Association between intraoperative tissue oxygenation, arterial blood pressure and noradrenaline use in urological patients

Background and Goal of Study: Inadequate tissue oxygenation should be prevented during surgery as it might cause postoperative morbidity. In this observational study we looked at factors that might influence tissue oxygenation (StO2) such as blood pressure and use of vasoactive drugs. Materials and Methods: In 159 urological patients receiving balanced anaesthesia and mechanical ventilation, we measured StO2 intraoperatively on the thenar eminescence (InSpectra, Hutchinson Tech., USA) along with (non)invasive blood pressure and recorded the use of noradrenaline. We correlated StO2 and MAP as recorded at defined moments during surgery. In addition, we looked at the effects of low blood pressure (as defined by the areas under the curve (AUC) while MAP was < 65 mmHg) on StO2. Furthermore, we related StO2 to different noradrenaline (NA) dosages: none, low (<0.03 mg/kg/min) and high dose (≥0.03 mg/kg/min). Results and Discussion: At the start and end of surgery, there was no correlation between StO2 and MAP. However, the lowest recorded MAP per patient did correlate with the corresponding StO2 (R=0.206, R2=0.043, p< 0.01; Pearson‘s correlation, 2-tailed). Similarly, a higher MAP AUC < 65 mmHg was associated with a lower average StO2 (R=-0.189, R2=0.036, p< 0.05; Spearman‘s correlation, 2-tailed). These results support the hypothesis of intact autoregulation of tissue blood flow to preserve StO2 in the normal range of blood pressure, whereas this autoregulation fails during lowest pressures. Furthermore, StO2 was higher without noradrenaline (no NA)(88.4%) than during high dose noradrenaline (85.2%) (p< 0.001; ANOVA, Bonferroni post hoc analysis). Possibly, the vasoconstrictive effect of noradrenaline decreases blood flow through the observed tissue (peripheral muscle). Conclusions: Tissue oxygenation is not influenced by blood pressure in the normal blood pressure range during anaesthesia, whereas it becomes dependent when MAP drops below the clinically relevant threshold of 65 mmHg. Noradrenaline use is associated with a decreased tissue oxygenation in the thenar eminescence. Further studies should identify other factors that may influence tissue oxygenation and assess the impact of tissue hypoxia on postoperative patients‘ outcome