The Pre-brief
HFOV uses supraphysiologic frequencies and low tidal volumes. It delivers a constant mean airway pressure without the high peak/plateau pressures and the large change in pressure with each breath of a conventional ventilator (Figure 1). These pressures can be injurious in stiff, noncompliant lungs. The result of oscillatory ventilation: improved lung recruitment, gas exchange, and oxygenation with less lung injury including reduction in barotrauma and atelectrauma.
Although early studies showed improved oxygenation with lower frequency of barotrauma1, several adult trials have failed to replicate improvement in outcomes – and some have shown increased morbidity and mortality with use of this device.2,3 Flaws in these papers include confounding by indication (i.e. the sickest patients are most likely to receive the intervention), study design, and lack of a consistent and optimal physiologic approach to management of this device in the acute phase of weaning phase of illness. More importantly, however, kids are not little adults. To name a few differences, the chest wall of a child is more cartilaginous and deformable with smaller overall lung volume. It is unclear what, if any, role these differences play on the benefit (or not) of HFOV in this patient population.
By and large, use of this device seems to be institution and provider dependent. Thankfully, the Prone and Oscillation Pediatric Clinical Trial (PROSpect) is currently enrolling to address the uncertainty of the role and optimal management of HFOV specifically for pediatric acute respiratory distress syndrome (PARDS). This trial is an adaptive randomized control trial randomizing patients with moderate-to-severe PARDS to test the hypothesis that prone versus supine positioning and HFOV versus conventional mechanical ventilation will result in a 2 day improvement in ventilator free days.3
Until then, when should we use this device?
I pull out HFOV in any pediatric patient when I’m nearing settings on a conventional ventilator or in APRV that I fear are doing more harm than good. For conventional ventilation, my threshold is typically a plateau pressure of 28cmH20 (or a peak inspiratory pressure around 30-32cmH20) with an FiO2 close to 100% and an inability to maintain SpO2 >88%.
I typically will NOT use HFOV with significant hemodynamic instability or airway obstruction.
How do you use it?
In this mode, you set a MEAN AIRWAY PRESSURE (MAP) creating a constant distending pressure to hold the lungs open. A piston moves in and out producing oscillations in the airway that move small volumes of gas toward and away from the patient. The set AMPLITUDE determines the height of those oscillations, and a set FREQUENCY (or HERTZ) determines how many of those oscillations occur per second.
Typically, you will start with a MAP about 3-5cmH2O above the MAP you were achieving with your prior form of ventilation. You can increase the MAP further after initiation, but a rapid increase in MAP resulting in higher intrathoracic pressure may impair preload to the right heart, lead to poor cardiac output, and precipitate hypotension or cardiac arrest.
Amplitude is generally set so that the patient’s chest and abdomen have a generous wiggle. Use 15-20cmH20 above your PIP on the conventional ventilator as a general starting point.
Frequency is often set based on age (although this is a question that will hopefully be answered by the PROSpect trial).
FiO2 start at 1.00.
How do you manipulate settings to improve oxygenation and ventilation?
To improve oxygenation with HFOV, you can either increase your MAP in 1-2cmH20 increments or increase FiO2.
To improve ventilation, you can either increase the amplitude (move more volume with each oscillation) OR you can DECREASE your frequency. Now, if you’re thinking the frequency is your RATE then you may be thinking it seems a bit backward to DECREASE the rate to blow off more CO2. Think of it this way: when you decrease the frequency you have fewer oscillations per second and MORE volume is moved within each of those oscillations; therefore, ventilation is improved! Another way to think about it, the unit of frequency is hertz which is (1/second) so it is the inverse of a normal ventilator rate.
Another way to improve ventilation is to let a little air out of the cuff of the endotracheal tube to allow some passive CO2 removal.
To improve secretion clearance, get a new ventilator. But seriously…this is not a great mode of ventilation for the bronchiolitic with massive amounts of secretions. Each time you suction you’ll lose recruitment. The volume diffusive respirator can be a better option for those patients.5
It is often necessary for a child to be paralyzed to tolerate this mode of ventilation.Â
Proning is also an option; although, this should be done very carefully with all hands on deck to prevent accidental extubation as the patient is turned over in bed.
If HFOV is so great at improving gas exchange AND is lung-protective, why don’t we use it all the time?
HFOV is not a normal way to breathe and in many cases a child must be paralyzed to tolerate it and allow the ventilator to do the work. This can lead to increased morbidity.
Additionally, if you are holding open the lungs it can have a negative effect on heart function. It can decrease return of venous blood to the right side of the heart and cause hypotension.
As previously mentioned, it is not a great mode of ventilation with lots of secretions because when you insert a suction catheter into the airway, you cut the airflow delivery and de-recruit all that lung you’ve gained!
The Debrief
In the right patient, HFOV is a good mode of ventilation to support oxygenation and decrease ventilator induced lung injury. Hopefully, in the coming years we will have more information to support (or not!) the use of this device in critically ill patients with respiratory failure.
References
- Arnold JH, Hanson JH, Toro-Figuero LO, Gutiérrez J, Berens RJ, Anglin DL. Prospective, randomized comparison of high-frequency oscillatory ventilation and conventional mechanical ventilation in pediatric respiratory failure. Crit Care Med. 1994 Oct;22(10):1530-9. PMID: 7924362.
- Ferguson ND, Cook DJ, Guyatt GH, Mehta S, Hand L, Austin P, Zhou Q, Matte A, Walter SD, Lamontagne F, Granton JT, Arabi YM, Arroliga AC, Stewart TE, Slutsky AS, Meade MO; OSCILLATE Trial Investigators; Canadian Critical Care Trials Group. High-frequency oscillation in early acute respiratory distress syndrome. N Engl J Med. 2013 Feb 28;368(9):795-805. doi: 10.1056/NEJMoa1215554. Epub 2013 Jan 22. PMID: 23339639.
- Young D, Lamb SE, Shah S, MacKenzie I, Tunnicliffe W, Lall R, Rowan K, Cuthbertson BH; OSCAR Study Group. High-frequency oscillation for acute respiratory distress syndrome. N Engl J Med. 2013 Feb 28;368(9):806-13. doi: 10.1056/NEJMoa1215716. Epub 2013 Jan 22. PMID: 23339638.
- Kneyber MCJ, Cheifetz IM, Curley MAQ. High-frequency oscillatory ventilation for PARDS: awaiting PROSPect. Crit Care. 2020;24(1):118. Published 2020 Mar 27. doi:10.1186/s13054-020-2829-3. PMID: 32216813.
- Carman B, Cahill T, Warden G, McCall J. A prospective, randomized comparison of the Volume Diffusive Respirator vs conventional ventilation for ventilation of burned children. 2001 ABA paper. J Burn Care Rehabil. 2002 Nov-Dec;23(6):444-8. Doi: 10.1097/00004630-200211000-00011. PMID: 12432322.
