Discrepancies in the measurement of changes in blood flow using flow- and velocity-sensitive electromagnetic probes

Blood flow was measured in the descending thoracic aorta of the anaesthetised dog using a perivascular flow probe and electromagnetic flowmeter. Blood velocity was measured in the same vessel with an intravascular velocity probe and electromagnetic flowmeter. Changes in blood pressure, flow and velocity, were induced by intravenous administration of noradrenaline, angiotensin, isoprenaline and aminophylline. If the velocity probe was within the cuff of the flow probe, percentage changes in flow and velocity were comparable, but if the velocity probe was "upstream" or "downstream" from the cuff, discrepancies between percentage drug-induced changes in flow and velocity were of the order of 30% and, in 12 out of 80 observations, in opposite directions. Measuring absolute flow with the velocity-sensitive device produced variable results showing a systematic underestimate of flow estimated from velocity, compared with flow measured with the perivascular probe. It is concluded that the measurement of blood flow and changes in blood flow, with the intravascular velocity probe, is liable to produce inaccurate data.