A model for differential volatile anesthetic delivery to the upper and lower torso of the rabbit

We have developed a model that permits differential delivery of volatile anesthetics to the upper and lower torso of the rabbit.

Rabbits were anesthetized with isoflurane (n = 4) or halothane (n = 3), and blood was drained from a carotid cannula into a membrane oxygenator and reinfused via a renal artery cannula into the lower torso circulation using a roller pump. Bypass of the lower torso circulation was achieved by tightening a ligature around the aorta at the level of the renal arteries. Blood concentrations of anesthetic (assessed by gas chromatography) and cardiovascular responses to noxious stimulation were determined with and without anesthetic delivery to the membrane oxygenator.

When the anesthetic was removed from the oxygenator gas flow, the arterial concentration of isoflurane in the lower torso was 28 +/- 15 microg/ml, while it was 133 +/- 28 microg/ml in the upper torso circulation; the corresponding values for the halothane-anesthetized rabbits were 63 +/- 8 and 270 +/- 49 microg/ml. There was a significant correlation (r=.92-.99) between pump flow and lower torso pressure in each individual rabbit. When anesthetic was delivered to both upper and lower torso, noxious electrical stimulation of the tail or hindpaw did not affect lower torso pressures (52 +/- 10 to 54 +/- 12 mmHg). Decreasing the anesthetic concentration in the lower torso resulted in significant increases in lower torso blood pressure during noxious stimulation (82 +/- 19 to 131 +/- 35 mmHg, P < .05).

The results indicate that volatile anesthetics isoflurane and halothane can be differentially delivered to the upper and lower torso of the rabbit, with an approximate 75-80% reduction in the anesthetic concentration in the lower torso when the anesthetic is eliminated from the gas flow to the oxygenator. This preparation can be used to study the pharmacological properties of volatile anesthetics.