A more detailed discussion of the Doppler ultrasound technique is given by Rgy.s.ton,,a^ and Espersen.eta/ (1995).
The Doppler equation enables the velocity of a moving object to be calculated from the shift in reflected frequency of a sound wave of known frequency:
where V is the velocity of moving blood corpuscles, D f is the Doppler frequency shift, C is the velocity of sound in tissue, ft is the transmitted frequency, and q is the angle between the ultrasound beam and the flow direction.
Doppler frequency shift signals can be displayed as velocity-time waveforms ( Fig, 1). High-frequency ultrasound waves (usually 2-4 MHz) are used for aortic blood flow measurement. A suprasternal approach, which is easy to perform, non-invasive, and painless, can be used. However, the probe cannot be fixed to enable continuous monitoring and signal acquisition is difficult in 5 per cent of cases (e.g. patients with a short neck, emphysema, or aortic valve disease). With sufficient experience accuracy is good.
Cycle Flo*. Tirw
Fig. 1 Doppler flow-velocity waveform.
Cycle Flo*. Tirw
Other approaches have been tried, of which descending thoracic aortic blood flow monitoring via the esophagus has proved the most successful. The Esophageal Doppler Monitor (marketed by Abbott Laboratories in Europe and Deltex in the United States) possesses a monitor to verify correct signal measurement and has undergone numerous single- and multicenter validation studies to ensure, with adequate training, reliable and reproducible results within minutes. A probe of diameter 6 mm inserted 35 to 40 cm into the distal esophagus is oriented to obtain a characteristic aortic flow signal. The area (integral) under each velocity-time waveform—the stroke distance—represents stroke volume flowing down the descending thoracic aorta. Applying a correction factor from a nomogram incorporating the patient's age, height, and weight enables an estimate of total left ventricular stroke volume to be determined with 85 to 90 per cent accuracy. Intra- and interobserver variability is low and trend-following is accurate despite wide variations in flow and blood pressure.
Only aortic coarctation and concurrent use of intra-aortic balloon counterpulsation preclude measurement. Aortic regurgitation produces a characteristic reverse flow throughout diastole. Caution should be observed in patients with esophageal varices or other local pathology, and in those with marked coagulopathies; however, no serious adverse event has yet been reported.
The waveform shape provides valuable information on left ventricular preload, afterload, and resistance. There are age-related normal ranges for peak velocity. Values outside this range are indicative of hypo- or hyperdynamic circulations. The flow time can be automatically corrected for heart rate by dividing it by the square root of the cycle time (FTc). This is inversely proportional to systemic vascular resistance. A short FTc is due to hypovolemia, flow obstruction, or excess arterial constriction (e.g. excessive vasopressor dosage). Preload changes predominantly affect the FTc, inotropic changes mainly affect the peak velocity, while afterload changes have an intermediate effect. The effects of therapy can be readily appreciated on a beat-by-beat basis, and Starling-like curves can be constructed to optimize fluid therapy.
Was this article helpful?