Situation: A 52-year-old man is in the intensive care unit (ICU) with septic shock and acute kidney injury. He is intubated and receives mechanical ventilation. Because his platelet count is low, he is on continuous kidney replacement therapy (CRRT) with regional citrate anticoagulation. By day 6 his condition improves. The team switches him to a ventilator mode in which he triggers every breath himself, there is no mandatory rate, and the ventilator boosts each breath to a preset inspiratory level; the mode is commonly used for weaning. Which mode is this?

In pressure support ventilation every breath is triggered by the client, and the ventilator augments each one with a preset inspiratory pressure; there is no mandatory rate. It is commonly used during weaning as the client takes over the work of breathing.

Mode identification

The clinical cue is that the patient triggers every breath himself, there is no mandatory rate, and the ventilator boosts each breath to a preset inspiratory level. This combination is the defining signature of pressure support ventilation (PSV).

PSV is a patient-triggered, pressure-limited, and normally flow-cycled mode ^[2]. The patient must initiate each breath; when that effort is detected, the ventilator delivers a clinician-set inspiratory pressure to augment the tidal volume and reduce the patient’s work of breathing. Because there is no backup mandatory rate, apnea is not supported by the mode itself, which is why PSV is reserved for patients with a reliable respiratory drive and is most often used during weaning ^[1][2].

The other options differ in one critical way each. Assist-control (AC) ventilation is also patient-triggered, but it delivers a preset tidal volume or pressure on every triggered breath and includes a backup mandatory rate, so it is not a weaning mode with no mandatory rate. Synchronized intermittent mandatory ventilation (SIMV) combines mandatory breaths at a set rate with spontaneous breaths between them; the mandatory component distinguishes it from PSV. Continuous positive airway pressure (CPAP) provides a constant positive airway pressure throughout the respiratory cycle but does not augment each breath with an additional inspiratory pressure above that baseline, so it does not “boost” the breath in the way described.

Mode	Trigger	Mandatory rate	Inspiratory augmentation	Typical use
PSV	Patient only	None	Preset pressure support on every breath	Weaning
AC	Patient or time	Yes (backup)	Preset volume or pressure	Full support
SIMV	Patient or time	Yes (set rate)	Mandatory breaths augmented; spontaneous breaths not	Partial support / weaning
CPAP	Patient only	None	No inspiratory boost above baseline	Oxygenation / spontaneous breathing trial

Key point! The phrase “no mandatory rate” is the discriminator that removes AC and SIMV from consideration. The phrase “boosts each breath to a preset inspiratory level” removes CPAP. Only PSV satisfies both conditions.

During PSV, the delivered flow and tidal volume are not fixed; they depend on the pressure support setting, the patient’s inspiratory effort, and respiratory system mechanics ^[2]. The breath ends when inspiratory flow decays to a preset percentage of peak flow, which is the flow-cycling criterion ^[2]. This makes PSV more comfortable for an awake patient because the patient retains control over inspiratory time and flow pattern.

Watch out! In PSV, tidal volume is variable. If the patient’s compliance worsens or effort weakens, tidal volume can fall despite the same pressure support level. Monitoring exhaled tidal volume and respiratory rate is therefore essential during weaning trials. Flow asynchrony can also occur when support is insufficient relative to the patient’s inspiratory effort, and visual waveform analysis helps identify this mismatch .

For the patient in this scenario, the transition to PSV on day 6 signals that the team is shifting from full ventilatory support toward spontaneous breathing. The absence of a mandatory rate means the patient’s own respiratory center is now driving ventilation, while the preset pressure support offsets the resistance of the endotracheal tube and the imposed work of the ventilator circuit ^[1][2].