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Discussion of Shunt: Qs/Qt


- shunt fraction, normally < 5%, represents the mismatch of ventilation to perfusion;
- it is ratio to non-ventilated alveoli to perfused pulmonary capillaries;
- alteration in the ventilation to perfusion ratio results from either relative obstruction of ventilation or relative obstruction of perfusion;
- rising shunt fraction is usually associatted with progressive respiratory failure or ARDS;

Decreased Ventilation: Decreased Perfusion:
- ARDS (See Resp. Failure)
- Massive PE
- Pulmonary Edema
- Micro Pulmonary Emboli
- Atelectasis Calculation: Qs/Qt = (Aa grad * 0.003) / (AVo diff + Aa grad * 0.003) Cco2 - Cao2 = ----------------- Cco2 - Cvo2 Cco2 = (Hgb * 1.34) + Pco2 (0.003) = pulm. capillary O2 content Cao2 = (Hgb * 1.34) (Sao2/100) + Pvo2 * 0.003 = art. O2 content Cvo2 = (Hgb * 1.34)

Causes of Increased Shunt in the Post Resusitation Period:
- cardiogenic pulmonary edema
- aspiration pneumonia
- non aspiration penumonitis
- pulmonary bone marrow emboli from trauma to the sternum during CPR
- ARDS (associatted w/ sepsis)
- atelectasis
- toxic chemical aspiration;
- Rx:
- begin therapeutic PEEP to decrease Shunt (inorder to incr PaO2)
- however, ensure that the apparent decrease in Shunt is not due to a decrease in Mixed Venous O2 thru the usual 5% shunt;
- this sends markedly desaturated blood thru the normal shunt making it appear as if the shunt were actually larger