Blood in the gastrointestinal tract delays and blunts the PCO2 response totransient mucosal ischaemia

Objective: To determine whether the presence of blood in the gastrointestin al tract impairs the detection of splanchnic mucosal ischaemia by reducing the rate and magnitude of rise in gut luminal PCO2. Design: Prospective observational study Setting: University Research Laboratory Subjects: Four adult male Wistar rats with four controls from a concurrent study Interventions: Four adult male Wistar rats were anaesthetised with sodium p entobarbitone and ventilated with oxygen and isoflurane to an initial PaCO2 of 30-40 torr. Electrochemical-fiberoptic gas sensors inserted into silast ic tubing placed in the ileal lumen measured luminal PCO2 at 1-s intervals. Distal aortic pressure was monitored continuously. Six 2-min episodes of a ortic hypotension were induced in each rat to a mean pressure of 30 mmHg by intermittent elevation of a silk sling placed around the proximal aorta. B efore the last three episodes in each rat 0.75-1.0 mi blood was injected in to the ileal lumen via a 25-gauge needle. Four control rats from a concurre nt experiment were treat-ed in an identical fashion except that the rats we re subjected to five ischaemic episodes, and there was no intraluminal inje ction of blood, although a 20-gauge cannula was placed in the proximal ilea l lumen Measurements ann Main results: The presence of blood in the lumen significa ntly decreased the ischaemic Delta PCO2 response from 33 +/- 8 to 15 +/- 4 torr (P < 0.001) and also significantly increased the peak response time fr om 188 +/- 12 to 227 +/- 24 s (P < 0.001). The corresponding measurements i n the concurrent controls differed only from the values after blood injecti on in the experimental group Conclusions: In this animal model the presence of intraluminal blood signif icantly delayed the rate and the amplitude of luminal PCO2 increases in res ponse to transient ischaemia. This raises questions about the validity of l uminal CO2 measurements as an indicator of splanchnic ischaemia in the pres ence of gastrointestinal haemorrhage.