Baroreflex-mediated cardiovascular responses to hyperbaric oxygen

Endothelial Dysfunction and Cardiovascular Disease: Hyperbaric Oxygen Therapy as an Emerging Therapeutic Modality?

Maintaining the physiological function of the vascular endothelium and endothelial glycocalyx is crucial for the prevention of cardiovascular disease, which is one of the leading causes of morbidity and mortality worldwide. Damage to these structures can lead to atherosclerosis, hypertension, and other cardiovascular problems, especially in individuals with risk factors such as diabetes and obesity. Endothelial dysfunction is associated with ischemic disease and has a negative impact on overall cardiovascular health. The aim of this review was to comprehensively summarize the crucial role of the vascular endothelium and glycocalyx in cardiovascular health and associated thrombo-inflammatory conditions. It highlights how endothelial dysfunction, influenced by factors such as diabetes, chronic kidney disease, and obesity, leads to adverse cardiovascular outcomes, including heart failure. Recent evidence suggests that hyperbaric oxygen therapy (HBOT) may offer therapeutic benefits in the treatment of cardiovascular risk factors and disease. This review presents the current evidence on the mechanisms by which HBOT promotes angiogenesis, shows antimicrobial and immunomodulatory effects, enhances antioxidant defenses, and stimulates stem cell activity. The latest findings on important topics will be presented, including the effects of HBOT on endothelial dysfunction, cardiac function, atherosclerosis, plaque stability, and endothelial integrity. In addition, the role of HBOT in alleviating cardiovascular risk factors such as hypertension, aging, obesity, and glucose metabolism regulation is discussed, along with its impact on inflammation in cardiovascular disease and its potential benefit in ischemia-reperfusion injury. While HBOT demonstrates significant therapeutic potential, the review also addresses potential risks associated with excessive oxidative stress and oxygen toxicity. By combining information on the molecular mechanisms of HBOT and its effects on the maintenance of vascular homeostasis, this review provides valuable insights into the development of innovative therapeutic strategies aimed at protecting and restoring endothelial function to prevent and treat cardiovascular diseases.

Apocynin and Hyperbaric Oxygen Therapy Improve Renal Function and Structure in an Animal Model of CKD

BACKGROUND/OBJECTIVES

Chronic kidney disease (CKD) is a progressive pathological condition which results in the severe fibrosis of the kidneys. However, the mechanisms of CKD progression and fibrogenesis remain unclear. We wanted to examine the effects that apocynin and hyperbaric oxygen therapy (HBOT) have on renal function and structure in animals with CKD induced through 5/6 nephrectomy (5/6 Nx-L).

METHODS

Male Wistar rats were divided in 5 groups (n = 8/group) as follows: control-sham-operated rats; Nx-L-rats with 5/6 Nx-L; APO-5/6 Nx-L + apocynin treatment; HBOT-5/6 Nx-L + hyperbaric oxygen treatment, and APO+HBOT-5/6 Nx-L, treated with both treatments. All treatments started 4 weeks after the final step of CKD induction and lasted for 4 weeks. At the end of the experiment, urine samples were collected for the proteinuria assessment and the mean arterial pressure (MAP) was measured. Kidneys were collected for histopathological, Western blot, and immunohistochemical analyses.

RESULTS

All treatments significantly decreased MAP compared to the Nx-L group (p < 0.001). In the APO and APO+HBOT groups, the level of proteinuria was decreased compared to the Nx-L group (p < 0.05 and p < 0.01, respectively). All examined treatments significantly decreased the intensity of lesions in the kidney compared to those observed in the Nx-L group (p < 0.001). Isolated treatments with apocynin and HBOT induced a significant decrease in desmin expression compared to the Nx-L group (p < 0.05); meanwhile, they did not affect the levels of fibronectin (FN) and hypoxia-inducible factor-1α (HIF-1α). Combined treatment did not affect desmin expression levels; however, it induced a significant increase in fibronectin expression compared to Nx-L (p < 0.001).

CONCLUSIONS

Apocynin treatment decreased BP and protein loss, and it improved renal morphology at least partly through the downregulation of desmin expression without changing FN and HIF-1α. Hyperbaric oxygen therapy improved hypertension but failed to significantly affect the level of proteinuria. Combined treatment (apocynin and HBOT) normalized blood pressure (BP) values, renal function, and improved kidney structure by modulating FN and HIF-1α, without affecting desmin protein expression. Further studies are needed to elucidate the mechanisms of slowing down the progression of CKD in this experimental model.

Acute psycho-physiological responses to submaximal constant-load cycling under intermittent hypoxia-hyperoxia vs. hypoxia-normoxia in young males

Background

Hypoxia and hyperoxia can affect the acute psycho-physiological response to exercise. Recording various perceptual responses to exercise is of particular importance for investigating behavioral changes to physical activity, given that the perception of exercise-induced pain, discomfort or unpleasure, and a low level of exercise enjoyment are commonly associated with a low adherence to physical activity. Therefore, this study aimed to compare the acute perceptual and physiological responses to aerobic exercise under intermittent hypoxia-hyperoxia (IHHT), hypoxia-normoxia (IHT), and sustained normoxia (NOR) in young, recreational active, healthy males.

Methods

Using a randomized, single-blinded, crossover design, 15 males (age: 24.5 ± 4.2 yrs) performed 40 min of submaximal constant-load cycling (at 60% peak oxygen uptake, 80 rpm) under IHHT (5 × 4 min hypoxia and hyperoxia), IHT (5 × 4 min hypoxia and normoxia), and NOR. Inspiratory fraction of oxygen during hypoxia and hyperoxia was set to 14% and 30%, respectively. Heart rate (HR), total hemoglobin (tHb) and muscle oxygen saturation (SmO2) of the right vastus lateralis muscle were continuously recorded during cycling. Participants' peripheral oxygen saturation (SpO2) and perceptual responses (i.e., perceived motor fatigue, effort perception, perceived physical strain, affective valence, arousal, motivation to exercise, and conflict to continue exercise) were surveyed prior, during (every 4 min), and after cycling. Prior to and after exercise, peripheral blood lactate concentration (BLC) was determined. Exercise enjoyment was ascertained after cycling. For statistical analysis, repeated measures analyses of variance were conducted.

Results

No differences in the acute perceptual responses were found between conditions (p ≥ 0.059, ηp 2 ≤ 0.18), while the physiological responses differed. Accordingly, SpO2 was higher during the hyperoxic periods during the IHHT compared to the normoxic periods during the IHT (p < 0.001, ηp 2 = 0.91). Moreover, HR (p = 0.005, ηp 2 = 0.33) and BLC (p = 0.033, ηp 2 = 0.28) were higher during IHT compared to NOR. No differences between conditions were found for changes in tHb (p = 0.684, ηp 2 = 0.03) and SmO2 (p = 0.093, ηp 2 = 0.16).

Conclusion

IHT was associated with a higher physiological response and metabolic stress, while IHHT did not lead to an increase in HR and BLC compared to NOR. In addition, compared to IHT, IHHT seems to improve reoxygenation indicated by a higher SpO2 during the hyperoxic periods. However, there were no differences in perceptual responses and ratings of exercise enjoyment between conditions. These results suggest that replacing normoxic by hyperoxic reoxygenation-periods during submaximal constant-load cycling under intermittent hypoxia reduced the exercise-related physiological stress but had no effect on perceptual responses and perceived exercise enjoyment in young recreational active healthy males.

Does Hyperbaric Oxygenation Improve Athletic Performance?

ABSTRACT

Šet, V, and Lenasi, H. Does hyperbaric oxygenation improve athletic performance? J Strength Cond Res 37(2): 482-493, 2023-Hyperbaric oxygen (HBO) has been suggested to affect oxygen availability and performance, and delay the onset of fatigue. Many mechanisms of HBO-induced alterations have been proposed, including modulation of various metabolic pathways, and the antioxidant defense mechanisms. As exercise per se affects similar aspects, it is tempting to speculate that simultaneous application of both, exercise and HBO might have synergistic effects. The aim of this review was to search through the currently available literature and evaluate the effect of acute exposure to HBO on exercise performance, potential effects of a combination of HBO and physical training, and to elucidate some possible mechanisms behind. We conducted searches in the PubMed and Scopus databases (search term: "hyperbaric" AND "oxygen" AND "exercise") and in relevant hyperbaric textbook and assessed potentially eligible full texts for details. Meta-analysis could not be performed because of a few available and rather heterogeneous studies. Twenty-seven studies were included in the final assessment (14 on exercise during HBO, 9 on exercise following HBO, 4 on applying HBO during recovery and rest between exercise bouts, and 3 on a combination of HBO and training). The results are contradictory, showing either positive or none ergogenic effects. There is some risk of bias and placebo effect. Discrepant findings of the available studies might partly be explained by different protocols applied, both regarding HBO and exercise intensity and regimen. There is a need for further research with well-designed trials to evaluate the effect of HBO on performance before recommending it to routine use in athletes.

Mild Hyperbaric Oxygen Exposure Enhances Peripheral Circulatory Natural Killer Cells in Healthy Young Women

Mild hyperbaric oxygen (HBO) enhances oxygen absorption in blood, relieving fatigue without causing oxidative stress. The benefits of mild HBO have been recognized in the treatment of lifestyle-related diseases and hypertension, but no research has been conducted on its effects on immunity. The aim of the present study is to investigate the effect of mild HBO on natural killer (NK) cells and cytokines in healthy young women. This crossover randomized control trial was conducted with 16 healthy young women. Participants were randomly exposed to normobaric oxygen (NBO; 1.0 atmospheres absolute (ATA), 20.8% oxygen) and mild HBO conditions (1.4 ATA, 35-40% oxygen, injected 18L oxygen per minute) in a hyperbaric oxygen chamber for 70 min. Heart rate, parasympathetic activity, NK cell count, interleukin (IL)-6, IL-12p70 and derivatives of reactive oxygen metabolites (d-ROMs) were measured before and after both exposures. In the NBO condition, parasympathetic activity remained unchanged, whereas after mild HBO exposure, parasympathetic activity was significantly increased. NK cells remained unchanged after NBO exposure, while NK cells were increased after exposure to mild HBO. Exposure to mild HBO did not increase d-ROM values, IL-6 and IL-12p70 protein levels. These findings suggest that exposure to mild HBO can be a useful protocol to increase NK cells by regulating parasympathetic activity via increasing oxygen delivery.

Carotid body chemosensitivity is not attenuated during cold water diving

Tonic carotid body (CB) activity is reduced during exposure to cold and hyperoxia. We tested the hypotheses that cold water diving lowers CB chemosensitivity and augments CO₂ retention more than thermoneutral diving. Thirteen subjects [age: 26 ± 4 yr; body mass index (BMI): 26 ± 2 kg/m²) completed two 4-h head-out water immersion protocols in a hyperbaric chamber (1.6 ATA) in cold (15°C) and thermoneutral (25°C) water. CB chemosensitivity was assessed with brief hypercapnic ventilatory response ([Formula: see text]) and hypoxic ventilatory response ([Formula: see text]) tests before dive, 80 and 160 min into the dive (D80 and D160, respectively), and immediately after and 60 min after dive. Data are reported as an absolute mean (SD) change from predive. End-tidal CO₂ pressure increased during both the thermoneutral water dive [D160: +2 (3) mmHg; P = 0.02] and the cold water dive [D160: +1 (2) mmHg; P = 0.03]. Ventilation increased during the cold water dive [D80: 4.13 (4.38) and D160: 7.75 (5.23) L·min^-1; both P < 0.01] and was greater than the thermoneutral water dive at both time points (both P < 0.01). [Formula: see text] was unchanged during the dive (P = 0.24) and was not different between conditions (P = 0.23). [Formula: see text] decreased during the thermoneutral water dive [D80: -3.45 (3.61) and D160: -2.76 (4.04) L·min·mmHg^-1; P < 0.01 and P = 0.03, respectively] but not the cold water dive. However, [Formula: see text] was not different between conditions (P = 0.17). In conclusion, CB chemosensitivity was not attenuated during the cold stress diving condition and does not appear to contribute to changes in ventilation or CO₂ retention.

Human Adaptations to Multiday Saturation on NASA NEEMO

Human adaptation to extreme environments has been explored for over a century to understand human psychology, integrated physiology, comparative pathologies, and exploratory potential. It has been demonstrated that these environments can provide multiple external stimuli and stressors, which are sufficient to disrupt internal homeostasis and induce adaptation processes. Multiday hyperbaric and/or saturated (HBS) environments represent the most understudied of environmental extremes due to inherent experimental, analytical, technical, temporal, and safety limitations. National Aeronautic Space Agency (NASA) Extreme Environment Mission Operation (NEEMO) is a space-flight analog mission conducted within Florida International University’s Aquarius Undersea Research Laboratory (AURL), the only existing operational and habitable undersea saturated environment. To investigate human objective and subjective adaptations to multiday HBS, we evaluated aquanauts living at saturation for 9–10 days via NASA NEEMO 22 and 23, across psychologic, cardiac, respiratory, autonomic, thermic, hemodynamic, sleep, and body composition parameters. We found that aquanauts exposed to saturation over 9–10 days experienced intrapersonal physical and mental burden, sustained good mood and work satisfaction, decreased heart and respiratory rates, increased parasympathetic and reduced sympathetic modulation, lower cerebral blood flow velocity, intact cerebral autoregulation and maintenance of baroreflex functionality, as well as losses in systemic bodyweight and adipose tissue. Together, these findings illustrate novel insights into human adaptation across multiple body systems in response to multiday hyperbaric saturation.

Magnitude and Clinical Predictors of Blood Pressure Changes in Patients Undergoing Hyperbaric Oxygen Therapy: A Retrospective Study

Hyperbaric oxygen therapy (HBOT) is widely used to treat several pathologies. The hemodynamic changes during HBOT, particularly the magnitude of arterial blood pressure (ABP) increase, are not completely understood. No clinical predictors for HBOT-induced ABP increase have been described. The purpose of this study was to quantify ABP changes in patients undergoing HBOT and to examine their predictors. This retrospective longitudinal cohort study examined 3291 elective HBOT sessions. Non-invasive ABP was recorded before and after each session. The primary outcome was to quantify the HBOT-induced ABP rise. The secondary outcome was to determine the ABP-rise predictors among demographic and clinical variables. Overall, ABP increased significantly after HBOT; this finding was more evident in the hypertensive subgroup compared to the normotensive one (+6 vs. +16.2 mmHg). Clinical predictors of significant post-HBOT ABP change were history of hypertension and pre-session baseline ABP classification. This study demonstrates an absolute HBOT-induced ABP rise. This change is clinically relevant in patients with history of hypertension. A higher baseline ABP seems a risk factor for clinically relevant ABP change. Pre-session ABP should be used clinically as an indicator for strict ABP monitoring during HBOT; future studies are recommended to explore the ABP optimization before starting an HBO treatment.

Hyperbaric oxygen therapy: Can it be a novel supportive therapy in COVID-19?

The coronavirus disease 2019 (COVID-19) is a pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV2). Although 85% of infected patients remain asymptomatic, 5% show severe symptoms such as hypoxaemic respiratory failure and multiple end organ dysfunction (MODS) requiring intensive care unit (ICU) admission with a mortality rate of about 2.8%. Since a definitive treatment is yet to be identified, preventive and supportive strategies remain the mainstay of management. Supportive measures such as oxygen therapy with nasal cannula, face mask, noninvasive ventilation, mechanical ventilation and even extreme measures such as extracorporeal membrane oxygenation (ECMO) fail to improve oxygenation in some patients. Hence, hyperbaric oxygen therapy (HBOT) has been proposed as a supportive strategy to improve oxygenation in COVID-19 patients. HBOT is known to increase tissue oxygenation by increasing the amount of dissolved oxygen in plasma. HBOT also mitigates tissue inflammation thus reducing the ill effects of cytokine storm in COVID-19 patients. Though there is limited literature available on HBOT in COVID-19 patients, considering the present need for additional supportive therapy to improve oxygenation, HBOT has been proposed as a novel supportive treatment in COVID-19 patients.

Carotid body chemosensitivity at 1.6 ATA breathing air versus 100% oxygen

Hyperoxia reduces the ventilatory response to hypercapnia by suppressing carotid body (CB) activation. This effect may contribute to CO₂ retention during underwater diving due to the high arterial O₂ content associated with hyperbaria. We tested the hypothesis that CB chemosensitivity to hypercapnia and hypoxia is attenuated during hyperbaria. Ten subjects completed two, 4-h dry dives at 1.6 atmosphere absolute (ATA) breathing either 21% O₂ (Air) or 100% O₂ (100% O₂). CB chemosensitivity was assessed using brief hypercapnic ventilatory response ([Formula: see text]) and hypoxic ventilatory response ([Formula: see text]) tests predive, 75 and 155 min into the dives, and 15 and 55 min postdive. End-tidal CO₂ pressure increased during the dive at 75 and 155 min [Air: +9 (SD 4) mmHg and +8 (SD 4) mmHg versus 100% O₂: +6 (SD 4) mmHg and +5 (SD 3) mmHg; all P < 0.01] and was higher while breathing Air (P < 0.01). [Formula: see text] was unchanged during the dive (P = 0.73) and was not different between conditions (P = 0.47). However, [Formula: see text] was attenuated from predive during the dive at 155 min breathing Air [-0.035 (SD 0.037) L·min·mmHg^-1; P = 0.02] and at both time points while breathing 100% O₂ [-0.035 (SD 0.052) L·min·mmHg^-1 and -0.034 (SD 0.064) L·min·mmHg^-1; P = 0.02 and P = 0.02, respectively]. These data indicate that the CB chemoreceptors do not appear to contribute to CO₂ retention in hyperbaria.NEW & NOTEWORTHY We demonstrate that carotid body chemosensitivity to brief exposures of hypercapnia was unchanged during a 4-h dive in a dry hyperbaric chamber at 1.6 ATA regardless of breathing gas condition [i.e., air (21% O₂) versus 100% oxygen]. Therefore, it appears that an attenuation of carotid body chemosensitivity to hypercapnia does not contribute to CO₂ retention in hyperbaria.

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 effect of hyperbaric oxygenation therapy on myocardial function

Hyperbaric oxygenation therapy is successfully implemented for the treatment of several disorders. Data on the effect of hyperbaric oxygenation on echocardiographic parameters in asymptomatic patients is limited. The current study sought to evaluate the effect of hyperbaric oxygenation therapy on echocardiographic parameters in asymptomatic patients. Thirty-one consecutive patients underwent a 60-sessions course of hyperbaric oxygenation therapy in an attempt to improve cognitive impairment. In all subjects, echocardiography examination was performed before and after a course of hyperbaric oxygenation therapy. Conventional and speckle tracking imaging parameters were calculated and analyzed. The mean age was 70 ± 9.5 years, 28 [90%] were males. History of coronary artery disease was present in 12 [39%]. 94% suffered from hypertension, 42% had diabetes mellitus. Baseline wall motion abnormalities were found in eight patients, however, global ejection fraction was within normal limits. During the study, ejection fraction [EF], increased from 60.71 ± 6.02 to 62.29 ± 5.19%, p = 0.02. Left ventricular end systolic volume [LVESV], decreased from 38.08 ± 13.30 to 35.39 ± 13.32 ml, p = 0.01. Myocardial performance index [MPi] improved, from 0.29 ± 0.07 to 0.26 ± 0.08, p = 0.03. Left ventricular [LV] global longitudinal strain increased from - 19.31 ± 3.17% to - 20.16 ± 3.34%, p = 0.036 due to improvement in regional strain in the apical and antero-septal segments. Twist increased from 18.32 ± 6.61° to 23.12 ± 6.35° p = 0.01, due to improvement in the apical rotation, from 11.76 ± 4.40° to 16.10 ± 5.56°, p = 0.004. Hyperbaric oxygen therapy appears to improve left ventricular function, especially in the apical segments, and is associated with better cardiac performance. If our results are confirmed in further studies, HBOT can be used in many patients with heart failure and systolic dysfunction.

Cardiorespiratory alterations in patients undergoing hyperbaric oxygen therapy

OBJECTIVE

To evaluate cardiorespiratory alterations due to a single session of hyperbaric oxygen therapy.

METHOD

Randomized study with patients: a control group and hyperbaric oxygen therapy. Evaluations occurred in the beginning, during, and after exposure to pure oxygen above atmosphere for 2 hours. Systemic blood pressure, peripheral oxygen saturation, pulse rate, lung volume and lung capacity, and maximal inspiratory and expiratory pressures were evaluated. Peripheral oxygen saturation, pulse rate, and systemic blood pressure were evaluated during the pressurizing in the first hour. Data were evaluated by means of ANOVA, Mann-Whitney, and independent t-test (p<0.05).

RESULTS

A total of 14 adult patients were evaluated. In the group under therapy (seven subjects), aged: 49.57±14.59 years, there was a decrease in the pulse rate of 16 beats per minute after 35 minutes of therapy (intragroup analysis), and the peripheral oxygen saturation was higher within the same period compared to the control group.

CONCLUSION

The hyperbaric oxygen therapy promotes cardiorespiratory alterations with the increase of the peripheral oxygen saturation and decrease of the pulse rate, without altering blood pressure levels and the strength, volumes, and respiratory capacities.

Preventive effects of ketone ester BD-AcAc2 on central nervous system oxygen toxicity and concomitant acute lung injury

BACKGROUND

Recent studies indicated that ketone ester R,S-1,3-butanediol acetoacetate diester (BD-AcAc₂) may be effective in preventing central nervous system oxygen toxicity (CNS-OT) and concomitant acute lung injury, a serious medical problem to be faced when breathing hyperbaric oxygen (HBO). This study aimed to further investigate the protective effects of BD-AcAc₂ against CNS-OT and concomitant acute lung injury (ALI) in mice.

METHODS

Mice were treated with BD-AcAc₂ in peanut oil vehicle (2.5, 5.0 or 10.0 g·kg⁻² body weight) by gavage 20 minutes before 600 kPa HBO exposure. Control mice received the vehicle only. Seizure latency was recorded. Malondialdehyde content in brain and lung tissues, total protein level in bronchoalveolar lavage fluid (BLF) and lung water content were measured 60 minutes after the hyperbaric exposure. Histopathology of lung tissue was undertaken.

RESULTS

Compared with the vehicle alone, BD-AcAc₂ prolonged seizure latency in a dose-dependent manner (P < 0.01). The HBO-induced increase in brain malondialdehyde, BLF protein and lung water were significantly reduced by BD-AcAc₂ (P < 0.01).

CONCLUSION

Oral administration of the ketone ester BD-AcAc₂ significantly protected against CNS-OT and concomitant ALI. Alleviation of oxidative stress may be one underlying mechanism providing this effect.

Adrenoceptor blockade modifies regional cerebral blood flow responses to hyperbaric hyperoxia: Protection against CNS oxygen toxicity

Exposure to extreme-hyperbaric oxygen (HBO2), > 5-6 atmospheres absolute (ATA), produces baroreflex impairment, sympathetic hyperactivation, hypertension, tachycardia, and cerebral hyperemia, known as Phase II, culminating in seizures. We hypothesized that attenuation of the effects of high sympathetic outflow would preserve regional cerebral blood flow (rCBF) and protect against HBO2-induced seizures. To explore this possibility, we tested four adrenoceptor antagonists in conscious and anesthetized rats exposed to HBO2 at 5 and 6 ATA, respectively: phentolamine (nonselective α1 and 2), prazosin (selective α1), propranolol (nonselective β1 and 2) and atenolol (selective β1). In conscious rats, 4 drug-doses were administered to rats prior to HBO2 exposures, and seizure latencies were recorded. Drug-doses that provided similar protection against seizures were administered before HBO2 exposures in anesthetized rats to determine the effects of adrenoceptor blockade on mean arterial pressure, heart rate, rCBF and EEG spikes. All four drugs modified cardiovascular and rCBF responses in HBO2 that aligned with epileptiform discharges, but only phentolamine and propranolol effectively increased EEG spike latencies by ~20 and 36 min, respectively. When phentolamine and propranolol were delivered during HBO2 at the onset of phase II, only propranolol led to sustained reductions in heart rate and rCBF, preventing the appearance of epileptiform discharges. The enhanced effectiveness of propranolol may extend beyond β-adrenoceptor blockade, i.e. membrane stability and reduced metabolic activity. These results indicate that adrenoceptor drug pre-treatment will minimize the effects of excessive sympathetic outflow on rCBF and extend HBO2 exposure time.

Attenuation of partial unilateral ureteral obstruction - induced renal damage with hyperbaric oxygen therapy in a rat model

OBJECTIVE

The objective of the present study was to evaluate the effectiveness of HBO therapy on biochemical parameters, renal morphology and renal scintigraphy in rats undergoing chronic unilateral partial ureteral obstruction (UPUO).

MATERIAL AND METHODS

Thirty-five rats were divided into five equal groups: Control group; Sham group; HBO group; UPUO group and UPUO/HBO group. The effects of HBO therapy were examined using biochemical parameters and histopathological changes. After calculating the score for each histopathological change, the total histopathological score was obtained by adding all the scores. In addition, dynamic renal scintigraphy findings were evaluated.

RESULTS

Serum parameters indicating inflammation, serum tumor necrosis factoralpha, ischemia modified-albumin, IMA/albumin ratio and Pentraxin-3 levels, were observed to be high in the UPUO group and low in the UPUO/HBO treatment group. Similarly, in the treatment group, the reduction in malondialdehyde, total oxidant status and oxidative stress index levels and increase in total antioxidant capacity values were observed to be statistically significant compared to the UPUO group (p<0.001, p=0.007, p<0.001, p=0.001, respectively). The total score and apoptosis index significantly decreased after administration of HBO treatment. Dynamic 99mTc-MAG3 renal scintigraphy also showed convincing evidence regarding the protective nature of HBO against kidney injury. In the UPUO/HBO therapy group, the percentage contribution of each operated kidney increased significantly compared to the UPUO group (41.73% versus 32.72%).

CONCLUSION

The findings of this study indicate that HBO therapy had a reno-protective effect by reducing inflammation and oxidative stress, and preserving renal function after renal tissue damage due to induction of UPUO.

Performance of the BBraun perfusor space syringe driver under hyperbaric conditions

BACKGROUND

The BBraun Perfusor Space™ syringe driver is already in use by ambulance services and retrieval teams but has not previously been assessed for hyperbaric chamber use.

METHODS

Pump flow accuracy was tested at rates between 1 and 40 ml· h⁻¹ using three different brands of 50 ml syringe. Function of the occlusion alarms was assessed using the same syringes. The hyperbaric profile involved pressurisation to 284 kPa at 30 kPa· min⁻¹, 30 min at 284 kPa and decompression at 30 kPa· min⁻¹. Output was recorded from differences in weight of collection containers. A single device was tested.

RESULTS

Performance was highly dependent on the syringe type used, with two of the three 50 ml syringes used demonstrating 'stiction' at both low and high occlusion pressure alarm settings, most marked during pressurisation. On decompression from 284 kPa all syringes alarmed at significantly lower pressures. Because of the stiction problems only the flow measurements for the BBrown Omni¬ x 50 ml syringes are reported. At a pressure of 284 kPa, the difference between programmed and delivered rates was within the manufacturer's specification of 10%: at 40 ml· h⁻¹ (median variation 1.25%, IQR 0.5-1.7%), 10 ml· h⁻¹ (8.6%, IQR 8-9.2%), 5 ml· h⁻¹ (-8.8%, IQR - 1.6-8.8%) and 1 ml· h⁻¹ (-4%, IQR 4-12%). Pressurisation was associated with significantly lower flow rates whilst decompression was associated with significantly increased rates. Limited testing at 405 kPa was also within the manufacturer's specifications.

CONCLUSION

A BBraun Infusor Space syringe driver performed within acceptable performance criteria but is highly dependent on syringe type and flow rates. The potential for the device to under deliver on pressurisation and over deliver on depressurisation, however, suggests vigilance and appropriate rate adjustments may be necessary during these phases.

Breathing 100% oxygen during water immersion improves postimmersion cardiovascular responses to orthostatic stress

Physiological compensation to postural stress is weakened after long‐duration water immersion (WI), thus predisposing individuals to orthostatic intolerance. This study was conducted to compare hemodynamic responses to postural stress following exposure to WI alone (Air WI), hyperbaric oxygen alone in a hyperbaric chamber (O₂HC), and WI combined with hyperbaric oxygen (O₂WI), all at a depth of 1.35 ATA, and to determine whether hyperbaric oxygen is protective of orthostatic tolerance. Thirty‐two healthy men underwent up to 15 min of 70° head‐up tilt (HUT) testing before and after a single 6‐h resting exposure to Air WI (N = 10), O₂ HC (N = 12), or O₂WI (N = 10). Heart rate (HR), blood pressure (BP), cardiac output (Q), stroke volume (SV), forearm blood flow (FBF), and systemic and forearm vascular resistance (SVR and FVR) were measured. Although all subjects completed HUT before Air WI, three subjects reached presyncope after Air WI exposure at 10.4, 9.4, and 6.9 min. HUT time did not change after O₂WI or O₂HC exposures. Compared to preexposure responses, HR increased (+10 and +17%) and systolic BP (−13 and −8%), and SV (−16 and −23%) decreased during HUT after Air WI and O₂WI, respectively. In contrast, HR and SV did not change, and systolic (+5%) and diastolic BP (+10%) increased after O₂HC. Q decreased (−13 and −7%) and SVR increased (+12 and +20%) after O₂WI and O₂HC, respectively, whereas SVR decreased (−9%) after Air WI. Opposite patterns were evident following Air WI and O₂HC for FBF (−26 and +52%) and FVR (+28 and −30%). Therefore, breathing hyperbaric oxygen during WI may enhance post‐WI cardiovascular compensatory responses to orthostatic stress.

Hyperbaric Environment: Oxygen and Cellular Damage versus Protection

The elevation of tissue pO2 induced by hyperbaric oxygen (HBO) is a physiological stimulus that elicits a variety of cellular responses. These effects are largely mediated by, or in response to, an increase in the production of reactive oxygen and nitrogen species (RONS). The major consequences of elevated RONS include increased oxidative stress and enhanced antioxidant capacity, and modulation of redox-sensitive cell signaling pathways. Interestingly, these phenomena underlie both the therapeutic and potentially toxic effects of HBO. Emerging evidence indicates that supporting mitochondrial health is a potential method of enhancing the therapeutic efficacy of, and preventing oxygen toxicity during, HBO. This review will focus on the cellular consequences of HBO, and explore how these processes mediate a delicate balance of cellular protection versus damage. © 2017 American Physiological Society. Compr Physiol 7:213-234, 2017.

An Update on the Appropriate Role for Hyperbaric Oxygen: Indications and Evidence

BACKGROUND

Among advanced therapeutic interventions for wounds, hyperbaric oxygen therapy (HBOT) has the unique ability to ameliorate tissue hypoxia, reduce pathologic inflammation, and mitigate ischemia reperfusion injury. Most of the conditions for which it is utilized have few successful alternative treatments, and the morbidity and mortality associated with treatment failure are significant. Data on the efficacy and effectiveness of HBOT were reviewed, comparative effectiveness research of HBOT was explained, and a new paradigm for the appropriate use of HBOT was described.

METHODS

Systematic reviews and randomized controlled trials that have evaluated HBOT were reviewed.

RESULTS

Although numerous small randomized controlled trials provide compelling support for HBOT, the physics of the hyperbaric environment create significant barriers to trial design. The electronic health record infrastructure created to satisfy mandatory quality and registry reporting requirements as part of healthcare reform can be harnessed to facilitate the acquisition of real world data for HBOT comparative effectiveness studies and clinical decision support.

CONCLUSIONS

Predictive models can identify patients unlikely to heal spontaneously and most likely to benefit from HBOT. Although electronic health records can automate the calculation of predictive models making them available at the point of care, using them in clinical decision making is complicated. It is not clear whether stakeholders will support the allocation of healthcare resources using mathematical models, but the current patient selection process mandates a 30-day delay for all patients who might benefit and allows treatment for at least some patients who cannot benefit.

T2* and T1 assessment of abdominal tissue response to graded hypoxia and hypercapnia using a controlled gas mixing circuit for small animals

PURPOSE

To characterize T2* and T1 relaxation time response to a wide spectrum of gas challenges in extracranial tissues of healthy rats.

MATERIALS AND METHODS

A range of graded gas mixtures (hyperoxia, hypercapnia, hypoxia, and hypercapnic hypoxia) were delivered through a controlled gas-mixing circuit to mechanically ventilated and intubated rats. Quantitative magnetic resonance imaging (MRI) was performed on a 3T clinical scanner; T2* and T1 maps were computed to determine tissue response in the liver, kidney cortex, and paraspinal muscles. Heart rate and blood oxygen saturation (SaO2 ) were measured through a rodent oximeter and physiological monitor.

RESULTS

T2* decreases consistent with lowered SaO2 measurements were observed for hypercapnia and hypoxia, but decreases were significant only in liver and kidney cortex (P < 0.05) for >10% CO2 and <15% O2 , with the new gas stimulus, hypercapnic hypoxia, producing the greatest T2* decrease. Hyperoxia-related T2* increases were accompanied by negligible increases in SaO2 . T1 generally increased, if at all, in the liver and decreased in the kidney. Significance was observed (P < 0.05) only in kidney for >90% O2 and >5% CO2 .

CONCLUSION

T2* and T1 provide complementary roles for evaluating extracranial tissue response to a broad range of gas challenges. Based on both measured and known physiological responses, our results are consistent with T2* as a sensitive marker of blood oxygen saturation and T1 as a weak marker of blood volume changes. J. Magn. Reson. Imaging 2016;44:305-316.

Microvascular oxygen partial pressure during hyperbaric oxygen in diabetic rat skeletal muscle

Hyperbaric oxygen (HBO) is a major therapeutic treatment for ischemic ulcerations that perforate skin and underlying muscle in diabetic patients. These lesions do not heal effectively, in part, because of the hypoxic microvascular O2 partial pressures (PmvO2 ) resulting from diabetes-induced cardiovascular dysfunction, which alters the dynamic balance between O2 delivery (Q̇o2) and utilization (V̇o2) rates. We tested the hypothesis that HBO in diabetic muscle would exacerbate the hyperoxic PmvO2 dynamics due, in part, to a reduction or slowing of the cardiovascular, sympathetic nervous, and respiratory system responses to acute HBO exposure. Adult male Wistar rats were divided randomly into diabetic (DIA: streptozotocin ip) and healthy (control) groups. A small animal hyperbaric chamber was pressurized with oxygen (100% O2) to 3.0 atmospheres absolute (ATA) at 0.2 ATA/min. Phosphorescence quenching techniques were used to measure PmvO2 in tibialis anterior muscle of anesthetized rats during HBO. Lumbar sympathetic nerve activity (LSNA), heart rate (HR), and respiratory rate (RR) were measured electrophysiologically. During the normobaric hyperoxia and HBO, DIA tibialis anterior PmvO2 increased faster (mean response time, CONT 78 ± 8, DIA 55 ± 8 s, P < 0.05) than CONT. Subsequently, PmvO2 remained elevated at similar levels in CONT and DIA muscles until normobaric normoxic recovery where the DIA PmvO2 retained its hyperoxic level longer than CONT. Sympathetic nervous system and cardiac and respiratory responses to HBO were slower in DIA vs. CONT. Specifically the mean response times for RR (CONT: 6 ± 1 s, DIA: 29 ± 4 s, P < 0.05), HR (CONT: 16 ± 1 s, DIA: 45 ± 5 s, P < 0.05), and LSNA (CONT: 140 ± 16 s, DIA: 247 ± 34 s, P < 0.05) were greater following HBO onset in DIA than CONT. HBO treatment increases tibialis anterior muscle PmvO2 more rapidly and for a longer duration in DIA than CONT, but not to a greater level. Whereas respiratory, cardiovascular, and LSNA responses to HBO are profoundly slowed in DIA, only the cardiovascular arm (via HR) may contribute to the muscle vascular incompetence and these faster PmvO2 kinetics.

Effects of striatal nitric oxide production on regional cerebral blood flow and seizure development in rats exposed to extreme hyperoxia

The endogenous vasodilator and signaling molecule nitric oxide has been implicated in cerebral hyperemia, sympathoexcitation, and seizures induced by hyperbaric oxygen (HBO2) at or above 3 atmospheres absolute (ATA). It is unknown whether these events in the onset of central nervous system oxygen toxicity originate within specific brain structures and whether blood flow is diverted to the brain from peripheral organs with high basal flow, such as the kidney. To explore these questions, total and regional cerebral blood flow (CBF) were measured in brain structures of the central autonomic network in anesthetized rats in HBO2 at 6 ATA. Electroencephalogram (EEG) recordings, cardiovascular hemodynamics, and renal blood flow (RBF) were also monitored. As expected, mean arterial blood pressure and total and regional CBF increased preceding EEG spikes while RBF was unaltered. Of the brain structures examined, the earliest rise in CBF occurred in the striatum, suggesting increased neuronal activation. Continuous unilateral or bilateral striatal infusion of the nitric oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester attenuated CBF responses in that structure, but global EEG discharges persisted and did not differ from controls. Our novel findings indicate that: 1) cerebral hyperemia in extreme HBO2 in rats does not occur at the expense of renal perfusion, highlighting the remarkable autoregulatory capability of the kidney, and 2) in spite of a sentinel increase in striatal blood flow, additional brain structure(s) likely govern the pathogenesis of HBO2-induced seizures because EEG discharge latency was unchanged by local blockade of striatal nitric oxide production and concomitant hyperemia.

Baroreceptor afferents modulate brain excitation and influence susceptibility to toxic effects of hyperbaric oxygen

Unexplained adjustments in baroreflex sensitivity occur in conjunction with exposures to potentially toxic levels of hyperbaric oxygen. To investigate this, we monitored central nervous system, autonomic and cardiovascular responses in conscious and anesthetized rats exposed to hyperbaric oxygen at 5 and 6 atmospheres absolute, respectively. We observed two contrasting phases associated with time-dependent alterations in the functional state of the arterial baroreflex. The first phase, which conferred protection against potentially neurotoxic doses of oxygen, was concurrent with an increase in baroreflex sensitivity and included decreases in cerebral blood flow, heart rate, cardiac output, and sympathetic drive. The second phase was characterized by baroreflex impairment, cerebral hyperemia, spiking on the electroencephalogram, increased sympathetic drive, parasympatholysis, and pulmonary injury. Complete arterial baroreceptor deafferentation abolished the initial protective response, whereas electrical stimulation of intact arterial baroreceptor afferents prolonged it. We concluded that increased afferent traffic attributable to arterial baroreflex activation delays the development of excessive central excitation and seizures. Baroreflex inactivation or impairment removes this protection, and seizures may follow. Finally, electrical stimulation of intact baroreceptor afferents extends the normal delay in seizure development. These findings reveal that the autonomic nervous system is a powerful determinant of susceptibility to sympathetic hyperactivation and seizures in hyperbaric oxygen and the ensuing neurogenic pulmonary injury.