The cardiopulmonary effects of severe blood loss in anesthetized horses.

This research explores the impact of severe blood loss on the heart and lung functions of anesthetized horses. The parameters monitored include cardiovascular variables, blood component volumes, and physical assessments. The results indicate that the heart rate and blood component volumes relatively remain stable during severe blood loss, while blood pressure and oxygen levels see significant changes.

Methods

• The experimental subjects included three male and six female horses, with an average age of 14.4 years and weight averaging 486 kg.
• The horses were put under anesthesia using a mixture of xylazine, guaifenesin, ketamine, and halothane or isoflurane.
• Data such as cardiovascular variables, hematocrit (proportion of blood volume that is occupied by red blood cells), total solids, capillary refill time (CRT – the time taken for color to return to an external capillary bed after pressure is applied), and appearance of mucous membranes were recorded.
• To simulate severe hemorrhage, blood was collected from the carotid artery into sterile plastic bags.
• An arterial blood gas analysis was performed to measure the amounts of oxygen and carbon dioxide in the blood.

Results

• The average amount of blood collected from the horses was about 53 ml per kg of body weight over a period of around 39 minutes.
• The hematocrit value decreased slightly after the onset of anesthesia but remained mostly consistent during the blood loss period.
• Induction of anesthesia and the subsequent blood loss resulted in a significant decrease of total solids in the blood.
• Systolic, mean, diastolic and pulse pressures dropped as blood was evacuated from the body.
• The heart rate did not show significant change during the severe hemorrhage.
• CRT increased considerably, from an average of 1.6 seconds to 4.8 seconds suggesting a slower pace of replenishment of blood capillaries during severe blood loss.
• The color of the mucous membrane worsened progressively mirroring the physiological stress experienced by the horses.
• As blood loss increased, the partial pressure of oxygen (PaO) in the blood, an indicator of the oxygen levels, decreased significantly.

Conclusions

• The research points out that acute severe hemorrhage in horses under anesthesia does not greatly affect the heart rate and hematocrit but significantly influences blood pressure, pulse pressure, and arterial oxygen levels.
• Other observable changes in physiological indicators like mucous membrane color and capillary refill time serve as important signs of increased blood loss.
• These findings suggest that during severe blood loss, horses under anesthesia display more significant changes in circulatory dynamics (blood pressure and arterial oxygen levels), while the relative stability of the heart rate and hematocrit might mask the severity of hemorrhage thus calling for extra clinical attention.