PEEP and Mechanical Ventilation: We Are Warned, We Cannot Ignore

To the Editor:

In an interesting study recently published in Respiratory Care, Natalini et al¹ analyzed in 186 subjects receiving mechanical ventilation (settings chosen by the attending physician) several potential causes of dynamic hyperinflation and intrinsic PEEP (auto-PEEP). Both intrinsic and extrinsic² auto-PEEP determinants as well as differences between low and high auto-PEEP cohorts (cutoff: 5 cm H₂O) were assessed. The results showed that expiratory flow limitation, the ratio between the expiratory time and the time constant of the respiratory system (T_E/τ_RS), the inspiratory resistance (R_RS), and body mass index not only were independently associated with auto-PEEP levels but also represented the strongest risk factors associated with auto-PEEP >5 cm H₂O. Surprisingly, T_E did not. The authors concluded that the ventilator settings play a marginal role in auto-PEEP generation in the absence of subjects' predisposing factors. As a clinical consequence, the authors suggested that auto-PEEP can be effectively reduced by acting on patients' respiratory mechanics impairment, with little/no additional effect obtained by breathing pattern manipulation.

We are indebted to the authors for several reasons. First, they pointed out the key role played by expiratory flow limitation in generating dynamic hyperinflation and auto-PEEP. As a matter of fact, their data show that <50% of actual auto-PEEP was accounted for by elastic and resistive properties of the respiratory system alone (comparing actual with theoretical auto-PEEP [ie, 1/maximum C_RS × trapped expiratory volume computed according to longest τ_RS]; data from Table 3). This reinforces the role of application of adequate CPAP levels to counterbalance auto-PEEP in the presence of expiratory flow limitation.³ Second, they stressed the importance of treating patients receiving mechanical ventilation with medical therapy. In our experience, too many physicians forget that mechanical ventilation has no therapeutic role in improving patients' respiratory mechanics impairment⁴; it only equilibrates the imbalance between respiratory muscle force-generating capacity and increased respiratory work load,^4,5 providing time to recover from respiratory illness, facts that warrant concomitant bronchodilator use. Third, they suggest considering T_E/τ_RS ratio instead of T_E alone when setting the ventilator. In this line, we suggest that τ_RS should be directly measured⁴ to account also for expiratory flow limitation, when present. In our opinion, few physicians recognize the relevance of setting T_E according to τ_RS. As a matter of fact, the breathing pattern was similar in both low and high auto-PEEP cohorts also in the present study.

This last fact is the cause of our major criticism of this worthy paper. The lack of relationship between auto-PEEP and T_E forced the authors to conclude that “manipulation of the breathing pattern might only have a negligible effect on the overall auto-PEEP value.” However, this result depended mainly on the quite constant T_E imposed by the attending physicians in the face of a wide range of auto-PEEP levels.¹ To test auto-PEEP response to changing T_E, ad hoc protocols are necessary (eg, different T_E values tested in the same patient). Thus, the claim that breathing pattern manipulation has negligible effects on auto-PEEP sounds wrong and misleading and conflicts with the authors' seminal observation that T_E should be chosen according to τ_RS.

In conclusion, also thanks to Natalini et al,¹ enough knowledge is currently available to identify patients prone to develop significant auto-PEEP during mechanical ventilation, to treat its intrinsic causes (pharmacologically), and to prevent/attenuate its onset by manipulating the ventilator settings. We are warned, we cannot ignore…

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