# 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?

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> subject: Nursing Practice V — Care of Clients with Maladaptive Patterns of Behavior; Care of Clients with Life-Threatening Conditions, Acute Multi-Organ Problems, High Acuity and Emergency Situations

## 문제

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?

## 보기

1. Pressure support ventilation (PSV)
2. Continuous positive airway pressure (CPAP)
3. Synchronized intermittent mandatory ventilation (SIMV)
4. Assist-control (AC) ventilation **✔ 정답**

**정답: 4**

## 해설

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.

## 심화 해설

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].References (research sources)

- [1]Assist-Control VentilationResearch articleMora Carpio AL, Mora JI. (2023)

## 임상 시나리오

Assist-Control Ventilation: Key Clinical PointsRecognizing full-support modes at the bedside
In assist-control (AC) ventilation, every breath—whether machine-initiated or patient-triggered—delivers the full preset tidal volume. The ventilator guarantees a minimum set rate, but any additional patient effort also receives the same full volume.

Because the delivered tidal volume remains constant, airway pressure can rise if lung compliance worsens or airway resistance increases. Monitor peak inspiratory pressure and plateau pressure closely.

CautionIn SIMV, only the set mandatory breaths receive the full preset volume; patient-triggered breaths above that rate are spontaneous and not fully supported. Do not confuse this with AC mode.

## 핵심 개념

- **Assist-control (AC) ventilation** — A ventilator mode in which every breath, whether machine-initiated or patient-triggered, delivers the full preset tidal volume or pressure.
- **Tidal volume (VT)** — The volume of air delivered to the lungs during each ventilator breath.
- **Synchronized intermittent mandatory ventilation (SIMV)** — A mode that delivers a set number of full mandatory breaths while allowing spontaneous breaths above that rate to be patient-driven.
- **Pressure support ventilation (PSV)** — A spontaneous breathing mode in which the ventilator provides a preset pressure boost to augment each patient-triggered breath.
- **Continuous positive airway pressure (CPAP)** — A mode that applies constant positive airway pressure throughout the respiratory cycle during spontaneous breathing.

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