(Minute/Alveolar) ventilation is the total rate of air movement into and out of the lungs, but does not account for physiologic dead space, and is expressed as volume per minute.
Content Reviewers:Rishi Desai, MD, MPH
The main job of the lungs is gas exchange, pulling oxygen into the body and getting rid of carbon dioxide.
Normally, during an inhale - the diaphragm and chest muscles contract to pull open the chest and suck in air like a vacuum cleaner, and then during an exhale - the muscles relax, allowing the lungs to spring back to their normal size pushing that air out.
Ventilation rates measure the volumes of air moving in and out of the lungs, over a period of time.
During normal quiet breathing, each breath of air that enters and leaves the lungs is about half a liter, which is called the tidal volume.
So the minute ventilation is the amount of air moved in and out of the lungs in a minute. So minute ventilation is given by
Minute Ventilation = (Tidal Volume) X (Respiratory Rate)
In a normal healthy adult, this means 500 ml per breath times 15 breaths per minute, or about 7.5 litres per minute.
However, not all the air that we breathe in reaches the alveoli, where gas exchange actually takes place.
Some air is trapped in the airways - an area called the anatomical dead space.
Also, some of the alveoli may be defective and can’t even participate in gas exchange.
So to calculate alveolar ventilation, it’s the tidal volume minus the physiologic dead space and that volume gets multiplied by the respiratory rate:
Alveolar ventilation = [(Tidal volume) - (Physiological dead space)] X (Respiratory Rate)
So the alveolar ventilation comes to about (500 - 150) ml or 350 ml per breath, times 15 breaths per minute or about 5.2 litres per minute.
A way of measuring the alveolar ventilation without actually measuring the dead spaces is by knowing inspired air contains almost zero carbon dioxide and all the carbon dioxide in the expired air comes from the functioning alveoli.
If we call the alveolar ventilation, VA. That’s the amount of air going in and out of the alveoli in a minute.
VCO2 = VA X FCO2 Or, VA = (VCO2) / (FCO2)
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