TIL: When a person exhales to their fullest extent, there is always some air left over in the lungs. This volume is important for certain clinical calculations, but cannot be determined through spirometry (measuring the volume of air going in and out of the lungs). To measure this last bit of air, known as the Residual Volume (RV), we have to use one of several other techniques. One such method is helium dilution, where the patient breaths from a known volume of an air/helium mixture. When the external mixture and the air remaining in the lungs has had time to equilibriate, the RV can be found using simple algebra.
In a normal person, the pressure in the chest is always negative. Put another way, the lungs are elastic and always trying to collapse. They continually pull on the fluid between the lungs and the chest wall, creating a suction-like effect. When a patient is on a ventilator, however, the machine is forcing air into the lungs without the chest cavity expanding, as it would with normal breath. The thoracic pressure is accordingly higher than normal. This affects fluid return to the heart, often resulting in peripheral edema (swelling of the limbs) amongst other problems. These side-effects are acceptable to keep the person breathing, but must be kept in check.
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