Applied respiratory physiology

Understanding and managing respiratory failure remains a cornerstone of critical care practice, as over half of all admissions to pediatric critical care units are related to respiratory issues. The unique aspects of a developing pulmonary system demand an in-depth knowledge of these changes and their impact on diagnostics and therapeutics. Only by understanding the normal function of the respiratory system is the critical care physician able to begin to formulate mechanisms for supporting a failing physiology. In this chapter, we describe the developmental anatomy of the lung, with emphasis on the fact that the number of alveoli continues to increase long after birth. We describe the developmental mechanics of breathing, with particular reference to elastic properties of the lung and chest wall, compliance of the lung and chest wall, airway resistance, and lung volumes. Next, we describe the physiologic effects of mechanical ventilation. Factors that affect the maintenance of oxygenation are discussed, and the alveolar gas equation is introduced. We describe the maintenance of alveolar ventilation with a discussion of the included components of tidal volume, dead space and respiratory frequency. This knowledge is applied to a simplified model of the lung allowing an examination of the mechanics of ventilation. Using the single compartment model of the lung, the derivation of the equation of motion for the respiratory system and its implications for artificial mechanical ventilation are explored. Developmental anatomy and physiology of the pulmonary circulation is reviewed including physiologic and pharmacologic factors affecting pulmonary vascular pressures, resistances and the resultant changes in blood flow. A brief discussion of ventilation and perfusion relationships including the difference between shunt and venous admixture concludes this chapter.