Tonometry

Peri-operative Gastrointestinal Tonometry

Monty G Mythen, Dr

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Numerous studies have shown that the measurement of gastrointestinal intramucosal pH (pHi) is predictive of a poor outcome following major surgery. However, this has not been a universal finding with some studies reporting no such relationship.

The data from cardiac surgical patients represents the largest and most consistent body of evidence. The majority of studies suggest that gastric luminal hypercarbia is common following (but not during) cardio-pulmonary by-pass and that if it persists to the end of surgery then the patient is at considerable risk of developing post-operative complications.

The data from major vascular surgery requiring cross clamping of the abdominal aorta is a little more confused. Gastric tonometry seems to perform less well as predictor of outcome than sigmoid tonometry. Considering the inevitable perfusion abnormalities associated with cross clamping the descending aorta this should probably be of no great surprise. One would anticipate that the level of the cross clamp and quality of collateral flow would be major determinants of sigmoid but not gastric perfusion.

The most disappointing data has come from major bowel surgery. Here the direct surgical bowel injury seems to be an overwhelming component and independent of gastric luminal CO2. Once again many would argue that this was a fairly predictable finding.

With the exception of major bowel surgery and abdominal aortic surgery gastric tonometry does seem to have predictive ability over and above more commonly measured variables for the development of post operative complications in major non-cardiac surgery. The incidence of catastrophic morbidity and death is relatively rare for most types of elective surgery,but minor morbidity associated with prolongation of hospital stay is disturbingly common; and it's economic impact is huge.

It has been shown that deliberately maintaining a normal pHi during both cardiac and non-cardiac surgery can be associated with a better outcome and shorter hospital stay. However these studies had pHi as an outcome variable and not a therapeutic trigger.

Therefore, we have the following questions to ask ourselves about the future of tonometry in major surgery particularly with the relatively recent introduction of automated air tonometry:

• Is gastrointestinal luminal PCO2 (PrCO2) an important physiological variable?
• Are there devices that can measure PrCO2 accurately, reliably and in a time frame that allows correction of an abnormality?
• Is a device that measures PrCO2 a "monitor"?
• Is an abnormal PrCO2 a bad thing?
• Is a normal PrCO2 gap a better thing?
• Can PrCO2 be corrected if an abnormal PrCO2 is detected?
• Do the benefits outweigh the risk?
• Can the cost of measuring PrCO2 be justified?
• Does automated air tonometry have a place in my practice?

Last updated: 1 September 2000
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