Circulatory and respiratory responses of spontaneously breathing, laterally recumbent horses to 12 hours of halothane anesthesia.

The research discusses the circulatory and respiratory effects on horses subjected to 12 hours of halothane anesthesia. It reveals cardiovascular and respiratory changes in these conditions, along with analysis of blood pressure, cardiac output, stroke volume, plasma total solids concentration, and parameters of respiratory function.

Methodology

• Eight horses that were breathing freely were studied in this research.
• They were placed in a left lateral recumbent position (lying on their left side) and anesthetized with halothane mixed with oxygen. The anesthesia was maintained at an end-tidal concentration of 1.06% (1.2 times the minimum alveolar concentration for horses).

Observations and Results

• The cardiovascular and respiratory measurements during the initial five hours of anesthesia were consistent with the previous findings by the same laboratory. There were significant time-related increases in several parameters including systemic arterial blood pressure, cardiac output, stroke volume, left ventricular work, PCV, and plasma total solids concentration.
• The respiratory system function showed little change in this period.
• Post the five-hour mark, the arterial blood pressure didn’t increase further but remained higher than the baseline level. The cardiac output continued to increase mainly because there was a significant increase in the heart rate.
• The peak inspiratory gas flow also significantly increased during the later stages of anesthesia, and there was a noticeable decrease in the inspiratory time starting from the fourth hour.

Additional Findings

• The study found no significant changes in PaO2 (partial pressure of oxygen in arterial blood) and PaCO2 (partial pressure of carbon dioxide in arterial blood) in the 12-hour span. However, PVO2 (mixed venous oxygen partial pressure) showed a significant and progressive increase beginning six hours after consistent conditions set in.
• With the progression of time, there was a rise in metabolic acidosis and this was significant from the ninth hour onwards, despite the supplementation of NaHCO3 (Sodium bicarbonate) through an IV.
• The research also studied the plasma concentrations of certain eicosanoids (fatty acid metabolites that perform specific physiological functions) in five of the eight horses before and during the anesthesia. No significant changes were detected from the pre-anesthetic values.
• Over time, there was a decrease in the dynamic compliance of both the thoracic wall and lungs.