Oxygen Consumption in Critical Illness

Overview

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.

This is a prospective longitudinal observational study in thirty intubated, ventilated patients. Patients will be recruited from the population of already intubated and ventilated individuals on an adult intensive care unit (AICU). Once patients have been recruited we will insert the optical gas analyser (OGA) into the ventilation circuit (tubing) for 48 hours. Data will be continuously collected until 48 hours from attachment of the OGA, death or extubation of the patient, and analysed retrospectively. A time series describing oxygen consumption with a resolution of one minute or less will be prepared and standard measures of dispersion calculated. In addition, variables that are routinely recorded on the computerised clinical record during the same period will then be examined and associations sought between these variables and changes in oxygen consumption as measured with the OGA. We have prospectively identified variables which, based on the published literature, are likely to be associated with variation in oxygen consumption (VO2) upon which attention will focus, though others may become apparent during the study: * Body temperature * Seizures * Change in haemoglobin concentration (including due to blood transfusion) * Bolus fluids * Vasoactive drugs * Beta-agonist drugs * Drugs affecting the angiotensin-converting enzyme system * Renal replacement therapy * Feeding * Sedation * Neuromuscular blockers * Mode of ventilation * Fraction of inspired oxygen (FiO2) * Physical activity * Surgery