To the Editor:
I would like to thank the authors for bringing to light the need for further education on airway pressure release ventilation (APRV) and how to manage this mode of ventilation.1 However, for Miller, Gentile, Davies, and MacIntyre to state that “there is only limited consensus among practitioners for initial APRV settings” and to infer that this is a problem for APRV may be misleading. Generally speaking, in terms of PEEP and ideal volume strategies, there is no overarching common consensus among practitioners on how to accurately set up conventional ventilation either. Had a similar survey been performed asking practitioners where they should set the tidal volume (VT) or PEEP on a conventional ventilator, I believe a similar limited consensus result would have been reached. The same result would occur when asking what mode of conventional ventilation should be used (eg, volume control, pressure control, or pressure-regulated volume control). In fact, it has been reported in a secondary analysis of a multi-center cohort2 that there was substantial variability in ventilator settings not entirely explained by subjects' risk for ARDS.
Additionally, this fact is demonstrated by the topics of the June 2016 issue of Respiratory Care. Within that publication, when asked about randomized, controlled trials, an author stated, “The results really only apply to the specific population studied (often a small fraction of the disease of interest) and only to the specific intervention strategy. This last point is particularly important in complex interventions like mechanical ventilation where potential settings and management strategies can be quite variable.”3
Miller et al1 state that “APRV is not an on–off switch … as a consequence, interpreting trials and making recommendations about APRV problematic.” However, one of the authors in 2016 stated the following about high-frequency oscillatory ventilation (HFOV), “[HFOV] is certainly not an on–off switch and involves not only numerous specific ventilator adjustments but also adjustments in other aspects of management (eg, fluids, pressors) that clearly can impact results. To draw the conclusion that HFOV managed differently in different patients is necessarily harmful seems excessive.”3 The question now becomes, why do the authors differ in their interpretation of APRV and HFOV not serving as an on–off switch? The evidence reported to date on APRV shows that it is no worse than conventional ventilation in the limited studies that compared the 2 methods, in contrast with a large clinical trial that demonstrated poor outcomes with HFOV.3,4
When you look at the authors'1 reported survey responses on the Phigh setting, a patient with conventional settings and measurements of PEEP 15 cm H2O, Ppl 28 cm H2O, and peak inspiratory pressure 30 cm H2O, the patient would have a mean of approximately 20 cm H2O. This would mean that 79% of the survey respondents would set their Phigh at 25–28 cm H2O. Are we really saying that this difference of 3 cm H2O for an initial Phigh setting is that bad and represents a poor understanding of APRV? We accept a range for an initial VT in conventional ventilation, so why do we not accept a range for Phigh? The fact that respondents didn't fit exactly as 2 protocols spelled out does not mean that there is a problem or a lack of understanding.
With regard to the limited consensus findings on Tlow, given how the survey question was asked, an initial setting of 0.2–0.8 s could be a correct statement depending on the reader's interpretation of the question. If a respondent read the question as asking where you would initially set the Tlow, you have to initially set it somewhere before you find out if it is at 50–75% of peak expiratory flow (PEF). Even one of the referenced APRV protocols5 states “we usually start with a Tlow of 0.6–0.8 s.” So, 0.2–0.8 s is a good place for an initial setting until it is on the patient, and then you adjust it. Therefore, 76% of the respondents accurately answered the question. Would we get the same type of consensus if you were to ask where to set the initial PEEP in conventional ventilation? The authors cited Amato et al6 as demonstrating that VTs are increasingly appreciated as an independent risk factor for ventilator-induced lung injury. However, Amato et al6 reported that individual changes in VT were not independently associated with survival and were only associated with improved survival if the VT changes led to decreases in ΔP. Amato et al6 only mention VT separately as an independent risk factor in their introduction and then reference other papers' positions, not their own findings. Additionally, APRV has been found to be able to deliver similar VTs as conventional ventilation with low ΔPs.7 Via a mathematical lung model, when using APRV on a simulated patient with ARDS, the VT achieved was 6.6 mL/kg with a total PEEP of 11 cm H2O.6 The lung model and vent settings resulted in a ΔP of 14 cm H2O and achieved the same PEEP as what would be recommend in the ARDSnet PEEP table (10–14 cm H2O) for the entered FIO2.7 Furthermore, the Tlow in the study was only set for approximately 50% of PEF. Had the Tlow been shortened to 75%, as proposed by others,8,9 the intrinsic PEEP would have been higher, VT would have been lower (thus closer to the ARDSnet recommended 6 mL/kg), and the ΔP would have been even lower, resulting in a potential decrease in mortality based on Amato et al6 These results demonstrate via a mathematical model that APRV could be seen as in keeping with Amato et al6 and even as current evidence for conventional ventilation.
Regarding PEEP and APRV, PEEP is another area of limited consensus. However, the different levels, high versus low, have not been shown to demonstrate changes in the outcomes of patients with ARDS.10 Therefore, APRV is being compared to something that also has no consensus. A recent study has demonstrated that using a Tlow of 75% of PEF has been found to produce less micro-strain and macro-strain on alveoli versus PEEP values of 5 cm H2O or 16 cm H2O.8 Furthermore, setting PEEP to higher levels has been reported to create significantly different alveolar size distribution within the lung versus setting a Tlow to 75% of PEF.9 So while a set PEEP may or may not generate a consistent end-expiratory pressure in the alveoli, it has been shown to create more strain and greater variance in alveolar size. Which is worse for the lung? Given the lack of data demonstrating a survival benefit for higher versus lower PEEP in conventional ventilation, maybe it's time we start to look at another way to stabilize the lung. A set PEEP value doesn't seem to be the answer, and if it is, the evidence has not shown us how to set it correctly.
The article by Miller et al1 is based on 60 responses and was limited to 1 per institution from individuals in the AARC Adult Acute Care Section, which has approximately 1,900 members. This represents 3% of that membership and an even lower percentage of practicing therapists. In addition, given the fact that the authors limited the number of responses to 1 per institution, it really only assesses the first person to see and respond to the survey versus a true assessment of how clinicians set up and manage APRV. With a 3% representation of a small subpopulation of therapists, can we truly assess how bedside clinicians are using APRV from this survey?
Why should APRV be held to a different standard than conventional ventilation or HFOV? We can't reach consensus on any ventilator settings, so why should/would APRV be any different? I also suggest that the agreement on APRV settings among clinicians might actually be higher than what would be found in conventional ventilation or HFOV, given the high degree of scrutiny it has endured over the years.
Footnotes
Dr Light discloses a relationship with Dräger.
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