Utility of dynamic preload indices of hypovolemia in isoflurane-anesthetized horses.

Overview

• This study investigated how well certain dynamic preload indices (SPV%, PPV%, PVI%) reflect hypovolemia and cardiac output changes in isoflurane-anesthetized horses.
• The primary goal was to evaluate their effectiveness during controlled blood loss and subsequent autotransfusion, and to assess the correlation between these indices and cardiac output.

Background and Purpose

• Hypovolemia, or low blood volume, can compromise cardiac function and blood pressure; thus, detecting it quickly in anesthetized animals is critical.
• Dynamic preload indices—systolic pressure variation (SPV%), pulse pressure variation (PPV%), and plethysmographic variability index (PVI%)—are parameters used to assess fluid responsiveness and volume status.
• The study aimed to determine how useful these indices are during hypovolemia in horses anesthetized with isoflurane, a common veterinary anesthetic.
• Researchers also wanted to find the best cut-off values for these indices that could accurately differentiate hypovolemia states.

Methodology

• Subjects: Eight healthy adult horses were selected for the experimental study.
• Anesthesia and Monitoring:
  • Horses were sedated with intravenous xylazine, then anesthetized with ketamine and diazepam, and maintained on isoflurane in 100% oxygen.
  • Animals were positioned on their backs (dorsal recumbency), paralyzed with atracurium, mechanically ventilated to maintain normal CO2 levels.
  • Instrumentation included two Swan-Ganz catheters for measuring cardiac output via thermodilution, and a facial artery catheter to monitor arterial blood pressure.
  • Dobutamine infusion was administered to maintain normal blood pressure (normotension).
• Experimental Procedure:
  • Dynamic indices (SPV%, PPV%, PVI%) were recorded at baseline (before blood loss).
  • Graded hemorrhage was induced by withdrawing blood incrementally (5 mL per kg body weight up to 25 mL/kg) over time.
  • Measurements were taken again 10 minutes after each hemorrhage increment.
  • After reaching the maximum blood loss, autotransfusion was performed in the same increments to reverse hypovolemia and indices were recorded.
• Data Analysis:
  • For each dynamic index, cut-off values were calculated to best distinguish baseline from non-baseline states while maximizing sensitivity and specificity.
  • Spearman’s correlation was used to analyze associations between dynamic indices and cardiac output.

Key Results

• Baseline median values:
  • SPV%: 9% (range 5-15%)
  • PPV%: 14% (range 8-37%)
  • PVI%: 12% (range 7-22%)
• Cut-off values at maximal hemorrhage (25 mL/kg):
  • SPV% = 16.1%, with an accuracy of 96%
  • PPV% = 28.5%, with an accuracy of 90%
  • PVI% = 15.4%, with an accuracy of 72%
• Accuracy of the indices improved as blood volume loss increased, indicating better discrimination at more severe hypovolemia.
• Correlations with cardiac output:
  • PPV% showed a significant negative correlation with cardiac output (Spearman’s rho = -0.49, p < 0.007), meaning higher PPV% indicates lower CO.
  • SPV% also showed a negative trend but was not statistically significant (rho = -0.34, p = 0.131).
  • PVI% had the weakest and non-significant negative correlation (rho = -0.20, p = 0.253).

Conclusions

• SPV% and PPV% are useful dynamic preload indices to detect hypovolemia and reflect decreases in cardiac output in horses anesthetized with isoflurane.
• PVI% was less reliable in this context and did not correlate strongly with cardiac output changes.
• The study demonstrates that these indices, especially SPV% and PPV%, can be valuable for monitoring volume status and guiding fluid therapy in anesthetized horses during surgery or critical care.
• Identified cut-off values provide practical benchmarks for clinicians to interpret these indices during blood volume changes.

Implications for Veterinary Anesthesia

• Use of SPV% and PPV% could allow anesthetists to dynamically assess fluid responsiveness and detect hypovolemia early during procedures.
• Monitoring these indices may improve cardiovascular management and outcomes in equine anesthesia, where large blood volume shifts can occur.
• Automated or real-time measurement of these indices could facilitate better decision-making regarding fluid administration and cardiovascular support.