Can mechanical ventilation strategies reduce chronic lung disease?
Introduction
Advances in neonatal intensive care and mechanical ventilation over the past 25 years have extended the survivability of premature infants to 24 weeks' gestation, and occasionally even earlier. Unfortunately, the developing lung is a delicate structure and is easily injured by the therapies necessary to sustain life outside the womb. Thus, an increasing number of infants are surviving with chronic lung disease (CLD). CLD, also referred to as bronchopulmonary dysplasia (BPD), was first described by Northway et al. in 1967 in 13 infants surviving hyaline membrane disease after mechanical ventilation. Their gestational ages ranged from 30 to 39 weeks and their birth weights ranged from 1474 to 3204 g.1It has been estimated that 30–40% of preterm infants requiring mechanical ventilation will develop CLD, and that there are 7500 new cases each year in the USA.2
The precise aetiology of CLD remains unknown. In 1975, Philip suggested that oxygen, positive pressure ventilation (PPV) and time were responsible for its causation,3and over subsequent decades, other factors, including inflammatory mediators, have also been implicated. The terms ‘barotrauma’ (implying injury caused by pressure), ‘volutrauma’ (implying injury caused by excessive tidal volume delivery) and, most recently, ‘atelectrauma’ (implying injury caused by alveolar collapse) have been applied to an overall concept of ‘ventilator-induced lung injury’ (VILI).4
As mechanical ventilation became more sophisticated, and as clinicians encountered choices beyond the traditional time-cycled, pressure-limited (TCPL) intermittent mandatory ventilation (IMV), which characterized the first three decades of the management of neonatal respiratory failure, the concept of lung-protective strategies began to evolve. The advent of exogenous surfactant therapy removed one of the major pathophysiologic abnormalities of respiratory distress syndrome (RDS), and focused attention on ways to avoid VILI by applying PPV more safely to the developing lung.
This review discusses ventilatory strategies aimed at protecting the premature neonatal lung and reducing the incidence of CLD. It is not meant to be exhaustive, but rather a summary of the approaches that have been used to counteract this increasing problem. There is not yet enough evidence-based data to make specific recommendations, but it is the authors' hope that thisreview will provide a stimulus for further clinical investigation.
Section snippets
Pathophysiology of respiratory distress syndrome
RDS is a disorder of the premature lung, characterized by biochemical and morphological immaturity. The lack of pulmonary surfactant leads to increased alveolar surface tension and a tendency for alveolar collapse, progressive atelectasis and decreased compliance. The pulmonary histology and cyto-architectural abnormalities include: insufficient alveolarization, decreasing the surface area available for gas exchange; an increased distance between the alveolus and its adjacent capillary,
Goals of mechanical ventilation
The use of either continuous distending pressure (CDP) or PPV is aimed at overcoming alveolar atelectasis and achieving sufficient lung expansion to facilitate adequate pulmonary gas exchange, while reducing the infant's work of breathing. This needs to be accomplished without excessive pressure, volume or flow, while maintaining a normal functional residual capacity and avoiding atelectasis. Complications of mechanical ventilation are well documented, and include injury to the structures of
Ventilatory strategies
The proliferation of technology in neonatal intensive care has provided the clinician with a wide range of choices for managing infants with respiratory failure. These range from continuous positive airway pressure (CPAP) to extracorporeal membrane oxygenation (ECMO). Even within the genre of conventional mechanical ventilation (CMV), multiple forms and modes of ventilation now exist.
Continuous positive airway pressure
CPAP, a form of CDP, was first introduced by Gregory et al. in 1971,7and is usually applied by nasal prongs. It is used in spontaneously breathing infants to maintain a degree of alveolar inflation during expiration, to prevent collapse and to decrease the work of breathing.8This technique was frequently used in the 1970s as the initial strategy in the management of RDS, but it was gradually supplanted by the use of mechanical ventilation in the presurfactant era, when it was used primarily as
High-frequency ventilation
First introduced into neonatal practice in the early 1980s, high-frequency ventilation (HFV) usesextremely small tidal volumes at rapid rates to affect gas exchange at lower alveolar pressures than CMV. There are two primary forms of HFV: high-frequency jet ventilation (HFJV) and high-frequency oscillatory ventilation (HFOV), as well as other hybrid forms.
Monitoring
Advances in biomedical engineering have also substantially changed the monitoring of babies receiving mechanical ventilation. Intermittent chest radiographs and blood gas sampling have been replaced by continuous monitoring of pulmonary mechanics and pulse oximetry or transcutaneous oxygen/carbon dioxide tensions.
Real-time pulmonary mechanics monitoring has become a valuable adjunct to CMV.36The same sensor technology utilized for PTV enables measurement of changes in airway flow or pressure,
Conclusions
Advances in surfactant therapy, mechanical ventilation and monitoring have revolutionized the management of preterm infants with RDS. With the improvement in the survival of these babies, the number of infants with CLD has increased. Various management strategies have been proposed, ranging from minimally invasive CPAP, to moderately invasive CMV and HFV to highly invasive ECMO.
The goals of mechanical ventilation are not only adequate gas exchange and minimization of the patient's work of
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2009, Paediatrics and Child HealthCitation Excerpt :Conversely, within limits, shorter expiratory times and thus faster ventilator rates improve both oxygenation and carbon dioxide elimination. There is little definitive outcome-related evidence for varying styles of IPPV ventilator settings, although current practice favours faster rates (shorter expiratory times), which may reduce the incidence of pulmonary air leak7,19 and help achieve entrainment of the infant’s spontaneous respiratory effort leading to improved ventilator synchrony (see below). It is customary to ventilate preterm infants without muscle relaxant medications (see below).
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