No abstract

At IPP levels below 50 mmHg, even moderate levels of PEEP impaired local blood flow enough to cause intestinal ischemia. Our data underscore the importance of considering regional circulatory adaptations during PEEP ventilation.

The available methods for tonometric pCO2 measurement only provide the possibility of performing intermittent registrations. A new method allowing continuous tonometric pCO2 measurement has been developed and tested in an in vitro model. A standard tonometer for intestinal pCO2 measurement was modified to allow continuous perfusion of the balloon with physiological saline solution in a closed system. The pCO2 in the system was determined in a specially constructed measurement chamber with a TCM20 percutaneous pCO2 monitor. In this in vitro model the tonometer balloon was placed in a saline bath with a constant pCO2 concentration and the measurements from the closed circulating system were compared with those obtained from a standard tonometer placed in the same bath. In 8 and 24 h experiments the circulating system measured the pCO2 value as accurately and reliably as traditional tonometry. This study indicates that the new method makes continuous monitoring of pCO2 possible.

Continuous saline tonometry detected intestinal ischemia as induced by graded reductions in IPP. A threshold could be defined above which intestinal ischemia does not occur.

This self-controlled experimental study was designed to test the hypothesis that dopexamine, a synthetic catecholamine that activates dopaminergic (DA-1) and beta2-adrenergic receptors, improves oxygenation in the jejunal mucosa during intestinal hypotension. In six normoventilated barbiturate-anesthetized pigs, controlled reductions in superior mesenteric arterial pressure (PSMA) was obtained by an adjustable clamp around the artery. Dopexamine infusions (0.5 and 1.0 microg.kg(-1).min(-1)) were administered at a freely variable PSMA (i.e., with the perivascular clamp fully open) and at a PSMA of 50 mmHg and 30 mmHg. We continuously measured superior mesenteric venous blood flow (QMES; transit-time ultrasonic flowmetry), jejunal mucosal perfusion (laser Doppler flowmetry), and tissue oxygen tension (PO2TISSUE; microoximetry). Jejunal luminal microdialysate of lactate, pyruvate, and glucose were measured every 5 min. Measurements of mucosal PCO2 (air tonometry), together with blood sampling and end-tidal PCO2 measurements, enabled calculations of pHi and PCO2 gap. Dopexamine reduced mesenteric vascular resistance and increased QMES at a PSMA of 50 mmHg and 30 mmHg. At a PSMA of 30 mmHg, dopexamine increased mesenteric oxygen delivery but did not influence mesenteric oxygen uptake or extraction. In this situation, dopexamine had no beneficial effect on jejunal mucosal blood flow. On the contrary, dopexamine increased mesenteric net lactate production and PCO2 gap, whereas PO2TISSUE and pHi decreased. Jejunal luminal microdialysate data demonstrated an increased lactate concentration and a pattern of decreased glucose concentration and increased luminal lactate-pyruvate ratio. These negative metabolic effects of dopexamine should be taken into account in situations of low perfusion pressures.

The equipment for continuous saline tonometry has a good reactivity, an accuracy comparable with standard tonometry.

Only intermittent and semi-continuous tonometric measurement of gastric and intestinal pHi is possible with the equipment available today. Earlier we developed a system for continuous saline tonometry and tested it in vitro. To assess the in vivo reliability of this method for continuous gastrointestinal saline tonometry, a standard tonometer for measurement of intestinal pCO2 and corresponding pHi was modified to allow continuous perfusion of physiological saline in a closed system and tested in a porcine model. In 11 anaesthetized and haemodynamically stable pigs, two continuous tonometry balloons were inserted into the distal small bowel, and a standard tonometry balloon was used as reference. To test long-term function of the continuous tonometers the research protocol lasted for eight hours. The two continuous saline tonometers performed well, and after an equilibration time of three hours the mean pHi values were stable between 7.35 and 7.43 and between 7.32 and 7.39 respectively. The standard tonometer measured stable pHi values. These preliminary studies indicate that continuous saline tonometry performs well over eight hours with a small bias and a good precision.

At very low intestinal perfusion pressures (approximately 30 mmHg) dopexamine produces intestinal vasodilation in excess of what is produced by intrinsic autoregulation. This suggests that there is a vasodilatory reserve in the intestine under such conditions and that a pharmacological vasodilator like dopexamine may improve intestinal circulation during regional severe hypotension.