PowerLab on Mount Everest

The Effects of High Altitude on Breathing and Cerebral Blood Flow

In a recent study supported by ADInstruments, Inc., scientists studied the effects of high altitude on breathing and cerebral blood flow. Using a PowerLab system, the 2005 Canadian Everest Expedition obtained data at the Base Camp of the world's highest mountain, Mount Everest. Their results will help physicians gain a better diagnostic understanding of sleep disturbances and cerebral complications encountered at high altitudes.

ADInstruments proudly sponsored Dr. Philip Ainslie (University of Calgary, now at the University of Otago, NZ) and the expedition in their research endeavors. The group used an 8 channel PowerLab, which proved to be robust, as it performed in one of the world's most unforgiving environments.

The group examined the effect of high altitude on breathing and cerebral blood flow associated with acute mountain sickness and cerebral edema. Dr. Ainslie's team conducted polysomnography sleep studies, collecting ventilatory control, beat-to-beat cerebral blood flow and blood pressure data in a laboratory in Kathmandu (1400m), at Everest Base Camp (5400m) and in a research laboratory Kunde (4000m). Cerebral blood flow and blood pressure responses to hypoxia and hypercapnia were measured by Transcranial Doppler Ultrasound and a beat-to-beat non-invasive blood pressure system, respectively. All cerebral blood flow and blood pressure data were collected by and analyzed using PowerLab hardware with LabChart software.

Interview with Dr. Ainslie

What environment were the climbers exposed to before making measurements? Did you attempt to replicate the climbing behavior usually followed on an Everest climb?

All measurements were made at rest and during sleep. Typically, climbers feel the worst in the morning upon awakening which coincide with the highest incidence of acute mountain sickness. Sleep is one of the most disturbed events at high altitude. At basecamp, most of the subjects we used had some form of acute mountain sickness, which in part, would have been related to their disturbed sleep.

Did the portability of the PowerLab help with the experimental situation?

Certainly. The size of the PowerLab was perfect as it fitted into my backpack! It was also one of the few pieces of equipment which worked well with the generator at basecamp. Many other pieces of equipment demanded too much power, short-circuiting the generator.

Were there any particular features in LabChart software that were crucial?

Since we were recording the waveforms from the cerebral blood flow and blood pressure measurements, it was crucial that meaningful data was extracted in the data manipulation. Of most importance, was the fact the all data was recorded on a time-aligned beat-by-beat basis - this is vital for the calculation of transfer functions analysis based on the relationship between cerebral blood flow and blood pressure.

Are there any general conclusions or any knock down observations from the data collected?

We are still working with data; however, preliminary analysis suggests that cerebral autoregulation (the ability of the brain to adapt to changes in blood pressure) is decreased at base-camp by about 35% and further decreased during sleep by about 64%. In other words, blood flow to the brain seemed to passively follow large surges in arterial blood pressure. Such surges in blood pressure and blood flow to the brain occurred regularly during sleep at high altitude, a phenomenon known as periodic breathing. This decrease in cerebral autoregulation may be one logical reason why symptoms of acute mountain sickness and, in particular, headache, are higher upon waking.

Dr. Ainslie is currently a lecturer and researcher in the Department of Physiology, University of Otago, Dunedin, New Zealand.

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