Hypoxemia

DDx of hypoxemia (note that several disease processes have more than one mechanism of hypoxia):

a. Hypoventilation: due to decreased clearance of CO2, decreased alveolar O2 tension, but has a nl A-a    gradient. Corrects easily with increased FiO2.  Usually secondary to obesity hypoventilation syndrome, CNS depression (ie. narcotics),  impaired neural conduction (ie. ALS, phrenic nerve paralysis), muscular weakness (ie myasthenia gravis)
b. VQ mismatch:  ventilation/perfusion mismatch.  Accounts for the normal A-a gradient, which is worse in the apices than the lung bases.  Common causes include COPD/obstructive lung disease, interstitial lung disease, pulmonary vascular disease (ie. PE).
c. Shunt: can be anatomic (ie. cardiac shunt, ASD/VSD) physiologic (ie. atelectasis, ARDS, PNA).  Note that R-->L shunts cause extreme VQ mismatch.
d. Impaired diffusion/DCLO: limited diffusion of O2 from alveoli to capillary, usually a consequence of alveolar or interstitial fibrosis (ILD). Again, usually co-occurs with VQ mismatch.  Hypoxia classically worsened by exercise due to decreased diffusion time with higher cardiac output.
e. decreased FiO2: reduced fraction of inspired O2, usually attributable to high altitude or suffocation

Measuring the degree of hypoxemia:

--remember than RA (at sea level) has an Fi02 of 21% (0.21)
--for O2 via nasal cannula, you can estimate a 4% increase for each 1L by NC (so 4L NC ~ 37%FiO2)
--the alveolar gas equation is used to calculate the PAO2 (alveolar O2 tension) :
PAO2= (FiO2 x Patm-PH2O) - (PaCO2/R), where R is the respiratory quotient, Patm is 760mmHg and PH2O (47mmHg at 37 C) and R=0.8
--A-a gradient is PAO2-PaO2; you can use online calculators to estimate it
OR, you can use an easier estimate...
--PaO2/FiO2 ratio: 300-500 is nl <300 is hypoxic, <200 is severe hypoxia
(in our patient her ratio was around 223)

(Victoria Kelly MD, 7/1/10)