High-altitude pulmonary edema (HAPE) is a potentially life-threatening condition involving a form of noncardiogenic pulmonary edema thought to be caused by an endothelial breakdown in the lungs secondary to unequal capillary pressure. The condition typically affects healthy individuals at altitudes greater than 8000 feet above sea level. However, individual susceptibility varies greatly. This condition is the leading cause of death attributed to high altitude but is largely preventable with proper acclimatization and rate of ascent, early recognition of symptoms, and appropriate treatment.
Risk Factors for high-altitude pulmonary edema include but are not limited to the following:
High-altitude pulmonary edema most often effects those at high altitudes (greater than 8000 feet above sea level), although it may occur at lower altitudes. Overall incidence is highly variable ranging from 0.2% to 6% at approximately 4500 meters above sea level to 2% to 15% at 5500 meters above sea level.
Pathophysiology of high altitude pulmonary edema is multifactorial and complex. The inciting event is believed to be an increase in pulmonary artery pressure related to the hypoxic pulmonary vasoconstrictor response to hypoxic conditions. In fact, all individuals with high altitude pulmonary hypertension have pulmonary hypertension, but not all those with pulmonary hypertension have high altitude pulmonary edema. Therefore, the initiating event is believed to be an increase in pulmonary artery pressure likely caused by the hypoxic pulmonary vasoconstrictor response. This response normally improves ventilation and perfusion matching at sea level, however, in global hypoxia from high altitude, the hypoxic pulmonary vasoconstrictor response does not improve ventilation and perfusion matching. Some capillary beds in the lungs that have less vasoconstriction than others are subject to unequal capillary pressure which can lead to over perfusion of that specific capillary bed. The capillary beds in the lungs are unequal in their micro perfusion, and therefore, this may result in capillary leakage and a patchy pulmonary edema. This edema is first composed of plasma and some red blood cells, but as the endothelial barrier is disrupted further, can lead to frank hemorrhage and spillage of protein. Other factors thought to affect the breakdown of the endothelial barrier include individual factors affecting the synthesis of nitric oxide and endothelin. Those individuals with decreased nitric oxide synthesis and increased endothelin synthesis, when exposed to hypoxic conditions, are more susceptible to high altitude pulmonary edema.
Alcohol use and sleeping pills are thought to increase the likelihood of developing high-altitude pulmonary edema. This is likely due to a decrease in the respiratory drive which can lead to an increase in hypoxia. An increase in hypoxia can lead to the inciting event of an increase in pulmonary artery pressure due to hypoxic pulmonary vasoconstriction.
Early symptoms of high altitude pulmonary edema include shortness of breath on exertion, decreased exercise capacity, and a non-productive cough. In more advanced illness, these symptoms can progress to shortness of breath at rest, productive cough of pink, frothy sputum, and orthopnea. Coexisting symptoms of acute mountain illness such as nausea, headache, difficulty sleeping, fatigue, and loss of appetite can also occur. Physical exam findings can include tachypnea, tachycardia, cyanosis, and rales heard on auscultation of the chest.
No specific laboratory or radiographic findings are necessary to establish the diagnosis of high altitude pulmonary edema as it is largely a clinical diagnosis. However, certain findings on chest x-ray, CT, ultrasound, and echocardiography can help support the diagnosis. Radiographic findings in high altitude pulmonary edema include patchy infiltrates, most often seen in the right middle lung. This can mimic that of infectious infiltrates and becomes more diffuse and bilateral as the disease progresses. On lung ultrasound, ultrasound lung comets or "B-lines" as they are commonly referred to, can be seen. This sonographic artifact is due to the presence of fluid and is caused by air-fluid interference. Echocardiography shows increased pulmonary artery pressures and may show right heart dysfunction. Laboratory testing may or may not show an elevated white blood cell count as well as elevated pro-BNP and troponin if there is heart strain.
Multiple treatment options are available for high altitude pulmonary edema including oxygen, rest and warmth, descent, hyperbaric treatment, positive pressure ventilation, and medications. The most effective treatment for high altitude pulmonary edema is descent and oxygen therapy. Both descent and oxygen therapy relieve hypoxemia and decrease the hypoxic pulmonary vasoconstrictor response which is the initiating factor in disease progression. Decreasing exertion and warming the patient can help to decrease pulmonary artery pressure which can help prevent exacerbation of pulmonary edema. If descent is not a possibility, hyperbaric treatment in a hyperbaric chamber with or without concurrent oxygen therapy can be a helpful temporizing measure until descent is possible. Several medications are effective in the treatment of high altitude pulmonary edema. Nifedipine, a calcium channel blocker, has been shown to potentially decrease pulmonary artery pressure and slightly improve PaO2. Dosing regimens vary, but 30 mg every 12 hours is a commonly used treatment strategy. Sildenafil and Tadalafil, both phosphodiesterase-5 inhibitors, function by increasing the amount of nitric oxide available. Nitric oxide is a pulmonary vasodilator and decreases hypoxic pulmonary vasoconstriction and pulmonary artery pressure. Dosages are not established for treatment. However, doses similar to those for prophylaxis are used (Sildenafil 50 mg every eight hours and Tadalafil 10 mg every 12 hours). Dexamethasone and beta agonists have not been adequately studied in the treatment of high altitude pulmonary edema, and it is no longer recommended to use morphine, diuretics, or nitrates.
The risk of developing high altitude pulmonary edema can be reduced by avoiding alcohol and sleeping pills, proper acclimatization, slow rate of ascent, and early recognition of signs and symptoms to provide early treatment and descent, if necessary. Certain medications have also been determined to be effective in preventing high altitude pulmonary edema in susceptible individuals. These medications include dexamethasone, nifedipine, acetazolamide, sildenafil, and tadalafil. These agents are thought to decrease the degree of hypoxic pulmonary hypertension, thereby blocking the first step in the pathway of developing high-altitude pulmonary edema.
Patients presenting to a hospital or other healthcare facility with high altitude pulmonary edema need to be monitored closely as respiratory status can dramatically worsen resulting in respiratory distress or failure. Anticipation of this decline in respiratory status is important as patients with severe high altitude pulmonary edema may require intubation if ventilation or oxygenation are severely impaired.