Chapter 30: Principles and Goals of Mechanical Ventilation

LEARNING OBJECTIVES

The Greek physician and philosopher Claudius Galen (129-199 ce) was the first to artificially reproduce the process of ventilation. He inflated the lungs of a dead animal with bellows during one of his numerous experiments, subsequently described in his iconic text, On the Parts of the Human Body. It was not until more than 1,500 years later that the paradigm of artificial respiration was used for human resuscitation. The first seemingly authentic report was by Tossach in 1744 concerning the resuscitation of a suffocated miner by mouth-to-mouth technique. Again, it was not until the dreaded poliomyelitis epidemics of the 1940s and 1950s in Europe and the United States that mechanical ventilation (MV) was developed as a major therapeutic strategy. Since then, the science of MV has rapidly expanded to encompass a variety of modalities that ensure adequate ventilation and gas exchange by a diverse array of mechanical and physiologic processes. Simultaneously, there has been increasing awareness that unregulated inflation of the lungs may lead to deleterious consequences. Those include excessive stretch and shear forces at the alveolar level that cause physiologically harmful effects on both the respiratory and cardiovascular systems (Chap. 28).

INDICATIONS FOR MECHANICAL VENTILATION

Mechanical ventilation is indicated in any clinical situation where hypoxemia or hypercarbia secondary to underlying abnormal or dysfunctional lung pathophysiology cannot be corrected by spontaneous respiration. Often MV is initiated when lung gas exchange is normal but protection of the anatomical airway by a cuffed endotracheal tube is necessary, to prevent aspiration of oropharyngeal or gastrointestinal contents. This commonly occurs in the setting of a decreased consciousness with depressed protective laryngeal reflexes and incomplete glottic inlet closure (Chap. 11). Finally, MV may be necessary when the work of breathing, while sufficient to eliminate CO₂ in cases of severe metabolic acidosis, leads to tachypnea and respiratory muscle fatigue. A prime example of such a scenario is severe sepsis, wherein endotracheal intubation and MV are initiated to avoid impending respiratory failure secondary to excessively high work of breathing. Used in this way, MV decreases both respiratory muscle fatigue and myocardial O₂ requirements and thereby promotes clinical recovery. Other indications for initiating MV are to regulate P_aco₂ in patients with increased intracranial pressure, or to provide an air splint in patients with massive chest injury and flail chest.

REVIEW OF THE MECHANICS OF ...