

Assessment of the Aortic Valve Area

Following are a few important points regarding the Aortic Valve Area Assessment.

• The continuity principle states that the flow in one area must equal the flow in a second area if there are no shunts in between the two areas.

• Using doppler velocities, aortic valve area can be calculated using the following continuity pinciple:

Aortic Valve Area (cms²) = { (Cross-sectional Area of LVOT x Time Velocity Integral across the LVOT) ÷ (Time Velocity Integral across Aortic Valve) }

• The weakest aspect of this calculation is the variability in measurement of cross-sectional area of LVOT, because it involves squaring the LVOT dimension. Such variations in the aortic valve area derived using doppler velocities may be observed during exercise or in conditions that increase the blood flow across the valve.

• Based on a study that simultaneously determined Gorlin formula and transesophageal echocardiography planimetry valve areas, demonstrated that acute changes in trans-valvular blood flow substantially altered valve area as calculated by the Gorlin formula but did not result in significant alterations of the anatomic valve area in aortic stenosis. This suggests that the flow-related variation in the Gorlin aortic valve area may be due to a disproportionate blood flow dependence of the formula itself and not a true change in valve area. Therefore, the advantage of continuity equation over Gorlin formula is that the former is less susceptible to blood flow across the valve.

Assessment of the Aortic Valve Resistance

• Doppler derived aortic valve resistance correlates well with catheterization derived aortic valve resistance and hence may provide an additional non-invasive parameter for the assessment of aortic stenosis severity.

• Although all doppler echocardiographic indexes of aortic stenosis are affected by blood flow, aortic valve resistance is more stable than aortic valve area under dobutamine-induced hemodynamic changes. However, baseline aortic valve area may be unreliable in patients with calcific degenerative aortic stenosis and low cardiac output states.

Severity of Aortic Stenosis

Aortic stenosis severity can be assessed by estimating both the pressure gradient across the valve and the aortic valve area.

• Using echocardiographic flow velocities, the aortic valve area can be calculated non-invasively to estimate the severity of aortic stenosis.

• Using the velocity of the blood through the valve, the pressure gradient across the valve can be calculated by the equation:

  Gradient = 4(velocity)² mm Hg

Normal Aortic Valve

• A normal aortic valve has no gradient, and
• Aortic valve surface area is of 2.5 to 3.5 cms²

Mild Aortic Stenosis

• Mean pressure gradient across the valve is less than 25 mm Hg, or
• Valve surface area is between 1.5 and 2.5 cms², or
• Jet velocity less than 3.0 m per second

Moderate Aortic Stenosis

• Mean pressure gradient across the valve is between 25 mm Hg and 40 mm Hg, or
• Valve surface area is between 1.0 and 1.5 cms², or
• Jet velocity 3.0 to 4.0 m per second

Moderate to Severe Aortic Stenosis

• Valve surface area is between 0.7 and 1.0 cms²

Severe Aortic Stenosis

• Mean pressure gradient across the valve is more than 40 mm Hg, or
• Valve surface area is less than 0.7 cm², or
• Jet velocity is greater than 4.0 m per second

Shown below is a table that summarizes the criteria for the classification of aortic stenosis into mild, moderate or severe:

Severity	mild	moderate	severe
Valve Surface Area	2.5 - 1.5	1.5 - 1.0	<1.0
Jet Velocity (m/s)	2 - 3	3 - 4	>4
Peak Gradient (mm Hg)	<25	25 - 40	>40
Mean Gradient (mm Hg)	<20	20-40	>40

Low Flow, Low Gradient Aortic Stenosis

• Echocardiography is fairly accurate in the assessment of the severity of aortic stenosis in patients with normal or near normal cardiac output. However, dobutamine stress echocardiography or cardiac catheterization may be required to accurately assess the severity of aortic stenosis in patients with low output aortic stenosis.

• Low dose (<20 micro-gram/Kg/min) dobutamine stress echocardiography allows:
  • The differentiation between severe and pseudo-severe low flow low gradient aortic stenosis through the degree of change of both the effective orifice area and the transvalvular gradient after dobutamine infusion. Changes following dobutamine infusion are due to the fact that dobutamine causes increase in the flow across the aortic valve.
    • In pseudosevere aortic stenosis, there is increase in the effective orifice area of the aortic valve, in a way that it is larger than 1.2 cm², and slight increase in the gradient, in a way that is inferior to 40 mm Hg.
    • In severe aortic stenosis, there is absence or little increase in the effective orifice area of the aortic valve, in a way that is inferior to 1.2 cm², and increase in the gradient to become greater than 40 mm Hg.
  • The assessment of the presence or absence of left ventricular flow reserve through the degree of change in stroke volume following dobutamine infusion.
    • Left ventricular flow reserve is an indicator of operative mortality. Left ventricle flow reserve does not predict recovery of left ventricle after aortic valve replacement nor the resolution of symptoms.
    • The aortic valve area should increase to more than 1-1.2 cm² with a dobutamine infusion and the mean pressure gradient should rise above 30 mm Hg. While early surgical mortality is 32–33% in patients who fail to to achieve these improvements with dobutamine infusions, it is only 5–7% in patients who demonstrate an increase in their contractility and gradient. Five years survival after surgery was 88% in patients with improved contractility and only 10–25% in patients with unimproved contractility.

Shown below is an algorithm summarizing the interpretation of the results of low dose dobutamine stress echocardiography in the evaluation of low flow, low gradient aortic stenosis:

Evaluation of low flow, low gradient aortic stenosis by low dose dobutamine stress echocardiography

Utilisation of Echocardiography

Demonstration of Diastolic Mitral Regurgitation due to Severe Aortic Stenosis:

Diastolic mitral regurgitation due to severe aortic stenosis

Demonstration of Aortic regurgitation combined with Aortic Stenosis:

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Demonstration of Calcific Aortic Stenosis with Mitral Annular Calcification and Moderate Mitral Regurgitation:

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Low flow Aortic Stenosis:

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Severe Aortic Stenosis without Aortic Regurgitation:

Continuous wave Doppler below shows a peak velocity across the aortic valve of a gradient of 85 mm Hg, which is compatible with severe aortic stenosis. No aortic regurgitation is present.

2020 ACC/AHA Guideline for the Management of Patients With Valvular Heart Disease: A Report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines

Recommendations for Diagnostic Testing: Initial Diagnosis of AS Referenced studies that support the recommendations are summarized in the Online Data Supplement

Class I

1.    In patients with signs or symptoms of AS or a BAV, TTE is indicated for accurate diagnosis of the cause of AS, assessment of hemodynamic severity, measurement of LV size and systolic function, and determination of prognosis and timing of valve intervention(Level of Evidence: A) 2.   In patients with suspected low-flow, low-gradient severe AS with normal LVEF (Stage D3), optimization of blood pressure control is recommended before measurement of AS severity by TTE, TEE, cardiac catheterization, or CMR.(Level of Evidence: B-NR)

Class IIa

3.   In patients with suspected low-flow, low-gradient severe AS with reduced LVEF (Stage D2), low-dose dobutamine stress testing with echocardiographic or invasive hemodynamic measurements is reasonable to further define severity and assess contractile reserve.(Level of Evidence: B-NR)

AHA/ACC 2014 Guideline for the Management of Patients With Valvular Heart Diseases

Diagnosis and Follow-Up

Class I

"1. TTE is indicated in patients with signs or symptoms of AS or a bicuspid aortic valve for accurate diagnosis of the cause of AS, hemodynamic severity, LV size, and systolic function, and for determining prognosis and timing of valve intervention. (Level of Evidence: B)"

Class IIa

"1. Low-dose dobutamine stress testing using echocardiographic or invasive hemodynamic measurements is reasonable in patients with stage D2 AS with all of the following: a. Calcified aortic valve with reduced systolic opening; b. LVEF less than 50%; c. Calculated valve area 1.0 cm2 or less; and d. Aortic velocity less than 4.0 m per second or mean pressure gradient less than 40 mm Hg. (Level of Evidence: B)"

2008 and Incorporated 2006 ACC/AHA Guidelines for the Management of Patients with Valvular Heart Disease (DO NOT EDIT)

Echocardiography (DO NOT EDIT)

Class I

"1. Echocardiography is recommended for the diagnosis and assessment of aortic stenosis severity. (Level of Evidence: B)"

"2. Echocardiography is recommended in patients with aortic stenosis for the assessment of left ventricular wall thickness, size, and function. (Level of Evidence: B)"

"3. Echocardiography is recommended for re-evaluation of patients with known aortic stenosis and changing symptoms or signs. (Level of Evidence: B)"

"4. Echocardiography is recommended for the assessment of changes in hemodynamic severity and left ventricular function in patients with known aortic stenosis during pregnancy. (Level of Evidence: B)"

"5. Transthoracic echocardiography is recommended for re-evaluation of asymptomatic patients: every year for severe aortic stenosis; every 1 to 2 years for moderate aortic stenosis; and every 3 to 5 years for mild aortic stenosis. (Level of Evidence: B)"

Class IIa

"1. Dobutamine stress echocardiography is reasonable to evaluate patients with low-flow/low-gradient AS and LV dysfunction. (Level of Evidence: B)"

Asymptomatic Adolescents (DO NOT EDIT)

Class I

"1. Doppler echocardiography is recommended yearly in the asymptomatic adolescent or young adult with AS who has a Doppler mean gradient greater than 30 mm Hg or a peak velocity greater than 3.5 m per second (peak gradient greater than 50 mm Hg) and every 2 years if the Doppler gradient is less than or equal to 30 mm Hg or the peak jet velocity is less than or equal to 3.5 m per second (peak gradient less than or equal to 50 mm Hg). (Level of Evidence: C)"