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[Gastric mucosal tonometry as a monitoring method in cardiac anesthesia. Empirical findings on the postoperative outcome under various volume controls].

OBJECTIVES
Several studies documented higher complication rates after cardiac surgery in patients with splanchnic hypoperfusion. Although it is prone to errors, gastric tonometry probably is the method of choice for detecting splanchnic hypoperfusion. While there are many reasons for splanchnic hypoperfusion, low cardiac output because of hypovolemia is one of the important ones in cardiac surgery. Thereby endogenous vasoactive substances, such as angiotensin II and the kinins, might be of special interest.


METHODS
Following approval from the local ethics committee, 40 patients undergoing elective cardiac surgery were studied. Every patient received a TRIP NGS Catheter (Tonometrics Division Instrumentarium Corp., Helsinki, Finland). Using radioimmunoassays and chromatography angiotensin II and bradykinin was measured before, during and immediately after cardiopulmonary bypass. Using saline tonometry gastric mucosal CO2 was measured ten times perioperatively. Patients were shifted into two groups by dichotomization at the median of gastric mucosal pH (pHi) and the pCO2 gap (gastric mucosal pCO2-arterial pCO2) before surgery. Volume substitution, use of vasoactive drugs, haemodynamic instability and time of extubation were documented.


RESULTS
During cardiopulmonary bypass group I (pHi < 7.32 and CO2 gap > 3.85 mmHg) showed higher expression of angiotensin II and lower expression of bradykinin then group II (pHi > 7.32 and CO2 gap < 3.85 mmHg). The most significant difference was found on bypass. Immediately post bypass there was still a difference in the bradykinin expression. Before bypass no differences was found. In group I significantly more volume had to be substituted for haemodynamic stabilisation. These patients needed more often vasoactive drugs and in tendency were extubated later. At the time of extubation no group-difference was found as in the pHi as in the CO2 gap as in the amount of substituted volume. Patients with previous high pHi and low CO2 gap had lowest respectively highest values at the time, when fluid-balance was most negative.


CONCLUSIONS
Splanchnic hypoperfusion in cardiac surgery probably correlates with hypovolemia and therefore leads to vasoconstriction, wich is shown in higher expression of angiotensin II and lower of bradykinin. Gastric mucosal tonometry in cardiac surgery probably detects hypovolemia and therefore predicts haemodynamic instability. Therefore gastric mucosal tonometry could probably be used as a therapeutical sign for a sufficient cardiac output and therefore for tissue oxygenation in general.