Cardiac outputnoninvasive 1 Doppler ultrasound

An ultrasound beam of known frequency is reflected by moving red blood corpuscles with a shift in frequency proportional to the blood flow velocity. The actual velocity can be calculated from the Doppler equation which requires the cosine of the vector between the direction of the ultrasound beam and that of blood flow. This has been applied to blood flow in the ascending aorta and aortic arch (via a suprasternal approach), descending thoracic aorta (oesophageal approach) and intracardiac flow (e.g. transmitral from an apical approach). Spectral analysis of the Doppler frequency shifts produces velocity-time waveforms, the area of which represents the 'stroke distance', i.e. the distance travelled by a column of blood with each left ventricular systole (see figure opposite). The product of stroke distance and aortic (or mitral valve) cross-sectional area is stroke volume. Cross-sectional area can be measured echocardiographically; however, as both operator expertise and equipment is required, this additional measurement can be either ignored or assumed from nomograms to provide a reasonable estimate of stroke volume.

Advantages

Quick, safe, minimally invasive, reasonably accurate, continuous (via oesophageal approach), other information on contractility, preload and afterload from waveform shape (see figure opposite).

Disadvantages

Non-continuous (unless via oesophagus), learning curve, operator dependent.

0 0

Post a comment