Accuracy and reproducibility of long-term implanted transit-time ultrasound flow probes in dogs

Abstract

Objective: To assess the accuracy and reproducibility of long-term implanted ultrasound transit-time flow probes for measuring cardiac output. Design: Prospective animal study. Settings: Animal research laboratory in a university department. Animals: Eleven anaesthetised dogs, 24–34 kg. Measurements and results: Flow probes (16–24 mm S-series, Transonic) were implanted around the pulmonary artery for a mean duration of 22 months (range 6–47 months). Comparisons (n = 147) were made between cardiac output thus obtained and that measured by the direct Fick principle using oxygen uptake (Deltatrac II Metabolic Monitor) and the arterial to mixed venous oxygen content difference measured by a galvanic cell (Lex-O₂-Con-TL). Measurements were made either during baseline conditions or during pharmacologically altered cardiac output (range 22–180 ml · kg^–1· min^–1). Regardless of the intervention, the two methods yielded the same results in half of the dogs. In the others, however, cardiac output was underestimated by the flow probes by up to 38 % (probably because of non-perpendicular position of the probe towards the vessel). This difference was constant for the whole range of cardiac output studied and remained constant over the entire observation period for each individual dog, so that a correction factor was used. Thereafter, the mean difference between the two methods was –1.1 ml · kg^–1· min^–1 with a precision (SD) of 14.2 ml · kg^–1· min^–1 for all experiments. Conclusions: After in vivo calibration, ultrasound transit-time flow probes measure cardiac output precisely for several years, regardless of the intervention.