In deciding which symptomatic subjects with aortic stenosis (AS) need surgery, most authorities would agree that a subject with aortic valve area less than 1 cm^2 ( or <0.6 cm^2/m^2, if indexed to body surface area) on transthoracic echo (TTE), along with a mean transaortic gradient of >40 mm Hg, or a transaortic valve peak velocity of 400 cm/s have severe AS, and should proceed to surgery. But what of those symptomatic subjects, who have a valve area less than 1 cm^2 on echo, but have a peak velocity of flow or a mean transaortic gradient below the thresholds described above in association with reduced left ventricular ejection fraction (LVEF)? In such patients, it can be difficult to determine whether the reduced LVEF is, in fact, a consequence of AS itself (and who would therefore benefit from valve replacement) or whether the apparently reduced orifice of the aortic valve is due to the reduced transvalvular flow caused by low ejection fraction (EF) caused by the underlying heart failure. The latter subjects would obviously not benefit from valve replacement.
The difficulty in distinguishing between these two possibilities is not confined to echocardiographic methods. The tradional method of determing the severity of aortic stenosis has been Gorlin and Gorlin's seminal approach described in the 1950s, and described by the following equation:
Aortic Valve Area= Cardiac Output/Systolic Ejection Period*Heart Rate*44.3*Sqroot Mean Pressure Gradient.
The problem with the Gorlin formula is that it uses a constant that is flow dependent, i.e. it assumes a normal LVEF. It is therefore likely to be less accurate in subjects with heart failure. Most cardiologists would think long and hard before subjecting such subjects to the ordeal of cardiac catheterisation. It has also been pointed out that while measuring the left ventricular pressure (as part of the calculation of the pressure gradient), the catheter itself is occupying an already stenotic aortic valve orifice and might therefore confound pressure measurements.
Fortunately, it is possible to avoid such tricky situations by skillful use of stress TTE with dobutamine. Before we go there, it might be useful to summarise the principles of transvalvular pressure measurement on Echo.
Measurement of transaortic pressure depends on a simplification of the Bernoulli principle. Thus, the pressure gradient Delta P is described as:
Delta P = [4* (transaortic valve peak velocity)^2] - [4* (subaortic peak velocity)^2]
The rationale for subtracting the component contributed by subaortic flow may seem confusing until you consider what happens to a subject who has both AS and HOCM. The accelerated upward flow from HOCM boosts the aortic flow, and if we are not careful, will lead to an overestimation of the transaortic gradient. Fortnately, in most subjects, the contribution from subaortic flow is negligible, and can be ignored. The Bernoulli equation therefore boils down to:
Delta P = 4V^2, where V is the transaortic valve peak velocity.
Upon stimulation with dobutamine (5 microgram/min, increasing to 20 microgram/min), subjects with severe AS but normal LVEF, i.e. those with "true" stenosis, fail to increase their aortic valve area. However, their transaortic gradient , and with it, the valve resistance, increases. The concept of valve resistance requires an explanation.
Pressure = Flow*Resistance
or Resistance = Pressure/Flow
Flow is of course cardiac output in cm^3/s. The unit for pressure is dyne/cm^2. The unit for Resistance is therefore dyne.second.cm^-5.
Most cardiologists do not feel that measurement of valve resistance adds to information already gained from measurement of transvalvular pressure gradient.
In subjects who have reduced LVEF due to underlying myocardial failure rather than due to AS itself, stressing with dobutamine increases flow and therefore leads to an increased valve area on TTE. The transvalvular pressure gradient and vascular resistance falls. This is described as "Pseudo-stenosis".
Some authorities make a distinction between subjects who have reduced LVEF in association with concentric LVH, and those who have reduced LVEF but no LVH. The former group tend to have a worse prognosis.
Surgical outcomes tend to be good after valve replacement in subjects with "True" stenosis but poor in subjects with "Pseudostenosis". This distinction is therefore important.
In those with true stenosis, a further distinction may be based on a concept called "Contractile Reserve" (CR). CR is the increase in stroke volume with dobutamine stress. Those who have a CR>20%, i.e. those who increase their stroke volume by >20% with dobutamine, have better surgical outcomes than those subjects who have a CR<20%. However, it is important to mention that even these latter subjects have better prognosis with surgery than with medical management alone. Surgery should therefore not be witheld from such subjects. Sometimes the effective area of a heavily calcified, stenotic valve area can be difficult to determine on TTE. In such subjects, the continuity principle may be used. The continuity principle is an extension of Pascal's Law and assumes that flow in a (periodically) closed system such as the heart is the same throughout. Flow = Area*Velocity Since flow through the aortic valve equals flow in the aortic outflow tract (the vena contracta), it follows that Aortic Valve Area (AVA)*Transaortic velocity (TaV) = Area of aortic outflow tract (AO) * Velocity in aortic outflow tract (AoV). Thus AVA= AO*AoV/TaV In some subjects, the area of the aortic outflow tract is easier to measure than that of the aortic valve itself. The continuity principle is useful in such cases. Most cardiologists prefer to use the velocity time integral (VTI) rather than the instantaneous peak velocity in such calculations. Thus, effective Aortic Valve Area = Area of Aortic outflow tract* VTI in aortic outflow tract/ VTI through aortic valve. Subjects with similar valve areas can often have different clinical outcomes. It is thought that the extent to which the energy imparted by the left ventricle is dissipated as the blood passes through the stenotic aortic valve, predicts prognosis in such subjects even better than the area of the stenotic valve itself. This is epitomised by the Energy loss index. Energy Loss index (ELI)= (Aortic valve area*Area of the aorta at the sino-tubular junction/Area at the sinotubular junction-Aortic valve area)/ Body surface area. Subjects with an ELI< 0.52 cm^2/m^2 have a significantly poorer prognosis than those with a higher ELI. References: 1. Uptodate 2. Simultaneous Determination of Aortic Valve Area by the Gorlin Formula and by Transesophageal Echocardiography Under Different Transvalvular Flow Conditions: Evidence That Anatomic Aortic Valve Area Does Not Change With Variations in Flow in Aortic Stenosis. J Am Coll Cardiol. 1997;29(6):1296-1302. doi:10.1016/S0735-1097(97)00060-0 (Open access).
