Cardiopulmonary and
acid-base variables recorded during long-term
balanced anesthesia lasting between 12.5 and 16.9 h were evaluated retrospectively in 15 healthy foxhounds that underwent experimental
bulla osteotomy with implantation of
hearing aids. After
premedication with
propionylpromazine (0.11 +/- 0.02 mg/kg of
body weight) and L-
methadone (0.71 +/- 0.06 mg/kg) intravenously (i.v.) and induction with
pentobarbital sodium (6.02 +/- 0.83 mg/kg i.v.),
anesthesia was maintained with
halothane (end-tidal concentration; ETHAL: 0.4 to 1.5%) in
nitrous oxide (2 L/min) and
oxygen (1 L/min). Because of positional changes from sternal to right lateral recumbency after presurgical brain stem electric response audiometry and differences in duration of surgery, data obtained between 4 (baseline) and 14 h after induction of
anesthesia were analyzed. Arterial (PaO2) and alveolar (PAO2) O2 tensions, arterial-to-alveolar O2 tension ratio (PaO2/PAO2), and arterial O2 content (CaO2) remained relatively stable throughout
anesthesia. Arterial
carbon dioxide tension (PaCO2) was significantly increased 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 induction. Because changes in PaCO2 were accompanied by significant increases in body temperature from baseline (36.3, 34.6 to 37.4 degrees C) between 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 arterial blood pressure, the PaCO2 increase may have been caused by increase in metabolic CO2 production and enhanced drainage of CO2 from the tissues into systemic circulation. Furthermore, mild
metabolic acidosis (pHa: 7.31, 7.26 to 7.38; HCO3-: 18.9, 16.7 to 21.8 mEq/L; base deficit [BD]: -6.3, -8.5 to -3.4 mEq/L) already existed at 4 h after induction and was related in part to tissue hypoperfusion. Small increases in pHa during the course of
anesthesia were accompanied by significant increases in HCO3- concentration and significant decreases in BD between 5 and 10 h after induction. Minor circumscribed swelling of the dependent triceps or masseter muscle was noticed on the first postoperative day in two dogs, and marked tissue swelling with
hematoma formation 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. The
anesthetic protocol and extent of monitoring used were adequate to provide safe long-term
anesthesia for an experimental
surgical procedure with a 100% survival rate and uneventful recovery in most of the dogs.