MONITORING OF HIGH-FREQUENCY VENTILATION

High-frequency ventilation has been variously defined but in general represents mechanical ventilation at high rates (usually 160 breaths/min). Several means of ventilating in this manner have been used experimentally and clinically. The use of conventional positive-pressure ventilators at high rates and small tidal volumes is called high-frequency positive-pressure ventilation (HFPV). Tidal volumes are usually on the order of 3 to 4 mL/kg of body weight, with a frequency of 60 to 100 breaths/min. The use of an oscillator providing positive and negative pressure fluctuations (e.g., a loudspeaker) is called high-frequency oscillatory ventilation (HFOV). Higher frequencies, upward of 3000 cycles/min, have been used with this modality. A bias flow of fresh gas at the level of the oscillator provides the source of the respiratory gas and washes out CO₂ . Injection of a high-velocity pulse of gas into the airway through a narrow cannula, entraining with it fresh gas, is called high-frequency jet ventilation (HFJV).

In all these forms of high-frequency ventilation, instantaneous gas flows and pressure fluctuations cannot

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Monitoring of patients receiving high-frequency ventilation requires the ability to monitor O₂ and CO₂ exchange, as well as mechanical safety, including airway disconnection and obstruction. Hoskyns and colleagues^[257] measured tidal volumes in 0.8- to 1.9-kg infants ventilated at 2 to 25 Hz using an external respiratory jacket. A side port of the jacket was used to monitor pressure changes, which correlated linearly with tidal volume.

Whereas patient oxygenation can readily be monitored with pulse oximetry, there is not reliable noninvasive monitor of CO₂ exchange. One way of monitoring CO₂ is to measure the mean waste gas CO₂ concentration by placing a capnograph in the expired circuit. If any condition that interferes with CO₂ exchange develops, the mean expired CO₂ decreases. although this method provides a fairly gross measure of adequacy of CO₂ exchange, the expired CO₂ concentration highly depends on fresh gas flow rate. A more satisfactory monitor would multiply fresh gas flow by expired CO₂ fraction to obtain V̇CO₂ . Changes in CO₂ could then reflect a mechanical problem with the ventilator. Unfortunately, other factors such as anesthesia or hypothermia may alter V̇CO₂ . The clinician cannot obtain a measure of PACO₂ or PaCO₂ by this method. The most commonly used method is to interject a conventional breath periodically to measure end-tidal CO₂ .^[258] Capnometry can be used to measure end-tidal CO₂ during HFJV without such a maneuver, provided that gas is sampled from a port situated at the tip of the endotracheal tube.^[259]

Monitoring of airway pressure is extremely important in high-frequency ventilation. In particular, HFJV uses high pressures and gas flows. Expiratory port occlusion can therefore result in extremely high airway pressures. Gas pressures commonly are measured on both sides of the jet valve (i.e., drive pressure and jet pressure), along with an independent pressure measurement in the airway ( Fig. 36-25 ). An automated feedback loop is required to interrupt the jet ventilation by closing the solenoid valve in the event of excessively high pressures in the airway or on the jet side of the valve. Low pressures can be used as indications of airway disconnection or ventilator malfunction.

In addition to safety concerns, airway pressure has been shown in several studies to correlate with gas exchange efficiency during HFJV.^[260] Increasing peak airway pressures result in lower PaCO₂ . A superior indicator of PaCO₂ is the difference between peak airway pressure and end-expiratory airway pressure.^[261] However, there is no unique relationship, and the PaCO₂ obtained for a given patient depends on properties of the lung. Position of the

Figure 36-25 High-frequency jet ventilation. The jet is created when a high-pressure air-O₂ supply is rapidly modulated by the solenoid valve. Fresh inspired gas is from a low-pressure source, typically an anesthesia circuit. Drive pressure (PD) and jet pressure (PJ) are customarily monitored to detect solenoid or jet malfunction. An independent monitor of airway pressure (Paw), which can reliably detect overpressurization of the airway, circuit disconnection, or ventilator malfunction, should also be available.

Jet ventilation for prolonged periods should ideally be performed on patients with the ability to monitor arterial blood gases directly. Periodic measurement of PaCO₂ may provide greater assurance of adequate pulmonary gas exchange than simple reliance on noninvasive measures.