Assist/Control (A/C) Mode of Mechanical Ventilation

Assist/control ventilation, commonly abbreviated as A/C, is a mechanical ventilation mode that provides a high level of respiratory support while allowing the patient to initiate breaths.

Every breath delivered in A/C receives the programmed level of assistance, whether the breath is triggered by the patient or initiated by the ventilator according to time. The mode can be delivered using either volume control or pressure control.

Because of its ability to guarantee a minimum respiratory rate while responding to spontaneous effort, A/C is frequently used for patients requiring substantial ventilatory support.

What Is Assist/Control Ventilation?

Assist/control ventilation is a form of continuous mandatory ventilation (CMV) in which the ventilator provides mandatory breaths throughout the course of mechanical ventilation. These breaths may be initiated in one of two ways:

• By the patient through an inspiratory effort
• By the ventilator when the preset time interval is reached

A patient-triggered breath is often referred to as an assisted breath, while a ventilator-triggered breath is referred to as a control breath. The major concept is that both receive the programmed mechanical support.

In other words, the patient may determine when some breaths begin, but the ventilator still provides the selected tidal volume or inspiratory pressure once the breath is triggered. This allows A/C to combine a guaranteed minimum level of ventilation with the ability to respond to the patient’s respiratory drive.

How Assist/Control Ventilation Works

A respiratory rate is programmed into the ventilator when A/C is initiated. This rate establishes the minimum number of breaths that will be delivered each minute.

If the patient does not initiate a breath before the next scheduled breath, the ventilator provides a time-triggered mandatory breath. If the patient makes a sufficient inspiratory effort before that time is reached, the ventilator senses the effort and immediately delivers an assisted breath.

For example, consider a patient whose ventilator rate is set at 12 breaths/min. If the patient makes no spontaneous respiratory effort, the ventilator delivers 12 breaths/min.

If the patient begins triggering additional breaths and reaches a total respiratory rate of 18 breaths/min, the ventilator delivers approximately 18 fully supported breaths. Therefore, the programmed respiratory rate acts primarily as a minimum or backup rate rather than as a maximum.

Set Respiratory Rate vs. Total Respiratory Rate

The difference between the set rate and the total respiratory rate is important during A/C ventilation.

The set rate is the minimum rate programmed into the ventilator. The total rate includes both:

• Time-triggered control breaths
• Patient-triggered assisted breaths

Note: A patient may therefore breathe considerably faster than the programmed rate. This can be beneficial when the patient needs additional ventilation, but it can also result in excessive minute ventilation, shortened expiratory time, respiratory alkalosis, or air trapping.

Assisted Breaths and Control Breaths

The terms assist and control refer primarily to how the breath begins.

Assisted Breath

An assisted breath begins when the patient makes an inspiratory effort that exceeds the ventilator’s trigger threshold. The ventilator recognizes the effort and delivers the programmed mechanical breath. The patient initiates the breath, but the ventilator provides the assistance.

Control Breath

A control breath occurs when the patient does not trigger the ventilator before the time interval associated with the programmed respiratory rate expires. The ventilator then initiates the breath automatically.

In both cases, the breath receives the programmed mechanical support. This feature distinguishes A/C from modes in which patients may take unsupported or partially supported spontaneous breaths between mandatory breaths.

Triggering in Assist/Control Ventilation

A ventilator must recognize when a patient is attempting to inhale. The variable used to detect this effort is called the trigger variable. Modern ventilators commonly use either:

• Pressure triggering
• Flow triggering

Note: Trigger sensitivity should be adjusted so that the patient can initiate a breath without excessive effort.

Pressure Triggering

Pressure triggering detects a small decrease in airway pressure caused by the patient’s inspiratory effort. A typical initial pressure sensitivity may be approximately:

• −0.5 to −1.5 cm H₂O
• Approximately −1 to −2 cm H₂O in some clinical approaches

Note: If the sensitivity requires too large a negative pressure, the patient must work harder to trigger the ventilator.

Flow Triggering

Flow triggering detects changes in flow through the ventilator circuit. A typical initial flow trigger may be approximately:

• 1 to 2 L/min
• Up to approximately 1 to 3 L/min depending on the ventilator and clinical situation

Note: Flow triggering generally requires relatively little patient effort when properly adjusted.

Trigger Sensitivity Problems

A trigger that is not sensitive enough can cause ineffective triggering. The patient makes an inspiratory effort, but the ventilator does not respond. This increases the work of breathing and can lead to patient-ventilator dyssynchrony.

A trigger that is excessively sensitive may result in autotriggering, where the ventilator delivers breaths even though the patient did not make a genuine inspiratory effort.

Autotriggering may result from:

• Circuit leaks
• Water in ventilator tubing
• Cardiac oscillations
• Excessive trigger sensitivity
• Movement of the ventilator circuit

Note: The goal is to provide a trigger setting that responds easily to real inspiratory efforts without generating false breaths.

Volume Control Assist/Control Ventilation

Volume control assist/control (VC-A/C) is a form of A/C in which tidal volume is the primary controlled variable. The clinician selects a tidal volume, and the ventilator attempts to deliver that volume with each mandatory breath. Because volume is controlled, the pressure required to deliver it may change.

Tidal Volume Selection

A common initial tidal volume in adults is approximately:

6 to 8 mL/kg of predicted or ideal body weight

Lower tidal volumes are often used for lung-protective ventilation, particularly in patients with acute respiratory distress syndrome (ARDS).

Patients with ARDS may receive tidal volumes closer to:

4 to 6 mL/kg of predicted body weight

Ventilator settings should be individualized according to lung mechanics, gas exchange, airway pressures, and the clinical condition.

Advantages of Volume Assist/Control

The primary advantage of volume control is predictable tidal volume delivery. Each supported breath is intended to provide the selected volume, which helps maintain relatively predictable minute ventilation when the respiratory rate is stable.

Other advantages include:

• Direct control of tidal volume
• Ability to monitor changes in airway pressure
• Reliable delivery of the selected volume despite changes in mechanics
• Familiarity among clinicians
• Wide availability on critical care ventilators

Note: Volume control can also provide useful information about changes in respiratory system resistance and compliance.

Disadvantages of Volume Assist/Control

Because tidal volume is maintained, airway pressure may rise when lung mechanics worsen. For example, pressure may increase because of:

• Decreased lung compliance
• Increased airway resistance
• Bronchospasm
• Airway secretions
• Pulmonary edema
• Atelectasis
• Pneumothorax
• Endotracheal tube obstruction
• Excessive inspiratory flow demand

Note: Volume-controlled ventilation can also contribute to flow asynchrony if the ventilator’s selected inspiratory flow does not meet the patient’s demand.

Airway Pressure During Volume Assist/Control

Airway pressures provide useful information during volume-controlled A/C.

Two important pressures include:

• Peak inspiratory pressure (PIP)
• Plateau pressure (Pplat)

Increased Peak Pressure With Stable Plateau Pressure

If peak inspiratory pressure increases while plateau pressure remains relatively unchanged, increased airway resistance should be suspected.

Possible causes include:

• Bronchospasm
• Secretions
• Kinking of the endotracheal tube
• Mucus plugging
• Water in the ventilator circuit
• Airway narrowing

Increased Peak and Plateau Pressures

If both peak and plateau pressures increase, decreased respiratory system compliance should be considered.

Possible causes include:

• Pulmonary edema
• ARDS
• Atelectasis
• Pneumothorax
• Abdominal distention
• Reduced chest wall compliance

Note: Monitoring both pressures can help clinicians distinguish between resistance-related and compliance-related problems.

Pressure Control Assist/Control Ventilation

Pressure control assist/control (PC-A/C) is a form of A/C in which inspiratory pressure is the primary controlled variable. Instead of selecting a fixed tidal volume, the clinician selects a pressure level that the ventilator maintains during inspiration.

The resulting tidal volume depends on:

• Lung compliance
• Airway resistance
• Inspiratory time
• Patient effort
• Selected pressure

Note: Because pressure is controlled, tidal volume is variable.

Initial Pressure Selection

An initial inspiratory pressure may be adjusted to produce a tidal volume of approximately:

6 to 8 mL/kg of predicted body weight

In patients requiring lung-protective ventilation, a smaller tidal volume may be targeted.

The pressure should be adjusted according to the patient’s exhaled tidal volume, gas exchange, mechanics, and clinical response rather than relying on one universal pressure setting.

Advantages of Pressure Assist/Control

Pressure control can provide several potential benefits.

These include:

• Controlled inspiratory pressure
• Variable inspiratory flow
• Decelerating flow pattern
• Improved matching of inspiratory flow to patient demand
• Potentially improved patient-ventilator synchrony
• Lower peak inspiratory pressure in some situations
• Ability to sustain pressure throughout inspiration

Note: Because inspiratory flow changes according to patient demand, pressure-targeted assisted ventilation may provide better synchrony than fixed-flow volume ventilation.

Disadvantages of Pressure Assist/Control

The major disadvantage is that tidal volume is not guaranteed. If lung compliance worsens, the same pressure may produce a smaller tidal volume. The same problem can occur when airway resistance increases.

For example, a patient receiving PC-A/C may suddenly develop bronchospasm. Inspiratory pressure remains unchanged, but less volume reaches the lungs.

As a result, tidal volume and minute ventilation may fall. This makes low tidal-volume and low minute-ventilation alarms especially important during pressure-controlled ventilation.

Volume A/C vs. Pressure A/C

The major difference between volume and pressure A/C involves which variable is controlled.

With volume A/C:

• Tidal volume is selected
• Pressure varies
• Flow may be fixed or programmed
• Changes in mechanics are often reflected by airway pressure changes

With pressure A/C:

• Inspiratory pressure is selected
• Tidal volume varies
• Inspiratory flow is variable
• Changes in mechanics are often reflected by changes in tidal volume

Note: Neither approach is automatically appropriate for every patient. The selection depends on the patient’s respiratory condition, lung mechanics, synchrony, oxygenation, ventilation, airway pressures, and clinician goals.

Initial Settings for Assist/Control Ventilation

Initial ventilator settings must be individualized, but several general ranges may be used as starting points.

Important settings include:

• Mode
• Tidal volume or inspiratory pressure
• Respiratory rate
• FiO₂
• PEEP
• Inspiratory flow or inspiratory time
• Trigger sensitivity
• Alarm limits

Respiratory Rate

An initial adult respiratory rate may commonly fall between approximately:

12 to 14 breaths/min or greater

Some patients may require rates outside this range.

For example, a patient with severe metabolic acidosis may require a higher minute ventilation, while a patient with severe obstructive disease may require a lower rate to allow more expiratory time.

Inspiratory Flow

During volume control, a typical initial inspiratory flow may be approximately:

60 to 80 L/min

Higher flows may be necessary if the patient has a strong inspiratory demand.

Increasing inspiratory flow can also shorten inspiratory time and increase the amount of time available for expiration.

Inspiratory Time

A typical inspiratory time may be approximately:

0.6 to 1.0 second

Inspiratory time should be selected according to the patient’s breathing pattern, respiratory mechanics, oxygenation requirements, and need for adequate exhalation.

I:E Ratio

A common goal is an inspiratory-to-expiratory ratio of approximately:

1:2 or less

Patients with obstructive lung disease may require longer expiratory times, such as:

• 1:3
• 1:4
• 1:5

Note: The objective is to prevent the next breath from beginning before expiration is complete.

FiO₂

When oxygenation status is initially unknown, a relatively high FiO₂ may be used temporarily. The oxygen concentration should then be reduced as clinically appropriate to provide adequate oxygenation while limiting unnecessary exposure to high oxygen concentrations.

PEEP

PEEP commonly begins around:

5 cm H₂O

Some patients may require higher levels.

PEEP is adjusted according to oxygenation, lung mechanics, hemodynamics, and the underlying pulmonary condition.

PEEP During Assist/Control Ventilation

PEEP can be used with either volume or pressure A/C. PEEP maintains positive pressure in the lungs at the end of expiration, helping prevent alveolar collapse and support oxygenation.

A/C and PEEP serve different purposes. A/C determines how mandatory breaths are triggered and delivered, while PEEP determines the pressure maintained at the end of expiration.

PEEP may be particularly important in conditions associated with alveolar collapse or severe oxygenation impairment, including ARDS and pulmonary edema. Higher PEEP levels must be used carefully because excessive intrathoracic pressure can reduce venous return and cardiac output.

Minute Ventilation During Assist/Control

Minute ventilation is determined by tidal volume multiplied by respiratory rate. Because patients can trigger additional fully supported breaths during A/C, total minute ventilation may rise significantly above the level predicted by the set respiratory rate.

For example, a ventilator may be programmed with a rate of 12 breaths/min, but the patient may trigger a total of 24 breaths/min. If each breath receives the full programmed tidal volume, minute ventilation may become excessive.

This can produce:

• Low PaCO₂
• Respiratory alkalosis
• Increased mechanical power
• Increased risk of air trapping
• Patient discomfort

Note: The clinician must therefore assess the total respiratory rate, not just the set rate.

PaCO₂ Management

Changes in alveolar ventilation directly influence PaCO₂.

In general:

• Increased alveolar ventilation lowers PaCO₂
• Decreased alveolar ventilation raises PaCO₂

During volume A/C, PaCO₂ can often be adjusted by modifying respiratory rate or tidal volume. However, changing the set rate may not produce a predictable change if the patient is actively triggering additional breaths.

For example, decreasing the set rate from 18 to 14 breaths/min may have little effect if the patient continues triggering at a total rate of 24 breaths/min. The patient’s respiratory drive and actual rate must always be considered when adjusting ventilation.

Assist/Control in Patients With COPD

Patients with COPD require particular attention because of their tendency toward prolonged exhalation and air trapping. High respiratory rates and insufficient expiratory time can produce dynamic hyperinflation and auto-PEEP.

Strategies may include:

• Lower respiratory rate
• Smaller tidal volume when appropriate
• Higher inspiratory flow in volume control
• Shorter inspiratory time
• Longer expiratory time
• Treatment of bronchospasm
• Removal of secretions
• Monitoring expiratory flow waveforms

Note: Patients with chronic hypercapnia should also not necessarily be ventilated to a normal PaCO₂ immediately. Excessive minute ventilation may produce marked respiratory alkalosis and potentially disturb the patient’s chronic acid-base compensation.

Assist/Control in ARDS

A/C is commonly used for patients with ARDS because it can provide a high degree of support while allowing lung-protective ventilation strategies.

Important goals often include:

• Low tidal volume ventilation
• Limiting plateau pressure
• Appropriate PEEP
• Adequate oxygenation
• Avoiding excessive tidal volumes
• Monitoring driving pressure and respiratory mechanics
• Controlling excessive patient effort when necessary

Note: Volume A/C may be used to ensure the desired low tidal volume, while pressure A/C may also be used if tidal volumes are carefully monitored. The mode itself is less important than maintaining lung-protective targets and reassessing the patient’s response.

Work of Breathing

A/C can substantially reduce the patient’s work of breathing because every successful trigger results in a mechanically supported breath. When sensitivity and inspiratory flow are properly adjusted, the patient’s muscular effort may be largely limited to initiating the breath.

This can be useful when:

• Respiratory muscles are fatigued
• Severe respiratory failure is present
• The patient requires substantial respiratory muscle rest
• Oxygen consumption associated with breathing needs to be reduced

However, A/C does not automatically eliminate all work of breathing. Patients may still experience increased effort because of:

• Inappropriate trigger sensitivity
• Inadequate inspiratory flow
• Auto-PEEP
• Flow starvation
• Delayed cycling
• Premature cycling
• Pain or anxiety
• Increased respiratory drive

Note: Patient comfort and ventilator graphics should therefore be evaluated continuously.

Controlled Ventilation Within A/C

A/C can function essentially as controlled ventilation when the patient makes no inspiratory efforts. In this situation, every breath is time-triggered by the ventilator.

This may occur because of:

• Apnea
• Severe neurologic impairment
• Deep sedation
• General anesthesia
• Neuromuscular blockade

Controlled ventilation can eliminate spontaneous respiratory muscle activity and allow the ventilator to assume the work of breathing. This may be useful temporarily in severe acute respiratory failure.

However, patients who are completely ventilator dependent are at significant risk if the ventilator disconnects or malfunctions.

Neuromuscular Blockade and Sedation

Neuromuscular blocking medications prevent skeletal muscle activity but do not produce sedation, pain relief, or loss of awareness.

A patient receiving paralytic medications must therefore also receive appropriate sedation and analgesia according to the clinical situation. This is essential when controlled ventilation is being achieved pharmacologically.

Patient-Ventilator Asynchrony

Patient-ventilator asynchrony occurs when the patient’s respiratory effort and ventilator assistance are poorly coordinated. Several forms may occur during A/C.

Ineffective Triggering

The patient attempts to inhale, but the ventilator does not deliver a breath. Potential causes include:

• Trigger sensitivity that is too difficult
• Auto-PEEP
• Weak respiratory muscles
• Excessive sedation

Flow Asynchrony

Flow asynchrony occurs when ventilator flow does not meet the patient’s inspiratory demand. It is particularly relevant during volume-controlled ventilation with fixed inspiratory flow.

Possible responses include:

• Increasing inspiratory flow
• Changing the flow pattern
• Evaluating tidal volume
• Treating anxiety or pain
• Considering pressure-targeted ventilation when appropriate

Autotriggering

Autotriggering occurs when the ventilator delivers a breath without a true patient inspiratory effort.

Causes may include:

• Leaks
• Water in tubing
• Excessively sensitive trigger setting
• Circuit movement
• Cardiac oscillations

Double Triggering

Double triggering occurs when two supported breaths are delivered with little or no expiration between them. The patient may continue inhaling after the first ventilator breath ends, causing a second breath to be triggered immediately.

During volume A/C, this may result in approximately twice the selected tidal volume entering the respiratory system before complete exhalation occurs. Double triggering is particularly concerning during lung-protective ventilation because it can produce unexpectedly large tidal volumes.

Auto-PEEP and Air Trapping

Auto-PEEP develops when expiration is incomplete before the next inspiration begins. Gas becomes trapped in the lungs, creating positive pressure above the selected external PEEP.

Common causes include:

• High respiratory rate
• Large tidal volume
• Insufficient expiratory time
• Bronchospasm
• Increased airway resistance
• Secretions
• Obstructive lung disease

Note: Patients with COPD and severe asthma are particularly susceptible.

Detecting Auto-PEEP

Ventilator flow graphics are useful for recognizing incomplete exhalation. If expiratory flow does not return to baseline before the next inspiration begins, air trapping should be suspected. An expiratory hold maneuver may also be used to measure total PEEP.

Managing Auto-PEEP

Management focuses on allowing more complete exhalation. Potential interventions include:

• Reducing respiratory rate
• Reducing tidal volume when appropriate
• Increasing inspiratory flow during volume control
• Shortening inspiratory time
• Treating bronchospasm
• Suctioning secretions
• Correcting airway obstruction

Note: The underlying cause should always be identified rather than adjusting ventilator settings without assessing the patient.

Hemodynamic Effects

Positive-pressure ventilation can affect cardiovascular function. Increasing intrathoracic pressure may reduce venous return to the heart.

This can decrease:

• Preload
• Stroke volume
• Cardiac output
• Blood pressure

The effect may become more significant with:

• High PEEP
• High mean airway pressure
• Long inspiratory times
• Hypovolemia
• Severe dynamic hyperinflation

Note: A patient who develops hypotension after ventilator changes should be assessed for possible cardiopulmonary causes, including excessive airway pressure, auto-PEEP, pneumothorax, and inadequate circulating volume.

Ventilator Alarms in Assist/Control

Alarm settings are essential for safe A/C ventilation.

High-Pressure Alarm

A high-pressure alarm may occur because of:

• Secretions
• Bronchospasm
• Endotracheal tube obstruction
• Patient coughing
• Decreased lung compliance
• Pneumothorax
• Water in the circuit
• Patient-ventilator dyssynchrony

Note: The patient should be assessed before assuming the alarm is caused by the ventilator.

Low-Pressure Alarm

A low-pressure alarm may suggest:

• Circuit disconnection
• Air leak
• Cuff leak
• Loose tubing
• Extubation
• Major change in respiratory mechanics

Low Minute-Ventilation Alarm

This alarm is particularly important during pressure control because tidal volume can decrease unexpectedly.

Causes include:

• Reduced tidal volume
• Circuit leak
• Disconnection
• Worsening compliance
• Increased airway resistance

High Minute-Ventilation Alarm

High minute ventilation may result from:

• Tachypnea
• Anxiety
• Pain
• Fever
• Hypoxemia
• Metabolic acidosis
• Inadequate ventilator support
• Excessive triggering

Note: Alarm values should be individualized to the patient rather than left at inappropriate default settings.

Monitoring a Patient on Assist/Control

A patient receiving A/C requires repeated clinical assessment.

Important parameters include:

• Set respiratory rate
• Total respiratory rate
• Tidal volume
• Minute ventilation
• Peak inspiratory pressure
• Plateau pressure
• PEEP
• FiO₂
• Oxygen saturation
• Arterial blood gases
• Breath sounds
• Chest movement
• Patient comfort
• Hemodynamics
• Ventilator waveforms
• Evidence of auto-PEEP
• Patient-ventilator synchrony

Note: Ventilator values should always be interpreted together with the patient’s clinical appearance. A normal number on the ventilator does not guarantee that ventilation is safe or adequate.

Advantages of Assist/Control Ventilation

A/C offers several important advantages:

• Guarantees a minimum respiratory rate
• Provides full support for patient-triggered breaths
• Reduces the work of breathing
• Can be used in patients with or without spontaneous respiratory effort
• Can be delivered using volume or pressure control
• Allows the patient to increase total respiratory rate
• Provides substantial respiratory muscle rest
• Is suitable for many forms of acute respiratory failure

Note: Its flexibility is one reason it is commonly used early in invasive mechanical ventilation.

Disadvantages and Complications

Potential disadvantages include:

• Respiratory alkalosis from excessive triggering
• Auto-PEEP
• Dynamic hyperinflation
• Double triggering
• Flow asynchrony
• Autotriggering
• Excessive airway pressures during volume control
• Reduced tidal volume during pressure control
• Hemodynamic compromise
• Respiratory muscle inactivity during prolonged full support

Note: Complications are often related to the interaction between ventilator settings, respiratory mechanics, and patient respiratory drive rather than to the mode alone.

Assist/Control vs. SIMV

A/C and synchronized intermittent mandatory ventilation (SIMV) differ primarily in how spontaneous breaths are handled. During A/C, every successfully triggered breath receives the programmed mandatory support.

During SIMV, the ventilator provides a selected number of mandatory breaths, but the patient may take spontaneous breaths between them. These spontaneous SIMV breaths do not automatically receive the same mandatory tidal volume or pressure as the scheduled breaths. Pressure support may be added to assist spontaneous breaths during SIMV.

Note: Because A/C supports every triggered breath fully, it generally provides more ventilatory assistance than SIMV when other settings are similar.

Assist/Control vs. Pressure Support Ventilation

Pressure Support Ventilation (PSV) requires the patient to initiate breaths.

There is no mandatory backup ventilation in pure PSV unless a separate apnea backup mode is activated.

The patient generally controls:

• Respiratory rate
• Inspiratory timing
• Expiratory cycling
• Overall breathing pattern

In A/C, a minimum respiratory rate is guaranteed and every triggered breath becomes a mandatory supported breath. Therefore, A/C is typically used when greater ventilatory support is required.

PSV is more commonly used when the patient has a reliable respiratory drive and can perform more of the work of breathing.

When Assist/Control May Be Used

A/C may be appropriate in many situations requiring invasive mechanical ventilation, including:

• Acute respiratory failure
• ARDS
• Severe pneumonia
• Pulmonary edema
• Respiratory muscle fatigue
• Neurologic impairment
• Drug overdose with respiratory depression
• Postoperative ventilatory support
• Severe obstructive lung disease when carefully adjusted
• Patients requiring deep sedation or neuromuscular blockade

Note: The specific settings and form of A/C should be matched to the underlying disease process.

Key Concepts to Remember

The most important principles of A/C ventilation include:

• A/C provides continuous mandatory ventilation.
• Breaths may be triggered by the patient or by time.
• Every successfully triggered breath receives programmed mechanical support.
• The set rate is a minimum, not necessarily the total respiratory rate.
• Volume A/C controls tidal volume while pressure varies.
• Pressure A/C controls inspiratory pressure while tidal volume varies.
• Trigger sensitivity should minimize patient effort without causing autotriggering.
• Total respiratory rate must be monitored for excessive ventilation.
• Expiratory time must be adequate to prevent auto-PEEP.
• Pressure, volume, flow, gas exchange, alarms, and patient synchrony require continuous reassessment.

Assist/Control Practice Questions

1. What type of mechanical ventilation is Assist/Control (A/C) classified as?
Continuous mandatory ventilation (CMV)

2. What are the two ways a mandatory breath can be initiated during Assist/Control ventilation?
By a patient trigger or a time trigger

3. What is a patient-triggered breath called during Assist/Control ventilation?
An assisted breath

4. What is a ventilator-initiated, time-triggered breath called during Assist/Control ventilation?
A control breath

5. What does the set respiratory rate represent during Assist/Control ventilation?
The minimum or backup respiratory rate

6. If an A/C ventilator is set at 12 breaths/min and the patient triggers 18 breaths/min, approximately how many supported breaths will the patient receive?
18 breaths/min

7. How does a patient-triggered breath in A/C differ from an unsupported spontaneous breath?
A patient-triggered breath receives the programmed mechanical support

8. What are the two common triggering methods used during Assist/Control ventilation?
Pressure triggering and flow triggering

9. What is a typical initial pressure-trigger sensitivity during A/C ventilation?
Approximately −0.5 to −1.5 cm H₂O

10. What is a typical initial flow-trigger sensitivity during A/C ventilation?
Approximately 1 to 2 L/min

11. What can occur if trigger sensitivity requires too much patient effort?
Ineffective triggering and increased work of breathing

12. What is autotriggering?
The delivery of a ventilator breath without a genuine patient inspiratory effort

13. What variable is directly controlled during volume control Assist/Control ventilation?
Tidal volume

14. What typically happens to airway pressure during VC-A/C when respiratory system compliance decreases?
Airway pressure increases

15. What is a common initial tidal-volume range for an adult receiving VC-A/C?
Approximately 6 to 8 mL/kg of predicted or ideal body weight

16. What tidal-volume range may be used as part of a lung-protective strategy for a patient with ARDS?
Approximately 4 to 6 mL/kg of predicted body weight

17. What does an increase in peak inspiratory pressure with a relatively unchanged plateau pressure generally suggest?
Increased airway resistance

18. What does an increase in both peak inspiratory pressure and plateau pressure generally suggest?
Decreased respiratory system compliance

19. What variable is directly controlled during pressure control Assist/Control ventilation?
Inspiratory pressure

20. Why can tidal volume change during PC-A/C even when the inspiratory pressure setting remains unchanged?
Tidal volume depends on factors such as compliance, airway resistance, inspiratory time, and patient effort

21. What is a major safety concern associated with pressure control A/C?
Tidal volume and minute ventilation may decrease if respiratory mechanics worsen

22. What initial inspiratory flow may commonly be selected during volume-controlled A/C?
Approximately 60 to 80 L/min

23. What is a typical initial inspiratory time during Assist/Control ventilation?
Approximately 0.6 to 1.0 second

24. Why may a patient with COPD require a longer expiratory time during A/C ventilation?
To reduce air trapping, dynamic hyperinflation, and auto-PEEP

25. What ventilator waveform finding suggests incomplete exhalation and possible auto-PEEP?
Expiratory flow fails to return to baseline before the next inspiration begins

26. What complication can occur if a patient repeatedly triggers additional fully supported breaths during A/C ventilation?
Excessive minute ventilation and respiratory alkalosis

27. Why is the total respiratory rate important to monitor during A/C ventilation?
Because the patient may trigger breaths above the set rate and substantially increase minute ventilation

28. How does increasing alveolar ventilation generally affect PaCO₂?
It lowers PaCO₂

29. How does decreasing alveolar ventilation generally affect PaCO₂?
It raises PaCO₂

30. Why might reducing the set respiratory rate fail to lower minute ventilation in an actively breathing patient on A/C?
Because the patient may continue triggering additional supported breaths above the set rate

31. What is a common starting PEEP level during Assist/Control ventilation?
Approximately 5 cm H₂O

32. What is the primary purpose of PEEP during A/C ventilation?
To maintain positive pressure at end expiration and help prevent alveolar collapse

33. How can high PEEP or high mean airway pressure affect cardiovascular function?
It can reduce venous return and cardiac output

34. What happens to tidal volume in PC-A/C if lung compliance worsens and the pressure setting remains unchanged?
Tidal volume usually decreases

35. Why are low tidal-volume alarms especially important during pressure control A/C?
Because the delivered tidal volume can fall when respiratory mechanics worsen

36. What type of inspiratory flow pattern is commonly associated with pressure control ventilation?
A decelerating inspiratory flow pattern

37. Why may pressure control improve patient-ventilator synchrony compared with fixed-flow volume control?
Because inspiratory flow can vary according to the patient’s demand

38. What is flow asynchrony during volume-controlled A/C?
A mismatch in which ventilator flow does not meet the patient’s inspiratory demand

39. What adjustment may help correct flow starvation during volume-controlled A/C?
Increasing inspiratory flow

40. What is ineffective triggering?
A patient inspiratory effort that fails to trigger a ventilator breath

41. What factors can contribute to ineffective triggering?
Auto-PEEP, weak respiratory effort, excessive sedation, or an insensitive trigger setting

42. What is double triggering during Assist/Control ventilation?
Two supported breaths delivered with little or no exhalation between them

43. Why is double triggering especially concerning during volume-controlled A/C?
Because the combined delivered volume may approach twice the selected tidal volume

44. What is auto-PEEP?
Unintended positive pressure remaining in the lungs at end expiration because exhalation is incomplete

45. What conditions make auto-PEEP particularly likely during A/C ventilation?
COPD and severe asthma

46. What ventilator adjustment can increase expiratory time during volume-controlled A/C?
Increasing inspiratory flow to shorten inspiratory time

47. What is one method used to measure total PEEP when auto-PEEP is suspected?
An end-expiratory hold maneuver

48. What can a high-pressure ventilator alarm indicate during A/C ventilation?
Increased airway resistance, decreased compliance, airway obstruction, coughing, secretions, or patient-ventilator dyssynchrony

49. What can a low-pressure alarm suggest during A/C ventilation?
Circuit disconnection, an air leak, cuff leak, loose tubing, or extubation

50. Why must ventilator alarm limits be individualized for each patient?
Because appropriate pressure, volume, and ventilation limits depend on the patient’s condition and current ventilator requirements

51. What happens during A/C ventilation when the patient makes no spontaneous inspiratory efforts?
All breaths are time-triggered by the ventilator

52. In what situations may A/C function essentially as controlled ventilation?
During apnea, deep sedation, general anesthesia, severe neurologic impairment, or neuromuscular blockade

53. Why can controlled ventilation reduce respiratory muscle oxygen consumption?
Because the ventilator assumes most or all of the work required for breathing

54. Why is ventilator disconnection especially dangerous in a completely apneic patient?
Because the patient cannot generate effective spontaneous ventilation without the ventilator

55. What important principle applies when neuromuscular blocking agents are used during mechanical ventilation?
They must be accompanied by appropriate sedation and analgesia because paralysis does not eliminate awareness

56. What is the main difference between A/C and SIMV regarding patient-triggered breaths?
In A/C, each successfully triggered breath receives full programmed support, while SIMV allows spontaneous breaths between mandatory breaths

57. Why does A/C generally provide more ventilatory assistance than SIMV?
Because every triggered breath in A/C receives mandatory mechanical support

58. What is a major difference between A/C and pure pressure support ventilation?
A/C guarantees a minimum respiratory rate, while pure PSV depends on the patient to initiate breaths

59. Why must a patient have an adequate respiratory drive to use pure pressure support ventilation?
Because the patient must initiate the supported breaths

60. What is one reason A/C may be selected early in acute respiratory failure?
It can provide substantial ventilatory support and reduce the patient’s work of breathing

61. What should be monitored along with ventilator settings to determine whether A/C is effective?
Gas exchange, patient effort, airway pressures, tidal volume, respiratory rate, and synchrony

62. Why should ventilator values not be interpreted without assessing the patient?
Because acceptable ventilator numbers do not always mean that ventilation or oxygenation is clinically adequate

63. What bedside findings should be assessed in a patient receiving A/C?
Breath sounds, chest movement, vital signs, comfort, oxygenation, and hemodynamic status

64. Why is exhaled tidal volume important to monitor during pressure-controlled A/C?
Because delivered tidal volume can change even when the pressure setting remains constant

65. What effect can worsening bronchospasm have during PC-A/C?
It can reduce tidal volume and minute ventilation

66. What effect can worsening bronchospasm have during VC-A/C?
It can increase the pressure required to deliver the selected tidal volume

67. What is one possible cause of a high-pressure alarm associated with the artificial airway?
Kinking or obstruction of the endotracheal tube

68. What is one possible cause of a low-pressure alarm related to the endotracheal tube cuff?
A cuff leak

69. Why may a patient with severe obstructive lung disease require a lower ventilator rate?
To provide more time for exhalation and reduce dynamic hyperinflation

70. What I:E ratios may be appropriate when a patient requires prolonged exhalation?
Approximately 1:3, 1:4, or 1:5

71. What is dynamic hyperinflation?
Progressive air trapping caused by incomplete exhalation before subsequent breaths

72. How can reducing respiratory rate help manage auto-PEEP?
It increases the time available for complete exhalation

73. How can shortening inspiratory time during PC-A/C help a patient with air trapping?
It increases the time available for expiration

74. Why should a chronically hypercapnic patient not necessarily be ventilated immediately to a normal PaCO₂?
Rapidly lowering PaCO₂ may produce respiratory alkalosis and disturb chronic acid-base compensation

75. What is the overall goal when selecting and adjusting A/C ventilator settings?
To provide adequate gas exchange and ventilatory support while minimizing excessive pressures, air trapping, asynchrony, and other complications

76. Why may an excessively sensitive trigger setting cause problems during A/C ventilation?
It can cause autotriggering and deliver breaths without a true patient inspiratory effort

77. What patient condition can increase the risk of excessive minute ventilation during A/C?
An abnormally high respiratory drive

78. What acid-base disturbance can result from excessive assisted breathing during A/C?
Respiratory alkalosis

79. How can frequent patient-triggered breaths contribute to air trapping?
They shorten the time available for exhalation between breaths

80. Why should inspiratory flow meet or exceed the patient’s inspiratory demand during A/C?
To reduce flow starvation and unnecessary work of breathing

81. What happens to airway pressure in volume-controlled A/C if airway resistance increases?
The pressure required to deliver the preset tidal volume increases

82. What happens to tidal volume in pressure-controlled A/C if airway resistance increases?
The delivered tidal volume may decrease

83. Why is pressure-controlled A/C considered a pressure-targeted mode?
Because the ventilator maintains a selected inspiratory pressure while tidal volume is allowed to vary

84. Why is volume-controlled A/C considered a volume-targeted mode?
Because the ventilator is programmed to deliver a selected tidal volume while airway pressure may vary

85. What ventilator parameter should be watched closely when using VC-A/C in a patient with worsening compliance?
Plateau pressure

86. What ventilator parameter should be watched closely when using PC-A/C in a patient with worsening compliance?
Exhaled tidal volume

87. Why can a patient’s actual respiratory rate be higher than the set rate in A/C?
Because the patient can trigger additional fully supported breaths

88. What is one advantage of allowing the patient to trigger breaths during A/C?
The ventilator can respond to the patient’s respiratory drive while still guaranteeing a minimum rate

89. Why can A/C substantially rest fatigued respiratory muscles?
Because the ventilator provides the programmed support after each successful trigger

90. What is one potential disadvantage of prolonged full ventilatory support with A/C?
Respiratory muscle inactivity

91. Why should FiO₂ be reduced after adequate oxygenation is established?
To avoid unnecessary exposure to high oxygen concentrations

92. What is the relationship between PEEP and alveolar collapse?
PEEP helps prevent alveolar collapse by maintaining positive pressure at end expiration

93. What may happen to blood pressure if intrathoracic pressure becomes excessively high during A/C?
Blood pressure may decrease because venous return and cardiac output can fall

94. What patient condition can make the hemodynamic effects of positive-pressure ventilation more pronounced?
Hypovolemia

95. Why should a sudden drop in blood pressure during A/C prompt assessment for pneumothorax?
Pneumothorax can impair ventilation and venous return and may cause acute hemodynamic compromise

96. What is one reason ventilator graphics are useful during A/C?
They can help identify abnormal flow, pressure, volume, auto-PEEP, and patient-ventilator asynchrony

97. What may a failure of expiratory flow to reach zero before the next breath indicate?
Incomplete exhalation with air trapping

98. Why is patient comfort an important part of monitoring A/C ventilation?
Discomfort may indicate inadequate flow, poor triggering, asynchrony, pain, anxiety, or other problems

99. What should be done when a ventilator alarm sounds during A/C?
Assess the patient and determine the underlying cause rather than assuming the ventilator itself is the problem

100. What is the central principle of safe Assist/Control ventilation?
Continuously match ventilator settings and support to the patient’s gas exchange, respiratory mechanics, breathing pattern, and clinical condition

Final Thoughts

Assist/control ventilation provides a high level of respiratory support by delivering mandatory breaths that may be initiated either by the patient or by the ventilator.

Volume-controlled A/C provides a selected tidal volume while allowing pressure to vary, whereas pressure-controlled A/C maintains a selected inspiratory pressure while tidal volume varies with respiratory mechanics.

Proper management requires more than selecting a mode. Respiratory rate, tidal volume, pressure, flow, inspiratory time, PEEP, FiO₂, trigger sensitivity, alarms, and patient-ventilator interaction must all be reassessed as the patient’s condition changes.