Bladder epithelial oxygen tension--a new means of monitoring regional perfusion? Preliminary study in a model of exsanguination/fluid repletion

Continuous bladder urinary oxygen tension as a new tool to monitor medullary oxygenation in the critically ill

Acute kidney injury (AKI) is common in the critically ill. Inadequate renal medullary tissue oxygenation has been linked to its pathogenesis. Moreover, renal medullary tissue hypoxia can be detected before biochemical evidence of AKI in large mammalian models of critical illness. This justifies medullary hypoxia as a pathophysiological biomarker for early detection of impending AKI, thereby providing an opportunity to avert its evolution. Evidence from both animal and human studies supports the view that non-invasively measured bladder urinary oxygen tension (PuO₂) can provide a reliable estimate of renal medullary tissue oxygen tension (tPO₂), which can only be measured invasively. Furthermore, therapies that modify medullary tPO₂ produce corresponding changes in bladder PuO₂. Clinical studies have shown that bladder PuO₂ correlates with cardiac output, and that it increases in response to elevated cardiopulmonary bypass (CPB) flow and mean arterial pressure. Clinical observational studies in patients undergoing cardiac surgery involving CPB have shown that bladder PuO₂ has prognostic value for subsequent AKI. Thus, continuous bladder PuO₂ holds promise as a new clinical tool for monitoring the adequacy of renal medullary oxygenation, with its implications for the recognition and prevention of medullary hypoxia and thus AKI.

Bladder tissue oxygen tension monitoring in pigs subjected to a range of cardiorespiratory and pharmacological challenges

PURPOSE

A fall in tissue oxygen tension (tPO(2)) is an early indicator of organ hypoxia in both patients and animal models. We previously demonstrated the utility of bladder tPO(2) in various rodent shock models. As a prelude to clinical testing, we aimed to provide further validation of bladder tPO(2) monitoring in a large animal model undergoing a range of cardiorespiratory insults and vasoactive drug interventions.

METHODS

Anaesthetized, mechanically ventilated, instrumented female pigs (n = 8) were subjected to a range of short-term cardiorespiratory (changes in inspired oxygen concentration (FiO(2)), haemorrhage, positive end-expiratory pressure) and pharmacologic (inotrope, pressor) challenges. Global haemodynamics, arterial and pulmonary blood gases and bladder tPO(2) were measured before and after each challenge.

RESULTS

Bladder tPO(2) values fell in line with increasing degrees of hypoxaemia and haemorrhage, and were restored during resuscitation. These changes often preceded those seen in global haemodynamics, arterial base excess and lactate. The rise in bladder tPO(2) with hyperoxia, performed as an oxygen challenge test, was incrementally blunted by progressive haemorrhage. Dobutamine and norepinephrine both increased cardiac output and global O(2) delivery, but had no effect on bladder tPO(2) or lactataemia in these healthy pigs.

CONCLUSIONS

In this pig model bladder tPO(2) provides a sensitive indicator of organ hypoxia compared to traditional biochemical markers during various cardiorespiratory challenges. This technique offers a potentially useful tool for clinical monitoring.

Tissue oxygen tension monitoring: will it fill the void?

PURPOSE OF REVIEW

The holy grail of circulatory monitoring is an accurate, continuous and relatively noninvasive means of assessing the adequacy of organ perfusion. This could be then advantageously used to direct therapeutic interventions to prevent both under-treatment and over-treatment and thus improve outcomes. However, in view of the heterogeneous response (adaptive or maladaptive) of different organs to various shock states, any monitor of perfusion adequacy cannot reflect every organ system, but should at least detect early deterioration in a 'canary' organ. Tissue oxygen tension reflects the balance between local oxygen supply and demand, and could thus be a potentially useful monitoring modality. This article examines the different technologies available and reviews the current literature regarding its utility as a monitor.

RECENT FINDINGS

Tissue oxygen tension, measured at a variety of sites in both human and laboratory studies, does appear to be a sensitive indicator of organ perfusion in different shock states. However, responses can vary not only between organs and between different shock states, but also over time. These changes reflect the particular oxygen supply-demand balance present in that tissue bed at that specific time point in the disease process. The response to a dynamic oxygen challenge test provides further information that allows severity to be more readily differentiated.

SUMMARY

Monitoring of tissue oxygen tension may offer a potentially useful tool for clinical management though significant validation needs to be first performed to confirm its promise.

Mitochondrial function in sepsis: acute phase versus multiple organ failure

OBJECTIVE

To describe temporal changes in mitochondrial function during the septic process, including the recovery phase.

DESIGN

Literature review.

SUBJECTS

Clinical studies and laboratory models.

MAIN RESULTS

Biochemical and ultrastructural mitochondrial abnormalities have been recognized in in vivo, ex vivo, and in vitro laboratory models of sepsis for >30 yrs. Short-term models show variable effects on mitochondrial function and structure; this is likely related to differences in model design, including species, organs studied, degree of septic insult, and degree of resuscitation. Longer-term models more consistently reveal mitochondrial dysfunction and damage. There is a rebound increase in oxygen consumption and resting energy expenditure in the recovery phase of sepsis. This could reflect mitochondrial recovery (biogenesis) that may restore the energy supply needed to fuel restorative metabolic processes and enable patient survival.

CONCLUSION

Mitochondrial dysfunction seems to be intrinsically involved in the pathogenesis of multiple organ failure. As a consequence of a progressive decrease in energy availability, metabolism must decrease or the cell will die. The interplay between adenosine 5'-triphosphate supply and demand, dictated by the degree of mitochondrial dysfunction and the level of metabolic shutdown (analogous to a hibernation-type response), seems to be crucial in determining outcome. Further studies are needed to confirm this hypothesis.

Tissue oxygen monitoring in rodent models of shock

Tissue Po2 (tPo2) reflects the balance between local O2 supply and demand and, thus, could be a useful monitoring modality. However, the consistency and amplitude of the tPo2 response in different organs during different cardiorespiratory insults is unknown. Therefore, we investigated the effects of endotoxemia, hemorrhage, and hypoxemia on tPo2 measured in deep and peripheral organ beds. We compared arterial pressure, blood gas and lactate levels, descending aortic and renal blood flow, and tPo2 in skeletal muscle, bladder epithelium, liver, and renal cortex during 1) LPS infusion (10 mg/kg), 2) sequential removal of 10% of circulating blood volume, and 3) reductions in inspired O2 concentration in an anesthetized Wistar rat model with values measured in sham-operated animals. Different patterns were seen in each of the shock states, with condition-specific variations in the degree of acidemia, lactatemia, and tissue O2 responses between organs. Endotoxemia resulted in a rise in bladder tPo2 and an early fall in liver tPo2 but no significant change in muscle and renal cortical tPo2. Progressive hemorrhage, however, produced proportional declines in liver, muscle, and bladder tPo2, but renal cortical tPo2 was maintained until profound blood loss had occurred. By contrast, progressive hypoxemia resulted in proportional decreases in tPo2 in all organ beds. This study highlights the heterogeneity of responses in different organ beds during different shock states that are likely related to local changes in O2 supply and utilization. Whole body monitoring is not generally reflective of these changes.

Tissue oxygen and hemodynamics in renal medulla, cortex, and corticomedullary junction during hemorrhage-reperfusion

Previous studies of intrarenal perfusion and tissue oxygenation have produced a wide range of results and have not matched tissue oxygen tension (tPo(2)) with concurrent changes in flow in three distinct regions. We thus used an anesthetized rat model of hemorrhage-reperfusion to address this question. Combined tpo(2)/laser-Doppler fiber-optic probes were simultaneously sited in cortical, corticomedullary (CMJ), and medullary regions of the left kidney. Total renal blood flow was measured in separate experiments. Recordings were made during exsanguination of 10 and 20% of estimated blood volume at 10-min intervals, followed by shed-blood resuscitation after a further 10 min. The decay in tpo(2) was then recorded following total cessation of blood flow, allowing estimation of local oxygen consumption. During exsanguination, tPo(2) was maintained in all intrarenal regions, despite significant falls in blood pressure and total renal blood flow. However, intrarenal flow was redistributed with reduced cortical, unchanged CMJ, and increased medullary blood flow. After resuscitation, significant rises above baseline were seen in blood pressure and in tpo(2) across all regions. Whereas cortical and medullary flows regained baseline values, CMJ flow fell. The ratio of tpo(2) to microvascular blood flow increased significantly in all regions during resuscitation, suggesting decreased oxygen consumption. On total cessation of blood flow, the cortex and CMJ showed significant increases in the oxygen decay half-life, consistent with decreased consumption. To our knowledge, this is the first quantitative demonstration of a markedly heterogeneous intrarenal cardiorespiratory response to a hemodynamic insult, with effects most marked at the corticomedullary junction.

Bladder Mucosa pH and Pco2 as a Minimally Invasive Monitor of Hemorrhagic Shock and Resuscitation

BACKGROUND

Continuous monitoring of pH, Pco2, and Po2 using fiberoptic sensor technology has been proposed recently as a clinical monitor of the severity of shock and impaired tissue perfusion. Surrogates of gut tissue perfusion such as gastric tonometry, although cumbersome, have been used to indirectly quantify the degree of gut ischemia. The purpose of this study was to demonstrate the feasibility of monitoring bladder mucosa (BM) and to compare urinary bladder mucosa and proximal jejunum mucosa interstitial pH and Pco2 during hemorrhagic shock and resuscitation.

METHODS

Eleven male miniature swine (25-35 kg) (control, n = 4; shock, n = 7) underwent jejunal tonometry and cystostomy. A multisensor probe was placed adjacent to the BM. Urine was diverted. Normocarbia was maintained. Animals were hemorrhaged and kept at a mean arterial pressure of 40 mm Hg. When a constant infusion was required to maintain the mean arterial pressure at 40 mm Hg (decompensation), animals were resuscitated with shed blood plus two times the shed volume in lactated Ringer's solution (20 minutes) and observed for 2 hours.

RESULTS

During decompensation, BM pH values decreased significantly from 7.33 +/- 0.08 to 7.01 +/- 0.2 (p < 0.01) and recovered to 7.11 +/- 0.19 at 120 minutes after completion of resuscitation. During decompensation, BM Pco2 values increased significantly compared with baseline (from 49 +/- 6 mm Hg to 71 +/- 19 mm Hg, p < 0.05) and returned to baseline with resuscitation. Jejunum mucosa and BM interstitial Pco2 correlated throughout shock and resuscitation (r = 0.49). Bland-Altman analysis demonstrated significant differences between jejunum mucosa (intramucosal pH) and BM interstitial pH.

CONCLUSION

Shock-induced changes in the Pco2 of the BM are comparable to tonometric changes in the gut. These data suggest that continuous fiberoptic multisensor probe monitoring of the BM could potentially provide a minimally invasive method for the assessment of impaired tissue perfusion of the splanchnic circulation during shock and resuscitation.

Hydrogen peroxide in the human body

Hydrogen peroxide (H(2)O(2)) is widely regarded as a cytotoxic agent whose levels must be minimized by the action of antioxidant defence enzymes. In fact, H(2)O(2) is poorly reactive in the absence of transition metal ions. Exposure of certain human tissues to H(2)O(2) may be greater than is commonly supposed: substantial amounts of H(2)O(2) can be present in beverages commonly drunk (especially instant coffee), in freshly voided human urine, and in exhaled air. Levels of H(2)O(2) in the human body may be controlled not only by catabolism but also by excretion, and H(2)O(2) could play a role in the regulation of renal function and as an antibacterial agent in the urine. Urinary H(2)O(2) levels are influenced by diet, but under certain conditions might be a valuable biomarker of 'oxidative stress'.

Hydrogen peroxide in human urine: implications for antioxidant defense and redox regulation

The presence of hydrogen peroxide, at levels sometimes exceeding 100 microM, in human urine samples was established by three different assay methods: 2-oxoglutarate decarboxylation and the ferrous oxidation-xylenol orange (FOX) assay and an oxygen electrode. Detected levels of H(2)O(2) were decreased by addition of superoxide dismutase. We conclude that urine contains autooxidizable molecules that, upon exposure to 21% O(2), undergo rapid superoxide-dependent autooxidation reactions to generate H(2)O(2). The exposure of human tissues to hydrogen peroxide may be greater than is commonly supposed, which has implications in relation to the proposed role of this species in cell signaling.

A novel approach to monitor tissue perfusion: bladder mucosal PCO2, PO2, and pHi during ischemia and reperfusion

PURPOSE

The purpose of this study is to determine if monitoring urinary bladder PCO2, PO2, and calculated intramucosal pH would be a reliable index of tissue perfusion.

MATERIALS AND METHODS

This nonrandomized controlled study was conducted in a laboratory at a university medical center. Eight immature female Yorkshire pigs were studied with T-9 aortic cross-clamping for 30 minutes followed by a 60-minute period of reperfusion. Cystotomy was performed for placement of a Foley catheter and Paratrend 7 O2/CO2 sensor.

RESULTS

Baseline hemodynamic and metabolic measurements were obtained along with measurements of bladder mucosal PO2 and PCO2 (mean+/-SEM). Blood flow measured with microspheres confirmed absence of blood flow during occlusion and hyperemia during reperfusion. Bladder mucosal PO2 decreased from 42+/-14.0 mm Hg (5.6 kPa) to 1.3+/-1.3 mm Hg (1.4 kPa) during the 30-minute interval of ischemia. This was followed by an increase of bladder PO2 to greater than baseline values at the end of the reperfusion period. Bladder mucosal Pco2 increased from 57+/-4.7 mm Hg (7.6 kPa) to 117+/-7.1 mm Hg (15.6 kPa) (P < .05) during ischemia. During reperfusion the Pco2 returned to baseline levels (55+/-4.0 mm Hg [7.3 kPa]). Calculated bladder mucosal pHi declined from 7.31+/-0.04 to 7.08+/-0.05 (P < .05) during the ischemic period and after reperfusion pHi was 7.17+/-0.03.

CONCLUSIONS

Monitoring urinary bladder PO2, PCO2, or calculating pHi may provide a simple and reliable means of monitoring tissue perfusion.