LFJV is most commonly associated with a manually controlled insufflation device (hand jet insufflator) so that the provider depressing a demand or on-off valve controls the rate. (See also chapter 29). Practically speaking then, rates during the use of LFJV are generally in the conventional ventilation range. HFJV is of course delivered through a sophisticated mechanical ventilator, with electronically controlled solenoid valves controlling the flow of gas. Just as in conventional ventilation, the respiratory rate can be set at a various levels, though most high-frequency jet ventilators marketed in the United States have a maximum rate of 150 cpm.

It should be noted that during HFJV, the adjustment of rate has the least impact on either oxygenation or ventilation because changes in rate do not directly affect minute ventilation (MV) as they do in conventional ventilation modes. MV is primarily determined by the set driving pressure and the inspiratory time.¹²

In LFJV, the inspiratory to expiratory ratio of delivered ventilations is, as with the rate, manually controlled by the provider. This frequently results in a great deal of breath-to-breath variability. Modern high-frequency jet ventilators allow for a range of I:E ratios. Most jet ventilators express this in terms of the inspiratory time alone, with the expiratory time implied (eg, a setting of 30% would indicate 30% of the respiratory cycle as inspiration and 70% as expiration). As would be anticipated in conventional ventilation, increasing the duration of the inspiratory phase will result in greater delivery of gas and an increased MV. This is true of both LFJV and HFJV.

In both LFJV and HFJV, the phrase driving pressure is used to denote the pressure measured at the gas delivery valve before it opens to the patient. This becomes a simple expression for minute volume adjustments: the higher the driving pressure, the higher the delivered minute volume. This pressure can be expressed in pounds per square inch (psi) or in bar (1 bar is approximately equal to atmospheric pressure at sea level and equal to 14.5037 psi).
In HFJV (and ideally in LFJV) a reducing regulator allows for the adjustment of this pressure up or down. The maximal pressure that one is able to obtain depends on the compressed gas source, but for most central gas delivery systems in the United States this is approximately 50 psi. For most patients, optimal driving pressure settings will be between 20 and 30 psi, but pulmonary-related comorbidities could result in the need for either higher or lower settings. In HFJV, driving pressure is one of the key determinants of MV, oxygenation, and carbon dioxide elimination.¹³,¹⁴

Airway pressures in jet ventilation are determined by the amount of volume delivered to the lungs. The primary determinants of this are the set driving pressure and the inspiratory time. Increased driving pressure provides increased “energy” to overcome the resistance of the small diameter delivery tubing and, thus, increased volumes. An increased inspiratory time functions just as it does in conventional ventilation to deliver greater volumes. In general, peak airway pressures during HFJV are lower than those generated during conventional positive pressure ventilation, and this can be of great advantage across the range of applications for HFJV.⁵,⁶,¹⁵,¹⁶,¹⁷ Because the lungs never fully exhale during HFJV, positive pressure is maintained throughout the respiratory cycle. As a result, although peak airway pressures are lower than conventional ventilation, mean airway pressures between the two modes are generally equivalent. Airway pressures in LFJV depends heavily on the operator manually controlling the rate and I:E ratio. Other significant contributing factors would include the cross-sectional area of the trachea and the ID and length of the delivery catheter. Animal studies have reported a range of pressures between 20 and 50 cm H20 using LFJV via transtracheal puncture with either a 14 g or 16 g catheter.

The most common complication of either type of jet ventilation is barotrauma, and the most common underlying etiology is unrecognized obstruction to outflow, either through the natural airways or in some circumstances through an endotracheal tube. This is counter to a common misconception that the high-pressure gas source itself is the cause of barotrauma. As discussed earlier, the high-pressure gas is essentially used as work to overcome the resistance of the small diameter ventilator delivery tubing and whatever jet device is attached to it. The pressures generated at the point of exit are significantly lower (consider that the hallmark of HFJV is lower peak airway pressures than in conventional ventilation) than the set driving pressure. A frequently used example is that with a set driving pressure of 20 psi, a standard length of delivery tubing, and incorporating a 14G catheter, one could expect to deliver between 500 and 600 mL with a 1 second inspiration.¹⁸,¹⁹ Pressure in the lungs, then, depends on the volume delivered during each breath. The greater the volume delivered, the greater the pressure. A review of the literature reveals that the most common occurrence in cases of barotrauma is the development of an obstruction, often in the upper airway, which impedes egress.²⁰,²¹,²²,²³,²⁴,²⁵,²⁶ If insufflation continues and this obstruction remains unrecognized, barotrauma ensues. This risk is higher in the setting of LFJV as the detection of impaired exhalation depends on the vigilance of the provider delivering the manual insufflations. HFJV offers the advantage of an integral alarm system designed to detect outflow obstruction. Modern high-frequency jet ventilators incorporate a sophisticated switching system that enables the delivery tubing itself to act as pressure tubing to a dedicated pressure transducer. At the end of the expiratory cycle, backpressure in the airway is measured, and if the set alarm limit is exceeded, delivery of additional breaths is stopped. This occurs at the end of each respiratory cycle, regardless of the set rate, providing breath-to-breath detection of potential outflow obstruction. The alarm level is adjustable but is often nondefeatable. A typical setting for this alarm limit is 20 cm H₂O.