Part 1: The Pathophysiology of Acute and Critical Illness

Acute Respiratory Dysfunction

INTRODUCTION

Acute respiratory dysfunction in children warrants prompt diagnosis and management, particularly in neonates and infants, as decompensation can be fast and respiratory arrest is the most common cause of cardiac arrest in children. Their smaller airways, increased metabolic demands (relative to body mass), and decreased respiratory reserve may put children at a disadvantage compared to adults.

Severe respiratory dysfunction necessitates admission to the intensive care unit (ICU). Since the polio epidemic of the 1950s, ICUs have developed in parallel with our increasing understanding of respiration in health and disease. Respiratory support is now extensively used and has become progressively more sophisticated.

In this chapter, we will explore the approach to a child with acute respiratory dysfunction, the initial considerations about the differential diagnosis of those conditions, and the broad principles of management strategies. In order to provide perspective, we first review some essentials of normal respiratory function.

NORMAL RESPIRATION

The principal purposes of respiration are to provide oxygen and remove carbon dioxide, and this normal function involves the following elements.

Control of Respiration

Respiratory centers in the medulla receive stimulatory input from central respiratory pacer cells, central and peripheral chemoreceptors, upper airway receptors, and volitional pathways. These signals are integrated into a combined output to respiratory muscles in order to regulate breathing frequency, as well as the durations of inspiration and expiration.

Neuromuscular Function

Central signals are carried by the cervical and thoracic nerves to the neck, thoracic muscles, and diaphragm. These signals maintain patency of the upper airway and drive the thoracic bellows to provide ventilation. Downward movement of the diaphragm and outward movement of internal intercostal muscles lead to negative intrathoracic pressure and movement of air into the lungs (inhalation). The relaxation of these muscles leads to passive recoil of the lung and moves the air out (exhalation). The more negative the intrathoracic pressure, the greater the tidal volume generated.

Resting Lung Volume

At rest, elastic recoil tends to move the thoracic cage outward and the lung inward. The balance of these opposing forces, along with a surfactant in the alveoli that reduces the surface tension (ie, lessens the force required to distend the lung). Elastic recoil determines the resting volume of the lung at the end of expiration, also known as functional residual capacity (FRC). This is the effective volume available for gas exchange.

Airflow in the Large and Small Airways

Airflow generated by mechanical movements passes through the airways. The nasal passages, nasopharynx, larynx, trachea, and main-stem bronchi constitute the large airways. The bronchial tree, from segmental to subsegmental bronchi and down to each alveolar unit, comprise the lower airways. Large airways offer less resistance and airflow is mostly laminar ...