The influence of hemorrhagic hypotension on spinal cord tissue oxygen tension

In order to investigate the spinal cord surface PO2 (sPO2) reaction to hypovolemic hypotension, nine female pigs (25 kg bw) were premedicated, anesthetized, intubated and artificially ventilated with N2O:O2 = 3:1. Following laminectomy from L3 to L5, sPO2 was measured on the dorsal side of the exposed spinal cord using six gold cathodes (luminal diameter 15 microns) while MAP (mean +/- SD) was lowered in steps of about 10 Torr by bleeding into a reservoir from 69.3 +/- 10.1 Torr to extreme low values of 13.3 +/- 3.1 Torr. Only a slight decrease of lumbar sPO2 (mean +/- SD) from 33.5 +/- 7.2 Torr to 27.6 +/- 4.8 Torr was evaluated from the data in response to MAP reduction to 50.2 +/- 3.5 Torr. Below that value, a marked proportional decrease of sPO2 and MAP was observed (0.65 Torr/Torr) corresponding to pressure passive flow behavior of the Hagen Poisseuille Law ("loss of autoregulation"). Four to five minutes after start of reinfusion both sPO2 and MAP showed an overshoot with maximum values of 54.0 +/- 11.1 Torr resp. 102.6 +/- 18 Torr. Initial values were approximated about 15 min. later. Histograms plotted from the individual sPO2 values of all pigs and of all the different experimental stages showed signs of severe hypoxia only if MAP was reduced below about 30 Torr. In general, this situation was reversible within reinfusion, only one of the nine pigs did not tolerate hemorrhagic hypoxia induced by MAP reduction to less than 30 Torr for at least 5-10 min. Nevertheless, the experiments showed a considerable circulatory stability in the investigated pigs.