A variety of methods have been employed to help wean patients from prolonged ventilatory support. Although synchronized intermittent mandatory ventilation is probably the most widely used, it has not been shown to be clearly superior to T piece or pressure support weaning. Regardless of the method you choose, begin weaning before the patient's lung function has returned to normal or baseline levels and end when the patient shows the minimum capacity necessary to sustain himself off the ventilator. The patient's response to the change in the level of ventilatory support governs the rapidity of weaning. The rapid shallow breathing index can be useful in predicting weaning outcome, as is the patient's ability to tolerate a weaning trial.
Potential indications for mechanical ventilation include hypoxemia unresponsive to oxygen administration, hypercapnia resulting in acidemia, and an unstable chest wall. For best results, carefully prepare the patient (both physically and emotionally) before instituting ventilation. Sedatives and local anesthesia can facilitate intubation; avoid paralytic agents unless you are experienced at intubation. The oral route is most commonly used. Once the patient circuit is attached to the endotracheal tube, reexamine the patient and double-check the inspiratory flow and I:E ratio; adjust the ventilator's settings as necessary. Monitor the patient frequently to ascertain the adequacy of alveolar ventilation and arterial oxygen.
Most modern ventilators have several key features in common: microprocessor control of operational and monitoring functions; electromechanical valves to control and adjust gas flow patterns; and extensive monitoring systems. In addition, these machines can provide a number of different modes of ventilation (including pressure support). Though not microprocessor-controlled, the Siemens Servo 900 series ventilators use feedback electronics to adjust inspiratory flow based on expiratory flow to meet preset volumes. In contrast, the Bennett 7200 units use microprocessor-regulated solenoid valves to deliver preset tidal volume. High-frequency ventilators deliver smaller tidal volumes at rates greater than 60 bpm.
