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INTRODUCTION

Key Clinical Questions

  • image How should I select an initial mode of mechanical ventilation? What are the distinguishing features of the various modes of mechanical ventilation?

  • image How can patient ventilator dyssynchrony be rapidly identified and managed?

  • image When and how should a patient on mechanical ventilation be assessed for liberation from mechanical ventilation?

HISTORY OF MECHANICAL VENTILATION

One of the earliest descriptions of artificial ventilation is credited to the Belgian physician Andrea Vesalius, who, in his 1653 work, De Humani Corporis Fabricia, stated that “an opening must be attempted in the trunk of the trachea, in which a tube of reed or cane should be put; you will then blow into this so that the lung may rise again and the animal take in air.” Over the ensuing decades, with the development of a bellows drive mechanism, the successful use of ventilators to treat victims of drowning and patients with neuromuscular weakness was reported.

Widespread use of mechanical ventilators, however, did not occur until the polio epidemic of the 1940s and 1950s. Thousands of patients were supported with negative pressure ventilators, which create a negative pressure outside the thorax so as to expand the lungs. With the development of the cuffed endotracheal tube (ETT), positive pressure ventilation became possible and increased in popularity.

Ventilator technology continued to improve, including a reduction in the size of the machines and the development of microchips able to respond to changes in patient characteristics. With these advancements, mechanical ventilation helped form the specialty of critical care medicine.

PHYSIOLOGIC CONSIDERATIONS

During quiet spontaneous breathing, inspiration is an active process, dependent on the contraction of the diaphragm and other respiratory muscles to expand the thoracic cage and draw air in. Exhalation is a passive process, during which relaxation of the same muscles returns the thoracic cage to its prior state to expel air out. During positive pressure ventilation, inspiration is an active process whereby the ventilator exerts a pressure at the tip of the endotracheal tube higher than the alveolar pressure such that air flows from the ventilator into the lungs and exhalation is a passive process whereby the pressure at the tip of the endotracheal tube is reduced and air passively flows back from the lungs into the ventilator.

To understand ventilatory air flow and the relation of pressure, flow, and volume, it is necessary to discuss two important physiologic variables—resistance and compliance.

Resistance

In mechanical ventilation, resistance refers to the degree to which the flow of air is obstructed.

Poiseuille’s law defines the variables which affect resistance to airflow through a hollow tube:

R = 8ηl/πr4

where R is resistance, η is the viscosity of air, l is the length of the tube, and r is the radius ...

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