Gas Exchange

Metabolic monitoring in critical care:
Clinical value of modular metabolic monitoring early in illness

Case Studies from Edinburgh Royal Infirmary, on an unrestricted research grant.
Dr. Timothy S. Walsh
Consultant in Intensive Care and Anesthesia,
Royal Infirmary of Edinburgh

The whole article available in PDF: 355 KB

Background

Oxygen consumption and carbon dioxide elimination by the body reflect the metabolic rate of tissues, and the magnitude of aerobic metabolism. Early research in critical illness suggested that oxygen consumption was supply-dependent in the critically ill, but this has now been shown to be unusual in clinically resuscitated patients. Our understanding of oxygen consumption during critical illness has been confused by the problems of accurately measuring this important physiological variable. Historically, the most straightforward and widely used method utilised the pulmonary artery catheter to determine cardiac output, and blood gas data to calculate arterial and mixed venous oxygen content (the inverse/reverse Fick method). This method is time-consuming, intermittent, and subject to considerable measurement error. The Gold Standard for metabolic measurement is gas analysis. Until recently this was only possible with bulky non-integrated systems such as Deltatrac. The COVX module is a new system that is compact, modular, and fully integrated with other ICU monitoring modalities. It measures oxygen consumption and carbon dioxide elimination on a breath-by-breath basis. This new technology offers the opportunity to routinely follow metabolic rate in mechanically ventilated patients.

The critical care oxygen consumption literature is confusing and contradictory, but the following appear to be consistently found:

1. Oxygen consumption early in critical illness predicts outcome. Patients with high oxygen consumption are more likely to survive.
2. The ability of patients to increase their oxygen consumption in response to a "metabolic challenge", such as dobutamine, is a strong predictor of outcome. Those that cannot increase oxygen consumption are likely to subsequently die.
3. Using oxygen consumption as a target for resuscitation in patients with established critical illness does not appear to improve outcome on an "intention to treat" basis

This information has not been widely used in the clinical management of patients because of the declining use of PA catheters and the inherent errors associated with the oxygen consumption measurements made. The following examples indicate some of the potential of COVX metabolic monitoring early in critical illness.

Last updated: 1 March 2001Created
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