Chapter 20: Conventional Ventilation

SCOPE

DISEASE/CONDITION(S)

Mechanical ventilatory support for neonates in respiratory failure.

GUIDELINE OBJECTIVE(S)

The objectives of this chapter are to review the concepts of mechanical ventilatory support in the neonate according to the various indications, describe the most common modalities of mechanical ventilation, and discuss their advantages and limitations.

BRIEF BACKGROUND

Mechanical ventilation remains the main supportive therapy for neonates in severe respiratory failure. This is more common in premature infants because of the structural and functional immaturity of their respiratory system.

Although mechanical ventilation is necessary to maintain adequate gas exchange, serious complications associated with this therapy make the decision to initiate mechanical ventilation very important and should be done considering all available alternatives. The severity of respiratory failure is usually assessed by the clinical presentation of the infant plus evaluation of respiratory function and arterial blood gases.

Mechanical ventilation is indicated based on the severity of respiratory failure and the underlying lung disease. In the extreme premature infant, mechanical ventilation is often initiated shortly after birth due to poor respiratory effort leading to hypoxemia and bradycardia. In the preterm infant, mechanical ventilation is frequently started because of recurrent episodes of apnea and hypoxemia. Alveolar hypoventilation with hypercapnia and respiratory acidosis is another common indication for mechanical ventilation to support the infant’s failing respiratory pump. In preterm infants with respiratory distress syndrome (RDS), mechanical ventilation contributes to the recruitment of distal air spaces with improvement in ventilation perfusion matching and gas exchange.

In full-term infants, the indication for mechanical ventilation is usually more conservative since these infants are better able to cope with the increased work of breathing. In term infants with lung hypoplasia, mechanical ventilation is usually initiated early after birth, whereas in infants with congenital pneumonia or meconium aspiration a more conservative approach can be adopted. These decisions are driven by the degree of hypoxia and hypercapnia.

Mechanical ventilation consists of the alternation of two levels of positive pressure at the infant’s airway. The positive end-expiratory pressure (PEEP) provides a continuous distending pressure that maintains lung volume. The pressure is increased intermittently to a set peak inspiratory pressure (PIP) during a predetermined inspiratory time (Ti). This produces a gradient with respect to the alveolar pressure that generates gas flow into the lung. The flow and the tidal volume (V_T) during each cycle for a given pressure gradient is determined by the compliance of the respiratory system, the resistance of the airways, and the infant’s inspiratory effort when the positive pressure is provided in synchrony with the effort.

The neonate’s respiratory system mechanical properties, that is, compliance and airway resistance, determine its time constant. The respiratory time constant, which is a measure of the time to achieve equilibrium between the applied pressure and ...