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Partial Pressure

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I am stumped?

We all know that we inspire roughly 79% Nitrogen and 21% oxygen at one atmosphere of 760mm/hg. Leaving water vapor out of the mix for now this brakes down to partial pressures of  600.4 mm/hg of Nitrogen and 159.6 mm/hg Oxygen. In a homeostatic body with a proper pH we normally absorb roughly 5% of that oxygen and none of the nitrogen leaving a "partial pressure" void of 5% which is commonly filled by C02 creating the value  we know as End Tidal CO2.  In addition our O2 sat has risen to 100% because our pH has become more alkolotic causing a left shift in the oxy-hemoglobin curve resulting in greater affinity for hemoglobin to bind with oxygen

The numbers may be a little off here but bear with me.

If CO2 diffuses to fill the void in the partial pressure, in a state of hyperventilation where our CO2 levels drop what files the void to maintain a partial pressure of 760 mm/hg in our exhaled air? Do we just absorb less Oxygen because the Hemi sites are full and we have saturated the plasma?

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I'm going to tackle this one without consulting some text books which may be a major error since I am decades removed from basic sciences of physiology.  You are presuming normal ventilatory patterns which is nice except you won't have a patient with these parameters because they would not have called EMS in the first place.  Yes, CO2 diffuses into the void but so does nitrogen.  Don't forget, most of the air inhaled is nitrogen and CO2 is produced internally and not externally.  Oxygen displaces nitrogen which is one reason we pre-oxygenate patients with 100% oxygen prior to intubation in order to prevent hypoxia during the apnea normally seen during intubation.  CO2 is involved with acid-base calculations but just because nitrogen is inert doesn't mean it is unimportant.  

Clear as mud?  Sorry.


May the tube (and ETCO2) be with you. 

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