Monday, April 21, 2014

Stowaway and Accidental Empiricist Humbles Physiological Theorists: The Boy in the Wheel Well

Kessler Peak in the Wasatch:  10,400 feet
Several years ago, I posted about empirical confirmation of West's theoretical blood gas results at altitude on Everest.  (Last week, an avalanche on Everest took more lives in a single day than any other in the history of the mountain.)  The remarkably low PaO2 values (mean 26 mm Hg) demonstrated by those authors, (and the correspondingly low estimated SaO2 values) are truly incredible and even bewildering especially from the perspective of clinical practice where we often get all bent out of shape with PaO2 values under 55 mm Hg or so.  Documentation of the PaO2 values in the "natural experiment" that mountaineers subject themselves to serves as fodder for ponder for those of us who are prone to daydreaming about physiology:  is tolerance of these low values possible only because of acclimatization and extreme physical fitness?  (but they're exercising, not just standing there!)  what is the lower safe limit of hypoxemia?  does it vary by age?  the presence of other illnesses?  is there a role for permissive hypoxemia in the practice of critical care?

But this story line contains another didactic:  Men climbing Everest in the last century did not rely on West and his calculations to predict the survivability of an Everest ascent.  They were empiricists, practicing in a bottom up (literally), trial and error, feedback and refinement style.  It mattered little to them what the theorists had to say or predict about how low their oxygen would be up there on the summit, or whether they could tolerate it - they were determined  to push the limits and see for themselves.  The theorists could figure it out later after the empiricists proved the concept.

I worry that in medicine with our increasing homage to "evidence", that we are losing sight of the history, value, and art of empiricism in medical practice.  As I have posted before, we never performed trials of mechanical ventilation, or many other therapies.  They represent what I have called Category 1 therapies, those with results which are so undeniable as that randomized controlled trials are not necessary or even ethical.  (The spoof in BMJ in 2003 about an RCT of parachutes to prevent death from gravitational challenge is a classic parody of overzealous reliance on "evidence".)  A pernicious corollary is the continued teaching of the pathophysiological model of disease and its implicit encouragement to take reductionist physiological principles and use them to form naive hypotheses to guide the treatment of disease, often in the face of contradictory empirical observations.  Take, for example, the adjustment of flow rates on the ventilator - in my experience, the only way to adjust the flow rate  so as to achieve any desired goal (say, less "breath stacking") is to just go and empirically adjust the flow rate up and down and see if it has any effect (most often it does not; when it does, it is in the direction opposite from what you predicted).  Naive hypotheses (usually based on observational associations) also drive other misguided, wasteful, and harmful aspects of practice such as f/Vt measurements, transfusions, electrolyte replacement, intraaortic balloon pump use, etc.

And so we have this unfortunate runaway 16-year-old boy who breached the non-existent security at an airport in California, crawled into the wheel well of a 767, and rode there as a stowaway at altitudes as high as 38,000 feet in temperatures as low as  -50 degrees F for 3700 miles to Hawaii where he crawled out and was found stumbling around the tarmac with a body temperature of 79 degrees.  The ABC news medical consultant Jim Nancy doesn't believe he could have survived.  I would not have predicted it either, but I'm going to wager that the evidence ultimately shows that he did indeed travel this way and survive, because there are few other credible explanations for how he got to Hawaii and was profoundly hypothermic when he was finally discovered.  At 38,000 feet, where the atmosphere is utterly bereft of oxygen (and other gasses) his PAO2 (alveolar) was probably on the order of 20mm Hg, his PaO2 (arterial) on the order of 10-15 mmHg, and this was sustained for over 5 hours.  It is truly amazing that he survived the ordeal without obvious brain damage, as news reports are indicating.

Questions to ponder are:  would neurophysiological testing reveal subclinical evidence of brain damage?  what was the role of hypothermia in his tolerance of extreme hypoxemia?  what does this say about the role of acclimatization and age in the tolerance of extreme hypoxemia?

Meanwhile, the pedagogical message is clear:  be careful of placing too much trust in your theoretical models, for they are just that - theories and models.

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