Modes of Mechanical Ventilation: Pressure Control Explained

PRVC is actually pressure control with volume guarantee, not true volume control. It is an adaptive dual control closed-loop ventilation that adjusts inspiratory pressure over several breaths to maintain a set target volume. The ventilator delivers a volume control test breath first to measure PEEP and plateau pressure, then converts to pressure control mode. It is useful for sedated, paralyzed patients with changing lung compliance and resistance. Advantages include physiological decelerating flow, patient comfort, no barotrauma, and guaranteed tidal volume. Disadvantages include inability to use in awake patients, potential for increased work of breathing during recovery, and pressure asynchrony when the patient needs more support.

PRVC is an adaptive mechanical ventilation mode that combines the guaranteed minute ventilation of volume control ventilation with the pressure limitation benefits of pressure control ventilation, automatically adjusting inspiratory pressure based on lung compliance to achieve target tidal volume while preventing barotrauma; the ventilator calculates compliance from an initial volume control breath and then adjusts pressure in small increments (approximately 3 mmHg) for subsequent breaths, making it suitable for patients with unstable lung mechanics.

Pressure Regulated Volume Control (PRVC) is an advanced ventilator mode that combines the advantages of both pressure control and volume control ventilation by automatically adjusting inspiratory pressure breath-by-breath to guarantee a preset tidal volume while maintaining the lowest possible airway pressure; unlike traditional pressure control where increasing resistance causes tidal volume to decrease, PRVC uses a microprocessor to monitor delivered tidal volume and adjust pressure accordingly, ensuring consistent ventilation delivery while protecting against high airway pressures that can damage the alveoli.

Pressure Regulated Volume Control (PRVC), also known as adaptive pressure ventilation or autoflow, is an adaptive control mode where the ventilator automatically adjusts inspiratory pressure between breaths to achieve a preset average tidal volume target, combining elements of both pressure-controlled and volume-controlled ventilation; however, unlike proportional assist ventilation (PAV) or Neurally Adjusted Ventilatory Assist (NAVA), PRVC has a counterintuitive characteristic where increased patient effort actually reduces ventilator support, potentially increasing patient work of breathing and causing dyssynchrony, though it offers benefits including guaranteed tidal volume delivery and reduced clinician intervention requirements.

PRVC combines pressure-controlled ventilation's decelerating flow pattern with volume-guaranteed delivery. The ventilator performs a systematic process: initial test breathes at minimum pressure calculate patient compliance, then adjusts inspiratory pressure by 1-3 cm H2O increments to achieve target tidal volume while staying 5 cm below upper pressure limits. This guarantees tidal volume for CO2 elimination while minimizing barotrauma risk. Volume support ventilation operates as a spontaneous mode where the ventilator adjusts inspiratory pressure to deliver preset tidal volume at the lowest possible pressure, automatically decreasing support as patient strength improves. Both modes represent inter-breath variation approaches that adapt to patient mechanics while maintaining safety features.

For ARDS patients, pressure control mode is preferred over volume control because it provides inherent safety limits against excessive pressure delivery. Lung protective ventilation requires tidal volumes of only 4-6 ml/kg of ideal body weight, significantly lower than normal lung settings. This reduced tidal volume prevents ventilator-induced lung injury. To maintain adequate minute ventilation (6-7 L/min) with low tidal volumes, respiratory rates must be increased to 20-35 breaths per minute. This strategy accepts some degree of hypercapnia (permissive hypercapnia) in exchange for protecting fragile lung tissue.

Pressure control ventilation delivers a preset pressure to the lungs, with tidal volume determined by patient compliance and resistance. This lung-protective strategy targets pressure rather than volume, making it ideal for patients with low compliance, particularly those with ARDS. Key advantages include improved oxygenation, better ventilation distribution through inspiratory pause, and precise control of mean airway pressure and plateau pressure. The mode is always pressure-limited and time-cycled, requiring specific settings including pressure control limit, respiratory rate, inspiratory time, PEEP, FiO2, and sensitivity. Unlike volume control, flow rate is patient-determined, making the mode highly responsive to inspiratory demands.

Lung protective ventilation in ARDS involves using low tidal volumes and low plateau pressures to prevent ventilator-induced lung injury. Prone positioning for 12-16 hours daily improves oxygenation by reducing cardiac compression on dependent lung regions and decreasing abdominal organ pressure on the diaphragm. Lung recruitment maneuvers help open collapsed alveoli and improve gas exchange.

This comprehensive section presents the evidence-based lung protective ventilation strategy for ARDS patients. The core strategy involves: low tidal volume ventilation (4-6 ml/kg ideal body weight), limiting plateau pressure to <30 cm H2O, and using moderate PEEP to keep alveoli open throughout the respiratory cycle. This accepts mild hypercapnia (permissive hypercapnia) rather than risking lung injury from aggressive ventilation. The ARDS Network study demonstrated this strategy significantly reduces mortality. Key arterial blood gas goals include pH ~7.3, PaO2 55-80 mmHg, and avoiding attempts to normalize blood gases which would require higher tidal volumes and increase lung injury risk. The section addresses managing obstructive lung disease where dynamic hyperinflation causes gas trapping and auto-PEEP, increasing work of breathing and reducing cardiac output. Prone positioning improves oxygenation by homogenizing ventilation-perfusion relationships, requiring at least 16 hours daily for benefit. Advanced ventilation modes including Bilevel Positive Airway Pressure (BIPAP) cycling between two pressure levels, and Pressure Regulated Volume Control (PRVC) which automatically adjusts pressure between breaths to achieve target tidal volume. Driving pressure (plateau minus PEEP) represents lung stress and should guide PEEP adjustments. Mean airway pressure reflects average pressure over the entire respiratory cycle and influences oxygenation and hemodynamics. Neuromuscular blockers may benefit severe ARDS patients during the first 48 hours to reduce dyssynchrony and allow uniform lung protective ventilation.

Two main ARDS phenotypes exist: EL (easy lung) - well-ventilated tissue predominates, responds to oxygen via mask, does not require recruitment or high PEEP; AJ (difficult lung) - predominantly non-aerated tissue, high recruitability, responds to recruitment and high PEEP. CT imaging during mechanical ventilation (with PEEP of 5 cm H2O) can distinguish these phenotypes. The 'wet sponge' model describes lung behavior: at the top, alveoli maintain air content; as one moves downward, moisture saturation increases due to gravity. In direct ARDS, the lung resembles a wet sponge. For these patients, prone positioning works effectively. Alveoli are classified into four types: normally ventilated, requiring 10-20 cm H2O pressure, requiring 20-30 cm H2O pressure, and filled with viscous secretum that cannot be opened. Recruitment maneuvers work by applying PEEP to open alveoli that collapse on exhalation. Patients are classified as recruitment-responsive (PEEP of 45 cm H2O ventilates nearly entire lung) or non-responsive (minimal effect). Experimental studies show fundamental differences between direct and indirect ARDS. In direct ARDS, lung damage is persistent and non-responsive to recruitment and PEEP. In indirect ARDS, lung damage is less severe, responds to recruitment and PEEP, and shows better recovery. Direct ARDS carries higher risk of ventilator-induced lung injury. The 'baby lung' concept describes the portion of lung tissue available for ventilation in ARDS patients. In severe ARDS, part of the lung is excluded from gas exchange, while the remaining ventilated portion is small and susceptible to damage. Lung-protective ventilation uses smaller tidal volumes (6-8 ml/kg ideal body weight) and higher PEEP to minimize ventilator-induced lung injury. The ARDSnet trial demonstrated that using tidal volumes of 6 ml/kg ideal body weight (compared to 12 ml/kg) significantly improved survival. Maximum damage occurs at the boundary between collapsed and non-collapsed alveoli. Tidal volume should be calculated based on ideal body weight (height in cm minus 100 for men, 105 for women), not actual body weight.

Pressure Support Mode (PSV) is a spontaneous mode where the patient triggers all breaths, and the ventilator delivers a set pressure to assist each breath. This mode is used during weaning from mechanical ventilation. The standard weaning protocol follows a sequence: Continuous Mandatory Ventilation (CMV) → Assist Control Mode (ACMV) → Synchronized Intermittent Mandatory Ventilation (SIMV) → Pressure Support Mode (PSV). Non-invasive ventilation includes CPAP (maintains equal pressure during inspiration and expiration) and BiPAP (provides different pressures for inspiration and expiration). The I:E ratio (Inspiratory:Expiratory ratio) determines the proportion of time spent in inspiration versus expiration, typically 1:2 in normal breathing, adjusted to 1:2.5 or 1:3 in COPD with CO2 retention.

Weaning from mechanical ventilation follows a progressive protocol: starting with fully controlled ventilation, transitioning to SIMV (Synchronized Intermittent Mandatory Ventilation) where the patient performs some breathing between mandatory breaths. The mandatory volume is gradually reduced while the patient's spontaneous breathing is monitored. The respiratory rate is decreased to allow the patient to take over more ventilation. When the patient can maintain adequate ventilation with minimal support, they can be transitioned to PSV (Pressure Support Ventilation) or spontaneous breathing.

Pressure support ventilation allows patient-initiated breaths with ventilator pressure assistance. Clinicians set pressure support level and PEEP; tidal volume, rate, and minute ventilation become patient-dependent. Cycling occurs at 25% of peak expiratory flow. This mode preserves respiratory muscle function and serves as a bridge between full ventilatory support and spontaneous breathing. The typical sequence progresses from controlled ventilation to assisted-control to pressure support before extubation, individualized to each patient's condition.

Pressure support ventilation is a mode where the patient triggers each breath and the ventilator delivers preset pressure to assist inspiration. The patient generates the respiratory cycle, and the ventilator provides additional pressure to reduce work of breathing. This mode is used during weaning to transition from full support to spontaneous breathing. Standard settings include pressure support of 12-13 cm H2O, FiO2 to maintain saturation 92-94%, and PEEP 5 cm H2O. Weaning involves gradual reduction of support while monitoring for fatigue (increased respiratory rate, retractions, tachycardia). The ventilator provides backup ventilation during apneas to prevent respiratory failure. Weaning is difficult in patients with prolonged sedoanalgesia, delirium, or underlying respiratory pathology.

The weaning process involves gradual reduction of ventilator support using Pressure Support Ventilation (PSV), starting at 80% of inspiratory pressure. PSV should be reduced while monitoring tidal volume and respiratory rate; if RR increases significantly or tidal volume drops, reduction should be reversed. Spontaneous Breathing Trial (SBT) should be performed daily in the morning using low-level PSV or CPAP for 30 minutes to 2 hours. During SBT, assess for air trapping (wheezing) and cardiac issues. Successful SBT followed by extubation has 13% reintubation rate, while direct extubation without SBT increases failure rate to 40%.