Continuous monitoring of the partial pressure of oxygen in cerebral venous blood

Oxygen-sensing mechanisms in development and tissue repair

Under normoxia, hypoxia inducible factor (HIF) α subunits are hydroxylated by PHDs (prolyl hydroxylase domain proteins) and subsequently undergo polyubiquitylation and degradation. Normal embryogenesis occurs under hypoxia, which suppresses PHD activities and allows HIFα to stabilize and regulate development. In this Primer, we explain molecular mechanisms of the oxygen-sensing pathway, summarize HIF-regulated downstream events, discuss loss-of-function phenotypes primarily in mouse development, and highlight clinical relevance to angiogenesis and tissue repair.

EPR and electrochemical quantification of oxygen using newly synthesized para-silylated triarylmethyl radicals

Novel silylated triarylmethyl (TAM) radicals based on TAM core CT-03 and their electron paramagnetic resonance (EPR) spectra are evaluated as a function of oxygen concentration. Combination of peak-to-peak linewidth of the EPR signal and electrochemical determination allows designing a method for oxygen quantification in phosphate buffer, dimethylsulfoxide, and dichloromethane, which can be extended to other solvents.

Inhaled carbon monoxide provides cerebral cytoprotection in pigs

Carbon monoxide (CO) at low concentrations imparts protective effects in numerous preclinical small animal models of brain injury. Evidence of protection in large animal models of cerebral injury, however, has not been tested. Neurologic deficits following open heart surgery are likely related in part to ischemia reperfusion injury that occurs during cardiopulmonary bypass surgery. Using a model of deep hypothermic circulatory arrest (DHCA) in piglets, we evaluated the effects of CO to reduce cerebral injury. DHCA and cardiopulmonary bypass (CPB) induced significant alterations in metabolic demands, including a decrease in the oxygen/glucose index (OGI), an increase in lactate/glucose index (LGI) and a rise in cerebral blood pressure that ultimately resulted in increased cell death in the neocortex and hippocampus that was completely abrogated in piglets preconditioned with a low, safe dose of CO. Moreover CO-treated animals maintained normal, pre-CPB OGI and LGI and corresponding cerebral sinus pressures with no change in systemic hemodynamics or metabolic intermediates. Collectively, our data demonstrate that inhaled CO may be beneficial in preventing cerebral injury resulting from DHCA and offer important therapeutic options in newborns undergoing DHCA for open heart surgery.

Small Volume Resuscitation: A Randomized Controlled Trial With Either Norepinephrine or Vasopressin During Severe Hemorrhage

BACKGROUND

The present study was designed to evaluate the effects of hypertonic-hyperoncotic hydroxyethyl starch solution (HHS) combined with either norepinephrine (NE) or arginine vasopressin (AVP) on cerebral perfusion pressure (CPP) and brain metabolism after hemorrhagic shock.

METHODS

Fourteen pigs were subjected to uncontrolled liver bleeding until hemodynamic decompensation followed by resuscitation using HHS (4 mL/kg) combined with either NE (bolus of 1000 microg; 60 microg/kg/hr; n = 7) or AVP (bolus of 10 U; 2 U/kg/hr; n = 7), respectively. Extracellular cerebral concentrations of glucose, glycerol, lactate, and the lactate/pyruvate ratio were assessed by microdialysis. After 30 minutes of therapy, bleeding was controlled by manual compression and all surviving animals were observed for 1 hour.

RESULTS

After hemodynamic decompensation, AVP resulted in a significantly higher increase of CPP (mean +/- SD; 47 +/- 19 versus 28 +/- 9 mm Hg; p < 0.01) and cerebral venous partial pressure of oxygen (66 +/- 8 versus 49 +/- 9 mm Hg; p < 0.05) compared with NE after 10 minutes of therapy. Hemodynamic data and blood gas variables were not different between groups during the remaining study period. Brain metabolism was found comparable in both groups at any time.

CONCLUSIONS

AVP was comparable to NE with respect to hemodynamics and blood gases, as well as brain metabolism in surviving animals throughout the study period. Our findings emphasize the importance of early resuscitation, as neuronal cell damage potentially starts immediately after onset of severe hemorrhage.

Is the renal production of erythropoietin controlled by the brain stem?

Although the structure and function of erythropoietin (Epo) are well documented, the mechanisms of the regulation of the renal synthesis of Epo are still poorly understood. Especially, the description of the localization and function of the O(2)-sensitive sensor regulating the renal synthesis of Epo is insufficient. A body of evidence suggests that extrarenal O(2)-sensitive sensors, localized particularly in the brain stem, play an important role in this connection. To support this concept, high cerebral pressure with consecutive hypoxia of the brain stem was generated by insufflation of synthetic cerebrospinal fluid into the catheterized cisterna magna of rats. When the cerebral pressure of the rats was above the level of their mean arterial blood pressure or the high cerebral pressure persisted for a longer period (>/=10 min), the Epo plasma concentration increased significantly. Bilateral nephrectomy or hypophysectomy before initiation of high intracranial pressure abolished this effect. Systemic parameters (heart rate, blood pressure, Pa(O(2)), Pa(CO(2)), arterial pH, renal blood flow, glucose concentration in blood) were not affected. Other stressors, like restricting the mobility of the rats, had no effect on Epo production. Hence, the effect of high cerebral pressure on renal synthesis of Epo seems to be specific. Increasing cerebral hydrostatic pressure leads to increased renal synthesis of Epo. Obviously, during hypoxia, cerebral O(2)-sensitive sensors release humoral factors, triggering the renal synthesis of Epo. The structure and function of these "Epo-releasing-factors" will have to be characterized in future experiments.

Scientific and technological aspects of oxygen-sensitive electrodes for measurements of oxygen partial pressure in patch-clamp experiments

Anoxia/hypoxia induces dramatic changes in brain activity leading to the damages in brain structure. Several minutes of decrease in environmental oxygen partial pressure (P(O2)) can irreversibly destroy nerve cells. Therefore, investigations of intracellular mechanisms responsible for hypoxia-induced changes of mammalian nerve system are very important. On-line adjustment and measurements of P(O2) in bath solution during patch-clamp experiments are especially topical. At the recent time, a special interest on the on-line measurements of oxygen contents in bath solution has appeared as a result of application of old approaches previously used for polarographic measurements of oxygen contents in the blood and tissues. Here we describe the simple method of manufacturing oxygen-sensitive microelectrodes, which can be used with standard patch-clamp amplifier. We also describe the main principles of polarographic method and properties of oxygen-sensitive electrodes used in patch-clamp experiments.

Resuscitation from severe hemorrhagic shock after traumatic brain injury using saline, shed blood, or a blood substitute

The original purpose of this study was to compare initial resuscitation of hemorrhagic hypotension after traumatic brain injury (TBI) with saline and shed blood. Based on those results, the protocol was modified and saline was compared to a blood substitute, diaspirin cross-linked hemoglobin (DCLHb). Two series of experiments were performed in anesthetized and mechanically ventilated (FiO2 = 0.4) pigs (35-45 kg). In Series 1, fluid percussion TBI (6-8 ATM) was followed by a 30% hemorrhage. At 120 min post-TBI, initial resuscitation consisted of either shed blood (n = 7) or a bolus of 3x shed blood volume as saline (n = 13). Saline supplements were then administered to all pigs to maintain a systolic arterial blood pressure (SAP) of >100 mmHg and a heart rate (HR) of <110 beats/min. In Series 2, TBI (4-5 ATM) was followed by a 35% hemorrhage. At 60 min post-TBI, initial resuscitation consisted of either 500 mL of DCLHb (n = 6) or 500 mL of saline (n = 5). This was followed by saline supplements to all pigs to maintain a SAP of >100 mmHg and a HR of <110 beats/min. In Series 1, most systemic markers of resuscitation (e.g., SAP, HR, cardiac output, filling pressures, lactate, etc.) were normalized, but there were 0/7 vs. 5/13 deaths within 5 h (P = 0.058) with blood vs. saline. At constant arterial O2 saturation (SaO2), mixed venous O2 saturation (SvO2), cerebral perfusion pressure (CPP), and cerebral venous O2 saturation (ScvO2) were all higher, intracranial pressure (ICP) was lower, and CO2 reactivity was preserved with blood vs. saline (all P < 0.05). In Series 2, SAP, ICP, CPP, and lactate were higher with DCLHb vs. saline (all P< 0.05). Cardiac output was lower even though filling pressure was markedly elevated with DCLHb vs. saline (both P< 0.05). Neither SvO2 nor cerebrovascular CO2 reactivity were improved, and ScvO2 was lower with DCLHb vs. saline (P < 0.05). All survived at least 72 h with neuropathologic changes that included sub-arachnoid hemorrhage, midline cerebellar necrosis, and diffuse axonal injury. These changes were similar with DCLHb vs. saline. Thus, whole blood was more effective than saline for resuscitation of TBI, whereas DCLHb was no more, and according to many variables, less effective than saline resuscitation. These experimental results are comparable to those in a recent multicenter trial using DCLHb for the treatment of severe traumatic shock. Further investigations in similar experimental models might provide some plausible explanations why DCLHb unexpectedly increased mortality in patients.