Normal Antegrade Intracardiac Flows
Normal antegrade intracardiac flows are the result of the complex process that leads the oxygenated blood from the pulmonary veins in the left atrium, then across mitral valve in the left ventricle and finally across aortic valve in the aorta (on the left side of the heart, but it applies to venous blood for the right heart). Although the intracardiac circulation is pulsatile with variable velocity profile during diastole and systole, the flow passing through normal heart valves and great vessel tends to be laminar. Since these characteristics, Doppler echocardiography can assess blood flow through quantification of blood velocity.
Usually we are able to visualize patterns of intracardiac flows with color-Doppler such as valvular regurgitation, intracardiac shunts or high velocity flows. Spectral Doppler can be performed to assess blood flow across mitral annulus, tricuspidal annulus and the left or right ventricular outflow tract (LVOT and RVOT) (by PW Doppler). In the absence of significant valvular regurgitation the range of blood velocities is narrow with a well-defined and dense spectral signal that can be traced (figure 1A). The flow across the four valves is virtually the same in the normal heart and corresponds to stroke volume; therefore intracardiac flow is crucial for calculation of cardiac output.
Figure 1. PW Doppler signal obtained at LVOT from the apical 5-chamber view (A). The spectral waveform is displayed below the baseline because the blood is directed away from the transducer. Since the flow tends to be laminar, it appears more dense at the edge than inside. Assuming a circular shape of LVOT, the precise measurement of LVOT diameter at the level of PW sampling (B) allows an assessment of stroke volume (see text for explanation). (LVOT: left ventricular outflow tract).
The flow coming from pulmonary veins to left atrium can be well appreciated, by color-Doppler, from apical view as red-coded stream emerging from atrial roof and lateral wall (usually the right inferior vein cannot be visualized by transthoracic echocardiography); instead the flow originating by inferior and superior vena cava can be visualized by subcostal view. Generally, the pulmonary venous flow is assessed, by PW Doppler, sampling the orifice of right superior vein; in normal subjects with sinus rhythm, the spectral Doppler displays a systolic, positive wave, that sometimes appears biphasic and a diastolic positive wave; finally a small negative wave can be recorded, related to atrial contraction (atrial flow reversal)(figure 2).
Figure 2. PW Doppler atrial inflow sampled at the drainage of right upper pulmonary vein in the left atrium. It can be recognized a systolic wave (S) above the baseline due to atrial relaxation and descent of mitral annulus toward the cardiac apex during systole; then, a diastolic wave (D) related to the atrial conduit function; at last a small reversal atrial wave (Ar) can be appreciated corresponding to atrial contraction.
Ventricular inflow can be evaluated from different views but the best way is the apical one, because the blood is directed toward the transducer. It is encoded, by color-Doppler, as a wide red column streaming from left or right atrium to the corresponding ventricular apex during diastole (Figure). By PW-Doppler, mitral inflow (and tricuspidal) is represented as a double-peaked velocity profile; it corresponds to the early high-velocity filling phase (called E-wave) and late atrial contraction phase (A-wave) of diastolic process (figure 3); in healthy subjects the E-wave exceeds the A-wave, whilst their relation changes in different stages of diastolic dysfunction.
Figure 3. The typical pattern of transmitral flow velocities recorded at the level of mitral leaflets tips by an apical 4-chamber view and displayed as spectral Doppler signal over the baseline. The higher wave (E-wave) reflects the high velocity filling flow; it follows a short tract near to baseline related to the diastasis; the diastolic process is concluded by the A-wave corresponding to atrial contraction.
Color Doppler is usually employed to study the flow in the LVOT (from apical 3- or 5-chamber view) and RVOT (from parasternal right ventricular view or short axis view at aortic valve level). It appears as a blue encoded (away from the transducer) flow column straight to the semilunar valve. By spectral Doppler modality is displayed as a negative trace with a steep slope and a less fast upstream (figure 1A)..
The non-invasive determination of intracardiac flow (and stroke volume) requires the integration of blood velocities signals over cardiac cycle (time-velocity integral or TVI) and calculation of cross-sectional area at the same point in which the Doppler signal is sampled (figure 1). TVI can easily achieved by tracing the Doppler velocity profile at the level of atrioventricular annulus (by four-chamber view, in diastole), LVOT (by five-chamber view in systole) or RVOT (parasternal short-axis view); a close alignment between ultrasound beam and blood direction (<20°) is essential for correct measurement of flow velocity. Cross-sectional area can be measured by planimetry of a valvular annulus, if feasible, or by determination of diameter if we assume a circular shape of orifice; care should be taken to ensure accurate determination of diameters because the potential error is determinant in the final calculation. Overall, intracardiac flow can be calculated as follows, i.e. at the level of LVOT:
LVOT Flow (cm3) = VTI LVOT (cm) * area LVOT (cm2).
3D Color-Doppler Quantification
Three-dimensional color-flow Doppler overcomes the inaccuracies in measurements of LVOT and mitral annulus, assumptions on geometric shape, flow profile and angle dependency of spectral Doppler and permits the contemporary and more reproducible quantification of mitral inflow and aortic stroke volume.
- ↑ Kilner PJ, Yang GZ, Wilkes AJ, et al. Asymmetric redirection of flow through the heart. Nature 2000; 404:759-761.
- ↑ 2.0 2.1 Kallmeyer A, Zamorano JL, Locorotondo G et al. Non-invasive haemodynamic assessment. In: Galiuto L, Badano L Fox K, Sicari R, Zamorano JL. The EAE Textbook of Echocardiography. Oxford University Press, London, 2011.
- ↑ Zoghbi WA, Enriquez-Sarano M, Foster E, et al. Recommendations for evaluation of the severity of valvular regurgitation with two-dimensional and Doppler echocardiography. J Am Soc Echocardiogr 2003;16:777-82.
- ↑ Nagueh SF, Appleton CP, Gillebert TC, et al. Recommendations for the Evaluation of Left Ventricular Diastolic Function by Echocardiography. Eur J Echocardiogr. 2009;10(2):165-93.
- ↑ Galderisi M, Henein MY, D’hooge J et al. Recommendations of the European Association of Echocardiography: how to use echo-Doppler in clinical trials: different modalities for different purposes. Eur J Echocardiogr. 2011;12(5):339-53.
- ↑ Feigenbaum H, Armstrong WF, Ryan T. Hemodynamics. In: Feigenbaum’s Echocardiography. 6th ed. Lippincott W&W, 2005.
- ↑ Thavendiranathan P, Liu S, Datta S et al. Automated quantification of mitral inflow and aortic outflow stroke volumes by three-dimensional real-time volume color-flow Doppler transthoracic echocardiography: comparison with pulsed-wave Doppler and cardiac magnetic resonance imaging. J Am Soc Echocardiogr 2012;25(1):56-65.