The physiologic determinants of ventilator dependence

The foregoing physiologic background provides the rationale for an approach to ventilator weaning. The rate of ventilator withdrawal can be limited by hypoxemia, by lung impedance, by a high minute-ventilation requirement, by depressed ventilatory drive, by neuromuscular weakness, or by psychogenic factors. Therapeutic interventions instrumental to successful weaning can be classified in several interactive categories. When hypoxemia or excessive chest impedance limits the rate of withdrawal, improved bronchodilation, diuresis, secretion clearance, and positioning are helpful. In addition, reduction of the minute-ventilation requirement and removal of high-resistance circuit elements may be beneficial. When minute ventilation is high, control of fever, pain, agitation, and the factors that increase dead space should receive primary attention. Depressed drive can be approached by optimizing nutrition, unloading respiration, correcting alkalosis, removing sedatives, ensuring sleep, or by correcting a deficiency of thyroid hormone. In this setting, progesterone may occasionally be helpful. Neuromuscular weakness is best approached by provision of adequate rest and by attention to acidbase, electrolyte, hormonal, and nutritional status. Assuring adequate oxygen delivery and ventilatory rest are of obvious importance. Physical therapy, ambulation, alteration in ventilatory mode, and muscle training are potentially beneficial. Finally, the importance of psychic distress should not be underestimated. Anxiety should be allayed by outlining and emphasizing the weaning plan, by maintaining a positive, reassuring attitude, and by encouraging as nearly normal activity as can be tolerated. Keeping the patient fully informed and involved in the weaning attempt will help ensure a successful outcome.