To the Editor:—
We read with interest the editorial by Bigatello et al. , 1which described “protective” ventilatory techniques as part of an integral approach to the treatment of adult respiratory distress syndrome.
Some points, however, need to be addressed. Although it is reasonable to use tidal volumes less than 8 ml/kg and to keep the plateau pressure less than 30 cm H2O, it is more important to avoid shear forces “opening-collapsed” that are repetitively generated in the small airway during the respiratory cycle. The use of different maneuvers to open a collapsed lung and to keep the lung open was postulated years ago. 2The optimal method of alveolar recruitment, however, is a subject of controversy.
Bigatello et al. discussed two possible methods to obtain alveolar recruitment. The first was the “open-lung approach” of Dr. Amato. 3We believe that this strategy does not obtain alveolar-opening pressures. A peak airway pressure of 40 cm H2O is enough to recruit completely a healthy lung; but higher pressures are necessary to open the lung of a patient with adult respiratory distress syndrome. In addition, to avoid lung collapse, the open-lung approach applies a positive end-expiratory pressure (PEEP) of 2 cm H2O above the lower inflexion point, as found on the inspiration curve of the volume–pressure loop. Rimensberg et al. , 4while studying the expiration part of the volume–pressure curve, clearly demonstrate that the level of PEEP needed to avoid collapse was lower than the lower inflexion point. The open lung approach, therefore, overestimates the level of PEEP necessary to avoid collapse, which increases the peak airway pressure and limits the appropriate carbon dioxide clearance.
The second method, described by Pelosi et al. , 5is the sigh. The sigh is a method to increase the functional residual capacity during general anesthesia. Later, by extension, it was applied to a critically ill patient undergoing mechanical ventilation. Even when this strategy did not prove to be beneficial for patients, 6it was included as a ventilatory mode in most of the ventilators at the time. To apply this strategy, volume-control ventilation was used, which doubled (and sometimes more than doubled) the tidal volume.
We believe this is an erroneous strategy. We believe the concept of recruiting the collapsed lung by increasing inspiratory pressures, but we disagree about the use of a volume-controlled method without limitation of the maximal level of pressure, which can increase ephitelial–alveolar damage. 7
We believe that an alveolar recruiting maneuver should be performed in consideration of the following key points:
Use a pressure-controlled method or a volume-controlled mode with limited pressure. This will allow the maximal peak inspiratory pressure to be set to avoid iatrogenic lung damage.
Reach the critical alveolar pressure by setting the peak inspiratory pressure. The critical alveolar-opening pressure is approximately 40 cm H2O in a healthy lung 8and approximately 55 cm H2O in a diseased lung. 2
Avoid shear-force lesions by limiting pressure and volume differences in the airway during the respiratory cycle. To do this, PEEP increments should be parallel to peak inspiratory pressure increments, and the respiratory rate should be adjusted to limit the tidal volume to 10 ml/kg.
Maintain ventilation with these parameters during an adequate period of time: 10–20 respiratory cycles are sufficient.
Decrease the peak airway pressure and return the ventilator setting to that used before maneuver; keep PEEP above the collapse pressure.
This alveolar recruitment strategy has proven to be useful in patients with healthy lungs undergoing general anesthesia, 8and it is used in many critical care units.