Saturday, February 22, 2014
Hypertension at Altitude - A Conundrum Without Evidence
I was asked the other day about blood pressure at altitude, a subject that I knew nothing about because I had never worked as a doctor in a country at altitude. No part of the UK is at a significant altitude so it was not a part of my medical school curriculum.
The person in question is a diabetic already on treatment for hypertension and he had just bought an automated sphygmomanometer to check his blood pressure. He bought it at about sea level, a place that he regularly visits to work and play but he lives at 2200 metres.
An altitude of 2200m is not exceptional worldwide. It is not high enough to give altitude sickness, like Cusco (Peru) at 3400m. However, it is significantly higher than any major, highly populated geographical area in any of the countries that do the vast majority of the medical research into such health issues.
His blood pressure, on treatment (Amlodipine/Valsartan and incidentally Prazosin for his prostate) is 160/95mmHg at altitude where he sees his cardiologist, who obviously, according to all of the protocols and guidelines, is increasing his medication. However, his blood pressure at sea level is repeatedly and consistently 110/55.
I wondered if it could be a problem with the sphygmomanometer that the pressure readings are erroneous because of the low atmospheric pressure at altitude. If the pressure difference of altitude were around 40 or 50mmHg, it would all make immediate sense. When I checked the numbers, I was shocked to find how much pressure has been lost in the atmosphere at 2200m- 160mmHg.
At 2200m, oxygen pressure is only 80% of the sea level value. Water boils at 93 degrees celcius.
Yes, at this altitude, tea tastes terrible. Water does not boil hard enough to extract the full taste from coffee and you have to add 50% cooking time to pasta, rice or whatever else it is that you are cooking!
But this discrepancy was simply too much to suggest that the problem is a simple error of baseline. Can anybody advise on this topic of the need to recalibrate automated sphygs at altitude?
Here is a bizarre thought that puts it all into perspective. Since we measure blood pressure relative to atmospheric pressure, his systolic pressure at altitude after compensation is effectively ZERO! He has a diastolic of minus 65 in absolute terms!
Does anybody know any evidence for the treatment of hypertension at altitude?
Should this blood pressure be treated? Or is this a natural compensatory mechanism that should not be interfered with?
Increased cardiac output, including increased pulse and blood pressure, is an acknowledged compensatory mechanism for high altitude. On treatment, this guy is almost 'narcoleptic' in his exhaustion as if not enough blood is getting to his brain. He also gets faintness and vertigo, symptoms of hypotension. He is generally weak and listless.
If the 110/55 blood pressure were representative, and with a normal pulse rate of 60-65 bpm, I would diagnose him as hypotensive! However, the numbers at altitude say he is hypertensive and in need of more hypotensive medication....
To complicate matters more, he has mild aortic stenosis....
Without this information that can be relied upon, I am left with educated guesswork from first principle theories.
So what do we know? Well, this is what I know on the matter....
We know that elevated BP is a normal compensatory mechanism for altitude in order to get adequate levels of oxygen to the brain.
As far back as he knows, his family has always lived at altitude. As well as making temporary adjustments through the 2 3 DPG system to change haemoglobin oxygen carrying, he may well have genetic/epigenetic changes for high altitude living. Such long term changes are recorded in terms of evidence (without mechanism) in the higher regions of the Andes where the conquistadores could never fully adjust like the locals as seen in their fertility and abortion rates (which were essentially 0% and 100% respectively, the latter when a woman arrived in the area already pregnant).
So he could be of a significantly different genetic/epigenetic stock to whom western averages and guidelines from low altitude do not apply.... but they are the only hard evidence we have to go on and they say that he needs MORE antihypertensives.... The evidence suggests that not treating him could be fatal.
But is that evidence conditional and only applicable in the population tested? Chronic hypotension is also statistically life threatening....
The risk of high blood pressure is in part a direct risk factor and treatment directly decreases risks. It is also an indicator of other risk factors such as atherosclerosis and renal disease where the treatment of blood pressure does not necessarily, directly improve outcomes. Certainly, in terms of intra-arterial damage, one would imagine that it is the pulse pressure rather than the absolute levels of diastolic and systolic that cause turbulent flow and trauma.
At least focusing on pulse pressure allows us to forget that his absolute BP is -65/0.... which would explain his hypotensive symptoms....
Or is it just the syphgmomanometer producing spuriour readings? Should we forget the high altitude readings and treat the low altitude readings....
I am confused and really glad that I am not in the position of making this life and death decision. I am appreciative that this is only a fun, academic exercise for me.
But really shows the conditionality of evidence and just how important it is to have the highest quality evidence specific to the situation on which to make these decisions....
Unless anybody knows of research in this area, research is desperately needed!