Airway Pressure Release Ventilation Letter—Reply

We thank Dr Light for his insights on airway pressure release ventilation (APRV)¹ and will address his comments one by one below. However, we first re-emphasize that the purpose of our study² was not to address the clinical value of APRV—that can only be accomplished with randomized clinical trials. In contrast, our goal was to illustrate the current practice of experienced clinicians using APRV and to point out that APRV is not a simple “on–off” switch; rather, it utilizes 4 non-conventional settings that can be adjusted over wide ranges. We believe we have shown that there is considerable practice variability among experienced APRV users when using the mode, and that there is the potential for untoward consequences.

Our thoughts on the specific points raised by Dr Light:

With regard to conventional ventilation settings, while we agree that settings for conventional mechanical ventilation (loosely defined as mimicking the normal breathing pattern) often lack consensus, there is a considerable evidence base driving consensus on ventilator management of the acutely injured lung. Starting with the ARDS Network's small tidal volume (V_T) trial³ and followed by several subsequent trials,⁴ a strong consensus has emerged to limit V_Ts and the end-inspiratory airway plateau pressures to physiologic ranges. Recent large surveys indicate that most ICUs have adopted this approach.⁵

With regard to similarities between APRV and high-frequency oscillatory ventilation (HFOV), we agree that similarities exist between APRV and HFOV and thank Dr Light for highlighting this point. Both strategies are based on CPAP principles, and both manipulate CPAP in non-conventional ways: APRV with periodic brief releases, HFOV with superimposed small-amplitude oscillations. Moreover, with both modes there are non-conventional settings that can be manipulated in multiple ways that, depending on patient characteristics, can clearly affect outcomes. Underscoring this point is the recent meta-analysis of adult randomized clinical trials demonstrating that, while HFOV may be an effective rescue strategy in very severe lung injury, it can cause considerable harm when delivered inappropriately to subjects doing well on conventional ventilation.⁶

With regard to the P_high setting, given that existing APRV guidelines^7,8 recommend limiting P_high to <30–35 cm H₂O (in agreement with many conventional ventilation guidelines), we were concerned that 36% of our respondents accepted values above that level. This result also heightened our concern that many respondents may not fully appreciate that spontaneous efforts occurring during P_high will add to the maximal transpulmonary pressure, further increasing the risk of lung injury.

With regard to the T_low setting, while it is always possible that respondents may have misunderstood our question regarding the initial T_low setting, we believe that most would interpret our question to be addressing the setting at which the patient is started and then is followed for a period of time before reassessing. We stand by our interpretation that there is limited consensus on whether to use an absolute time setting or to use a variety of expiratory flow analyses to set T_low.

With regard to tidal pressure and V_T, large changes in tidal pressure and V_T clearly affect ventilator-induced lung injury. Indeed, the whole basis for the initial ARDS Network trial³ was to limit V_T to the physiologic range of 4–8 mL/kg predicted body weight. We agree that this approach does not address V_T distribution in heterogeneous lung injury, and we also agree that driving pressure (end-inspiratory pressure − end-expiratory pressure) might be a better V_T target.⁹ However, our concern from the responses to our survey was that there seemed to be a number of respondents for whom tidal lung distention was not important with APRV (ie, accepting V_T > 8 mL/kg predicted body weight).

With regard to T_low versus set PEEP, our survey and analysis of responses were designed to illustrate current APRV clinical practice and was not designed to compare APRV approaches to other strategies for setting expiratory pressure. We agree that there is a lack of consensus on setting the best PEEP with conventional ventilation in many diseases. PEEP/F_IO2 tables targeting both P_aO2 and plateau pressure limitations are commonly used with conventional ventilation. A recent meta-analysis of high versus low PEEP/F_IO2 tables in ARDS suggests that more aggressive PEEP appears to work better in very severe injury and that less aggressive PEEP appears to work better in less severe injury.¹⁰ However, whether these approaches to setting applied PEEP are better than manipulating T_low and auto-PEEP with APRV requires randomized clinical trials.

Finally, with regard to respondent numbers, we certainly agree that 60 respondents is a low number. However, APRV is routinely used in only a small fraction of institutions and thus clinicians comfortable with the mode are likely few in number.¹¹ Importantly, we feel that respondents from the AARC Adult Acute Care Section likely represent a subset of respiratory therapists with significant interest in and experience with APRV.

In summary, Dr Light makes a number of important points, and we appreciate the opportunity to discuss them. We certainly do not want to be viewed as “taking shots” at APRV. We are only reporting current clinical practice. APRV has intriguing physiologic features that are worthy of serious study and discussion. However, our results show that there is currently substantial variability in its application and support the notion that APRV's ultimate value will require well-conducted clinical trials using consistent approaches to management.

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