Venous Admixture and effect of Supplemental oxygen

 Venous Admixture and Effect of Supplemental Oxygen

Written by Dr Madhuri Anupindi

1. a) Venous admixture is the volume of mixed venous blood that would be required to be added to ideal pulmonary end-capillary blood in order to obtain the pulmonary end-capillary to arterial P02 difference.

– calculated volume of deoxygenated blood from venous circulation which seems to have bypassed the lung and not participated in gas exchange

Diagnosing venous admixture requires calculating the A-a gradient using the alveolar gas equation:

P_A0₂ = (Fio2 x (P_atmos – P_H20) – (P_aC0₂/RQ))

A-a gradient = P_A0₂ – P_a0₂ (ideal alveolar partial pressure of oxygen minus arterial partial pressure of oxygen)  normally 5-10mmHg but increases with age (different equations including (Age/4 + 4) or (2.5 + (0.21 x age))

Venous admixture can be then quantified using the shunt equation:

Q_s/Q_t = (C_c02 – C_a02)/(C_c02- C_v02)

Where:

Q_s/Q_t = shunt fraction  shunt flow divided by total cardiac output

C_c02 = oxygen content of pulmonary end-capillary blood which is assumed to be equal to alveolar oxygen content

C_a02 = oxygen content of systemic arterial blood

C_v02 = oxygen content of mixed venous blood

And oxygen content of blood can be calculated by (1.34 x Hb x sats) + (0.003 x P_a02)

The normal shunt fraction is up to 5%.

Sources of venous admixture:

• Physiological shunt
  • Anatomical
    • Bronchial veins: blood from deep bronchial veins drain into the pulmonary veins and left atrium
    • Thebesian veins: small group of valveless veins that drain the inner surface of the myocardium directly into the cardiac chambers
  • Pathological shunt
    • Intra-cardiac shunt e.g. VSD  right-left shunting of blood
    • Intrapulmonary shunt
      • True shunt: V/Q = 0  areas of lung with no ventilation e.g. pulmonary AVM
      • V/Q scatter: blood which passes through lung regions where V/Q < 1 e.g. pneumonia, lung tumours, atelectasis

1. b) Arterial hypoxaemia: abnormally low concentration of oxygen in the arterial blood (variably defined as P_a02 < 60 -80mmHg)

Supplemental oxygen effect on arterial hypoxaemia:

• Will increase the P_A02 (ideal alveolar partial pressure of oxygen)  see alveolar gas equation
• Effect on arterial Pa02 will depend on the cause
  • If due to hypoventilation or low inspiratory oxygen pressure then there is no impairment of gas exchange and no increase in venous admixture  no increase in A-a gradient  supplemental oxygen will therefore increase P_a02
  • If due to VQ mismatch, supplemental 02 should increase P_a02 as it increases P_A02, even in low VQ regions as some ventilation is still occurring
  • If due to diffusion limitation then supplemental o2 will increase P_A02 which will generally increase P_a02 due to the raised driving pressure for diffusion of oxygen into the blood
  • If due to true shunt (V/Q = 0) then increasing fio2 will not affect the P_A02 within these lung units. The increased fio2 will increase the P_A02 within the ventilated lung units, however the effect of this on P_a02 will diminish as the shunt fraction increases (see iso-shunt diagram)