Using the gli spirographic prediction equations to revisit the allometric relationships between lung volumes, height and age in adults

The determination the forced vital capacity (FVC) and the forced expiratory volume in 1 second (FEV1) during spirometry studies, is at the core of the evaluation of the pulmonary function of patients with respiratory diseases. The Global Lung Function Initiative (GLI) offers the most extensive data set of normal lung functions available, which is currently used to determine the average expected/predicted FEV1 and FVC (predV), and their lower limit of normal (LLN, 5th percentile) at any given height and age for women and men. These prediction equations are currently expressed in a rather complex form: predV = exp [p+ (a x Ln (height) + (n x Ln (age)) + spline] and LLN = exp(Ln (predV) + Ln (1 - 1.645 x S x CV)/S); and are currently used to generate interpretations in commercialized spinographic system. However, as shown in this paper, these equations contain physiological and fundamental allometric information on lung volumes that become obvious when rewriting mean predicted values as a "simple" power function of height and LLN as a percentage of the mean predicted values. We therefore propose to present the equations of prediction obtained from the GLI data using simplified expressions in adults (18-95 years old) to reveal some of their physiological and allometric meaning. Indeed, when predicted FEV1 and FVC (predV) were expressed under the form predV= αx heightax b(age), the resulting exponent (a) ranges between 2 and 3, transforming the one dimension of a length (size) into a volume, akin to the third-order power (cubic) function of height historically used to predict lung volumes. Only one function, b (age), is necessary to replace all the factors related to age, including the tables of discrete data of spline functions original equations. Similarly, LLN can be expressed as LLN = c (age) xpredV to become a simple percentage of the predicted values, as a function of age. The equations with their respective new polynomial functions were validated in 52,764 consecutive spirometry tests performed in 2022 in 22,612 men and 30,152 women at the Cleveland Clinic. Using these equations, it become obvious that for both women and men, FEV1/FVC ratio decreases with the size as the exponent of the power function of height is lower for FEV1 than FVC. We conclude that rewriting the GLI predicted equations with simpler formulations restitutes to the GLI data some of their original allometric meaning, without altering the accuracy of their prediction.

Sleep-Disordered Breathing, Hypoxia, and Pulmonary Physiologic Influences in Atrial Fibrillation

Background We leverage a large clinical cohort to elucidate sleep-disordered breathing and sleep-related hypoxia in incident atrial fibrillation (AF) development given the yet unclear contributions of sleep-related hypoxia and pulmonary physiology in sleep-disordered breathing and AF. Methods and Results Patients who underwent sleep studies at Cleveland Clinic January 2, 2000, to December 30, 2015, comprised this retrospective cohort. Cox proportional hazards models were used to examine apnea hypopnea index, percentage time oxygen saturation <90%, minimum and mean oxygen saturation, and maximum end-tidal carbon dioxide on incident AF adjusted for age, sex, race, body mass index, cardiopulmonary disease and risk factors, antiarrhythmic medications, and positive airway pressure. Those with spirometry were additionally adjusted for forced expiratory volume in 1 second, forced vital capacity, and forced expiratory volume in 1 second/forced vital capacity. This cohort (n=42 057) was 50.7±14.1 years, 51.3% men, 74.1% White individuals, had median body mass index 33.2 kg/m2, and 1947 (4.6%) developed AF over 5 years. A 10-unit apnea hypopnea index increase was associated with 2% higher AF risk (hazard ratio [HR], 1.02 [95% CI, 1.00-1.03]). A 10-unit increase in percentage time oxygen saturation <90% and 10-unit decreases in mean and minimum oxygen saturation were associated with 6% (HR, 1.06 [95% CI, 1.04-1.08]), 30% (HR, 1.30 [95% CI, 1.18-1.42]), and 9% (HR, 1.09 [95% CI, 1.03-1.15]) higher AF risk, respectively. After adjustment for spirometry (n=9683 with available data), only hypoxia remained significantly associated with incident AF, although all coefficients were stable. Conclusions Sleep-related hypoxia was associated with incident AF in this clinical cohort, consistent across 3 measures of hypoxia, persistent after adjustment for pulmonary physiologic impairment. Findings identify a strong role for sleep-related hypoxia in AF development without pulmonary physiologic interdependence.

Treatment of life-threatening H2S intoxication: Lessons from the trapping agent tetranitrocobinamide

We sought to evaluate the efficacy of trapping free hydrogen sulfide (H2S) following severe H2S intoxication. Sodium hydrosulfide solution (NaHS, 20 mg/kg) was administered intraperitoneally in 69 freely moving rats. In a first group (protocol 1), 40 rats were randomly assigned to receive saline (n = 20) or the cobalt compound tetranitrocobinamide (TNCbi) (n = 20, 75 mg/kg iv), one minute into coma, when free H2S was still present in the blood. A second group of 27 rats received TNCbi or saline, following epinephrine, 5 min into coma, when the concentration of free H2S has drastically decreased in the blood. In protocol 1, TNCbi significantly increased immediate survival (65 vs 20 %, p < 0.01) while in protocol 2, administration of TNCbi led to the same outcome as untreated animals. We hypothesize that the decreased efficacy of TNCbi with time likely reflects the rapid spontaneous disappearance of the pool of free H2S in the blood following H2S exposure.

Methylene blue induced O2 consumption is not dependent on mitochondrial oxidative phosphorylation: Implications for salvage pathways during acute mitochondrial poisoning

While administration of the cyclic redox agent methylene blue (MB) during intoxication by mitochondrial poisons (cyanide, hydrogen sulfide, rotenone) increases survival, the mechanisms behind these antidotal properties remain poorly understood. The objective of the studies presented in this paper was to characterize the interactions between the redox properties of MB, the intermediate metabolism and the mitochondrial respiration. We first show that intra-venous administration of micromolar levels of methylene blue in sedated and mechanically ventilated rats, increases not only resting oxygen consumption but also CO₂ production (by ~ 50%), with no change in their ratio. This hypermetabolic state could be reproduced in a cellular model, where we found that the rate of electron transfer to MB was of the same order of magnitude as that of normal cellular metabolism. Notably, the large increase in cellular oxygen consumption caused by MB was relatively indifferent to the status of the mitochondrial respiratory chain: oxygen consumption persisted even when the respiratory chain was inhibited or absent (using inhibitors and cells deficient in mitochondrial oxidative phosphorylation); yet MB did not impede mitochondrial ATP production in control conditions. We present evidence that after being reduced into leuco-methylene blue (LMB) in presence of reducing molecules that are physiologically found in cells (such as NADH), the re-oxidation of LMB by oxygen can account for the increased oxygen consumption observed in vivo. In conditions of acute mitochondrial dysfunction, these MB redox cycling properties allow the rescue of the glycolysis activity and Krebs cycle through an alternate route of oxidation of NADH (or other potential reduced molecules), which accumulation would have otherwise exerted negative feedback on these metabolic pathways. Our most intriguing finding is that re-oxidization of MB by oxygen ultimately results in an in vivo matching between the increase in the rate of O₂ consumed, by MB re-oxidation, and the rate of CO₂, produced by the intermediate metabolism, imitating the fundamental coupling between the glycolysis/Krebs cycle and the mitochondrial respiration.

Effects of fentanyl overdose-induced muscle rigidity and dexmedetomidine on respiratory mechanics and pulmonary gas exchange in sedated rats

The objective of our study was to establish in sedated rats the consequences of high-dose fentanyl-induced acute muscle rigidity on the mechanical properties of the respiratory system and on the metabolic rate. Doses of fentanyl that we have previously shown to produce persistent rigidity of the muscles of the limbs and trunk in the rat (150 -300 microg/kg iv), were administered in 23 volume-controlled mechanically ventilated and sedated rats. The effects of a low dose of the FDA approved central alpha-2 agonist, dexmedetomidine (3 microg/kg iv), which has been suggested to oppose fentanyl-induced muscle rigidity, were determined after fentanyl administration. Fentanyl produced a significant decrease in Crs in the 23 rats that were studied. In 13 rats, an abrupt response occurred within 90 seconds, consisting in rapid rhythmic contractions of most skeletal muscles, that were replaced by persistent tonic/tetanic contractions leading a significant decrease of Crs (from 0.51 ± 0.11 ml/cmH₂O to 0.36 ± 0.08 ml/cmH₂O, 3 minutes after fentanyl injection). In the other 10 animals, a Crs progressively decreased to 0.26 ± 0.06 ml/cmH₂O at 30 minutes. There was a significant rise in V̇O₂ during muscle tonic contractions (from 8.48 ± 4.31 to 11.29 ± 2.57 ml/min), which contributed to a significant hypoxemia, despite ventilation being held constant. Dexmedetomidine provoked a significant and rapid increase in Crs towards baseline levels, while decreasing the metabolic rate and restoring normoxemia. We propose that the changes in respiratory mechanics and metabolism produced by opioid-induced muscle rigidity contribute to fentanyl lethality.

Respiratory effects of low and high doses of fentanyl in control and β-arrestin 2-deficient mice

We have investigated the potential acute desensitizing role of the β arrestin 2 (β-arr2) pathway on the ventilatory depression produced by levels of fentanyl ranging from analgesic to life-threatening (0.1 to 60 mg/kg ip) in control and β-arr2-deficient nonsedated mice. Fentanyl at doses of 0.1, 0.5, and 1 mg/kg ip-corresponding to the doses previously used to study the role of β-arr2 pathway-decreased ventilation, but along the V̇e/V̇co₂ relationship established in baseline conditions. This reduction in ventilation was therefore indistinguishable from the decrease in breathing during the periods of spontaneous immobility. Above 1.5 mg/kg, however, ventilation was depressed out of proportion of the changes in metabolic rate, suggesting a specific depression of the drive to breathe. The ventilatory responses were similar between the two groups. At high doses of fentanyl (60 mg/kg ip) 1 out of 20 control mice died by apnea versus 8 out of 20 β-arr2-deficient mice (P = 0.008). In the surviving mice, ventilation was however identical in both groups. The ventilatory effects of fentanyl in β-arr2-deficient mice, reported in the literature, are primarily mediated by the "indirect" effects of sedation/hypometabolism on breathing control. There was an excess mortality at very high doses of fentanyl in the β-arr2-deficient mice, mechanisms of which are still open to question, as the capacity of maintaining a rhythmic, although profoundly depressed, breathing activity remains similar in all of the surviving control and β-arr2-deficient mice.NEW & NOTEWORTHY When life-threatening doses of fentanyl are used in mice, the β-arrestin 2 pathway appears to play a critical role in the recovery from opioid overdose. This observation calls into question the use of G protein-biased μ-opioid receptor agonists, as a strategy for safer opioid analgesic drugs.

Azure B as a novel cyanide antidote: Preclinical in-vivo studies

We have determined the effects of azure B (AzB), the main demethylated metabolite of methylene blue (MB), on a model of lethal cyanide intoxication. Our rationale was the following: AzB 1- possesses redox properties very similar to those of MB, which is a potent cyanide antidote, 2- may present a higher intracellular diffusibility than MB, 3- is already present in commercially available solutions of MB, and 4- appears very quickly in the blood after MB administration. AzB could therefore be a member of the phenothiazium chromophore family of interest to treat cyanide intoxication. We found, in spontaneously breathing urethane sedated rats, that AzB mimicked the effects of MB by increasing metabolism, ventilation and cardiac contractility up to 30-40 mg/kg. AzB had a lethal toxicity when the dose of 60 mg/kg was reached. Doses of AzB were therefore chosen in keeping with these data and the doses of MB previously used against cyanide intoxication (4-20 mg/kg) in the rat - doses corresponding to those used in humans to treat methemoglobinemia. KCN, infused at the rate of 0.375 mg/kg/min iv for 13 min, was fatal within 15 min in 100 % of our un-anesthetized rats. AzB at the dose of 4 mg/kg (n = 5) or 10 mg/kg (n = 5) administered 3 min into cyanide infusion allowed 100 % of the animals to survive with no clinical sequelae. The onset of coma was also significantly delayed and no apnea or gasping occurred. At the dose of 20 mg/kg, AzB was much less effective. At 4 mg/kg, the antidotal effects of AzB were significantly better than those produced by MB at the same dose and were not different from the effects produced by 20 mg/kg MB. We conclude that AzB is a potent cyanide antidote at relatively low doses.

Antidotal Effects of the Phenothiazine Chromophore Methylene Blue Following Cyanide Intoxication

Our study was aimed at (1) determining the efficacy of the dye methylene blue (MB), following a rapidly lethal cyanide (CN) intoxication in un-sedated rats; (2) clarifying some of the mechanisms responsible for the antidotal properties produced by this potent cyclic redox dye. Sixty-nine awake rats acutely intoxicated by CN (IP, KCN 7 mg/kg) received saline, MB (20 mg/kg) or hydroxocobalamin (HyCo, 150 mg/kg) when in deep coma. Survival in this model was very low, reaching 9% at 60 min without any treatment. Methylene blue significantly increased survival (59%, p < .001) at 60 min, versus 37% with HyCo (p < .01). In addition, 8 urethane-anesthetized rats were exposed to a sublethal CN intoxication (KCN, 0.75 mg/kg/min IV for 4 min); they received MB (20 mg/kg, IV) or saline, 5 min after the end of CN exposure. All MB-treated rats displayed a significant reduction in hyperlactacidemia, a restoration of pyruvate/lactate ratio-a marker of NAD/NADH ratio-and an increase in CO2 production, a marker of the activity of the TCA cycle. These changes were also associated with a 2-fold increase in the pool of CN in red cells. Based on series of in vitro experiments, looking at the effects of MB on NADH, as well as the redox effects of MB on hemoglobin and cytochrome c, we hypothesize that the antidotal properties of MB can in large part be accounted for by its ability to readily restore NAD/NADH ratio and to cyclically re-oxidize then reduce the iron in hemoglobin and the electron chain complexes. All of these effects can account for the rapid antidotal properties of this dye following CN poisoning.

Antidotal effects of methylene blue against cyanide neurological toxicity: in vivo and in vitro studies

The aim of the present study was to determine whether methylene blue (MB) could directly oppose the neurological toxicity of a lethal cyanide (CN) intoxication. KCN, infused at the rate of 0.375 mg/kg/min intravenously, produced 100% lethality within 15 min in unanaesthetized rats (n = 12). MB at 10 (n = 5) or 20 mg/kg (n = 5), administered 3 min into CN infusion, allowed all animals to survive with no sequelae. No apnea and gasping were observed at 20 mg/kg MB (P < 0.001). The onset of coma was also significantly delayed and recovery from coma was shortened in a dose-dependent manner (median of 359 and 737 seconds, respectively, at 20 and 10 mg/kg). At 4 mg/kg MB (n = 5), all animals presented faster onset of coma and apnea and a longer period of recovery than at the highest doses (median 1344 seconds, P < 0.001). MB reversed NaCN-induced resting membrane potential depolarization and action potential depression in primary cultures of human fetal neurons intoxicated with CN. MB restored calcium homeostasis in the CN-intoxicated human SH-SY5Y neuroblastoma cell line. We conclude that MB mitigates the neuronal toxicity of CN in a dose-dependent manner, preventing the lethal depression of respiratory medullary neurons and fatal outcome.

Evidence for the emergence of an opioid-resistant respiratory rhythm following fentanyl overdose

Breathing resumes within one to two minutes following fentanyl overdose induced apnea in spontaneously breathing rats. As this regular rhythm is produced at a time wherein fentanyl concentrations and receptor occupancy are likely to be extremely high, the mechanisms initiating and sustaining such a respiratory activity remain unclear. Forty-four un-anesthetized adult rats were studied in an open-flow plethysmograph. Regardless of the dose of fentanyl that was used, i.e. 50 μg.kg^-1 (n = 8), 100 μg.kg^-1 (n = 8) or 300 μg.kg^-1 (n = 7), all rats developed an immediate central apnea followed by a depressed regular rhythm that was produced 118, 97 and 81 s (median) later, respectively. Only one rat did not recover. This inspiratory and regular activity consisted of a low frequency and tidal volume pattern with a significant reduction in V̇E/V̇CO2 ratio, which persisted for at least 30 min and that was not different between 100 or 300 μg.kg^-1. The time at which this respiratory rhythm emerged, following the highest dose of fentanyl, was not affected by 100 % O₂ or 8% CO₂/15 % O₂. The absolute level of ventilation was however higher in hypercapnic and moderately hypoxic conditions than in hyperoxia. When a second injection of the highest dose of fentanyl (300 μg.kg^-1) was performed at 10 min, ventilation was not significantly affected and no apnea was produced in major contrast to the first injection. When a similar injection was performed 30 min after the first injection, in a separate group of rats, an apnea and breathing depression was produced in 30 % of the animals, while in the other rats, ventilation was unaffected. We conclude that the depressed regular respiratory activity emerging during and following fentanyl overdose is uniquely resistant to fentanyl.

The central role of protein kinase C epsilon in cyanide cardiotoxicity and its treatment

In adult mouse myocytes, brief exposure to sodium cyanide (CN) in the presence of glucose does not decrease ATP levels, yet produces profound reduction in contractility, intracellular Ca2+ concentration ([Ca2+]i) transient and L-type Ca2+ current (ICa) amplitudes. We analyzed proteomes from myocytes exposed to CN, focusing on ionic currents associated with excitation-contraction coupling. CN induced phosphorylation of α1c subunit of L-type Ca2+ channel and α2 subunit of Na+-K+-ATPase. Methylene blue (MB), a CN antidote that we previously reported to ameliorate CN-induced reduction in contraction, [Ca2+]i transient and ICa amplitudes, was able to reverse this phosphorylation. CN decreased Na+-K+-ATPase current contributed by α2 but not α1 subunit, an effect that was also counteracted by MB. Peptide consensus sequences suggested CN-induced phosphorylation was mediated by protein kinase C epsilon (PKCε). Indeed, CN stimulated PKC kinase activity and induced PKCε membrane translocation, effects that were prevented by MB. Pre-treatment with myristoylated PKCε translocation activator or inhibitor peptides mimicked and inhibited the effects of CN on ICa and myocyte contraction, respectively. We conclude that CN activates PKCε, which phosphorylates L-type Ca2+ channel and Na+-K+-ATPase, resulting in depressed cardiac contractility. We hypothesize that this inhibition of ion fluxes represents a novel mechanism by which the cardiomyocyte reduces its ATP demand (decreased ion fluxes and contractility), diminishes ATP turnover and preserves cell viability. However, this cellular protective effect translates into life-threatening cardiogenic shock in vivo, thereby creating a profound disconnect between survival mechanisms at the cardiomyocyte level from those at the level of the whole organism.

Hydrogen sulfide intoxication induced brain injury and methylene blue

Hydrogen sulfide (H₂S) remains a chemical hazard in the gas and farming industry. It is easy to manufacture from common chemicals and thus represents a potential threat for the civilian population. It is also employed as a method of suicide, for which incidence has recently increased in the US. H₂S is a mitochondrial poison and exerts its toxicity through mechanisms that are thought to result from its high affinity to various metallo-proteins (such as - but not exclusively- the mitochondrial cytochrome c oxidase) and interactions with cysteine residues of proteins. Ion channels with critical implications for the cardiac and the brain functions appear to be affected very early during and following H₂S exposure, an effect which is rapidly reversible during a light intoxication. However, during severe H₂S intoxication, a coma, associated with a reduction in cardiac contractility, develops within minutes or even seconds leading to death by complete electro-mechanical dissociation of the heart. If the level of intoxication is milder, a rapid and spontaneous recovery of the coma occurs as soon as the exposure stops. The risk, although probably very small, of developing long-term debilitating motor or cognitive deficits is present. One of the major challenges impeding our effort to offer an effective treatment against H₂S intoxication after exposure is that the pool of free/soluble H₂S almost immediately disappears from the body preventing agents trapping free H₂S (cobalt or ferric compounds) to play their protective role. This paper (1) presents and discusses the neurological symptoms and lesions observed in various animals models and in humans following an acute exposure to sub-lethal or lethal levels of H₂S, (2) reviews the potential interest of methylene blue (MB), a potent cyclic redox dye - currently used for the treatment of methemoglobinemia - which has potential rescuing effects on the mitochondrial activity, as an antidote against sulfide intoxication.

Hydrogen Sulfide Toxicity: Mechanism of Action, Clinical Presentation, and Countermeasure Development

INTRODUCTION

Hydrogen sulfide (H₂S) is found in various settings. Reports of chemical suicide, where individuals have combined readily available household chemicals to produce lethal concentrations of H₂S, have demonstrated that H₂S is easily produced. Governmental agencies have warned of potential threats of use of H₂S for a chemical attack, but currently there are no FDA-approved antidotes for H₂S. An ideal antidote would be one that is effective in small volume, readily available, safe, and chemically stable. In this paper we performed a review of the available literature on the mechanism of toxicity, clinical presentation, and development of countermeasures for H₂S toxicity.

DISCUSSION

In vivo, H₂S undergoes an incomplete oxidation after an exposure. The remaining non-oxidized H₂S is found in dissolved and combined forms. Dissolved forms such as H₂S gas and sulfhydryl anion can diffuse between blood and tissue. The combined non-soluble forms are found as acid-labile sulfides and sulfhydrated proteins, which play a role in toxicity. Recent countermeasure development takes into account the toxicokinetics of H₂S. Some countermeasures focus on binding free hydrogen sulfide (hydroxocobalamin, cobinamide); some have direct effects on the mitochondria (methylene blue), while others work by mitigating end organ damage by generating other substances such as nitric oxide (NaNO2).

CONCLUSION

H₂S exists in two main pools in vivo after exposure. While several countermeasures are being studied for H₂S intoxication, a need exists for a small-volume, safe, highly effective antidote with a long shelf life to treat acute toxicity as well as prevent long-term effects of exposure.

Methylene Blue Administration During and After Life-Threatening Intoxication by Hydrogen Sulfide: Efficacy Studies in Adult Sheep and Mechanisms of Action

Exposure to toxic levels of hydrogen sulfide (H2S) produces an acute cardiac depression that can be rapidly fatal. We sought to characterize the time course of the cardiac effects produced by the toxicity of H2S in sheep, a human sized mammal, and to describe the in vivo and in vitro antidotal properties of methylene blue (MB), which has shown efficacy in sulfide intoxicated rats. Infusing NaHS (720 mg) in anesthetized adult sheep produced a rapid dilation of the left ventricular with a decrease in contractility, which was lethal within about 10 min by pulseless electrical activity. MB (7 mg/kg), administered during sulfide exposure, maintained cardiac contractility and allowed all of the treated animals to recover. At a dose of 350 mg NaHS, we were able to produce an intoxication, which led to a persistent decrease in ventricular function for at least 1 h in nontreated animals. Administration of MB, 3 or 30 min after the end of exposure, whereas all free H2S had already vanished, restored cardiac contractility and the pyruvate/lactate (P/L) ratio. We found that MB exerts its antidotal effects through at least 4 different mechanisms: (1) a direct oxidation of free sulfide; (2) an increase in the pool of "trapped" H2S in red cells; (3) a restoration of the mitochondrial substrate-level phosphorylation; and (4) a rescue of the mitochondrial electron chain. In conclusion, H2S intoxication produces acute and long persisting alteration in cardiac function in large mammals even after all free H2S has vanished. MB exerts its antidotal effects against life-threatening sulfide intoxication via multifarious properties, some of them unrelated to any direct interaction with free H2S.

Methylene Blue Counteracts H2S-Induced Cardiac Ion Channel Dysfunction and ATP Reduction

We have previously demonstrated that methylene blue (MB) counteracts the effects of hydrogen sulfide (H₂S) cardiotoxicity by improving cardiomyocyte contractility and intracellular Ca²⁺ homeostasis disrupted by H₂S poisoning. In vivo, MB restores cardiac contractility severely depressed by sulfide and protects against arrhythmias, ranging from bundle branch block to ventricular tachycardia or fibrillation. To dissect the cellular mechanisms by which MB reduces arrhythmogenesis and improves bioenergetics in myocytes intoxicated with H₂S, we evaluated the effects of H₂S on resting membrane potential (E_m), action potential (AP), Na⁺/Ca²⁺ exchange current (I_NaCa), depolarization-activated K⁺ currents and ATP levels in adult mouse cardiac myocytes and determined whether MB could counteract the toxic effects of H₂S on myocyte electrophysiology and ATP. Exposure to toxic concentrations of H₂S (100 µM) significantly depolarized E_m, reduced AP amplitude, prolonged AP duration at 90% repolarization (APD₉₀), suppressed I_NaCa and depolarization-activated K⁺ currents, and reduced ATP levels in adult mouse cardiac myocytes. Treating cardiomyocytes with MB (20 µg/ml) 3 min after H₂S exposure restored E_m, APD₉₀, I_NaCa, depolarization-activated K⁺ currents, and ATP levels toward normal. MB improved mitochondrial membrane potential (∆ψ_m) and oxygen consumption rate in myocytes in which Complex I was blocked by rotenone. We conclude that MB ameliorated H₂S-induced cardiomyocyte toxicity at multiple levels: (1) reversing excitation-contraction coupling defects (Ca²⁺ homeostasis and L-type Ca²⁺ channels); (2) reducing risks of arrhythmias (E_m, APD, I_NaCa and depolarization-activated K⁺ currents); and (3) improving cellular bioenergetics (ATP, ∆ψ_m).

Intramuscular cobinamide versus saline for treatment of severe hydrogen sulfide toxicity in swine

INTRODUCTION

Hydrogen sulfide (H₂S) is found in petroleum, natural gas, and decaying organic matter. Terrorist groups have attempted to use it in enclosed spaces as a chemical weapon. Mass casualty scenarios have occurred from industrial accidents and release from oil field sites. There is no FDA approved antidote for sulfide poisoning. We have previously reported that intravenous cobinamide is effective for sulfide poisoning. A rapid-acting antidote that is easy to administer intramuscularly (IM) would be ideal for use in a prehospital setting. In this study, we assessed survival in sulfide-poisoned swine treated with IM cobinamide.

METHODS

Eleven swine (45-55 kg) were anesthetized, intubated, and instrumented with continuous femoral and pulmonary artery pressure monitoring. After stabilization, anesthesia was adjusted such that animals ventilated spontaneously with a FiO₂ of 0.21. Sodium hydrosulfide (NaHS, 8 mg/mL) was infused intravenously at 0.9 mg/kg.min until apnea or severe hypotension. Animals were randomly assigned to receive cobinamide (4 mg/kg), or no treatment at the apnea/hypotension trigger. The NaHS infusion rate was sustained for 1.5 min post trigger, decreased to 0.2 mg/kg.min for 10 min, and then discontinued.

RESULTS

The amount of NaHS required to produce apnea or hypotension was not statistically different in both groups (cobinamide: 9.0 mg/kg ±6.1; saline: 5.9 mg/kg ±5.5; mean difference: -3.1, 95% CI: -11.3, 5.0). All of the cobinamide treated animals survived (5/5), none of the control (0/6) animals survived (p < .01). Mean time to return to spontaneous ventilation in the cobinamide treated animals was 3.2 (±1.1) min. Time to return to baseline systolic blood pressure (±5%) in cobinamide-treated animals was 5 min.

CONCLUSION

Intramuscular cobinamide was effective in improving survival in this large swine model of severe hydrogen sulfide toxicity.

Methylene blue counteracts cyanide cardiotoxicity: cellular mechanisms

In adult left ventricular mouse myocytes, exposure to sodium cyanide (NaCN) in the presence of glucose dose-dependently reduced contraction amplitude, with ~80% of maximal inhibitory effect attained at 100 µM. NaCN (100 µM) exposure for 10 min significantly decreased contraction and intracellular Ca2+ concentration ([Ca2+]i) transient amplitudes, systolic but not diastolic [Ca2+]i, and maximal L-type Ca2+ current ( ICa) amplitude, indicating acute alteration of [Ca2+]i homeostasis largely accounted for the observed excitation-contraction abnormalities. In addition, NaCN depolarized resting membrane potential ( Em), reduced action potential (AP) amplitude, prolonged AP duration at 50% (APD50) and 90% repolarization (APD90), and suppressed depolarization-activated K+ currents but had no effect on Na+-Ca2+ exchange current ( INaCa). NaCN did not affect cellular adenosine triphosphate levels but depolarized mitochondrial membrane potential (ΔΨm) and increased superoxide (O2·-) levels. Methylene blue (MB; 20 µg/ml) added 3 min after NaCN restored contraction and [Ca2+]i transient amplitudes, systolic [Ca2+]i, Em, AP amplitude, APD50, APD90, ICa, depolarization-activated K+ currents, ΔΨm, and O2·- levels toward normal. We conclude that MB reversed NaCN-induced cardiotoxicity by preserving intracellular Ca2+ homeostasis and excitation-contraction coupling ( ICa), minimizing risks of arrhythmias ( Em, AP configuration, and depolarization-activated K+ currents), and reducing O2·- levels. NEW & NOTEWORTHY Cyanide poisoning due to industrial exposure, smoke inhalation, and bioterrorism manifests as cardiogenic shock and requires rapidly effective antidote. In the early stage of cyanide exposure, adenosine triphosphate levels are normal but myocyte contractility is reduced, largely due to alterations in Ca2+ homeostasis because of changes in oxidation-reduction environment of ion channels. Methylene blue, a drug approved by the U.S. Food and Drug Administration, ameliorates cyanide toxicity by normalizing oxidation-reduction state and Ca2+ channel function.

Revisiting the physiological effects of methylene blue as a treatment of cyanide intoxication

BACKGROUND

Although methylene blue (MB) had long been proposed to counteract the effects of cyanide (CN) intoxication, research on its mechanisms of action and efficacy has been abandoned for decades. Recent studies on the benefits of MB in post-anoxic injuries have prompted us to reexamine the relevance of this historical observation.

METHODS

Our study was performed in adult male Sprague-Dawley rats and on HEK293T epithelial cells. First, the effects and toxicity of MB (0-80 mg/kg) on circulation and metabolism were established in four urethane-anesthetized rats. Then nine rats received a lethal infusion of a solution of KCN (0.75 mg/kg/min) and were treated by either saline or MB, at 20 mg/kg, a dose that we found to be innocuous in rat and to correspond to a dose of about 4 mg/kg in humans. MB was also administered 5 min after the end of a sub-lethal exposure to CN in a separate group of 10 rats. In addition, ATP/ADP ratio, ROS production, mitochondrial membrane potential (Δψm) and cellular O₂ consumption rate (OCR) were determined in HEK293T cells exposed to toxic levels of CN (200 µM for 10 min) before and after applying a solution containing MB (1-100 µM for 10 min).

RESULTS

Methylene blue was found to be innocuous up to 50 mg/kg. KCN infusion (0.75 mg/kg/min) killed all animals within 7-8 min. MB (20 mg/kg) administered at the same time restored blood pressure, cardiac contractility and limited O₂ deficit, allowing all the animals to survive, without any significant methemoglobinemia. When administered 5 min after a non-lethal CN intoxication, MB sped up the recovery of lactate and O₂ deficit. Finally, MB was able to decrease the production of ROS and restore the ATP/ADP ratio, Δψm as well as OCR of epithelial cells intoxicated by CN.

CONCLUSIONS

The present observations should make us consider the potential interest of MB in the treatment of CN intoxication. The mechanisms of the antidotal properties of MB cannot be accounted for by the creation of a cyanomethemoglobinemia, rather its protective effects appears to be related to the unique properties of this redox dye, which, depending on the dose, could directly oppose some of the consequences of the metabolic depression produced by CN at the cellular level.

Hydrogen Sulfide Specifically Alters NAD(P)H Quinone Dehydrogenase 1 (NQO1) Olfactory Neurons in the Rat

The regions of the olfactory epithelium affected by hydrogen sulfide (H₂S) toxicity in the rat present a striking similarity with the developmental olfactory zone 1 described in the mouse. This zone which is the only region containing neurons expressing NAD(P)H quinone dehydrogenase 1 (NQO1) is involved in complex behavioral responses in rodents, and other mammals, triggered by specific olfactory stimuli. We therefore sought to determine whether (1) olfactory neurons expressing NQO1 are located in the same regions in the rats and in the mice and (2) there is an overlap between olfactory neurons expressing this protein and those affected by the toxicity of H₂S. Rats were exposed to H₂S - 200 ppm during 3 h, three consecutive days- and displayed symmetric acute segmental necrosis of the neurons and sustentacular cells of the olfactory epithelium in the dorsomedial nasal cavity. We found that expression of NQO1 in Sprague-Dawley rats spatially recapitulated that of the mouse. The degree of agreement or overlap between these two populations of neurons (necrosis vs. NQO1 expression) reached 80.2%. Although the underlying mechanisms accounted for the high sensitivity for NQO1 neurons -or the relative protection of non NQO1 neurons- to sulfide toxicity remain to be established, this observation is offering an intriguing approach that could be used to acutely suppress the pool of neural cells in olfactory zone I and to understand the mechanisms of toxicity and protection of other populations of neurons exposed to sulfide.

Circulatory Failure During Noninhaled Forms of Cyanide Intoxication

Our objective was to determine how circulatory failure develops following systemic administration of potassium cyanide (KCN). We used a noninhaled modality of intoxication, wherein the change in breathing pattern would not influence the diffusion of CN into the blood, akin to the effects of ingesting toxic levels of CN. In a group of 300 to 400 g rats, CN-induced coma (CN i.p., 7 mg/kg) produced a central apnea within 2 to 3 min along with a potent and prolonged gasping pattern leading to autoresuscitation in 38% of the animals. Motor deficits and neuronal necrosis were nevertheless observed in the surviving animals. To clarify the mechanisms leading to potential autoresuscitation versus asystole, 12 urethane-anesthetized rats were then exposed to the lowest possible levels of CN exposure that would lead to breathing depression within 7 to 8 min; this dose averaged 0.375 mg/kg/min i.v. At this level of intoxication, a cardiac depression developed several minutes only after the onset of the apnea, leading to cardiac asystole as PaO2 reached value approximately 15 Torr, unless breathing was maintained by mechanical ventilation or through spontaneous gasping. Higher levels of KCN exposure in 10 animals provoked a primary cardiac depression, which led to a rapid cardiac arrest by pulseless electrical activity (PEA) despite the maintenance of PaO2 by mechanical ventilation. These effects were totally unrelated to the potassium contained in KCN. It is concluded that circulatory failure can develop as a direct consequence of CN-induced apnea but in a narrow range of exposure. In this "low" range, maintaining pulmonary gas exchange after exposure, through mechanical ventilation (or spontaneous gasping), can reverse cardiac depression and restore spontaneous breathing. At higher level of intoxication, cardiac depression is to be treated as a specific and spontaneously irreversible consequence of CN exposure, leading to a PEA.

Sulfide Intoxication-Induced Circulatory Failure is Mediated by a Depression in Cardiac Contractility

Hydrogen sulfide (H2S) intoxication produces a rapid cardio-circulatory failure leading to cardiac arrest. In non-lethal forms of sulfide exposure, the presence of a circulatory shock is associated with long-term neurological sequelae. Our aim was to clarify the mechanisms of H2S-induced circulatory failure. In anesthetized, paralyzed, and mechanically ventilated rats, cardiac output, arterial pressure and ventricular pressures were determined while NaHS was infused to increase arterial concentration of soluble H2S (CgH2S) from undetectable to levels leading to circulatory failure. Compared to control/saline infusion, blood pressure started to decrease significantly along with a modest drop in peripheral vascular resistance (-19 ± 5%, P < 0.01), when CgH2S reached about 1 μM. As CgH2S exceeded 2-3 μM, parameters of ventricular contractility diminished with no further reduction in peripheral resistance. Whenever H2S exposure was maintained at a higher level (CgH2S over 7 μM), a severe depression of cardiac contractility was observed, leading to asystole within minutes, but with no evidence of peripheral vasoplegia. The immediate and long-term neurological effects of specifically counteracting sulfide-induced cardiac contractility depression following H2S exposure remain to be investigated.

H2S concentrations in the heart after acute H2S administration: methodological and physiological considerations

In this study, we have tried to characterize the limits of the approach typically used to determine H₂S concentrations in the heart based on the amount of H₂S evaporating from heart homogenates-spontaneously, after reaction with a strong reducing agent, or in a very acidic solution. Heart homogenates were prepared from male rats in control conditions or after H₂S infusion induced a transient cardiogenic shock (CS) or cardiac asystole (CA). Using a method of determination of gaseous H₂S with a detection limit of 0.2 nmol, we found that the process of homogenization could lead to a total disappearance of free H₂S unless performed in alkaline conditions. Yet, after restoration of neutral pH, free H₂S concentration from samples processed in alkaline and nonalkaline milieus were similar and averaged ∼0.2-0.4 nmol/g in both control and CS homogenate hearts and up to 100 nmol/g in the CA group. No additional H₂S was released from control, CS, or CA hearts by using the reducing agent tris(2-carboxyethyl)phosphine or a strong acidic solution (pH < 2) to "free" H₂S from combined pools. Of note, the reducing agent DTT produced a significant sulfide artifact and was not used. These data suggest that 1) free H₂S found in heart homogenates is not a reflection of H₂S present in a "living" heart and 2) the pool of combined sulfides, released in a strong reducing or acidic milieu, does not increase in the heart in a measurable manner even after toxic exposure to sulfide.

On the inaccuracy of breath-by-breath metabolic gas exchange systems

This technical note is presenting and discussing a severe limitation of the breath-by-breath (BBB) determination of pulmonary gas exchange routinely used as a surrogate for the metabolic gas exchange rate at rest and during exercise. We are presenting the view that continuous airway gas sampling at the mouth used for the determination of O₂ and CO₂ content is inaccurate in the range of a low-to-medium expiratory flows, due to the discrepancy between the cross section surfaces of the sampling line and the expiratory tube (mask or mouth piece). This difference results in the sampling of a mixed exhaled gas at low expiratory flow, for which any temporal relationship between the instantaneous expired CO2 and O₂ signals and instantaneous expiratory flow is lost. Further analysis of this mechanism points to the difference between the exhalation (ve) and sampling (vs) gas velocities, where the ratio of ve/vs must be equal or higher than 1 to enable proper analysis of the respiratory gas concentrations during exhalation. Moreover, the above requirement is particularly crucial in monitoring the tidal concentration of the respiratory gases in experiments with small animals.

Is exogenous hydrogen sulfide a relevant tool to address physiological questions on hydrogen sulfide?

This review challenges the use of solutions of dissolved exogenous H2S in the literature as a tool to determine the potential physiological functions of endogenous H2S as well as its putative therapeutic applications. Our major point of contention is that solutions of dissolved H2S are used in vitro at concentrations, within the high microM range, which are above the concentrations of dissolved H2S found in blood and tissues during lethal H2S exposure in vivo. In addition, since the levels of toxicity are extremely variable among cell types, a property that is seldom acknowledged, the physiological relevance of data obtained after local or in-vitro administrations of H2S at concentrations of few microM is far from certain. Conversely, the rate of disappearance of the dissolved pool of H2S in the body (being trapped or oxidized), which we found to be at least of several micromoles/kg/min, is so rapid in vivo that if relatively low quantities of H2S, i.e. few micromoles for instance, are administered, no change in H2S concentrations in the body is to be expected, unless toxic levels are used. Protocols looking at the effects of compounds slowly releasing H2S must also resolve a similar conundrum, as their effects must be reconciled with the unique ability of the blood and tissues to get rid of H2S and the steepness of the dose-toxic effects relationship. Only by developing a comprehensive framework in which H2S metabolism and toxicity will be used as a rationale to justify any experimental approach will we be able to bring definitive evidence supporting a protective role for exogenous H2S, if any, and its putative function as an endogenous mediator.

Methylene blue counteracts H2S toxicity-induced cardiac depression by restoring L-type Ca channel activity

We have previously reported that methylene blue (MB) can counteract hydrogen sulfide (H2S) intoxication-induced circulatory failure. Because of the multifarious effects of high concentrations of H2S on cardiac function, as well as the numerous properties of MB, the nature of this interaction, if any, remains uncertain. The aim of this study was to clarify 1) the effects of MB on H2S-induced cardiac toxicity and 2) whether L-type Ca(2+) channels, one of the targets of H2S, could transduce some of the counteracting effects of MB. In sedated rats, H2S infused at a rate that would be lethal within 5 min (24 μM·kg(-1)·min(-1)), produced a rapid fall in left ventricle ejection fraction, determined by echocardiography, leading to a pulseless electrical activity. Blood concentrations of gaseous H2S reached 7.09 ± 3.53 μM when cardiac contractility started to decrease. Two to three injections of MB (4 mg/kg) transiently restored cardiac contractility, blood pressure, and V̇o2, allowing the animals to stay alive until the end of H2S infusion. MB also delayed PEA by several minutes following H2S-induced coma and shock in unsedated rats. Applying a solution containing lethal levels of H2S (100 μM) on isolated mouse cardiomyocytes significantly reduced cell contractility, intracellular calcium concentration ([Ca(2+)]i) transient amplitudes, and L-type Ca(2+) currents (ICa) within 3 min of exposure. MB (20 mg/l) restored the cardiomyocyte function, ([Ca(2+)]i) transient, and ICa The present results offer a new approach for counteracting H2S toxicity and potentially other conditions associated with acute inhibition of L-type Ca(2+) channels.

Developing effective countermeasures against acute hydrogen sulfide intoxication: challenges and limitations

Hydrogen sulfide (H2 S) is a chemical hazard in the gas and farming industry. As it is easy to manufacture from common chemicals, it has also become a method of suicide. H2 S exerts its toxicity through its high affinity with metalloproteins, such as cytochrome c oxidase and possibly via its interactions with cysteine residues of various proteins. The latter was recently proposed to acutely alter ion channels with critical implications for cardiac and brain functions. Indeed, during severe H2 S intoxication, a coma, associated with a reduction in cardiac contractility, develops within minutes or even seconds leading to death by complete electromechanical dissociation of the heart. In addition, long-term neurological deficits can develop owing to the direct toxicity of H2 S on neurons combined with the consequences of a prolonged apnea and circulatory failure. Here, we review the challenges impeding efforts to offer an effective treatment against H2 S intoxication using agents that trap free H2 S, and present novel pharmacological approaches aimed at correcting some of the most harmful consequences of H2 S intoxication.

Immediate and Long-Term Outcome of Acute H2S Intoxication Induced Coma in Unanesthetized Rats: Effects of Methylene Blue

BACKGROUND

Acute hydrogen sulfide (H2S) poisoning produces a coma, the outcome of which ranges from full recovery to severe neurological deficits. The aim of our study was to 1--describe the immediate and long-term neurological effects following H2S-induced coma in un-anesthetized rats, and 2--determine the potential benefit of methylene blue (MB), a compound we previously found to counteract acute sulfide cardiac toxicity.

METHODS

NaHS was administered IP in un-sedated rats to produce a coma (n = 34). One minute into coma, the rats received MB (4 mg/kg i.v.) or saline. The surviving rats were followed clinically and assigned to Morris water maze (MWM) and open field testing then sacrificed at day 7.

RESULTS

Sixty percent of the non-treated comatose rats died by pulseless electrical activity. Nine percent recovered with neurological deficits requiring euthanasia, their brain examination revealed major neuronal necrosis of the superficial and middle layers of the cerebral cortex and the posterior thalamus, with variable necrosis of the caudate putamen, but no lesions of the hippocampus or the cerebellum, in contrast to the typical distribution of post-ischemic lesions. The remaining animals displayed, on average, a significantly less effective search strategy than the control rats (n = 21) during MWM testing. Meanwhile, 75% of rats that received MB survived and could perform the MWM test (P<0.05 vs non-treated animals). The treated animals displayed a significantly higher occurrence of spatial search than the non-treated animals. However, a similar proportion of cortical necrosis was observed in both groups, with a milder clinical presentation following MB.

CONCLUSION

In conclusion, in rats surviving H2S induced coma, spatial search patterns were used less frequently than in control animals. A small percentage of rats presented necrotic neuronal lesions, which distribution differed from post-ischemic lesions. MB dramatically improved the immediate survival and spatial search strategy in the surviving rats.

H2S induced coma and cardiogenic shock in the rat: Effects of phenothiazinium chromophores

CONTEXT

Hydrogen sulfide (H2S) intoxication produces an acute depression in cardiac contractility-induced circulatory failure, which has been shown to be one of the major contributors to the lethality of H2S intoxication or to the neurological sequelae in surviving animals. Methylene blue (MB), a phenothiazinium dye, can antagonize the effects of the inhibition of mitochondrial electron transport chain, a major effect of H2S toxicity.

OBJECTIVES

We investigated whether MB could affect the immediate outcome of H2S-induced coma in un-anesthetized animals. Second, we sought to characterize the acute cardiovascular effects of MB and two of its demethylated metabolites-azure B and thionine-in anesthetized rats during lethal infusion of H2S.

MATERIALS AND METHODS

First, MB (4 mg/kg, intravenous [IV]) was administered in non-sedated rats during the phase of agonal breathing, following NaHS (20 mg/kg, IP)-induced coma. Second, in 4 groups of urethane-anesthetized rats, NaHS was infused at a rate lethal within 10 min (0.8 mg/min, IV). Whenever cardiac output (CO) reached 40% of its baseline volume, MB, azure B, thionine, or saline were injected, while sulfide infusion was maintained until cardiac arrest occurred.

RESULTS

Seventy-five percent of the comatose rats that received saline (n = 8) died within 7 min, while all the 7 rats that were given MB survived (p = 0.007). In the anesthetized rats, arterial, left ventricular pressures and CO decreased during NaHS infusion, leading to a pulseless electrical activity within 530 s. MB produced a significant increase in CO and dP/dtmax for about 2 min. A similar effect was produced when MB was also injected in the pre-mortem phase of sulfide exposure, significantly increasing survival time. Azure B produced an even larger increase in blood pressure than MB, while thionine had no effect.

CONCLUSION

MB can counteract NaHS-induced acute cardiogenic shock; this effect is also produced by azure B, but not by thionine, suggesting that the presence of methyl groups is a prerequisite for producing this protective effect.

Persistent reduced oxygen requirement following blood transfusion during recovery from hemorrhagic shock

Our study intended to determine the effects on oxygen uptake (VO2) of restoring a normal rate of O2 delivery following blood transfusion (BT) after a severe hemorrhage (H). Spontaneously breathing urethane anesthetized rats were bled by removing 20 ml/kg of blood over 30 min. Rats were then infused with their own shed blood 15 min after the end of H. At mid-perfusion, half of the rats received a unique infusion of the decoupling agent 2,4-dinitrophenol (DNP, 6 mg/kg). VO2 and arterial blood pressure (ABP) were continuously measured throughout the study, along with serial determination of blood lactate concentration [La]. Animals were euthanized 45 min after the end of reperfusion; liver and lungs were further analyzed for early expression of oxidative stress gene using RT-PCR. Our bleeding protocol induced a significant decrease in ABP and increase in [La], while VO2 dropped by half. The O2 deficit progressively accumulated during the period of bleeding reached -114 ± 53 ml/kg, just before blood transfusion. Despite the transfusion of blood, a significant O2 deficit persisted (-82 ± 59 ml/kg) 45 min after reperfusion. This slow recovery of VO2 was sped up by DNP injection, leading to a fast recovery of O2 deficit after reperfusion, becoming positive (+460 ± 132 ml/kg) by the end of the protocol, supporting the view that O2 supply is not the main controller of VO2 dynamics after BT. Of note is that DNP also enhanced oxidative stress gene expression (up-regulation of NADPH oxidase 4 in the lung for instance). The mechanism of slow recovery of O2 requirement/demand following BT and the resulting effects on tissues exposed to relatively high O2 partial pressure are discussed.

High-dose hydroxocobalamin administered after H2S exposure counteracts sulfide-poisoning-induced cardiac depression in sheep

CONTEXT

Severe H2S poisoning leads to death by rapid respiratory and cardiac arrest, the latter can occur within seconds or minutes in severe forms of intoxication.

OBJECTIVES

To determine the time course and the nature of H2S-induced cardiac arrest and the effects of high-dose hydroxocobalamin administered after the end of sulfide exposure.

MATERIALS AND METHODS

NaHS was infused in 16 sedated mechanically ventilated sheep to reach concentrations of H2S in the blood, which was previously found to lead to cardiac arrest within minutes following the cessation of H2S exposure. High-dose hydroxocobalamin (5 g) or saline solution was administered intravenously, 1 min after the cessation of NaHS infusion.

RESULTS

All animals were still alive at the cessation of H2S exposure. Three animals (18%) presented a cardiac arrest within 90 s and were unable to receive any antidote or vehicle. In the animals that survived long enough to receive either hydroxocobalamin or saline, 71% (5/7) died in the control group by cardiac arrest within 10 min. In all instances, cardiac arrest was the result of a pulseless electrical activity (PEA). In the group that received the antidote, intravenous injection of 5 g of hydroxocobalamin provoked an abrupt increase in blood pressure and blood flow; PEA was prevented in all instances. However, we could not find any evidence for a recovery in oxidative metabolism in the group receiving hydroxocobalamin, as blood lactate remained elevated and even continued to rise after 1 h, despite restored hemodynamics. This, along with an unaltered recovery of H2S kinetics, suggests that hydroxocobalamin did not act through a mechanism of H2S trapping.

CONCLUSION

In this sheep model, there was a high risk for cardiac arrest, by PEA, persisting up to 10 min after H2S exposure. Very high dose of hydroxocobalamin (5 g), injected very early after the cessation of H2S exposure, improved cardiac contractility and prevented PEA.

Effects of infusion of human methemoglobin solution following hydrogen sulfide poisoning

RATIONALE

We have recently reported that infusion of a solution containing methemoglobin (MetHb) during exposure to hydrogen sulfide results in a rapid and large decrease in the concentration of the pool of soluble/diffusible H2S in the blood. However, since the pool of dissolved H2S disappears very quickly after H2S exposure, it is unclear if the ability of MetHb to "trap" sulfide in the blood has any clinical interest and relevance in the treatment of sulfide poisoning.

METHODS

In anesthetized rats, repetition of short bouts of high level of H2S infusions was applied to allow the rapid development of an oxygen deficit. A solution containing MetHb (600 mg/kg) or its vehicle was administered 1 min and a half after the end of H2S intoxication.

RESULTS

The injection of MetHb solution increased methemoglobinemia to about 6%, almost instantly, but was unable to affect the blood concentration of soluble H2S, which had already vanished at the time of infusion, or to increase combined H2S. In addition, H2S-induced O2 deficit and lactate production as well as the recovery of carotid blood flow and blood pressure were similar in treated and control animals.

CONCLUSION

Our results do not support the view that administration of MetHb or drugs-induced methemoglobinemia during the recovery phase following severe H2S intoxication in sedated rats can restore cellular oxidative metabolism, as the pool of diffusible sulfide, accessible to MetHb, disappears rapidly from the blood after H2S exposure.