OXYGEN-TRANSPORT IN THE CRITICALLY ILL PATIENT

The traditional view of oxygen transport as the product of haemoglobin, oxygenation and cardiac output holds up well, but because the efficiency of oxygen delivery (the extraction ratio) is sometimes a fixed parameter in the critically ill, absolute values for each determinant are difficult to define and seem to have a large range for adequate function. The main factor in effective delivery to the tissues is cardiac output when extraction is limited. In most acute circumstances the blood volume is not easy to estimate without direct measurement. Hypovolaemia may influence the distribution of oxygen transport unfavourably away from splanchnic and renal circulations. Monitoring of the oxygen transport variables may be indirect, looking at the compensations for inadequate delivery, such as comparing transcutaneous oxygen tension to arterial oxygen tension, intramural gastric pH or lactate concentrations. The direct monitoring of oxygen delivery and uptake can be done with conventional intensive care unit monitoring (cardiac output, and arterial and mixed venous blood gases). The direct airway measurement of oxygen uptake also includes pulmonary oxygen utilization, which can be high in ARDS and pneumonia. Prehospital emergency management should maintain oxygenation and blood volume, and aim to reduce additional burdens of pain, agitation and shivering. This can then be developed into full critical care monitoring and therapy in the emergency department. The chief priority for any emergency department should be to develop the organization and protocols to maximize survival. © 1992 Baillière Tindall.