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newSchoolWorks.com › VoTechWorks › Health & Personal Services › Respiratory Therapy › Mechanical Ventilation: Principles and Clinical Application › Lesson 5
HLT-RESP80-03 · Lesson 5 · 40 clock hours · demo

Ventilator Modes: Volume Control, Pressure Control, and Assist-Control

Objective. Students will demonstrate correct setup and verification of volume-control assist-control, pressure-control assist-control, and SIMV modes on a full-featured training ventilator, confirming delivered parameters match ordered settings within acceptable clinical tolerance.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Ventilator Modes: Volume Control, Pressure Control, and Assist-Control

Why Modes Matter

A mechanical ventilator can deliver a breath in several different ways depending on what you tell it to control. The mode defines two things: (1) what variable the ventilator targets with each breath—volume or pressure—and (2) how the machine and the patient share the work of triggering, cycling, and sustaining ventilation. Choosing and verifying the correct mode is one of the most safety-critical tasks in respiratory therapy. A wrong mode or a misconfigured setting can cause lung injury, patient distress, or hemodynamic instability within minutes.

Volume-Control Ventilation (VC)

In volume-control (VC) mode the ventilator delivers a preset tidal volume (Vt) with every mandatory breath. The clinician sets the Vt, respiratory rate (RR), inspiratory flow rate or inspiratory time, and PEEP. Because volume is the controlled variable, the pressure required to deliver that volume varies with the patient's lung compliance and airway resistance. If compliance drops or resistance rises, airway pressure climbs—potentially to dangerous levels.

Key parameter to monitor in VC: peak inspiratory pressure (PIP) and plateau pressure. Plateau pressure reflects alveolar pressure and should generally be kept below 30 cm H₂O to minimize ventilator-induced lung injury (VILI).

The flow-time waveform in VC is typically a square (constant) wave when constant flow is selected. The pressure-time waveform rises sharply at the start of inspiration (resistive pressure) and levels off—a distinctive shape that differs from pressure-control modes.

Pressure-Control Ventilation (PC)

In pressure-control (PC) mode the ventilator targets a preset inspiratory pressure above PEEP with every mandatory breath. The clinician sets the inspiratory pressure level, RR, inspiratory time (or I:E ratio), and PEEP. Because pressure is the controlled variable, the tidal volume delivered varies with lung compliance and resistance. If the patient's lungs stiffen, Vt will fall even though pressure stays constant.

Key parameter to monitor in PC: delivered tidal volume—measured by the expiratory flow sensor. A sudden drop in Vt warns of worsening compliance or a leak.

The flow-time waveform in PC is a decelerating ramp: flow is highest at the start of inspiration and tapers toward zero as alveolar pressure approaches the set pressure. This decelerating pattern tends to distribute gas more evenly and is often better tolerated. The pressure-time waveform shows a flat plateau at the set pressure throughout inspiration—easy to recognize on the screen.

Assist-Control (AC): Combining a Mode with a Trigger Strategy

Assist-Control is a breath sequence, not a separate breath type. In AC, every breath the patient triggers receives full ventilator support at the set Vt (VC-AC) or set pressure (PC-AC). The ventilator also delivers mandatory backup breaths at the set rate if the patient does not trigger within the time window.

AC modes unload the work of breathing almost completely. This is ideal for patients who need maximum support but can be problematic if a patient with a high spontaneous drive triggers many extra breaths (auto-PEEP, respiratory alkalosis).

Synchronized Intermittent Mandatory Ventilation (SIMV)

SIMV delivers a set number of mandatory breaths (either VC or PC) synchronized to the patient's own inspiratory effort when possible. Between mandatory breaths the patient may breathe spontaneously, often with pressure support (PS) added to assist those spontaneous breaths. SIMV is commonly used during weaning because it lets the patient gradually take over more work as mandatory rate is reduced.

Monitor spontaneous Vt carefully in SIMV: without adequate pressure support, spontaneous breaths may be small and labored, increasing the patient's work of breathing.

Setting Up and Verifying Modes: Safety Steps

1. Confirm the order before touching the ventilator—read the mode, Vt or pressure level, RR, FiO₂, PEEP, flow/I-time, and any pressure support.

2. Select the mode from the ventilator menu; confirm the screen displays the correct mode label.

3. Enter each parameter one at a time and verify the displayed value matches the ordered value.

4. Set alarms before connecting to the patient: high pressure, low pressure/apnea, low Vt, high RR at minimum.

5. Test on the test lung before patient connection: observe delivered Vt, PIP, plateau (VC), or Vt variation with compliance changes (PC).

6. Verify waveforms: confirm the shape matches the expected pattern for the selected mode.

7. Document mode and all settings per institutional policy.

Never bypass the alarm-set step. An unalarmed ventilator during a circuit disconnect is life-threatening.

Reading Waveforms During Verification

When annotating waveforms between VC-AC and PC-AC, look for:

These differences directly affect patient monitoring priorities and guide your clinical interventions.

Key terms10

Volume-Control (VC) ventilationA ventilation mode in which the ventilator delivers a preset tidal volume with each mandatory breath; the resulting airway pressure varies with patient lung mechanics.
Pressure-Control (PC) ventilationA ventilation mode in which the ventilator maintains a preset inspiratory pressure for a set inspiratory time; the resulting tidal volume varies with patient lung mechanics.
Assist-Control (AC)A breath sequence in which every breath—whether patient-triggered or machine-initiated—receives full ventilator support at the set volume or pressure target.
SIMV (Synchronized Intermittent Mandatory Ventilation)A mode that delivers a set number of mandatory breaths synchronized to patient effort; between mandatory breaths the patient breathes spontaneously, usually with pressure support.
Plateau pressureThe airway pressure measured during a brief inspiratory hold in volume-control ventilation; reflects alveolar distending pressure and is used to assess the risk of ventilator-induced lung injury.
Decelerating flow waveformThe characteristic flow pattern in pressure-control ventilation where inspiratory flow is highest at breath onset and tapers toward zero as alveolar pressure equilibrates with the set pressure.
PEEP (Positive End-Expiratory Pressure)Pressure maintained in the airway at the end of exhalation to prevent alveolar collapse and improve oxygenation; set on the ventilator regardless of mode.
Ventilator-Induced Lung Injury (VILI)Lung damage caused by excessive pressure (barotrauma), volume (volutrauma), or repetitive opening and closing of alveoli during mechanical ventilation.
TriggerThe signal—patient effort (pressure or flow drop) or a timer—that initiates a ventilator breath; sensitivity is set by the clinician.
Pressure Support (PS)An adjunct mode that augments spontaneous breaths with a set pressure boost; commonly combined with SIMV to reduce the work of spontaneous breathing.

Worked examples4

1. A physician orders VC-AC with a tidal volume of 450 mL. After setup, the ventilator's expiratory volume monitor reads 432 mL. Is this within acceptable clinical tolerance, and what is the percentage difference?
  1. Identify delivered Vt: 432 mL. Identify ordered Vt: 450 mL.
  2. Calculate the difference: 450 − 432 = 18 mL.
  3. Calculate percentage difference: (18 ÷ 450) × 100 = 4.0%.
  4. A common clinical benchmark for acceptable delivered volume accuracy on a training ventilator is within ±10% of the set value.
  5. 4.0% is less than 10%, so the delivered volume is within acceptable tolerance.
Answer: The percentage difference is 4.0%, which is within the ±10% acceptable clinical tolerance. No adjustment is required, but the finding should be documented and monitored.
2. In PC-AC mode, the set inspiratory pressure is 18 cm H₂O above PEEP of 5 cm H₂O. What is the expected peak airway pressure displayed on the ventilator during a normal mandatory breath?
  1. In pressure-control mode, the ventilator targets the set inspiratory pressure above the PEEP baseline.
  2. Peak airway pressure = Inspiratory pressure + PEEP = 18 + 5 = 23 cm H₂O.
  3. The pressure waveform should show a flat plateau at 23 cm H₂O throughout inspiration.
Answer: The expected peak (and plateau) airway pressure is 23 cm H₂O. If the displayed PIP is significantly higher or lower, recheck mode selection, circuit integrity, and test lung connections.
3. A patient on VC-AC receives a set Vt of 500 mL. During a ventilator check, an inspiratory hold shows a plateau pressure of 32 cm H₂O and a PIP of 42 cm H₂O. What is the calculated airway resistance pressure component (the difference between PIP and plateau), and what does an elevated plateau pressure indicate?
  1. Airway resistance pressure = PIP − Plateau pressure = 42 − 32 = 10 cm H₂O.
  2. This resistive component reflects the pressure needed to overcome airway resistance (e.g., narrow ETT, secretions, bronchospasm).
  3. The plateau pressure of 32 cm H₂O exceeds the generally recommended threshold of 30 cm H₂O.
  4. An elevated plateau pressure indicates excessive alveolar distending pressure, raising the risk of VILI (volutrauma/barotrauma).
  5. Clinical action: notify the provider; consider reducing Vt or adjusting other parameters per protocol.
Answer: The resistive pressure component is 10 cm H₂O. The plateau pressure of 32 cm H₂O exceeds 30 cm H₂O, indicating a lung protection concern; notify the provider promptly.
4. In SIMV mode, the mandatory rate is set at 10 breaths/min. The patient is making 4 additional spontaneous breaths per minute. What is the patient's total respiratory rate, and how many of those breaths per minute receive full mandatory ventilator support?
  1. Total RR = mandatory breaths + spontaneous breaths = 10 + 4 = 14 breaths/min.
  2. In SIMV, only the set mandatory rate receives full volume or pressure support: 10 breaths/min.
  3. The 4 spontaneous breaths are supported only by the set pressure support level (if active) or are entirely unsupported.
  4. Monitor spontaneous Vt to ensure the patient's own breaths are adequate.
Answer: The total respiratory rate is 14 breaths/min. Only 10 breaths/min receive full mandatory support; the remaining 4 spontaneous breaths rely on pressure support or the patient's own effort.

The math in this lesson1

Learners compute percent difference between ordered and delivered tidal volume against ±10%.
7.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates setting up each mode on the training ventilator connected to a test lung, explaining the graphical waveform changes displayed on the ventilator screen as each control knob or menu selection is changed. · Students rotate through three stations, each with a different training ventilator and a written order sheet; they program the ordered mode and settings, then verify delivered tidal volume and pressure readings with an instructor observing. · Class debrief using side-by-side waveform screenshots printed from each station's demonstration: students annotate differences in the pressure and flow curves between VC-AC and PC-AC and record implications for patient monitoring.
Materials: Training ventilators (minimum two models), test lungs with adjustable compliance and resistance, waveform screenshot printouts, ventilator order sheets, mode setup checklist
Check: Mode setup checklist with 12 steps per mode; student must complete all safety-critical steps and at least 10 of 12 total steps correctly for each mode to pass the demonstration.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Equipment and Ventilator Management
1. A patient is on VC-AC with a set tidal volume of 480 mL. The clinician notices the peak inspiratory pressure has increased from 28 to 38 cm H₂O over the past hour while the plateau pressure remains 24 cm H₂O. What is the most likely cause?
Equipment and Ventilator Management
2. Which waveform pattern best identifies that a ventilator is operating in pressure-control mode?
Patient Safety and Infection Control
3. A student sets up PC-AC mode but forgets to set the high-pressure alarm before connecting to the patient. Which statement best describes the patient safety risk?
Pathophysiology and Disease Management
4. In assist-control ventilation, a patient with a high respiratory drive triggers 30 breaths per minute. The backup rate is set at 12. Which complication is MOST likely to develop?
Equipment and Ventilator Management
5. When verifying VC-AC settings on a training ventilator connected to a test lung, the student observes a delivered Vt of 412 mL against an ordered Vt of 500 mL. What is the percentage error, and should the student flag this finding?
Pathophysiology and Disease Management
6. During a class debrief, a student asks why plateau pressure rather than peak inspiratory pressure is used to guide lung-protective ventilation in VC mode. The best answer is:
Patient Assessment and Monitoring
7. A patient on SIMV (VC) at a rate of 8 breaths/min with pressure support of 10 cm H₂O has an SpO₂ of 94% and appears to be working hard to breathe between mandatory breaths. The MOST appropriate initial assessment is to check:
Patient Safety and Infection Control
8. Which step on the mode setup checklist is correctly identified as 'safety-critical,' meaning failure to complete it automatically disqualifies the student's demonstration attempt?
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