Problems with an electromagnetic blood flowmeter.

Recurrent unexplained interference and day-to-day variation were encountered during preliminary studies with a sine-wave electromagnetic flowmeter. Using in vitro techniques, contributory causes were found to be defective leads to the flowmeter cuffs, inadequate contact between cuff and vessel, and failure to use an earth lead. When these faults were corrected, a study was made of the effects of haematocrit, temperature and the comparison of blood vessel or dialysis tubing in vitro. Final in vivo confirmation of the validity of the cuff was obtained by comparison with a cannula probe.