Basics of Mechanical Ventilation
Mechanical ventilation is a life-saving intervention that delivers positive pressure breaths to patients with respiratory failure, requiring understanding of ventilator modes, respiratory mechanics, initial settings, and patient-ventilator interaction to prevent harm while supporting gas exchange 1, 2.
Core Operational Principles
Mechanical ventilators work by applying positive pressure to deliver breaths, fundamentally different from normal negative pressure breathing 3. The ventilator can either fully control breathing (mandatory ventilation) or assist patient-initiated breaths (triggered ventilation) 4.
Key Ventilator Modes
The three most commonly used modes are 2:
Assist-Control (AC): Delivers a set tidal volume or pressure for every breath, whether patient-triggered or machine-triggered. If the patient doesn't initiate a breath within the set time, the ventilator delivers one automatically.
Synchronized Intermittent Mandatory Ventilation (SIMV): Provides a minimum number of mandatory breaths synchronized with patient effort, allowing spontaneous breaths between mandatory ones.
Pressure Support Ventilation (PSV): Patient-triggered mode that augments each spontaneous breath with a preset pressure level, reducing work of breathing while maintaining patient control.
Initial Ventilator Settings Strategy
When initiating mechanical ventilation 1, 2:
Tidal Volume: Start with 6-8 mL/kg of predicted body weight (lower for ARDS) Respiratory Rate: 12-20 breaths/minute, adjusted for target pH and PaCO2 FiO2: Begin at 100% then titrate down to maintain SpO2 88-95% PEEP: Start at 5 cm H2O, then adjust based on oxygenation needs
ARDS-Specific Considerations
For patients with moderate-to-severe ARDS (PaO2/FiO2 < 200), use higher PEEP levels rather than lower PEEP to reduce mortality 5. The individual patient data meta-analysis showed adjusted relative risk of 0.90 (95% CI, 0.81-1.00) for mortality reduction with higher PEEP in this population, though no benefit exists for mild ARDS 5.
Consider recruitment maneuvers in ARDS patients as they reduce mortality (RR 0.81,95% CI 0.69-0.95) and decrease need for rescue therapy 5. However, exercise caution in hypovolemic patients due to risk of transient hypotension 5.
Respiratory Mechanics Monitoring
The ventilator provides diagnostic data 2:
- Static Compliance: Measure of lung and chest wall stiffness (normal 50-100 mL/cm H2O)
- Airway Resistance: Indicates obstruction or bronchospasm (normal 5-10 cm H2O/L/sec)
- Plateau Pressure: Should remain <30 cm H2O to prevent barotrauma
- Driving Pressure: Difference between plateau and PEEP; lower is better
Critical Pitfalls to Avoid
Ventilator-induced lung injury: Excessive tidal volumes, high plateau pressures (>30 cm H2O), and inadequate PEEP cause alveolar damage and worsen outcomes 5, 1.
Patient-ventilator dyssynchrony: Occurs when patient effort conflicts with ventilator delivery, causing distress and prolonged ventilation. Monitor waveforms and adjust trigger sensitivity, flow rates, and cycling criteria 1, 4.
Hemodynamic compromise: Positive pressure reduces venous return and cardiac output, especially with high PEEP or in hypovolemic patients. Monitor blood pressure closely during ventilator adjustments 5.
Barotrauma: While recruitment maneuvers don't significantly increase pneumothorax risk (RR 0.84,95% CI 0.46-1.55), remain vigilant for sudden deterioration 5.
Patient-Ventilator Interaction
Understanding the trigger, limit, and cycle variables is essential 4:
- Trigger: What initiates the breath (time, pressure, or flow)
- Limit: Maximum pressure or volume during breath delivery
- Cycle: What terminates the breath (time, volume, or flow)
Modern ventilators are predominantly patient-triggered, requiring nurses and physicians to recognize when settings don't match patient needs 6, 4.
Effect on Other Organ Systems
Mechanical ventilation affects kidney function through multiple mechanisms 1:
- Reduced cardiac output decreases renal perfusion
- Increased intrathoracic pressure reduces venous return
- Neurohormonal activation affects fluid balance
Monitor urine output and renal function closely in ventilated patients.
When Ventilation Becomes Toxic
Mechanical ventilation should be withdrawn when clinically appropriate 2. Prolonged unnecessary ventilation increases pneumonia risk, causes ventilator-associated diaphragm dysfunction, and prolongs ICU stay. Daily spontaneous breathing trials identify patients ready for liberation from mechanical support.