Medicina

Practice Strategies

Mechanical Ventilation: A Review and Update for Clinicians
Sonia F. Howman, MD

T

he ancient Greek physician and philosopher Claudius Galen (129 – 210) was the first to describe the artificial ventilation of an animal. 1 More than 1000 years later, in the 16th century, this technique was applied to human resuscitation.2 Excluding these historical anecdotes, mechanicalventilation did not become a major therapeutic intervention until the poliomyelitis epidemic swept through Europe and the United States in the 1940s and 1950s. Since the middle of the 20th century, a wide variety of ventilatory techniques have been developed for the treatment of patients with respiratory failure. This article reviews the available modalities of mechanical ventilation in terms ofclinical indications and practical applications. Ventilator settings, alternative modes of ventilation, ventilation complications, and weaning from mechanical ventilation are also discussed. CLINICAL GOALS The basic purpose of mechanical ventilation is to support patients whose respiratory systems have failed until adequate function returns (Table 1). Reversal of acute, severe hypoxemia orrespiratory acidosis with mechanical ventilation can be life-saving.3 Mechanical ventilation can also relieve respiratory distress in patients for whom the work of breathing has become intolerable. In addition, prevention or reversal of atelectasis and reversal of respiratory muscle fatigue can be accomplished by mechanical ventilation. Even in patients with healthy lungs, mechanical ventilation is oftenemployed when sedation or neuromuscular blockade is necessary (eg, operative anesthesia). By decreasing systemic or myocardial oxygen consumption, mechanical ventilation may also assist patients who experience compromised myocardial function when the work of breathing becomes excessive. Other objectives of

mechanical ventilation include reduction in intracranial pressure by controlledhyperventilation for patients with closed head injury, and stabilization of the chest wall, as in cases of massive flail chest.4 Generally, mechanical ventilation supplies only symptomatic relief, not actual therapy for respiratory failure or acute lung injury. A notable exception is the state of congestive heart failure, during which mechanical ventilation may augment cardiac output and therefore betherapeutic. Iatrogenic lung injury may be inflicted during mechanical ventilation, and preventive measures should be taken. MODES OF MECHANICAL VENTILATION Studies have demonstrated that suppression of spontaneous breathing and complete dependence on controlled mechanical ventilation lead to rapid respiratory muscle atrophy.5 Therefore, modes of mechanical ventilation that allow spontaneousbreathing, or patient-triggered modes, are favored when feasible. Assist/Control Mode Ventilation Assist/control ventilation (ACV) is a combined mode of ventilation. The mechanical ventilator delivers a positive pressure breath of a predetermined tidal volume in response to each of the patient’s inspiratory efforts (termed assisted ventilation). However, should a patient fail to initiate a breath within aspecific time period, the ventilator automatically delivers a mechanical breath to maintain a minimum or “backup” respiratory rate (termed controlled ventilation).6 To trigger an

Dr. Howman is Clinical Assistant Professor of Medicine, University of Hawaii, Honolulu, HI, and Director, Intensive Care Unit, St. Francis Hospital, Honolulu.

26 Hospital Physician December 1999

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assisted breath, the patient must lower the airway pressure by a preset amount, called the trigger sensitivity. Figure 1 demonstrates examples of pressure and volume waveforms during volume-cycled ACV. Intermittent Mandatory Ventilation Intermittent mandatory ventilation (IMV) is a type of ventilatory support in which mandatory positive...