2.3 Mechanical Ventilation Modes and Settings

Mechanical ventilation provides life-saving respiratory support but carries significant risks, including ventilator-induced lung injury (VILI) and ventilator-associated pneumonia (VAP). Understanding the interplay of ventilator settings is crucial for the critical care pharmacist managing sedation, analgesia, and neuromuscular blockade.

Basic Ventilator Settings

Regardless of the specific mode, several core parameters are configured on the ventilator:

• Fraction of Inspired Oxygen (FiO2): The percentage of oxygen delivered. Room air is 21%. The goal is usually an SpO2 > 88-92% or PaO2 > 60 mmHg while minimizing FiO2 (ideally < 60%) to prevent oxygen toxicity.
• Positive End-Expiratory Pressure (PEEP): The pressure maintained in the lungs at the end of exhalation. Normal physiological PEEP is ~5 cm H2O. Higher PEEP levels (e.g., 10-20 cm H2O) are used to stent open collapsed alveoli (recruitment), improving oxygenation and compliance.
• Respiratory Rate (RR) or Frequency: The set number of breaths delivered per minute. Controls ventilation and PaCO2 clearance.
• Tidal Volume (Vt): The volume of gas delivered per breath (in volume-controlled modes). Typically dosed based on predicted body weight (PBW), not actual body weight, at 6-8 mL/kg.
• Peak Inspiratory Pressure (PIP): The maximum pressure reached in the airways during inspiration.

Primary Modes of Mechanical Ventilation

Ventilator modes dictate how the machine initiates, sustains, and terminates a breath, and how it interacts with the patient's spontaneous efforts.

Assist-Control (AC) or Continuous Mandatory Ventilation (CMV)

In AC modes, the ventilator delivers a guaranteed minimum minute ventilation. The machine provides a set number of breaths, but if the patient triggers an additional breath, the ventilator delivers a full assisted breath at the set volume or pressure.

1. Volume Control (AC-VC): The clinician sets the Tidal Volume (Vt). The ventilator delivers this exact volume, meaning the Peak Inspiratory Pressure (PIP) will vary based on the patient's airway resistance and lung compliance.
   • Risk: Barotrauma if lung compliance suddenly decreases, causing pressures to spike.
2. Pressure Control (AC-PC): The clinician sets the Inspiratory Pressure. The ventilator delivers this exact pressure, meaning the Tidal Volume will vary.
   • Risk: Volutrauma or hypoventilation if lung compliance changes, resulting in tidal volumes that are too high or too low.

Synchronized Intermittent Mandatory Ventilation (SIMV)

SIMV delivers a set number of mandatory breaths synchronized with the patient's effort. However, unlike AC, if the patient takes spontaneous breaths between the mandatory breaths, the ventilator does not provide full support; the patient dictates the volume of those spontaneous breaths (though pressure support is usually added to help overcome the resistance of the endotracheal tube). SIMV is often used as a weaning mode.

Pressure Support Ventilation (PSV)

This is a spontaneous mode. The patient dictates the respiratory rate and inspiratory time. The ventilator simply provides a set positive pressure during inspiration to assist the patient's effort, increasing the tidal volume and decreasing the work of breathing. The patient must have a consistent, reliable respiratory drive to use PSV safely. It is the primary mode used for Spontaneous Breathing Trials (SBTs).

Pressure Regulated Volume Control (PRVC)

PRVC is a dual-control mode. The clinician sets a target Tidal Volume. The ventilator delivers pressure-controlled breaths, continuously adjusting the inspiratory pressure breath-by-breath to achieve the target volume while using the lowest possible pressure. It offers the safety of volume control with the comfort and flow dynamics of pressure control.

The ARDSNet Protocol: Lung-Protective Ventilation

Acute Respiratory Distress Syndrome (ARDS) is characterized by severe inflammation, pulmonary edema, and collapsed alveoli. Traditional ventilation strategies often worsened ARDS by causing overdistension of the healthy alveoli.

The landmark ARDSNet trial established the standard of care for "lung-protective ventilation":

• Low Tidal Volumes: 4-6 mL/kg of predicted body weight (PBW). This prevents overstretching and barotrauma/volutrauma.
• Plateau Pressure Goal: Keep plateau pressure (the pressure in the alveoli at the end of inspiration) < 30 cm H2O.
• Permissive Hypercapnia: Because tidal volumes are low, PaCO2 often rises. This respiratory acidosis is tolerated (usually maintaining pH > 7.20-7.25) to protect the lungs.
• High PEEP: Used to recruit and maintain open alveoli, guided by standardized PEEP/FiO2 tables.

Liberation from Mechanical Ventilation

Weaning involves assessing the patient's readiness to breathe independently. This is typically evaluated via a Spontaneous Breathing Trial (SBT), where the patient is placed on minimal support (e.g., PSV of 5/5 cm H2O) for 30-120 minutes.

Rapid Shallow Breathing Index (RSBI)

The RSBI is a classic predictor of successful extubation.

• Formula: RSBI = Respiratory Rate (breaths/min) / Tidal Volume (Liters)
• Interpretation: An RSBI < 105 suggests a high likelihood of successful extubation. Rapid, shallow breathing (high rate, low volume) yields a high RSBI, indicating respiratory muscle fatigue and likely failure.