Peak Expiratory Flow During MI-E: ETT vs Face Mask. Which Is the Optimal PEF Cutoff to Choose?

To the Editor:

We read with great interest the manuscript by Hyun SE et al¹ who compared the effect of mechanical insufflation-exsufflation (MI-E) on peak expiratory flows (PEF), through endotracheal tube (ETT) or facial mask, and assessed expiratory flows during different pressure levels set on the device. The authors found a significant decrease in PEF when MI-E was applied through the ETT (with inner diameter ranging from 6.5 mm–8 mm) and that expiratory flows were significantly influenced by MI-E pressure setting, achieving the highest PEFs when +50/−50 cm H₂O of insufflation-exsufflation pressure was applied (177.4 L/min [165.9–188.8]). These interesting results are in line with most previous laboratory studies^2-4; nevertheless, we would like to comment on a few critical aspects of this work.

Hyun SE et al¹ concluded that insufflation-exsufflation pressures of at least +50/−50 cm H₂O should be necessary to achieve an efficient PEF, previously defined as ≥ 160 L/min. Indeed, this cutoff has been the standard since 1996 when Bach JR et al⁵ proposed it as criteria to extubate and decannulate patients with ventilatory failure although, importantly, PEF measurement was performed after removal of the artificial airway. More recently, Jiang J et al⁶ reviewed values from 9 studies that assessed PEF cutoffs during weaning to predict successful extubation. Most of the studies used PEF cutoffs < 160 L/min, ranging from 35 L/min in mixed ICU subjects⁷ to 113.7 L/min in medical ICU subjects,⁸ with significant prediction of extubation success. Similarly, we recently evaluated PEF during MI-E set at +40/−40 cm H₂O with mid-inspiratory flow in intubated and sedated critically ill patients and found that PEFs of 96.9 ± 20.6 L/min⁹ were sufficient to improve sputum clearance. We agree with Hyun SE¹ that, in critically ill patients on invasive mechanical ventilation, higher MI-E pressures may be necessary to reach efficient PEFs; however, considering the aforementioned discrepancies in PEF cutoffs, it would be inadequate to conclude that MI-E set at insufflation-exsufflation pressures < +50/−50 cm H₂O are unable to produce an efficient PEF.

Sputum clearance is improved when flow bias, which results from the difference between PEF and peak inspiratory flow (PIF), favors expiratory flows.^10,11 Thus, inspiratory flow should also be considered as a determinant factor when assessing the most effective setting during MI-E. For instance, Volpe M et al¹⁰ compared a standard MI-E maneuver with equal insufflation-exsufflation pressure and high inspiratory flow against an optimized maneuver, including higher exsufflation pressure than insufflation pressure and low inspiratory flow. Interestingly, the standard maneuver generated a higher PEF; but flow bias and, consequently, cephalad sputum displacement were better during the optimized setting. Unfortunately, in the present study there is a lack of information regarding the set inspiratory flow, PIF, and the resulting flow bias, which limits the interpretation of the results and the impact of the purposed setting.

To date, scientific evidence on the efficacy and safety of MI-E during invasive mechanical ventilation is scant. Nevertheless, the study by Hyun SE et al¹ as well as previous studies demonstrates that critical factors such as the studied population, inspiratory-expiratory pressure and flow settings, or PEF cutoff should be considered when MI-E is implemented. Additional clinical studies are needed to elucidate the role of MI-E and setting adequacy in intubated and critically ill patients in ICU.

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