What is the basic principle of mechanical ventilation and how to manage a patient on a ventilator?

Basic Principles and Management of Mechanical Ventilation

Core Principle of Mechanical Ventilation

The primary purpose of mechanical ventilation is to decrease work of breathing by providing external support for gas exchange when the patient's respiratory system cannot adequately maintain oxygenation and ventilation. 1

Mechanical ventilation works by moving gas toward and from the lungs through an external device connected directly to the patient, using intermittent positive pressure rather than the normal negative pressure generated by spontaneous breathing. 2, 3

Indications for Initiating Mechanical Ventilation

Start mechanical ventilation when patients meet these criteria:

• Refractory hypoxemia: PaO₂ < 60 mmHg despite high-flow oxygen (FiO₂ ≥ 0.40) 4
• Severe respiratory distress: Respiratory rate > 35 breaths/min 4
• Inadequate ventilation: Persistent or increasing hypercapnia with exhaustion 4
• Impaired consciousness: Inability to protect airway (coma, severe mental status depression) 4
• Vital capacity: < 15 mL/kg 4

Pre-Intubation Considerations

• Place patients in semi-recumbent position (head of bed 30-45°) unless hemodynamically unstable to reduce aspiration risk 4
• Consider non-invasive ventilation first if staff is adequately trained and patient has intact consciousness 4
• Use rapid sequence intubation with the largest endotracheal tube available (typically 8-9 mm for adults) to decrease airway resistance 4

Initial Ventilator Settings

Mode Selection

Start with Assist-Control (AC) ventilation for most patients requiring full ventilatory support. 5, 6

• AC mode delivers a preset number of mandatory breaths and allows patient-triggered breaths, with each triggered breath receiving the same tidal volume as mandatory breaths 5, 6
• AC prevents central apneas during sleep due to the backup respiratory rate, making it superior to pressure support ventilation (PSV) for initial support 6, 7
• Volume-controlled AC is preferred initially as it guarantees minute ventilation 5

Volume Control Settings

Set tidal volume at 6-8 mL/kg ideal body weight (not actual body weight) to minimize ventilator-induced lung injury. 4, 6, 3

Calculate ideal body weight:

• Men: 50 + 2.3 × (height in inches - 60) 4
• Women: 45.5 + 2.3 × (height in inches - 60) 4

Set respiratory rate at 10-12 breaths/min for adults (20-30 for children, 30 for neonates). 4

Pressure Settings

• Keep plateau pressure ≤ 30 cmH₂O to prevent barotrauma 4, 3
• Start with PEEP 5-8 cmH₂O and titrate based on oxygenation needs 4
• Target SpO₂ 88-94% (approximately PaO₂ 60 mmHg) 4, 6

FiO₂ Settings

• Start with FiO₂ 1.0 (100%) initially, then rapidly wean to maintain target SpO₂ 4
• Adjust FiO₂ before increasing PEEP when oxygenation is inadequate 4

Post-Intubation Management

Immediate Confirmation Steps

1. Inflate cuff to 20-30 cmH₂O immediately after intubation 4
2. Confirm placement with continuous waveform capnography - this is mandatory 4
3. Watch for equal bilateral chest wall expansion (auscultation is unreliable with PPE) 4
4. Record tube depth prominently 4
5. Obtain chest radiograph 4

Circuit Setup

• Attach high-quality HEPA filter to ventilator expiratory port to prevent aerosolization 4
• Use closed-suction systems if available 4
• Minimize circuit disconnections 4

Ongoing Monitoring

Monitor these parameters continuously:

• Peak inspiratory pressure and plateau pressure 3
• Tidal volume delivery 3
• Minute ventilation 8
• SpO₂ and end-tidal CO₂ 4
• Patient-ventilator synchrony 6

Common Pitfalls and Troubleshooting

Auto-PEEP (Breath Stacking)

This is especially dangerous in patients with severe bronchoconstriction (asthma, COPD). 4

If auto-PEEP develops:

• Disconnect patient from ventilator to allow passive exhalation 4
• Reduce respiratory rate or tidal volume 4
• Increase expiratory time (inspiratory:expiratory ratio 1:4 or 1:5 for asthma) 4
• Consider sedation or paralysis if patient-ventilator dyssynchrony persists 4

Acute Deterioration (DOPE Mnemonic)

Check for: 4

• Displacement of endotracheal tube
• Obstruction (mucous plug, kinking)
• Pneumothorax
• Equipment failure
• Auto-PEEP (additional consideration for obstructive disease) 4

Patient-Ventilator Asynchrony

• Adjust trigger sensitivity to prevent auto-triggering with chest compressions or missed triggers 4, 6
• Consider switching to pressure control if volume control causes excessive peak pressures 5
• Ensure adequate sedation initially 4

Special Considerations

Prone Positioning

• Improves oxygenation in approximately 65% of ARDS patients 4
• Maintain improvements for up to 18 hours after returning to supine 4
• If cardiac arrest occurs while prone, provide compressions over T7-T10 vertebrae until safe supination can be achieved 4

Fluid Management

Use judicious fluid resuscitation in patients with altered capillary permeability (sepsis, ARDS). 4

• Fluid restriction improves physiology and outcomes in established lung injury 4
• Consider albumin plus furosemide in hypo-oncotic patients with established lung injury 4

Avoiding Complications

• Insert nasogastric tube after intubation to minimize need for later interventions 4
• Avoid nebulized medications when possible; use metered-dose inhalers instead 4
• Minimize transport outside ICU; perform procedures at bedside when possible 4

Weaning Considerations

Screen for weanability early to minimize ventilator time and complications. 1

Prerequisites before considering weaning:

• PaO₂ > 55 mmHg on FiO₂ ≤ 0.40 4
• Maximum inspiratory pressure more negative than -30 cmH₂O 4
• Hemodynamic stability 4

Use T-tube trials rather than pressure support to assess true patient work of breathing before extubation. 1