Plasma colloid osmotic pressure and total protein trends in horses during anesthesia

Objective To investigate the changes in colloid osmotic pressure (COP) and total protein concentrations during routine general anesthesia in horses. Study design Prospective, clinical study. Animals Twelve adult healthy horses aged 9.1 ± 4.7 years and weighing 474 ± 79 kg presented for elective surgery and 14 adult horses aged 8.7 ± 7.3 years and weighing 510 ± 85 kg. Methods All horses were premedicated with xylazine and anesthesia induced with ketamine, diazepam and guaifenesin, and maintained with isoflurane for 2.5 hours. Lactate Ringer's solution was administered at 11 mL kg-1 hour-1. Osmolality, COP, electrolytes, glucose, and lactate were measured with specific commercial analyzers. Total protein (TP) was determined with a refractometer and packed cell volume with centrifuged capillary tubes. In the second group of 14 horses samples were taken from both venous and arterial sites simultaneously and the above measurements performed. Results Before anesthesia, COP and TP were 22.2 ± 2 mmHg and 6.9 ± 0.4 g dL-1, respectively. Within 15 minutes of anesthetic induction, COP and TP decreased significantly (19.9 ± 1.9 mmHg and 6.3 ± 1.9 g dL-1; p < 0.01). During anesthesia COP and TP decreased in a linear form (COP r2 = 0.96 and TP r2 = 0.97). The COP and TP were 15 ± 1.3 mmHg and 5.1 ± 0.2 g dL-1 at the end of anesthesia. Calculation of COP from TP values failed to accurately predict measured COP. Simultaneous arterial and venous samples in the 14 anesthetized horses yielded no differences for COP or TP. Conclusions and clinical relevance The data indicate that COP, like TP, decreases over the course of routine anesthetic management of horses and venous versus arterial samples should reveal comparable information.