Tuesday, March 19, 2019

Right to left shunt causing Hypoxemia

A right-to-left shunt exists when blood passes from the right to the left side of the heart without being oxygenated. There are two types of right-to-left shunts:

●Anatomic shunts exist when the alveoli are bypassed. Examples include intracardiac shunts, pulmonary arteriovenous malformations (AVMs), and hepatopulmonary syndrome.

●Physiologic shunts exist when non-ventilated alveoli are perfused. Examples include atelectasis and diseases with alveolar filling (eg, pneumonia, acute respiratory distress syndrome).

Right-to-left shunts cause extreme V/Q mismatch, with a V/Q ratio of zero in some lung regions. The net effect is hypoxemia, which is difficult to correct with supplemental oxygen.

The degree of shunt can be quantified from the shunt equation:

Qs/Qt  =  (CcO2  -  CaO2)  ÷  (CcO2  -  CvO2)

where Qs/Qt is the shunt fraction, CcO2 is the end-capillary oxygen content, CaO2 is the arterial oxygen content, and CvO2 is the mixed venous oxygen content. CaO2 and CvO2 are calculated from arterial and mixed venous blood gas measurements, respectively. CcO2 is estimated from the PAO2.

Source: UpToDate

Bhopalwala. H

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