tidal volume
Tidal volume is the amount of air that moves in or out in a single ordinary breath, the gentle in-and-out you do without thinking. The name borrows from the sea: like a quiet tide, the same modest volume washes in and out again and again.
In a resting adult it is roughly 500 millilitres, or about 7 millilitres for each kilogram of ideal body weight. Only part of each tidal breath reaches the alveoli to take part in gas exchange; the rest fills the conducting airways, which hold air but do no exchanging — the so-called dead space.
Tidal volume is not fixed. It rises with exertion, fever, anxiety, or anything that increases the body's demand for oxygen and its production of carbon dioxide, and it is one of the two dials — together with respiratory rate — that the body turns to set how much air it breathes per minute.
In intensive care, the tidal volume delivered by a ventilator is set deliberately. Lung-protective ventilation uses small tidal volumes (around 6 millilitres per kilogram of ideal body weight) to avoid over-stretching injured lungs.
For a 70 kg patient on a ventilator, lung-protective settings target a tidal volume near 420 millilitres (6 mL/kg of ideal body weight), well below the 500 mL of a relaxed natural breath.
Set tidal volume in lung-protective ventilation.
Breathing the same minute volume with larger, slower breaths is more efficient for gas exchange than rapid shallow breaths, because dead space is wasted on every breath: many small breaths waste it many times over.