5. Measure of organ blood flow

Direct cannulation and measurement: Doppler ultrasonography

this is possible, but impractical in any clinical context.

Doppler effect describes the change in the frequency of sound (including ultrasound) if either the emitter or the receiver is moving. If a noise source, for example a siren, moves towards a listener, the wavelength of the sound decreases, its frequency increases, and so its pitch rises. This principle is utilized in Doppler ultrasonography, in which ultrasound is directed at a diagonal from one crystal and is sensed by a second crystal as it reflects off red blood cells. The frequency of these reflected waves increases by an amount that is proportional to the velocity of flow towards the receiving crystal. It is difficult to calibrate a Doppler ultrasound probe to provide accurate quantitative measurements because determinations of vessel calibre may be inaccurate, and the shape of the flow profile may not be uniform. Nonetheless, the technique can provide some assessment of the adequacy of flow, particularly after vascular surgery to the carotid arteries or to vessels of the lower limb. It can also be used to give a non-invasive determination of cardiac output by measuring velocity in the arch of the aorta and relating it to aortic diameter. Transcranial Doppler ultrasonography can be used to give a measure of flow through large cerebral arteries.

Fd= 2FtVCosθ/C Fd is flow; Ft is transmitted Doppler frequency; V is the speed of blood flow; Cosθ is the cosine of the blood flow to beam angle; C is the speed of sound in tissue.

This is the basis of several methods which are used to measure both cardiac output and regional blood flow. It underlies thermal and chemical indicator dilution tests, renal clearance estimations and measurement of cerebral blood flow. It has been described as an application of the law of conservation of matter, in that the uptake or excretion of a substance by an organ or tissue must be equal to the difference between the amount entering the organ (arterial flow x concentration) and the amount leaving the organ (venous flow x venous concentration). .

Rearrangement of this relationship gives the familiar formula, namely that blood flow to an organ = rate of uptake or rate of excretion of a substance/ arteriovenous concentration. If oxygen is used as the substance, for example, then cardiac output is given by: VO2(ml/min) CO = \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ CaO2 - CvO2 The Fick principle applies only to situations in which the arterial supply presents the only source of the substance that is taken up

This was the commonest method in clinical use during the era in which pulmonary artery catheters were popular. The thermodilution technique for measuring cardiac output involves both injection and sampling carried out via a catheter in the right side of the heart. Cold fluid (such as glucose 5% at 0 C) is injected into the right atrium and the temperature change is detected by a thermistor at the distal end of the flotation catheter in the pulmonary artery. The recorded temperatures generate a concentration against time dilution curve analogous to that which would be seen had a chemical indicator been used.

The equation that is used is as follows: Flow (cardiac output) = ‘heat dose’ x 60/average concentration (AUC) x time (s). The injectate–blood temperature difference multiplied by the density, specific heat and volume of the injectate gives the numerator (the heat dose). The area under the curve (AUC) multiplied by the density and specific heat of blood gives the denominator. The potential complexity of these calculations means that the cardiac output determinations are computer generated.

If a conductor (such as blood) flows at right angles to a magnetic field, then an electromotive force is induced which is perpendicular to the magnetic field and to the direction of fluid flow. The induced voltage is proportional to the strength of the field and to the velocity of blood flow. A determination of the diameter of the vessel allows calculation of flow.