Monday, 27 August 2012

Day 10 - “Parinacota Physiology” by DM Bailey

Today sees the last of the experimental tests conducted at our make-shift high-altitude laboratory, Parinacota Hostel, based at 4,600 m. It’s been a long haul and we’re excited at the prospect of completing a unique study that has built on the best part of 10 years worth of research. In essence, we’re trying to understand precisely what makes the female brain more resistant to the inspiratory hypoxia (lack of oxygen) of high-altitude. Much to the chagrin of the macho-male mountaineer, females seem to cope better at high-altitude and suffer with less acute mountain sickness, a syndrome characterised by the dreaded “hangover-headache” and associated symptoms. We’re slowly unravelling the mechanisms that allow them to acclimatise so much faster than us (lesser!) males. There are clinical parallels to be made too; females outlive us males and the incidence of brain disease is far lower. Could there be a common link explained by their superior ability to preserve oxygen delivery to the brain? It all sounds quite intuitive though you can never under-estimate the challenges posed by field research. My colleagues Dr Julien Brugniaux and Luke Liddle (baptism of fire as a research student!) turn up to the Brain-Lab dressed ready for action; hats, mits, down-jackets and Sportivas! It’s freezing and equipment and experimenter alike don’t take kindly to a cold start! Our participant is sitting quietly with eyes closed (still snoozing!) as we busy ourselves around her. Electrodes to monitor heart rate (electrocardiography), optodes on the forehead to measure changes in cortical oxygenation (near infra-red spectroscopy), a finger cuff that measures beat-by-beat arterial blood pressure (photoplethysmography) and the joys of freezing gel at the side of her head (not for the purposes of hairstyling) to measure cerebral blood flow (transcranial Doppler ultrasonography). Ten minutes later and our participant looks like an extra out of Star Wars! First challenge is to simply keep her rested, eyes closed (no brain activation) as Luke whispers “breathe through your mouth” to avoid the complications of carbon dioxide retention and prevent blood flow to the brain from bouncing up and down. We’re using a technique known as transfer function analysis to assess cerebral autoregulation, a test that will reveal how well her brain buffers acute surges in blood pressure. We know that the male brain’s “shock-absorber” doesn’t work quite so well at high-altitude which may be tied in to their increased susceptibility to acute mountain sickness. Flanked by Julien and Luke (since most of the males become faint), we ask her to stand as part of an orthostatic stress test and note the initial drop in brain blood flow and oxygen, encouraged by such a rapid recovery to baseline. The males’ recovery is way more “sluggish” as we move on to the next challenge; an ambitious 45 second breath-hold designed to measure how reactive her (cerebral) circulation is to carbon dioxide. Brain blood flow increases with every passing second and then comes the heavy-breathing challenge; three minutes of controlled hyperventilation designed to swing carbon dioxide in the opposite direction and see how quickly brain blood flow drops. Again, it’s clear to see that her circulation is impressively “reactive” as we recover her back to baseline prior to de-instrumentation and then on with the next slew of complementary tests designed to assess arterial stiffness and mental agility (to determine if females have floppier vessels and improved cognitive function). All-in-all, the various tests take the best part of 4 hours and participant compliance is quite impressive! We’ve completed all experimentation and the research objective has been accomplished. Our data set is quite impressive. We’ve modelled the (male versus female) brain’s acute response to simulated high-altitude (4,600 m) in the environmental chamber based at the University of Glamorgan and its longer-term ability to acclimatise to the same hypoxic stimulus to the terrestrial high-altitude of Parinacota, which incidentally is far more aesthetic (see Jess Christley’s blog of her sobering experience as a research participant in what she affectionately coins the silver-lift!). With the science complete, an enduring question percolates through the camp. Will these physiological tests be able to predict our group’s summit success on Mt. Sajama? Will the females “outperform” the males? Only time will tell...though male nerves are already jangling for fear of being outgunned!

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