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How and Why HFJV Works
During High Frequency Jet Ventilation (HFJV), gas is propelled into the lungs at a very high velocity. The incoming gas is forced to stream into the airways in a long spike. Because the abundant energy of the "jet stream" also causes the gas to spiral as it flows, it easily splits into two streams at every bifurcation. Thus, fresh gas penetrates through the anatomic dead space, leaving much of the CO2 in the dead space compressed against the airway walls.
This key to achieving adequate gas exchange using small tidal volumes was graphically illustrated as early as 1915. Henderson and associates were intrigued by the apparent shallow breathing of panting dogs. To explain how these animals could pant indefinitely without becoming hypoxic, they explored flow streaming using tobacco smoke and a glass tube 2-m long and a few cm in diameter. Filling his mouth with smoke, Henderson blew a brief, sharp puff of smoke into the tube. His associates observed that the smoke streaked down the center of the tube as a long, sharp spike. The faster he blew this single mouthful of smoke, the farther down the tube the thin spike progressed. The instant he stopped the flow with his tongue, diffusion caused the smoke to fill the tube axially and the jet-stream effect disappeared.
This classic experiment demonstrates the relative importance of convection and diffusion during panting and high-frequency jet ventilation. Convection carries fresh gas deeply into the lungs extremely quickly. Once the flow stops, diffusion completes the gas exchange process as usual. It also reveals how the effective or physiologic dead space in the lungs can be reduced to less than the volume of the anatomic dead space at high breathing rates.
The creation of such a jet stream is only effective for a relatively short distance and a brief time. Longer distances and times allow for development of turbulent flow, which quickly mixes incoming gas with resident dead space gas.
The best way to maximize the jet-stream effect with a mechanical ventilator is to place the inhalation valve as close to the patient as possible. This is accomplished with the Life Pulse™ High Frequency Jet Ventilator by placing the valve and pressure transducer in the small plastic "patient box" that resides close to the infant's head.
Gas flow through the pinch valve is stopped almost as soon as it starts by closing the valve almost as quickly as it is opened. The abrupt cessation of the incoming HFJV breath also helps prevent the development of turbulence so that a crisp jet stream of fresh gas can penetrate deep into the tracheobronchial tree.
With HFJV, as with conventional mechanical ventilation, inhalation is active or forced, and exhalation is passive. Using rates that bring in as many as 11 breaths per second, one might be concerned that there is insufficient time for breaths to get back out. However, two factors allow exhalation to occur relatively easily. The size of each breath (1-3 mL/kg) is much smaller than usual, and the natural or resonant frequency of the infant lungs is close to the frequency range being used by HFJV. Thus, the lungs recoil readily during HFJV under almost all conditions.
Passive exhalation is undoubtedly the safest way to get gas back out of the lungs. It ensures that mean airway pressure will overestimate mean alveolar pressure during HFJV.
Pressure drops as gas advances into the lungs on inhalation, and there is not time for the pressure in the alveoli to equilibrate with that in the upper airways because of the short inspiratory time. Furthermore, the highest pressure during exhalation will be in the alveoli so gas flows naturally toward the trachea during exhalation until the beginning of the next inhalation.
Active exhalation or withdrawal of gas from the lungs, as with high-frequency oscillation (HFO), can lead to gas trapping by lowering intraluminal pressure disproportionately below pressure in surrounding alveoli, thereby collapsing more proximal airways before exhalation is complete. For that reason, users of HFO typically operate at higher mean airway pressures than those used with HFJV. Elevating the baseline pressure during HFO, "splints" the airways open while gas is actively withdrawn from alveoli.
The role of conventional ventilation during HFJV with the Life Pulse is limited to oxygenation. Conventional ventilators can deliver oxygenated gas directly to the alveolar level. They do this by using relatively long (eg, 0.5 second) inspiratory times and large tidal volumes (eg, 7 to 15mL/kg body weight), and they have the capability of controlling end-expiratory pressure. These are the factors that most readily control PO2. Unfortunately, they are also the factors most closely associated with barotrauma. Thus, it is useful to minimize these factors by running the conventional ventilator at minimal rates (ie, from 1 to 3 BPM) while the Jet ventilator is providing the bulk of the ventilation. Using the conventional ventilator to gradually recruit collapsed alveoli allows the Jet to achieve the best possible blood gases with the lowest possible airway pressures.
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