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Gastric Tonometry

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Author(s): Kamat V

Journal: Indian Journal of Critical Care Medicine
ISSN 0972-5229

Volume: 7;
Issue: 1;
Start page: 26;
Date: 2003;
Original page

Keywords: critically ill | multi organ failure | Gastric tonometry | splanchnic hypoperfusion | anaerobic metabolism | Saline tonometry | pHi | Air tonometry | PgCO2.

ABSTRACT
Early warning of end organ hypoperfusion in the critically ill, will help the intensivist in initiating corrective measures to prevent multi organ failure. Common end points of tissue perfusion like cardiac output, serum lactate and mixed venous oxygen saturation indicates global oxygen delivery and do not reflect regional perfusion and oxygenation. In low cardiac output states, the gut mucosa is the first to be affected by poor perfusion due to the counter current blood flow pattern in the intestinal villi. Gastric tonometry, by indirectly measuring the gut mucosal PCO2, gives an indication of the gastric mucosal perfusion. It is assumed that the increased gastric mucosal CO2 leading to gastric mucosal acidosis is a result of anaerobic metabolism consequent to splanchnic hypo perfusion. An increase in gut wall CO2 occurs due to anaerobic metabolism as well as decreased CO2 wash out secondary to a poor perfusion state. Gastric tonometry measures the balance between gut metabolism (CO2 production) and gut perfusion (CO2 removal). Saline tonometry is useful in assessing the gastric intramucosal pH (pHi). This is calculated from the measured intramucosal CO2 and the calculated arterial bicarbonate using the Henderson Hasselbach equation. Air tonometry which is a more recent development assesses the difference between the gastric mucosal CO2 (PgCO2) and arterial / end tidal CO2. The normal gap is 7-10 mmHg. A gap of >23 mmHg indicates anaerobic metabolism. There are several practical limitations to the application of gastric tonometry to assess splanchnic perfusion. Despite these limitations it is an easy and relatively non invasive method in following trends. Further refinements in technique could make it a more reliable monitor in predicting outcome in the critically ill patient. However the complexities of gastrointestinal physiology are yet to be resolved and we await large randomised studies on air tonometry to provide scientific proof that it is a prognostic marker in critically ill patients

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