INTRALUMINAL BALLOONLESS AIR TONOMETRY - A NEW METHOD FOR DETERMINATION OF GASTROINTESTINAL MUCOSAL CARBON-DIOXIDE TENSION

Objective: To determine if air introduced directly into the lumen of a hollow viscus can be used instead of fluid in a Silastic balloon to e stimate gastrointestinal mucosal PCO2. Design: Prospective, unblinded comparison of two methods of mucosal PCO2 measurement. Setting: Animal surgery suite at a large, university-affiliated medical center. Inter ventions: Standard, commercially available, tonometric catheters were positioned in the ileum (n = 4) or the stomach and ileum (n = 12) of a nesthetized, immature Yorkshire swine. Using gas-tight purse-string su tures, plastic cannulas were inserted into the lumen of the stomach (n = 12) and the lumen of a 10-cm isolated segment of ileum (n = 16). Da ta were collected after equilibration periods of 30 or 60 mins. Before each equilibration period, the ''air tonometers'' (i.e., the lumens o f the stomach and/or the isolated heal segment) were lavaged with 200 mt (stomach) or 20 mL (ileum) of air. In group 1 (n = 4) and group 2 ( n = 3), graded degrees of mesenteric hypoperfusion were achieved by me chanical mesenteric occlusion or pericardial tamponade, respectively. In group 3 (n = 8), graded degrees of respiratory acidosis were induce d. At various intervals, PCO2 was determined simultaneously in arteria l blood, gastric air, saline from the gastric tonometric balloon, ilea l air, and saline from the ileal tonometric balloon. Measurements and Main Results: In pigs with ischemia created by mesenteric vascular occ lusion (group 1), there was a moderate correlation between PCO2 values in air samples from the ileal lumen and samples of saline from the st andard tonometer (r(2) = .61, p < .001). In pigs with mesenteric ische mia secondary to pericardial tamponade (group 2), air and saline tonom etry were well-correlated in the stomach (r(2) = .71, p < .001) and il eum (r(2) = .83, p < .001). In pigs with normal mesenteric perfusion ( group 3) and PaCO2 >40 torr (5.3 kPa), PaCO2 correlated with heal muco sal PCO2, determined using air (r(2) = .93, p < .001) or saline (r(2) = .91, p < .001) tonometry, or gastric mucosal PCO2, determined using air (r(2) = 1.00, p < .001) or saline (r(2) = .97, p < .001) tonometry . Values obtained by air tonometry were highly correlated with values obtained using standard saline tonometry in the stomach (r(2) = .98, p < .001; bias = -5 +/- 5 torr [-0.65 +/- 0.65 kPal]) or ileum (r(2) = .96, p < .001; bias = 1 +/- 9 torr [0.13 +/- 1.17 kPa]). Conclusions: a) Under stable hemodynamic and respiratory conditions, air tonometry (which, in theory, can be performed using a conventional nasogastric o r nasoenteric feeding tube) estimates gastrointestinal mucosal PCO2 as accurately as standard saline tonometry in the stomach or ileum; b) r espiratory acidosis leads to tissue hypercarbia, a phenomenon that mus t be considered when tonometry is used to guide therapy in the clinica l setting; c) under stable, nonischemic conditions, gastric or intesti nal tonometry can be used to estimate PaCO2.