Aortic valve area

Overview
The aortic valve area is the size of the orifice for blood to flow from the left ventricle to the aorta. The aortic valve area is reduced in aortic stenosis, and the aortic valve area is the metric that is used to gauge the need for aortic valve replacement surgery. The pressure gradient across a narrowed aortic valve cannot be used to gauge the need for valve replacement as the gradient may be low in patients with impaired left ventricular function.

Pathophysiology
The normal aortic valve offers little or no resistance to the blood flow across the valve despite the high flow velocities. With progressive narrowing of the aortic valve (aortic stenosis), the aortic valve orifice offers progressively greater resistance to the blood flow through the valve with a consequent in the pressure gradient between the left ventricle and the aorta. The latter can be calculated using echocardiographic flow velocities while the trans-valvular pressure gradient can be calculated using the following equation: "Pressure Gradient = 4 x (velocity of blood through the valve)2 mmHg"

However, the absence of a large gradient across the aortic valve does not exclude the presence of critical aortic stenosis. Patients with left ventricular dysfunction can no longer generate a pressure gradient, and they are referred to as having low pressure, low flow, low ejection fraction aortic stenosis. Therefore, it is for this reason that the best measure of the severity of aortic stenosis is the aortic valve area and not the aortic valve gradient.

Cardiac Catheterization

 * Simultaneous measurement of left ventricular output (measures the flow through the aortic valve) and the pressure gradient across the aortic valve provides the variables that are required to calculate the aortic valve area and resistance.


 * Fluid dynamic mediated subvalvular pressure gradients are often present in patients with severe aortic stenosis in the absence of an anatomic subvalvular obstruction and constitute ~50% of the total measured transvalvular gradient. The extent of increase in cardiac output during exercise is inversely related to the magnitude of subvalvular gradient.

Aortic Valve Area:

 * According to the current recommendations, following dobutamine infusion, if the aortic valve area increases to >1.2 cm2, and the mean pressure gradient rises above 30 mmHg, such patients may benefit from aortic valve replacement. Failure to achieve these improvements has shown to be associated with higher early surgical mortality in comparison to patients who can augment their contractility and gradient: 32-33% versus 5-7%, respectively. Additionally, 5-year survival was lower in patients who could not augment their contractility in comparison to those who could: 10–25% versus 88%, respectively.


 * Aortic valve area can be calculated by the following two equations:

Gorlin Equation:
"Aortic Valve Area (cms2) = (Stroke volume (mL/beat) ÷ Systolic ejection period (secs/beat)) ÷ ( 44.3 x square root of mean systolic pressure gradient between the left ventricle and aorta (mmHg))"

Hakki Equation:
"Aortic Valve Area (cms2) = (Cardiac output (liters/minute)) ÷ (Square root of mean systolic pressure gradient between the left ventricle and aorta (mmHg))"

Aortic Valve Resistance:

 * Cannon JD Jr et al, described the benefit of calculating aortic valve resistance in conjunction with the Gorlin formula to differentiate patients with true severe aortic stenosis from those with mild aortic stenosis. The clinical implication of this differentiation is that patients with mild aortic stenosis may not benefit from aortic valve replacement.


 * Patients with high cardiac output with minimal or no increase in pressure gradient also fall into the category of mild aortic stenosis and may not benefit from aortic valve replacement.


 * Furthermore, aortic valve resistance is less flow-dependent than aortic valve area which is of particular benefit in patients with low output aortic stenosis.


 * Aortic valve resistance can be calculated using the equation:

"Aortic Valve Resistance (dyne seconds per cms5) = { (Mean Pressure Gradient between the left ventricle and aorta (mmHg) x Heart Rate (beats/min) x Systolic ejection period (secs/beat) ) ÷ Cardiac output } x 1.33"


 * Unsteady fluid-dynamics support the use of aortic valve area calculation over other measures of aortic stenosis including aortic valve resistance.

Echocardiography

 * 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.

Aortic Valve Area:

 * 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.

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.