Microcirculation of the intestinal mucosa

Defining Physiological Normoxia for Improved Translation of Cell Physiology to Animal Models and Humans

The extensive oxygen gradient between the air we breathe (Po₂ ~21 kPa) and its ultimate distribution within mitochondria (as low as ~0.5-1 kPa) is testament to the efforts expended in limiting its inherent toxicity. It has long been recognized that cell culture undertaken under room air conditions falls short of replicating this protection in vitro. Despite this, difficulty in accurately determining the appropriate O₂ levels in which to culture cells, coupled with a lack of the technology to replicate and maintain a physiological O₂ environment in vitro, has hindered addressing this issue thus far. In this review, we aim to address the current understanding of tissue Po₂ distribution in vivo and summarize the attempts made to replicate these conditions in vitro. The state-of-the-art techniques employed to accurately determine O₂ levels, as well as the issues associated with reproducing physiological O₂ levels in vitro, are also critically reviewed. We aim to provide the framework for researchers to undertake cell culture under O₂ levels relevant to specific tissues and organs. We envisage that this review will facilitate a paradigm shift, enabling translation of findings under physiological conditions in vitro to disease pathology and the design of novel therapeutics.

Pathophysiology of Acute Illness and Injury

The pathophysiology of acute illness and injury recognizes three main effectors: infection, trauma, and ischemia-reperfusion injury. Each of them can act by itself or in combination with the other two in developing a systemic inflammatory reaction syndrome (SIRS) that is a generalized reaction to the morbid event. The time course of SIRS is variable and influenced by the number and severity of subsequent insults (e.g., reparative surgery, acquired hospital infections). It occurs simultaneously with a complex of counter-regulatory mechanisms (compensatory anti-inflammatory response syndrome, CARS) that limit the aggressive effects of SIRS. In adjunct, a progressive dysfunction of the acquired (lymphocytes) immune system develops with increased risk for immunoparalysis and associated infectious complications. Both humoral and cellular effectors participate to the development of SIRS and CARS. The most important humoral mediators are pro-inflammatory (IL-1β, IL-6, IL-8, IL-12) and anti-inflammatory (IL-4, IL-10) cytokines and chemokines, complement, leukotrienes, and PAF. Effector cells include neutrophils, monocytes, macrophages, lymphocytes, and endothelial cells. The endothelium is a key factor for production of remote organ damage as it exerts potent chemo-attracting effects on inflammatory cells, allows for leukocyte trafficking into tissues and organs, and promotes further inflammation by cytokines release. Moreover, the loss of vasoregulatory properties and the increased permeability contribute to the development of hypotension and tissue edema. Finally, the disseminated activation of the coagulation cascade causes the widespread deposition of microthrombi with resulting maldistribution of capillary blood flow and ultimately hypoxic cellular damage. This mechanism together with increased vascular permeability and vasodilation is responsible for the development of the multiple organ dysfunction syndrome (MODS).

Effect of Remote Ischemic Preconditioning on Intestinal Ischemia-Reperfusion Injury in Adults Undergoing On-Pump CABG Surgery: A Randomized Controlled Pilot Trial

OBJECTIVE

Cardiopulmonary bypass (CPB) surgery commonly threatens the heart and remote organs with ischemia-reperfusion injury. Transient episodes of ischemia to nonvital tissue, known as remote ischemic preconditioning (RIPC), is thought to help local and remote vital organs to withstand subsequent ischemic insults.

DESIGN

Prospective, randomized, double-blinded control trial.

SETTING

Tertiary referral academic teaching hospital.

PARTICIPANTS

Thirty patients undergoing elective CPB surgery INTERVENTION: RIPC was achieved via three 5-minute cycles of upper limb ischemia using a blood pressure cuff or control (sham cuff).

MEASUREMENTS AND MAIN RESULTS

Primary outcome was the occurrence of intestinal injury, as measured by an increase in intestinal fatty acid binding protein (I-FABP). Secondary outcomes included incidence of gastrointestinal complications and duration of intensive care unit (ICU) stay. RIPC did not affect serum IFABP levels at the end of surgery and on the first postoperative day (p = 0.697 and p = 0.461, respectively). For all patients, mean I-FABP levels significantly increased at the end of surgery and decreased to under baseline levels on the first postoperative day (from a mean [± standard deviation] baseline value of 764 ± 492 pg/mL to 2,002 ± 974 pg/mL and decreased to 568 ± 319 pg/mL, p < 0.001). All patients remained clinically absent of gastrointestinal complications until hospital discharge. Duration of ICU stay was not correlated with I-FABP levels at the end of surgery. Neither duration of CPB nor duration of aortic clamping significantly correlated with postoperative I-FABP levels.

CONCLUSIONS

These findings suggest that RIPC does not affect intestinal injury in patients undergoing CPB surgery. In patients undergoing cardiac surgery, intestinal injury appears to be moderate and transient without any clinical relevant complication.