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>Dueker says residual volume in "young males" is 20% of total lung >capacity (TLC), not tidal volume. residual volume and deadspace are different things >Anatomic dead space is usually >assumed to be around 150ml, which agrees with your figure from a >different source (USN says "less than 200cc" on p3-12). Anotomic dead space and physiological deadspace are different things. >The USN >manual also cites vital capacity as averaging between 4-5l. (p 3-11) vital capacity (full inhalation to full exhalation) is different from tidal volume (normal inhalation to normal exhalation >Further, none of these divers are at rest. I assume these folks are >working, not at rest, or CO2 buildup wouldn't be an issue. Physiological dead space and tidal volume are a fixed ratio. >Here's a >chart from the USN manual, p3-15, to give you an idea why I disagree >with your 4l/min RMV figure: > >Activity RMV (l/min) >--------------------- ----------- >Bed Rest 6 >Standing Still 9 >Walking, 2mph 16 >Swimming 0.5kt (slow) 18 >Swimming 0.85kt (ave) 30 These are about right, 4 or 6 litres RMV, I guessed, it depends on your height. >Finally, I bet dollars to donuts none of the divers in question are >breathing with tidal volumes of 500cc. "Good breathers" refers to a >long, slow, deep inhalation/exhalation cycle, to reduce turbulence and >work of breathing. It also implies a larger tidal volume, typically >somewhere near vital capacity. I agree that it will be larger than resting tidal volume but not near vital capacity. For instance, my swimming gas use is about 12litres/min, a typical resting ventilation rate is 12/min, I have never measured my swimming ventilatory parametres, but since tidal volume increases rather than respiratory rate until this can no longer compensate, and since divers try to ventilate slowly a will guess at 12 (I'll count next time I am diving), this works out to 1 litre tidal volume. This is a rough estimate, but I doubt a tidal volume near vital capacity, the work of breathing at a tidal volume equal to vital capacity would be exhausting. 100ml extra apparatus dead space additional to 300ml physiological deadspace in this 1000ml tidal volume is still large, whether or not this has anything to do with the regulator in question. The symptoms described sound like CO2 buildup, whether due entirely to the regulator or the divers being CO2 retainers. regards, David Doolette ddoolett@me*.ad*.ed*.au*
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