Hyperoxia-induced arterial compliance decrease in healthy man

Contribution of Adenosine in the Physiological Changes and Injuries Secondary to Exposure to Extreme Oxygen Pressure in Healthy Subjects

Climbers and aviators are exposed to severe hypoxia at high altitudes, whereas divers are exposed to hyperoxia at depth. The aim of this study was to report changes in the adenosinergic system induced by exposure to extreme oxygen partial pressures. At high altitudes, the increased adenosine concentration contributes to brain protection against hypoxia through various mechanisms such as stimulation of glycogenolysis for ATP production, reduction in neuronal energy requirements, enhancement in 2,3-bisphosphoglycerate production, and increase in cerebral blood flow secondary to vasodilation of cerebral arteries. In the context of mountain illness, the increased level of A_2AR expression leads to glial dysfunction through neuroinflammation and is involved in the pathogenesis of neurological disorders. Nonetheless, a high level of adenosine concentration can protect against high-altitude pulmonary edema via a decrease in pulmonary arterial pressure. The adenosinergic system is also involved in the acclimatization phenomenon induced by prolonged exposure to altitude hypoxia. During hyperoxic exposure, decreased extracellular adenosine and low A_2A receptor expression contribute to vasoconstriction. The resulting decrease in cerebral blood flow is considered a preventive phenomenon against cerebral oxygen toxicity through the decrease in oxygen delivery to the brain. With regard to lung oxygen toxicity, hyperoxia leads to an increase in extracellular adenosine, which acts to preserve pulmonary barrier function. Changes in the adenosinergic system induced by exposure to extreme oxygen partial pressures frequently have a benefit in decreasing the risk of adverse effects.

Hyperoxia During Exercise: Impact on Adenosine Plasma Levels and Hemodynamic Data

Introduction

Adenosine is an ATP derivative that is strongly implicated in the cardiovascular adaptive response to exercise. In this study, we hypothesized that during exercise the hyperemia, commonly observed during exercise in air, was counteracted by the downregulation of the adenosinergic pathway during hyperoxic exposure.

Methods

Ten healthy volunteers performed two randomized sessions including gas exposure (Medical air or Oxygen) at rest and during exercise performed at 40% of maximal intensity, according to the individual fitness of the volunteers. Investigations included the measurement of adenosine plasma level (APL) and the recording of hemodynamic data [i.e., cardiac output (CO) and systemic vascular resistances (SVR) using pulsed Doppler and echocardiography].

Results

Hyperoxia significantly decreased APL (from 0.58 ± 0.06 to 0.21 ± 0.05 μmol L^–1, p < 0.001) heart rate and CO and increased SVR in healthy volunteers at rest. During exercise, an increase in APL was recorded in the two sessions when compared with measurements at rest (+0.4 ± 0.4 vs. +0.3 ± 0.2 μmol L^–1 for medical air and oxygen exposures, respectively). APL was lower during the exercise performed under hyperoxia when compared with medical air exposure (0.5 ± 0.06 vs. 1.03 ± 0.2 μmol L^–1, respectively p < 0.001). This result could contribute to the hemodynamic differences between the two conditions, such as the increase in SVR and the decrease in both heart rate and CO when exercises were performed during oxygen exposure as compared to medical air.

Conclusion

Hyperoxia decreased APLs in healthy volunteers at rest but did not eliminate the increase in APL and the decrease in SVR during low intensity exercise.

The role of the clinical laboratory in diagnosing acid-base disorders

Acid-base homeostasis is fundamental for life. The body is exceptionally sensitive to changes in pH, and as a result, potent mechanisms exist to regulate the body's acid-base balance to maintain it in a very narrow range. Accurate and timely interpretation of an acid-base disorder can be lifesaving but establishing a correct diagnosis may be challenging. The underlying cause of the acid-base disorder is generally responsible for a patient's signs and symptoms, but laboratory results and their integration into the clinical picture is crucial. Important acid-base parameters are often available within minutes in the acute hospital care setting, and with basic knowledge it should be easy to establish the diagnosis with a stepwise approach. Unfortunately, many caveats exist, beginning in the pre-analytical phase. In the post-analytical phase, studies on the arterial reference pH are scarce and therefore many different reference values are used in the literature without any solid evidence. The prediction models that are currently used to assess the acid-base status are approximations that are mostly based on older studies with several limitations. The two most commonly used methods are the physiological method and the base excess method, both easy to use. The secondary response equations in the base excess method are the most convenient. Evaluation of acid-base disorders should always include the assessment of electrolytes and the anion gap. A major limitation of the current acid-base laboratory tests available is the lack of rapid point-of-care laboratory tests to diagnose intoxications with toxic alcohols. These intoxications can be fatal if not recognized and treated within minutes to hours. The surrogate use of the osmolal gap is often an inadequate substitute in this respect. This article reviews the role of the clinical laboratory to evaluate acid-base disorders.

Baseline values of cardiovascular and respiratory parameters predict response to acute hypoxia in young healthy men

The majority of the available works have studied distinct hypoxic responses of respiratory and cardiovascular systems. This study examines how these systems interact while responding to hypoxia and whether baseline metrics moderate reactions to a hypoxic challenge. Central hemodynamic, aortic wave reflection, and gas exchange parameters were measured in 27 trained young men before and after 10-min normobaric isocapnic hypoxia (10 % O2). Associations were assessed by correlation and multiple regression analyses. Hypoxic changes in the parameters of pulse wave analysis such as augmentation index (-114 %, p=0.007), pulse pressure amplification (+6 %, p=0.020), time to aortic reflection wave (+21 %, p<0.001) report on the increase in arterial distensibility. Specifically, initially compliant arteries blunt the positive cardiac chronotropic response to hypoxia and facilitate the myocardial workload. The degree of blood oxygen desaturation is directly correlated with both baseline values and hypoxic responses of aortic and peripheral blood pressures. The hypoxia-induced gain in ventilation (VE), while controlling for basal VE and heart rate (HR), is inversely associated with deltaHR and deltasystolic blood pressure. The study suggests that cardiovascular and respiratory systems mutually supplement each other when responding to hypoxic challenge.

Hyperoxia Improves Hemodynamic Status During Head-up Tilt Testing in Healthy Volunteers: A Randomized Study

Head-up tilt test is useful for exploring neurally mediated syncope. Adenosine is an ATP derivative implicated in cardiovascular disturbances that occur during head-up tilt test. The aim of the present study was to investigate the impact of hyperoxia on adenosine plasma level and on hemodynamic changes induced by head-up tilt testing.Seventeen healthy male volunteers (mean age 35 ± 11 years) were included in the study. The experiment consisted of 2 head-up tilt tests, 1 session with subjects breathing, through a mask, medical air (FiO2 = 21%) and 1 session with administration of pure oxygen (FiO2 = 100%) in double-blind manner. Investigations included continuous monitoring of hemodynamic data and measurement of plasma adenosine levels.No presyncope or syncope was found in 15 of the 17 volunteers. In these subjects, a slight decrease in systolic blood pressure was recorded during orthostatic stress performed under medical air exposure. In contrast, hyperoxia led to increased systolic blood pressure during orthostatic stress when compared with medical air. Furthermore, mean adenosine plasma levels decreased during hyperoxic exposure before (0.31 ± 0.08 μM) and during head-up tilt test (0.33 ± 0.09 μM) when compared with baseline (0.6 ± 0.1 μM). Adenosine plasma level was unchanged during medical air exposure at rest (0.6 ± 0.1 μM), and slightly decreased during orthostatic stress. In 2 volunteers, the head-up tilt test induced a loss of consciousness when breathing air. In these subjects, adenosine plasma level increased during orthostatic stress. In contrast, during hyperoxic exposure, the head-up tilt test did not induce presyncope or syncope. In these 2 volunteers, biological study demonstrated a decrease in adenosine plasma level at both baseline and during orthostatic stress for hyperoxic exposure compared with medical air.These results suggest that hyperoxia was able to increase blood pressure during head-up tilt test via a decrease in plasma adenosine concentration. Our results also suggest that adenosine receptor antagonists are worth trying in neurocardiogenic syncope.

Effects of hyperoxia on resting state functional magnetic resonance imaging

We studied the effect of oxygen inhalation during resting state functional MRI scanning in healthy control individuals. We hypothesized that resting state networks would be modified under hyperoxic conditions. Thirty-four normal volunteers were recruited for this study. All participants were scanned twice: once while breathing atmospheric air and once under hyperoxic conditions in a randomized order. Hyperoxic conditions were produced by administering 100% O2. Blood oxygen level-dependent T2* scans were obtained for each of the scans. Resting state networks were extracted using independent component analysis. A paired t-test showed that the resting state networks scans (default mode network, attention network and executive network) acquired under hyperoxic conditions had significantly higher Z-scores than scans performed under atmospheric air. Spectral analysis of the time-course signal in these networks also showed a difference in the total power of low frequencies between the two conditions. These results were reversed in the visual network. Clinical or research applications of oxygen-enhanced MRI need to take into account the modularly effects that hyperoxia exerts on the networks resting state functional MRI.

Acute application of antioxidants protects against hyperoxia-induced reduction of plasma nitrite concentration

We investigated the effects of acute intake of antioxidants on hyperoxia-induced oxidative stress, reduction of plasma nitrite and change in arterial stiffness. Twelve healthy males randomly consumed either placebo or an oral antioxidant cocktail (vitamin C, 1000 mg; vitamin E, 600 IU; alpha-lipoic acid, 600 mg). Every therapy was consumed once, a week apart, in a cross-over design, 30 min before the experiment. The volunteers breathed 100% normobaric oxygen between 30th and 60th min of 1-h study protocol. Plasma levels of nitrite, lipid peroxides (LOOH) and vitamin C, arterial stiffness (indicated by augmentation index, AIx) and arterial oxygen (Ptc O2 ) pressure were measured before and after hyperoxia. Exposure to oxygen caused a similar increase of Ptc O2 in both placebo and antioxidants groups, confirming comparable exposure to hyperoxia (438 ± 100 versus 455 ± 83 mm Hg). Vitamin C was increased in the antioxidants group confirming successful application of antioxidants (69 ± 14 versus 57 ± 15 μm). Hyperoxia resulted in increased AIx and LOOH and decreased nitrite in placebo (-32 ± 11 versus -47 ± 13%, 72 ± 7 versus 62 ± 6 μm H2 O2 and 758 ± 184 versus 920 ± 191 nm, respectively), but not in the antioxidants group (-42 ± 13 versus -50 ± 13%, 64 ± 9 versus 61 ± 8 μm H2 O2 and 847 ± 156 versus 936 ± 201 nm, respectively). The acute intake of selected antioxidants was effective in preserving bioavailabity of ˙NO and vascular function, against hyperoxia-induced oxidative stress.

Plasma nitrite concentration decreases after hyperoxia-induced oxidative stress in healthy humans

The aim of this study was to measure plasma nitrite, the biochemical marker of endothelial nitric oxide ((•)NO) synthesis, before and after hyperoxia, in order to test the hypothesis that hyperoxia-induced vasoconstriction is a consequence of reduced bioavailability of (•)NO caused by elevated oxidative stress. Ten healthy men breathed 100% normobaric O(2) for 30 min between 15th and 45th min of the 1-h study protocol. Plasma nitrite and malondialdehyde (MDA), arterial stiffness (indicated by augmentation index, AIx) and arterial oxygen (P(tc)O(2)) pressure were measured at 1st, 15th, 45th and 60th minute of the study. Breathing of normobaric 100% oxygen during 30 min caused an increase in P(tc)O(2) (from 75 ± 2 to 412 ± 25 mm Hg), AIx (from -63 ± 4 to -51 ± 3%) and MDA (from 152 ± 13 to 218 ± 15 nm) values and a decrease in plasma nitrite (from 918 ± 58 to 773 ± 55 nm). During the 15-min recovery phase, plasma nitrite, AIx and MDA values remained altered. This study suggests that the underlying mechanism of hyperoxia-induced vasoconstriction may involve reduced (•)NO bioavailability caused by elevated and sustained oxidative stress.

CYP450 4A inhibition attenuates O2 induced arteriolar constriction in chronic but not acute Goldblatt hypertension

We explored the role of 20-hydroxy-5Z, 8Z, 11Z, 14Z-eicosatetraenoic acid (20-HETE) in oxygen-induced vasoconstriction in a normal renin form of hypertension [the 1 kidney-1 clip Goldblatt hypertensive rat (1K1C)] and a high renin form of hypertension [the 2 kidney-1 clip Goldblatt hypertensive rat (2K1C)]. A silver clip was placed around the left renal artery of adult Sprague-Dawley males. The right kidney was removed in the 1K1C group and left intact in the 2K1C group. Arteriolar responses to elevation of O(2) concentration in the superfusion solution from 0% O(2) to 21% O(2) were determined in the in situ cremaster muscle before and after inhibition of cytochrome P450 4A omega-hydroxylase (CYP450 4A) with N-methyl-sulfonyl-12, 12-dibromododec-11-enamide (DDMS). Arteriolar constriction to elevated PO(2) was enhanced in the chronic 1K1C but not the acute 1K1C or 2K1C. DDMS eliminated O(2)-induced arteriolar constriction in the 9-week 1K1C, but had no effect in the 2-week 1K1C, and only partially inhibited O(2)-induced constriction of arterioles in the 4-week 2K1C rat. These findings indicate that although the CYP4A/20-HETE system contributes to arteriolar constriction in response to elevated PO(2) in the established stage of 1K1C renovascular hypertension, physiological alterations in other mechanisms are the primary determinants of O(2)-induced constriction of arterioles in the early and developing stages of 1K1C and 2K1C hypertension.

Cardiovascular changes induced by cold water immersion during hyperbaric hyperoxic exposure

The present study was designed to assess the cardiac changes induced by cold water immersion compared with dry conditions during a prolonged hyperbaric and hyperoxic exposure (ambient pressure between 1.6 and 3 ATA and PiO(2) between 1.2 and 2.8 ATA). Ten healthy volunteers were studied during a 6 h compression in a hyperbaric chamber with immersion up to the neck in cold water while wearing wet suits. Results were compared with measurements obtained in dry conditions. Echocardiography and Doppler examinations were performed after 15 min and 5 h. Stroke volume, left atrial and left ventricular (LV) diameters remained unchanged during immersion, whereas they significantly fell during the dry session. As an index of LV contractility, percentage fractional shortening remained unchanged, in contrast to a decrease during dry experiment. Heart rate (HR) significantly decreased after 5 h, although it had not changed during the dry session. The changes in the total arterial compliance were similar during the immersed and dry sessions, with a significant decrease after 5 h. In immersed and dry conditions, cardiac output was unchanged after 15 min but decreased by almost 20% after 5 h. This decrease was related to a decrease in HR during immersion and to a decrease in stroke volume in dry conditions. The hydrostatic pressure exerted by water immersion on the systemic vessels could explain these differences. Indeed, the redistribution of blood volume towards the compliant thoracic bed may conceal a part of hypovolaemia that developed in the course of the session.

Alterations in the peripheral circulation in COPD patients

RATIONALE

Although various factors influence peripheral circulation in chronic obstructive pulmonary disease (COPD) patients, little is known about the vasomotor changes in these subjects.

OBJECTIVES

The present study was designed to assess alterations in the brachial circulation of COPD patients.

METHODS

Twenty-five COPD patients and 25 healthy subjects were studied. Brachial artery (BA) blood flow and indices of BA stiffness were investigated by two-dimensional ultrasonography and pulsed Doppler. Cardiac dimensions, left ventricular (LV) function and cardiac output were assessed by pulsed Doppler echocardiography.

MAIN RESULTS

A significant increase in LV mass was observed in the COPD group despite normal arterial pressure. Total arterial compliance and BA compliance were significantly decreased in COPD patients in comparison with healthy subjects. Heart rate was increased in COPD patients and was inversely correlated with PaO(2) and forced expiratory volume in the first second (FEV(1)). A decrease in LV preload was expressed by a reduction in LV diastolic diameters and LV stroke volume. Patients with severe COPD have a lower BA surface area than patients with moderate COPD. FEV(1) and PaO(2) were significantly related to BA compliance.

CONCLUSION

In COPD patients, significant alterations in the peripheral circulation were observed. Moreover, the magnitude of changes in the peripheral circulation was related to the severity of COPD.

Could hyperoxic ventilation impair oxygen delivery in septic patients?

OBJECTIVE

In critically ill patients, a decrease in whole body oxygen consumption under hyperoxia has been reported and this could be related to hyperoxia-induced arterial changes. We investigated changes in brachial artery circulation and tone during short-term hyperoxic ventilation in septic patients.

DESIGN AND SETTING

Prospective clinical study in the intensive care unit of a university hospital.

PATIENTS

Fourteen patients (severe sepsis n=3 and septic shock n=11) requiring mechanically controlled ventilation due to sepsis syndrome were investigated under stable clinical conditions.

INTERVENTIONS

After a 20-min period of hyperoxic ventilation (inspired oxygen fraction = 100%), two-dimensional images of brachial artery cross-sectional area and brachial blood flow velocities were recorded using conventional ultrasonography and pulsed Doppler simultaneously with invasive arterial pressure measurements.

MEASUREMENTS AND MAIN RESULTS

Hyperoxia did not affect heart rate, but increased mean arterial pressure and decreased cross-sectional areas both at the end of diastole and at the end of systole. Haemodynamic study showed an increase in resistance index, and a decrease in distensibility and compliance coefficients. Furthermore, a decrease in brachial artery blood flow and arterial oxygen delivery was observed during hyperoxic exposure.

CONCLUSIONS

Hyperoxia was paradoxically demonstrated to decrease oxygen delivery in upper limbs during septic shock.

Hyperoxic brain effects are normalized by addition of CO2

BACKGROUND

Hyperoxic ventilation (>21% O2) is widely used in medical practice for resuscitation, stroke intervention, and chronic supplementation. However, despite the objective of improving tissue oxygen delivery, hyperoxic ventilation can accentuate ischemia and impair that outcome. Hyperoxia results in, paradoxically, increased ventilation, which leads to hypocapnia, diminishing cerebral blood flow and hindering oxygen delivery. Hyperoxic delivery induces other systemic changes, including increased plasma insulin and glucagon levels and reduced myocardial contractility and relaxation, which may derive partially from neurally mediated hormonal and sympathetic outflow. Several cortical, limbic, and cerebellar brain areas regulate these autonomic processes. The aim of this study was to assess recruitment of these regions in response to hyperoxia and to determine whether any response would be countered by addition of CO2 to the hyperoxic gas mixture.

METHODS AND FINDINGS

We studied 14 children (mean age 11 y, range 8-15 y). We found, using functional magnetic resonance imaging, that 2 min of hyperoxic ventilation (100% O2) following a room air baseline elicited pronounced responses in autonomic and hormonal control areas, including the hypothalamus, insula, and hippocampus, throughout the challenge. The addition of 5% CO2 to 95% O2 abolished responses in the hypothalamus and lingual gyrus, substantially reduced insular, hippocampal, thalamic, and cerebellar patterns in the first 48 s, and abolished signals in those sites thereafter. Only the dorsal midbrain responded to hypercapnia, but not hyperoxia.

CONCLUSIONS

In this group of children, hyperoxic ventilation led to responses in brain areas that modify hypothalamus-mediated sympathetic and hormonal outflow; these responses were diminished by addition of CO2 to the gas mixture. This study in healthy children suggests that supplementing hyperoxic administration with CO2 may mitigate central and peripheral consequences of hyperoxia.