The potential of high-frequency ventilation in humans has been studied since the observation that adequate gas exchange occurred in panting dogs with tidal volumes lower than the anatomic dead space [27
]. In the 1970s, groups in Germany and Canada found a system that oscillated gas into and out of an animal's lungs was effective at CO2
]. Commercial products are now available for children and for adults.
These ventilators operate on the following principle (Fig. ). A bias flow of fresh, heated, humidified gas is provided across the proximal endotracheal tube. The bias flow is typically set at 20–40 l/m in, and the Paw at the proximal endotracheal tube is set at a relatively high level (25–35 cmH2O). An oscillating piston pump akin to the woofer of a loudspeaker vibrates this pressurized, flowing gas at a frequency that is generally set between 3 and 10 Hz. A portion of this flow is thereby pumped into and out of the patient by the oscillating piston. The Paw achieved is sensitive to the rate of bias flow but can be adjusted by varying the back pressure on the mushroom valve through which the bias flow vents into the room. The Paw can thus be modified by either adjusting the bias flow rate or the back pressure.
Figure 1 Schematic representing the major functioning parts of the high-frequency oscillatory ventilator. See text for a detailed explanation. Reproduced with permission from SensorMedics, Yorba Linda, California, USA http:\\www.viasyshealthcare.com.
The set power on the ventilator controls the distance that the piston pump moves and, hence, controls the tidal volume. The result is a visible wiggle of the patient's body, which is typically titrated to achieve acceptable CO2 elimination. The oscillatory pressure amplitude (ΔP) is measured in the ventilator circuit and is therefore only a surrogate of the actual pressure oscillations in the airways. These pressures are generally greatly attenuated through the endotracheal tube and larger airways so the press ure swings in the alveoli are much less. The Paw, on the other hand, is believed to be similar in the ventilator circuit and the alveoli.
The operator uses the parameters of power (which results in ΔP) and frequency (reductions in which improve CO2 clearance) to manipulate the Vt. It seems counterintuitive that reductions in frequency would improve alveolar ventilation; however, HFOV differs from conventional ventilation in that the lung never achieves an equilibrium volume during inspiration and expiration. Lowering the frequency therefore allows more time for a larger Vt to occur. With HFOV, CO2 elimination is proportional to the Vt and the frequency, but increases in the Vt achieved by lowering the frequency are thought to more than compensate for the reduction in frequency. It is also important to note that the actual Vt received by the patient depends on a number of factors, including the size of the endotracheal tube, the airway resistance, and the compliance of the total respiratory system. Unfortunately, there are no predictable relationships between power and ΔP with the Vt received by the patient. In addition, the Vt can change on a breath-to-breath basis, and therefore ventilator settings are used with clinical factors such as the amount of wiggle in monitoring the patient.
As with conventional ventilation, oxygenation is primarily determined by the Paw
, by the lung volume, and by the fractional inspired concentration of oxygen (FiO2
). The initial settings are typically chosen to achieve a Paw
value roughly 5 cmH2
O greater than that achieved with conventional ventilation. Failure to adequately oxygenate the patient is frequently remedied by increasing the Paw
or the FiO2
. There is no evidence guiding exactly how ventilator adjustments should be made in the hypoxemic patient on HFOV. Generally, when FiO2
> 0.6, our approach has been to increase the Paw
. These increases are made slowly to give time for alveolar recruitment and to assess for cardiovascular impairment. In addition, these increases are frequently made in conjunction with a recruitment maneuver. Paw
values as high as 35–45 cmH2
O have been used and tolerated [30
]. In our experience, a higher Paw
may result in hemodynamic impairment, especially if the intravascular volume is inadequate. Should significant derecruitment from oscillator disconnects or circuit changes occur, our experience suggests that recruitment maneuvers are also helpful in this situation. Many pediatric and adult trials using HFOV (discussed later), however, have not utilized such an approach. Once the patient improves and the FiO2
can be decreased to below 0.6–0.4, the Paw
is generally weaned slowly, decreasing Paw
by 1–2 cmH2
O and assessing response.
As already described, one of the theoretical advantages of HFOV over other high-frequency modes is the decoupling of oxygenation and CO2
elimination. Ventilation is determined by changes in power (a surrogate for Vt
) and in frequency. Simply increasing the power will often result in improved ventilation. Once this is maximized, the frequency can be reduced. One must, however, keep in mind that these steps may lead to larger tidal volumes (as already mentioned) and to larger pressure swings at the alveoli, and as a result may lead to the potential to negatively impact on lung protection [30
]. Finally, deflation of the endotracheal tube cuff may help eliminate CO2
by allowing the front of fresh gas to be advanced to the distal end of the endotracheal tube, allowing a slight reduction of the anatomic dead space, which may be significant in situations when the Vt
is small. However, this may sacrifice the ability to maintain a high Paw