What is Pressure Support Ventilation?

     
       

 

         
       

Pressure support is a method of assisting spontaneous breathing in a ventilated patient. It can be used as a partial or full support mode(1-3). The patient controls all parts of the breath except the pressure limit. The patient triggers the ventilator – the ventilator delivers a flow up to a preset pressure limit (for example 10cmH2O) depending on the desired minute volume, the patient continues the breath for as long as they wish, and flow cycles off when a certain percentage of peak inspiratory flow (usually 25%) has been reached. Tidal volumes may vary, just as they do in normal breathing.

The purpose of using CPAP (PEEP) is to restore functional Residual capacity to what is normal for the patient, when lung volumes are low: this reduces the workload of early inspiration. When lungs lose their compliance, higher intrapleural pressures are required to inflate the lungs to a normal tidal volume, even with CPAP. Consequently, pressure support can be added, to assist the patient up the volume pressure curve.
Figure 1 is a simplified volume pressure curve for a normal lung. The lung rests at FRC, which is about 2litres, and inspiration is relatively easy, as the lungs are compliant.

In the same patient with low lung volume and stiff non-compliant lungs (figure 2), there is a very high workload required just to bring the lung volume to the FRC level, above which the lung is reasonably compliant. This extra work, that which brings the lung to P1 on the diagram, may be enough to cause respiratory distress, muscle fatigue and failure to ventilate. There are two solutions to this problem. The first is to return the resting volume to FRC by applying a pressure at end expiration (PEEP) and keeping it there (CPAP).

If you look at figure 3, you can see that application of CPAP has returned the resting FRC to normal, but the work of breathing remains high due to the loss of lung compliance (P3 is required to achieve the target tidal volume in this patient of 500ml. The solution to this problem is to administer pressure support in inspiration, in order to reduce the workload of breathing, and achieve the targeted tidal volume, with lower intrapleural pressures (P4). The vast majority of patients in intensive care can be given ventilatory assistance in this way, and it is called “pressure support ventilation” (4).


The presence of an endotracheal tube (as a minimum) increases the resistance to inspiration, add to this a lung injury and the patient incurs a high workload to breathing. Pressure support offsets this work – it offloads the respiratory muscles in order to return the tidal volume to normal. A normal individual who is intubated and not attached to a ventilator will have a lower functional residual volume (FRC) – the lungs tend to collapse inwards – and a lower tidal volume. Positive end expiratory pressure (PEEP) re-recruits FRC and places the patient on the steep part (lower work required to inflate the lung) of the pressure volume curve. Pressure support overcomes the resistance to inspiration and reduces the workload of that part of the ventilatory cycle. The term “pressure support ventilation” describes the combination of pressure support and PEEP. Pressure support on mechanical ventilators is “above PEEP”, which is an incorrect term – it is really the pressure above “CPAP”. Thus if a patient is on PEEP 5cmH2O and pressure support of 10cmH2O what is the peak/plateau pressure? Click here for answer.


Pressure support is used to assist spontaneous breaths in SIMV ventilation. The patient can be easily weaned using this technique, as the backup rate is weaned initially, and then the pressure support.
 

Screen display of a patient on pressure support ventilation. The pressure support is set at 12cmH2O. Note the decelerating flow pattern and the termination of flow before the end of inspiration. The flat topped appearance of the pressure waveforms indicates a pressure controlled breath, and the slight variance in tidal volumes is typical of pressure support and, indeed, normal breathing

CLICK HERE TO LEARN A STRATEGY FOR USING PRESSURE SUPPORT VENTILATION

References

(1) Banner MJ, Kirby RR, MacIntyre NR. Patient and ventilator work of breathing and ventilatory muscle loads at different levels of pressure support ventilation. Chest 1991; 100(2):531-533.
(2) Brochard L, Pluskwa F, Lemaire F. Improved efficacy of spontaneous breathing with inspiratory pressure support. Am Rev Respir Dis 1987; 136(2):411-415.
(3) Brochard L. Inspiratory pressure support. Eur J Anaesthesiol 1994; 11(1):29-36.
(4) MacIntyre NR. Respiratory function during pressure support ventilation. Chest 1986; 89(5):677-683.

         
                   
       

         
     

       
       

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