35. Ventilation-perfusion mismatch (VQ). Causes and consequences

V?

alveolar ventilation

Q?

perialveolar capillary blood perfusion

how does ventilation increase within the lung?

increases from the apex to the base of the lung

20% larger in the base, than in the apex

how does perfusion change within the lung?

higher in the base of the lung, lower in the apex

100% larger in the base, than in the apex

V/Q ratio in a healthy lung?

below 1

in which disorders can we see decreased V/Q ratio?

• chronic bronchitis

• atelectasis

• bronchial obstruction

• asthma

• hepatopulmonary synndrome

• pulmonary edema

V/Q = 0?

lung has perfusion, but no ventilation

“shunt” : as the blood passes through this area, but there is no gas exchange

low V/Q?

blood flowing around the alveoli will be incompletely oxygenated, causing hypoxaemia and hypercapnia

the hypercapnia will induce hyperventilation to compensate

how can the lung compensate if a part of it has a low V/Q?

local vasoconstriction in the area → reduces the perfusion to increase the V/Q, and redirects the blood to other, well-ventilated areas of the lung

anatomical shunt?

A small degree of shunting is normal.

2-3% of the total pulmonary perfusion is supplied by the bronchial arteries and will be drained by the pulmonary veins.

This blood won’t be exposed to alveolar air, so it doesn’t contribute to gas exchange

Functional shunt?

the basal part of the lung naturally has a low V/Q ratio.

The presence of these two shunts shows means that around 4-6% of the pulmonary circulation is shunted (not exposed to air and oxygenated).

In which conditions can we see an increased V/Q ratio?

• emphysema

  • Increases the volume of the lungs that can be ventilated, however this extra volume doesn’t receive any extra blood supply, meaning that the V/Q ratio increases.

• pulmonary embolism

  • the capillaries supplying the alveoli will become obstructed, reducing the blood supply and increasing the V/Q ratio.

increased V/Q?

When the capillaries supplying the alveoli is obstructed the perfusion (Q) will be very low or even 0, meaning that the V/Q ratio increases toward infinity.

Because there is no gas exchange the air inside the alveoli will have the same gas composition as the outside air.

When perfusion of an area becomes zero will the area effectively become dead space.

consequences of V/Q mismatching?

Whether V/Q is increased or decreased there will be hypoxaemia.

In the beginning there be will normocapnia, meaning there is only a partial respiratory failure.

A severe V/Q mismatching may eventually yield hypercapnia, meaning that the respiratory failure becomes global instead of partial.