Oxygen Consumption in Critical Illness

Many serious illnesses are characterised by a lack of oxygen delivery to the body\'s tissues. This can be due to problems with the circulation, such as when the heart fails to pump blood efficiently or if the blood pressure is very low, or due to changes in the lungs preventing them from transferring oxygen effectively from the air into the blood. Many of the treatments used in critically ill patients are aimed at improving the supply of oxygen-rich blood to the tissues. These include drugs to increase the blood pressure or make the heart pump more forcibly, blood transfusions to increase the amount of oxygen that the blood can carry, and ventilators (breathing machines) to help the lungs introduce more oxygen into the bloodstream. Decisions to use such treatments are based on a number of factors. One of the most important is an assessment of how much oxygen an individual patient is using at a given time. Whilst it is possible to measure average oxygen consumption over a long period of time in healthy individuals, the equipment and techniques needed are simply not practical for routine use in the clinical setting of a critical care unit. Critical care doctors and nurses therefore have to rely on a number of different indicators of the adequacy of the amount of oxygen that is being provided in order to make decisions about how best to treat patients. Unfortunately, all of these have their limitations, are affected by other treatments patients may be receiving, and can be difficult to interpret. Examples include \'mixed venous oxygen saturation\' (SvO2), that is the amount of oxygen in the blood coming back to the lungs from the body, ready to have more oxygen added, and blood lactate levels, which give an indication of whether the tissues have enough oxygen to produce the energy they need to function in an efficient manner. Our group has developed a device which can be added easily to the normal equipment used on a critical care ward when a patient is on a ventilator, which can accurately and rapidly measure the amount of oxygen the individual is consuming. This study will provide us with data so the investigators can define a \"normal\" range of oxygen consumption in these patients, and treatments which alter consumption. In a future study, once the investigators know what the normal range is, they can introduce new techniques to improve oxygen consumption in those patients whose consumption is less than ideal.