Measuring Transvalvular Flow Rate Adds to Prognostication About Aortic Stenosis

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Aortic valve area (AVA) is a central criterion for assessing the severity of aortic stenosis (AS). Severe AS is defined as both low AVA (≤1.0 cm² in Europe, <1.0 cm² in the U.S.) and high mean gradient (≥40 mm Hg).

AVA is highly dependent on the transvalvular flow rate (Q), the average volume of blood passing through the aortic valve per unit of time during ventricular ejection. Q is calculated as the ratio of stroke volume to ejection time. Therefore, at low Q, maximal ("true") AVA may not be induced due to inadequate valve opening.

In the Journal of the American College of Cardiology, Mayooran Namasivayam, MBBS, PhD, research fellow in Medicine, and Judy Hung, MD, director of the Echocardiography Lab in the Corrigan Minehan Heart Center at Massachusetts General Hospital, and colleagues confirmed that low AVA measured at low Q does not reliably detect severe AS. This novel finding has the potential to widely affect clinical care, and the researchers offer an algorithm for incorporating Q into the assessment of AS.

Study Methods

The researchers calculated Q for each of 1,131 patients with moderate or severe AS (AVA ≤1.5 cm² or mean gradient ≥20 mm Hg) who underwent echocardiography between 2006 and 2016.

Most patients had normal ejection fraction, which is clinically important. Low-gradient AS with preserved EF is far more prevalent than low-gradient AS with reduced EF and is an area of diagnostic uncertainty in severe AS.

Effect of Q on Prognostic Value of AVA

Median follow-up time was 3.9 years (maximum, 7.3 years). Q determined the prognostic value of AVA:

• For patients with Q below the median (≤242 mL/s), AVA ≤1.0 cm² was not prognostic for mortality
• For patients with Q above the median, AVA ≤1.0 cm² was highly prognostic for mortality (HR, 1.66; 95% CI, 1.19–2.33; P = .003)

This difference was not affected by age, sex, valve replacement, comorbidity, medication or other echocardiographic data. The optimal cutoff for Q as a prognostic marker was 210 mL/s.

The results were similar in a validation cohort of 939 patients from the Quebec Heart and Lung Institute for whom follow-up was longer (median, 5.8 years; maximum, 13.5 years).

Reconsidering the Algorithm

In light of these study findings, the definition of severe aortic stenosis based on low AVA cannot be applied uniformly. Q should be incorporated into the classification algorithm for AS diagnosis and severity:

• AVA ≤1.0 cm² and mean gradient ≥40 mm Hg: severe AS
• AVA ≤1.0 cm² and mean gradient <40 mm Hg—consider Q as follows:
  • Q ≤ 210 mL/s: AVA is not prognostic for mortality; assess the patient with an alternative modality such as computed tomography calcium score
  • Q > 210 mL/s: severe AS

As this algorithm shows, Q assessment can help with the common problem of having to clarify true AS severity when measures are discordant (AVA ≤1.0 cm² but mean gradient <40 mm Hg).

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