BACKGROUND Anesthesia may represent a considerable bias in experimental medicine, particularly in conditions of stress (such as hemorrhage). Sodium pentobarbital (PB), widely used for cardiovascular investigations, may impair oxygen delivery by hemodynamic and respiratory depression. The critical issue, however, is whether the microcirculation can still maintain tissue oxygenation during anesthesia. To answer this question, the authors studied the effect of PB anesthesia on subcutaneous microvascular oxygen delivery and interstitial oxygenation in Syrian golden hamsters. METHODS Sodium pentobarbital anesthesia was induced by intravenous injection (30 mg/kg body weight) and maintained by a 15-min infusion (2 mg.kg-1.min-1), with animals breathing spontaneously (PB-S) or ventilated with air (PB-V). Systemic parameters evaluated were mean arterial pressure (MAP), heart rate, cardiac index (CI), arterial oxygen tension (PaO2), arterial carbon dioxide tension (PaCO2), base excess, and pH. Microvascular and interstitial oxygen tension (PO2), vessel diameter, red blood cell velocity (vRBC), and blood flow (Qb) were measured in a dorsal skinfold preparation. Microcirculatory PO2 values were determined by phosphorescence decay. RESULTS Sodium pentobarbital anesthesia significantly decreased CI, MAP, vRBC, and Qb. During PB infusion, PaO2 values were 56 +/- 12.8 mmHg (PB-S) and 115.9 +/- 14.6 mmHg (PB-V) compared with 69.4 +/- 18.2 mmHg and 61.4 +/- 12.6 mmHg at baseline. However, microvascular PO2 was reduced by 25-55% in both groups, resulting in an interstitial PO2 decrease from 23.9 +/- 5.6 mmHg (control) to 13.1 +/- 9.1 mmHg (PB-S) and 15.2 +/- 7 mmHg (PB-V). Microcirculatory PO2 values were restored 30 min after PB infusion, even though hemodynamic depression and a light anesthetic plane were maintained. CONCLUSIONS Sodium pentobarbital anesthesia caused impairment of microvascular oxygen delivery and interstitial oxygenation, effects that were not prevented by mechanical ventilation. Although these effects were restricted to deep anesthetic planes, prolonged hemodynamic depression suggests that caution is warranted when using PB as an anesthetic in cardiovascular investigations.