Pressure Support Ventilation (PSV) is a widely used mode of breathing assistance that supports a patient’s own effort to inhale. The primary goal of PSV is to reduce the patient’s work of breathing while allowing them to control the timing and frequency of their breaths. This mode delivers a consistent, preset level of pressure to augment the volume of air the patient draws into their lungs. PSV is instrumental in helping patients transition from full mechanical support back to breathing completely on their own.
The Mechanics of Pressure Support Ventilation
The operation of Pressure Support Ventilation is defined by three distinct phases: triggering, limitation, and cycling. Each phase relies on the patient’s interaction with the ventilator, making the mode responsive to individual needs. The breathing cycle begins with triggering, where the machine senses the patient’s inspiratory effort, typically through a slight drop in pressure or a change in flow. This signal tells the ventilator to begin assisting the breath.
Once triggered, the ventilator delivers a flow of air, increasing the pressure up to a set maximum level. This is the limitation phase, where the ventilator maintains a constant, preset positive pressure throughout the inspiration. This pressure boost, often set between 5 and 20 cm H₂O, provides support that overcomes resistance and reduces the patient’s muscle effort.
The breath is terminated by the cycling phase, known as flow-cycling. The ventilator monitors the decreasing inspiratory flow rate and ends the pressure support when this flow drops below a specific, preset percentage of the peak flow, typically 25%. This mechanism allows the patient to determine their own inspiratory time and volume. The ventilator then cycles into the passive exhalation phase, where the pressure drops to allow the patient to breathe out.
When and Why Pressure Support Ventilation is Used
PSV is often introduced once a patient has stabilized after acute respiratory failure and has regained a sufficient spontaneous breathing drive. The mode supports patients recovering from conditions like pneumonia, post-surgical complications, or exacerbations of chronic lung disease. By providing a pressure boost, PSV reduces the work of breathing and helps prevent respiratory muscle fatigue.
A primary goal of using PSV is to enhance patient comfort and improve synchronization with the machine. Unlike modes where the ventilator dictates the timing, PSV follows the patient’s own respiratory rhythm, which can minimize anxiety and the need for heavy sedation.
PSV is considered an excellent stepping stone as patients move toward independence from mechanical ventilation. It allows clinicians to provide precise, adjustable support tailored to the patient’s specific needs. This helps ensure adequate oxygenation and carbon dioxide removal as they recover.
Patient Control and Respiratory Effort
The defining characteristic of Pressure Support Ventilation is that the patient retains significant control over their breathing pattern. The patient initiates every breath, determining their own respiratory rate and the duration of each inhalation. The ventilator simply delivers a fixed, supportive pressure for the duration of that effort.
This active role is crucial for maintaining the function of respiratory muscles, such as the diaphragm. By requiring the patient to exert some effort, PSV helps prevent the muscle weakness that can occur when the ventilator does all the work. This spontaneous effort helps to recondition the muscles, which is a necessary step for successful liberation from the machine.
The volume of air inhaled is directly influenced by the patient’s effort and the level of pressure support set on the machine. A stronger spontaneous effort combined with the set pressure results in a larger breath volume. This variability contrasts with fully controlled modes of ventilation, where the machine delivers a mandatory, fixed volume or time regardless of the patient’s own breathing efforts.
Reducing Support and Weaning Success
Pressure Support Ventilation is considered a preferred mode for “weaning,” the gradual transition of a patient off full mechanical ventilation. The pressure support level can be slowly decreased as the patient’s condition improves and their respiratory muscle strength increases. Each reduction in pressure requires the patient to take on a greater proportion of the work of breathing.
This titration continues until the patient is comfortable breathing at a very low level of support, often 5 to 8 cm H₂O. This minimal setting is typically just enough to offset the resistance imposed by the artificial breathing tube and the ventilator circuit itself. Once this minimal support level is reached, the patient undergoes a Spontaneous Breathing Trial (SBT), which is the final test of readiness for extubation.
A successful SBT, usually lasting between 30 minutes and two hours, requires the patient to maintain stable vital signs and adequate gas exchange. Clinicians monitor for signs of distress and use measurements like the Rapid Shallow Breathing Index (RSBI). A value less than 105 is often predictive of a successful outcome. If the patient demonstrates they can sustain their own breathing effort, they are considered ready for the removal of the breathing tube and full independence from mechanical support.