BACKGROUND Closed-circuit anesthesia (CCA) has been suggested to provide better hemodynamic stability, but the relative contribution of CCA to the control of blood flow in microcirculation is not defined. It is hypothesized that isoflurane-based CCA provides a favorable skin blood flow mediating via lesser circulatory response to surgical stimulation. This study was purposed to compare the effects of isoflurane conveyed by CCA with that by semiclosed anesthesia (SCA) on the preservation of skin blood flow in the forearm. METHODS Twenty seven patients scheduled for colorectal surgery were enrolled for the study and randomly assigned to one of two groups, either receiving CCA (n=14) or SCA (n=13). Anesthesia was induced with fentanyl (100 ig) and thiopental (4-5 mg/kg), and intubation was facilitated with pancuronium priming (0.015 mg/kg) and succinylcholine (1.25 mg/kg). Two percent isoflurane in high O2 flow (3 L/min) was given for 10 min to each patient initially to wash isoflurane in the functional residual capacity of both lungs and the breathing circuit. In the CCA group, after insoflurane wash-in the 02 flow was reduced less than 250 mL/min with isoflurane vaporizer setting adjusted to 3-5% to keep inspired isoflurane at 2% for maintenance of anesthesia. In patients of in the SCA group, anesthesia was maintained with isoflurane in 3 L/min O2 flow keeping the inspired isoflurane concentration at 2% throughout the surgery. A laser Doppler flowmetry (Moore instrument, Axminster, England) was used to measure skin blood flow (SBF) at the thenar eminence of left hand. Skin blood flow was determined and was taken as baseline (100%) at the 10th min after tracheal intubation. Thereafter, the magnitude of SBF was determined likewise every 30 minutes over the two-hour study. The hemodynamic and respiratory parameters were measured at every 5 minutes in both groups. RESULTS In comparison with the SCA group, the CCA group had a greater magnitude of skin blood flow (106 +/- 13% vs. 75 +/- 17%; 104 +/- 14% vs. 68 +/- 14% P < 0.01) at the 60th and 120th min. Also, patients who received SCA had higher mean arterial pressure (MABP) (82 +/- 8 vs. 78 +/- 8; 89 +/- 6 vs. 77 +/- 10, P < 0.05) and heart rate (HR) (80 +/- 9 vs. 73 +/- 9; 82 +/- 15 vs. 71 +/- 14, P < 0.05) at the 60th and 120th min. CONCLUSIONS Closed circuit technique for isoflurane anesthesia is feasible and advantageous. It not only facilitates favorable skin blood flow but also provides better hemodynamic stability in comparison with semiclosed technique for isoflurane anesthesia.