Microvascular response in cancellous bone to halothane-induced hypotension in pigs

The microvascular response in cancellous bone to halothane-induced hypotension was studied simultaneously using Laser Doppler Flowmetry (LDF) and clearance of locally injected 133Xe in the subapical mandibular bone of 16 young pigs. During halothane inhalation, a significant reduction in mean blood pressure to values ranging from 22 to 60 mm Hg was observed. The blood flow, as measured with LDF, increased in more than 60% of the animals with hypotension levels above 30 mm Hg. At hypotension levels below 30 mm Hg, blood flow decreased in all cases. The individual regional vascular resistance, calculated as the quotient between blood pressure and recorded blood flow value, was significantly reduced (p less than 0.001) during hypotension. Rhythmical flow variations, observed in some animals at normotension, disappeared immediately following the start of halothane inhalation, but returned when blood pressure was stabilized at a lower level. For recordings made with 133Xe-clearance, a significant reduction (p less than 0.001) in the logarithmic decay of the washout curves was observed during hypotension. This logarithmic decay was correlated to the reduced blood pressure levels (r = 0.73). No average change in regional vascular resistance could, however, be demonstrated. The lack of correlation between results obtained by the 2 methods may be due to the fact that LDF is sensitive to the total blood flow in a limited tissue volume, while the 133Xe isotope seems to be washed out mainly by the capillary blood flow. It can be further concluded that halothane-induced hypotension levels, applicable to clinical routines, may produce an increase rather than a decrease in total blood flow in cancellous bone tissue.