What is pressure control ventilation in a ventilator?

Pressure Control Ventilation: Definition and Mechanism

Pressure control (PC) ventilation is a mode in which the clinician sets a fixed inspiratory pressure, expiratory pressure (PEEP), respiratory rate, and inspiratory time; the delivered tidal volume then varies according to the patient's lung compliance, airway resistance, and chest-wall mechanics. 1

Core Characteristics of Pressure Control

In pressure-targeted ventilation, the operator sets the inspiratory pressure as the primary control variable, and the ventilator generates whatever volume results from that pressure over the set inspiratory time. 2 This contrasts fundamentally with volume control, where tidal volume is fixed and pressure varies. 2

Fixed Parameters Set by the Clinician

• Inspiratory pressure (IPAP) – the target pressure delivered during inspiration 1
• Expiratory pressure (EPAP/PEEP) – maintains alveolar recruitment and offsets intrinsic PEEP 1
• Respiratory rate – backup mandatory rate (typically 10–15 breaths/min for obstructive disease, 15–25 breaths/min for restrictive disease) 1
• Inspiratory time (Ti) – duration of the inspiratory phase, which should be long enough to achieve adequate volume but allow complete exhalation 2, 1
• FiO₂ – fraction of inspired oxygen 1

Variable Parameters (Patient-Dependent)

• Tidal volume – determined by the interaction between set pressure, lung/chest-wall compliance, and airway resistance 1, 3
• Peak inspiratory flow – the ventilator delivers a high initial flow that decelerates throughout inspiration to maintain the target pressure 3, 4
• Minute ventilation – product of tidal volume and total respiratory rate 3

Physiological Advantages of Pressure Control

Pressure control offers several clinical benefits over volume control, particularly in patients with variable respiratory mechanics or when using non-invasive interfaces. 2

Constant Pressure Delivery

• The pressure delivered remains constant throughout inspiration, avoiding the sudden uncomfortable pressure spikes that occur with volume control. 2 This improves patient comfort and synchrony. 3, 5

Compensation for Air Leak

• Pressure-targeted ventilation automatically compensates for air leaks, which is essential when using non-invasive ventilation (NIV) interfaces such as full-face masks. 2 The ventilator increases flow to maintain target pressure despite leak. 2

Decelerating Flow Pattern

• PC ventilation incorporates a variable decelerating flow waveform with high initial peak flow as inspiration begins. 4, 6 This high early flow better matches patient demand in acute respiratory failure, reducing work of breathing compared to the constant flow of traditional volume control. 4
• In patients with acute lung injury (ALI) and ARDS, pressure control significantly reduced patient work of breathing (0.59 ± 0.42 J/L) compared to volume control (0.70 ± 0.58 J/L, p < 0.05), attributed to higher ventilator peak inspiratory flow (103.2 ± 22.8 L/min vs 43.8 L/min, p < 0.01). 4

Positive Expiratory Pressure Throughout the Cycle

• Maintaining positive pressure during expiration (EPAP) flushes exhaled CO₂ from the mask and distal tubing, aids triggering, and counteracts upper airway collapse during expiration. 2 This is particularly important in obstructive lung disease with intrinsic PEEP. 1

Pressure Control vs Volume Control: Clinical Equivalence

For the same tidal volume, there is no outcome advantage of pressure-controlled versus volume-controlled ventilation in terms of lung stress and strain generated. 7 Most comparative studies were not well-controlled and offer little guidance on when to use each mode. 6

When to Choose Pressure Control

• Volume control is recommended in the early passive ventilation phase because it facilitates measurement of respiratory mechanics and driving pressure. 7 Accurate plateau pressure measurement requires an inspiratory pause >0.5 seconds, which is standard in volume control. 1
• Pressure control may provide better respiratory comfort during assisted breathing at later stages because it does not limit inspiratory flow. 7 The variable flow accommodates changing patient demand breath-to-breath. 5
• Any benefit associated with pressure control regarding gas exchange probably results from the decelerating-flow waveform, which is now available during volume control on many modern ventilators. 6

Critical Safety Considerations and Pitfalls

Tidal Volume Variability

• Pressure control does not guarantee a fixed tidal volume; changes in compliance or resistance will alter delivered volume. 7, 3 Clinicians must continuously monitor exhaled tidal volume and adjust inspiratory pressure accordingly. 6
• The clinician should titrate inspiratory pressure to achieve the target tidal volume of 6–8 mL/kg ideal body weight. 1, 6

Inspiratory Time Adjustment

• Inspiratory time should be of sufficient length to achieve adequate volume and at a frequency that allows the patient time to fully exhale. 2 Inadequate expiratory time causes air trapping and dynamic hyperinflation. 1
• For obstructive disease, use I:E ratios of 1:2 to 1:4 (shorter inspiration, prolonged expiration); for restrictive disease, use 1:1 to 1:2 (longer inspiration). 1

Pressure Limitation and Lung Protection

• Pressure control inherently limits maximum airway pressure, which may reduce the risk of ventilator-induced lung injury. 5, 6 However, this does not eliminate the need to monitor plateau pressure and driving pressure. 7
• Maintain plateau pressure ≤30 cmH₂O and use low tidal volumes (6 mL/kg ideal body weight) to prevent overdistension. 1

Flow and Waveform Control

• During pressure control, inspiratory flow and flow waveform are determined by the ventilator as it attempts to maintain a square inspiratory pressure profile; the clinician cannot directly set flow. 6 This contrasts with volume control, where flow rate and waveform are explicitly programmed. 6

Terminology and Mode Confusion

The terminology for pressure-targeted ventilation causes confusion because different manufacturers use different names for similar modes. 2

• "Bi-level ventilation" typically refers to setting one pressure for inspiration (IPAP) and a second for expiration (EPAP); the difference is the level of pressure support. 2 This term is most commonly used for NIV. 2
• "CPAP with Pressure Support" can describe both invasive and non-invasive ventilation on ICU ventilators, where the operator sets an incremental inspiratory pressure above the CPAP baseline. 2
• Pressure control (PC), pressure-controlled ventilation (PCV), and pressure assist-control (PAC) all refer to the same fundamental mode with fixed pressure, rate, and inspiratory time. 3, 5