Basic views and diagnostic approaches to the AV were presented in the introduction to echocardiography at the start of this text and should be briefly reviewed before proceeding with this section. The role of echocardiography in the management of these patients cannot be underestimated.1
The aortic valve can be examined in the midesophageal short-axis (MESAX) and long-axis views (MELAX) of the aortic valve. The MESAX provides an ideal opportunity to examine the three leaflets of the valve. By using color flow Doppler turbulent flow across the AV can be seen. Planimetry (Figure 6–5) of the stenotic valve permits a rough estimate of the valve area. Planimetry of the AV is performed by obtaining the midesophageal short-axis view. Once the leaflets are imaged, the freeze button is pressed. Using the trace function the valvular orifice is highlighted. Estimates of AV orifice area by planimetry are at times unreliable secondary to poor visualization of the valve or secondary to leaflet thickening and calcification.11
The aortic valve short-axis view seen here permits planimetry of the aortic valve. By freezing the image the aortic valve orifice can be traced estimating its valve area. Often planimetry is not accurate as calcifications in the valve can make tracing its true outline difficult.
Stenotic orifice area can be calculated using the "continuity equation" which follows the conservation of flow or mass. As water flows down a river its velocity is slow where the river is wide and its velocity is fast where it is narrow—the rapids!!! The flow of blood thorough the LVOT and the aortic valve follows the same conservation law.
The aortic valve area can be calculated using either the conservation of flow or the conservation of mass.
According to the conservation of flow:
LVOT flow = LVOT CSA × LVOT Vmax
AV flow = AV CSA × AV Vmax
AV CSA = (LVOT CSA × LVOT Vmax)/AV Vmax
CSA = cross-sectional area
So if the velocity of blood in the LVOT is known and the cross-sectional area of the LVOT is also known, then the flow through the LVOT can be calculated. Obtaining the velocity of flow through the stenotic AV permits calculation of the stenotic valve area.
According to the conservation of mass, the same volume of blood that goes through the LVOT will traverse the AV:
LVOT volume = LVOT CSA × LVOT TVI
Recall that the TVI (time-velocity integral) represents the distance that the blood travels during systole as the result of the machine integrating the velocities with the systolic ejection time. The LVOT TVI is measured in the deep transgastric long-axis view, by placing the pulsed wave sample gate in the LVOT and tracing the spectral envelope obtained (Figure 6–6). The TVI is expressed in centimeters.
In this midesophageal long-axis view of the aortic valve is seen the anterior leaflet of the mitral valve, the aortic valve, the left ventricular outflow track, and the mitral valve. The image has been frozen permitting measurement of the diameter of the LVOT. This measurement is essential for use in the continuity equation to determine the area of the aortic valve.
CSA is estimated by approximating the LVOT with a cylinder. CSA in this setting is estimated for that of a circle or πr2, where r is the radius of the circle. By measuring the LVOT diameter (d) (Figure 6–7), the CSA of the LVOT can be calculated:
LVOT CSA = 0.785 × d2 (cm2)
Pulse waved Doppler is employed to determine the time-velocity integral (TVI) of the left ventricular outflow tract. A deep transgastric view is used to best align the Doppler beam with the direction of blood flow to minimize the angle of incidence between the two. Pulse wave Doppler is used because the maximal velocity is slow—under 60 cm/s. Tracing the velocity flow envelope allows the machine to calculate the time-velocity integral. Note that in this image there is no pressure gradient in the LVOT as would normally be expected.
Knowing both the CSA and the TVI of the LVOT permits calculation of the volume of blood going across the LVOT during systole.
Next, using the deep transgastric long-axis view, the Doppler beam is aligned in parallel with flow across the stenotic aortic valve and continuous wave Doppler is used to obtain the maximal velocity and the TVI across the AV (Figure 6–8).
The TVI of the aortic valve is calculated using continuous wave Doppler. Recall that pulse wave Doppler is useful for measurements at lower blood velocities. Here continuous wave Doppler has been aligned parallel to that aortic valve flow as imaged using the deep transgastric view. Of note, the blood velocity across the aortic valve is greater than 4 m/sec.
With these three elements it is possible to calculate the orifice area of the AV valve.
LVOT volume = LVOT CSA × LVOT TVI
AV volume = AV CSA × LVOT TVI
AV CSA = LVOT CSA × LVOT TVI/AV TVI
Pressure gradients across a narrowed aortic valve are also used as an estimate of the severity of AS. Here, too, echocardiography is essential in determining the maximal and mean pressure gradients across a narrowed orifice.
Recall from the introductory chapter the Bernoulli equation:
where V is the maximal velocity.
Generally flow within the heart proceeds at a velocity < 1.2 m/s. In areas of narrowing, velocity can increase to upward of 5 to 6 m/s. Using Bernoulli equation it is clear that a 5 m/s velocity would be associated with a 100 mm Hg pressure gradient.
It must be remembered that a low-pressure gradient does not exclude a narrowed aortic valve. The ventricle must be capable of doing sufficient pressure work to generate pressures of this degree. As heart failure ensues the gradient can fall simply because the LV can no longer generate such dramatic intracavitary pressures.
Echocardiography is likewise essential in the evaluation of AR.12,13 There are a number of approaches available to determine just how leaky the AV is. Anesthesiologists use everything from visual estimates of the regurgitant jet to more quantitative approaches.
AR can be evaluated in both MESAX and MELAX views of the aortic valve. Application of color flow Doppler in both views provides an image of the regurgitant flow during diastole. Both qualitative and quantitative methods are used to assess the severity of chronic AR.
One of the quantitative parameters is the width of the regurgitant jet relative to the width of the LVOT. Severe AR is associated with jets which occupy greater than 65% of the LVOT width.11 Unfortunately, the AR regurgitant jet can be subject to many factors unrelated to the severity of AR making it a less reliable estimate for AR severity.
Another quantitative parameter is the "vena contracta." Figure 6–9 demonstrates the vena contracta of a regurgitant jet. The vena contracta is the smallest diameter of the regurgitant jet measured at the level of the aortic valve. The size of the vena contracta should correspond to the area of the effective regurgitant orifice (EROA). Simply, vena contracta permits an estimation of how big is the leak through the aortic valve. A vena contracta greater than 6 mm is considered indicative of severe AR. An EROA greater than 0.30 cm2 is considered severe.
This color flow Doppler image of the midesophageal aortic valve long-axis view demonstrates measurement of the vena contracta of aortic regurgitation. The vena contracta represents the smallest diameter of the regurgitant jet at the level of the aortic valve. A vena contracta of 6.2 mm grades the aortic regurgitation in this case as severe.
Most patients encountered by the cardiac anesthesiologist will already have their diagnosis and a surgical plan at the time of surgery and determination of the severity of AV disease is usually unnecessary perioperatively (Figure 6–10). Occasionally, moderate aortic stenosis or aortic regurgitation is detected during the course of surgery, which necessitates a discussion between the anesthesiologist and surgeon regarding course of action. The recent guidelines of the American Heart Association1 provide definitions for mild, moderate, and severe AV disease.1 Moreover, they provide guidelines for indications for AV replacement (AVR). While incidentally discovered severe AS will generally be addressed by the surgeon, the risks and benefits of surgical intervention in the setting of incidentally discovered moderate disease should be considered by the surgeon in consultation with the patient's cardiologist.
Survival of patients with severe aortic stenosis with and without aortic valve replacement. Survival curves of patients with severe aortic stenosis by echocardiographic examination who were symptomatic (sym) or not symptomatic (no sym) and if they underwent aortic valve replacement. Survival clearly favors valve replacement. (From: Brown ML, Schaff HV, Lahr BD, et al. The benefits of early valve replacement in asymptomatic patients with severe aortic stenosis. J Thorac Cardiovasc Surg. 2008;135(2):308-315, with permission.)