Getting your patient in tune with pressure support ventilation.

When mechanical ventilation is initially instituted on a patient, dys-synchrony is not usually problematic, providing the patient is adequately sedated. These patients are on a control mode of ventilation such as volume or pressure assist-control. As a patient begins to improve and they are allowed to wake up, usually during the "weaning" phase, a support mode is sometimes applied. If the patient is uncomfortable and not in tune with the ventilator during a support mode of ventilation, more sedation is needed and the time spent on the ventilator along with the resulting complications will rise. More work is performed by the patient as his condition begins to improve and the patient is allowed more freedom to breath spontaneously. Pressure support ventilation (PSV) as a stand-alone mode (all breaths are pressure-supported) can assist with patient-ventilator synchrony, if used correctly. Pressure-supported (PS) breaths can also be used in for the supported breaths as SIMV. This article will describe proper use of stand alone PSV as it relates to optimizing patient comfort especially when weaning from ventilatory support. When describing PSV, it is best to refer to each part of the breath as it relates to the mechanical ventilation mode classification system. A pressure-supported breath is patient-triggered, pressure-limited, and ideally, flow-cycled.

All PS breaths are patient-triggered and require the patient to breathe spontaneously. Sensing the initiation of a breath is not usually an issue. However, COPD patients may have trouble triggering a breath from the ventilator due to an increased trigger threshold secondary to auto-PEEP. Early collapse of the small airways, prevalent in obstructive diseases, may be decreased by maximizing pulmonary hygiene and bronchodilation, and by decreasing the PS level. The increased triggering threshold caused by auto-PEEP can be counteracted by increasing the set PEEP by 1 cm [H.sub.2]O until the machine responds to the patient's inspiratory effort. This maneuver opens the collapsed airways so that the patient doesn't have to overcome their auto-PEEP to trigger the ventilator.

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PS breaths are pressure-limited. The pressure rises quickly to a clinician set pressure level. Choosing the appropriate level is not always easy. It is typically set to achieve a minute volume comprised of a comfortable respiratory rate and adequate tidal volume. A physician's order might read "set PS level to sustain a respiratory rate below 25 breaths per minute." There are situations when the pressure may need to be adjusted even if the desired respiratory rate has been achieved. When using pressure support the resultant tidal volume from each breath depends on several variables. They include: lung compliance and airway resistance, the set pressure level, the amount of auto-PEEP and the patient's effort. Excessive work to generate adequate tidal volume may be tiring and detrimental. If the use of sternocleiod-mastoid accessory muscles of the neck is observed, the PS level may need to be increased. Another clue that the pressure may need to be increased can be found in the pressure-time graphic. If the pressure waveform is not square, as it should be, the patient is pulling on his own lung impedance because there is not enough flow available with the set pressure to satisfy the patient's inspiratory drive. If a patient has signs of increased work of breathing but increasing the PS level makes it worse, auto-PEEP could be the cause and an attempt should be made to decrease the level.

Cycling from inspiration to expiration can cause a significant dys-synchrony issue. Matching a ventilator's inspiratory time to the patient's inspiratory time should be the goal. Ideally, a PS breath should be flow-cycled. As the patient inspires, the variable inspiratory flow required to deliver the set pressure for a particular patient decelerates throughout inspiration. The breath cycles to expiration when the inspiratory flow decreases to a certain percentage of the initial peak flow. The percentage depends on the particular algorithm on the ventilator used. Some ventilators use a low percentage such as 5%. Most use a value of 25% which is adequate for most patients. The inspiratory flow in patients who have high airway resistance or a chest tube leak will take longer than normal to decrease, resulting in a longer than desired inspiratory time. They may need a higher percentage inspiratory flow-cycle criteria. Most ventilators have time and pressure backup cycling criteria. In other words, if the inspiratory flow doesn't decrease to the internally set percentage, in the allotted time, the ventilator will cycle into expiration. Also, if the patient attempts to exhale, forcing a pressure rise, the breath will change to expiration. This can be seen on the pressure-time waveform as a pressure spike at the end of the inspiratory phase, indicating that the patient is working hard to exhale during a PS breath.

Newer ventilators have adjustable flow-cycling percentage criteria. This assists clinicians in helping the patient exhale more naturally by flow. If the patient is pressure spiking at the end of the inspiration the flow-cycle percentage should be increased so that the patient will exhale with a shortened inspiratory time. Keep increasing it until the spike is no longer there. If the patient appears to cycle to expiration too soon, lower the percentage criteria which will allow the patient to have a longer inspiratory time. Each clinician should know the cycling criteria for their ventilator and the backup cycling features because they are individual on each ventilator.

The gold standard for ventilatory support discontinuation is the spontaneous breathing trial. 75% of patients with respiratory failure who are ready to be discontinued from the ventilator will do well with this method. The remaining 25% will need true weaning. An awake patient on assist-control with only minimal sedation may have difficulty due to his variable inspiratory time conflicting with the set nonvariable inspiratory time from the ventilator. PSV is a good option to make him more comfortable while he is being weaned.

The successful use of this mode depends on the patient's condition, the ventilator and most importantly, the clinicians understanding of the ventilator being used and the mode itself. It is our job to enhance patient comfort and minimize mechanical ventilation complications by proceeding with the discontinuation process as soon as feasible. PSV is a beneficial tool we can utilize to meet this goal.

by Dana Oakes RRT-NPS & Sean Shortall RRT-NPS