"Should I take Viagra to help me climb Everest?"
Yes, I've been asked that question once or twice! Keep a straight face and take a deep breathe before giving your answer. To understand where Viagra fits into high altitude medicine it helps to know a little about the right side of the heart. Of course, most of the time it is the opposite side of the heart that grabs the attention. Oxygenated blood returns from the lungs and is deposited into the left atrium. From there it is pulled and pushed into the left ventricle and fired towards the vital organs. As everyone knows, without a healthy left ventricle, the flow of blood stalls and the body simply fails. But hidden in the shadows is the right heart and it is just as important. The right heart receives deoxygenated blood from the veins and pumps it towards the lungs. Thanks to evolution, flow through the pulmonary arteries, arterioles and capillaries meets with the minimum of resistance. Nevertheless without the effort of the right heart nothing would reach the other side!
The low resistance of the right heart can be seen most clearly when you compare the blood pressures on both sides. Whilst the systolic blood pressure leaving the left ventricle is 100-140mmHg, on the right it is just 15-30mmHg. A 5 fold difference! However this all changes in hypoxia. As oxygen levels fall, pulmonary blood vessels begin to constrict and pressure starts to rise. Whilst some vessels shut completely, others barely change. This results in a patchy flow of blood to different parts of the lung. Whilst some parts dry up completely, others become stretched and distended with blood. At high pressure, the thin membrane that separates the lungs from the blood vessels can split apart and cause fluid to leak into the airways. This creates a mechanical barrier that prevents oxygen from binding to red blood cells and entering the blood stream. At present it's not clear to what extent this occurs in healthy individuals who ascend to high altitude. However in High Altitude Pulmonary Edema (HAPE) this process is so widespread that victims simply drown in the fluid that collects in their lungs.
The increase in pressure also has important consequences for the right heart. Since the pressure in the right ventricle is normally low, the muscle layer is thin and poorly developed. Unfortunately, if there is a sudden increase in pressure the right heart struggles to cope and blood backs up into the veins. Without enough blood entering the lungs, oxygen levels within the arteries fall and tissues start to fail.
The long vertical lines on this lung ultrasound scan are "Comet Tail Artefacts" or "B Lines". These are only visible when water escapes from the capillaries and accumulates in the airways.
What about Viagra? Viagra, otherwise known as sildenafil, prevents the pulmonary blood vessels from constricting. This is done by increasing the availability of a molecule called nitric oxide (NO). NO is a powerful vasodilator contained within the lining of blood vessels. In some individuals this can fall sharply when exposed to hypoxic conditions.
Could this be of benefit at high altitude?
A fall in pulmonary artery pressure could reduce the leakage of fluid into the lungs AND improve the contraction of the right heart. Together, these could lead to an increase in the amount of oxygen that reaches the muscles and an improvement in physical performance. Unfortunately, a series of small studies, using different doses of sildenafil over a range of different altitudes, real and simulated, have not been able to confirm this and have instead generated a rather confusing picture!
Nevertheless pulmonary artery pressure certainly falls. In a review of five randomized controlled trials of healthy individuals given sildenafil at high altitude, the pulmonary artery pressure fell by a mean of 5 mmHg.
But what everyone really wants to know is – can we climb better if we take it?
Researchers have looked at this by measuring physical performance in three different ways - maximum workload, VO2 max and time taken to complete a physical task. Early studies revealed positive results. Researchers studying 14 healthy volunteers at Mt Everest Base Camp (5300m) found that those taking sildenafil were able to pedal 40W harder on a cycle ergometer compared to those taking placebo. In an early pressure chamber study to a simulated altitude of 4350m, the fall in VO2 max amongst six participants taking sildenafil was 10% lower than that found in the same number who took placebo. Amongst ten cyclists completing a 5km timed ride at a simulated altitude of 3874m, those taking sildenafil were 15% quicker.
However as the years have passed, positive results have become increasingly hard to find. In a recent PubMed search I was able to find details of ten studies that examined the impact of sildenafil upon physical performance in the hypoxic environment. Only four showed a positive effect. The six most recent studies were not able to replicate the results of the earlier work.
To add a further note of caution, all ten studies highlighted side effects from taking the drug. Headache was commonly reported. Therefore it's perhaps not surprising that those taking sildenafil sometimes had a higher AMS score! From personal experience I can concur. As part of a high altitude study (naturally!) I remember suffering several hours of headache, upset stomach and facial flushing following a dose of sildenafil. I wouldn't be keen to take it again!
So when asked,"Should I take Viagra to help me climb Everest?" my answer goes something like this...
"Early research showed that taking Viagra led to improvements in high altitude performance amongst a number of small groups of healthy volunteers.
However in recent years, study after study has been unable to confirm these early findings. No differences in maximum workload, VO2 max or time taken to perform physical tasks have been found between those taking sildenafil and placebo.
A great deal of uncertainty now exists.
Given these findings, I would not advise taking Viagra to boost physical performance at high altitude. But if you do take it you need to be aware that side effects are common and these may have a detrimental effect on your time in the mountains."