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Dr. Dantzker is Professor of Medicine, Albert Einstein College of Medicine.
Correspondence to: David R. Dantzker, MD, FCCP, 64 East 86th St, New York, NY 10028; e-mail: david{at}dantzker.com
In 1997, I wrote an editorial for CHEST regarding the search for a method to monitor the adequacy of tissue oxygenation that could be used by clinicians to guide the treatment of physiologically unstable patients.1 At that time, I likened this to the quest for the Holy Grail. Here it is 4 years later and, like the search for the grail, the goal appears to remain tantalizingly out of reach. While elusiveness is a wonderfully romantic part of the fascination with a quest, it is disappointing that more progress has not been made. The article by Marik in this issue of CHEST (see page 923) provides additional evidence that monitoring changes in luminal PCO2, reflecting the interstitial hydrogen ion concentration, signals a change in overall homeostasis that correlates with a bad outcome. This additional proof of principal is welcomed, but begs the question of whether this variable can be put to use in a clinically useful and reliable manner.
Over the years, many indexes have been suggested as ways of monitoring tissue oxygenation. Most of these, such as venous PO2, lactate, and O2 consumption-O2 delivery relationships have fallen by the way either because they could not stand up to rigorous clinical appraisal or because they were successfully challenged by newer theoretical understandings of the microcirculation and tissue metabolism. Some techniques, such as direct magnetic resonance or near infrared spectroscopic measurements of tissue metabolic state, would probably be quite useful, but have not yet been developed to the point of clinical applicability.
In 1964, Bergofsky showed that hollow organs such as the bladder could be used as an in vivo tonometer to approximate tissue gas tensions.2 Fiddian-Green et al3 in the early 1980s used this approach to measure interstitial PCO2 in the gut and offered this measurement as an index of tissue hydrogen ion concentration (pHi) and by inference the adequacy of tissue oxygenation. The principle is quite simple. As the oxygen-deprived tissue depends increasingly on glycolysis for adenosine triphosphate generation, buffering of the rising hydrogen ion concentration leads to CO2 production, which diffuses into the lumen of the gut and can be measured. Subsequent studies4 in animals validated the concept, and human studies5 6 showed that a fall in pHi correlated with a poor outcome in a large number of clinical situations, such as high-risk and cardiac surgery, as well as in the ICU.
Weil and his coworkers,7 allowing for the use of sublingual measurements of PCO2 to replace gastric or small-bowel sampling, accomplished subsequent modification of the technique of monitoring pHi. Their data plus those presented in the study by Marik suggest that an easier measure of tissue anaerobiosis is now available. Are we now closer to a clinically useful tool?
Physicians taking care of physiologically unstable patients have always been fascinated with exotic means of monitoring bodily functions. In the days of seemingly endless health-care resources, we could indulge this intellectual curiosity and hope that sufficient information could be extracted to alter outcome and justify the added cost in care time and capital investment. Over time, many of these "black boxes" have been relegated to the hospital storeroom as the initial enthusiasm waned. Measurement of pHi, as originally described by Fiddian-Green et al3 or in the form of sublingual capnography, has been enticing us long enough. Before it meets a similar fate, it is time for a coordinated effort to study its clinical usefulness. We already have a number of ways of predicting outcome, including the clinical acumen of seasoned clinicians. What we need is an objective tool to direct therapy.
The only attempt thus far to prospectively study pHi as a guide to therapy was a heroic, but less than satisfying, effort by Gutierrez and his colleagues.8 Despite some methodologic flaws, their data did suggest that patients treated using pHi as a guide had a better outcome than those treated using standard monitoring techniques. What is called for now is a well-planned, multicenter, prospective trial comparing outcome in a relatively well-defined category of patients using pHi in the study group to guide clearly specified interventions.
One should not underestimate the difficulty of such a clinical trial. Time, money, courage, and fortitude will be required. However, only with such an effort, do we have any chance at turning a quest into a real journey of discovery.
References
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