Physiology, monitoring, and optimisation of perioperative tissue oxygenation: a narrative review

Maintenance of tissue oxygenation in patients having surgery is important as tissue hypoxia is a major determinant of organ failure. Tissue oxygenation follows a stepwise physiological pathway involving the macrocirculation, the microcirculation, and the cellular oxygen metabolism. This narrative review endorsed by the Network for the Advancement of Patient Blood Management, Haemostasis and Thrombosis outlines the physiology of tissue oxygenation, evaluates methods for intraoperative tissue oxygenation monitoring, and summarises therapeutic strategies to ensure adequate tissue oxygenation. In the macrocirculation, oxygen is delivered to peripheral organs by convection (through the bulk flow of oxygenated blood generated by cardiac output). Effective tissue perfusion requires both sufficient blood flow and perfusion pressure. Interventions targeting the macrocirculation include fluid therapy, blood transfusions, and targeted management of arterial pressure and cardiac output. Within the microcirculation, oxygen diffuses from capillaries into the surrounding tissues. The microcirculation distributes blood flow according to local metabolic demands. Most techniques for intraoperative microcirculation monitoring, such as handheld vital microscopy, near-infrared spectroscopy, laser Doppler, laser speckle imaging, fluorescence angiography, or the urethral perfusion index, are not implemented in clinical practice. The role of therapeutic interventions specifically targeting the microcirculation remains uncertain. At the cellular level, oxygen is consumed within the mitochondria, where it serves as the final electron acceptor in oxidative phosphorylation to generate adenosine triphosphate. Direct monitoring of cellular oxygen metabolism remains experimental and is not routinely available. Therapeutic strategies aiming to directly improve cellular oxygen metabolism are evolving. Future research is needed to better understand how to optimise tissue oxygenation during surgery to improve patient-centred outcomes.