Pressure support ventilation is an assisted ventilatory mode, which is patient triggered (by pressure, airflow, or both), pressure limited, and flow cycled. Thus inspiratory assistance is only provided when the patient makes a breathing effort. The machine then provides a certain amount of inspiratory flow (a decelerated and servoregulated airflow pattern) so as to maintain the preset level of airway pressure (the pressure support level) almost constant throughout the inspiration. The velocity of pressurization, which depends on the shape of the inspiratory flow waveform, can be adjusted in some ventilators. Inspiration ends when the inspiratory airflow reaches a certain threshold value, which theoretically coincides with the end of the inspiratory muscle effort. This flow value is variable and depends on the manufacturer; while some ventilators use a percentage of peak flow (i.e. 25 per cent of peak flow), others use a fixed level (i.e. 5 l/min). Some modern ventilators even allow modification of the flow threshold value to cycle from inspiration to expiration. Additionally, cycling from inspiration to expiration occurs when an increment in airway pressure is detected (i.e. there is an active expiration to stop inspiratory flow) or when a certain time has elapsed. This is important, for example, in case of leaks and non-invasive ventilation. Tracings obtained during pressure support ventilation are shown in Fig 1.
Fig. 1 From top to bottom, tracings of air flow, airway pressure (Paw), esophageal pressure (Peso), gastric pressure (Pga), and tidal volume obtained in a patient ventilated with pressure support mode. The breath-by-breath variability in breathing pattern can be clearly seen.
This ventilatory mode allows the patient to breathe at his or her own respiratory rate. It also provides considerable flexibility regarding the duration of inspiratory and expiratory time, inspiratory flow, and tidal volume. Although these features may render this ventilatory mode more comfortable and provide better synchronization between ventilator and patient compared with volume-controlled modes, these theoretical advantages have not been demonstrated.
Tidal volume depends on the preset level of pressure support, the magnitude of inspiratory effort, and the mechanical characteristics (resistance and compliance) of the respiratory system. Although patients have more flexibility with regard to their own breathing pattern than during assist-control ventilation, they certainly do not have full control of volumes and flows. This emphasizes the importance of how the pressure support is actually set by the clinician.
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