Continuous measurement of gut luminal PCO2 in the rat: responses to transient episodes of graded aortic hypotension

Abstract

Objectives: To test the rapidity of a continuous PCO2 measurement system response to brief reductions in gut perfusion from transient episodes of graded aortic hypotension, and to investigate the relationship between the increase in ileal luminal PCO2 and mean aortic pressure during the episodes.

Design: Prospective, experimental animal study.

Setting: University research laboratory.

Subjects: Adult male Sprague-Dawley rats, weighing 430 to 510 g.

Interventions: Five Sprague-Dawley rats were anesthetized with intraperitoneal sodium phenobarbital and ventilated with 100% oxygen via tracheostomy to a PaCO2 of 30 to 50 torr (4.0 to 6.7 kPa). Distal aortic pressure was monitored invasively, and a sensor was inserted into the ileal lumen. Luminal PCO2 measurements were recorded every 2 secs. Normal saline was infused at 3 mL/hr, and isoflurane was titrated to a mean aortic pressure of 80 to 100 mm Hg. In each rat, paired 2-min inductions of distal aortic hypotension were induced by digital elevation of an aortic silk sling above the celiac artery to as many as possible of the following pressures (mm Hg): 60, 50, 40, 30, 20, and 10. The experiment was stopped if instability of luminal PCO2 or hypotension persisted through the intervening 8-min recovery periods.

Measurements and main results: One rat completed paired inductions of all six goal aortic pressures. Two rats completed five inductions. One rat completed four inductions, and one rat completed three inductions. The times to onset of luminal hypercapnia and to peak luminal hypercapnia were highly consistent and independent of the degree of hypotension. Onset of hypercapnia was usually detected < 1 min after aortic elevation, but peak luminal hypercapnia occurred approximately 1 min after release of the aortic sling. Regression analysis showed an inverse linear relationship between the maximum increase in luminal PCO2 above baseline and mean aortic pressure during induced hypotension (r2 = .6; p < .001).

Conclusions: Continuous ileal luminal PCO2 measurement by the sensor is rapidly responsive to brief reductions in aortic pressure in a rat model. Maximum luminal PCO2 increase during such perturbations is inversely related to mean aortic pressure.