Ventilation

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.

The respiratory rate is the number breath a person takes per minute. In an adult this is normally around 15 breath per minute at rest.

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.

When you add the volume of air lost in these malfunctioning alveoli to the anatomical dead space, you get the physiological dead space.

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)

In a normal healthy person, almost all the alveoli are functioning properly, and the physiological dead space is about equal to the anatomic dead space which is about 150 ml.

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.

A fraction of this volume is carbon dioxide, so let’s call that fraction FCO2. So, the volume of carbon dioxide, VCO2, is:

 	VCO2 = VA X FCO2
			        Or, VA = (VCO2) / (FCO2)