Splanchnic tonometry: a review of physiology, methodology, and clinical applications

Abstract

The objective of this article is to review Splanchnic tonometry. The English literature, involving both animal and human studies, was used for review, with emphasis on papers on physiological and methodological principles and clinical applications. Tonometry involves the measurement of intraluminal Pco₂ as a measure of mucosal Pco₂ in the gastrointestinal tract via a catheter in, for instance, stomach or sigmoid colon, and the calculation, with help of the blood bicarbonate content and the Henderson-Hasselbalch equation, of the mucosal pH (pH_i). The latter is considered as a relatively simple index of the adequacy of mucosal blood flow. Concerning methodology, it is still unclear whether acid secretion should be inhibited for proper assessment of Pco₂ in the stomach. Buffering of bicarbonate by gastric acid may elevate the intraluminal Pco₂ independently from mucosal Pco₂, thereby confounding pH; as a measure of perfusion adequacy. This can be prevented by inhibition of acid secretion. Authors have raised doubts whether the composite variable pH_i is of additive value to the acid-base status of arterial blood, so that it is unclear whether a subnormal pH_i is a specific and sensitive indicator of mucosal ischemia, as suggested by others on the basis of a decline in the pH_i along the gastrointestinal tract in animals subjected to vascular occlusion or circulatory shock. Moreover, tissue Pco2 depends on the Pco₂ of supplying blood. Conversely, the bicarbonate concentration in ischemic mucosa may not equal that in arterial blood. Taken together, an elevated tonometer fluid arterial blood Pco₂-gradient might be a more sensitive and specific indicator of mucosal ischemia than a decrease in the pH_i, analogous to an increase in tissue Pco₂ and widening of the venoarterial Pco₂ gradient during various types of hypoperfusion, in animals and humans. Although splanchnic ischemia is an early event in shock, the sensitivity and specificity of this index for mucosal ischemia and its clinical value, relative to that of the pH_i, have not been formally evaluated yet. Nevertheless, the pH_i has been suggested to be of predictive value for gastrointestinal complications, multiple organ failure, success or failure of weaning from mechanical ventilation, and outcome in critically ill patients. Tonometry may be a useful monitoring technique to guide treatment and to improve survival. Splanchnic tonometry is a relatively simple, noninvasive, and thereby promising technique to monitor the critically ill. However, some aspects need further evaluation before the technique can be advocated for routine use.