Blood gas partition coefficient and pulmonary extraction ratio for propofol in goats and pigs

The interpretation of continuously measured propofol concentration in respiratory gas demands knowledge about the blood gas partition coefficient and pulmonary extraction ratio for propofol. In the present investigation we compared both variables for propofol between goats and pigs during a propofol anaesthesia. In ten goats and ten pigs, expired alveolar gas and arterial and mixed venous blood samples were simultaneously drawn during total intravenous anaesthesia with propofol. The blood gas partition coefficient and pulmonary extraction ratio were calculated for both species. Non-parametric methods were used for statistical inference. The blood gas partition coefficient ranged between 7000 and 646,000 for goats and between 17,000 and 267,000 for pigs. The pulmonary extraction ratio ranged between 32.9% and 98.1% for goats and was higher for pigs, which ranged between -106.0% and 39.0%. The blood gas partition coefficient for propofol exceeded those for other known anaesthetic compounds so that it takes longer to develop a steady-state. The different pulmonary extraction rates in two species suggest that there are different ways to distribute propofol during the lung passage on its way from the blood to breathing gas. This species-specific difference has to be considered for methods using the alveolar gas for monitoring the propofol concentration in plasma.