Ventilation and Perfusion
When the rate and depth of ventilation is reduced (hypoventilation), less oxygen is delivered per minute to the alveoli to replace that taken up by the blood. This causes the partial pressure of oxygen within the alveoli to fall. Hypoventilation may be mitigated by increasing the concentration of oxygen in the inspired gas.
Learning bite
Hypoventilation always reduces PAO2 unless there is an increase in the inspired oxygen concentration sufficient to compensate.
What happens if hypoventilation persists?
A point will eventually be reached where compensation is no longer possible. For any subsequent reduction in inspired concentration, the PAO2 will fall precipitously.
If the alveolar capillary suffers impaired delivery of blood, as occurs in pulmonary embolism, the situation is referred to as hypoperfusion.
Further information on ventilation/perfusion (V/Q) mismatch
Under normal conditions, ventilation (V, 5 l/min) and perfusion (Q, 5 l/min) are well matched (despite minor variations at the extremes of the apices and bases) and the ventilation/perfusion ratio is approximately 1. If ventilation exceeds perfusion, for example in shock (low cardiac output, V/Q>1), there is not enough haemoglobin circulating to accept all the oxygen available with any consequent increase coming form O2 going into solution which is marginal.
Conversely, when perfusion exceeds ventilation – for example a lung contusion or pneumonia (reduced ventilation, V/Q <1) – there is an inadequate amount of oxygen to fully saturate the haemoglobin. The reduced oxygen in the blood leaving these areas of the lung has the same effect as “shunting”. Areas of normal, high and low V/Q can co-exist in the injured or infected lung. Consequently the final oxygen content of arterial blood depends upon the combined influence of all three. Areas of high V/Q cannot compensate for areas of low V/Q. As the latter tends to predominate hypoxaemia occurs. Unsurprisingly, V/Q mismatch is responsible for many deaths in trauma victims.
Learning bite
Once 30% or more of the blood in the pulmonary circulation passes through an area of low V/Q, the hypoxia cannot be corrected by simply increasing the oxygen content of the inspired gas.