Lung Recruitment for Ventilation: Does It Work, and is It Safe?
Article Outline
Ventilation strategies for infants, children, and adults with lung injury is an active area of translational and clinical research. Development continues on the concept of the “open lung.” A large body of experimental literature clearly indicates that severe lung injury results from ventilating a lung from collapse with each ventilator cycle. The lung will not collapse unless surfactant function is deficient and/or positive end-expiratory pressure (PEEP) is not used. The distribution of injury/pneumonia (or another primary abnormality) is seldom uniform across the lung, however. An open lung is a lung that is inflated as uniformly as possible, with reversal of any atelectasis and with aeration of fluid-filled alveoli. A consolidated lung volume will not open despite the application of pressures that would severely overdistend a more-normal lung.1 Thus, opening an injured lung is a relative goal—to open the regions that can be opened. Another avenue to lung injury is overdistention. As reported in the experimental literature, overstretching a lung will cause stress–strain injuries to the airspace epithelium and vascular endothelium.
See related article, p 651
In adults and children with acute respiratory distress syndrome (RDS), the lung can be opened by evaluating volume recruitment above the inflection point of a quasi-static pressure volume curve—a difficult maneuver to perform. Another approach to opening the lung is to empirically measure the oxygenation responses to progressive increases in mean airway pressure. There is no absolute volume that defines a lung as open, because much of the lung may not be recruitable.1 In adult medicine, the approach of attempting to open the lung and then ventilate using relatively low tidal volumes (about 6 mL/kg) is becoming established. With newer approaches to ventilating the injured lung with lower tidal volumes and higher mean airway pressures, the lung may be ventilated at a volume above the normal functional residual capacity (FRC) for the normal lung.
The practical difficulties encountered with this approach include defining when the lung is open in an individual patient based on measurement of pressure-volume curves, imaging, or oxygenation response, and then determining how best to keep it open. One approach involves using recruitment maneuvers to increase mean airway pressure for variable periods, a variant of sigh cycles with ventilators. Despite the theoretical attractiveness of such maneuvers and their ability to recruit lung volume and increase oxygenation in some circumstances, 2 large randomized controlled trials of patients with ARDS and a meta-analysis of recruitment maneuvers in adults with lung injury demonstrated no long-term benefits.2, 3, 4
Respiratory treatment strategies that do not work consistently in adults with lung injury may work in infants, however. The prime example of this is the use of surfactant to treat RDS in preterm infants and pneumonia or meconium aspiration in term infants.5 Surfactant treatment has not been effective for RDS in adults. The open lung concept is certainly relevant for newborns. Gas exchange is inadequate in an atelectatic or fluid-filled lung. The major effect of surfactants is to open the lung by increasing FRC and total lung capacity.5 Similarly, inhaled nitric oxide is ineffective unless the lung is open to allow the gas to reach the distal airspaces. Clinicians have attempted to gauge the status of lung inflation based on chest films, especially in infants on high-frequency oscillation. Empirically, neonatologists have increased PEEP or continuous positive airway pressure delivered noninvasively in attempts to improve oxygenation in infants with RDS. The use of lung recruitment procedures has seldom been reported in infants and children, however.
Several years ago, de Jaegere et al6 demonstrated that the lungs of ventilated preterm infants with RDS could be recruited by using the improved oxygenation to define when the lung was open. These investigators used a high-frequency oscillator and a protocol to increase mean airway pressure from a mean value of about 7 cm H2O in increments of 2 cm H2O every 2 to 3 minutes until the fraction of inspired oxygen (FiO2) reached ≤ 0.25 or no further improvement in oxygenation was seen. A poor oxygenation response was presumably associated with a right-to-left shunt or a nonrecruitable lung. The surprising finding was that the mean airway pressure of 20.5 cm H2O was required to decrease the FiO2 from 0.7 to 0.24. The maneuver was performed at a median age of 3 hours in newborns with RDS with a birth weight of 1.3 ± 0.5 kg. The average time needed to perform this maneuver was about 20 minutes. Subsequently, the mean airway pressure was reduced until decreased oxygenation was noted, which defined the closing pressure of the lung (about 12 cm H2O). The lung was then “reopened” by increasing the mean airway pressure to its opening pressure, after which the pressure was reduced to 2 cm above the closing pressure for subsequent ventilation.
After surfactant treatment, the recruitment maneuver was then repeated, with a mean airway pressure of about 15 cm H2O to open the lung.6 The closing pressure was about 7 cm H2O after surfactant treatment. The total period required to assess these pressure and oxygenation responses must have been about an hour of continuous intensive adjustments of ventilation. In the original report, the newborns' blood pressure and heart rate did not change.6 However, the concept that blood pressure and heart rate alone do not define cardiovascular performance, particularly in preterm infants, has been well developed in The Journal in recent years.7 In this issue of The Journal, de Waal et al8 report a study that used these same lung recruitment maneuvers in preterm infants with RDS on high- frequency oscillation. Their evaluation of these infants' cardiovascular status revealed a small 17% decrease in right ventricular output but no consistent changes in superior vena cava flow or ductal shunting when pressures were increased to the opening pressure. The effects of overdistention of the adult lung on cardiac output have been well documented in animal models and patients. The gas volume of the fully recruited preterm lung is small (30 to 50 mL/kg) relative to the adult lung, and the chest wall is very compliant. The increased mean airway pressures did not influence cardiovascular function in these preterm infants with RDS.
This recruitment maneuver works in the sense that lung oxygenation is improved and an “optimal” pressure for oscillation can be determined. There were no acute safety concerns; cardiovascular status was not compromised, and no air leaks occurred. The information provided by these physiological measurements in infants is valuable to the field; however, I believe that such a recruitment maneuver is not something the average neonatologist should take home and try, for several reasons. First, these are demanding and time-consuming assessments. The mean airway pressures used to define the open lung are high, and in my experience with ventilation of preterm animals, they can cause lung blebs and pneumothorax. Although not evaluated in this clinical experience, lung stretch may transduce inflammatory signals.9 Such recruitment maneuvers should not be necessary when the infant is treated with surfactant once RDS requiring a high FiO2 is recognized. Surfactant treatment will mitigate the need to open the surfactant-deficient lung using high pressures. However, the other extreme of clinical practice, using a PEEP of 5 for all infants with RDS, also is not responsive to optimization of oxygenation and lung recruitment.
References
- Lung recruitment in patients with the acute respiratory distress syndrome. N Engl J Med. 2006;354:1775–1786
- Ventilation strategy using low tidal volumes, recruitment maneuvers, and high positive end-expiratory pressure for acute lung injury and acute respiratory distress syndrome: a randomized controlled trial. JAMA. 2008;299:637–645
- Positive end-expiratory pressure setting in adults with acute lung injury and acute respiratory distress syndrome: a randomized controlled trial. JAMA. 2008;299:646–655
- Recruitment maneuvers for acute lung injury: a systematic review. Am J Respir Crit Care Med. 2008;178:1156–1163
- . Why surfactant works for respiratory distress syndrome. NeoReviews 7. 2006;e95–e105
- . Lung recruitment using oxygenation during open-lung high-frequency ventilation in preterm infants. Am J Respir Crit Care Med. 2006;174:639–645
- . Cardiovascular support in the preterm: treatments in search of indications. J Pediatr. 2006;148:289–291
- . Effect of lung recruitment on pulmonary, systemic and ductal blood flow in preterm infants. J Pediatr. 2009;154:651–655
- Brief, large tidal volume ventilation initiates lung injury and a systemic response in fetal sheep. Am J Respir Crit Care Med. 2007;176:575–581
The concepts described herein were developed in part based on research supported by National Institutes of Health Grant HD-12714.
PII: S0022-3476(09)00055-9
doi:10.1016/j.jpeds.2009.01.059
© 2009 Mosby, Inc. All rights reserved.
Refers to article:
- Effect of Lung Recruitment on Pulmonary, Systemic, and Ductal Blood Flow in Preterm Infants
