Furthermore, there is little known of the acute physiological response to undertaking this type of exercise in this patient population. Metabolic monitoring can provide information on the energy expenditure of patients. However, direct methods of data acquisition such as use of pulmonary artery catheters to measure whole-body oxygen consumption can be unreliable and their use in clinical practice has diminished in recent years. The non-invasive technique of indirect calorimetry (IC) has therefore emerged as a potential alternative that may be more practically applicable in the clinical setting. Indirect calorimetry measures total gas exchange, oxygen consumption and carbon dioxide production which can in turn determine energy expenditure. However the widespread clinical utility of the IC technique, can be offset by the number of clinical factors that may preclude its use across all patients e.g. those with cardiovascular or ventilator instability, those with open chest drainage or receiving renal haemofiltration.