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ANESTHETIC PHARMACOLOGY

Isoflurane Inhalation Enhances Increased Physiologic Deadspace Volume Associated with Positive Pressure Ventilation and Compromises Arterial Oxygenation

Claudia Praetel, MD^*, Michael J. Banner, PhD^*,, Terri Monk, MD^*, and Andrea Gabrielli, MD^*,

Departments of *Anesthesiology, Physiology, and Surgery, University of Florida College of Medicine, Gainesville, Florida

Address correspondence and reprint requests to Michael J. Banner, PhD, University of Florida College of Medicine, Department of Anesthesiology, PO Box 100254, Gainesville, FL 32610-0254. Address e-mail to MBanner{at}anest.ufl.edu

Abnormally increased physiologic deadspace volume (VD_phys), consisting of alveolar deadspace volume and airway deadspace volume, is one of several causative factors predisposing to compromised arterial blood gas exchange. We compared the effects of two methods of general anesthesia on VD_phys when combined with positive pressure ventilation (PPV): total IV anesthesia (TIVA) and inhaled anesthesia with isoflurane. Forty patients with no history of pulmonary pathology undergoing elective surgery in the supine position were studied. A crossover design was used, and all patients received both anesthetic methods sequentially in randomized order. PPV and TIVA significantly increased VD_phys compared with baseline (preoperative and breathing spontaneously) from 164 ± 60 mL to 264 ± 79 mL (P < 0.05). Isoflurane inhalation combined with PPV significantly enhanced this increase, resulting in a twofold increase in VD_phys to 315 ± 80 mL (P < 0.05). Also, alveolar deadspace volume increased by more than 200% with isoflurane. Furthermore, isoflurane inhalation (1.15% end-tidal concentration) resulted in impaired arterial oxygenation, as evidenced by a significant decrease in the PaO₂/fractional inspired oxygen concentration ratio compared with baseline values from 387 ± 35 to 310 ± 70 (P < 0.05). Although significant increases in VD_phys resulted with PPV combined with TIVA, these adverse changes were much less compared with isoflurane inhalation and PPV. These findings may apply to subjects with compromised pulmonary function (i.e., acute respiratory distress syndrome or severe inhalational burn injury).

IMPLICATIONS: The effects of positive pressure ventilation in combination with inhaled isoflurane or total IV anesthesia on physiologic deadspace volume (VD_phys) were investigated in a crossover study. Inhaled isoflurane resulted in a significantly larger increase in VD_phys and an associated decrease in arterial oxygenation. This result may be important in patients with preexisting severe ventilation/perfusion abnormalities and impaired gas exchange.