Effect of thermodilution injectate volume and temperature on the accuracy and precision of cardiac output measurements for healthy anesthetized horses.

The research examines the effect of injectate temperature and volume on the accuracy of cardiac output (CO) measurements in healthy adult horses, using four thermodilution protocols. The study finds that smaller injectate volumes or injectate at room temperature resulted in a significant change in the variance of CO measurements, suggesting these are not recommended owing to potential inaccuracies.

Experiment Overview

The procedure for measuring cardiac output (CO) in this study featured the following steps:

• Eight healthy adult horses were anesthetized and fitted with Swan-Ganz catheters.
• Four thermodilution protocols were used, varying in temperature and volume of the injectate. This injectate was a saline solution, and the varying temperatures and volumes were protocol A (control at 5° C, 1 mL/15 kg), protocol B (5° C, 1 mL/25 kg), protocol C (5° C, 1 mL/35 kg), and protocol D (17° C, 1 mL/15 kg).
• Each protocol was performed three times during each of five measurement cycles, each separated by 30 minutes. Protocol A was always conducted first, while the others were done in random order.

Research Findings

The results of the experiment illuminated the following facts:

• The mean CO did not differ significantly across the four different protocols.
• The within-subject variance for CO measurements from protocols C and D was significantly larger than that from Protocol A (control). However, the injectate volume decrease to 1 mL/25 kg in Protocol B did not affect accuracy or precision in a negative way.

Conclusions

The concluding points from this study were:

• In healthy adult horses, decreasing the volume of the thermodilution injectate from the recommended 1 mL/15 kg down to 1 mL/25 kg does not appear to negatively impact the accuracy or precision of CO measurements.
• However, the use of smaller injectate volumes (as in protocol C) or the use of injectate maintained at approximately room temperature (as in Protocol D) resulted in greater variability in CO measurements and is not recommended because it may lead to less accurate results.