Respiratory Potpourri: Recruitment Maneuvers During High-Frequency Jet Ventilation (HFJV): High, Low, Long, or Short?

May 31, 2020

15, Health, Medical Practices, Respiratory Care, Volume

Rob Graham, R.R.T./N.R.C.P.

Those who have been utilising high-frequency jet ventilation (HFJV) for some time have most likely used conventional breaths (CMV)) superimposed on HFJV either to reverse atelectasis or for initial lung recruitment. Traditionally the term conventional breath was an apt description as their parameters were just that; relatively high peak inspiratory pressure (PIP) of 20 cmH2O (or higher) and inspiratory time (Ti) of 0.5 seconds or so.

The past debate has revolved around whether to set CMV PIP below HFJV PIP, or above. The difference between the two, aside from the obvious difference in pressure, is with the jet itself. CMV PIP set higher than HFVJ PIP will cause the jet ventilator to pause for the duration of the CMV breath, while CMV PIP set below HFJV PIP will not; HFJV breaths will be superimposed on CMV breaths.

PIP differences aside, clinicians were instructed to use CMV during initial HFJV to recruit the lungs since the low PIP and short Ti of HFJV are not powerful enough to do so on their own. These were started at rates of 5-10 and were reduced as FiO2 improved with the aim to stop them entirely and run HJFV in CPAP mode. CMV breaths were (and still are) advised to help determine optimal PEEP settings. If FiO2 increases when CMV rate is reduced or CMV breaths are discontinued, PEEP is increased until FiO2/ SpO2 is stable when CMV is discontinued. This approach seems to work, and it is still the standard practice in many NICUs.

The burning question behind the use of CMV with HFJV is “why?”. After all, if the benefit of HFJV is its gentleness, why use high PIP CMV breaths at all? This strategy works in the short term, but there are known sequelae associated with CMV, mainly inflammatory response and lung injury stemming from sheer forces, volutrauma, and conducting airway/alveolar duct rupture or tears. The short-term gain from the use of CMV may come at the cost of pulmonary damage later.

I do not use the traditional style CMV breaths in my personal practice. Initially, rather than use CMV to recruit the lung, I prefer to start with higher PEEP instead. This has worked well for me and is how HFJV is done in the NICU I practice in. I believe it is one of the reasons our CLD rates are remarkably low.

There are clinical situations, however, which do not respond well enough to increasing PEEP. Regional atelectasis is one, and unilateral collapse is another. The question is how to manage these pathologies clinically without causing further damage to the lung, and without over-distending well-functioning areas of higher compliance.

Physics dictate that gas takes the path of least resistance; compliant areas accept volume more readily, and gas will preferentially fill these areas until they become less compliant from over-distention. Once this happens, gas will begin to enter less compliant/ higher resistance areas.

There is an inherent problem with the standard CMV breath in this situation: time. Time constants dictate how long it takes for gas to fill a space, and the most compliant areas of the lung take the longest to fill; a standard Ti of 0.5 seconds likely does not afford enough time for this to happen, let alone time for pendelluft to redistribute volume within the lung. The result is areas of higher compliance being over-distended, resulting in volutrauma, and collapsed/atelectatic areas suffering damage from the inflammatory response with surfactant impairment that follows atelectasis. (1,2) The clinical response may be good, but it comes at a cost.

Contrast this with a different form of CMV, one which ostensibly protects compliant areas while gently opening up areas of collapse. How is this accomplished? A combination of relatively low CMV PIP combined with a longer CMV Ti.

Limiting PIP reduces the volume that enters compliant areas, thus giving some protection against volutrauma while increasing the CMV Ti gives more time for pendelluft to occur once compliant areas have accepted as much volume as they will at a given PIP. The lower PIP also slowly and gently exerts a force against collapsed areas and eventually recruits them. I refer to these breaths as recruitment maneuvers (RMs) to differentiate from the standard CMV breaths of old.

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It is fortunate that here in Ontario as in California, closing down non-essential businesses and institutions as well as social distancing and stay at home advisories have thus far prevented our system from being overwhelmed. Knock wood, but at this point, we in Ontario have a surplus of adult ICU bed capacity and no shortage of ventilators. As with the rest of the world, ensuring PPE availability is still a challenge, and the first wave of this pandemic is not yet over. How any of us fair with the predicted second wave is at this point unknown and will largely depend on our collective ability to ramp up production of PPE to ensure all involved in the care of COVID-19 patients are properly protected.

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Disclosures: The author receives compensation from Bunnell Inc for teaching and training users of the LifePulse HFJV in Canada. He is not involved in sales or marketing of the device nor does he receive more than per diem compensation. Also, while the author practices within Sunnybrook H.S.C. this paper should not be construed as Sunnybrook policy per se. This article contains elements considered “off label” as well as maneuvers, which may sometimes be very effective but come with inherent risks. As with any therapy, the risk-benefit ratio must be carefully considered before they are initiated.

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