Let's Talk Pulm

Let's Talk Pulm

Valerie Pallos, PGY3

Causes of Hypoxemia

• V/Q mismatch: major cause of hypoxemia in chronic lung diseases; responds well to O2 supplementation

• R to L shunting: hypoxemia due to perfusion of nonventilated alveoli (ie: PNA, pulm edema, AVM)

• Decreased alveolar ventilation: ALWAYS see high P_aCO₂

• Decreased diffusion: plays a role in exercise-induced desats; occurs with interstitial lung disease

• High altitude: reduced P_AO_2; Normal A-a gradient

Oxyhemoglobin Dissociation Curve

• P_aO₂ of 60mmHg results in S_aO₂ of 90%

• Three major factors affect shift to L or R

• Temperature, Acidosis and 2,3-DPG: increase when curve shifts to the right

• When the curve is shifted to the RIGHT, it reflects a decrease in hemoglobin affinity (promotes off loading of oxygen to the tissue)

• Think of a shift to the R as a shift down so when P_aO₂ of 60mmHg is shifted to the R on the curve, S_aO₂ decreases

CO vs MetHgB

• CO binds tightly to Hgb preventing O₂ from binding; causes a LEFT shift in the curve

• A pulse ox CANNOT distinguish between oxyHb and COHb, but appears to quickly get 100% saturated

• Methemoglobin is produced when Fe is oxidized so the resulting HgB molecule CANNOT hold onto O₂ or CO₂ which is disastrous for tissues

• Curve is also shifted to the LEFT, because HgB hold onto O₂ much more tightly

• Results in a dissociation curve that reaches 100% at a much lower P_aO₂