The goal of mechanical ventilation, as for any respiratory support, is simply to achieve adequate gas exchange to maintain physiologic neutrality. Mechanical ventilation is indicated when noninvasive forms of respiratory support such as nasal continuous positive airway pressure (NCPAP) are inadequate to support the infant’s respiratory needs. This may manifest as hypoxia, hypercarbia, or apnea. Far less-common indications for mechanical ventilation include airway anomalies and central or pharmacologically induced apnea. Mechanical ventilation of the neonate can seem intimidating to even the most seasoned practitioner. This chapter provides a systematic approach to management of the ventilated neonate, addressing the basic physiology underlying neonatal pulmonary pathology.
In the neonate, the cause of respiratory distress does not change the mode of support to be employed, with the potential exception of air leak syndromes (pneumothorax, pulmonary interstitial emphysema [PIE]). The majority of neonatal patients requiring respiratory support have either an absolute or a functional deficiency of surfactant. Whether treating an infant with respiratory distress syndrome (true surfactant deficiency) or meconium aspiration or pneumonia (surfactant inactivation), we are faced with a lung with decreased compliance and prone to atelectasis.
Conventional, or tidal, ventilation is the most frequently used mode of respiratory support and is addressed in this chapter. Gas is delivered in positive pressure breaths, with exhalation occurring by the lung’s elastic recoil. Simply stated, breath goes in, breath comes out like the tide, hence the term tidal. One can think of conventional ventilation in terms of bulk flow gas exchange.
Before proceeding, it is worth simply reviewing the concept of compliance because this guides the decisions we make. The lung as a unit can be thought of as any other elastic spherical structure and will behave as shown in Figure 85-1.
Although overly simplistic and not entirely correct, the lung can be thought of as a balloon. Blowing up a balloon is something we all have experienced. There is a certain amount of work that must be done to inflate the balloon; often, the initial inflation is the hardest part of blowing it up. The first inflection point of the curve represents this, and we refer to this point of inflation as “opening pressure.” This point of the curve can also be thought of as our ideal functional residual capacity (FRC), as at this point we exert the minimal amount of pressure to maintain inflation. Maintaining this pressure will ensure that we avoid collapse and minimize work of breathing because at this point in the curve, additional volume can be achieved with little added pressure. The slope of the curve at any point is lung compliance. As you reach the point of overinflation, the balloon becomes relatively inelastic, and with further inflation, you risk rupture. This is ...