Ventilators that deliver IPPV are set to deliver either a certain volume or a certain pressure. This is termed "volume-control" or "pressure-control" ventilation and the different characteristics have advantages and disadvantages (Figure 4.2).
In volume-controlled ventilation, inhalation proceeds until a preset tidal volume is delivered and this is followed by passive exhalation. A feature of this mode is that gas is delivered at a constant inspiratory flow, resulting in peak pressures applied to the airways higher than that required for lung distension. Since the volume delivered is constant, airway pressures vary with changing pulmonary compliance and airway resistance. A major disadvantage is that excessive airway pressure may be generated, resulting in barotrauma.
In pressure-control ventilation a peak inspiratory pressure is applied and the pressure difference between the ventilator and lung results in inflation until that pressure is attained. Passive exhalation follows. The delivered volume is dependent on pulmonary and thoracic compliance. A major advantage of pressure control is the decelerating inspiratory flow pattern, in which inspiratory flow tapers off as the lung inflates. This usually results in a more homogeneous gas distribution throughout the lungs. A major disadvantage is that dynamic changes in pulmonary mechanics may result in varying tidal volumes.
More sophisticated ventilators have been manufactured, mainly for intensive care units, which incorporate the characteristics of both modes (for example, pressure-limited volume control). Alarms have been added for excessive leaks and abnormally high or low pressures and volumes. Some examples are illustrated below:
• Pressure support ventilation (PSV) - PSV is patient-initiated, pressure-control ventilation. The ventilator assists in response to the patient's spontaneous inspiratory effort. With each inspiratory effort the ventilator delivers a preset pressure.
• Synchronised intermittent mandatory ventilation (SIMV) -the ventilator delivers preset breaths (for example, 12 per minute) in coordination with the respiratory effort of the patient. Spontaneous breathing is allowed between breaths. Synchronisation attempts to limit barotrauma, which can occur if a preset breath is delivered to a patient who is already in maximal inhalation or is forcefully exhaling.
• Biphasic or bilevel positive airway pressure (BiPAP) - this delivers a continuous positive airway pressure (CPAP) that cycles between a higher and a lower positive pressure.
The pressure waveforms of different ventilation modes are shown in Figure 4.4.
Additional ventilator settings typically include breaths per minute, inspired oxygen concentration, the ratio of inspiration to expiration, inspiratory and expiratory pressures, pressure limits, and tidal volume measurements. In this way the dual lung functions of oxygenation and ventilation can be manipulated.
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