Isoflurane Preserves Central Nervous System Blood Flow During Intraoperative Cardiac Tamponade in Dogs

Purpose: The present study tested the hypothesis that the anesthetic technique will influence the changes in regional blood flow (RBF) during intraoperative cardiac tamponade.

Methods: Twenty-four dogs were divided into three equal groups: Group I, anesthesia was maintained with ketamine (25 mg·kg^–1·hr^–1); Group II, with fentanyl and midazolam (F-M; 10 µg·kg^–1·hr^–1 and 0.5 mg·kg^–1·hr^–1, respectively); Group III with 1 minimum alveolar concentration (MAC; 1.4%) isoflurane. Radioactive microspheres were used to measure RBF in myocardium, brain, spinal cord, abdominal viscera, skeletal muscle and skin. Cardiac output (CO) was measured by thermodilution and arterial pressure with a catheter situated in the thoracic aorta. Catheters were introduced into the pericardial cavity to infuse isotonic saline and to measure intrapericardial pressure (IPP). Measurements were obtained under control conditions and during tamponade, as defined by an increase in IPP sufficient to reduce mean arterial pressure by 40%.

Results: Tamponade caused decreases in CO and RBF that were comparable under the three anesthetics, except that RBF in subcortical regions of the brain and in the spinal cord were maintained under isoflurane but decreased under ketamine or F-M.

Conclusions: In dogs, intraoperative cardiac tamponade caused comparable changes in RBF under the different anesthetic techniques except that autoregulation was effective in maintaining RBF within the central nervous system only under isoflurane anesthesia. Our findings provide no compelling reason to recommend one anesthetic over the others for maintenance of anesthesia in situations with increased risk for intraoperative cardiac tamponade. However, they cannot be extrapolated to anesthesia induction in the presence of cardiac tamponade.