Hemodynamic and respiratory responses to variable arterial partial pressure of oxygen in halothane-anesthetized horses during spontaneous and controlled ventilation.

This research study investigated how different levels of arterial blood oxygen pressure (PaO2) influences the cardiovascular and respiratory responses in horses anesthetized with halothane. The testing was done under conditions of self-regulation (spontaneous ventilation, SV) and regulated (controlled ventilation, CV) breathing.

Study Setup

• Six horses who had been anesthetized using only halothane gas were studied during spontaneous ventilation (where the horse controls its own breathing) and during controlled ventilation (where the breathing is regulated externally).
• The minimal alveolar concentration (MAC) of halothane needed for effective anesthesia was determined for each horse while they were spontaneously breathing.
• Even at varied oxygen pressures (denoted as > 250, 120, 80, and 50 mm of Hg), each horse was maintained at a anesthetic level of 1.2 times its individual MAC. The body position of the horses was uniformly maintained in left lateral recumbency.

Observations

• Where PaO2 was reduced, this effected SV by lowering blood pressure and total peripheral resistance. Heart rate, on the other hand, increased. As a result, there was a slight increase in overall cardiac output.
• Contrastingly, controlled ventilation resulted in generally lower cardiovascular function than comparable PaO2 levels during spontaneous ventilation. The heart rate, cardiac output, and overall work done by the left ventricle were higher when PaO2 was lowered, while there was a decrease in total peripheral resistance.
• Under spontaneous ventilation, as anesthesia duration increased at PaO2 levels greater than 250 mm of Hg, there was an observed increase in cardiac output and stroke volume, while pressure and peripheral resistance dropped.
• Additionally, respiratory changes were also observed as a decrease in PaO2 led to an increase in expired volume per minute. Concurrently, the partial pressure of carbon dioxide (PaCO2) in the blood was decreased.
• Lastly, while oxygen use remained constant throughout the study, oxygen delivery dropped with a reduction in PaO2 and was lower during controlled ventilation compared to spontaneous ventilation at similar PaO2 levels.

Conclusion

• The study suggests that levels of oxygen pressure in the blood can significantly impact both cardiovascular and respiratory responses in horses under halothane anesthesia, and these effects differ based on whether ventilation is spontaneous or controlled.