The Effects of Flow Waveform on Oxygenation in Pediatric Patients.

I appreciate the comments by Drs. Ferrando and Javier Belda1 on the use of pressure versus volume-controlled ventilation to support lung recruitment. Their comments help to distinguish the information we know well about lung-protective ventilation from the information that is less well documented. It is also an opportunity to clarify any misconceptions from the original article because the goal was to provide the pediatric anesthesiologist with a rationale for optimal ventilation of the anesthetized pediatric patient. There is much more literature in patients with acute lung injury regarding ventilator-induced lung injury to guide the ventilation strategy than there is literature in surgical patients, especially pediatric surgical patients. Nevertheless, there is general agreement that limiting tidal volume and using positive end-expiratory pressure are essential components of a lung-protective ventilation strategy. Less well documented is the best mode of ventilation for any given patient. As I stated in the original article, “All of these (ventilation) modes are potentially useful for the pediatric surgical patient, but one must understand the benefits and limitations of each when selecting a ventilation mode and adjusting the ventilator settings.”2 The traditional pressure control ventilation (PCV) delivers a constant pressure for the duration of the set inspiratory time. As a result, the inspiratory pressure inflating the lung is available for the entire inspiratory time. One of the important limitations of PCV is that tidal volume To the Editor: We agree with 2 points made by Feldman in his excellent review of optimal ventilation in anesthetized pediatric patients. First, we agree with his recommendations for limiting tidal volume and using positive end-expiratory pressure (PEEP) to prevent alveolar collapse and recruitment maneuvers to reverse atelectasis. Second, we agree that these recommendations are not well supported by studies in pediatric surgical patients.1 However, we have some doubts regarding the relative advantages of pressure-controlled ventilation compared with volume-controlled ventilation. Feldman comments that the squared wave produced by pressure-controlled ventilation favors lung recruitment. Lung recruitment is helpful when atelectasis is present, because alveolar lung recruitment decreases shunt and improves oxygenation.2 Pediatric patients are prone to alveolar collapse,3 so the resulting square wave pressure of pressure-controlled ventilation should improve oxygenation compared with volume-controlled ventilation. However, the superiority of pressure-controlled ventilation has not been demonstrated in small infants. Even in adults, the relative benefits of pressure-controlled ventilation have been questioned in clinical studies.4,5 Moreover, as our group recently showed in recruited lungs with PEEP,6 or even only with PEEP without a recruitment,7 flow or pressure waveform did not affect oxygenation or ventilation in infants. These results suggest that pressure-controlled ventilation is not necessarily advantageous compared with volume-controlled ventilation. The expected beneficial effect of pressure-controlled ventilation is tidal recruitment, which may be useful if atelectasis is present. However, tidal recruitment increases mechanical stress on the lungs, and this may promote ventilator-induced lung injury. This may be particularly important in children whose lungs are especially vulnerable to injury.8,9 Therefore, when lungs are treated to prevent the ventilator-induced lung injury by reversing atelectasis with recruitment maneuvers and PEEP, pressure-controlled ventilation is not necessarily better than volume-controlled ventilation in maintaining oxygenation during pediatric ventilation.6,7