RETROSPECTIVE EVALUATION OF CARDIOPULMONARY AND ACID-BASE VARIABLES DURING LONG-TERM BALANCED ANESTHESIA FOR EXPERIMENTAL SURGERY IN DOGS

Cardiopulmonary and acid-base variables recorded during: long-term bal anced anesthesia lasting between 12.5 and 16.9 h were evaluated retros pectively in 15 healthy foxhounds that underwent experimental bulla os teotomy with implantation of hearing aids. After premedication with pr opionylpromazine (0.11 +/- 0.02 mg/kg of body weight) and L-methadone (0.71 +/- 0.06 mg/kg) intravenously (i.v.) and induction with pentobar bital sodium (6.02 +/- 0.83 mg/kg i.v.), anesthesia was maintained wit h halothane (end-tidal concentration; ETHAL: 0.4 to 1.5%) in nitrous o xide (2 L/min) and oxygen (1 L/min). Because of positional changes fro m sternal to right lateral recumbency after presurgical brain stem ele ctric response audiometry and differences in duration of surgery, data obtained between 4 (baseline) and 14 h after induction of anesthesia were analyzed. Arterial (Pao(2)) and alveolar (PAo(2),) O-2, tensions, arterial-to-alveolar O-2, tension ratio (Pao(2)/PAo(2),), and arteria l O-2, content (Cao(2)) remained relatively stable throughout anesthes ia. Arterial carbon dioxide tension (Paco(2)) was significantly increa sed above baseline (39, 33 to 46 mm Hg [median, range]) between 7 (39. 8, 36.5 to 48.9 mm Hg) and 9 (42, 37.5 to 49.5 mm Hg) h after inductio n. Because changes in Paco(2) were accompanied by significant increase s in body temperature from baseline (36.3, 34.6 to 37.4 degrees C) bet ween 8 (37.1, 35 to 38 degrees C) and 11 (37.6, 35.3 to 38.1 degrees C ) h after anesthesia induction as well as by slight increases in arter ial blood pressure, the Paco(2), increase may have been caused by incr ease in metabolic CO2 production and enhanced drainage of CO2 from the tissues into systemic circulation. Furthermore, mild metabolic acidos is (pHa: 7.31, 7.26 to 7.38; HCO3-: 18.9, 16.7 to 21.8 mEq/L; base def icit [BD]: -6.3, -8.5 to -3.4 mEq/L) already existed at 4 h after indu ction and was related in part to tissue hypoperfusion. Small increases in pHa during the course of anesthesia mere accompanied by significan t increases in HCO3- concentration and significant decreases in ED bet ween 5 and 10 h after induction. Minor circumscribed swelling of the d ependent triceps or masseter muscle was noticed on the first postopera tive day in two dogs, and marked tissue swelling with hematoma formati on at the medial side of one hind limb was noticed in a third dog. All dogs recovered completely and were submitted to follow-up studies. Th e anesthetic protocol and extent of monitoring used were adequate to p rovide safe long-term anesthesia for an experimental surgical procedur e with a 100% survival rate and uneventful recovery in most of the dog s.