High-altitude pulmonary edema is initially caused by an increase in capillary pressure

Hypoxia-induced inhibition of whole cell membrane currents and ion transport of A549 cells

In excitable cells, hypoxia inhibits K channels, causes membrane depolarization, and initiates complex adaptive mechanisms. It is unclear whether K channels of alveolar epithelial cells reveal a similar response to hypoxia. A549 cells were exposed to hypoxia during whole cell patch-clamp measurements. Hypoxia reversibly inhibited a voltage-dependent outward current, consistent with a K current, because tetraethylamonium (TEA; 10 mM) abolished this effect; however, iberiotoxin (0.1 microM) does not. In normoxia, TEA and iberiotoxin inhibited whole cell current (-35%), whereas the K-channel inhibitors glibenclamide (1 microM), barium (1 mM), chromanol B293 (10 microM), and 4-aminopyridine (1 mM) were ineffective. (86)Rb uptake was measured to see whether K-channel modulation also affected transport activity. TEA, iberiotoxin, and 4-h hypoxia (1.5% O(2)) inhibited total (86)Rb uptake by 40, 20, and 35%, respectively. Increased extracellular K also inhibited (86)Rb uptake in a dose-dependent way. The K-channel opener 1-ethyl-2-benzimidazolinone (1 mM) increased (86)Rb uptake by 120% in normoxic and hypoxic cells by activation of Na-K pumps (+60%) and Na-K-2Cl cotransport (+170%). However, hypoxic transport inhibition was also seen in the presence of 1-ethyl-2-benzimidazolinone, TEA, and iberiotoxin. These results indicate that hypoxia, membrane depolarization, and K-channel inhibition decrease whole cell membrane currents and transport activity. It appears, therefore, that a hypoxia-induced change in membrane conductance and membrane potential might be a link between hypoxia and alveolar ion transport inhibition.

Whole-body hypoxic preconditioning protects mice against acute hypoxia by improving lung function

Survival in severe hypoxia such as occurs in high altitude requires previous acclimatization, which is acquired over a period of days to weeks. It was unknown whether intrinsic mechanisms existed that could be rapidly induced and could exert immediate protection on unacclimatized individuals against acute hypoxia. We found that mice pretreated with whole-body hypoxic preconditioning (WHPC, 6 cycles of 10-min hypoxia-10-min normoxia) survived significantly longer than control animals when exposed to lethal hypoxia (5% O2, survival time of 33.2 ± 6.1 min vs. controls at 13.8 ± 1.2 min, n = 10, P < 0.005). This protective mechanism became operative shortly after WHPC and remained effective for at least 8 h. Accordingly, mice subjected to WHPC demonstrated improved gas exchange when exposed to sublethal hypoxia (7% O2, arterial blood Po2 of 49.9 ± 4.2 vs. controls at 39.7 ± 3.6 Torr, n = 6, P < 0.05), reduced formation of pulmonary edema (increase in lung water of 0.491 ± 0.111 vs. controls at 0.894 ± 0.113 mg/mg dry tissue, n = 10, P < 0.02), and decreased pulmonary vascular permeability (lung lavage albumin of 7.63 ± 0.63 vs. controls at 18.24 ± 3.39 mg/dl, n = 6–10, P < 0.025). In addition, the severity of cerebral edema caused by exposure to sublethal hypoxia was also reduced after WHPC (increase in brain water of 0.254 ± 0.052 vs. controls at 0.491 ± 0.034 mg/mg dry tissue, n = 10, P < 0.01). Thus WHPC protects unacclimatized mice against acute and otherwise lethal hypoxia, and this protection involves preservation of vital organ functions.

High-altitude-related disorders—part I: pathophysiology, differential diagnosis, and treatment

As increasing numbers of people choose to sojourn or retire to the mountains, high-altitude illness is becoming a pathological phenomenon about which healthcare providers should have greater awareness. Hypoxia is the primary cause of high-altitude illness, but other stressors on the sympathetic nervous system, such as cold and exertion, also contribute to disease development and progression. Although variable across persons, symptoms of high-altitude disorders usually occur at altitudes over 7000 feet, and typically in 1 of 3 forms: acute mountain sickness (AMS), high-altitude cerebral edema (HACE), or high-altitude pulmonary edema (HAPE). Major symptoms include nausea, poor sleep, headache, lassitude, cough, dyspnea on exertion and at rest, ataxia, and mental status changes. As a rule, illness occurring at high altitude should be attributed to the altitude until proven otherwise. Treatment is best accomplished by descent and by oxygen or pharmacologic intervention if necessary. Under no circumstances should a person with worsening symptoms of high-altitude illness delay descent. As will be discussed in part II of this article, gradual ascent and subsequent acclimatization to altitude is the most effective prevention, though acetazolamide (Diamox) may be a useful prophylactic measure in some.

Pulmonary edema and pleural effusion in norepinephrine-stimulated rats--hemodynamic or inflammatory effect?

Stimulation with norepinephrine (NE) leads to pulmonary edema and pleural effusion in rats. These pulmonary fluid shifts may result from pulmonary congestion due to the hemodynamic effects of NE and/or inflammation with an increase in vascular permeability. The contribution of these two factors were investigated in the present study. Female Sprague-Dawley rats received continuous i.v. NE infusion (0.1 mg/kg/h) over time intervals between 90 min and 72 h. After heart catheterization, pleural fluid (PF) and lung tissue were obtained. In some of the animals, a bronchoalveolar lavage (BAL) was performed. Pulmonary edema and inflammation were shown histologically. We determined the expression of interleukin (IL)-6 as one of the most potent acute-phase protein mediators in serum, PF and BAL supernatant fluid (BALF) using ELISA as well as in the lung tissue using Western blotting. Total protein concentration in BALF and PF served as indicators of increased capillary permeability. Pulmonary edema and pleural effusion appeared coincidentally with an increase in total peripheral resistance (TPR) after 6 h of NE infusion. PF reached a maximum between 8 and 16 h (2.2 +/- 0.3 ml, controls < 0.5 ml) and disappeared within 48 h. Activation of IL-6 in the fluids was observed after 8 h of NE stimulation. In the lung tissue it started after 12 h and reached 330% of the control value after 48 h. Pulmonary inflammation was documented histologically. It was accompanied by increased protein concentration in BALF after 24 h of NE treatment. Hemodynamic effects of NE are the main causative factors in the initial phase of the pulmonary fluid shifts. Additionally, NE leads to an activation of cytokines such as IL-6 and to inflammation and to an increase in capillary permeability. However, inflammation and increased capillary permeability occurred later than pulmonary edema and pleural effusion. Hence, we conclude that they are secondary factors which may contribute to maintain the fluid shifts over a longer period of time.

Clinical relevance of time of onset, duration, and type of pulmonary edema after liver transplantation

We investigated the clinical significance of time of onset, duration, and type of pulmonary edema after orthotopic liver transplantation by retrospectively reviewing 93 consecutive recipients. Pulmonary edema was diagnosed by means of radiographic criteria and Pao(2)/Fio(2) ratio <300. Type was identified by pulmonary artery wedge pressure (hydrostatic, >18 mm Hg; permeability, < or =18 mm Hg). Of 91 evaluable patients, 44 (48%) had no pulmonary edema, 23 (25%) had immediate pulmonary edema resolving within 24 hours, 8 (9%) had late pulmonary edema (developing de novo in the first 16 to 24 hours), and 16 (18%) had persistent pulmonary edema (developing immediately and persisting for at least 16 hours). At 16 to 24 hours, mean arterial pressure was lower with persistent permeability-type edema than without pulmonary edema (75 versus 87 mm Hg, P <.01). Patients with persistent permeability-type edema had higher mean pulmonary arterial pressure (23 versus 16 mm Hg, P <.01) and higher pulmonary vascular resistance (103 versus 53 dyn. second. m(-5), P <.05), consistent with a resistance-dependent mechanism. Patients with persistent hydrostatic-type edema did not differ from those without edema in mean arterial pressure (84 versus 87 mm Hg, P >.05) or pulmonary vascular resistance (67 versus 53 dyn. second. m(-5), P >.05), but had increased mean pulmonary arterial pressure (27 versus 16, P <.01), suggesting a flow volume-dependent mechanism. Duration of mechanical ventilation, intensive care, and hospital stay were prolonged in patients with late or persistent permeability-type edema but not in patients with immediate pulmonary edema of any type. In conclusion, immediate pulmonary edema resolving within 24 hours after liver transplantation had little clinical consequence; persistent permeability-type pulmonary edema portended a worse outcome.

High-altitude illness

High-altitude illness is the collective term for acute mountain sickness (AMS), high-altitude cerebral oedema (HACE), and high-altitude pulmonary oedema (HAPE). The pathophysiology of these syndromes is not completely understood, although studies have substantially contributed to the current understanding of several areas. These areas include the role and potential mechanisms of brain swelling in AMS and HACE, mechanisms accounting for exaggerated pulmonary hypertension in HAPE, and the role of inflammation and alveolar-fluid clearance in HAPE. Only limited information is available about the genetic basis of high-altitude illness, and no clear associations between gene polymorphisms and susceptibility have been discovered. Gradual ascent will always be the best strategy for preventing high-altitude illness, although chemoprophylaxis may be useful in some situations. Despite investigation of other agents, acetazolamide remains the preferred drug for preventing AMS. The next few years are likely to see many advances in the understanding of the causes and management of high-altitude illness.

Serial changes in nasal potential difference and lung electrical impedance tomography at high altitude

Recent work suggests that treatment with inhaled beta(2)-agonists reduces the incidence of high-altitude pulmonary edema in susceptible subjects by increasing respiratory epithelial sodium transport. We estimated respiratory epithelial ion transport by transepithelial nasal potential difference (NPD) measurements in 20 normal male subjects before, during, and after a stay at 3,800 m. NPD hyperpolarized on ascent to 3,800 m (P < 0.05), but the change in potential difference with superperfusion of amiloride or isoprenaline was unaffected. Vital capacity (VC) fell on ascent to 3,800 m (P < 0.05), as did the normalized change in electrical impedance (NCI) measured over the right lung parenchyma (P < 0.05) suggestive of an increase in extravascular lung water. Echo-Doppler-estimated pulmonary artery pressure increases were insufficient to cause clinical pulmonary edema. There was a positive correlation between VC and NCI (R(2) = 0.633) and between NPD and both VC and NCI (R(2) = 0.267 and 0.418). These changes suggest that altered respiratory epithelial ion transport might play a role in the development of subclinical pulmonary edema at high altitude in normal subjects.

Environmental insults: smoke inhalation, submersion, diving, and high altitude

In the expanding search for recreation, we spend more and more of our time in various environments. Whether the air is thin or compressed or smoke-filled or there is no air at all, emergency physicians continue to meet and treat the various pulmonary emergencies that the environment may create. The authors present the background, diagnosis, and management of a few of the more common pulmonary emergencies that the environment may produce.

Catecholamine-induced pulmonary edema and pleural effusion in rats--alpha- and beta-adrenergic effects

We investigated the contribution of alpha- and beta-adrenergic pathways to catecholamine-induced pulmonary edema and the role of pleural effusion in preventing alveolar edema. Female Sprague-Dawley rats received continuous intravenous infusion of norepinephrine and of separate alpha- or beta-adrenergic stimulation over 6-24 h. We performed heart catheterization in vivo and excised post mortem lung tissue for histological analysis. Interleukin (IL)-6 and total protein concentrations were determined in serum, pleural fluid (PF) and bronchoalveolar lavage fluid. alpha-Adrenergic treatment increased right ventricular systolic pressure (RVSP) and total peripheral resistance (TPR) and caused severe alveolar edema associated with IL-6 activation in serum and diffuse pulmonary inflammation. PF amounts were moderate (0.9+/-0.2 ml). beta-Adrenergic stimulation also increased RVSP but decreased TPR. Interstitial but not alveolar edema and focal inflammation without IL-6 activation developed. Large PF amounts (6.2+/-1.5 ml) occurred which were considered to prevent alveolar edema. We conclude that both alpha- and beta-adrenergic stimulation contribute to pulmonary fluid shifts in rats, but alpha-adrenergic pathways cause more acute and more severe lung injury than beta-adrenergic mechanisms.

Cardio-Pulmonary Interactions at High Altitude

The purpose of this review is to find the evidence that a disproportionate pulmonary vasoconstriction persisting for days, weeks and years during residence at high altitude is the common pathophysiologic mechanism of high altitude pulmonary edema (HAPE), subacute mountain sickness and chronic mountain sickness. A recent finding in early HAPE suggests that transmission of excessively elevated pulmonary artery pressure to the pulmonary capillaries leading to alveolar hemorrhage as the pathophysiologic mechanism of HAPE. The elevated incidence of HAPE in Indian soldiers led the Indian Army to extend the acclimatization period from a few days to 5 weeks. Using this protocol, HAPE was prevented, but after several weeks of residence at an altitude of 6000m dyspnea, anasarca and pleuro-pericardial effusion developed. Clinical examination revealed severe congestive right heart failure. This condition has been previously described in long-term high altitude residents of the Himalaya and the Andes. In rats, smooth muscle cells appear in normally non-muscular arterioles within days of simulated altitude. Rapid remodeling of the small precapillary arteries may prevent HAPE but increase pulmonary vascular resistance leading to pulmonary hypertension in long-term high altitude residents. Symptoms and signs of HAPE, subacute mountain sickness and chronic mountain sickness reverse completely after residents are transfered to low altitude. In conclusion, these findings strongly suggest that pulmonary hypertension at high altitude, which could be named "high altitude pulmonary hypertension", is the principal and common pathogenic factor of all three cardio-pulmonary manifestations of high altitude illness. Accordingly, subacute mountain sickness and chronic mountain sickness could be renamed in "acute-" and "chronic right heart failure of high altitude", respectively.

No association between high-altitude tolerance and the ACE I/D gene polymorphism

PURPOSE

The absence (deletion allele [D]) of a 287 base-pair fragment in the ACE gene is associated with higher ACE tissue activity than its presence (insertion allele [I]) and, as such, may enhance vasoconstriction and fluid retention through increased levels of angiotensin II and aldosterone. Because fluid retention is found in acute mountain sickness (AMS) and exaggerated pulmonary hypertension is essential in the pathophysiology of high-altitude pulmonary edema (HAPE), we hypothesized that the DD genotype is associated with increased susceptibility to these illnesses.

METHODS

ACE genotype was thus determined in 83 mountaineers staying over night at 4559 m and related to AMS symptoms. Genotype was similarly determined in 76 mountaineers who had participated in previous studies at 4559 m; 38 of the latter group had a history of HAPE, and 25 had developed HAPE again during these studies.

RESULTS

The allele frequency was in Hardy-Weinberg equilibrium in both investigations. Neither the history nor the observed episodes of HAPE nor the prevalence of AMS defined as an AMS-C score >/= 0.70 (environmental symptom questionnaire) in the first study or in both studies taken together were significantly different between the genotypes DD, ID, and II.

CONCLUSION

We conclude that I/D-ACE gene polymorphism has no important effect on susceptibility to AMS or HAPE.

Update: High altitude pulmonary edema

Recent high altitude studies with pulmonary artery (PA) catheterization and broncho-alveolar lavage (BAL) in early high altitude pulmonary edema(HAPE) have increased our understanding of the pathogenetic sequence in HAPE. High preceding PA and pulmonary capillary pressures lead to a non-inflammatory leak of the alveolar-capillary barrier with egress of red cells, plasma proteins and fluid into the alveolar space. The mechanisms accounting for an increased capillary pressure remain speculative. The concept that hypoxic pulmonary vasoconstriction (HPV) is uneven so that regions with less vasoconstriction are over-perfused and become edematous remains compelling but unproved. Also uncertain is the role and extent of pulmonary venoconstriction. With disruption of the normal alveolar-capillary barrier, some individuals may later develop a secondary inflammatory reaction. A high incidence of preceding or concurrent respiratory infection in children with HAPE has been used to support a causative role of inflammation in HAPE. However, alternatively even mild HPV may simply lower the threshold at which inflammation-mediated increases in alveolar capillary permeability cause significant fluid flux into the lung. Other major questions to be addressed in future research are: 1.) What is the mechanism of exaggerated hypoxic pulmonary vasoconstriction? Is there a link to primary pulmonary hypertension? Several observations suggest that susceptibility to HAPE is associated with endothelial dysfunction in pulmonary vessels. This has not yet been studied adequately. 2.) What is the nature of the leak? Is there structural damage, i. e. stress failure, or does stretch cause opening of pores? 3.) What is the pathophysiologic significance of a decreased sodium and water clearance across alveolar epithelial cells in hypoxia? 4.) What is the role of exercise? Do HAPE-susceptible individuals develop pulmonary edema when exposed to hypoxia without exercise? Answers to these questions will increase our understanding of the pathophysiology of HAPE and also better focus research on the genetic basis of susceptibility to HAPE.

Pathological features of the lung in fatal high altitude pulmonary edema occurring at moderate altitude in Japan

In order to characterize the pathological features of high altitude pulmonary edema (HAPE) occurring at moderate altitude in Japan, we performed routine hematoxylin and eosin (HE) staining in lung materials from HAPE autopsied cases. We also undertook advanced immunohistochemical staining for observation of type II pneumocytes, pulmonary surfactant (PS), and mast cells in the lung of HAPE cases to examine the biological changes within the lung parenchyma. The pathological findings of HAPE were characterized by alveolar edema, congestion of pulmonary vessels, alveolar hyaline membranes, alveolar hemorrhage, and multithrombi and fibrin clots, but maintained alveolar structure. The immunostaining results showed that the type II pneumocytes were cellular fusion, deformity, and exfoliation from the walls of alveoli; the PS not only lined the alveolar surface, but was also patchily distributed within alveoli; and the number of mast cells were increased (9.0 +/- 0.9 cells/mm(2)) compared to that in controls (1.1 +/- 0.4 cells/mm(2)) (p < 0.01). We conclude that the pathological features of HAPE at moderate altitude in Japan are similar to others reported worldwide, and that the type II pneumocytes, PS, and mast cells may contribute to some extent to pathophysiological parts in the development and progression of HAPE.

Pulmonary edema in 6 children with Down syndrome during travel to moderate altitudes

OBJECTIVE

Children with Down syndrome (DS) are living longer and are increasingly participating in recreational activities. When a child with DS was diagnosed with high-altitude pulmonary edema (HAPE), this study was undertaken to determine whether and under what circumstances children with DS develop HAPE.

DESIGN

A retrospective review of the medical records of Children's Hospital, Denver, Colorado was performed for children with a discharge diagnosis of HAPE. Diagnostic criteria for HAPE included the presence of crackles or frothy sputum production on examination, hypoxemia, chest radiograph findings consistent with pulmonary edema, and rapid clinical improvement after descent or oxygen therapy.

RESULTS

A total of 52 patients with HAPE were found of whom 6 also had DS. The age range of the children with DS was 2 to 14 years. HAPE developed at altitudes ranging from 1738 to 3252 m. Four children developed HAPE within 24 hours of arrival to altitude. Three children had chronic pulmonary hypertension, and 4 had either an existing cardiac defect with left-to-right shunt or previously had a defect with left-to-right shunt that had been repaired. One child had Eisenmenger syndrome with chronic right-to-left shunting of blood. Five children had preexisting illnesses before travel to altitude.

CONCLUSION

Children with DS often have medical problems such as chronic pulmonary hypertension, frequent infections, and pulmonary vascular overperfusion and injury from existing or previous cardiac defects. These problems all may be viewed as risk factors for HAPE and thus result in the rapid development of HAPE at low altitudes. Care should be taken when traveling to even moderate altitudes with children with DS.

Physiology in medicine: importance of hypoxic pulmonary vasoconstriction in maintaining arterial oxygenation during acute respiratory failure

Hypoxic pulmonary vasoconstriction continues to attract interest more than half a century after its original report because of persistent mystery about its biochemical mechanism and its exact physiological function. Recent work suggests an important role for pulmonary arteriolar smooth muscle cell oxygen-sensitive voltage-dependent potassium channels. Inhibition of these channels by decreased PO2 inhibits outward potassium current, causing membrane depolarization, and calcium entry through voltage-dependent calcium channels. Endothelium-derived vasoconstricting and vasodilating mediators modulate this intrinsic smooth muscle cell reactivity to hypoxia. However, refined modeling of hypoxic pulmonary vasoconstriction operating as a feedback mechanism in inhomogeneous lungs, using more realistic stimulus-response curves and confronted with direct measurements of regional blood flow distribution, shows a more effective than previously assessed ability of this remarkable intrapulmonary reflex to improve gas exchange and arterial oxygenation. Further studies could show clinical benefit of pharmacological manipulation of hypoxic pulmonary vasoconstriction, in circumstances of life-threatening hypoxemia.