The first mention of artificial respiration was made in 1555 by Andreas Vesalius, who described inserting a reed through an opening made in the trachea and blowing into it, causing the lung to rise again. Not until 1928 did Vesalius' vision become a reality, when the first iron lung, developed by Philip Drinker, was used to assist patients who were paralyzed as a result of poliomyelitis. Advances in science and technology since the days of the iron lung have led to the modern era of mechanical ventilation and, with it, the era of ICUs and critical care medicine.
More than 5 million people are admitted annually to the nearly 6,000 ICUs in the United States, and admissions are expected to rise as the population ages.1 This large and growing number of patients, coupled with the increasing use of PAs to take care of them, means that PAs need to understand the objectives of mechanical ventilation, the indications for initiating it, and the protocols for weaning patients from it. Clinicians should also be familiar with the most common ventilation modes available.
PHYSIOLOGY
Mechanical ventilation is designed to assist patients with the most fundamental function of the lungs: exchanging oxygen (O2) and carbon dioxide (CO2) with the external environment. Several factors influence the ability of the lungs to perform this function, including airway resistance, mechanical resistance of the chest wall and abdomen, and pulmonary compliance.2 All of these can be greatly affected in the postoperative, acutely ill, or traumatically injured patient, resulting in impaired gas exchange and acute respiratory failure. In turn, acute respiratory failure can quickly lead to hypoxemia and/or acute respiratory acidosis, both of which are life-threatening conditions that demand immediate intervention.
Clinically, hypoxemia is defined as Pao2 less than 60 mm Hg and acute respiratory acidosis as arterial blood pH less than 7.25 in a patient breathing room air. Acute respiratory acidosis typically occurs as the Paco2 rises higher than 50 mm Hg without a compensatory rise in arterial bicarbonate (HCO3). Advanced age and chronic respiratory conditions may alter these parameters, but for most patients, arterial blood gas (ABG) values consistent with the aforementioned criteria are indications for respiratory assistance, most often requiring endotracheal intubation and mechanical ventilation.
Functionally, mechanical ventilation takes over the process of respiration for the patient and provides support to help correct the acute respiratory abnormalities that cause hypoxemia and hypercapnia. Correction of hypoxemia is accomplished through the delivery of air with an O2 content greater than normal and the application of positive end-expiratory pressure (PEEP). Correction of acute respiratory acidosis is achieved by controlling the patient's respiratory rate and tidal volume (VT) to achieve adequate minute ventilation (MV). MV is the volume of air expired per minute and is calculated by multiplying the VT of each breath by the number of respirations per minute. Normal MV values range from 5 to 8 L/min and vary in response to the production of CO2. In postoperative, severely ill, or injured patients, increased metabolic demand often leads to CO2 production in excess of the body's ability to regulate, resulting in hypercapnia, acute respiratory acidosis, and respiratory failure.
INITIATING MECHANICAL VENTILATION
The objective of mechanical ventilation is to reduce the patient's work of breathing and reverse life-threatening hypoxemia and/or acute respiratory acidosis. Although mechanical ventilation is the most common intervention used in the treatment of critically ill and postoperative patients with impaired gas exchange, the indications for initiation remain the subject of some disagreement. According to a large international study, the most common reasons for initiating mechanical ventilation are acute respiratory failure, coma, complications of chronic obstructive pulmonary disease, and neuromuscular disorders.3 Airway protection, airway obstruction, the use of general anesthesia, hemodynamic instability, hypoxemia, metabolic acidosis, and the need for pulmonary toilet are also cited as general indications for mechanical ventilation.4 Many of these clinical indications are determined by the discovery of abnormal ABG values.
Practical considerations also are involved in the decision to initiate mechanical ventilation. While many health care practitioners regard the initiation of mechanical ventilation as an extreme measure, 
Paul L. Marino, MD, PhD, FCCM, a leading expert in intensive care medicine, gives three principles to guide its use: (1) consideration of mechanical ventilation is indication enough for it, (2) use of intubation is not a sign of weakness, (3) initiation of mechanical ventilation does not doom a patient to prolonged mechanical ventilation and, ultimately, death.5
Once the decision has been made to initiate mechanical ventilation, the patient is preferentially intubated via the orotracheal route, and the initial mode and settings for the ventilator are determined. Several different protocols for initial settings have been established; the most commonly used settings are listed in Table 1.