Ventilatory modes of weaning are based on a progressive reduction in the contribution of the ventilator and a progressive increase in the patient's contribution to ventilation. While moderate muscle activity is desirable for successful weaning, the extent of muscle exertion required and the value of muscle training remain unknown. The ventilatory modes commonly used for weaning are T-tube breathing and the two patient-assisted modes, intermittent mandatory ventilation and pressure support ventilation. Mandatory minute volume ventilation, which incorporates intermittent mandatory ventilation or pressure support ventilation, can be classified as another weaning mode (see below).
Studies to determine the optimal weaning mode have been reported (see below), but the numbers of patients studied were small. Weaning protocols, definitions of weaning success/failure, and study populations were not standardized. Despite much enthusiasm for a particular mode, there is no consistent evidence to show that any mode is superior to the others when each mode is properly used. Indeed, modes are often used in combination, and the traditional mode, T-tube weaning, is still widely used.
This weaning mode compels the patient to breathe spontaneously through a T-tube circuit after disconnection from the ventilator. Periods of unassisted breathing are gradually lengthened according to the patient's capability. The duration of each T-tube 'trial' and the integration and mode of alternating ventilator support are empirical. Assist-control ventilation is frequently used, and overventilation with respiratory alkalosis may occur. If that happens, the patient's PCO2 and serum bicarbonate will be inappropriately low when T-tube weaning resumes. The circuit itself is simple and valveless, and imposes insignificant work of breathing. Unfortunately, the change from ventilator support to unassisted spontaneous breathing is an abrupt 'sink or swim' physiological adjustment, which some patients tolerate poorly. Esteb§D eLal: (1995) recently reported that a once-daily T-piece trial of up to 2 h resulted in more rapid extubation than intermittent mandatory ventilation and pressure support ventilation.
Intermittent mandatory ventilation avoids the drastic change to full unassisted breathing by allowing spontaneous breathing between preset ventilator-delivered breaths. The ventilator rate is progressively reduced as the patient's breathing ability improves, at a pace determined clinically. Advantages claimed for intermittent mandatory ventilation include better venous return via the thoracic pump mechanism and an ability to exercise respiratory muscles. Synchronized intermittent mandatory ventilation is a modification whereby the mechanical breaths are provided by patient-triggered ventilation. Advantages over intermittent mandatory ventilation are unproven. Intermittent mandatory ventilation may induce increased work of breathing due to the circuit demand valve and imprecise breath-to-breath co-ordination between ventilator and patient.
Pressure support ventilation also allows a transition from mechanical to spontaneous ventilation. Each breath is initiated by the patient but is supported by a constant preset airway pressure. The pressure support ceases after a given fraction of inspiratory time, or when inspiratory flow falls below a predetermined fraction of the initial inspiratory flow. Expiration is passive. The pressure support is gradually decreased to a level that will compensate for circuit impedance (about 2-5 cmH 2O);
however, effort is still required to open the demand valve. Brochard.eLal (1994) found that pressure support ventilation was associated with a shorter duration of weaning and a higher success rate than T-tube breathing or intermittent mandatory ventilation.
Mandatory minute volume ventilation is a general term used to describe a variety of ventilatory support modes that interact with the patient to deliver a predetermined minimum minute volume. The basic ventilator mode is intermittent mandatory ventilation or pressure support ventilation. Spontaneous breathing is monitored by the ventilator, which then self-adjusts to deliver the preset minute volume. Hence ventilator assistance automatically decreases as the patient's spontaneous efforts improve, i.e. the patient 'self-weans'. Mandatory minute volume ventilation was originally described with intermittent mandatory ventilation as the basic mode and minute volume as the targeted variable. Some newer ventilators use respiratory rate or tidal volume as the targeted variable, which is regulated by changing pressure support ventilation or flow pattern. Forms of mandatory minute volume ventilation are varied and can be confusing; they include volume-assured pressure support, volume support, and pressure-regulated volume control. The concept is appealing, but the technology for low-resistance rapid-response mandatory minute volume ventilation is complicated. Rapid shallow breathing may not be recognized, and the ventilator algorithm may induce non-physiological breathing patterns. Clinical experience is limited.
Airway pressure release ventilation and bi-positive airway pressure
These are newer ventilatory modes that have also been used for weaning. With airway pressure release ventilation, the ventilator applies a high continuous positive airways pressure (CPAP) and intermittently 'releases' the airway pressure to a lower level using a time-controlled release valve. The higher CPAP level and lung volume are re-established when the release valve is closed. The airway pressure release ventilation pressure versus time profile is the inverse of that for intermittent mandatory ventilation. Gas exchange, and thus ventilator support, is provided by lung deflation, which is superimposed on spontaneous CPAP breathing. Peak airway pressures never exceed the higher CPAP level.
Bi-positive airway pressure is similar to airway pressure release ventilation, except that the pressure release rate depends on the patient's spontaneous breathing rate (i.e. airway pressure is released every two to six spontaneous breaths). Better co-ordination between spontaneous and pressure release breaths may result. Pressure support ventilation can be added to augment spontaneous breathing. The role of airway pressure release ventilation and bi-positive airway pressure in weaning is unclear, and clinical experience is limited. Proportional assist ventilation is a reported new mode that augments spontaneous breathing by increasing pressure delivered to the airway in proportion to inspiratory effort. Both experience and availability of proportional assist ventilation remain limited.
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