Effect of hypoxia on regional lung perfusion, by scanning

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.

Magnetic Resonance Imaging of Uneven Pulmonary Perfusion in Hypoxia in Humans

RATIONALE

Inhomogeneous hypoxic pulmonary vasoconstriction causing regional overperfusion and high capillary pressure is postulated for explaining how high pulmonary artery pressure leads to high-altitude pulmonary edema in susceptible (HAPE-S) individuals.

OBJECTIVE

Because different species of animals also show inhomogeneous hypoxic pulmonary vasoconstriction, we hypothesized that inhomogeneity of lung perfusion in general increases in hypoxia, but is more pronounced in HAPE-S. For best temporal and spatial resolution, regional pulmonary perfusion was assessed by dynamic contrast-enhanced magnetic resonance imaging.

METHODS

Dynamic contrast-enhanced magnetic resonance imaging and echocardiography were performed during normoxia and after 2 h of hypoxia (Fi(O2) = 0.12) in 11 HAPE-S individuals and 10 control subjects. As a measure for perfusion inhomogeneity, the coefficient of variation for two perfusion parameters (peak signal intensity, time-to-peak) was determined for the whole lung and isogravitational slices.

RESULTS

There were no differences in perfusion inhomogeneity between the groups in normoxia. In hypoxia, analysis of coefficients of variation indicated a greater inhomogeneity in all subjects, which was more pronounced in HAPE-S compared with control subjects. Discrimination between HAPE-S and control subjects was best in gravity-dependent lung areas. Pulmonary artery pressure during hypoxia increased from 22 +/- 3 to 53 +/- 9 mm Hg in HAPE-S and 24 +/- 4 to 33 +/- 6 mm Hg in control subjects (mean +/- SD; p < 0.001), respectively.

CONCLUSION

This study shows that hypoxic pulmonary vasoconstriction is inhomogeneous in hypoxia in humans, particularly in HAPE-S individuals where it is accompanied by a greater increase in pulmonary artery pressure compared with control subjects. These findings support the hypothesis of exaggerated and uneven hypoxic pulmonary vasoconstriction in HAPE-S individuals.

Heterogeneous pulmonary blood flow in response to hypoxia: A risk factor for high altitude pulmonary edema?

High altitude pulmonary edema (HAPE) is a rapidly reversible hydrostatic edema that occurs in individuals who travel to high altitude. The difficulties associated with making physiologic measurements in humans who are ill or at high altitude, along with the idiosyncratic nature of the disease and lack of appropriate animal models, has meant that our understanding of the mechanism of HAPE is incomplete, despite considerable effort. Bronchoalveolar lavage studies at altitude in HAPE-susceptible subjects have shown that mechanical stress-related damage to the pulmonary blood gas barrier likely precedes the development of edema. Although HAPE-susceptible individuals have increased pulmonary arterial pressure in hypoxia, how this high pressure is transmitted to the capillaries has been uncertain. Using functional magnetic resonance imaging of pulmonary blood flow, we have been able to show that regional pulmonary blood flow in HAPE-susceptible subjects becomes more heterogeneous when they are exposed to normobaric hypoxia. This is not observed in individuals who have not had HAPE, providing novel data supporting earlier suggestions by Hultgren that uneven hypoxic pulmonary vasoconstriction is an important feature of those who develop HAPE. This brief review discusses how uneven hypoxic pulmonary vasoconstriction increases regional pulmonary capillary pressure leading to stress failure of pulmonary capillaries and HAPE. We hypothesize that, in addition to the well-documented increase in pulmonary vascular pressure in HAPE-susceptible individuals, increased perfusion heterogeneity in hypoxia results in lung regions that are vulnerable to increased mechanical stress.

Sildenafil improves cardiac output and exercise performance during acute hypoxia, but not normoxia

Sildenafil causes pulmonary vasodilation, thus potentially reducing impairments of hypoxia-induced pulmonary hypertension on exercise performance at altitude. The purpose of this study was to determine the effects of sildenafil during normoxic and hypoxic exercise. We hypothesized that 1) sildenafil would have no significant effects on normoxic exercise, and 2) sildenafil would improve cardiac output, arterial oxygen saturation (SaO2), and performance during hypoxic exercise. Ten trained men performed one practice and three experimental trials at sea level (SL) and simulated high altitude (HA) of 3,874 m. Each cycling test consisted of a set-work-rate portion (55% work capacity: 1 h SL, 30 min HA) followed immediately by a time trial (10 km SL, 6 km HA). Double-blinded capsules (placebo, 50, or 100 mg) were taken 1 h before exercise in a randomly counterbalanced order. For HA, subjects also began breathing hypoxic gas (12.8% oxygen) 1 h before exercise. At SL, sildenafil had no effects on any cardiovascular or performance measures. At HA, sildenafil increased stroke volume (measured by impedance cardiography), cardiac output, and SaO2 during set-work-rate exercise. Sildenafil lowered 6-km time-trial time by 15% (P<0.05). SaO2 was also higher during the time trial (P<0.05) in response to sildenafil, despite higher work rates. Post hoc analyses revealed two subject groups, sildenafil responders and nonresponders, who improved time-trial performance by 39% (P<0.05) and 1.0%, respectively. No dose-response effects were observed. During cycling exercise in acute hypoxia, sildenafil can greatly improve cardiovascular function, SaO2, and performance for certain individuals.

Unraveling the mechanism of high altitude pulmonary edema

During the last decade, major advances in the understanding of the mechanism of high altitude pulmonary edema (HAPE) have supplemented the landmark work done in the previous 30 years. A brief review of the earlier studies will be described, which will then be followed by a more complete treatise on the subsequent research, which has elucidated the role of accentuated pulmonary hypertension in the development of HAPE. Vasoactive mediators, such as nitric oxide (NO) and endothelin-1, have played a major role in this understanding and have led to preventive and therapeutic interventions. Additionally, the role of the alveolar epithelium and the Na-K ATPase pump in alveolar fluid clearance has also more recently been understood. Direction for future work will be given as well.

Negative pressure pulmonary edema after acute upper airway obstruction

STUDY OBJECTIVES

To review the clinical characteristics and the pathogenesis of negative pressure pulmonary edema, and to determine its incidence in surgical patients.

DESIGN

Retrospective case-report study.

SETTING

Operating room, postanesthesia care unit and surgical intensive care of a teaching hospital.

PATIENTS

30 surgical adult ASA physical status I, II, III, IV, and V patients who suffered from negative pressure pulmonary edema during the period 1992-1995.

MEASUREMENTS AND MAIN RESULTS

This study showed a rapid onset of negative pressure pulmonary edema after acute upper airway obstruction, due mainly to laryngospasm in the postoperative period and to upper airway pathology in the preoperative period. Negative pressure pulmonary edema appeared more frequent in healthy (ASA physical status I and II), middle-aged and male patients, with a general incidence of 0.094%. The resolution was relatively rapid after reestablishment of the airway, adequate oxygenation, and positive airway pressure application. The clinical course was uncomplicated in all the patients.

CONCLUSIONS

In this study, negative pressure pulmonary edema presented a relatively high incidence. Prevention, early diagnosis, and prompt treatment allowed a rapid and uncomplicated resolution.

Variable radiomorphologic data of high altitude pulmonary edema. Features from 60 patients

The purpose of the study was to collect radiomorphologic data of a large population of subjects with high altitude pulmonary edema. A blinded retrospective analysis of 60 patients severe enough to warrant hospital admission is reported. Immediately after rescue to low altitude, the severity of HAPE was graded using a quadrant-based scoring system (0-4 each quadrant). Its distribution and the morphologic features were noted. HAPE was more severe in the base, and specifically, the right lower quadrant, as compared to the other quadrants. It was often located both centrally and peripherally (60 percent) and in 92 percent was characterized by air space disease of homogeneous (n = 40) rather than patchy distribution (n = 15). In recurrent HAPE (n = 13), radiomorphologic data were as variable as among different HAPE patients. We conclude that HAPE does not have one common radiomorphologic condition. Based on the literature, earlier experience, and follow-up observations, we hypothesize that it may start patchy and peripheral, supporting the concept of uneven vasoconstriction with overperfusion and/or permeability leak. Later on, such as in the severe cases studied, it becomes homogeneous. Recurrent episodes generally do not show an identical distribution of HAPE, suggesting that structural abnormalities are not involved in the pathogenesis of HAPE.

Recurrent cyanotic episodes with severe arterial hypoxaemia and intrapulmonary shunting: a mechanism for sudden death

The pathophysiology of recurrent cyanotic episodes has been investigated in 51 infants and children. Episodes began at a median age of 7 weeks (range 1 day to 22 months, 39 at less than 4 months). They were characterised by the rapidity of onset and progression of severe hypoxaemia with early loss of consciousness from cerebral hypoxia. The most common precipitating factor was a sudden naturally occurring stimulus from pain, fear, or anger. In uncontrolled trials, cyanotic episodes were reduced in frequency and severity by tetrabenazine (n = 15) and additional inspired oxygen (n = 10). Eight patients died suddenly and unexpectedly (four during cyanotic episodes). Twenty eight patients underwent physiological studies during cyanotic episodes. There was no evidence of seizure activity at the onset and although prolonged absence of inspiratory effort with continued expiratory efforts was common, breathing sometimes continued. Episodes were not caused by upper airway obstruction and sometimes occurred during positive airway pressure ventilation. The rapidity of fall in arterial oxygen pressure and continued breathing suggested a right to left shunt of sudden onset. The results of contrast echocardiography and lung imaging studies confirmed that this was occurring within the lungs. These cyanotic episodes included both intrapulmonary shunting and prolonged expiratory apnoea. They are best explained by interactions between central sympathetic activity, brainstem control of respiration and vasomotor activity, reflexes arising from around and within the respiratory tract, and the matching of ventilation to perfusion in the lungs. They are a cause of sudden unexpected death in infancy and early childhood.

Pulmonary oedema associated with airway obstruction

The purpose of this review is to describe the pathogenesis of pulmonary oedema associated with upper airway obstruction, summarize what is known of its clinical presentation, and reflect upon its implications for the clinical management of airway obstruction. The pathogenesis of pulmonary oedema associated with upper airway obstruction is multifactorial. However, as the phrase "negative pressure pulmonary oedema" suggests, markedly negative intrapleural pressure is the dominant pathophysiological mechanism involved in the genesis of pulmonary oedema associated with upper airway obstruction. The frequency of the event is impossible to ascertain from the literature but paediatric cases requiring airway intervention for croup or epiglottitis and adults requiring airway intervention for emergence laryngospasm or upper airway tumours account for over 50 per cent of the documented cases in each age group, respectively. Individuals at risk should be observed closely while they remain at risk. The majority of cases present within minutes either of the development of acute severe upper airway obstruction or of relief of the obstruction. Resolution is typically rapid, over a period of a few hours. Rarely is anything more required for management than the maintenance of a patent airway, supplemental oxygen, and, in approximately 50 per cent of cases, mechanical ventilation and positive end-expiratory pressure.

High-altitude pulmonary edema: a collective review

In summary, HAPE is a potentially fatal form of noncardiogenic PE seen in a small number of individuals visiting above 9,000 ft in elevation. The pathophysiology is uncertain but is probably due, at least in part, to hydrostatic and capillary permeability abnormalities of the pulmonary vascular bed in response to hypobaric hypoxia. A subclinical form above 14,000 ft is common (15% to 23% incidence), but the incidence of HAPE itself is unclear. Possible risk factors include rapid ascent, strenuous activity on arrival, reascent to altitude by highlanders after a short stay lower, previous HAPE, cold, respiratory tract infections, sedation, youth, and the peripheral edema of AMS. Clinical presentation is similar to that of pneumonia: tachypnea, tachycardia, cyanosis, cough, fever, and chest discomfort. Symptoms often worsen with sleep. WBC count is usually elevated, and arterial blood gases reveal a respiratory alkalosis and an alarmingly low hemoglobin saturation. Chest radiographs reveal bilateral patchy infiltrates. Radiographic findings are dissimilar to those from cardiogenic PE. Differential diagnosis includes pneumonia, PE and HAB. Treatment modalities include early descent, bed rest, oxygen therapy, and EPAP. Mortalities range from 4% to 27% depending on the rapidity of descent and evacuation.

High-altitude pulmonary edema: pathophysiology and clinical review

High-altitude pulmonary edema (HAPE) affects young, healthy climbers in an unpredictable fashion. It is potentially fatal, and its underlying pathophysiology is not thoroughly understood. The history and clinical presentation of HAPE, as well as the known underlying pathophysiology, are reviewed. For instance, in HAPE there is an association with blunted respiratory drives to hypoxia and accentuated hypoxic pulmonary vasoconstriction. Recent data show that HAPE is a high permeability leak of protein into the alveolar space associated with an influx of alveolar macrophages. These data have been obtained recently by fiberoptic bronchoscopy in the field setting of Mt McKinley at 4,400 m. The approach to recognition and treatment that involves primarily descent and/or oxygen is discussed.

Effects of asphyxia on lung fluid balance in baby lambs

The purpose of this study was to assess the effects of combined hypoxia and hypercapnia and of severe asphyxia on lung water balance and protein transport in newborn lambs. We studied ten 2-4-wk-old anesthetized lambs which were mechanically ventilated first with air for 2-3 h, then with 10-12% oxygen in nitrogen for 2-4 h, and then with 10-12% oxygen and 10-12% carbon dioxide in nitrogen for 2-4 h. Next we stopped their breathing for 1-2 min to produce severe asphyxia, after which we followed their recovery in air for 2-4 h. In 5 of the 10 lambs we intravenously injected radioactive albumin and measured its turnover time between plasma and lymph during the baseline period and after recovery from asphyxia. During alveolar hypoxia alone, mean pulmonary arterial pressure increased 60% and lung lymph flow increased 74%, whereas lymph protein concentration decreased from 3.47 +/- 0.13 to 2.83 +/- 0.15 g/dl. Cardiac output, left atrial pressure, and plasma protein concentration did not change. When carbon dioxide was added to the inspired gas mixture, pulmonary arterial pressure increased 22%, cardiac output increased 13%, lung lymph flow increased 33%, and lymph protein concentration decreased from 2.83 +/- 0.15 to 2.41 +/- 0.13 g/dl. Left atrial pressure and plasma protein concentration did not change. After 60-90 s of induced asphyxia, vascular pressures and lung lymph flow rapidly returned to values the same as those obtained during the baseline period. The turnover time for radioactive albumin between plasma and lymph was the same between the baseline and recovery periods (185 +/- 16 vs. 179 +/- 12 min). The ratio of albumin to globulin in lymph relative to the same ratio in plasma did not change during any phase of these experiments. Five lambs killed after recovery from asphyxia had significantly less blood and extravascular water in their lungs than control lambs had. We conclude that in the newborn lamb both alveolar hypoxia and alveolar hypoxia with hypercapnia increase lung lymph flow by increasing filtration pressure in the microcirculation, but neither hypoxia with hypercapnia nor brief severe asphyxia alters the protein permeability of the pulmonary microcirculation.