Gastrointestinal tonometry is perhaps the most important advance in monitoring to appear over the past two decades. Increased gut mucosal PCO2 is an excellent physiological signal of tissue hypoperfusion or hypoxia. Newer methods are being developed to ease the process of monitoring mucosal PCO2. One such method uses air in conjunction with an infrared capnometer. Air is pumped in and out of the balloon as frequently as every 10 min. This system obviates the use of blood gas analyzers and markedly decreases the opportunities to introduce errors into the measurement. An example of the use of intermittent gas tonometry is shown in Fig.,2. This anecdotal example serves to illustrate the potential of this technique as a monitor of tissue oxygenation. Prospective trials of air tonometry are required to determine its place among the monitoring techniques in use in the ICU.
Fig. 2 Changes in mucosal PCO2 in a patient with sepsis, measured with a closed-loop air tonometer system: l arterial PCO2; n cardiac index. Gastric mucosal PCO2 increased following the institution of intravenous epinephrine (adrenaline) to treat hypotension. This was accompanied by a decrease in cardiac index. Mucosal PCO2 increased to approximately 150 mmHg, a value compatible with the onset of bowel ischemia, when intravenous epinephrine was stopped and a norepinephrine (noradrenaline) infusion started. Tonometer PCO2 decreased rapidly as the cardiac index increased to baseline values. As regional CO 2 increased, a PCO2 gap developed that was greater than 20 mmHg and at one point reached 100 mmHg. Over the next hour, regional PCO2 returned to baseline values and the PCO2 gap disappeared as the patient improved.
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