Respiratory support is the mainstay in caring for children in respiratory failure (RF) and an integral part of pediatric critical care. Children, especially in the first few years of life, are more prone to RF than are adults, due to anatomic and physiologic differences and, thus, it continues to be a common indication for hospitalization in this age group.
Respiratory failure is defined as the failure to maintain adequate oxygenation (hypoxemia), ventilation (hypercarbia), or both (mixed). The etiology and pathophysiology of acute respiratory dysfunction and RF are discussed in Chapter 96. In this chapter, the fundamentals of respiratory physiology will briefly be addressed in order to understand the modes of respiratory support and their application in children with RF and will be followed by discussions of modes of respiratory support in children with RF.
BASIC RESPIRATORY PHYSIOLOGY
The respiratory system must overcome elastic forces (lung and chest wall) and resistive forces (airways) to achieve gas flow. During spontaneous breathing, diaphragmatic contraction causes a drop in pleural and intrathoracic pressure resulting in air being “pulled” into the lungs, followed by passive recoil of the lungs and chest wall during exhalation. During positive-pressure ventilation (PPV), on the other hand, air is “pushed” into the lungs during inspiration, followed by passive recoil of the lungs and chest wall during expiration. Positive-pressure ventilation is, thus, by definition, nonphysiologic.
In healthy people, the work of breathing (WOB) is minimal, and the respiratory system works efficiently to meet the metabolic demands of the body. In disease states, however, the compliance and resistance of the respiratory system changes, causing increased demands on the respiratory system with resultant increased WOB. Elevated airway resistance in conditions such as bronchiolitis and asthma causes airflow limitation during expiration and hyperinflation with resultant hypercarbia due to increased pulmonary dead space; acute respiratory distress syndrome (ARDS) is associated with poor lung compliance with a tendency of alveolar units to collapse (atelectasis), contributing to ventilation-perfusion mismatch and intrapulmonary shunt, resulting in hypoxemia.
MODES OF RESPIRATORY SUPPORT
The primary aim of respiratory support is to improve gas exchange while minimizing the patient’s work of breathing. The full spectrum of commonly used respiratory support includes simple supplemental oxygen therapy; mechanical ventilation, including noninvasive positive pressure ventilation; invasive mechanical ventilation; and advanced and adjunct ventilator therapies. Extracorporeal membrane oxygenation (ECMO) is the therapy of choice when conventional respiratory support has failed (see Chapter 108) While mechanical ventilation can be achieved using negative or positive pressure, negative pressure ventilation (NPV), although appealing in concept and having inherent potential hemodynamic advantages, has failed to gain popularity as a practical mode of support in the intensive care unit (ICU), largely due to the disadvantages of limited physical access to the patient, lack of a protected airway, and challenges with secretion clearance. Thus, the discussion of MV in this chapter will ...