3.3 Mechanical Ventilation: Modes, Settings, and Monitoring
Key Takeaways
- Tidal volume must always be calculated using Ideal Body Weight (6-8 mL/kg for standard, 4-6 mL/kg for ARDS) to avoid volutrauma and barotrauma.
- Peak Inspiratory Pressure (PIP) reflects resistance and compliance, whereas Plateau Pressure (Pplat) measures alveolar pressure; Pplat must be kept under 30 cm H2O.
- An elevated PIP with a normal Pplat indicates an airway resistance issue (e.g., tube kink or secretions), while elevation in both indicates a compliance issue.
- The DOPES mnemonic (Displacement, Obstruction, Pneumothorax, Equipment, Stacked breaths/Auto-PEEP) is the standard algorithm for troubleshooting ventilator alarms.
- Auto-PEEP is managed by allowing complete exhalation, decreasing respiratory rate, increasing expiratory time (1:3 or 1:4 I:E ratio), and matching vent PEEP.
Last updated: July 2026
Mechanical Ventilation: Modes, Settings, and Monitoring
Managing mechanical ventilation during transport requires a deep understanding of pulmonary physiology, ventilator modes, and troubleshooting techniques. The primary goals of mechanical ventilation are to optimize oxygenation and ventilation, protect the lung parenchyma from ventilator-induced lung injury (VILI), and maintain hemodynamic stability.
Key Terminology and Settings
- Tidal Volume (Vt): The volume of gas delivered with each breath. Vt must always be calculated based on the patient's Ideal Body Weight (IBW) rather than actual body weight. Standard settings are 6 to 8 mL/kg of IBW for most patients, and 4 to 6 mL/kg of IBW for patients with Acute Respiratory Distress Syndrome (ARDS) to prevent volutrauma.
- Devine Formula for IBW:
- Male: 50 + 2.3 * (Height in inches - 60)
- Female: 45.5 + 2.3 * (Height in inches - 60)
- Devine Formula for IBW:
- Respiratory Rate (RR): The number of breaths delivered per minute (typically 12–20 bpm).
- Minute Ventilation (Ve): Calculated as Vt * RR. Normal Ve is 5 to 8 L/min. It directly correlates with carbon dioxide clearance and pH.
- Positive End-Expiratory Pressure (PEEP): The pressure maintained in the alveoli at the end of expiration. PEEP prevents alveolar collapse (atelectasis) and recruits closed alveoli, improving oxygenation and V/Q matching. Standard settings start at 5 cm H2O and can be titrated up to 15–20 cm H2O in severe ARDS.
- Fraction of Inspired Oxygen (FiO2): The percentage of oxygen in the delivered gas (21% to 100%). Titrate to maintain target oximetry (92–96% for standard patients, 88–92% for ARDS/COPD).
- Inspiratory to Expiratory (I:E) Ratio: The ratio of the duration of inspiration to expiration, normally set at 1:2. In obstructive lung disease (asthma/COPD), it is lengthened to 1:3 or 1:4 to allow complete exhalation.
Ventilator Modes
Modern transport ventilators offer multiple modes, categorized by what triggers, limits, and cycles the breath:
| Mode | Control Variable | Trigger | Breath Characteristics | Clinical Use |
|---|---|---|---|---|
| Volume Control-Assist Control (VC-AC) | Volume (preset Vt) | Time or Patient | Preset Vt is delivered for every breath (machine or patient-initiated). | Standard mode for paralyzed/sedated transport patients. Risk of respiratory alkalosis if patient hyperventilates. |
| Synchronized Intermittent Mandatory Ventilation (SIMV) | Volume or Pressure | Time or Patient | Preset rate and Vt. Spontaneous breaths between mandatory breaths are not assisted, though they can be pressure supported. | Historically used for weaning. Can increase work of breathing and cause patient-ventilator dyssynchrony. |
| Pressure Control-Assist Control (PC-AC) | Pressure (preset Pi) | Time or Patient | Preset inspiratory pressure is delivered. Vt is variable and depends on lung compliance and airway resistance. | Lung-protective strategy in severe ARDS. Avoids high peak pressures. |
| Pressure Support Ventilation (PSV) | Pressure | Patient only | Patient triggers all breaths. The ventilator provides a preset pressure support to overcome circuit resistance. | Used for spontaneously breathing patients, weaning trials, or non-invasive ventilation (BiPAP). |
Airway Pressures and Compliance Monitoring
Monitoring airway pressures is crucial for detecting pulmonary changes and preventing barotrauma:
- Peak Inspiratory Pressure (PIP): The maximum pressure recorded during inspiration. PIP is the sum of resistive pressure (airway resistance, endotracheal tube diameter) and elastic pressure (alveolar compliance). Keep PIP < 35 cm H2O.
- Plateau Pressure (Pplat): The pressure within the alveoli, measured during an inspiratory hold maneuver when gas flow stops. Pplat directly reflects static compliance of the lungs. The target Pplat is < 30 cm H2O to prevent barotrauma and VILI.
- Interpretation:
- Elevated PIP with Normal Pplat: Indicates an airway resistance problem (e.g., tube kink, secretions, patient biting the tube, bronchospasm).
- Elevated PIP with Elevated Pplat: Indicates a lung compliance problem (e.g., mainstem intubation, pneumothorax, pulmonary edema, ARDS, worsening bronchospasm causing auto-PEEP).
Troubleshooting Ventilator Alarms
When a ventilator alarm sounds or a patient acutely desaturates, the clinician must systematically troubleshoot using the DOPES mnemonic:
- D – Displacement: Check for ETT dislodgement, accidental extubation, or right mainstem bronchus intubation. Confirm with EtCO2 and bilateral breath sounds.
- O – Obstruction: Check for secretions blocking the tube, patient biting the tube, or a kinked circuit. Suction the patient and place a bite block.
- P – Pneumothorax: Look for signs of a tension pneumothorax (asymmetrical chest rise, decreased breath sounds on the affected side, tracheal deviation, hemodynamic collapse). Perform immediate needle decompression or finger thoracostomy.
- E – Equipment Failure: Check for ventilator malfunction, disconnected circuit, oxygen source depletion, or leaks. If equipment failure is suspected, disconnect the patient from the ventilator and manually ventilate using a BVM with 100% oxygen.
- S – Stacked Breaths (Auto-PEEP): Common in obstructive diseases (asthma/COPD) where expiratory time is insufficient, leading to progressive air trapping. This increases intrathoracic pressure, decreases venous return, and causes severe hypotension or cardiac arrest.
- Management of Auto-PEEP: Disconnect the ventilator circuit from the ETT and allow the patient to fully exhale (manually compress the chest if necessary). Once hemodynamics stabilize, adjust ventilator settings: decrease the respiratory rate, decrease the tidal volume, increase the expiratory time (lengthen I:E ratio to 1:3 or 1:4), and increase inspiratory flow rate.
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Test Your Knowledge
While transporting an intubated patient on a ventilator in Volume Control-Assist Control (VC-AC) mode, the high pressure alarm sounds. You note that the Peak Inspiratory Pressure (PIP) is 42 cm H2O, but the Plateau Pressure (Pplat) remains normal at 22 cm H2O. Which of the following conditions is the most likely cause of this pressure discrepancy?
A
B
C
D
Test Your Knowledge
An intubated patient with severe status asthmaticus begins to exhibit sudden, severe hypotension during transport. You note that the expiratory flow curve on the ventilator waveform does not return to baseline before the next breath begins. What is the immediate first action you should take?
A
B
C
D