Doppler US accurate for spectrum of aortic valve measurements

Jan 25, 2006

Doppler ultrasound is accurate for measuring aortic stenosis, even in the small subgroup of patients with severely reduced left ventricular ejection fraction, according to French and Canadian researchers in the Journal of the American College of Cardiology.

Some earlier reports had concluded that the technique might significantly underestimate the size of the valvular opening, and thus overestimate the severity of stenosis in these patients.

"Although patients with severe aortic stenosis (AS) and severely reduced left ventricular (LV) ejection fraction represent approximately 5% of AS patients, they also represent the most challenging subset," wrote Lyes Kadem, Ph.D.; Régis Rieu Ph.D.; Dr. Jean Dumesnil; Dr. Philippe Pibarot; and colleagues from the Université de la Méditerranée in Marseilles, France, and the Institut de Recherches Cliniques in Montreal. "It is thus difficult to separate patients with true anatomically severe AS and concomitant LV systolic dysfunction from those with pseudosevere AS, i.e., reduced valve opening caused by poor LV function in the setting of incidental mild to moderate valve obstruction" (JACC, January 3, 2006, Vol. 47:1, pp. 131-137).

The complex experiment to assess ultrasound's accuracy in the presence of both AS and low EF involved a bioprosthetic valve and three rigid orifices tested in a mock flow circulation model using a wide spectrum of flow rates approximating the range of patients from severe LV function to normal resting output.

Continuous-wave US Doppler echocardiography was performed using an ATL Ultramark 9 scanner (Philips Medical Systems, Andover, MA) and a 2.5-mHz probe applied at the surface of the aorta and oriented to optimize the alignment of the Doppler beam and flow across the stenosis. Transvalvular Doppler velocity was performed over five to seven cycles and averaged.

In addition, particle image velocity (PIV) measurements were performed using a double-pulsed miniyttrium-aluminum garnet laser to image circulatory fluid seed with Amberlite particles. The PIV measurements were performed at 13 time points during systole to measure mean EOA. The velocity field was calculated by measuring the displacement of the particles in two consecutive images that were cross-correlated, the authors explained.

First, the results showed excellent overall correlation (r2 = 0.94) and agreement between EOA measured by Doppler and PIV, the group reported. The mean absolute and relative differences between the two measurement methods were 0.003 ± 0.103 cm² and 7.6 ± 8.5%, respectively. For rigid orifices measuring 0.5 and 1.0 cm, increasing flow rate yielded no significant change in EOA.

However, significant EOA increases were seen in both Doppler US and PIV when the stroke volume increased from 20 to 70 mL, both in the 1.5-cm rigid orifices (+52% for Doppler; +54% for PIV) and the bioprosthetic valve (+62% for Doppler; +63% for PIV). "The changes can be explained either by the presence of unsteady effects at low flow rates and/or by an increase in valve leaflet opening," Kadem and colleagues reported.

"Our hypothesis in this study was that the flow-related changes in EOA observed in rigid orifices are likely attributable to the predominance of unsteady effects at low flow rates, and it is corroborated by our results as well as those previously published by Voelker et al (Circulation, February 15, 1995, Vol. 91:4, pp. 1196-1204).

The EOA measurements support the concept that "the kinetic energy (proportional to the velocity squared) of the fluid crossing the orifice is sufficient to break down the vortex structures that are generated downstream from the stenosis, and thus enable the formation of a large and well-established flow jet, whereas at low flow rates ... the reduction in kinetic energy predisposes to the formation of vortices, which tend to squeeze the flow jet and thus the vena contracta, resulting in a smaller EOA," the team wrote.

On the other hand, when the same equation is applied to data obtained with the bioprosthetic valve, measured EOA was far lower than predicted EOA. "This finding suggests that in flexible orifices, only a part of the flow dependence of EOA may be explained by the unsteady effects, and the remaining part is likely attributable to an increase in valve leaflet opening occurring with increasing flow rate," the group wrote.

The study's major clinical impact is that changes in valve area calculated with Doppler continuity equation are not artifacts but represent real changes in EOA, the researchers wrote. In rigid orifices, the results confirm that for a given geometric orifice area, the EOA may vary as much as 50% depending on flow. As a result, the geometric orifice area measured by transthoractic or transesophageal echocardiography has a limited ability to predict the hemodynamic burden caused by stenotic valve orifice, the group concluded.

The two main mechanisms that appear to be responsible for EOA increase seen during dobutamine perfusion -- an increase in leaflet opening with increase flow rate and the predominance of unsteady effects at low flow rates -- mean that increasing EOA during dobutamine perfusion "should not necessarily be equated with a flexible valve, because it could also be caused by a change in the ratio between the unsteady effects and the inertial forces that might occur at low flow rates," they wrote.

Further studies will be needed to confirm the results, the authors wrote. However, the research will be undertaken with the knowledge that Doppler-derived EOAs are accurate over a wide range of valve sizes, functions, and flow rates, while geometric orifice area measurements are inherently limited, particularly in the setting of low flow rates, the group concluded.

By Eric Barnes
AuntMinnie.com staff writer
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