It has been known for several decades that it is possible to adequately ventilate the lungs with tidal volumes smaller than the anatomical dead space using breathing frequencies much higher than those at which a person normally breathes. This is actually a common occurrence. Dogs do not sweat. They regulate their temperature when they are hot by panting during which they take very shallow, very fast, quickly repeated breaths. The size of these panting breaths is much smaller than the animal's anatomical dead space. Yet, the dog feels no worse for this type of breathing.
Devices have been developed to produce high frequency, low amplitude breaths. These are generally used on patients with respiratory distress syndrome, whose lungs will not expand properly. These are most often neonates whose lungs have not been developed fully, but can also be older patients whose lungs have been injured.
The very low tidal volumes produced put less stress on fragile lungs that may not be able to withstand the stretch required for normal tidal volume. Convection plays a minor role in gas transport with these ventilators while various forms of enhanced diffusion predominate.
The three basic modes of high-frequency ventilation are:
High-frequency positive pressure ventilation (HFPPV): which uses respiratory rates of about 60 to 100 breaths/ min.
High-frequency jet ventilation (HFJV): which uses rates between about 100 and 400 to 600 breaths/ min.
High-frequency oscillatory ventilation (HFOV) : which uses rates into the thousands, up to about 4000 breaths/min.
HFPPV uses a conventional positive pressure ventilator set at high rates with lower than normal tidal volumes. HFJV ventilators use a nozzle or an injector; the small-diameter tube creates a high-velocity jet of air that is directed into the lungs. Exhalation is passive. HFOV ventilators use either a small piston or a device similar to a stereo speaker to deliver gas in a & quot;to-and-fro & quot; motion, pushing gas in during inspiration and drawing gas out during exhalation. Ventilation with high-frequency oscillation has been used primarily in infants with respiratory distress and in adults or infants with open air leaks, such as bronchopleural fistulas.