Alveolo-arterial oxygen pressure differential

Alveolo-arterial oxygen pressure difference (AaDO ₂ ) is the difference between the oxygen partial pressures (pO ₂ ) in the breathing gas mixture of the alveoli and the arterial blood.

physiology

The pulmonary gas exchange in the alveoli depends on the alveolar partial pressures of oxygen and carbon dioxide. Only if there is a partial pressure gradient between the alveoli and the blood can a respiratory gas component (e.g. O ₂ , CO ₂ etc.) diffuse. Gases not only exert (partial) pressures in a gas mixture, but also when dissolved in liquids ( Henry's law ). The concentration of the dissolved gases depends on their specific solubility with their partial pressure .

With a complete gas exchange, the arterial oxygen partial pressure (p _a O ₂ ) would be just as high as the alveolar one. In fact, however, due to incomplete gas exchange, the arterial p _a O _{2 is} lower than the alveolar p _A O ₂ ; thus there is an alveolo-arterial O ₂ partial pressure difference. For room air (approx. 21% by volume O ₂ ) the average oxygen pressure difference (AaDO ₂ ) is approx. 10–15 mmHg (1.33–1.99 kPa); the upper limit is given as 25 mmHg (3.33 kPa). If 100% oxygen is inhaled over a certain period of time, the difference increases and is around 50–60 mmHg (6.66–7.99 kPa).

A small part of the cardiac output does not take part in gas exchange in the lungs. This is about 2% of the blood that feeds the lung tissue via the bronchial arteries (branches of the thoracic artery ) and then returns directly to the left heart via the pulmonary veins ( venae bronchiales or vena pulmonalis ), as does the blood from small coronary vessels ( venae cordis minimae or Venae Thebessii ), which open directly into the left atrium or ventricle. These so-called anatomical shunts (short circuits ) lower the arterial p _a O ₂ by 5-8 mmHg (0.66-1.07 kPa).

In the so-called physiological shunt , blood flows from the lung sections that are well supplied with blood (perfusion) but poorly ventilated (ventilation) and thus have a lower oxygen content into the pulmonary veins (pulmonary veins) to the left heart. The lung sections therefore have a low ventilation / perfusion quotient (see also Euler-Liljestrand mechanism ).

calculation

The alveolo-arterial oxygen pressure difference (AaDO ₂ ) can be calculated from the measured values ​​of a blood gas analysis . It is defined as:

${\displaystyle AaDO_{2}=P_{A}O_{2}-P_{a}O_{2}\quad }$

Here, P _A O ₂ of the alveolar partial pressure of oxygen, P _a O ₂ is the oxygen partial pressure of arterial blood gas analysis. The alveolar oxygen partial pressure (P _A O ₂ ) can be calculated approximately from the alveolar gas equation :

${\displaystyle P_{A}O_{2}=F_{i}O_{2}(P_{atm}-P_{H_{2}O})-{\frac {P_{a}CO_{2}}{RQ}}}$

Here F _i O _{2 is} the oxygen content of the air we breathe, P _atm is the air pressure, P _{H 2 O} is the water vapor partial pressure, P _a CO ₂ is the carbon dioxide partial pressure from blood gas analysis, and RQ is the respiratory quotient .

If you use the usual values ​​under normal conditions (P _atm = 760 mmHg, P _{H 2 O} = 47 mmHg at 100% humidity in the alveoli, RQ = 0.8), you get:

${\displaystyle AaDO_{2}=(F_{i}O_{2}\times 713{\text{mmHg}}-P_{a}CO_{2}/0.8)-P_{a}O_{2}}$

literature

• RF Schmidt, G. Thews (Hrsg.): Physiologie des Menschen. Springer publishing house.

Individual evidence

1. ↑ Alveolar-arterial gradient . Retrieved December 30, 2017.