Monitoring tissue gas tensions in critical illness.

OBJECTIVE To review the technology and the role of monitoring tissue oxygenation in critical illness. DATA SOURCES Articles and published peer review abstracts on monitoring tissue oxygenation. SUMMARY OF REVIEW Miniaturised optodes and electrode systems are the commonly used technology for measurement of tissue gas tensions. Reductions in tissue perfusion frequently leads to a decrease in tissue PO2 and an increase in tissue PCO2 which has been confirmed in a number of animal and human trials in hypovolaemic shock. Monitoring tissue oxygenation has also enabled the delineation of cytopathic hypoxia, which is one of the important pathophysiological mechanisms of sepsis. Although these devices have improved our understanding of pathophysiological mechanisms of critical illness, at a clinical level titrating oxygen therapy to tissue oxygen tensions has only been shown to be useful in patients with impaired wound healing. A number of questions remain unanswered in relation to the monitoring of tissue oxygenation in critical illness. These include establishing normal values of PO2 and PCO2 in humans at the various tissue beds, establishing dysoxic thresholds for the various tissues, identifying optimal sites for monitoring and improving measurement accuracy. Furthermore, the nature of microcirculatory blood flow and tissue gas exchange in critical illness is complex and incompletely understood, limiting our ability to interpret changes from the baseline. Knowing critical tissue PO2 thresholds will provide the clinician with practical resuscitation endpoints in hypoxia and shock, and may even modify the practice of 'permissive hypoxia' in severe respiratory failure. These questions need answers in the years to come. CONCLUSIONS Monitoring of tissue oxygenation is largely a research tool. For its application in the critically ill patient there needs to be a greater understanding of normal values of PO2 and PCO2 at the various tissue beds, dysoxic thresholds for the various tissues and optimal sites for monitoring.