Effects of thoracic epidural anaesthesia on microvascular gastric mucosal oxygenation in physiological and compromised circulatory conditions in dogs

Background. The effects of thoracic epidural anaesthesia (TEA) on gastric mucosal microvascular haemoglobin oxygenation (μHbO₂) are unclear. At the splanchnic level, reduction of sympathetic tone may promote vasodilation and increase μHbO₂. However, these splanchnic effects are counteracted by systemic effects of TEA (e.g., decreased cardiac output (CO) and mean arterial pressure (MAP)), thus making the net effect on μHbO₂ difficult to predict. In this respect, effects of TEA on μHbO₂ may differ between physiological and compromised circulatory conditions, and additionally may depend on adequate fluid resuscitation. Furthermore, TEA may alter the relationship between regional μHbO₂ and systemic oxygen-transport (DO₂). Methods. Chronically instrumented dogs (flow probes for CO measurement) were anaesthetized, their lungs ventilated and randomly received TEA with lidocaine (n=6) or epidural saline (controls, n=6). Animals were studied under physiological and compromised circulatory conditions (PEEP 10 cm H₂O), both with and without fluid resuscitation. We measured gastric mucosal μHbO₂ by reflectance spectrophotometry, systemic DO₂, and systemic haemodynamics (CO, MAP). Results. Under physiological conditions, TEA preserved μHbO₂ (47 (3)% and 49 (5)%, mean (SEM)) despite significantly decreasing DO₂ (11.3 (0.8) to 10.0 (0.7) ml kg^-1 min^-1) and MAP (66 (2) to 59 (3) mm Hg). However, during compromised circulatory conditions, TEA aggravated the reduction in μHbO₂ (to 32 (1)%), DO₂ (to 6.7 (0.8) ml kg^-1 min^-1) and MAP (to 52 (4) mm Hg), compared with controls. During TEA, fluid resuscitation completely restored these variables. TEA preserved the correlation between μHbO₂ and DO₂, compared with controls. Conclusions. TEA maintains μHbO₂ under physiological conditions, but aggravates the reduction of μHbO₂ induced by cardiocirculatory depression, thereby preserving the relationship between gastric mucosal and systemic oxygenation.