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We feel that a mPaw ≥ 24 cmH2O while on conventional ventilation is a reasonable threshold at which to consider changing to HFOV because adult patients on HFOV can generally be transitioned back to conventional ventilation when the mPaw on HFOV has been weaned to this level (assuming FiO2 has been weaned to 40 – 50% first). Early institution of HFOV in patients deteriorating on CV may be important to improved survival. In both the Wilford Hall and Toronto non-randomized studies, prolonged time on CV (e.g. > 7 days) predicted a worse outcome. Preparing to initiate HFOV Prior to starting a patient on HFOV it is imperative that the patient’s airway is suctioned and known to be patent. If bronchoscopy is contemplated (e.g. to evaluate for opportunistic infection) it should be performed prior to initiating HFOV. This will also allow direct visualization of the airway to document the degree of patency of the endotracheal tube. Narrowing or obstruction of the endotracheal tube with mucus or blood clots may greatly impede delivery of the oscillatory waveform and make ventilation difficult. Adequate titration of sedation, analgesia, and neuromuscular blockade should be performed while the patient is still on conventional ventilation. The patient’s intravascular volume status should be reassessed keeping in mind the higher mPaw that will be used with HFOV and the potential for hypotension secondary to elevated intrathoracic pressures and reduced preload. Initial HFOV Settings Oxygenation The main determinant of oxygenation during HFOV is the mPaw, which is generally initiated at 5 cmH2O higher than the mPaw noted during conventional ventilation. Hemodynamically unstable patients may be started on a mPaw either the same or 2 – 3 cmH2O above mPaw during conventional ventilation. Brief hypotension shortly after starting HFOV is usually managed with a trial of fluid boluses to improve preload. Whether patients should be started on even higher mPaw (a “sustained inflation” as an alveolar recruiting maneuver) such as 40 cmH2O for 40 – 60 seconds when HFOV is first initiated is not yet clear. If the patient requires suctioning for gross secretions visible in the endotracheal tube or has a disconnect from the circuit, we favor doing a brief sustained inflation in the hopes of re-inflating areas of atelectasis that may have occurred even with short periods of suction or circuit disconnection. FiO2% is usually set at 100% after the transition to HFOV, and then tapered using oximetry guidance to maintain SpO2 ≥ 88%. If the SpO2 (or PaO2) has not improved enough to allow weaning of FiO2%, the mPaw is raised in 3-5 cmH2O increments at 30 – 60 minute intervals in the hopes of improving lung recruitment. The time course of oxygenation change after initiation of HFOV (or after a given increase in mPaw) is quite variable. Some patients may slowly improve oxygenation only after a period of several hours. Vigilance and patience are required during the early phase of treatment. The maximum mPaw we can generally obtain with the SensorMedics 3100B is 45 – 55 cmH2O. Patients with large bronchopleural fistulas or endotracheal cuff leaks may have difficulty achieving a desired mPaw without increasing the bias flow. In some patients with very severe airleaks we have had to use maximum bias flow (60 lpm) on the oscillator.
3100B Front Panel Controls
1. Bias Flow 2. Mean Pressure Adjust 3. Mean Airway Pressure 4. Power 5. Amplitude (Delta P) 6. % Inspiratory Time 7. Frequency 8. Start / Stop Ventilation The main determinants of PaCO2 elimination are the pressure amplitude of oscillation (∆P) and the frequency setting (hertz). Increasing the ∆P and decreasing the frequency (Hz) increase delivered tidal volume and lower PaCO2. Conversely, decreasing ∆P and increasing frequency (Hz) will reduce delivered tidal volume and allow PaCO2 to rise.