Effect of implant duration on in vivo sensitivity of electromagnetic flow transducers

Twenty-three electromagnetic flow transducers with lumen diameters of 3.5-6.0 mm were implanted in rhesus monkeys and baboons for 12 h to 120 days. Each flow transducer was calibrated (1) in vitro on dialysis tubing with saline before implantation, (2) in vivo the last day of the implanted period, and (3) again in vitro after the flow transducer was recovered. Three other flow transducers were implanted on femoral arteries of baboons just central to an arteriovenous silastic shunt, and were calibrated in vivo daily for 23-47 days. In vitro sensitivity was not affected by implant durations of up to 120 days. In vivo sensitivity fluctuated unpredictably for the first 3-4 wk of implant, after which it followed a systematic course that depended on the lumen size. In vivo sensitivity at any time during implant (after the initial period) could be accurately predicted by knowing either the in vitro sensitivity or the terminal in vivo sensitivity.