4.2 Mechanical Ventilation Modes & Settings

Principles of Mechanical Ventilation

Mechanical ventilation provides life-saving respiratory support by maintaining gas exchange and reducing the work of breathing in patients with acute respiratory failure. A fundamental understanding of ventilator modes, settings, and the complex interaction between the patient and the machine is essential for the Acute Care Nurse Practitioner. The primary goals are to optimize oxygenation and ventilation, unload the respiratory muscles, and minimize ventilator-induced lung injury (VILI).

The ventilator breath is defined by four distinct phases: the trigger variable (what initiates the breath, typically time, pressure, or flow), the target or limit variable (what regulates gas flow during inspiration, such as a set volume or pressure limit), the cycle variable (what terminates inspiration and allows exhalation, such as volume, time, or flow), and the baseline variable (the pressure maintained during exhalation, i.e., Positive End-Expiratory Pressure or PEEP).

Basic Ventilator Settings

Regardless of the chosen mode, several key parameters must be set:

• Fraction of Inspired Oxygen (FiO2): The percentage of oxygen delivered in the inspired gas mixture (21% to 100%). The goal is generally to maintain SpO2 > 92% or PaO2 > 60 mm Hg using the lowest possible FiO2 to prevent oxygen toxicity and absorption atelectasis.
• Respiratory Rate (RR or 'f'): The number of mandatory breaths delivered per minute. Along with tidal volume, it determines minute ventilation (VE = RR × Vt) and thus PaCO2 clearance.
• Tidal Volume (Vt): The volume of gas delivered with each mandatory breath, typically set at 6-8 mL/kg of predicted body weight (PBW). In ARDS, lung-protective ventilation targets 4-6 mL/kg PBW.
• Positive End-Expiratory Pressure (PEEP): The positive pressure maintained in the airways at the end of exhalation. PEEP prevents alveolar collapse (atelectasis), increases functional residual capacity (FRC), and improves oxygenation by redistributing lung water and decreasing intrapulmonary shunting. Physiological PEEP is 5 cm H2O; higher levels are used therapeutically in ARDS or severe hypoxemia.
• Inspiratory Flow Rate (Vmax) and Flow Pattern: Determines how fast the tidal volume is delivered. Normal rates are 40-60 L/min. A higher flow rate decreases inspiratory time and increases expiratory time (I:E ratio), which is crucial in obstructive lung diseases (e.g., asthma, COPD) to prevent air trapping.

Primary Modes of Mechanical Ventilation

Ventilator modes dictate how the machine and patient interact. They are broadly classified by how the breaths are controlled (volume vs. pressure) and the degree of patient spontaneity allowed.

Volume-Controlled Modes

In volume-controlled ventilation, the tidal volume (Vt) is guaranteed, ensuring a consistent minute ventilation regardless of changes in lung compliance or airway resistance. However, the airway pressure varies and can reach dangerously high levels if compliance drops.

1. Assist-Control Volume Ventilation (AC-VC) / Continuous Mandatory Ventilation (CMV): The ventilator delivers a preset tidal volume at a preset minimum respiratory rate. If the patient triggers a spontaneous breath, the ventilator delivers the full preset tidal volume. Every breath, whether patient-triggered or machine-initiated, is identical in volume.

   • Advantage: Guarantees minute ventilation and fully rests respiratory muscles.
   • Disadvantage: Risk of hyperventilation and respiratory alkalosis if the patient is tachypneic. High risk of elevated peak pressures.

2. Synchronized Intermittent Mandatory Ventilation (SIMV): The ventilator delivers a preset number of mandatory breaths (with a set Vt) synchronized with the patient's spontaneous efforts. In between the mandatory breaths, the patient can take unassisted spontaneous breaths with their own tidal volume.

   • Advantage: Less interference with normal cardiovascular hemodynamics; prevents respiratory muscle atrophy by allowing some spontaneous work.
   • Disadvantage: Spontaneous breaths may be too shallow, leading to increased work of breathing if not supported with pressure support. Often used as a weaning mode, though evidence does not support it over pressure support weaning.

Pressure-Controlled Modes

In pressure-controlled ventilation, a set inspiratory pressure is delivered for a set inspiratory time. The tidal volume is variable and depends on lung compliance and airway resistance.

1. Assist-Control Pressure Ventilation (AC-PC): The ventilator delivers a preset inspiratory pressure at a preset rate. All triggered breaths (mandatory or patient-initiated) receive the same set pressure support.

   • Advantage: Limits peak airway pressures, reducing the risk of barotrauma. Improves oxygenation by providing a decelerating flow pattern.
   • Disadvantage: Tidal volume and minute ventilation are not guaranteed. Sudden decreases in compliance (e.g., pneumothorax, bronchospasm) will result in hypoventilation.

2. Pressure Support Ventilation (PSV): A purely spontaneous mode. The patient controls the respiratory rate, inspiratory time, and tidal volume. The ventilator provides a preset positive pressure during inspiration to overcome the resistance of the endotracheal tube and assist the patient's effort.

   • Use: Primarily used for weaning trials and for patients who are stable and require minimal support.
   • Requirement: The patient must have an intact respiratory drive and stable hemodynamics.

Advanced Modes

• Airway Pressure Release Ventilation (APRV): A mode that provides continuous positive airway pressure (CPAP) at a high level (Phigh) for a prolonged period (Thigh), with brief, intermittent releases to a lower pressure (Plow) for a short duration (Tlow). This promotes alveolar recruitment and allows for spontaneous breathing throughout the cycle. It is often used as a rescue mode for severe ARDS.
• Pressure-Regulated Volume Control (PRVC): A dual-control mode that attempts to provide the benefits of both volume and pressure control. The clinician sets a target tidal volume, and the ventilator automatically adjusts the inspiratory pressure breath-by-breath to deliver that volume using the lowest possible pressure limit.

Troubleshooting the Ventilator: Alarm Interpretation

A systematic approach to alarms is critical for patient safety.

• High-Pressure Alarm (Peak Inspiratory Pressure > Limit): Indicates increased resistance or decreased compliance.
  • Causes: Secretions in the airway, biting the endotracheal tube, bronchospasm, tension pneumothorax, mainstem intubation, or worsening pulmonary edema/ARDS.
• Low-Pressure or Low-Volume Alarm: Indicates a leak or disconnection.
  • Causes: Patient disconnection from the ventilator circuit, a leak around the endotracheal tube cuff, or an open exhalation valve.

Weaning from Mechanical Ventilation

Weaning is the process of discontinuing ventilatory support. It begins with daily spontaneous awakening trials (SATs) followed by Spontaneous Breathing Trials (SBTs).

• SBT Protocol: The patient is placed on minimal support, typically PSV (e.g., 5-8 cm H2O) with a PEEP of 5 cm H2O, or a T-piece, for 30 to 120 minutes.
• Rapid Shallow Breathing Index (RSBI): Calculated as Respiratory Rate / Tidal Volume in liters (f/Vt). An RSBI < 105 measured during a spontaneous breath without pressure support is highly predictive of successful extubation.
• Extubation Criteria: The patient must successfully pass the SBT, have a strong cough, minimal secretions, an intact gag reflex, adequate mentation to protect their airway, and resolution of the underlying condition that necessitated intubation.