Haemodynamics and oxygen tension in the microcirculation of ischaemic skin flaps after neural blockade and haemodilution

The aim of this study was to evaluate the effects of neural blockade and extended normovolaemic haemodilution on haemodynamics and oxygenation in the microcirculation of ischaemic skin flaps using a recently developed island flap on the back of Syrian golden hamsters. One part of the flap was made ischaemic by inducing a random perfusion pattern following interruption of the axial blood supply. The model permitted quantitative assessment of the microhaemodynamics and oxygen tension in all microvascular segments by the use of intravital microscopy. Oxygen tension was measured with the phosphorescence decay method. Neural blockade was induced by applying 2% lidocaine to the neurovascular flap pedicle. Haemodilution was achieved by isovolaemic exchange of 50% of the total blood volume with dextran 70. One hour after surgery (baseline), centreline velocity was significantly reduced to 20-44% in all the microvessels in the randomly as compared to the axially perfused part, whereas the diameters were slightly larger (ns). In the control group, blood flow declined by 20-75% (P < 0.01 vs. baseline) over time in the entire flap. Flow reduction was significantly attenuated by haemodilution in the entire flap, but more pronounced in the ischaemic part. Neural blockade caused marked vasodilatation and significantly improved blood flow in the axially but not in randomly perfused microvessels. After 8 h, oxygen tension ranged from 4.0 to 6.1 mmHg in the axial part (means, ns between groups), whereas in the ischaemic part, it was 0.8-1.0 mmHg (P < vs. axial) in the control and neural blockade groups, and 1.7 mmHg (ns vs. axial and between groups) after haemodilution. Our findings indicate that neural blockade does not improve microcirculation and oxygenation in randomly perfused flap tissue because the sympathetic regulation of its microcirculation is overruled by autoregulatory mechanisms. Normovolaemic haemodilution, even after a 50% exchange, augments oxygenation in ischaemic flap tissue due to increased blood flow particularly in the randomly perfused tissue.