Question regarding continuity equation for aortic valve area

[cardio]

Hello,
I have a clinical question in which you may be able to help. Are there clinical settings in which the continuity equation for aortic valve areas should not be used, such as LVOT obstruction and so forth? I have heard every "method " under the sun to regulate the variance in the equation such as, Always use 2 for the LVOT diameter and always make the LVOT velocity 1 by backing either in or out of the valve. These solutions do not make sense nor due justice to the study. There are many times the LVOT is 1.5 m/s throwing the eqaution off and things like that. Thanks
Gary

[cardio]

The continuity equation is a reliable and accurate method for measuring aortic valve area (effective orifice area) in most aortic stenosis (AS) patients. The continuity equation states that flow in the left ventricular outflow tract (LVOT) must equal flow through the stenotic aortic valve.

LVOT flow is obtained by measuring the LVOT diameter and LVOT velocity. However, several assumptions are required to derive LVOT flow. First, we assume that that LVOT is circular and therefore use the equation of a circle to obtain an area from the diameter measurement. Second, we assume that the LVOT velocity has a flat spatial profile, such that the same velocity is present across the LVOT from septum to mitral leaflet aspect. Third, we assume that the LVOT diameter and velocity measurements are obtained at the same site. Fourth, we assume that the transvalvular velocity, which we measure by CW Doppler, also has a flat spatial velocity profile across the narrowed orifice area. While studies have questioned the validity of each of these assumptions, derived aortic valve areas using this technique have correlated well with valve area measurements by other techniques and with clinical outcome.

The continuity equation is accurate in AS patients with concomitant aortic regurgitation, mitral valve disease and left ventricular dysfunction. However, problems can be encountered in patients with a sigmoid septum or septum that protrudes into the LVOT. It is inherent in the continuity equation that the LVOT velocity and LVOT diameter are measured at the same site to derive LVOT flow. Unfortunately, the LVOT may not have a nice cylindrical appearance in these patients and the LVOT diameter may vary widely from the annulus to the site of septal protrusion. LVOT velocity may also differ significantly within the LVOT, being much larger at the site of septal protrusion. Ultimately, you need to decide where you obtained your “best quality” LVOT velocity, and use the corresponding diameter measurement from the same site. Using an assumed LVOT velocity or LVOT diameter is not logical and should not be employed in this situation. It is important to check that the continuity equation valve area measurement is consistent with the appearance of the aortic valve on 2D imaging. Other techniques such as planimetry of the aortic valve area should be attempted in these patients.

The continuity equation cannot be used in AS patients with dynamic LVOT obstruction, such as concomitant hypertrophic obstructive cardiomyopathy (HOCM), or in AS patients with a subvalvular membrane. In HOCM patients, laminar LVOT velocity signals with minimal spectral dispersion cannot be obtained and CW Doppler velocity signals may originate from the LVOT rather than the stenotic aortic valve. In the presence of a subvalvular membrane, LVOT flow measurements are inaccurate as the assumption of a flat LVOT spatial velocity profile is invalid. Furthermore, high CW Doppler velocity signals may originate from the subvalvular membrane rather than the aortic valve. Finally, continuity equation aortic valve area may be inaccurate when the LVOT spatial velocity profile is skewed, as can occur in the AS patient with a concomitant peri-membraneous ventricular septal defect. In these situations, planimetry of the aortic valve area by transthoracic or transesophageal echocardiography is the preferred technique to measure aortic valve area.

Dr. Ian Burwash
Ottawa Heart Institute