Oxidative Stress Reaction to Hypobaric-Hyperoxic Civilian Flight Conditions

BACKGROUND

In military flight operations, during flights, fighter pilots constantly work under hyperoxic breathing conditions with supplemental oxygen in varying hypobaric environments. These conditions are suspected to cause oxidative stress to neuronal organ tissues. For civilian flight operations, the Federal Aviation Administration (FAA) also recommends supplemental oxygen for flying under hypobaric conditions equivalent to higher than 3048 m altitude, and has made it mandatory for conditions equivalent to more than 3657 m altitude.

AIM

We hypothesized that hypobaric-hyperoxic civilian commercial and private flight conditions with supplemental oxygen in a flight simulation in a hypobaric chamber at 2500 m and 4500 m equivalent altitude would cause significant oxidative stress in healthy individuals.

METHODS

Twelve healthy, COVID-19-vaccinated (third portion of vaccination 15 months before study onset) subjects (six male, six female, mean age 35.7 years) from a larger cohort were selected to perform a 3 h flight simulation in a hypobaric chamber with increasing supplemental oxygen levels (35%, 50%, 60%, and 100% fraction of inspired oxygen, FiO2, via venturi valve-equipped face mask), switching back and forth between simulated altitudes of 2500 m and 4500 m. Arterial blood pressure and oxygen saturation were constantly measured via radial catheter and blood samples for blood gases taken from the catheter at each altitude and oxygen level. Additional blood samples from the arterial catheter at baseline and 60% oxygen at both altitudes were centrifuged inside the chamber and the serum was frozen instantly at -21 °C for later analysis of the oxidative stress markers malondialdehyde low-density lipoprotein (M-LDL) and glutathione-peroxidase 1 (GPX1) via the ELISA test.

RESULTS

Eleven subjects finished the study without adverse events. Whereas the partial pressure of oxygen (PO2) levels increased in the mean with increasing oxygen levels from baseline 96.2 mm mercury (mmHg) to 160.9 mmHg at 2500 m altitude and 60% FiO2 and 113.2 mmHg at 4500 m altitude and 60% FiO2, there was no significant increase in both oxidative markers from baseline to 60% FiO2 at these simulated altitudes. Some individuals had a slight increase, whereas some showed no increase at all or even a slight decrease. A moderate correlation (Pearson correlation coefficient 0.55) existed between subject age and glutathione peroxidase levels at 60% FiO2 at 4500 m altitude.

CONCLUSION

Supplemental oxygen of 60% FiO2 in a flight simulation, compared to flying in cabin pressure levels equivalent to 2500 m-4500 m altitude, does not lead to a significant increase or decrease in the oxidative stress markers M-LDL and GPX1 in the serum of arterial blood.

Acute Hyperoxia Improves Spinal Cord Oxygenation and Circulatory Function Following Cervical Spinal Cord Injury in Rats

Spinal cord injury is associated with spinal vascular disruptions that result in spinal ischemia and tissue hypoxia. This study evaluated the therapeutic efficacy of normobaric hyperoxia on spinal cord oxygenation and circulatory function at the acute stage of cervical spinal cord injury. Adult male Sprague Dawley rats underwent dorsal cervical laminectomy or cervical spinal cord contusion. At 1-2 days after spinal surgery, spinal cord oxygenation was monitored in anesthetized and spontaneously breathing rats through optical recording of oxygen sensor foils placed on the cervical spinal cord and pulse oximetry. The arterial blood pressure, heart rate, blood gases, and peripheral oxyhemoglobin saturation were also measured under hyperoxic (50% O2) and normoxic (21% O2) conditions. The results showed that contused animals had significantly lower spinal cord oxygenation levels than uninjured animals during normoxia. Peripheral oxyhemoglobin saturation, arterial oxygen partial pressure, and mean arterial blood pressure are significantly reduced following cervical spinal cord contusion. Notably, spinal oxygenation of contused rats could be improved to a level comparable to uninjured animals under hyperoxia. Furthermore, acute hyperoxia elevated blood pressure, arterial oxygen partial pressure, and peripheral oxyhemoglobin saturation. These results suggest that normobaric hyperoxia can significantly improve spinal cord oxygenation and circulatory function in the acute phase after cervical spinal cord injury. We propose that adjuvant normobaric hyperoxia combined with other hemodynamic optimization strategies may prevent secondary damage after spinal cord injury and improve functional recovery.

Arterial oxygen and carbon dioxide tension and acute brain injury in extracorporeal cardiopulmonary resuscitation patients: Analysis of the extracorporeal life support organization registry

BACKGROUND

Acute brain injury (ABI) remains common after extracorporeal cardiopulmonary resuscitation (ECPR). Using a large international multicenter cohort, we investigated the impact of peri-cannulation arterial oxygen (PaO2) and carbon dioxide (PaCO2) on ABI occurrence.

METHODS

We retrospectively analyzed adult (≥18 years old) ECPR patients in the Extracorporeal Life Support Organization registry from 1/2009 through 12/2020. Composite ABI included ischemic stroke, intracranial hemorrhage (ICH), seizures, and brain death. The registry collects 2 blood gas data pre- (6 hours) and post- (24 hours) cannulation. Blood gas parameters were classified as: hypoxia (<60mm Hg), normoxia (60-119mm Hg), and mild (120-199mm Hg), moderate (200-299mm Hg), and severe hyperoxia (≥300mm Hg); hypocarbia (<35mm Hg), normocarbia (35-44mm Hg), mild (45-54mm Hg) and severe hypercarbia (≥55mm Hg). Missing values were handled using multiple imputation. Multivariable logistic regression analysis was used to assess the relationship of PaO2 and PaCO2 with ABI.

RESULTS

Of 3,125 patients with ECPR intervention (median age=58, 69% male), 488 (16%) experienced ABI (7% ischemic stroke; 3% ICH). In multivariable analysis, on-ECMO moderate (aOR=1.42, 95%CI: 1.02-1.97) and severe hyperoxia (aOR=1.59, 95%CI: 1.20-2.10) were associated with composite ABI. Additionally, severe hyperoxia was associated with ischemic stroke (aOR=1.63, 95%CI: 1.11-2.40), ICH (aOR=1.92, 95%CI: 1.08-3.40), and in-hospital mortality (aOR=1.58, 95%CI: 1.21-2.06). Mild hypercarbia pre-ECMO was protective of composite ABI (aOR=0.61, 95%CI: 0.44-0.84) and ischemic stroke (aOR=0.56, 95%CI: 0.35-0.89).

CONCLUSIONS

Early severe hyperoxia (≥300mm Hg) on ECMO was a significant risk factor for ABI and mortality. Careful consideration should be given in early oxygen delivery in ECPR patients who are at risk of reperfusion injury.

Association of Arterial Hyperoxia With Outcomes in Critically Ill Children: A Systematic Review and Meta-analysis

IMPORTANCE

Oxygen supplementation is a cornerstone treatment in pediatric critical care. Accumulating evidence suggests that overzealous use of oxygen, leading to hyperoxia, is associated with worse outcomes compared with patients with normoxia.

OBJECTIVES

To evaluate the association of arterial hyperoxia with clinical outcome in critically ill children among studies using varied definitions of hyperoxia.

DATA SOURCES

A systematic search of EMBASE, MEDLINE, Cochrane Library, and ClinicalTrials.gov from inception to February 1, 2021, was conducted.

STUDY SELECTION

Clinical trials or observational studies of children admitted to the pediatric intensive care unit that examined hyperoxia, by any definition, and described at least 1 outcome of interest. No language restrictions were applied.

DATA EXTRACTION AND SYNTHESIS

The Meta-analysis of Observational Studies in Epidemiology guideline and Newcastle-Ottawa Scale for study quality assessment were used. The review process was performed independently by 2 reviewers. Data were pooled with a random-effects model.

MAIN OUTCOMES AND MEASURES

The primary outcome was 28-day mortality; this time was converted to mortality at the longest follow-up owing to insufficient studies reporting the initial primary outcome. Secondary outcomes included length of stay, ventilator-related outcomes, extracorporeal organ support, and functional performance.

RESULTS

In this systematic review, 16 studies (27 555 patients) were included. All, except 1 randomized clinical pilot trial, were observational cohort studies. Study populations included were post-cardiac arrest (n = 6), traumatic brain injury (n = 1), extracorporeal membrane oxygenation (n = 2), and general critical care (n = 7). Definitions and assessment of hyperoxia differed among included studies. Partial pressure of arterial oxygen was most frequently used to define hyperoxia and mainly by categorical cutoff. In total, 11 studies (23 204 patients) were pooled for meta-analysis. Hyperoxia, by any definition, showed an odds ratio of 1.59 (95% CI, 1.00-2.51; after Hartung-Knapp adjustment, 95% CI, 1.05-2.38) for mortality with substantial between-study heterogeneity (I2 = 92%). This association was also found in less heterogeneous subsets. A signal of harm was observed at higher thresholds of arterial oxygen levels when grouped by definition of hyperoxia. Secondary outcomes were inadequate for meta-analysis.

CONCLUSIONS AND RELEVANCE

These results suggest that, despite methodologic limitations of the studies, hyperoxia is associated with mortality in critically ill children. This finding identifies the further need for prospective observational studies and importance to address the clinical implications of hyperoxia in critically ill children.

Effects of high-intensity interval exercise under hyperoxia on HSP27 and oxidative stress responses

Hypoxia in working muscles during exercise may be associated with increased oxidative stress. Inhalation of hyperoxic gas diminishes the hypoxia within working muscles during exercise. Exposure to hyperoxia increases the expression of the antioxidant HSP27. We investigated the effects of acute high-intensity interval exercise (HIE) under hyperoxia on HSP27 levels and oxidative stress responses. Eight male subjects participated in two experiments: 1) normoxic HIE (NHIE) and 2) hyperoxic (60 % oxygen) HIE (HHIE). HIE consisted of four 30-s all-out cycling bouts with 4-min rest between bouts. Levels of serum oxidative stress markers (d-ROMs and LPO), HSP27, BAP, IL-6, and TNF-α significantly increased after both trials. The HIE-induced changes in d-ROMs, LPO, and HSP27 levels were significantly lower in the HHIE trial than in the NHIE trial. These findings suggest that inhaling hyperoxic gas during exercise might diminish oxidative stress induced by all-out HIE.

Oxidative Stress Markers to Investigate the Effects of Hyperoxia in Anesthesia

Oxygen (O₂) is commonly used in clinical practice to prevent or treat hypoxia, but if used in excess (hyperoxia), it may act as toxic. O₂ toxicity arises from the enhanced formation of Reactive Oxygen Species (ROS) that exceed the antioxidant defenses and generate oxidative stress. In this study, we aimed at assessing whether an elevated fraction of inspired oxygen (FiO₂) during and after general anesthesia may contribute to the unbalancing of the pro-oxidant/antioxidant equilibrium. We measured five oxidative stress biomarkers in blood samples from patients undergoing elective abdominal surgery, randomly assigned to FiO₂ = 0.40 vs. 0.80: hydroperoxides, antioxidants, nitrates and nitrites (NO_x), malondialdehyde (MDA), and glutathionyl hemoglobin (HbSSG). The MDA concentration was significantly higher 24 h after surgery, and the body antioxidant defense lower, in the FiO₂ = 0.80 group with respect to both the FiO₂ = 0.40 group and the baseline values (p ≤ 0.05, Student’s t-test). HbSSG in red blood cells was also higher in the FiO₂ = 0.80 group at the end of the surgery. NO_x was higher in the FiO₂ = 0.80 group than the FiO₂ = 0.40 group at t = 2 h after surgery. MDA, the main end product of the peroxidation of polyunsaturated fatty acids directly influenced by FiO₂, may represent the best marker to assess the pro-oxidant/antioxidant equilibrium after surgery.

Hyperoxia-induced lung structure-function relation, vessel rarefaction, and cardiac hypertrophy in an infant rat model

BACKGROUND

Hyperoxia-induced bronchopulmonary dysplasia (BPD) models are essential for better understanding and impacting on long-term pulmonary, cardiovascular, and neurological sequelae of this chronic disease. Only few experimental studies have systematically compared structural alterations with lung function measurements.

METHODS

In three separate and consecutive series, Sprague-Dawley infant rats were exposed from day of life (DOL) 1 to 19 to either room air (0.21; controls) or to fractions of inspired oxygen (FiO2) of 0.6, 0.8, and 1.0. Our primary outcome parameters were histopathologic analyses of heart, lungs, and respiratory system mechanics, assessed via image analysis tools and the forced oscillation technique, respectively.

RESULTS

Exposure to FiO2 of 0.8 and 1.0 resulted in significantly lower body weights and elevated coefficients of lung tissue damping (G) and elastance (H) when compared with controls. Hysteresivity (η) was lower due to a more pronounced increase of H when compared with G. A positive structure-function relation was demonstrated between H and the lung parenchymal content of α-smooth muscle actin (α-SMA) under hyperoxic conditions. Moreover, histology and morphometric analyses revealed alveolar simplification, fewer pulmonary arterioles, increased α-SMA content in pulmonary vessels, and right heart hypertrophy following hyperoxia. Also, in comparison to controls, hyperoxia resulted in significantly lower plasma levels of vascular endothelial growth factor (VEGF). Lastly, rats in hyperoxia showed hyperactive and a more explorative behaviour.

CONCLUSIONS

Our in vivo infant rat model mimics clinical key features of BPD. To the best of our knowledge, this is the first BPD rat model demonstrating an association between lung structure and function. Moreover, we provide additional evidence that infant rats subjected to hyperoxia develop rarefaction of pulmonary vessels, augmented vascular α-SMA, and adaptive cardiac hypertrophy. Thus, our model provides a clinically relevant tool to further investigate diseases related to O2 toxicity and to evaluate novel pharmacological treatment strategies.

Sex Hormones Response to Physical Hyperoxic and Hyperbaric Stress in Male Scuba Divers: A Pilot Study

The use of hyperbaric oxygen plays a significant role in many aspects of medicine. However, there are few studies that analyzed the role of hyperbaric oxygen, in addition to physical exercise, on the endocrine profile. The aim of this study was to compare changes in plasma male sex hormones after hyperbaric physical exercise with different hyperbaric oxygen pre-conditionings. We recruited six healthy, well-trained recreational male divers. Concentrations of prolactin (PRL), follicle-stimulating hormone (FSH), luteotrophic hormone (LH), cortisol, 17-β estradiol (E2), and total testosterone (TT) were measured in venous blood immediately after four different study conditions. Exercise increased PRL and hyperbaric oxygen potentiated this effect. Hyperbaria stimulated the E2 reduction and hyperoxia partially inhibited this reduction. Hyperbaria, but not hyperoxia, stimulated the TT reduction. There were no changes in FSH, LH, and cortisol. The increase in PRL likely reflects a stress response after physical exercise, amplified by hyperbaric oxygen. TT reduction may be interpreted as an acute and transient fertility impairment. Age, blood pressure, and BMI were taken into account as covariates for statistical analyses, and they significantly affected the results, in particular TT. These data open new insight into the role of E2 and PRL in male endocrine adaptive responses.

Intraoperative cerebral oxygenation, oxidative injury, and delirium following cardiac surgery

BACKGROUND

Delirium affects 20-30% of patients after cardiac surgery and is associated with increased mortality and persistent cognitive decline. Hyperoxic reperfusion of ischemic tissues increases oxidative injury, but oxygen administration remains high during cardiac surgery. We tested the hypothesis that intraoperative hyperoxic cerebral reperfusion is associated with increased postoperative delirium and that oxidative injury mediates this association.

METHODS

We prospectively measured cerebral oxygenation with bilateral oximetry monitors in 310 cardiac surgery patients, quantified intraoperative hyperoxic cerebral reperfusion by measuring the magnitude of cerebral oxygenation above baseline after any ischemic event, and assessed patients for delirium twice daily in the ICU following surgery using the confusion assessment method for ICU (CAM-ICU). We examined the association between hyperoxic cerebral reperfusion and postoperative delirium, adjusted for the extent of cerebral hypoxia, the extent of cerebral hyperoxia prior to any ischemia, and additional potential confounders and risk factors for delirium. To assess oxidative injury mediation, we examined the association between hyperoxic cerebral reperfusion and delirium after further adjusting for plasma levels of F₂-isoprostanes and isofurans at baseline and ICU admission, the association between hyperoxic cerebral reperfusion and these markers of oxidative injury, and the association between these markers and delirium.

RESULTS

Ninety of the 310 patients developed delirium following surgery. Every 10%·hour of intraoperative hyperoxic cerebral reperfusion was independently associated with a 65% increase in the odds of delirium (OR, 1.65 [95% CI, 1.12-2.44]; P=0.01). Hyperoxia prior to ischemia was also independently associated with delirium (1.10 [1.01-1.19]; P=0.02), but hypoxia was not (1.12 [0.97-1.29]; P=0.11). Increased hyperoxic cerebral reperfusion was associated with increased concentrations of F₂-isoprostanes and isofurans at ICU admission, increased concentrations of these markers were associated with increased delirium, and the association between hyperoxic cerebral reperfusion and delirium was weaker after adjusting for these markers of oxidative injury.

CONCLUSIONS

Intraoperative hyperoxic cerebral reperfusion was associated with increased postoperative delirium, and increased oxidative injury following hyperoxic cerebral reperfusion may partially mediate this association. Further research is needed to assess the potential deleterious role of cerebral hyper-oxygenation during surgery.

Real-Time Monitoring of Placental Oxygenation during Maternal Hypoxia and Hyperoxygenation Using Photoacoustic Imaging

Purpose

This preclinical study aimed to evaluate placental oxygenation in pregnant rats by real-time photoacoustic (PA) imaging on different days of gestation and to specify variations in placental oxygen saturation under conditions of maternal hypoxia and hyperoxygenation.

Material and methods

Placentas of fifteen Sprague-Dawley rats were examined on days 14, 17, and 20 of pregnancy with a PA imaging system coupled to high-resolution ultrasound imaging. Pregnant rats were successively exposed to hyperoxygenated and hypoxic conditions by changing the oxygen concentration in inhaled gas. Tissue oxygen saturation was quantitatively analyzed by real-time PA imaging in the skin and 3 regions of the placenta. All procedures were performed in accordance with applicable ethical guidelines and approved by the animal care committee.

Results

Maternal hypoxia was associated with significantly greater decrease in blood oxygen saturation (ΔO₂ Saturation) in the skin (70.74% ±7.65) than in the mesometrial triangle (32.66% ±5.75) or other placental areas (labyrinth: 18.58% ± 6.61; basal zone: 13.13% ±5.72) on different days of pregnancy (P<0.001). ΔO₂ Saturation did not differ significantly between the labyrinth, the basal zone, and the decidua. After the period of hypoxia, maternal hyperoxygenation led to a significant rise in oxygen saturation, which returned to its initial values in the different placental regions (P<0.001).

Conclusions

PA imaging enables the variation of blood oxygen saturation to be monitored in the placenta during maternal hypoxia or hyperoxygenation. This first preclinical study suggests that the placenta plays an important role in protecting the fetus against maternal hypoxia.

Regional Reproducibility of BOLD Calibration Parameter M, OEF and Resting-State CMRO2 Measurements with QUO2 MRI

The current generation of calibrated MRI methods goes beyond simple localization of task-related responses to allow the mapping of resting-state cerebral metabolic rate of oxygen (CMRO₂) in micromolar units and estimation of oxygen extraction fraction (OEF). Prior to the adoption of such techniques in neuroscience research applications, knowledge about the precision and accuracy of absolute estimates of CMRO₂ and OEF is crucial and remains unexplored to this day. In this study, we addressed the question of methodological precision by assessing the regional inter-subject variance and intra-subject reproducibility of the BOLD calibration parameter M, OEF, O₂ delivery and absolute CMRO₂ estimates derived from a state-of-the-art calibrated BOLD technique, the QUantitative O2 (QUO2) approach. We acquired simultaneous measurements of CBF and R2* at rest and during periods of hypercapnia (HC) and hyperoxia (HO) on two separate scan sessions within 24 hours using a clinical 3 T MRI scanner. Maps of M, OEF, oxygen delivery and CMRO₂, were estimated from the measured end-tidal O₂, CBF_0, CBF_HC/HO and R2*_HC/HO. Variability was assessed by computing the between-subject coefficients of variation (bwCV) and within-subject CV (wsCV) in seven ROIs. All tests GM-averaged values of CBF₀, M, OEF, O₂ delivery and CMRO₂ were: 49.5 ± 6.4 mL/100 g/min, 4.69 ± 0.91%, 0.37 ± 0.06, 377 ± 51 μmol/100 g/min and 143 ± 34 μmol/100 g/min respectively. The variability of parameter estimates was found to be the lowest when averaged throughout all GM, with general trends toward higher CVs when averaged over smaller regions. Among the MRI measurements, the most reproducible across scans was R2*₀ (wsCV_GM = 0.33%) along with CBF₀ (wsCV_GM = 3.88%) and R2*_HC (wsCV_GM = 6.7%). CBF_HC and R2*_HO were found to have a higher intra-subject variability (wsCV_GM = 22.4% and wsCV_GM = 16% respectively), which is likely due to propagation of random measurement errors, especially for CBF_HC due to the low contrast-to-noise ratio intrinsic to ASL. Reproducibility of the QUO2 derived estimates were computed, yielding a GM intra-subject reproducibility of 3.87% for O₂ delivery, 16.8% for the M value, 13.6% for OEF and 15.2% for CMRO₂. Although these results focus on the precision of the QUO2 method, rather than the accuracy, the information will be useful for calculation of statistical power in future validation studies and ultimately for research applications of the method. The higher test-retest variability for the more extensively modeled parameters (M, OEF, and CMRO₂) highlights the need for further improvement of acquisition methods to reduce noise levels.

Adjusting Oxygen Fraction to Avoid Hyperoxemia during Cardiopulmonary Bypass

Although an adverse influence of hyperoxemia during cardiopulmonary bypass is well documented, there is a wide range of oxygen settings during cardiopulmonary bypass, based mostly on trial and error. The aim of this study was to determine the optimal inspired oxygen fraction during cardiopulmonary bypass. Ninety patients undergoing isolated coronary artery bypass operations were randomly allocated to one of 3 groups of 30 each. In group 1, cardiopulmonary bypass was started with an inspired oxygen fraction of 0.40, increased to 0.60 during rewarming. These settings were 0.40 and 0.50 in group 2, and 0.35 and 0.45 in group 3. Samples for blood gas analysis were collected at defined time periods during the operation. PaO2 was significantly higher in groups 1 and 2 compared to group 3. All patients in group 1 and 88% of patients in group 2 suffered at least one episode of hyperoxemia during cardiopulmonary bypass, compared to 30% of patients in group 3. The differences were significant, and we concluded that to avoid hyperoxemia, inspired oxygen fraction should be kept at 0.35 during cardiopulmonary bypass and increased to 0.45 during rewarming.

High cumulative oxygen levels are associated with improved survival of children treated with mild therapeutic hypothermia after cardiac arrest

AIM

The aim of this study was to analyze the relationship between the partial pressure of arterial oxygen (PaO2) and in-hospital (IH) mortality in children after cardiac arrest (CA) using the conventional cutoff analysis, which was compared with the cumulative analysis, a new method in PaO2 analysis. Additionally, we analyzed this relationship for children with and without mild therapeutic hypothermia (MTH; 32-34 °C).

METHODS

This observational cohort study included all children (aged >28 days) with CA and return of spontaneous circulation (ROSC) between 2002 and 2011. The first research question was the association between PaO2 and IH mortality after ROSC. This was analyzed for three hyperoxia cutoff values, and for three time intervals using the cumulative PaO2 determined with the area under the curve (AUC). For the second research question, these analyses were repeated for children with and without MTH.

RESULTS

Of the 200 patients included (median age 2.6 years), 84 (42%) survived to hospital discharge. Fifty-eight children (29%) were treated with MTH. With the cutoff analysis and the AUC analysis we found no relationship between PaO2 and IH mortality. However, analysis of the MTH-group showed a lower IH mortality in children with high cumulative PaO2 levels on two of the three time intervals. Multivariable analysis showed significantly higher odds of survival (0.643 (95% confidence interval (CI) 0.424-0.976), 0.554 (95% CI 0.335-0.916)).

CONCLUSIONS

Cumulative PaO2 analysis showed that the IH mortality is significantly lower in MTH-treated children with high PaO2 levels. The effects of cumulative PaO2 on the outcome need to be studied further, and this will help us to achieve individualized goal-directed therapy.

High level of oxygen treatment causes cardiotoxicity with arrhythmias and redox modulation

Hyperoxia exposure in mice leads to cardiac hypertrophy and voltage-gated potassium (Kv) channel remodeling. Because redox balance of pyridine nucleotides affects Kv function and hyperoxia alters cellular redox potential, we hypothesized that hyperoxia exposure leads to cardiac ion channel disturbances and redox changes resulting in arrhythmias. In the present study, we investigated the electrical changes and redox abnormalities caused by 72h hyperoxia treatment in mice. Cardiac repolarization changes were assessed by acquiring electrocardiogram (ECG) and cardiac action potentials (AP). Biochemical assays were employed to identify the pyridine nucleotide changes, Kv1.5 expression and myocardial injury. Hyperoxia treatment caused marked bradycardia, arrhythmia and significantly prolonged (ms) the, RR (186.2 ± 10.7 vs. 146.4 ± 6.2), PR (46.8 ± 3.1 vs. 39.3 ± 1.6), QRS (10.8 ± 0.6 vs. 8.5 ± 0.2), QTc (57.1 ± 3.5 vs. 40 ± 1.4) and JT (13.4 ± 2.1 vs. 7.0 ± 0.5) intervals, when compared with normoxia group. Hyperoxia treatment also induced significant increase in cardiac action potential duration (APD) (ex-APD90; 73.8 ± 9.5 vs. 50.9 ± 3.1 ms) and elevated levels of serum markers of myocardial injury; cardiac troponin I (TnI) and lactate dehydrogenase (LDH). Hyperoxia exposure altered cardiac levels of mRNA/protein expression of; Kv1.5, Kvβ subunits and SiRT1, and increased ratios of reduced pyridine nucleotides (NADH/NAD & NADPH/NADP). Inhibition of SiRT1 in H9C2 cells using Splitomicin resulted in decreased SiRT1 and Kv1.5 expression, suggesting that SiRT1 may mediate Kv1.5 downregulation. In conclusion, the cardiotoxic effects of hyperoxia exposure involve ion channel disturbances and redox changes resulting in arrhythmias.

Quantitative blood oxygenation level-dependent (BOLD) response of the left ventricular myocardium to hyperoxic respiratory challenge at 1.5 and 3.0 T

The aim of this study was to quantify the response of the myocardial transverse relaxation times (ΔT2*) to hyperoxic respiratory challenge (HRC) at different field strengths in an intra-individual comparison of healthy volunteers and in a patient with coronary artery disease. Blood oxygenation level-dependent (BOLD) cardiovascular MR (CMR) data were acquired in 10 healthy volunteers (five women, five men; mean age, 29 ± 3 years; range, 22-35 years) at 1.5 and 3.0 T. Medical air (21% O2 ), pure oxygen and carbogen (95% O2 , 5% CO2 ) were administered in a block-design temporal pattern to induce normoxia, hyperoxia and hyperoxic hypercapnia, respectively. Average T2* times were derived from measurements by two independent and blind readers in 16 standard myocardial segments on three short-axis slices per patient. Inter- and intra-reader correlations of T2* measurements were good [intra-class correlation coefficient (ICC) = 0.75 and ICC = 0.79, both p < 0.001]. During normoxia, the mean T2* times were 29.9 ± 6.1 ms at 1.5 T and 27.1 ± 6.6 ms at 3.0 T. Both hyperoxic gases induced significant (all p < 0.01) T2* increases (∆T2* hyperoxia: 1.5 T, 12.7%; 3.0 T, 11.2%; hyperoxic hypercapnia: 1.5 T, 13.1%; 3.0 T, 17.7%). Analysis of variance (ANOVA) results indicated a significant (both p < 0.001) effect of the inhaled gases on the T2* times at both 1.5 T (F = 17.74) and 3.0 T (F = 39.99). With regard to the patient imaged at 1.5 T, HRC induced significant T2* increases during hyperoxia and hyperoxic hypercapnia in normal myocardial segments, whereas the T2* response was not significant in ischemic segments (p > 0.23). The myocardial ∆T2* response to HRC can reliably be imaged and quantified with BOLD CMR at both 1.5 and 3.0 T. During HRC, hyperoxia and hyperoxic hypercapnia induce a significant increase in T2*, with ∆T2* being largest at 3.0 T and during hyperoxic hypercapnia in normal myocardial segments.

Oxygen-induced frequency shifts in hyperoxia: a significant component of BOLD signal

In comparison to the well-documented significance of intravascular deoxyhemoglobin (deoxyHgb), the effects of dissolved oxygen on the blood-oxygen-level-dependent (BOLD) signal have not been widely reported. Based on the fact that the prolonged inspiration of high oxygen fraction gas can result in up to a sixfold increase of the baseline tissue oxygenation, the current study focused on the influence of dissolved oxygen on the BOLD signal during hyperoxia. As results, our in vitro study revealed that the r1 and r2 (relaxivities) of the oxygen-treated serum were 0.22 mM(-1) · s(-1) and 0.19 mM(-1) · s(-1) , respectively. In an in vivo experiment, hyperoxic respiration induced negative BOLD contrast (i.e. signal decrease) in 18-42% of measured brain regions, voxels with accompanying significant decreases in both the T(*)2 (-12.1% to -19.4%) and T1 (-5.8% to -3.3%) relaxation times. In contrast, the T(*)2 relaxation time significantly increased (11.2% to 14.0%) for the voxels displaying positive BOLD contrast (in 41-50% of the measured brain), which reflected a hyperoxygenation-induced reduction in tissue deoxyHgb concentration. These data imply that hyperoxia-driven BOLD signal changes are primarily determined by the counteracting effects of extravascular oxygen and intravascular deoxyHgb. Oxygen-induced magnetic susceptibility was further demonstrated by the study of 1 min hypoxia, which induced BOLD signal changes opposite to those under hyperoxia. Vasoconstriction was more common in voxels with negative BOLD contrast than in voxels with positive contrast (% change of blood volume, -9.8% to -12.8% versus 2.0% to 2.2%), which further suggests that negative BOLD contrast is mainly evoked by an increase in extravascular oxygen concentration. Conclusively, frequency shifts, which are induced by the accumulation of oxygen molecules and associated magnetic field inhomogeneity, are a significant source of the negative BOLD contrast during hyperoxia.

Oxidative stress in breath-hold divers after repetitive dives

INTRODUCTION

Hyperoxia causes oxidative stress. Breath-hold diving is associated with transient hyperoxia followed by hypoxia and a build-up of carbon dioxide (CO₂), chest-wall compression and significant haemodynamic changes. This study analyses variations in plasma oxidative stress markers after a series of repetitive breath-hold dives.

METHODS

Thirteen breath-hold divers were asked to perform repetitive breath-hold dives to 20 metres' depth to a cumulative breath-hold time of approximately 20 minutes over an hour in the open sea. Plasma nitric oxide (NO), peroxinitrites (ONOO⁻) and thiols (R-SH) were measured before and after the dive sequence.

RESULTS

Circulating NO significantly increased after successive breath-hold dives (169.1 ± 58.26% of pre-dive values; P = 0.0002). Peroxinitrites doubled after the dives (207.2 ± 78.31% of pre-dive values; P = 0.0012). Thiols were significantly reduced (69.88 ± 19.23% of pre-dive values; P = 0.0002).

CONCLUSION

NO may be produced by physical effort during breath-hold diving. Physical exercise, the transient hyperoxia followed by hypoxia and CO₂ accumulation would all contribute to the increased levels of superoxide anions (O₂²⁻). Since interaction of O₂²⁻ with NO forms ONOO⁻, this reaction is favoured and the production of thiol groups is reduced. Oxidative stress is, thus, present in breath-hold diving.

Why hyperoxia matters during acute anemia

BACKGROUND

Given the low physical solubility of oxygen (O2) in plasma, little value is attached to hyperoxic ventilation (FiO2 1.0) as a modality for improving O2 transport and tissue oxygen supply when hypoxemia (i.e., O2 partial pressure (paO2) <60 mmHg) is absent. Because recent experimental and clinical data conflict with this notion, we used mathematical modeling to reevaluate efficacy of hyperoxic ventilation in improving tissue oxygenation in the absence of hypoxemia by specifying its theoretical efficacy in terms of hemoglobin (Hb) equivalents.

METHODS

A mathematical approach was used based on the assumption that efficacy of hyperoxic ventilation depends on the additional amount of O2 dissolved in plasma and is influenced by the high biological availability of the additional O2 provided at high paO2. This approach was used to calculate the amount of additional Hb necessary to increase the amount of utilizable O2 to the same extent as hyperoxic ventilation (the so-called "Hb equivalent").

RESULTS

Although former estimations strongly underestimated efficacy of hyperoxic ventilation (Hb equivalent, 1-2 g/dL), our more exact mathematical approach revealed a theoretical Hb equivalent of hyperoxic ventilation in the range of 3-7 g/dL, which depended on basic physiological conditions like pulmonary function, Hb concentration, and peripheral shunt perfusion.

CONCLUSION

Hyperoxic ventilation establishes a highly available source of O2 that can be utilized effectively for tissue oxygenation. Although further experimental studies are required to quantify this theoretically calculated amount of utilizable O2, these results suggest that the tissue oxygenation efficacy of hyperoxic ventilation, even in absence of hypoxemia, is grossly underestimated in daily clinical practice.

Physical exercise might influence the risk of oxygen-induced acute neurotoxicity

OBJECTIVE

Hyperoxia can induce acute neurotoxicity with generalized seizures. Hyperoxia-induced reduction in cerebral blood flow velocity (CBFV) might be protective. It is unclear whether dynamic exercise during hyperoxia can overcome CBFV-reduction and thus possibly increase the risk of neurotoxicity.

METHODS

We studied CBFV with both-sided transcranial Doppler with fixed transducer-position and heart rate under increasing hyperoxic conditions in nine professional military oxygen divers. The divers performed dynamic exercise on a bicycle-ergometer in a hyperbaric chamber (ergometries I-III, 21kPa, 100kPa, 150kPa pO2), with continuous blood pressure (ergometries I, II), end-tidal CO2 (PetCO2; ergometry I) being measured.

RESULTS

Systolic (CBFVsyst) and diastolic CBFV (CBFVdiast) readings at rest decreased with increasing pO2. During exercise, CBFVsyst and CBFVdiast significantly increased in parallel with increasing pO2, despite reduced flow velocities at rest.

ERGOMETRY I

CBFVsyst increased from 65.0 +/- 11.3 cm/second at rest to 80.2 +/- 23.4cm/s during maximum workload (n.s.), diastolic from 14.5 +/- 4.1 cm/second to 15.6 +/- 7.5 cm/s (n.s.). PetCO2 increased from 43.4 +/- 7.8mmHg to 50.0 +/- 7.5mmHg.

ERGOMETRY II

CBFVsyst increased from 58.2 +/- 16.5 cm/second to 99.7 +/- 17.0 cm/s (p<0.001), diastolic from 14.0 +/- 10.7 cm/second to 29.4 +/- 11.1 cm/second (p<0.01).

ERGOMETRY III

CBFVsyst increased from 54.4 +/-15.0cm/second to 109.4 +/- 22.3cm/s (p<0.001), diastolic from 14.7 +/- 10.4 cm/second to 35.5 +/- 9.3 cm/second (p<0.01).

INTERPRETATION

Physical exercise overrules the decrease in CBFV during hyperoxia and leads to even higher CBFV-increases with increasing pO2. A tendency towards CO2 retainment with elevated PetCOz may be causative and thus heighten the risk of oxygen-induced neurotoxicity.

Video-rate functional photoacoustic microscopy at depths

We report the development of functional photoacoustic microscopy capable of video-rate high-resolution in vivo imaging in deep tissue. A lightweight photoacoustic probe is made of a single-element broadband ultrasound transducer, a compact photoacoustic beam combiner, and a bright-field light delivery system. Focused broadband ultrasound detection provides a 44-μm lateral resolution and a 28-μm axial resolution based on the envelope (a 15-μm axial resolution based on the raw RF signal). Due to the efficient bright-field light delivery, the system can image as deep as 4.8 mm in vivo using low excitation pulse energy (28 μJ per pulse, 0.35  mJ/cm² on the skin surface). The photoacoustic probe is mounted on a fast-scanning voice-coil scanner to acquire 40 two-dimensional (2-D) B-scan images per second over a 9-mm range. High-resolution anatomical imaging is demonstrated in the mouse ear and brain. Via fast dual-wavelength switching, oxygen dynamics of mouse cardio-vasculature is imaged in realtime as well.

Absolute quantification of resting oxygen metabolism and metabolic reactivity during functional activation using QUO2 MRI

We have recently described an extension of calibrated MRI, which we term QUO2 (for QUantitative O(2) imaging), providing absolute quantification of resting oxidative metabolism (CMRO(2)) and oxygen extraction fraction (OEF(0)). By combining BOLD, arterial spin labeling (ASL) and end-tidal O(2) measurements in response to hypercapnia, hyperoxia and combined hyperoxia/hypercapnia manipulations, and the same MRI measurements during a task, a comprehensive set of vascular and metabolic measurements can be obtained using a generalized calibration model (GCM). These include the baseline absolute CBF in units of ml/100g/min, cerebrovascular reactivity (CVR) in units of %Δ CBF/mm Hg, M in units of percent, OEF(0) and CMRO(2) at rest in units of μmol/100g/min, percent evoked CMRO(2) during the task and n, the value for flow-metabolic coupling associated with the task. The M parameter is a calibration constant corresponding to the maximal BOLD signal that would occur upon removal of all deoxyhemoglobin. We have previously shown that the GCM provides estimates of the above resting parameters in grey matter that are in excellent agreement with literature. Here we demonstrate the method using functionally-defined regions-of-interest in the context of an activation study. We applied the method under high and low signal-to-noise conditions, corresponding respectively to a robust visual stimulus and a modified Stroop task. The estimates fall within the physiological range of literature values, showing the general validity of the GCM approach to yield non-invasively an extensive array of relevant vascular and metabolic parameters.

The association between early arterial oxygenation and mortality in ventilated patients with acute ischaemic stroke

BACKGROUND

There are conflicting data that suggest that hyperoxia may be associated with either worse or better outcomes in patients suffering a stroke.

OBJECTIVES

To investigate the association between PaO(2) in the first 24 hours in the intensive care unit and mortality among ventilated patients with acute ischaemic stroke.

DESIGN

Retrospective cohort study.

SETTING

Data were extracted from the Australian and New Zealand Intensive Care Society Adult Patient Database.

PARTICIPANTS

Adults ventilated for ischaemic stroke in 129 ICUs in Australia and New Zealand, 2000-2009.

MAIN OUTCOME MEASURES

The primary outcome was the odds ratio for in hospital mortality associated with "worst" PaO(2) considered as a categorical variable, with data divided into deciles and compared with the mortality of the 10th decile. For patients on an FiO(2) of _50% at any time in the first 24 hours, "worst" PaO(2) was defined as the PaO(2) associated with the highest alveolar-arterial (A-a) gradient. For patients on an FiO(2) of <50%, it was defined as the lowest PaO(2). Secondary outcomes were ICU and hospital length of stay and the proportion of patients in each decile discharged home.

RESULTS

Of the 2643 patients eligible for study inclusion, 1507 (57%) died in hospital. The median "worst" PaO(2) was 117mmHg (interquartile range, 87-196mmHg). There was no association between worst PaO(2) and mortality, length of stay or likelihood of discharge home.

CONCLUSIONS

We found no association between worst arterial oxygen tension in the first 24 hours in ICU and outcome in ventilated patients with ischaemic stroke.

[Normobaric hyperoxia affects osteogenesis in rats]

We studied the influence of normobaric hyperoxia on bone metabolism in 3- and 12-month-old male Wistar rats. Maintaining young rats (3 months) during the 14-hour daily sessions under normobaric hyperoxia (90% O2) was accompanied by a significant decrease in the concentration of C-terminal propeptides of collagen type I (by 36%), the acid phosphatase activity (by 32%), an increased activity of alkaline phosphatase (by 64%), an increased concentration of pyridinoline (by 37%) and 3-CrossLaps (by 8%), glycosaminoglycans (by 72%) in the blood serum. We believe that normobaric hyperoxia (90%) is accompanied by the disturbance of collagen synthesis. In adult rats under the same conditions for 14 sessions, the concentration ofglycosaminoglicans significantly increased by 60% relative to the control values. After 28 sessions of breathing the normobaric gas mixture containing 90% O 2 this parameter increased by 195%. Breathing normobaric gas mixture containing both 40% and 90% of O2 for 14 and 28 days decreased the acid phosphatase activity and the tartrat-resistant acid phosphatase activity by 18-25%. We believe that in adult animals 90% hyperoxia does not affect the activity of osteoblasts, but breaks the link between glycosaminoglicans and collagen fibrils, decreases the activity oflysosomal enzymes which are produced by osteoclasts and which can inhibit osteogenesis.

[The state of acid-alkaline balance and oxygen-transport function of blood in patients with acute carotid ischemic stroke]

The gas composition, acid-alkaline state of arterial and venous blood and oxygen-transport function in carotid ischemic stroke was studied in 97 patients admitted to a hospital in the first 24h after stroke. Measurements were made at admission and after 5-7 days and 21-23 days. The relative hyperoxia, which reached maximal values to the first day, was found in patients in the acute stage of ischemic stroke. The increase in partial pressure of CO2 (pCO2) and relative acidosis seen to 5-7 days represent the compensatory reaction and lead to the increase in affinity of hemoglobin to oxygen thus improving the tissue oxygenation. We found the inverse correlations between the parameters of oxygen delivery (OD) and oxygen consumption (OC) in the first day and the severity of neurological deficit assessed with the NIHSS in the 21-23 days (r = -0.42; p < 0.01 and r = -0.55; p < 0.01 for OD and OC, respectively), i.e., the decrease in oxygen delivery and consumption corresponded to the greater severity of the stroke course.

Responses of peripheral blood flow to acute hypoxia and hyperoxia as measured by optical microangiography

Oxygen availability is regarded as a critical factor to metabolically regulate systemic blood flow. There is a debate as to how peripheral blood flow (PBF) is affected and modulated during hypoxia and hyperoxia; however in vivo evaluating of functional PBF under oxygen-related physiological perturbation remains challenging. Microscopic observation, the current frequently used imaging modality for PBF characterization often involves the use of exogenous contrast agents, which would inevitably perturb the intrinsic physiologic responses of microcirculation being investigated. In this paper, optical micro-angiography (OMAG) was employed that uses intrinsic optical scattering signals backscattered from blood flows for imaging PBF in skeletal muscle challenged by the alteration of oxygen concentration. By utilizing optical reflectance signals, we demonstrated that OMAG is able to show the response of hemodynamic activities upon acute hypoxia and hyperoxia, including the modulation of macrovascular caliber, microvascular density, and flux regulation within different sized vessels within skeletal muscle in mice in vivo. Our results suggest that OMAG is a promising tool for in vivo monitoring of functional macro- or micro-vascular responses within peripheral vascular beds.

Prognostic implications of arterial blood gases in acute decompensated heart failure

BACKGROUND

The prognostic value of arterial blood gases (ABG) in patients with acute decompensated heart failure (ADHF) is not well-established. We therefore conducted the present study to determine the relationship between ABG on admission and long-term mortality in patients with ADHF.

METHODS

We studied 588 patients consecutively admitted to our department with ADHF. ABG and classical prognostic variables were determined at patients' arrival to the emergency department. The independent association among the main variables of ABG (pO2, pCO2 and pH) and mortality was assessed with Cox regression analysis.

RESULTS

At a median follow-up of 23months, 221 deaths (37.6%) were registered. 308 (52.4%), 54 (9.2%) and 50 (8.5%) patients showed hypoxemia (pO2<60mmHg), hypercapnia (pCO2>50mmHg) and acidosis (pH<7.35), respectively. Patients with hypoxemia, hypercapnia and acidosis did not show higher mortality rates (38% vs. 37.1%, 42.6% vs. 37.1%, and 48% vs. 36.6%, respectively; p-value=ns for all comparisons). In multivariate analysis, after adjusting for well-known prognostic covariates, pO2, pCO2 and pH did not show a significant association with mortality. Hazard ratios (HR) for these variables were: pO2, per increase in 10mmHg: 0.99 (95% CI: 0.90-1.09), p=0.861; pCO2, per increase in 10mmHg: 1.12 (95% CI: 0.91-1.39), p=0.262; pH per increase in 0.1: 1.01 (95% CI: 0.99-1.04), p=0.309. When dichotomizing these variables according to established cut-points, the HR were: hypoxemia (pO2<60mmHg):1.07 (95% CI: 0.81-1.40), p=0.637; hypercapnia (pCO2>50mmHg): 0.98 (95% CI: 0.62-1.57), p=0.952; acidosis (pH<7.35): 1.38 (95% CI: 0.87-2.19), p=0.173.

CONCLUSION

In patients admitted with ADHF, admission arterial pO2, pCO2 and pH were not associated with all-cause long-term mortality.

Acute normobaric hyperoxia transiently attenuates plasma erythropoietin concentration in healthy males: evidence against the 'normobaric oxygen paradox' theory

AIM

The purpose of the present study was to evaluate the 'normobaric oxygen paradox' theory by investigating the effect of a 2-h normobaric O(2) exposure on the concentration of plasma erythropoietin (EPO).

METHODS

Ten healthy males were studied twice in a single-blinded counterbalanced crossover study protocol. On one occasion they breathed air (NOR) and on the other 100% normobaric O(2) (HYPER). Blood samples were collected Pre, Mid and Post exposure; and thereafter, 3, 5, 8, 24, 32, 48, 72 and 96 h, and 1 and 2 weeks after the exposure to determine EPO concentration.

RESULTS

The concentration of plasma erythropoietin increased markedly 8 and 32 h after the NOR exposure (approx. 58% and approx. 52%, respectively, P ≤ 0.05) as a consequence of its natural diurnal variation. Conversely, the O(2) breathing was followed by approx. 36% decrement of EPO 3 h after the exposure (P ≤ 0.05). Moreover, EPO concentration was significantly lower in HYPER than in the NOR condition 3, 5 and 8 h after the breathing intervention (P ≤ 0.05).

CONCLUSION

In contrast to the 'normobaric oxygen paradox' theory, the present results indicate that a short period of normobaric O(2) breathing does not increase the EPO concentration in aerobically fit healthy males. Increased O(2) tension suppresses the EPO concentration 3 and 5 h after the exposure; thereafter EPO seems to change in a manner consistent with natural diurnal variation.

[Effect of normobaric hyperoxia on the markers of bone tissue metabolism]

We studied the influencing of normobaric hyperoxia one hour daily during 14 and 28 days on the bone metabolism for 3- and 12-monthly Wistar male-rats (n =80). It was shown, that breathed normobaric gas mixture with 90% O2 during 14 days significant reducing of C-terminal propeptides of type I collagen levels for 36%, an increasing piridinolin levels for 37% and concentration of C-terminal telopeptides of type I collagen for 8% in the blood serum. We found reliable decreasing the concentration ofpiridinolin levels for 27% in adult rats in the same conditions. The C-terminal propeptides of type I collagen levels did not change in any of the groups' research. We believe that 14 days normobaric hyperoxia (90%) in young animals accompanied by violation of collagen synthesis. The adult rats were more stable and had no changes after influence the two hyperoxia levels.

[Changes in lipid composition of human plasma and erythrocyte membranes due to sodium hydroxybutyrate and normobaric hyperoxia during bed rest]

Gas chromatography was employed to study effects of sodium hydroxybutyrate (GHBA) and normobaric hyperoxia on the fatty-acid composition of total lipids of blood plasma and erythrocyte membranes in 5 normal male volunteers in the baseline data collection period and during 14-d bed rest. Both in plasma and erythrocyte membranes saturated palmitic acid was found increased and polyunsaturated linoleic acid and arachidonic acid reduced.

Oxygen therapy-use and abuse in acute myocardial infarction patients

In August 2008, an article was published in Heart entitled, "The Routine Use of Oxygen in the Treatment of Myocardial Infarction." This article stimulated me to opine on this topic, which has been an interest of mine for many years.

Endothelial colony-forming cells from preterm infants are increased and more susceptible to hyperoxia

RATIONALE

Preterm birth and hyperoxic exposure increase the risk for bronchopulmonary dysplasia (BPD), a chronic lung disease characterized by impaired vascular and alveolar growth. Endothelial progenitor cells, such as self-renewing highly proliferative endothelial colony-forming cells (ECFCs), may participate in vascular repair. The effect of hyperoxia on ECFC growth is unknown.

OBJECTIVES

We hypothesize that umbilical cord blood (CB) from premature infants contains more ECFCs with greater growth potential than term CB. However, preterm ECFCs may be more susceptible to hyperoxia.

METHODS

ECFC colonies were quantified by established methods and characterized by immunohistochemistry and flow cytometry. Growth kinetics were assessed in room air and hyperoxia (FI(O(2)) = 0.4).

MEASUREMENTS AND MAIN RESULTS

Preterm CB (28-35 wk gestation) yielded significantly more ECFC colonies than term CB. Importantly, we found that CD45(-)/CD34(+)/CD133(+)/VEGFR-2(+) cell number did not correlate with ECFC colony count. Preterm ECFCs demonstrated increased growth compared with term ECFCs. Hyperoxia impaired growth of preterm but not term ECFCs. Treatment with superoxide dismutase and catalase enhanced preterm ECFC growth during hyperoxia.

CONCLUSIONS

Preterm ECFCs appear in increased numbers and proliferate more rapidly but have an increased susceptibility to hyperoxia compared with term ECFCs. Antioxidants protect preterm ECFCs from hyperoxia.

Changes of plasma asymmetric dimethylarginine levels after coronary artery bypass grafting

OBJECTIVES

We investigated whether coronary artery bypass grafting affects plasma asymmetric dimethylarginine (ADMA) concentrations and whether precardioplegic hyperoxia influences ADMA release from the heart.

DESIGN

Twenty two patients were randomized into control (n = 11) and hyperoxia (n = 11, ventilated with >96% oxygen before cardiopulmonary bypass) groups. Arterial and coronary sinus blood was sampled before cardioplegia and during early reperfusion. Arterial samples were drawn 60 min after declamping of the aorta, and on the first postoperative day.

RESULTS

Baseline arterial values of ADMA were not different between groups (0.59+/-0.18 micro mol/l control, 0.63+/-0.13 micro mol/l hyperoxia group). Negligible release of ADMA into coronary sinus was detected 20 min after cardioplegia. A significant decrease of arterial ADMA was observed by the first postoperative morning (0.42+/-0.16 micro mol/l in control, and 0.38+/-0.07 in hyperoxia group, p < 0.01 compared to baseline).

CONCLUSIONS

CABG with cardioplegia is associated with decrease of ADMA by the first postoperative morning. Reperfusion of cardioplegic heart did not result in significant release of ADMA. Pretreatment with hyperoxia had no influence on myocardial release and arterial levels of ADMA.

Hyperoxaemia does not change concentrations of serotonin and beta‐thromboglobulin in blood of healthy humans

BACKGROUND

The mechanisms of oxygen-induced effects on blood vessels (vasoconstriction in hyperoxaemia and vasodilatation during hypoxaemia) are uncertain. Many investigators have suggested that the vasoconstriction seen during hyperoxia/hyperoxaemia is mediated through the endothelium as a result of either increased release or activity of vasoconstrictors (oxygen radicals, endothelin, norepinephrine, angiotensin II, or serotonin (5-HT)), or reduced activity of vasodilators (prostaglandin E2 and nitric oxide). Serotonin has been assumed to have a central role.

METHODS

Eight healthy volunteers were exposed to FiO2 of 1.0 for 20 min and serum concentrations of serotonin and activated platelets were measured (indicated by concentrations of beta-thromboglobulin (beta-TG)).

RESULTS

During hyperoxaemia in humans, serum concentrations of scrotonin and beta-TG remained unchanged.

CONCLUSION

If serotonin is involved in oxygen-induced vasoconstriction, the mechanism is more likely to be either a potentiating effect of serotonin on other vasoconstrictors or increased activity of serotonin on its receptor.

Significance of pulmonary vagal afferents for respiratory muscle activity in the cat

The influence of vagal stretch receptor afferents on respiratory motor-output and respiratory changes in esophageal pressure (DeltaP(es)) was studied in anaesthetized cats. Tracheal occlusions and lung inflations were performed during hyperoxic normocapnia, during electrical stimulation of one carotid sinus nerve (CSN) or the intracranial medullary chemosensitivity (MCS), during hypercapnia or the combination of CSN and hypercapnia. Tracheal occlusions during inspiration led to increased and prolonged inspiratory muscle (IM) activity. Moderate hyperinflation in inspiration decreased and shortened inspiratory motor output. Changes in esophageal pressure and in amplitude and discharge duration of IM are largely proportional (0.84>r<0.98) to lung volume above normal endexpiratory volume (FRC). The effects are described as the Hering-Breuer inspiration inhibitory reflex (HB-IIR). Tracheal occlusion or hyperinflation in end-inspiratory position not only prolonged expiration but also activated expiratory muscles (EM). The effects linearly (0.86>r<0.98) increased with elevation of lung volume. We refer to these effects as the Hering-Breuer expiration facilitatory reflex (HB-EFR). Severe hyperinflation or rapid inflation of the lungs during inspiration, however, led to an inspiratory facilitation with increased IM activity. During concomitant chemoreflex activation, CSN or MCS stimulation, respiratory hypercapnia, or the combination of both, the extent of the above described responses of IM and EM activity were significantly (0.05>p<0.0002) enlarged. The changes in the discharge period of IM and EM following lung inflation were smaller in the presence of the increased chemical respiratory drive (0.01>p<0.005). The relative changes in EM responses to lung inflations during increased respiratory drive were greater than those of IM. Bilateral vagotomy abolished the respiratory responses to tracheal occlusion and hyperinflation of the lungs. The results of the present investigation show that aside from the well-known inhibition of inspiration, vagal slowly adapting lung stretch receptors facilitate expiration. The sensitivity of the lung reflexes is enhanced with increasing respiratory drive. The HB-inspiration inhibitory reflex limits the depth of lung inflation, whereas the HB-expiration facilitatory reflex promotes an effective lung deflation. Both reflex mechanisms, the inspiratory and expiratory one, are present in eupnoeic breathing, but play an important role during increased chemoreflex drive and obstruction of expiration, e.g., with increased external airway resistance.

Pre-treatment with hyperoxia before coronary artery bypass grafting - effects on myocardial injury and inflammatory response

BACKGROUND

In experimental studies, exposure to hyperoxia for a limited time before ischaemia induces a low-grade systemic oxidative stress and evokes an (ischaemic) preconditioning-like effect of the myocardium. We hypothesised that hyperoxia before cardioplegia could protect the myocardium against necrosis and stunning caused by ischaemia-reperfusion.

METHODS

Forty patients undergoing coronary artery bypass grafting were randomly exposed to an oxygen fraction of 0.4 or > 0.96 in inspired air on an average of 120 min before cardioplegia. Blood for troponin I, creatine kinase-MB, lactate, glutathione and interleukin-6 was sampled from arterial and coronary sinus cannulae during 20 min of reperfusion. Additional arterial samples were drawn 60 min after declamping and in the first post-operative morning. The cardiac index and right and left ventricular stroke work indices were measured before sternotomy and up to 12 h post-operatively.

RESULTS

Troponin I, creatine kinase-MB and lactate did not differ between the groups. Hyperoxic pre-treatment had no impact on the post-operative haemodynamic indices measured with the thermodilution pulmonary artery catheter. More oxidised glutathione was released in the hyperoxia group in the first minute of reperfusion (P = 0.015). Hyperoxic pre-treatment abolished the myocardial release of interleukin-6 during 20 min of reperfusion (P = 0.021 vs. controls). In the first post-operative morning, interleukin-6 was higher in the hyperoxia group [127.0 (86.0-140.0) vs. 85.2 pg/ml (66.6-94.5 pg/ml); P = 0.016].

CONCLUSIONS

Exposure to >96% oxygen before cardioplegia did not attenuate ischaemia-reperfusion injury of the heart in patients undergoing coronary artery bypass grafting. The only potentially beneficial effect observed was the decreased transmyocardial release of interleukin-6.

Ventilatory response to a hyperoxic test is related to the frequency of short apneic episodes in late preterm neonates

Chemoreception is frequently involved in the processes underlying apnea in premature infants. Apnea could result from a decrease in carotid body effectiveness. However, increased carotid body activity could also initiate apnea through hypocapnia following hyperventilation when the receptors are stimulated. The aim of this study was to analyze the relationship between carotid body effectiveness and short apneic episodes in older preterm neonates. Carotid body effectiveness was assessed at thermoneutrality in 36 premature neonates (2.07 +/- 0.26 kg) by performing a 30-s hyperoxic test during sleep, the oxygen inhalation involving a ventilation decrease. Blood O(2) saturation (Sp(o2)) and ventilatory parameters were monitored before and during the hyperoxic test. Short episodes of apnea (frequency and mean duration) were recorded during the morning's 3-h interfeeding interval. Pretest Sp(o2) was not related to any of the measured respiratory parameters. A higher frequency of short apneic episodes was linked to a greater ventilation decrease in response to the hyperoxic test (rho = -0.32; p = 0.01). Increased carotid body response is correlated with greater apneic episodes frequency, even in the absence of concomitant oxygen desaturation. Fetal or early postnatal hypoxemia could have increased peripheral chemoreceptor activity, which could initiate a "overshoot/undershoot" situation, which in turn could induce a critical P(o2)/P(co2) combination and apnea.

Effects of glutamine and hyperoxia on pulmonary oxygen uptake and muscle deoxygenation kinetics

The aim of the present study was to determine whether glutamine ingestion, which has been shown to enhance the exercise-induced increase in the tricarboxylic acid intermediate (TCAi) pool size, resulted in augmentation of the rate of increase in oxidative metabolism at the onset of exercise. In addition, the potential interaction with oxygen availability was investigated by completing exercise in both normoxic and hyperoxic conditions. Eight male cyclists cycled for 6 min at 70% VO2max following consumption of a drink (5 ml kg body mass(-1)) containing a placebo or 0.125 g kg body mass(-1) of glutamine in normoxic (CON and GLN respectively) and hyperoxic (HYP and HPG respectively) conditions. Breath-by-breath pulmonary oxygen uptake and continuous, non-invasive muscle deoxygenation (via near infrared spectroscopy: NIRS) data were collected throughout exercise. The time constant of the phase II component of pulmonary oxygen uptake kinetics was unchanged between trials (CON: 21.5 +/- 3.0 vs. GLN: 18.2 +/- 1.3 vs. HYP: 18.9 +/- 2.0 vs. HPG: 18.6 +/- 1.2 s). There was also no alteration of the kinetics of relative muscle deoxygenation as measured via NIRS (CON: 5.9 +/- 0.7 vs. GLN: 7.3 +/- 0.8 vs. HYP: 6.5 +/- 0.9 vs. HPG: 5.2 +/- 0.4 s). Conversely, the mean response time of pulmonary oxygen uptake kinetics was faster (CON: 33.4 +/- 1.2 vs. GLN: 29.8 +/- 2.3 vs. HYP: 33.2 +/- 2.6 vs. HPG: 31.6 +/- 2.6 s) and the time at which muscle deoxygenation increased above pre-exercise values was earlier (CON: 9.6 +/- 0.9 vs. GLN: 8.7 +/- 1.1 vs. HYP: 8.5 +/- 0.8 vs. HPG: 8.4 +/- 0.7 s) following glutamine ingestion. In normoxic conditions, plasma lactate concentration was lower following glutamine ingestion compared to placebo. Whilst the results of the present study provide some support for the present hypothesis, the lack of any alteration in the time constant of pulmonary oxygen uptake and muscle deoxygenation kinetics suggest that the normal exercise induced expansion of the TCAi pool size is not limiting to oxidative metabolism at the onset of cycle exercise at 70% VO2max.

[The heart and underwater diving]

Cardiovascular examination of a certain number of candidates for underwater diving raises justifiable questions of aptitude. An indicative list of contraindications has been proposed by the French Federation of Underwater Studies and Sports but a physiopathological basis gives a better understanding of what is involved. During diving, the haemodynamic changes due not only to the exercise but also to cold immersion, hyperoxaemia and decompression impose the absence of any symptomatic cardiac disease. Moreover, the vasoconstriction caused by the cold and hyperoxaemia should incite great caution in both coronary and hypertensive patients. The contraindication related to betablocker therapy is controversial and the debate has not been settled in France. The danger of drowning makes underwater diving hazardous in all pathologies carrying a risk of syncope. Pacemaker patients should be carefully assessed and the depth of diving limited. Finally, the presence of right-to-left intracardiac shunts increases the risk of complications during decompressionand contraindicates underwater diving. Patent foramen ovale is a special case but no special investigation is required for its detection. The cardiologist examining candidates for underwater diving should take all these factors into consideration because, although underwater diving is a sport associated with an increased risk, each year there are more and more people, with differing degrees of aptitude, who wish to practice it.

Cardiovascular responses to oxygen inhalation after hemorrhage in anesthetized rats: hyperoxic vasoconstriction

Oxygen inhalation is recommended for the initial care of trauma victims. The improved survival seen in early hemorrhage is normally associated with an increase in blood pressure. Although clinical use of oxygen can occur late after hemorrhage, the effects of late administration have not been specifically examined. Anesthetized rats were studied using an isobaric hemorrhage model with target pressures of either 70 or 40 mmHg. At various times after hemorrhage, the feedback control of the blood pressure was stopped and the inspired gas was changed from room air to 100% oxygen. The results show that shortly after hemorrhage to 70 mmHg, oxygen inhalation results in an increase in mean arterial blood pressure of 60 +/- 3 mmHg, which is associated with a large increase in total peripheral resistance from 0.89 +/- 0.05 to 1.25 +/- 0.1 peripheral resistance units. The blood pressure response is essentially unchanged with time, and it is not altered by a 10-min exposure to N(G)-nitro-l-arginine methyl ester. At a target pressure of 40 mmHg, the initial blood pressure response to oxygen is the same, but it gradually decreases as the animal develops a lactic acidosis. We conclude that the therapeutic value of oxygen needs to be separately evaluated for late hemorrhage.

Endothelial Progenitor Cell Release into Circulation Is Triggered by Hyperoxia-Induced Increases in Bone Marrow Nitric Oxide

Endothelial progenitor cells (EPC) are known to contribute to wound healing, but the physiologic triggers for their mobilization are often insufficient to induce complete wound healing in the presence of severe ischemia. EPC trafficking is known to be regulated by hypoxic gradients and induced by vascular endothelial growth factor-mediated increases in bone marrow nitric oxide (NO). Hyperbaric oxygen (HBO) enhances wound healing, although the mechanisms for its therapeutic effects are incompletely understood. It is known that HBO increases nitric oxide levels in perivascular tissues via stimulation of nitric oxide synthase (NOS). Here we show that HBO increases bone marrow NO in vivo thereby increasing release of EPC into circulation. These effects are inhibited by pretreatment with the NOS inhibitor l-nitroarginine methyl ester (l-NAME). HBO-mediated mobilization of EPC is associated with increased lower limb spontaneous circulatory recovery after femoral ligation and enhanced closure of ischemic wounds, and these effects on limb perfusion and wound healing are also inhibited by l-NAME pretreatment. These data show that EPC mobilization into circulation is triggered by hyperoxia through induction of bone marrow NO with resulting enhancement in ischemic limb perfusion and wound healing.

Arterial oxygenation influences central motor output and exercise performance via effects on peripheral locomotor muscle fatigue in humans

Changing arterial oxygen content (C(aO(2))) has a highly sensitive influence on the rate of peripheral locomotor muscle fatigue development. We examined the effects of C(aO(2)) on exercise performance and its interaction with peripheral quadriceps fatigue. Eight trained males performed four 5 km cycling time trials (power output voluntarily adjustable) at four levels of C(aO(2)) (17.6-24.4 ml O(2) dl(-1)), induced by variations in inspired O(2) fraction (0.15-1.0). Peripheral quadriceps fatigue was assessed via changes in force output pre- versus post-exercise in response to supra-maximal magnetic femoral nerve stimulation (DeltaQ(tw); 1-100 Hz). Central neural drive during the time trials was estimated via quadriceps electromyogram. Increased C(aO(2)) from hypoxia to hyperoxia resulted in parallel increases in central neural output (43%) and power output (30%) during cycling and improved time trial performance (12%); however, the magnitude of DeltaQ(tw) (-33 to -35%) induced by the exercise was not different among the four time trials (P > 0.2). These effects of C(aO(2)) on time trial performance and DeltaQ(tw) were reproducible (coefficient of variation = 1-6%) over repeated trials at each F(IO(2)) on separate days. In the same subjects, changing C(aO(2)) also affected performance time to exhaustion at a fixed work rate, but similarly there was no effect of Delta C(aO(2)) on peripheral fatigue. Based on these results, we hypothesize that the effect of C(aO(2)) on locomotor muscle power output and exercise performance time is determined to a significant extent by the regulation of central motor output to the working muscle in order that peripheral muscle fatigue does not exceed a critical threshold.

The role of endothelin-1 in hyperoxia-induced lung injury in mice

Background

As prolonged hyperoxia induces extensive lung tissue damage, we set out to investigate the involvement of endothelin-1 (ET-1) receptors in these adverse changes.

Methods

Experiments were performed on four groups of mice: control animals kept in room air and a group of mice exposed to hyperoxia for 60 h were not subjected to ET-1 receptor blockade, whereas the dual ETA/ETB-receptor blocker tezosantan (TEZ) was administered via an intraperitoneal pump (10 mg/kg/day for 6 days) to other groups of normal and hyperoxic mice. The respiratory system impedance (Zrs) was measured by means of forced oscillations in the anesthetized, paralyzed and mechanically ventilated mice before and after the iv injection of ET-1 (2 μg). Changes in the airway resistance (Raw) and in the tissue damping (G) and elastance (H) of a constant-phase tissue compartment were identified from Zrs by model fitting.

Results

The plasma ET-1 level increased in the mice exposed to hyperoxia (3.3 ± 1.6 pg/ml) relative to those exposed to room air (1.6 ± 0.3 pg/ml, p < 0.05). TEZ administration prevented the hyperoxia-induced increases in G (13.1 ± 1.7 vs. 9.6 ± 0.3 cmH₂O/l, p < 0.05) and H (59 ± 9 vs. 41 ± 5 cmH₂O/l, p < 0.05) and inhibited the lung responses to ET-1. Hyperoxia decreased the reactivity of the airways to ET-1, whereas it elevated the reactivity of the tissues.

Conclusion

These findings substantiate the involvement of the ET-1 receptors in the physiopathogenesis of hyperoxia-induced lung damage. Dual ET-1 receptor antagonism may well be of value in the prevention of hyperoxia-induced parenchymal damage.

[The mathematical model of the blood circulation and external respiration functional state during high-pressure oxygenation or hyperoxia]

Based on the minimal energy uptake, the proposed mathematical model of blood circulation and external respiration functioning during high-pressure oxygenation or hyperoxia allows solution of the optimization task with restrictions dictated by adequate functioning of the two systems. Optimal levels of oxygen and carbon dioxide pressure in arterial and venous blood, minute blood volume and alveolar ventilation as a function of O2 partial pressure in inspired gas mixture were determined. Calculations are compared with experimentally derived values.

Serum erythropoietin levels in healthy humans after a short period of normobaric and hyperbaric oxygen breathing: the "normobaric oxygen paradox"

Renal (peritubular) tissue hypoxia is a well-known physiological trigger for erythropoietin (EPO) production. We investigated the effect of rebound relative hypoxia after hyperoxia obtained under normo- and hyperbaric oxygen breathing conditions. A group of 16 healthy volunteers were investigated before and after a period of breathing 100% normobaric oxygen for 2 h and a period of breathing 100% oxygen at 2.5 ATA for 90 min (hyperbaric oxygen). Serum EPO concentration was measured using a radioimmunoassay at various time points during 24-36 h. A 60% increase (P < 0.001) in serum EPO was observed 36 h after normobaric oxygen. In contrast, a 53% decrease in serum EPO was observed at 24 h after hyperbaric oxygen. Those changes were not related to the circadian rhythm of serum EPO of the subjects. These results indicate that a sudden and sustained decrease in tissue oxygen tension, even above hypoxia thresholds (e.g., after a period of normobaric oxygen breathing), may act as a trigger for EPO serum level. This EPO trigger, the "normobaric oxygen paradox," does not appear to be present after hyperbaric oxygen breathing.