Aortic valvular flow : a clinical and experimental doppler echocardiographic study
Sammanfattning: Aortic blood velocity determination by ultrasound Doppler is a safe and valuable tool when evaluating patients with heart disease. The time-course of the velocity signal can be used for detemnining left ventricular function. In a mixed patient material, including patientswith ischaemic and valvular heart disease as well as heart transplants, stroke volume was noninvasively and accurately calculated during rest and exercise (-1 ± 7 m! [diffmean ±I SD]) using suprasternal pulsed Doppler registrations and parasternal 2D aortic anulus dimension imaging. Early during supine exercise (at 40% of upright maximal load) middleaged healthy women increased their cardiac output by 79%, stroke volume by 10%, aortic maximal flow velocity by 27% and maximal acceleration by 43%. This is in contrast to the reaction in women with coronary artery disease and previous infarction where no increase or, in certain patients, a fall in these variables was observed. In this way aortic ultrasound Doppler adds haemodynamic information to the conventional exercise test noninvasively.However, whether aortic ultrasound Doppler is used by itself (e.g. in calculations of velocity-changes, integrals and instantaneous pressure drop) or in combination with ultrasound tissue imaging (e.g. in calculations of cardiac output and stenotic valve areas), there are assumptions and simplifications made in the analysis. Theoretical and experimental analysis shows that a three-component Windkessel model is relevant when modelling early proximal aortic flow. Maximal aortic flow velocity and acceleration are not only influenced by the rate of pressure change - and thereby left ventricular con tractility - but also by aortic vessel characteristics. Besides the influence of aortic pressure change, maximal velocity is related to the compliance of aorta and great arteries and maximal acceleration inversely related to the characteristic impedance. This knowledge is of importance when interpreting the spectral Doppler signal. It also gives the future opportunity of assessing aortic compliance and characteristic impedance noninvasively if the aortic pressure change is known. For flow determinations by ultrasound Doppler, knowledge of the 3D flow profile is important. In calculations within the heart this profile is often assumed to be flat. Using 2D colour Doppler it is possible to .reconstruct a true spatial velocity profile using an external time delay device. A nearly flat, but slightly antero-septally skewed, flow profile was found in the subvalvular area in patients with moderate to severe aortic valvular stenosis and in these patients a parasternally measured flow diameter is a good estimate for the mean of two transverse flow axises. Stroke volume determination, using pulsed Doppler ultrasound in this area, may therefore be favourable in patients with aortic stenosis. In contrast to this, flow may be overestimated by more than 40% by using centerline velocity as an estimate of spatial mean velocity in certain patients with aortic regurgitation and in certain normals.
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