Situation: A 64-year-old man is in the intensive care unit (ICU) on day 2 after emergency intubation for severe community-acquired pneumonia. He is sedated with a propofol infusion and receives mechanical ventilation. His care team includes ICU nurses, a resident, an attending intensivist, respiratory therapists and the hospital dialysis nurses who treat ICU clients. The ventilator is set so that every breath, whether the machine starts it or the client triggers it, delivers the full preset tidal volume. Which mode is this?

In assist-control ventilation every breath, mandatory or client-triggered, receives the set volume (or pressure), so it gives full support. In synchronized intermittent mandatory ventilation only the set number of mandatory breaths are full breaths; breaths the client takes above that rate are the client's own.

Ventilator mode identification

The question describes a ventilator pattern in which every breath—whether initiated by the machine or triggered by the patient—delivers the full preset tidal volume. This is the defining feature of assist-control (AC) ventilation.

In AC mode, the clinician sets a fixed tidal volume (VT), a minimum respiratory rate, and other parameters such as inspiratory flow and FiO₂. The ventilator guarantees that set number of mandatory breaths per minute. If the patient initiates an additional breath above that rate, the ventilator senses the negative inspiratory effort and delivers the same full preset VT for that patient-triggered breath as well ^[1]. This means the patient receives full ventilatory support on every breath, regardless of whether it is machine-initiated or patient-triggered.

Key point! In AC ventilation, the delivered tidal volume remains constant from breath to breath, even if lung compliance or airway resistance changes. The ventilator will still attempt to deliver the set VT, which can result in rising airway pressures if compliance worsens ^[1].

The other options do not match the description:

Mode	What each breath delivers	Patient-triggered breaths
Assist-control (AC)	Full preset tidal volume (or pressure)	Also receive the full preset VT
Synchronized intermittent mandatory ventilation (SIMV)	Mandatory breaths deliver full preset VT	Spontaneous breaths above the set rate are the patient's own effort, not full preset VT
Pressure support ventilation (PSV)	Patient-triggered breaths augmented by a set inspiratory pressure	All breaths are spontaneous; no mandatory rate is set
Continuous positive airway pressure (CPAP)	Continuous positive pressure during spontaneous breathing	No mandatory breaths; no additional inspiratory pressure above baseline

The distinction between AC and SIMV is especially important for nursing licensure examinations. In SIMV, the ventilator delivers a set number of mandatory breaths at the full preset VT. However, if the patient breathes above that set rate, those extra breaths are spontaneous and depend on the patient's own respiratory effort—they do not receive the full preset VT unless pressure support is added . In contrast, AC mode delivers the full preset VT on every single breath, whether mandatory or patient-triggered.

Watch out! A common source of confusion is the term "assist-control." The "assist" component refers to patient-triggered breaths that are fully supported, while the "control" component refers to machine-initiated breaths at the set rate. Both components deliver the same full VT ^[1].

From a clinical monitoring perspective, a patient on AC ventilation who is breathing above the set rate is receiving the full preset VT with each breath. This can lead to respiratory alkalosis if the patient is tachypneic, because minute ventilation rises with each additional fully supported breath. In SIMV, by contrast, tachypnea above the set rate produces smaller spontaneous breaths, so minute ventilation does not increase as dramatically . This difference has implications for arterial blood gas interpretation and sedation management in the ICU.

The scenario also mentions sedation with propofol. In AC mode, because every breath is fully supported, patients who are sedated or have reduced respiratory drive will still receive adequate ventilation as long as the mandatory rate is set appropriately. This is one reason AC is commonly selected for patients requiring deep sedation or neuromuscular blockade ^[1].