The cardiopulmonary effects of severe blood loss in anesthetized horses

To characterize the acute cardiopulmonary effects of severe hemorrhage in anesthetized horses.

Prospective experimental study.

Three geldings and six mares, aged 14.4 ± 2.7 years, weighing 486 ± 41 kg (range: 425-550 kg).

Horses were anesthetized using xylazine, guaifenesin, ketamine and halothane or isoflurane. Cardiovascular variables, hematocrit, total solids, capillary refill time (CRT) and color of mucous membranes were measured as blood was collected from the carotid artery into sterile plastic bags. Arterial blood gas analysis was also performed.

The average amount of blood collected from these horses was (mean ± SD) 53 ± 4.8 mL kg^-1 bodyweight (range: 23-32 kg) over 39 ± 4 minutes. Hematocrit decreased from 38 ± 3 to 32 ± 2% after induction of anesthesia and did not change significantly over the period of blood loss. Total solids decreased significantly after induction of anesthesia, and over the period of blood loss. Systolic, mean, diastolic and pulse pressures decreased as blood was lost. Heart rate did not change significantly. Capillary refill time increased from 1.6 ± 0.4 seconds to 4.8 ± 1.3 seconds as blood loss increased. Mucous membrane color deteriorated progressively. Arterial PO₂ decreased significantly over the period of blood loss.

Hematocrit and heart rate do not change significantly during acute severe hemorrhage in the anesthetized horse. Arterial blood pressure, pulse pressure and PaO₂ decrease as blood loss increases. Changes in mucous membrane color and CRT also occur as blood loss increases.

During severe hemorrhage in the inhalant-anesthetized horse, both heart rate and hematocrit remain unchanged. Blood pressure decreases and changes in arterial PO₂ correlate most strongly with volume of blood lost.