機械的人工呼吸 I: 適応と設定

Mechanical ventilation is indicated when spontaneous breathing cannot sustain life. Common indications include acute respiratory failure, surgery, inhalation injury, drug overdose, multiple trauma, coma, neuromuscular disorders, and circulatory shock. It provides oxygen delivery, removes carbon dioxide, and allows respiratory muscles to rest until the patient can breathe independently.

Ventilator settings are individualized based on the patient's condition, level of consciousness, respiratory muscle strength, chest X-ray findings, and arterial blood gases. Healthcare providers adjust respiratory rate, tidal volume, oxygen concentration, positive-end-expiratory pressure, pressure support, and inspiratory-expiratory ratio to optimize oxygenation and correct hypoxemia and hypercarbia.

Negative-pressure ventilators, or iron lungs, are rarely used today. Positive-pressure ventilators inflate the lungs by exerting positive pressure on the airway, pushing air in like a bellows mechanism and forcing alveoli to expand during inspiration. Expiration occurs passively. Positive-pressure ventilators are widely used in hospital settings and include volume-cycled, pressure-cycled, and high-frequency oscillatory support ventilators.

Invasive positive-pressure ventilators include volume-cycled, pressure-cycled, and high-frequency oscillatory support ventilators, typically requiring endotracheal intubation. Noninvasive positive-pressure ventilation, such as continuous positive airway pressure and bilevel positive airway pressure, delivers support without invasive airway placement. Each type is selected based on patient needs, health status, and associated benefits and risks.

Tidal volume is the volume of gas delivered to the patient during each ventilator breath. It is one of the critical settings adjusted based on patient assessment findings. The inspiratory flow rate and time determine the speed at which tidal volume is delivered, and peak inspiratory pressure represents the maximal pressure the ventilator generates to deliver this volume.

Positive-end-expiratory pressure applies positive pressure at the end of expiration during ventilator breaths, preventing alveolar collapse and improving oxygenation. This setting is adjusted based on patient condition and arterial blood gases. As the patient's condition improves, PEEP and other settings can be modified to allow greater patient control over breathing work.

Sensitivity determines the patient's effort required to initiate a breath from the ventilator. This setting is adjusted based on respiratory muscle strength and level of consciousness. Proper sensitivity settings allow patients to trigger breaths spontaneously, promoting active participation in breathing and reducing the exhausting effort required during mechanical ventilation support.