Repeatability, reproducibility, and effect of head position on central venous pressure measurement in standing adult horses.

Background: Central venous pressure (CVP) is a used as an estimation of intravascular volume status in various species. Techniques for measuring CVP in horses have been described, but the repeatability of these readings at a single time point or over time has not been established. Hypothesis: That CVP measurements in healthy adult horses would be repeatable at each time point, that these readings would be reproducible over time, and that alteration in head position relative to the heart would alter CVP. Animals: Ten healthy adult research horses. Methods: In an experimental study, horses were instrumented with a central venous catheter. Readings were taken in triplicate q6h for 2 days by water manometry, and twice daily with the head in neutral, elevated, and lowered positions by electronic manometry. Results: Variation in the “neutral” measurements obtained at each time point was <0.1 ± 1.0 cmH2O (P= .718). There was a significant decrease in CVP over time (P= .015), which was eliminated when results were controlled for acute decrease in body weight of −1.35% (presumed hypohydration because of lack of acclimatization and decreased water intake). Head height had a significant and directional effect on CVP in that the elevated head position decreased CVP −2.0 ± 6.5 cmH2O (P < .001) while the lowered head position increased CVP by 3.7 ± 5.5 cmH2O (P < .001). Conclusions and Clinical Importance: CVP values obtained by water manometry were repeatable in adult horses, but were reproducible only when controlled for changes in hydration. Care should be taken to maintain consistency in head position to prevent erroneous readings.