High Altitude Pulmonary Oedema (HAPE)

AUTHOR: DR PERSIA BOWATER

What is High Altitude Pulmonary Oedema ?

HAPE is a condition which can occur in climbers who have rapidly ascended in altitude and must be considered in all individuals who develop shortness of breath and a cough.

Pathophysiology

The pathophysiology of High Altitude Pulmonary Oedema (HAPE) is thought to be multifactorial. The trigger is the rapid change in atmospheric composition accompanied by change in altitude. As we ascend, atmospheric pressure decreases, and therefore the partial pressure of oxygen in the atmospheric pressure decreases. This results in effective hypoxia. 

Atmospheric hypoxia leads to a reduction in nitric oxide production in the pulmonary circulation, as the body attempts to increase flow of deoxygenated blood to the lungs by increasing the pulmonary pressure. Although this is an important physiological element of altitude acclimatisation, during rapid ascent it can cause inappropriate vasoconstriction in the pulmonary capillaries, leading to an increase in pressure and resultant leakage of fluid into the alveolar and interstitial spaces.(1,2,4)

There is also evidence that some people are more likely to suffer from High Altitude Pulmonary Oedema (HAPE) through a genetic predisposition to impairment in alveolar transepithelial sodium transport, which is aggravated at high altitudes and causes reduced alveolar water clearance.(3)

A combination of these factors leads to pulmonary hypertension and oedema.

Presentation

Climbers who have rapidly ascended to greater than 2500m may experience a sudden worsening of their functional ability, accompanied by shortness of breath and a cough. As High Altitude Pulmonary Oedema (HAPE) progresses, this is followed by the other hallmark features of pulmonary oedema – a ‘wet’ sounding chest, coughing up pink, frothy sputum, cyanosis and orthopnoea. Without rapid treatment, hypoxia is inevitable and HAPE can prove fatal in 50% of cases if not treated. 

This ‘textbook’ presentation of High Altitude Pulmonary Oedema (HAPE) may not always be the case, and it can be far more insidious with mountaineers or trekkers having a shortness of breath as the predominant symptom, associated with general lethargy and mild cough. As an expedition medic, one should be wary of anyone who cannot, for example, do up their shoe lace, or is persistently lagging behind the group. 

Investigations

On the mountain, recognition of the clinical signs mentioned above and basic observations are the most important and often only form of investigation.

Once in hospital, a chest x-ray to confirm and quantify the pulmonary oedema may be useful – ‘cotton wool’ infiltrates in the mid and lower zones may be seen. Arterial blood gases may show respiratory alkalosis. Cardiac ECHO may have comet tail artefacts, and ECG will likely show sinus tachycardia and signs suggestive of pulmonary hypertension (right axis deviation and bundle branch block; peaked P waves in leads II, III, and aVF; and an increase in the depth of precordial S waves).

Figure 1: Chest X-ray showing cotton wool infiltrates in HAPE. (10)

Management

Prevention is always better than cure. A slow ascent allows the body to acclimatise to changes in atmospheric oxygen. Initially, the Hypoxic Ventilatory Response (HVR) increases the rate and depth of respiration, raising alveolar ventilation by 25–30%. Hypoxic Pulmonary Vasoconstriction (HPV), which can also be a main contributor to High Altitude Pulmonary Oedema, is essential in improving blood oxygenation. Cardiac output increases due to increased rate and stroke volume. The kidneys produce more erythropoietin to stimulate production of red blood cells, and diuresis also increases to concentrate the blood – together these processes improve the oxygen transport capacity of the blood. However, all these processes take time. If acclimatisation is given time to occur, the risk of all major high altitude sicknesses are significantly reduced.

Of all the altitude illnesses, High Altitude Pulmonary Oedema is known to catch people out: those who have previously ascended to altitude, or have followed a very conservative ascent profile can experience High Altitude Pulmonary Oedema later in their trip, thus catching expedition medics unaware until the HAPE is severe. 

If symptoms appear and if it is possible, Descent is the first-line treatment.

If immediate descent is not possible, the Wilderness Medical Society (WMS) recommend giving oxygen to saturations of >90% if available, or during descent if the patient is severely unwell. A portable hyperbaric/altitude chamber (PAC) can be used if oxygen is not available – this has a greater impact on patients with HACE, but guidelines have suggested the PAC can also be of benefit in HAPE. If none of these options are available, Pharmacological Management is recommended. The first line drug is Nifedipine MR 20mg TDS – a calcium channel blocker which induces pulmonary vasodilation. Tadalafil or Sildenafil can be used if Nifedipine is not available – these are Phosphodiesterase 5- inhibitors which increase availability of nitric oxide.

If climbers have previously suffered from High Altitude Pulmonary Oedema (HAPE), Pharmacological Prophylaxis may be indicated as the likelihood of recurrence is as high as 60%. Nifedipine is again the drug of choice and should be taken from the day before ascent begins, to the day of descent.

A patient’s symptoms must have completely resolved, and they should have stable oxygenation levels during exercise, before considering whether they can continue to ascend.

Differentials to consider

It is important to remember that there are other causes of illness that are not due to high altitude, and you should always consider other causes of respiratory distress such as pneumonia, asthma exacerbation, pulmonary embolism, pleural effusion and pneumothorax. Also consider alternative causes for pulmonary oedema, such as an acute exacerbation of chronic heart failure. Past medical history and adequate pre screening should have been undertaken prior to departure. 

Take home messages

• High Altitude Pulmonary Oedema (HAPE) is pulmonary oedema which occurs as a result of physiological pulmonary vasoconstriction in response to altitude related hypoxia.
• A slow ascent is key to prevention – above 3000m, do not increase sleeping elevation by more than 500m each day. Try to include a rest day every 3-4 days. Avoid overexertion, alcohol and sleeping pills.
• If signs of High Altitude Pulmonary Oedema (HAPE) appear (sudden onset cough, shortness of breath or severe fatigue), do not ascend further! Descend if possible as first line treatment.
• Nifedipine is the first line pharmacological treatment and prophylaxis.
• Pharmacological prophylaxis is only indicated if patients have previously had HAPE.

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