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

Oxygen supplement during general anaesthesia During general anaesthesia the use of supplemental oxygen to avoid life-threatening hypoxaemia has been common practice for many years. This lead to supranormal levels of oxygen in the lungs (hyperoxia) and most patients also have supranormal levels of partial pressure of arterial oxygen (PaO2) in their blood (hyperoxaemia). The same iatrogen hyperoxia is also common in mechanically ventilated patients in the intensive care unit. Is seems to be forgotten that supplemental oxygen is a medicine and like all medication it should not be administered in excess.

During general anaesthesia a fixed fraction of inspired oxygen ranging from 0.3 to 1.0 is often used. Patients are monitored with continuously measurement of peripheral oxygen saturation and if this is low or an arterial gas shows low PaO2 then FiO2 is further increased. However, a decrease beyond the prefixed FiO2 is seldom done even if oxygen saturation is 100% or the arterial gas shows a high PaO2.

More and more evidence question the safety of this liberal use of hyperoxia as high oxygen supplement and the formation of ROS can lead to cell dysfunction and thereby contribute to pulmonary dysfunction postoperatively.

Adverse effects of oxygen FiO2 exceeding the atmospheric content of 0.21 can have direct toxic effects on lung tissue especially for FiO2 exceeding 0.60. Studies from the 1970's showed that when breathing FiO2 1.0 for more than four hours people experienced mild symptoms like tracheobronchitis and pleuritic. This is a mild irritation behind the sternum, in the airways or in the lungs. This discomfort is aggravated by deep inspiration and can cause cough. They also showed that a high FiO2 of 0.95-1.0 given for several days lead to pulmonary edema and eventually lung fibrosis. High FiO2 also induce collapse of lung areas leading to pulmonary shunt because of reabsorption atelectasis and induce pulmonary vasodilatation but otherwise induce vasoconstriction in all others vascular beds (except in uterus) and thereby reduce cardiac output and end organ perfusion.

The precise mechanism of the direct cellular damage and vasoconstriction is unknown but is believed to be due to increased production of ROS5.

In the clinical setting with critical ill patients hyperoxia and hyperoxaemia has been associated with increased mortality specifically in subgroups of patients with stroke, traumatic brain injury and those resuscitated from cardiac arrest and it is also shown to increased infarct size and mortality after cerebral and myocardial infarction. Perioperative treatment with high FiO2 has also been associated with increased mortality and major respiratory complications.

Metabonomics Lately there has been a lot of attention on the formation of ROS during hyperoxic conditions. Besides induction oxidative stress ROS also affects a number of signal-transduction-pathways that leads to cellular changes. These changes can be detected using metabonomics.

Metabonomics is the analysis of all small molecules or metabolites present in a given sample using nuclear magnetic resonance (NMR) or mass spectrometry (MS). Metabonomics gives a "snapshot" of all metabolites in a given sample and thus has great potential to find several new biomarkers by analysis of the continuous changes in the metabolic profile in response to a given exposure.

Oscillometry Anaesthesia and high FiO2 lead to collapse of the small airways. A new airway oscillometry system (TremoFlo C-100) enables a simple and non-invasive measurement of changes in the small airways. The system measure resistance and reactance non-invasively during normal breathing.

The trial We 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 45 minutes following standard settings. Exhaled breath condensate (EBC) and arterial blood will be collected prior to the surgery and repeated during surgery after 45 minutes of mechanical ventilation with a FiO2 0.50. 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 45 minutes of oxygen exposure. All specimens will be destroyed after the NMR analyses. Collapse of the small airways induced by anaesthesia and FiO2 will be evaluated by measuring resistance and reactance after surgery compared to a baseline measurement before surgery.