Volume Controlled Ventilation (VC-AC)

Clinical Context:
Volume Control Assist/Control (VC-AC) is a common initial mode of ventilation. It provides a guaranteed minute ventilation, making it the safety standard for paralyzed or heavily sedated patients. However, because volume is fixed, pressure becomes the variable—meaning the clinician must vigilantly monitor airway pressures to prevent barotrauma.

Learner Objectives:

1. Define VC-AC using the Phase Variables (Trigger, Limit, Cycle).
2. Categorize ventilator settings into Independent (set by you and patient specific factors) and Dependent (the results of the ventilator and patient factors).
3. Use the Equation of Motion to predict how changes in airway resistance or lung compliance alter the Peak Inspiratory Pressure (PIP).
4. Demonstrate how manipulating Flow Rate impacts Inspiratory Time ($T_I$) and Resistive Pressure.

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1. Definition and Mechanics

What is it?
In VC-AC, you specify the volume of breath delivered to the patient ($V_T$). The ventilator will do whatever work is necessary (generate whatever pressure is required) to deliver that specific volume.

Phase Variables (The "Logic" of the Mode):

• Trigger (Start):
  • Patient: Flow or Pressure drop (assisted breath).
  • Time: Backup rate acts if the patient makes no effort (mandatory breath).
  • Note: In "Assist/Control," every breath—whether patient-triggered or time-triggered—receives the full set tidal volume.
• Limit (Sustain): Flow.
  • The clinician sets the flow rate to reach the target volume. This is why the Flow waveform looks like a square (constant flow) or a decelerating ramp, depending on settings.
• Cycle (End): Volume.
  • The breath ends the moment the set Tidal Volume ($V_T$) is delivered.

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2. The Variables: Who Controls What?

In VC mode, you must understand which variables you control and which variables the patient's lungs dictate.

Independent Variables (We Set)	Patient Factors (Physiology)	Dependent Variables (We Monitor)
Tidal Volume ($V_T$)	Airway Resistance ($R$)	Peak Inspiratory Pressure ($PIP$)
Respiratory Rate ($RR$)	Lung Compliance ($C_{L}$)	Plateau Pressure ($P_{plat}$)
Flow Rate ($\dot{V}$)	Chest Wall Compliance ($C_{CW}$)	Mean Airway Pressure
PEEP & $FiO_2$		Inspiratory Time ($T_I$)

Key Takeaway: You cannot set the Pressure in Volume Control. Pressure is the result of pushing your set Volume and flow through the patient's Resistance and Compliance.

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3. The Equation of Motion: Predicting Pressure Changes

To understand why alarms ring in VC mode, we look at the Equation of Motion.

$P_{vent} = \underbrace{(\dot{V} \times R)}_{\text{Resistive Load}} + \underbrace{\left( \frac{V_T}{C_{stat}} \right)}_{\text{Elastic Load}} + PEEP$

In VC-AC, $\dot{V}$ (Flow) and $V_T$ (Volume) are constant (because you set them). Therefore, if $P_{vent}$ (seen as Peak Pressure) rises, it means either Resistance ($R$) increased or Compliance ($C_{stat}$) decreased.

Tool Exploration: Visualizing the Fixed vs. The Variable>

VC-AC

Adult (70kg)

405

0

cmH2OPpeak

600400

0

mlVTI

122

0.0

l/minExpMinVol

600400

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mlVTE

3510

0

b/minfTotal

1:9.0

I:E

PawcmH2O

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Flowl/min

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Volml

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12

500mL

Vt Set

5cmH2O

PEEP

40%

Oxygen

Controls

Patient Model

Compliance40 mL/cmH2O

Resistance10 cmH2O/L/s

Chest Wall1 x

Mode Settings

No editable settings enabled for this mode.

Equations

Independent variables (used across equations)

V_T500 mL

Tidal volume

V_T: 0.5 L

C40 mL/cmH2O

Compliance

C: 0.04 L/cmH2O

R10 cmH2O·s/L

Resistance

\dot V60 L/min

Inspiratory flow

\dot V: 1 (from 60 L/min ÷ 60)

PEEP5 cmH2O

Positive end-expiratory pressure

P_el12.5 cmH2O

Elastic pressure (V_T/C)

P_res10 cmH2O

Resistive pressure (R·Vdot)

Equation of Motion (VC)

Pressure from elastic + resistive load + PEEP.

Equation + work

$$P_{aw} = \frac{V_T}{C} + R \cdot \dot V + PEEP \\ \frac{V_T}{C} = \frac{0.5\,\mathrm{L}}{0.04\,\mathrm{L/cmH_2O}} = 12.5\,\mathrm{cmH_2O} \\ R \cdot \dot V = 10 \cdot 1 = 10\,\mathrm{cmH_2O} \\ P_{aw} = 12.5 + 10 + 5 = 27.5\,\mathrm{cmH_2O}$$

Activity:

1. Identify the Constants: Look at the Flow (middle) and Volume (bottom) waveforms. They are identical for every breath.
2. Identify the Variable: Look at the Pressure (top) waveform. The height of this wave is the $P_{peak}$.
3. Scenario A (Resistance Problem): Imagine the patient bites the ETT or develops bronchospasm (Asthma).
   • $R$ increases.
   • Since Flow ($\dot{V}$) is constant, the resistive work ($\dot{V} \times R$) increases.
   • Result: The PIP shoots up. The ventilator alarms "High Pressure."
4. Scenario B (Compliance Problem): Imagine the patient develops pulmonary edema or pneumothorax.
   • $C_{stat}$ decreases (lungs get stiff).
   • Since Volume ($V_T$) is constant, the elastic work ($V_T / C$) increases.
   • Result: The PIP shoots up.

Clinical Decision Point:
In both scenarios, the monitor shows high pressure. To distinguish between a Resistance problem (biting tube) and a Compliance problem (pneumothorax), you would perform an Inspiratory Hold to measure the Plateau Pressure ($P_{plat}$).

• High PIP + Normal $P_{plat}$ = Resistance problem.
• High PIP + High $P_{plat}$ = Compliance problem.

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4. Modifying Ventilation: Rate and Volume

If an ABG shows Respiratory Acidosis (High $CO_2$), you need to increase Minute Ventilation ($\dot{V}_E = RR \times V_T$).

Tool Exploration - Ventilation>

VC-AC

Adult (70kg)

405

0

cmH2OPpeak

600400

0

mlVTI

122

0.0

l/minExpMinVol

600400

0

mlVTE

3510

0

b/minfTotal

1:9.0

I:E

PawcmH2O

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Flowl/min

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Volml

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800

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12

500mL

Vt Set

5cmH2O

PEEP

40%

Oxygen

Controls

Mode Settings

Tidal Vol

-

500mL

+

Rate

-

12b/min

+

Equations

Independent variables (used across equations)

V_T500 mL

Tidal volume

V_T: 0.5 L

RR12 breaths/min

Respiratory rate

\dot V_E6 L/min

Minute ventilation

Minute Ventilation

Ventilation per minute from Vt and RR.

Equation + work

$$\dot V_E = V_T \cdot RR \\ V_T = 500\,\mathrm{mL} = 0.5\,\mathrm{L} \\ \dot V_E = 0.5 \cdot 12 = 6\,\mathrm{L/min}$$

1. Increase RR: Change RR from 12 to 18.
   • Outcome: More breaths per minute $\to$ higher $\dot{V}_E$ $\to$ lower $CO_2$.
   • Risk: Less time for exhalation (watch for Auto-PEEP).
2. Increase $V_T$: Change $V_T$ from 500 to 600.
   • Outcome: Larger breaths $\to$ higher $\dot{V}_E$.
   • Risk: Look at the Equation of Motion. Increasing $V_T$ increases the Elastic Load term ($V_T/C$). Your Peak Pressures will rise, risking lung injury.

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5. Advanced Mechanics: The Role of Flow

Flow rate is often the most misunderstood setting in VC-AC. It determines how fast the breath is shoved into the lungs.

The Relationship:
$T_I = \frac{V_T}{\dot{V}}$
(Inspiratory Time = Volume / Flow Rate)

Tool Exploration: Flow>

VC-AC

Adult (70kg)

405

0

cmH2OPpeak

600400

0

mlVTI

122

0.0

l/minExpMinVol

600400

0

mlVTE

3510

0

b/minfTotal

1:9.0

I:E

PawcmH2O

0

20

40

60

2

4

6

8

10

12

Flowl/min

-50

0

50

2

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Volml

0

200

400

600

800

2

4

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12

500mL

Vt Set

5cmH2O

PEEP

40%

Oxygen

Controls

Mode Settings

Tidal Vol

-

500mL

+

Insp Flow

-

60L/min

+

Flow Pattern

SqDec

Equations

Independent variables (used across equations)

V_T500 mL

Tidal volume

V_T: 0.5 L

C40 mL/cmH2O

Compliance

C: 0.04 L/cmH2O

R10 cmH2O·s/L

Resistance

\dot V60 L/min

Inspiratory flow

\dot V: 1 (from 60 L/min ÷ 60)

PEEP5 cmH2O

Positive end-expiratory pressure

P_el12.5 cmH2O

Elastic pressure (V_T/C)

P_res10 cmH2O

Resistive pressure (R·Vdot)

Equation of Motion (VC)

Pressure from elastic + resistive load + PEEP.

Equation + work

$$P_{aw} = \frac{V_T}{C} + R \cdot \dot V + PEEP \\ \frac{V_T}{C} = \frac{0.5\,\mathrm{L}}{0.04\,\mathrm{L/cmH_2O}} = 12.5\,\mathrm{cmH_2O} \\ R \cdot \dot V = 10 \cdot 1 = 10\,\mathrm{cmH_2O} \\ P_{aw} = 12.5 + 10 + 5 = 27.5\,\mathrm{cmH_2O}$$

1. Baseline: Note the I:E Ratio and the width of the inspiration on the waveform. With a Flow of 40 L/min, the breath takes longer to deliver.
2. Adjustment: Change Flow Rate to 80 L/min (doubling the speed).
   • Observe: The inspiratory phase (width of the "mountain") becomes narrower. The $T_I$ decreases.
   • Physics impact:
     • Recall the Resistive Load: $(\dot{V} \times R)$.
     • If you double Flow ($\dot{V}$), you double the pressure required to push that air through the airways.
   • Result: Peak Pressure increases, but Plateau Pressure (distending pressure) remains unchanged because the volume is the same.

Why does this matter?

• Air Hunger: If a patient is trying to inhale deeply and the flow is set too low (e.g., 40 L/min), they will feel "flow starved" and fight the ventilator (dyssynchrony). Increasing flow can improve comfort.
• Oxygenation impact: Higher flow $\to$ shorter $T_I$ $\to$ less time for gas exchange. Sometimes we lower flow to increase inspiratory time and improve oxygenation.

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6. Summary of Clinical Utility

When to use VC-AC:

• Pros: Guarantees a minimum $CO_2$ clearance (Minute Ventilation); standard for ARDS lung protection (using low $V_T$).
• Cons: Fixed flow can be uncomfortable for awake patients; pressures are variable and must be watched.

Reflection Checkpoint:
You have a patient with severe asthma (high resistance) on VC-AC. You notice their Peak Pressures are high (45 cmH2O).

1. Is this dangerous to the alveoli? (Hint: Check a plateau pressure).
2. If you increase the Flow Rate to shorten their inspiratory time, what will happen to the Peak Pressure? (Answer: It will rise further, because $P_{resistive} = Flow \times Resistance$).

Exploration
Below is a ventilator with more controls to play around and learn more about this mode. This is a work in project and there may be errors