Pulmonary vasodilation by acetazolamide during hypoxia is unrelated to carbonic anhydrase inhibition

Membrane Permeability Is Required for the Vasodilatory Effect of Carbonic Anhydrase Inhibitors in Porcine Retinal Arteries

It has been demonstrated previously that a variety of carbonic anhydrase inhibitors (CAIs) can induce vasodilation in pre-contracted retinal arteriolar segments although with different efficacy and potency. Since the CAIs tested so far are able to permeate cell membranes and inhibit both intracellular and extracellular isoforms of the enzyme, it is not clear whether extra- or intracellular isoforms or mechanisms are mediating their vasodilatory effects. By means of small wire myography, we have tested the effects of four new CAIs on wall tension in pre-contracted retinal arteriolar segments that demonstrably do not enter cell membranes but have high affinity to both cytosolic and membrane-bound isoforms of CA. At concentrations between 10^-6 M to 10^-3 M, none of the four membrane impermeant CAIs had any significant effect on arteriolar wall tension, while the membrane permeant CAI benzolamide (10^-3 M) fully dilated all arteriolar segments tested. This suggests that CAI act as vasodilators through cellular mechanisms located in the cytoplasm of vascular cells.

The Impact of Acetazolamide and Methazolamide on Exercise Performance in Normoxia and Hypoxia

Doherty, Connor J., Jou-Chung Chang, Benjamin P. Thompson, Erik R. Swenson, Glen E. Foster, and Paolo B. Dominelli. The impact of acetazolamide and methazolamide on exercise performance in normoxia and hypoxia. High Alt Med Biol. 24:7-18, 2023.-Carbonic anhydrase (CA) inhibitors are commonly prescribed for acute mountain sickness (AMS). In this review, we sought to examine how two CA inhibitors, acetazolamide (AZ) and methazolamide (MZ), affect exercise performance in normoxia and hypoxia. First, we briefly describe the role of CA inhibition in facilitating the increase in ventilation and arterial oxygenation in preventing and treating AMS. Next, we detail how AZ affects exercise performance in normoxia and hypoxia and this is followed by a discussion on MZ. We emphasize that the overarching focus of the review is how the two drugs potentially affect exercise performance, rather than their ability to prevent/treat AMS per se, their interrelationship will be discussed. Overall, we suggest that AZ hinders exercise performance in normoxia, but may be beneficial in hypoxia. Based upon head-to-head studies of AZ and MZ in humans on diaphragmatic and locomotor strength in normoxia, MZ may be a better CA inhibitor when exercise performance is crucial at high altitude.

Pathophysiology and Therapy of High-Altitude Sickness: Practical Approach in Emergency and Critical Care

High altitude can be a hostile environment and a paradigm of how environmental factors can determine illness when human biological adaptability is exceeded. This paper aims to provide a comprehensive review of high-altitude sickness, including its epidemiology, pathophysiology, and treatments. The first section of our work defines high altitude and considers the mechanisms of adaptation to it and the associated risk factors for low adaptability. The second section discusses the main high-altitude diseases, highlighting how environmental factors can lead to the loss of homeostasis, compromising important vital functions. Early recognition of clinical symptoms is important for the establishment of the correct therapy. The third section focuses on high-altitude pulmonary edema, which is one of the main high-altitude diseases. With a deeper understanding of the pathogenesis of high-altitude diseases, as well as a reasoned approach to environmental or physical factors, we examine the main high-altitude diseases. Such an approach is critical for the effective treatment of patients in a hostile environment, or treatment in the emergency room after exposure to extreme physical or environmental factors.

Effects of acetazolamide on pulmonary artery pressure and prevention of high altitude pulmonary edema after rapid active ascent to 4,559 m

Acetazolamide prevents acute mountain sickness (AMS) by inhibition of carbonic anhydrase. Since it reduces acute hypoxic pulmonary vasoconstriction (HPV), it may also prevent high-altitude pulmonary edema (HAPE) by lowering pulmonary artery pressure. We tested this hypothesis in a randomized, placebo-controlled, double-blind study. Thirteen healthy, non-acclimatized lowlanders with a history of HAPE ascended (<22h) from 1,130 to 4,559m with one overnight stay at 3,611m. Medications started 48h before ascent (acetazolamide: n=7, 250mg 3x/d; placebo: n=6, 3x/d). HAPE was diagnosed by chest radiography, and pulmonary artery pressure by measurement of right ventricular to atrial pressure gradient (RVPG) by transthoracic echocardiography. AMS was evaluated with the Lake Louise Score (LLS) and AMS-C Score. Incidence of HAPE was 43% vs. 67% (acetazolamide vs. placebo, p=0.39). Ascent to altitude increased RVPG from 20±5 to 43±10mmHg (p<0.001) without a group difference (p=0.68). Arterial PO₂ fell to 36±9mmHg (p<0.001) and was 8.5mmHg higher with acetazolamide at high altitude (p=0.025). At high altitude, the LLS and AMS-C score remained lower in those taking acetazolamide (both p<0.05). Although acetazolamide reduced HAPE incidence by 35%, this effect was not statistically significant, and considerably less than reductions of about 70-100% with prophylactic dexamethasone, tadalafil, and nifedipine performed with the same ascent profile at the same location. We could not demonstrate a reduction in RVPG compared to placebo treatment despite reductions in AMS severity and better arterial oxygenation. Limited by a small sample size, our data do not support recommending acetazolamide for prevention of HAPE in mountaineers ascending rapidly to over 4,500m.

Acetazolamide prevents hypoxia-induced reactive oxygen species generation and calcium release in pulmonary arterial smooth muscle

Upon sensing a reduction in local oxygen partial pressure, pulmonary vessels constrict, a phenomenon known as hypoxic pulmonary vasoconstriction. Excessive hypoxic pulmonary vasoconstriction can occur with ascent to high altitude and is a contributing factor to the development of high-altitude pulmonary edema. The carbonic anhydrase inhibitor, acetazolamide, attenuates hypoxic pulmonary vasoconstriction through stimulation of alveolar ventilation via modulation of acid-base homeostasis and by direct effects on pulmonary vascular smooth muscle. In pulmonary arterial smooth muscle cells (PASMCs), acetazolamide prevents hypoxia-induced increases in intracellular calcium concentration ([Ca²⁺]_i), although the exact mechanism by which this occurs is unknown. In this study, we explored the effect of acetazolamide on various calcium-handling pathways in PASMCs. Using fluorescent microscopy, we tested whether acetazolamide directly inhibited store-operated calcium entry or calcium release from the sarcoplasmic reticulum, two well-documented sources of hypoxia-induced increases in [Ca²⁺]_i in PASMCs. Acetazolamide had no effect on calcium entry stimulated by store-depletion, nor on calcium release from the sarcoplasmic reticulum induced by either phenylephrine to activate inositol triphosphate receptors or caffeine to activate ryanodine receptors. In contrast, acetazolamide completely prevented Ca²⁺-release from the sarcoplasmic reticulum induced by hypoxia (4% O₂). Since these results suggest the acetazolamide interferes with a mechanism upstream of the inositol triphosphate and ryanodine receptors, we also determined whether acetazolamide might prevent hypoxia-induced changes in reactive oxygen species production. Using roGFP, a ratiometric reactive oxygen species-sensitive fluorescent probe, we found that hypoxia caused a significant increase in reactive oxygen species in PASMCs that was prevented by 100 μM acetazolamide. Together, these results suggest that acetazolamide prevents hypoxia-induced changes in [Ca²⁺]_i by attenuating reactive oxygen species production and subsequent activation of Ca²⁺-release from sarcoplasmic reticulum stores.

Ion Channels, Transporters, and Sensors Interact with the Acidic Tumor Microenvironment to Modify Cancer Progression

Solid tumors, including breast carcinomas, are heterogeneous but typically characterized by elevated cellular turnover and metabolism, diffusion limitations based on the complex tumor architecture, and abnormal intra- and extracellular ion compositions particularly as regards acid-base equivalents. Carcinogenesis-related alterations in expression and function of ion channels and transporters, cellular energy levels, and organellar H⁺ sequestration further modify the acid-base composition within tumors and influence cancer cell functions, including cell proliferation, migration, and survival. Cancer cells defend their cytosolic pH and HCO₃^- concentrations better than normal cells when challenged with the marked deviations in extracellular H⁺, HCO₃^-, and lactate concentrations typical of the tumor microenvironment. Ionic gradients determine the driving forces for ion transporters and channels and influence the membrane potential. Cancer and stromal cells also sense abnormal ion concentrations via intra- and extracellular receptors that modify cancer progression and prognosis. With emphasis on breast cancer, the current review first addresses the altered ion composition and the changes in expression and functional activity of ion channels and transporters in solid cancer tissue. It then discusses how ion channels, transporters, and cellular sensors under influence of the acidic tumor microenvironment shape cancer development and progression and affect the potential of cancer therapies.

Carbonic anhydrase inhibitors suppress seizures in a rat model of birth asphyxia

OBJECTIVE

Seizures are common in neonates recovering from birth asphyxia but there is general consensus that current pharmacotherapy is suboptimal and that novel antiseizure drugs are needed. We recently showed in a rat model of birth asphyxia that seizures are triggered by the post-asphyxia recovery of brain pH. Here our aim was to investigate whether carbonic anhydrase inhibitors (CAIs), which induce systemic acidosis, block the post-asphyxia seizures.

METHODS

The CAIs acetazolamide (AZA), benzolamide (BZA), and ethoxzolamide (EZA) were administered intraperitoneally or intravenously to 11-day-old rats exposed to intermittent asphyxia (30 min; three 7+3 min cycles of 9% and 5% O₂ at 20% CO₂ ). Electrode measurements of intracortical pH, Po₂ , and local field potentials (LFPs) were made under urethane anesthesia. Convulsive seizures and blood acid-base parameters were examined in freely behaving animals.

RESULTS

The three CAIs decreased brain pH by 0.14-0.17 pH units and suppressed electrographic post-asphyxia seizures. AZA, BZA, and EZA differ greatly in their lipid solubility (EZA > AZA > BZA) and pharmacokinetics. However, there were only minor differences in the delay (range 0.8-3.7 min) from intraperitoneal application to their action on brain pH. The CAIs induced a modest post-asphyxia elevation of brain Po₂ that had no effect on LFP activity. AZA was tested in freely behaving rats, in which it induced a respiratory acidosis and decreased the incidence of convulsive seizures from 9 of 20 to 2 of 17 animals.

SIGNIFICANCE

AZA, BZA, and EZA effectively block post-asphyxia seizures. Despite the differences in their pharmacokinetics, they had similar effects on brain pH, which indicates that their antiseizure mode of action was based on respiratory (hypercapnic) acidosis resulting from inhibition of blood-borne and extracellular vascular carbonic anhydrases. AZA has been used for several indications in neonates, suggesting that it can be safely repurposed for the treatment of neonatal seizures as an add-on to the current treatment regimen.

Pulmonary Hypertension in Acute and Chronic High Altitude Maladaptation Disorders

Alveolar hypoxia is the most prominent feature of high altitude environment with well-known consequences for the cardio-pulmonary system, including development of pulmonary hypertension. Pulmonary hypertension due to an exaggerated hypoxic pulmonary vasoconstriction contributes to high altitude pulmonary edema (HAPE), a life-threatening disorder, occurring at high altitudes in non-acclimatized healthy individuals. Despite a strong physiologic rationale for using vasodilators for prevention and treatment of HAPE, no systematic studies of their efficacy have been conducted to date. Calcium-channel blockers are currently recommended for drug prophylaxis in high-risk individuals with a clear history of recurrent HAPE based on the extensive clinical experience with nifedipine in HAPE prevention in susceptible individuals. Chronic exposure to hypoxia induces pulmonary vascular remodeling and development of pulmonary hypertension, which places an increased pressure load on the right ventricle leading to right heart failure. Further, pulmonary hypertension along with excessive erythrocytosis may complicate chronic mountain sickness, another high altitude maladaptation disorder. Importantly, other causes than hypoxia may potentially underlie and/or contribute to pulmonary hypertension at high altitude, such as chronic heart and lung diseases, thrombotic or embolic diseases. Extensive clinical experience with drugs in patients with pulmonary arterial hypertension suggests their potential for treatment of high altitude pulmonary hypertension. Small studies have demonstrated their efficacy in reducing pulmonary artery pressure in high altitude residents. However, no drugs have been approved to date for the therapy of chronic high altitude pulmonary hypertension. This work provides a literature review on the role of pulmonary hypertension in the pathogenesis of acute and chronic high altitude maladaptation disorders and summarizes current knowledge regarding potential treatment options.

Extrinsic acidosis suppresses glycolysis and migration while increasing network formation in pulmonary microvascular endothelial cells

Acidosis is common among critically ill patients, but current approaches to correct pH do not improve disease outcomes. During systemic acidosis, cells are either passively exposed to extracellular acidosis that other cells have generated (extrinsic acidosis) or they are exposed to acid that they generate and export into the extracellular space (intrinsic acidosis). Although endothelial repair following intrinsic acidosis has been studied, the impact of extrinsic acidosis on migration and angiogenesis is unclear. We hypothesized that extrinsic acidosis inhibits metabolism and migration but promotes capillary-like network formation in pulmonary microvascular endothelial cells (PMVECs). Extrinsic acidosis was modeled by titrating media pH. Two types of intrinsic acidosis were compared, including increasing cellular metabolism by chemically inhibiting carbonic anhydrases (CAs) IX and XII (SLC-0111) and with hypoxia. PMVECs maintained baseline intracellular pH for 24 h with both extrinsic and intrinsic acidosis. Whole cell CA IX protein expression was decreased by extrinsic acidosis but not affected by hypoxia. When extracellular pH was equally acidic, extrinsic acidosis suppressed glycolysis, whereas intrinsic acidosis did not. Extrinsic acidosis suppressed migration, but increased Matrigel network master junction and total segment length. CRISPR-Cas9 CA IX knockout PMVECs revealed an independent role of CA IX in promoting glycolysis, as loss of CA IX alone was accompanied by decreased hexokinase I and pyruvate dehydrogenase E1α expression and decreasing migration. 2-deoxy-d-glucose had no effect on migration but profoundly inhibited network formation and increased N-cadherin expression. Thus, we report that while extrinsic acidosis suppresses endothelial glycolysis and migration, it promotes network formation.

Carbonic Anhydrase Inhibitors of Different Structures Dilate Pre-Contracted Porcine Retinal Arteries

Carbonic anhydrase inhibitors (CAIs), such as dorzolamide (DZA), are used as anti-glaucoma drugs to lower intraocular pressure, but it has been found that some of these drugs act as vasodilators of retinal arteries. The exact mechanism behind the vasodilatory effect is not yet clear. Here we have addressed the issue by using small vessel myography to examine the effect of CAIs of the sulfonamide and coumarin type on the wall tension in isolated segments of porcine retinal arteries. Vessels were pre-contracted by the prostaglandin analog U-46619, and CAIs with varying affinity for five different carbonic anhydrase (CA) isoenzymes found in human tissue tested. We found that all compounds tested cause a vasodilation of pre-contracted retinal arteries, but with varying efficacy, as indicated by the calculated mean EC50 of each compound, ranging from 4.12 µM to 0.86 mM. All compounds had a lower mean EC50 compared to DZA. The dilation induced by benzolamide (BZA) and DZA was additive, suggesting that they may act on separate mechanisms. No clear pattern in efficacy and affinity for CA isoenzymes could be discerned from the results, although Compound 5, with a low affinity for all isoenzymes except the human (h) CA isoform IV, had the greatest potency, with the lowest EC50 and inducing the most rapid and profound dilation of the vessels. The results suggest that more than one isozyme of CA is involved in mediating its role in controlling vascular tone in retinal arteries, with a probable crucial role played by the membrane-bound isoform CA IV.

Interventions for treating acute high altitude illness

BACKGROUND

Acute high altitude illness is defined as a group of cerebral and pulmonary syndromes that can occur during travel to high altitudes. It is more common above 2500 metres, but can be seen at lower elevations, especially in susceptible people. Acute high altitude illness includes a wide spectrum of syndromes defined under the terms 'acute mountain sickness' (AMS), 'high altitude cerebral oedema' and 'high altitude pulmonary oedema'. There are several interventions available to treat this condition, both pharmacological and non-pharmacological; however, there is a great uncertainty regarding their benefits and harms.

OBJECTIVES

To assess the clinical effectiveness, and safety of interventions (non-pharmacological and pharmacological), as monotherapy or in any combination, for treating acute high altitude illness.

SEARCH METHODS

We searched CENTRAL, MEDLINE, Embase, LILACS, ISI Web of Science, CINAHL, Wanfang database and the World Health Organization International Clinical Trials Registry Platform for ongoing studies on 10 August 2017. We did not apply any language restriction.

SELECTION CRITERIA

We included randomized controlled trials evaluating the effects of pharmacological and non-pharmacological interventions for individuals suffering from acute high altitude illness: acute mountain sickness, high altitude pulmonary oedema or high altitude cerebral oedema.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed the eligibility of study reports, the risk of bias for each and performed the data extraction. We resolved disagreements through discussion with a third author. We assessed the quality of evidence with GRADE.

MAIN RESULTS

We included 13 studies enrolling a total of 468 participants. We identified two ongoing studies. All studies included adults, and two studies included both teenagers and adults. The 13 studies took place in high altitude areas, mostly in the European Alps. Twelve studies included participants with acute mountain sickness, and one study included participants with high altitude pulmonary oedema. Follow-up was usually less than one day. We downgraded the quality of the evidence in most cases due to risk of bias and imprecision. We report results for the main comparisons as follows.Non-pharmacological interventions (3 studies, 124 participants)All-cause mortality and complete relief of AMS symptoms were not reported in the three included trials. One study in 64 participants found that a simulated descent of 193 millibars versus 20 millibars may reduce the average of symptoms to 2.5 vs 3.1 units after 12 hours of treatment (clinical score ranged from 0 to 11 ‒ worse; reduction of 0.6 points on average with the intervention; low quality of evidence). In addition, no complications were found with use of hyperbaric chambers versus supplementary oxygen (one study; 29 participants; low-quality evidence).Pharmacological interventions (11 trials, 375 participants)All-cause mortality was not reported in the 11 included trials. One trial found a greater proportion of participants with complete relief of AMS symptoms after 12 and 16 hours when dexamethasone was administered in comparison with placebo (47.1% versus 0%, respectively; one study; 35 participants; low quality of evidence). Likewise, when acetazolamide was compared with placebo, the effects on symptom severity was uncertain (standardized mean difference (SMD) -1.15, 95% CI -2.56 to 0.27; 2 studies, 25 participants; low-quality evidence). One trial of dexamethasone in comparison with placebo in 35 participants found a reduction in symptom severity (difference on change in the AMS score: 3.7 units reported by authors; moderate quality of evidence). The effects from two additional trials comparing gabapentin with placebo and magnesium with placebo on symptom severity at the end of treatment were uncertain. For gabapentin versus placebo: mean visual analogue scale (VAS) score of 2.92 versus 4.75, respectively; 24 participants; low quality of evidence. For magnesium versus placebo: mean scores of 9 and 10.3 units, respectively; 25 participants; low quality of evidence). The trials did not find adverse events from either treatment (low quality of evidence). One trial comparing magnesium sulphate versus placebo found that flushing was a frequent event in the magnesium sulphate arm (percentage of flushing: 75% versus 7.7%, respectively; one study; 25 participants; low quality of evidence).

AUTHORS' CONCLUSIONS

There is limited available evidence to determine the effects of non-pharmacological and pharmacological interventions in treating acute high altitude illness. Low-quality evidence suggests that dexamethasone and acetazolamide might reduce AMS score compared to placebo. However, the clinical benefits and harms related to these potential interventions remain unclear. Overall, the evidence is of limited practical significance in the clinical field. High-quality research in this field is needed, since most trials were poorly conducted and reported.

Carbonic anhydrase is not a relevant nitrite reductase or nitrous anhydrase in the lung

KEY POINTS

Carbonic anhydrase (CA) inhibitors such as acetazolamide inhibit hypoxic pulmonary vasoconstriction (HPV) in humans and other mammals, but the mechanism of this action remains unknown. It has been postulated that carbonic anhydrase may act as a nitrous anhydrase in vivo to generate nitric oxide (NO) from nitrite and that this formation is increased in the presence of acetazolamide. Acetazolamide reduces HPV in pigs without evidence of any NO generation, whereas nebulized sodium nitrite reduces HPV by NO formation; however; combined infusion of acetazolamide with sodium nitrite inhalation did not further increase exhaled NO concentration over inhaled nitrite alone in pigs exposed to alveolar hypoxia. We conclude that acetazolamide does not function as either a nitrous anhydrase or a nitrite reductase in the lungs of pigs, and probably other mammals, to explain its vasodilating actions in the pulmonary or systemic circulations.

ABSTRACT

The carbonic anhydrase (CA) inhibitors acetazolamide and its structurally similar analogue methazolamide prevent or reduce hypoxic pulmonary vasoconstriction (HPV) in dogs and humans in vivo, by a mechanism unrelated to CA inhibition. In rodent blood and isolated blood vessels, it has been reported that inhibition of CA leads to increased generation of nitric oxide (NO) from nitrite and vascular relaxation in vitro. We tested the physiological relevance of augmented NO generation by CA from nitrite with acetazolamide in anaesthetized pigs during alveolar hypoxia in vivo. We found that acetazolamide prevents HPV in anaesthetized pigs, as in other mammalian species. A single nebulization of sodium nitrite reduces HPV, but this action wanes in the succeeding 3 h of hypoxia as nitrite is metabolized and excreted. Pulmonary artery pressure reduction and NO formation as measured by exhaled gas concentration from inhaled sodium nitrite were not increased by acetazolamide during alveolar hypoxia. Thus, our data argue against a physiological role of carbonic anhydrase as a nitrous anhydrase or nitrite reductase as a mechanism for its inhibition of HPV in the lung and blood in vivo.

Carbonic anhydrase inhibitors modify intracellular pH transients and contractions of rat middle cerebral arteries during CO2/HCO3- fluctuations

The CO₂/HCO₃^- buffer minimizes pH changes in response to acid-base loads, HCO₃^- provides substrate for Na⁺,HCO₃^--cotransporters and Cl^-/HCO₃^--exchangers, and H⁺ and HCO₃^- modify vasomotor responses during acid-base disturbances. We show here that rat middle cerebral arteries express cytosolic, mitochondrial, extracellular, and secreted carbonic anhydrase isoforms that catalyze equilibration of the CO₂/HCO₃^- buffer. Switching from CO₂/HCO₃^--free to CO₂/HCO₃^--containing extracellular solution results in initial intracellular acidification due to hydration of CO₂ followed by gradual alkalinization due to cellular HCO₃^- uptake. Carbonic anhydrase inhibition decelerates the initial acidification and attenuates the associated transient vasoconstriction without affecting intracellular pH or artery tone at steady-state. Na⁺,HCO₃^--cotransport and Na⁺/H⁺-exchange activity after NH₄⁺-prepulse-induced intracellular acidification are unaffected by carbonic anhydrase inhibition. Extracellular surface pH transients induced by transmembrane NH₃ flux are evident under CO₂/HCO₃^--free conditions but absent when the buffer capacity and apparent H⁺ mobility increase in the presence of CO₂/HCO₃^- even after the inhibition of carbonic anhydrases. We conclude that (a) intracellular carbonic anhydrase activity accentuates pH transients and vasoconstriction in response to acute elevations of pCO₂, (b) CO₂/HCO₃^- minimizes extracellular surface pH transients without requiring carbonic anhydrase activity, and (c) carbonic anhydrases are not rate limiting for acid–base transport across cell membranes during recovery from intracellular acidification.

Interventions for preventing high altitude illness: Part 1. Commonly-used classes of drugs

BACKGROUND

High altitude illness (HAI) is a term used to describe a group of cerebral and pulmonary syndromes that can occur during travel to elevations above 2500 metres (8202 feet). Acute hypoxia, acute mountain sickness (AMS), high altitude cerebral oedema (HACE) and high altitude pulmonary oedema (HAPE) are reported as potential medical problems associated with high altitude. In this review, the first in a series of three about preventive strategies for HAI, we assess the effectiveness of six of the most recommended classes of pharmacological interventions.

OBJECTIVES

To assess the clinical effectiveness and adverse events of commonly-used pharmacological interventions for preventing acute HAI.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE (OVID), Embase (OVID), LILACS and trial registries in January 2017. We adapted the MEDLINE strategy for searching the other databases. We used a combination of thesaurus-based and free-text terms to search.

SELECTION CRITERIA

We included randomized-controlled and cross-over trials conducted in any setting where commonly-used classes of drugs were used to prevent acute HAI.

DATA COLLECTION AND ANALYSIS

We used standard methodological procedures as expected by Cochrane.

MAIN RESULTS

We included 64 studies (78 references) and 4547 participants in this review, and classified 12 additional studies as ongoing. A further 12 studies await classification, as we were unable to obtain the full texts. Most of the studies were conducted in high altitude mountain areas, while the rest used low pressure (hypobaric) chambers to simulate altitude exposure. Twenty-four trials provided the intervention between three and five days prior to the ascent, and 23 trials, between one and two days beforehand. Most of the included studies reached a final altitude of between 4001 and 5000 metres above sea level. Risks of bias were unclear for several domains, and a considerable number of studies did not report adverse events of the evaluated interventions. We found 26 comparisons, 15 of them comparing commonly-used drugs versus placebo. We report results for the three most important comparisons: Acetazolamide versus placebo (28 parallel studies; 2345 participants)The risk of AMS was reduced with acetazolamide (risk ratio (RR) 0.47, 95% confidence interval (CI) 0.39 to 0.56; I² = 0%; 16 studies; 2301 participants; moderate quality of evidence). No events of HAPE were reported and only one event of HACE (RR 0.32, 95% CI 0.01 to 7.48; 6 parallel studies; 1126 participants; moderate quality of evidence). Few studies reported side effects for this comparison, and they showed an increase in the risk of paraesthesia with the intake of acetazolamide (RR 5.53, 95% CI 2.81 to 10.88, I² = 60%; 5 studies, 789 participants; low quality of evidence). Budenoside versus placebo (2 parallel studies; 132 participants)Data on budenoside showed a reduction in the incidence of AMS compared with placebo (RR 0.37, 95% CI 0.23 to 0.61; I² = 0%; 2 studies, 132 participants; low quality of evidence). Studies included did not report events of HAPE or HACE, and they did not find side effects (low quality of evidence). Dexamethasone versus placebo (7 parallel studies; 205 participants)For dexamethasone, the data did not show benefits at any dosage (RR 0.60, 95% CI 0.36 to 1.00; I2 = 39%; 4 trials, 176 participants; low quality of evidence). Included studies did not report events of HAPE or HACE, and we rated the evidence about adverse events as of very low quality.

AUTHORS' CONCLUSIONS

Our assessment of the most commonly-used pharmacological interventions suggests that acetazolamide is an effective pharmacological agent to prevent acute HAI in dosages of 250 to 750 mg/day. This information is based on evidence of moderate quality. Acetazolamide is associated with an increased risk of paraesthesia, although there are few reports about other adverse events from the available evidence. The clinical benefits and harms of other pharmacological interventions such as ibuprofen, budenoside and dexamethasone are unclear. Large multicentre studies are needed for most of the pharmacological agents evaluated in this review, to evaluate their effectiveness and safety.

The noncarbonic anhydrase inhibiting acetazolamide analog N-methylacetazolamide reduces the hypercapnic, but not hypoxic, ventilatory response

Previous studies have shown that the carbonic anhydrase (CA) inhibitors acetazolamide (AZ) and methazolamide (MZ) have inhibiting actions on breathing. Classically these have been attributed to CA inhibition, but other effects unrelated to CA inhibition have been identified in other tissues. To explore this possibility in the control of ventilation by the central nervous system, we investigated whether an AZ-analog without CA inhibiting properties, by virtue of a single methylation on the sulfonamide moiety, N-methylacetazolamide (NMA), would still display similar actions to acetazolamide and methazolamide. NMA (20 mg kg⁻¹) was given intravenously to anesthetized cats and we measured the responses to steady-state isocapnic hypoxia and stepwise changes in end-tidal pco₂ before and after infusion of this AZ analog using the technique of end-tidal forcing. NMA caused a large decrease in the apneic threshold and CO₂ sensitivity very similar to those previously observed with AZ and MZ, suggesting that these effects are mediated independently of CA inhibition. In contrast to acetazolamide, but similar to methazolamide, NMA did not affect the steady-state isocapnic hypoxic response. In conclusion, our data reveal complex effects of sulfonamides with very similar structure to AZ that reveal both CA-dependent and CA-independent effects, which need to be considered when using AZ as a probe for the role of CA in the control of ventilation.

Pharmacology of acute mountain sickness: old drugs and newer thinking

Pharmacotherapy in acute mountain sickness (AMS) for the past half century has largely rested on the use of carbonic anhydrase (CA) inhibitors, such as acetazolamide, and corticosteroids, such as dexamethasone. The benefits of CA inhibitors are thought to arise from their known ventilatory stimulation and resultant greater arterial oxygenation from inhibition of renal CA and generation of a mild metabolic acidosis. The benefits of corticosteroids include their broad-based anti-inflammatory and anti-edemagenic effects. What has emerged from more recent work is the strong likelihood that drugs in both classes act on other pathways and signaling beyond their classical actions to prevent and treat AMS. For the CA inhibitors, these include reduction in aquaporin-mediated transmembrane water transport, anti-oxidant actions, vasodilation, and anti-inflammatory effects. In the case of corticosteroids, these include protection against increases in vascular endothelial and blood-brain barrier permeability, suppression of inflammatory cytokines and reactive oxygen species production, and sympatholysis. The loci of action of both classes of drug include the brain, but may also involve the lung as revealed by benefits that arise with selective administration to the lungs by inhalation. Greater understanding of their pluripotent actions and sites of action in AMS may help guide development of better drugs with more selective action and fewer side effects.

New insights into carbonic anhydrase inhibition, vasodilation, and treatment of hypertensive-related diseases

Carbonic anhydrase (CA) and its inhibitors are relevant to many physiological processes and diseases. The enzyme is differentially expressed throughout the body, in concentration and subcellular location, and as 13 catalytically active isoforms. Blood vessels contain small amounts of CA, but the enzyme's role in vascular physiology and blood pressure regulation is uncertain. However, considerable recent evidence points to vasodilation by CA inhibitors. CA inhibition in vascular smooth muscle, endothelium, heart, blood cells, and nervous system could all contribute. It is equally plausible that other targets besides CA for all known CA inhibitors may account for their vascular effects. I will review this knowledge and important remaining gaps relating to treatment of hypertensive-related diseases with potent sulfonamide inhibitors, such as acetazolamide; but also the possibility that CA inhibition by thiazides and loop diuretics, although generally weaker, may have antihypertensive effects beyond their inhibition of renal sodium transporters.

Hypoxia, not pulmonary vascular pressure, induces blood flow through intrapulmonary arteriovenous anastomoses

KEY POINTS

Blood flow through intrapulmonary arteriovenous anastomoses (IPAVA) is increased by acute hypoxia during rest by unknown mechanisms. Oral administration of acetazolamide blunts the pulmonary vascular pressure response to acute hypoxia, thus permitting the observation of IPAVA blood flow with minimal pulmonary pressure change. Hypoxic pulmonary vasoconstriction was attenuated in humans following acetazolamide administration and partially restored with bicarbonate infusion, indicating that the effects of acetazolamide on hypoxic pulmonary vasoconstriction may involve an interaction between arterial pH and PCO2. We observed that IPAVA blood flow during hypoxia was similar before and after acetazolamide administration, even after acid-base status correction, indicating that pulmonary pressure, pH and PCO2 are unlikely regulators of IPAVA blood flow.

ABSTRACT

Blood flow through intrapulmonary arteriovenous anastomoses (IPAVA) is increased with exposure to acute hypoxia and has been associated with pulmonary artery systolic pressure (PASP). We aimed to determine the direct relationship between blood flow through IPAVA and PASP in 10 participants with no detectable intracardiac shunt by comparing: (1) isocapnic hypoxia (control); (2) isocapnic hypoxia with oral administration of acetazolamide (AZ; 250 mg, three times a day for 48 h) to prevent increases in PASP; and (3) isocapnic hypoxia with AZ and 8.4% NaHCO3 infusion (AZ + HCO3 (-) ) to control for AZ-induced acidosis. Isocapnic hypoxia (20 min) was maintained by end-tidal forcing, blood flow through IPAVA was determined by agitated saline contrast echocardiography and PASP was estimated by Doppler ultrasound. Arterial blood samples were collected at rest before each isocapnic-hypoxia condition to determine pH, [HCO3(-)] and Pa,CO2. AZ decreased pH (-0.08 ± 0.01), [HCO3(-)] (-7.1 ± 0.7 mmol l(-1)) and Pa,CO2 (-4.5 ± 1.4 mmHg; P < 0.01), while intravenous NaHCO3 restored arterial blood gas parameters to control levels. Although PASP increased from baseline in all three hypoxic conditions (P < 0.05), a main effect of condition expressed an 11 ± 2% reduction in PASP from control (P < 0.001) following AZ administration while intravenous NaHCO3 partially restored the PASP response to isocapnic hypoxia. Blood flow through IPAVA increased during exposure to isocapnic hypoxia (P < 0.01) and was unrelated to PASP, cardiac output and pulmonary vascular resistance for all conditions. In conclusion, isocapnic hypoxia induces blood flow through IPAVA independent of changes in PASP and the influence of AZ on the PASP response to isocapnic hypoxia is dependent upon the H(+) concentration or Pa,CO2.

Effect of acetazolamide and gingko biloba on the human pulmonary vascular response to an acute altitude ascent

Acetazolamide and gingko biloba are the two most investigated drugs for the prevention of acute mountain sickness (AMS). Evidence suggests that they may also reduce pulmonary artery systolic pressure (PASP). To investigate whether these two drugs for AMS prevention also reduce PASP with rapid airlift ascent to high altitude, a randomized controlled trial was conducted on 28 healthy young men with acetazolamide (125 mg bid), gingko biloba (120 mg bid), or placebo for 3 days prior to airlift ascent (397 m) and for the first 3 days at high altitude (3658 m). PASP, AMS, arterial oxygen saturation (Sao2), mean arterial pressure (MAP), heart rate (HR), forced vital capacity (FVC), forced expiratory volume in the first second (FEV1), and peak expiratory flow (PEF) were assessed both at 397 m and 3658 m. HR, PEF, and PASP increased with altitude exposure (p<0.05), and SaO2 decreased (p<0.05). PASP with acetazolamide (mean at 3658 m, 26.2 mm Hg; incremental change, 4.7 mm Hg, 95% CI., 2.6-6.9 mm Hg) was lower than that with ginkgo biloba (mean at 3658 m, 33.7 mm Hg, p=0.001; incremental change, 13.1 mm Hg, 95%CI., 9.6-16.5 mm Hg, p=0.002), and with placebo (mean at 3658 m, 34.7 mm Hg, p<0.001; 14.4 mm Hg, 95% CI., 8.8-20.0 mm Hg, p=0.001). The data show that a low prophylactic dosage of acetazolamide, but not gingko biloba, mitigates the early increase of PASP in a quick ascent profile.

Pulmonary vasodilation by acetazolamide during hypoxia: impact of methyl-group substitutions and administration route in conscious, spontaneously breathing dogs

Acetazolamide (ACZ) prevents hypoxic pulmonary vasoconstriction (HPV) in isolated lungs, animals, and humans, but not by carbonic anhydrase (CA) inhibition. We studied administration routes in, and certain structural aspects of, ACZ critical to HPV inhibition. Analogs of ACZ during acute hypoxia were tested in unanesthetized dogs. Dogs breathed normoxic gas for 1 h (inspired O2 fraction = 0.21), followed by 10% O2 for 2 h (hypoxia) in these protocols: 1) controls; 2) ACZ intravenously (2 mg · kg(-1) · h(-1)); 3) ACZ orally (5 mg/kg, 12 and 1 h before the experiment); 4) inhaled ACZ (750 mg); 5) methazolamide (MTZ) intravenously (3 mg · kg(-1) · h(-1)); and 6) N-methyl-acetazolamide (NMA) intravenously (10 mg · kg(-1) · h(-1)). In controls, mean pulmonary arterial pressure (MPAP) increased 7 mmHg, and pulmonary vascular resistance (PVR) 224 dyn · s · cm(-5) with hypoxia (P < 0.05). With intravenous and inhaled ACZ, MPAP and PVR did not change during hypoxia. With oral ACZ, HPV was only slightly suppressed; MPAP increased 5 mmHg and PVR by 178 dyn · s · cm(-5) during hypoxia. With MTZ and NMA, the MPAP rise (4 ± 2 mmHg) was reduced, and PVR did not increase during hypoxia compared with normoxia (MTZ intravenous: 81 ± 77 and 68 ± 82 dyn · s · cm(-5) with NMA intravenous). Inhaled ACZ prevents HPV, but not without causing systemic CA inhibition. NMA, a compound lacking CA inhibiting effects by methylation at the sulfonamide moiety, and MTZ, a CA-inhibiting analog methylated at the thiadiazole ring, are only slightly less effective than ACZ in reducing HPV.

High-altitude pulmonary edema

High-altitude pulmonary edema (HAPE), a not uncommon form of acute altitude illness, can occur within days of ascent above 2500 to 3000 m. Although life-threatening, it is avoidable by slow ascent to permit acclimatization or with drug prophylaxis. The critical pathophysiology is an excessive rise in pulmonary vascular resistance or hypoxic pulmonary vasoconstriction (HPV) leading to increased microvascular pressures. The resultant hydrostatic stress causes dynamic changes in the permeability of the alveolar capillary barrier and mechanical injurious damage leading to leakage of large proteins and erythrocytes into the alveolar space in the absence of inflammation. Bronchoalveolar lavage and hemodynamic pressure measurements in humans confirm that elevated capillary pressure induces a high-permeability noninflammatory lung edema. Reduced nitric oxide availability and increased endothelin in hypoxia are the major determinants of excessive HPV in HAPE-susceptible individuals. Other hypoxia-dependent differences in ventilatory control, sympathetic nervous system activation, endothelial function, and alveolar epithelial active fluid reabsorption likely contribute additionally to HAPE susceptibility. Recent studies strongly suggest nonuniform regional hypoxic arteriolar vasoconstriction as an explanation for how HPV occurring predominantly at the arteriolar level causes leakage. In areas of high blood flow due to lesser HPV, edema develops due to pressures that exceed the dynamic and structural capacity of the alveolar capillary barrier to maintain normal fluid balance. This article will review the pathophysiology of the vasculature, alveolar epithelium, innervation, immune response, and genetics of the lung at high altitude, as well as therapeutic and prophylactic strategies to reduce the morbidity and mortality of HAPE.

Acetazolamide attenuates transvascular fluid flux in equine lungs during intense exercise

  During intense exercise in horses the transvascular fluid flux in the pulmonary circulation (Jv-a) represents 4% of cardiac output (Q). This fluid flux has been attributed to an increase in pulmonary transmural hydrostatic forces, increases in perfused microvascular surface area, and reversible alterations in capillary permeability under conditions of high flow and pressure. Erythrocyte fluid efflux, however, accounts for a significant fraction of Jv-a. In the lung the Jacobs-Stewart cycle occurs with diffusion of CO2 into alveolar space with possible accompanying chloride (Cl-) and water movement from the erythrocyte directly into the pulmonary interstitium. We hypothesised that inhibition of carbonic anhydrase in erythrocytes inhibits the Jacobs-Stewart cycle and attenuates Jv-a. Five horses were exercised on a treadmill until fatigue without (control) and with acetazolamide treatment (30 mg kg(-1) 30 min before exercise). Erythrocyte fluid efflux, plasma fluid flux across the lung and Jv-a were calculated using haemoglobin, haematocrit, plasma protein and Q. Fluid fluxes were used to calculate erythrocyte, plasma and whole blood Cl- fluxes across the lung. Cardiac output was not different between control and acetazolamide treatment. During exercise erythrocyte fluid efflux and Jv-a increased in control (9.3±3.3 and 11.0±4.4 l min(-1), respectively) and was higher than after acetazolamide treatment (3.8±1.6 and 1.2±1.2 l min(-1), respectively) (P<0.05). Plasma fluid flux did not change from rest in control and decreased after acetazolamide treatment (-4.5±1.5 l min(-1)) (P<0.05). Erythrocyte Cl- flux increased during exercise in control and after acetazolamide treatment (P<0.05). During exercise plasma Cl- flux across the lung did not change in control; however, it increased with acetazolamide treatment (P=0.0001). During exercise whole blood Cl- flux increased across the lung in control (P<0.05) but not after acetazolamide treatment. The results indicate that Jv-a in the lung is dependent on the Jacobs-Stewart cycle and mostly independent of transmural hydrostatic forces. It also appears that Jv-a is mediated by Cl- and water egress from erythrocytes directly into the interstitium without transit through plasma.

Methazolamide is a new hepatic insulin sensitizer that lowers blood glucose in vivo

We previously used Gene Expression Signature technology to identify methazolamide (MTZ) and related compounds with insulin sensitizing activity in vitro. The effects of these compounds were investigated in diabetic db/db mice, insulin-resistant diet-induced obese (DIO) mice, and rats with streptozotocin (STZ)-induced diabetes. MTZ reduced fasting blood glucose and HbA(1c) levels in db/db mice, improved glucose tolerance in DIO mice, and enhanced the glucose-lowering effects of exogenous insulin administration in rats with STZ-induced diabetes. Hyperinsulinemic-euglycemic clamps in DIO mice revealed that MTZ increased glucose infusion rate and suppressed endogenous glucose production. Whole-body or cellular oxygen consumption rate was not altered, suggesting MTZ may inhibit glucose production by different mechanism(s) to metformin. In support of this, MTZ enhanced the glucose-lowering effects of metformin in db/db mice. MTZ is known to be a carbonic anhydrase inhibitor (CAI); however, CAIs acetazolamide, ethoxyzolamide, dichlorphenamide, chlorthalidone, and furosemide were not effective in vivo. Our results demonstrate that MTZ acts as an insulin sensitizer that suppresses hepatic glucose production in vivo. The antidiabetic effect of MTZ does not appear to be a function of its known activity as a CAI. The additive glucose-lowering effect of MTZ together with metformin highlights the potential utility for the management of type 2 diabetes.

Sildenafil citrate for the prevention of high altitude hypoxic pulmonary hypertension: double blind, randomized, placebo-controlled trial

Exaggerated hypoxic pulmonary vasoconstriction is a key factor in the development of high altitude pulmonary edema (HAPE). Due to its effectiveness as a pulmonary vasodilator, sildenafil has been proposed as a prophylactic agent against HAPE. By conducting a parallel-group double blind, randomized, placebo-controlled trial, we investigated the effect of chronic sildenafil administration on pulmonary artery systolic pressure (PASP) and symptoms of acute mountain sickness (AMS) during acclimatization to high altitude. Sixty-two healthy lowland volunteers (36 male; median age 21 years, range 18 to 31) on the Apex 2 research expedition were flown to La Paz, Bolivia (3650 m), and after 4-5 days acclimatization ascended over 90 min to 5200 m. The treatment group (n=20) received 50 mg sildenafil citrate three times daily. PASP was recorded by echocardiography at sea level and within 6 h, 3 days, and 1 week at 5200 m. AMS was assessed daily using the Lake Louise Consensus symptom score. On intention-to-treat analysis, there was no significant difference in PASP at 5200 m between sildenafil and placebo groups. Median AMS score on Day 2 at 5200 m was significantly higher in the sildenafil group (placebo 4.0, sildenafil 6.5; p=0.004) but there was no difference in prevalence of AMS between groups. Sildenafil administration did not affect PASP in healthy lowland subjects at 5200 m but AMS was significantly more severe on Day 2 at 5200 m with sildenafil. Our data do not support routine prophylactic use of sildenafil to reduce PASP at high altitude in healthy subjects with no history of HAPE. TRIALS REGISTRATION NUMBER: NCT00627965.

High-Altitude Pulmonary Edema

High-altitude pulmonary edema (HAPE) is an uncommon form of pulmonary edema that occurs in healthy individuals within a few days of arrival at altitudes above 2,500–3,000 m. The crucial pathophysiology is an excessive hypoxia-mediated rise in pulmonary vascular resistance (PVR) or hypoxic pulmonary vasoconstriction (HPV) leading to increased microvascular hydrostatic pressures despite normal left atrial pressure. The resultant hydrostatic stress can cause both dynamic changes in the permeability of the alveolar capillary barrier and mechanical damage leading to leakage of large proteins and erythrocytes into the alveolar space in the absence of inflammation. Bronchoalveolar lavage (BAL) and pulmonary artery (PA) and microvascular pressure measurements in humans confirm that high capillary pressure induces a high-permeability non-inflammatory-type lung edema; a concept termed “capillary stress failure.” Measurements of endothelin and nitric oxide (NO) in exhaled air, NO metabolites in BAL fluid, and NO-dependent endothelial function in the systemic circulation all point to reduced NO availability and increased endothelin in hypoxia as a major cause of the excessive hypoxic PA pressure rise in HAPE-susceptible individuals. Other hypoxia-dependent differences in ventilatory control, sympathetic nervous system activation, endothelial function, and alveolar epithelial sodium and water reabsorption likely contribute additionally to the phenotype of HAPE susceptibility. Recent studies using magnetic resonance imaging in humans strongly suggest nonuniform regional hypoxic arteriolar vasoconstriction as an explanation for how HPV occurring predominantly at the arteriolar level can cause leakage. This compelling but not yet fully proven mechanism predicts that in areas of high blood flow due to lesser vasoconstriction edema will develop owing to pressures that exceed the structural and dynamic capacity of the alveolar capillary barrier to maintain normal alveolar fluid balance. Numerous strategies aimed at lowering HPV and possibly enhancing active alveolar fluid reabsorption are effective in preventing and treating HAPE. Much has been learned about HAPE in the past four decades such that what was once a mysterious alpine malady is now a well-characterized and preventable lung disease. This chapter will relate the history, pathophysiology, and treatment of HAPE, using it not only to illuminate the condition, but also for the broader lessons it offers in understanding pulmonary vascular regulation and lung fluid balance.

Generation of nitric oxide from nitrite by carbonic anhydrase: a possible link between metabolic activity and vasodilation

In catalyzing the reversible hydration of CO2 to bicarbonate and protons, the ubiquitous enzyme carbonic anhydrase (CA) plays a crucial role in CO2 transport, in acid-base balance, and in linking local acidosis to O2 unloading from hemoglobin. Considering the structural similarity between bicarbonate and nitrite, we hypothesized that CA uses nitrite as a substrate to produce the potent vasodilator nitric oxide (NO) to increase local blood flow to metabolically active tissues. Here we show that CA readily reacts with nitrite to generate NO, particularly at low pH, and that the NO produced in the reaction induces vasodilation in aortic rings. This reaction occurs under normoxic and hypoxic conditions and in various tissues at physiological levels of CA and nitrite. Furthermore, two specific inhibitors of the CO2 hydration, dorzolamide and acetazolamide, increase the CA-catalyzed production of vasoactive NO from nitrite. This enhancing effect may explain the known vasodilating effects of these drugs and indicates that CO2 and nitrite bind differently to the enzyme active site. Kinetic analyses show a higher reaction rate at high pH, suggesting that anionic nitrite participates more effectively in catalysis. Taken together, our results reveal a novel nitrous anhydrase enzymatic activity of CA that would function to link the in vivo main end products of energy metabolism (CO2/H+) to the generation of vasoactive NO. The CA-mediated NO production may be important to the correlation between blood flow and metabolic activity in tissues, as occurring for instance in active areas of the brain.

Methazolamide does not impair respiratory work performance in anesthetized rabbits

In human medicine, the carbonic anhydrase (CA) inhibitor acetazolamide is used to treat irregular breathing disorders. Previously, we demonstrated in the rabbit that this substance stabilized closed-loop gain properties of the respiratory control system, but concomitantly weakened respiratory muscles. Among others, the highly diffusible CA-inhibitor methazolamide differs from acetazolamide in that it fails to activate Ca2+-dependent potassium channels in skeletal muscles. Therefore, we aimed to find out, whether or not methazolamide may exert attenuating adverse effects on respiratory muscle performance as acetazolamide. In anesthetized spontaneously breathing rabbits ( n = 7), we measured simultaneously the CO2 responses of tidal phrenic nerve activity, tidal transpulmonary pressure changes, and tidal volume before and after intravenous application of methazolamide at two mean (± SE) cumulative doses of 3.5 ± 0.1 and 20.8 ± 0.4 mg/kg. Similar to acetazolamide, low- and high-dose methazolamide enhanced baseline ventilation by 52 ± 10% and 166 ± 30%, respectively ( P < 0.01) and lowered the base excess in a dose-dependent manner by up to 8.3 ± 0.9 mmol/l ( P < 0.001). The transmission of a CO2-induced rise in phrenic nerve activity into volume and/or pressure and, hence, respiratory work performance was 0.27 ± 0.05 ml·kg−1·kPa·unit−1 under control conditions, but remained unchanged upon low- or high-dose methazolamide, at 0.30 ± 0.06 and 0.28 ± 0.07 ml·kg−1·kPa·unit−1, respectively. We conclude that methazolamide does not cause respiratory muscle weakening at elevated levels of ventilatory drive. This substance (so far not used for medication of respiratory diseases) may thus exert stabilizing influences on breathing control without adverse effects on respiratory muscle function.

Medication and dosage considerations in the prophylaxis and treatment of high-altitude illness

With increasing numbers of people traveling to high altitude for work or pleasure, there is a reasonable chance that many of these travelers have preexisting medical conditions or are receiving various medications at the time of their sojourn. As with all travelers to high altitude, they are at risk for altitude illnesses such as acute mountain sickness, high-altitude cerebral edema, and high-altitude pulmonary edema. While there are clear recommendations for pharmacologic measures to prevent or treat these illnesses, these recommendations are oriented toward healthy individuals and do not take into account the presence of preexisting medical conditions. In this review, we consider how the choice and dose of the medications used in the management of altitude illness-acetazolamide, dexamethasone, nifedipine, tadalafil, sildenafil, and salmeterol-are affected by a patient's underlying medical conditions. We discuss the indications and current dosing recommendations for individuals without underlying disease, and then consider how drug selection or dosing regimens will be affected by the presence of renal insufficiency, hepatic insufficiency, other important medical conditions, and the potential for serious drug interactions. We include comments about interactions with antimalarial medications and antibiotics used in the treatment of traveler's diarrhea, as well as the safety of use during pregnancy. By giving these issues adequate consideration, clinicians can increase the chances that properly evaluated patients with underlying medical conditions will enjoy a safe trip to high altitude.

Effects of acetazolamide on aerobic exercise capacity and pulmonary hemodynamics at high altitudes

Aerobic exercise capacity is decreased at altitude because of combined decreases in arterial oxygenation and in cardiac output. Hypoxic pulmonary vasoconstriction could limit cardiac output in hypoxia. We tested the hypothesis that acetazolamide could improve exercise capacity at altitude by an increased arterial oxygenation and an inhibition of hypoxic pulmonary vasoconstriction. Resting and exercise pulmonary artery pressure (Ppa) and flow (Q) (Doppler echocardiography) and exercise capacity (cardiopulmonary exercise test) were determined at sea level, 10 days after arrival on the Bolivian altiplano, at Huayna Potosi (4,700 m), and again after the intake of 250 mg acetazolamide vs. a placebo three times a day for 24 h. Acetazolamide and placebo were administered double-blind and in a random sequence. Altitude shifted Ppa/Q plots to higher pressures and decreased maximum O2 consumption (V̇o2max). Acetazolamide had no effect on Ppa/Q plots but increased arterial O2 saturation at rest from 84 ± 5 to 90 ± 3% ( P < 0.05) and at exercise from 79 ± 6 to 83 ± 4% ( P < 0.05), and O2 consumption at the anaerobic threshold (V-slope method) from 21 ± 5 to 25 ± 5 ml·min−1·kg−1 ( P < 0.01). However, acetazolamide did not affect V̇o2max (from 31 ± 6 to 29 ± 7 ml·kg−1·min−1), and the maximum respiratory exchange ratio decreased from 1.2 ± 0.06 to 1.05 ± 0.03 ( P < 0.001). We conclude that acetazolamide does not affect maximum exercise capacity or pulmonary hemodynamics at high altitudes. Associated changes in the respiratory exchange ratio may be due to altered CO2 production kinetics.

Carbonic anhydrase II and alveolar fluid reabsorption during hypercapnia

Carbonic anhydrase II (CAII) plays an important role in carbon dioxide metabolism and intracellular pH regulation. In this study, we provide evidence that CAII is expressed in both type I (AECI) and type II (AECII) alveolar epithelial cells by RT-PCR and Western blotting in freshly isolated rat cells. These results were further confirmed by double immunostaining with CAII antibodies and AECI- or AECII-specific markers in freshly isolated alveolar epithelial cells and rat lung tissues. Inhibition of CAII by acetazolamide or methazolamide delayed the decrease in the intracellular pH observed during hypercapnia in cultured AECI, AECII, and AECI-like cells. In an isolated-perfused rat lung model, alveolar fluid reabsorption significantly decreased during high CO(2) exposure, which was not prevented by carbonic anhydrase inhibition. Thus, we provide evidence that CAII is expressed in rat alveolar epithelial cells and does not regulate lung alveolar fluid reabsorption during hypercapnia.

Effects of acetazolamide on ventilatory, cerebrovascular, and pulmonary vascular responses to hypoxia

RATIONALE

Acute mountain sickness (AMS) may affect individuals who (rapidly) ascend to altitudes higher than 2,000-3,000 m. A more serious consequence of rapid ascent may be high-altitude pulmonary edema, a hydrostatic edema associated with increased pulmonary capillary pressures. Acetazolamide is effective against AMS, possibly by increasing ventilation and cerebral blood flow (CBF). In animals, it inhibits hypoxic pulmonary vasoconstriction.

OBJECTIVES

We examined the influence of acetazolamide on the response to hypoxia of ventilation, CBF, and pulmonary vascular resistance (PVR).

METHODS

In this double-blind, placebo-controlled, randomized study, nine subjects ingested 250 mg acetazolamide every 8 h for 3 d. On the fourth test day, we measured the responses of ventilation, PVR, and CBF to acute isocapnic hypoxia (20 min) and sustained poikilocapnic hypoxia (4 h). Ventilation was measured with pneumotachography. Hypoxia was achieved with dynamic end-tidal forcing. The maximum pressure difference across the tricuspid valve (DeltaPmax, a good index of PVR) was measured with Doppler echocardiography. CBF was measured by transcranial Doppler ultrasound.

RESULTS

In normoxia, acetazolamide increased ventilation and reduced DeltaPmax, but did not influence CBF. The ventilatory and CBF responses to acute isocapnic hypoxia were unaltered, but the rise in DeltaPmax was reduced by 57%. The increase in DeltaPmax by sustained poikilocapnic hypoxia observed after placebo was reduced by 34% after acetazolamide, the ventilatory response was increased, but the CBF response remained unaltered.

CONCLUSIONS

Acetazolamide has complex effects on ventilation, PVR, and CBF that converge to optimize brain oxygenation and may be a valuable means to prevent/treat high-altitude pulmonary edema.