Is APRV just another option?
At the time, the University of Florida (Gainesville), their home institution, had been a hotbed of innovation in mechanical ventilation directed toward oxygenation failure, with such techniques as intermittent mandatory ventilation (IMV), high-level CPAP and Super PEEP having been clinical offshoots of work performed there. Stock and Downs reasoned that the principal management problem in most patients with injured lungs was failure to maintain sufficient lung volume, not one of providing enough ventilation.
Back then, the airway pressures applied were routinely high, lung rupture occurred commonly, and valves and flow controllers on ventilators of the day made gas delivery difficult to coordinate with the patient's breathing rhythm. An "open circuit" design that allowed the patient to have unfettered access to the gas source while limiting peak airway pressure was a welcome innovation. Unassisted breathing with continuous positive airway pressure (CPAP) or CPAP with periodic decompressions (release cycles) (i.e., APRV) offered the apparent answer.
Using high-level CPAP assigned greater priority to keeping the lung open than to relieving the breathing workload, which in all but the most severe cases they considered a tolerable burden. For the latter group of very distressed patients, the periodic release and near immediate subsequent build-up of airway pressure (APRV) would assist the patient's own breathing efforts with energy from the machine while not allowing the lung to collapse.
A comparative analogy was made between APRV and IMV, but with what was perceived an important difference. In IMV, mean airway pressure was considerably lower, and peak airway pressure was considerably higher. (Whether airway rather than trans-alveolar pressure should be the focus of attention is another issue left unaddressed.)
It should be noted that using APRV fits right in with today's open lung approach to the problem of limiting ventilator-induced lung injury (VILI). This really comes as no surprise, as interest in inverse ratio ventilation (IRV) for improving oxygenation by sustained opening of alveolar units was running high at that time.
IRV was considered a closed circuit method for raising mean airway pressure and arterial blood gases, and as emphasized by Lachmann (an early "open lung" advocate), it was imperative not to allow the lung to collapse during the relatively brief expiratory phase. Indeed, APRV can be viewed as IRV applied with a very high I:E ratio and delivered using an open circuit. The lower requirement for sedation was justifiably seen a big advantage favoring APRV over IRV.
About the same time as APRV was being described, Marcel Baum in Austria proposed alternating CPAP between two levels of adjustable magnitude and duration, with much the same rationale. Unlike APRV, however, multiple breathing cycles could occur at the lower CPAP level before re-establishing the upper CPAP baseline.
The essential differences between APRV and Baum's Bi-Phasic ventilation--which he termed BIPAP--rest in the mean airway pressure and in the duration and the nature of the floor under the low pressure phase. In concept, Baum's BIPAP closely resembled today's bi-level ventilation. (This is not to be confused with the commercial "Bi-PAP", which does not alternate CPAP but rather combines PEEP and pressure support on each machine cycle.)
By setting a lower airway pressure, BIPAP reduces the mean airway pressure while it guarantees a certain lower airway pressure will be maintained. This airway pressure "floor" must be set sufficiently high so that widespread closure doesn't happen. In theory, the risk of serious ventilator induced lung damage is greatest during the transitions to the higher pressure baseline.
APRV is open to the same concern regarding tidal collapse during circuit decompression. In using very brief release times, APRV seeks to maintain autoPEEP of a level sufficient to prevent widespread airway closure. However, unless PEEP is used with it (thereby limiting the exhalation driving pressure for airflow and ventilation efficiency), APRV does not assure that such closure of lung units does not occur.
Making the release cycle shorter generates more auto-PEEP but compromises the ventilation boost, especially in patients with tightly obstructed airways. (APRV is definitely a poor choice for ventilating patients with exacerbated COPD, bronchospasm or acute asthma.) While lung unit opening and closure may or may not occur in ARDS, relatively high stretch of open units is virtually guaranteed.
Almost from the start, opponents to APRV voiced their concern regarding several potentially important issues. First, airway pressure was set to mean values somewhat higher than customary, even for these patients. This high pressure baseline not only presented questions of overstretching lung tissue within the heterogeneously damaged lung, but raised concern regarding the potential for sustained hyperinflation to disadvantage the respiratory muscles assigned the bulk of the breathing workload during APRV.
Second, the possibility of impeding venous return and augmenting right ventricular afterload by high airway pressure needed to be confronted. Finally, and perhaps most worrisome, it was pointed out that the trans-alveolar pressure and not simply the airway pressure is the variable of interest. It seemed plausible that the combined stretching forces of high airway pressure with those from low pleural pressures associated with vigorous breathing effort might be tissue damaging.
To some extent, each of these concerns is valid, depending on the patient, the pressures and the settings in use. Although strong opinions exist to this day, none of these potential problems has been research-proven to be an insurmountable deal-breaker.
After more than 20 years, neither APRV nor BIPAP (bi-level) has taken over as the dominant mode of choice for ventilating ARDS. Whether this lack of general acceptance should be attributed to irrational fear, ignorance, inexperience or actual shortcomings of these modes is not completely clear. Maybe a little of each, thrown in with the continuing controversy as to whether striving for the fully open lung is wise.
Vocal supporters from a relatively small number of centers experienced with routine APRV use are convinced that most of us are missing out on a great and underappreciated management tool. To be sure, initial enthusiasm seems to have faded with passing time. Yet modern equipment regulates pressures, flows and circuit resistance much better than previously, and they these options offer two uncontestable advantages.
By dropping the higher pressure and stretching the period between release cycles, APRV can be used as one mode for all phases of ventilatory support, from the initial acute period right through the weaning phase into low level CPAP. Furthermore, the "open circuit," though not unique to either APRV or BIPAP, encourages synchrony, even in patients with severe respiratory distress.
The same cannot be said for commonly used time-cycled modes (pressure or flow regulated assist-control). To the extent that maintaining a spontaneous breathing pattem is important to patient comfort and outcome, this is a big plus. Less clear is whether we should encourage "open lung" approach to ventilating all patients with ARDS.
As time passes, it impresses me how slowly we move toward better care for our sickest patients. It took roughly two decades (1965-1995) for us to recognize that large tidal volumes that are appropriate for healthy lungs cause barotrauma when their delivery required high pressure. It has also taken a maddeningly long time (roughly 1985 to the present) for us to agree that VILI actually exists in the clinical setting and can be avoided by more intelligently selecting our ventilation targets and employing rational strategies established by convincing laboratory research many years ago.
Now 22 years downstream from Stock and Downs' initial publication, we remain conflicted as to whether we should finally adopt APRV as a first-line mode for ventilating the acutely injured lung. In my view, when the patient can be made comfortable and does not work excessively, well-adjusted APRV is an attractive choice for the initial phase of ARDS management. But the same might be said for high frequency ventilation and any other available mode that is applied wisely with modern equipment and with careful attention to the underlying physiology that determines both benefit and hazard.
by John J. Marini, MD
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|Title Annotation:||RESPIRATORY CARE|
|Author:||Marini, John J.|
|Publication:||FOCUS: Journal for Respiratory Care & Sleep Medicine|
|Date:||Jun 22, 2015|
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