Chapter 14

### INTRODUCTION

The care of patients with life-threatening conditions requires a detailed understanding of human physiology and the pathophysiology of major illnesses, as well as an understanding of and experience with the rapidly changing technologies available in a modern intensive care unit (ICU). In addition, the science of caring for the critically ill patient has evolved rapidly in recent years as the molecular mediators of illness have become better defined and new therapies have been devised based on those advances. As a result, critical care is a multidisciplinary field and optimal outcomes for critically ill patients require a team-oriented approach, including critical care physicians and nurses, respiratory therapists, and pharmacists, as well as consulting specialists, physical, occupational and recreational therapists and social services specialists.

### RESPIRATORY CRITICAL CARE

#### ACUTE RESPIRATORY FAILURE

ESSENTIALS OF DIAGNOSIS & TYPICAL FEATURES

• Inability to deliver oxygen or remove carbon dioxide.

• Pao2 is low while Paco2 is normal in hypoxemic respiratory failure (V/Q mismatch, diffusion defects, and intrapulmonary shunt).

• Pao2 is low and Paco2 is high in hypercapnic respiratory failure (alveolar hypoventilation seen in CNS dysfunction, over-sedation, neuromuscular disorders).

• Noninvasive mechanical ventilation can be an effective treatment for hypercapnic respiratory failure and selected patients with hypoxemic failure.

• Conventional mechanical ventilation should be accomplished within a strategy of "lung-protective" ventilation.

• HFOV and ECMO are viable options for patients failing conventional mechanical ventilation.

##### Pathogenesis

Acute respiratory failure, defined as the inability of the respiratory system to adequately deliver oxygen or remove carbon dioxide, causes significant morbidity and mortality in critically ill children, accounting for approximately 50% of deaths in children younger than 1 year of age. Anatomic and developmental differences place infants at higher risk than adults for respiratory failure. An infant's thoracic cage is more compliant than that of the adult or older child, allowing a greater tendency toward alveolar collapse. The intercostal muscles are poorly developed and unable to achieve the "bucket-handle" motion characteristic of adult breathing, and the diaphragm is shorter and relatively flat with fewer type I muscle fibers, making it less effective and more easily fatigued. The infant's airways are smaller in caliber than those in older children and adults, resulting in greater resistance to inspiratory and expiratory airflow and greater susceptibility to occlusion by mucus plugging and mucosal edema. Compared with adults, the alveoli of children are also smaller and have less collateral ventilation, again resulting in a greater tendency to collapse and develop atelectasis. Finally, young infants may have an especially reactive pulmonary vascular bed, impaired immune system, or residual effects from prematurity, all of which increase the risk of respiratory failure.

Respiratory failure can be due to inadequate oxygenation (hypoxemic respiratory failure) or inadequate ventilation (hypercapnic respiratory failure) or both. Hypoxemic respiratory failure occurs in three situations: (1) V/Q mismatch, which occurs when blood flows to parts of the ...

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