© 2003 American Thoracic Society
Recruitment maneuvers in ARDSTo the Editor:Villagra and colleagues evaluate a recruitment maneuver in severe primary acute respiratory distress syndrome (ARDS) patients (1). Patients are ventilated according to the principle of lung protective ventilation. A recruitment maneuver is applied by using a pressure controlled mode of ventilation (PCV) with an inspiratory plateau pressure of 50 cm H2O and a positive end-expiratory pressure (PEEP) level higher than the upper inflexion point. The conclusion is: when an "ideal" lung protective ventilation is applied, surimposed recruitment maneuvers do not improve oxygenation in the majority of these ARDS patients (1). Primary ARDS is characterized by lung consolidation, and PEEP and recruitment maneuvers may induce overdistension phenomena of normal lung regions (with diversion of blood flow to diseased regions) and only marginal alveolar recruitment. High PEEP levels and recruitment maneuvers in "primary" ARDS patients may be either ineffective or detrimental (2–4).
During PCV, alveolar recruitment is dependent on the inspiratory time constant ( Are the authors using a high level of PEEP (30 cm H2O) with the aim to recruit the lungs or to maintain alveolar recruitment? For the first aim, this PEEP level is probably too low; for the second one, this PEEP level is too high (6). Recruitment maneuver has to open the lungs and place the ventilatory system on the deflation part of the pressure–volume curve and PEEP has to keep the ventilatory system above the critical lung closing pressure (2). The study shows that recruitment maneuvers may be ineffective and deleterious. Are the authors really applying ventilatory parameters promoting alveolar recruitment and keeping the lungs open? And is it possible to recruit consolidated lung area without inducing dangerous phenomenon of hyperinflation? Recruitment maneuvers may be unnecessary or dangerous according to the modality of application, the etiology of ARDS, but also when an "ideal" lung protective ventilation is applied (6). We urgently need further evaluations concerning the modalities of application, the expected benefits/hazards and the accurate indications of recruitment maneuvers.
Gosselies Hospital Belgium REFERENCES
From the Authors:We thank Dr. Sottiaux for his interest regarding our study (1). Although lung protective strategies to provide mechanical ventilation for the acute respiratory distress syndrome (ARDS) patient are widely accepted, the role of recruitment maneuvers as an adjunct to mechanical ventilation remains to be determined. At present, many methods are proposed to recruit the lungs but lack guidance regarding which should be used (2). We performed alveolar recruitment by switching ventilatory mode from volume control constant flow to pressure control decelerating flow (mean peak pressure 47 ± 4.5 cm H2O) and by using high positive end-expiratory pressure (PEEP) levels (30 ± 4.9 cm H2O) to prevent alveolar derecruitment during expiration. In patients with ARDS ventilated at constant peak inspiratory pressure, when hysteresis is reduced with increasing PEEP, the inflation curve approximates the deflation curve, and pressure–volume slope may decrease with optimum PEEP (3). Consequently, compliance is nonlinear over inflation volume and it is very low in the upper segment of the lung pressure–volume relationship (4). Therefore, despite appropriate driving pressures during pressure control ventilation (difference between airway pressure at end inspiration and PEEP) low tidal ventilation and respiratory acidosis will invariably occur. Mathematical models (3) and human studies (5) predict almost the end of alveolar recruitment in those conditions. To accomplish full alveolar recruitment of the whole lung during pressure control ventilation at high PEEP levels would require potentially dangerous peak inspiratory pressures. Our study showed that recruitment maneuvers in ARDS patients superimposed to high PEEP ventilation had minimal and short lived effect on oxygenation (1). However, we agree with Dr. Sottiaux that further studies are needed to argue for or against the use of recruitment maneuvers in patients with ARDS.
Hospital de Sabadell Corporació Parc Taulí Sabadell, Spain REFERENCES
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
) of the respiratory system: passive equilibration between airways and lung needs an inspiratory time (TI) equal to three to four times