V/Q mismatch

Return to the restaurant: trouble starts when waiters and customers fall out of step. Some tables get heaps of food but no diners; others fill with diners but no food arrives. V/Q mismatch is that breakdown in the lung — regions where the supply of fresh air and the supply of blood no longer line up, so gas exchange in those spots becomes inefficient.

Mismatch comes in two flavours that sit at opposite ends of a spectrum. Where ventilation outstrips blood flow (a high ratio), air is partly wasted because there is too little blood to receive its oxygen — the dead-space direction. Where blood flow outstrips ventilation (a low ratio), blood is partly wasted because it slides past poorly aired sacs and leaves under-oxygenated — the shunt direction. Most lung diseases scatter the lung with patches of both, and it is by far the commonest mechanism of low blood oxygen.

Mismatch from low-ratio units usually responds well to supplemental oxygen, because raising the inhaled oxygen lifts the pressure even in the under-aired sacs. A useful nuance: this is what distinguishes ordinary mismatch from a true shunt, where blood bypasses ventilated lung entirely and added oxygen helps far less.