Oxygen

      
   

 

      
     

What problems are associated with right to left shunting?

  • Right to left shunting causes hypoxemia resistant to oxygen therapy.

When blood passes through the lungs without coming in contact with air, a right to left shunt exists. This deoxygenated blood mixes with well oxygenated blood on the far side of the lung, and reduces the percentage saturation of hemoglobin. In all individuals a small physiologic shunt is present, principally arising from blood in the bronchial circulation. This has little effect on blood oxygen content. Larger shunts may cause significant problems, however. The reason for this is the curious shape of the oxyhemoglobin dissociation curve, as you can see from the diagram below:

The effect of shunt on the oxyhemoglobin dissociation curve
CLICK ON THUMBNAIL TO VIEW IMAGE

The addition of mixed venous blood, slides the patient down the curve to the steep slope, where severe hypoxemia may result. Shunt classically does not respond to oxygen, although the administration of 100% oxygen may increase the dissolved oxygen content and increase the mixed venous oxygen saturation. The higher the SVO2, the less damaging a shunt is. The PaCO2 is usually normal, as the patient increases minute ventilation to blow off CO2 derived from the shunt, due to activation of chemoreceptors.

The shunt equation is used to calculate the magnitude of a shunt:

Qs/Qt  =  CcO2 – CaO2/CcO2 – CvO2 where CcO2 is the capillary oxygen content in the ideal capillary, CaO2 is the arterial oxygen content, and CvO2 is the mixed venous oxygen content. The content is calculated by using the equation discussed above: CnO2 is (1.34 x Hb x SnO2/100) + 0.003 x PnO2, where n = a or v or c. The PO2 is derived from the alveolar gas equation.

As one would expect, the greater the magnitude of the shunt, the larger the PAO2 – PaO2 difference.

A 17 year old male presents to the emergency room after being stabbed in the chest, on chest x-ray his right lung was fully collapsed, and yet his SpO2 was 94% on room air - why?

        
   

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