Metabolic and Physiological Changes During Minor Orthopaedic Surgery in Otherwise Healthy Patients

The air we breathe contains 21% of oxygen. Oxygen is vital for the cells ability to produce energy and without it we could not survive. Oxygen normally exists as a molecule consisting of two atoms, O2. It has two unpaired electrons and thus is unstable and willing to accept electrons to become stable. During the formation of ATP a transportation of electrons happens over the inner membrane of the mitochondria's. Oxygen can accept these and is thereby reduced to water. Normally about 4% is not fully reduced and instead produces superoxide. Superoxide is transformed to hydrogen peroxide by superoxide dismutase (SOD) and then into oxygen and water by catalase and glutathione peroxidase. It is also possible for hydrogen peroxide to be converted to hydroxyl radicals by Fenton reactions. All these radicals are called reactive oxygen species (ROS) and they are highly reactive and capable to induce damage to cellular components as proteins, DNA and lipids. Under normal conditions SOD, catalase and glutathione peroxidase work as anti-oxidative compounds to prevent oxidative stress and damage. However, under hyperoxic conditions these defences can be overwhelmed, resulting in the formation of excess ROS and thus oxidative damage.

During general anaesthesia the use of supplemental oxygen to avoid life-threatening hypoxaemia has been common practice for many years and a fixed fraction of inspired oxygen (FiO2) ranging from 0.3 to 1.0 is often used. This lead to supranormal levels of oxygen in the lungs and most of the patients also have supranormal levels of partial pressure of arterial oxygen in their blood.

This study will examine otherwise healthy ambulant patients undergoing minor orthopaedic surgery during general anaesthesia to elucidate metabolic and physiological changes caused by ventilation with FiO2 0.50 for at least 45 minutes using standard respiratory settings. Exhaled breath condensate (EBC) and arterial blood will be collected prior to and after surgery. The two EBCs and two blood samples will be stored at -80°C for analysis after all patients have been included. The metabolic changes will be measured with NMR technique and multivariate statistical analysis comparing baseline values with values obtained after oxygen exposure.

Collapse of the small airways induced by anaesthesia and FiO2 will be evaluated by measuring resistance and reactance with airway oscillometry after surgery compared to a baseline measurement before surgery.