This article was written in collaboration with Dr. Nelson Schiller, Founder of the UCSF Adult Doppler Echocardiography Laboratory and the Adult Congenital Heart Disease clinic.
Since its development in the 1970s, Doppler echocardiography has occupied a position of increasing importance in the assessment of patients with valvular heart disease. Continuous-wave and pulsed Doppler measurements, as well as color Doppler, have been used to monitor the progression of chronic valvular disease. Then, in 1980, Trondheim Norway pioneer Liv Hatle showed that by applying continuous Doppler echocardiography to regurgitant and obstructive jets, accurate hemodynamic data could be derived. This application of continuous-wave Doppler has replaced measurements that had previously been limited to the invasive cardiac catheterization laboratory. It is now rare for a cardiologist to use catheters to measure the severity of aortic stenosis: this application has been completely supplanted by a non-invasive Doppler.
Another great clinical contribution of the Doppler echocardiography has been the estimation of the pulmonary artery pressure in clinical practice. In a revolutionary study, Stanford researchers Paul Yock and Richard Popp validated the use of the sum of continuous-wave Doppler tricuspid gradient and right atrial pressure to estimate peak and mean pulmonary pressure. In estimating right atrial pressure, echocardiographic imaging of the inferior vena cava has improved our estimation of right atrial pressure. Accurate knowledge of right atrial pressure is a keystone in the treatment of congestive heart failure.
DOPPLER ECHOCARDIOGRAPHIC EVALUATION OF VALVULAR REGURGITATION:
Mitral regurgitation is a common valvular lesion often leading to excessive left ventricular dilatation and dysfunction and the timing for surgical or non-surgical repair is difficult and controversial. Echocardiography, particularly with TEE, allows for the identification of the mechanism of mitral regurgitation. The timing of the regurgitation is important as late systolic regurgitation seen in MVP may not be as severe as regurgitation occurring during the whole systole. Quantification of the amount of regurgitation remains a challenge and can be influenced by hemodynamics. The blood pressure should always be documented as part of the evaluation of the patient. There are semi-quantitative and quantitative methods. Doppler evaluated the jet extent and how far it reaches into the left atrium. When mitral regurgitation results in pulmonary venous systolic flow reversal, it is severe in the majority of cases (>85%). Doppler also allows the visualization of the origin and the size of the regurgitant jet. This is called the “vena contracta”. The cross-sectional area of the vena contracta represents a measure of the effective orifice area (EROA). These can be affected by several technical factors that include Nyquist limit, Doppler gain, transducer, and pulse repetition frequency as well as jet constraints such as very eccentric jets.
In valvular regurgitation, blood flow converges toward the regurgitant orifice, forming concentric and hemispheric shells of increasing velocity and decreasing surface area. This convergence method also called proximal iso velocity surface area (PISA) can be used to calculate the EROA and regurgitant volume. It is relatively easy, widely used, and can provide prognostic data. However, it is somewhat limited in asymmetric orifices, non-holosystolic regurgitation, functional regurgitation and small errors in its measurement can lead to large percent errors in regurgitation volumes. For these reasons, a volumetric approach to quantitate regurgitation may be desirable.
In the 1980s, we validated a method for the quantitation of the regurgitant fraction by using a combination of 2D echocardiographic LV stroke volume and the forward stroke volume determined by the Doppler signal across the aortic valve. A more practical method of using pulsed Doppler signal across the LV outflow tract would become standard across echo labs since it could be applied to patients with aortic valve pathologies. The total stroke volume can also be calculated by integrating the doppler mitral inflow and the mitral valve annular area. The volumetric methods have no geometric assumptions, they work in eccentric jets and after mitral clip where multiple jets can be present. They are more time-consuming however and they allow for only one valvular regurgitation to be calculated (Mitral or Aortic). The 3D approach should be used for the calculation of the left ventricular size and function since it is more accurate.
In patients with isolated aortic insufficiency, it is important to visualize the three components of the color jet (jet area, vena contracta, and flow convergence) as well as document the patient’s blood pressure which can influence the degree of regurgitation. The width of the jet in the LV outflow tract can be measured in centrally directed jets in a semiquantitative way with ratios of >65% indicating significant regurgitation.
The regurgitant volume is responsible for both the hemodynamic changes and the clinical presentation. In the 80s we evaluated a Color Doppler flow mapping to measure maximal regurgitant volume in early diastole. When correlated to LV volume and mass, can evaluate the appropriateness of adaptation of the left ventricle to the volume load. An imbalance between the increased volume and mass may herald a poor prognosis. The regurgitant volume can be calculated by subtracting the forward stroke volume in the LV outflow tract and the inflow stroke volume at the mitral orifice area.
THE FUTURE: AUTOMATED ANALYSIS OF DOPPLER ECHOCARDIOGRAPHY AND STREAMLINING MANAGEMENT.
Deep learning frameworks to automatically analyze echocardiographic data and videos for the presence of valvular heart disease have been tested and shown to increase the efficiency of the clinical workflow for screening for valvular heart disease and for quantifying metrics of disease severity. This has the possibility to reduce inter-observer variability and remove disparities in the quality of care provided by different healthcare professionals. The automated measurements showed good agreement with the expert reads. These could be easily integrated with a 3D acquisition of the echo and Doppler parameters. Potentially, AI be integrated with the Heart team approach and facilitate referrals to expert centers for treating valvular heart disease?