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



Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Tuesday, May 22, 2012 1:03 pm