Comprehensive Overview of Mechanical Ventilators and Ventilation Modes
Fundamental Principles of Mechanical Ventilation
Mechanical ventilators deliver respiratory support by controlling either pressure, volume, or flow to maintain adequate gas exchange while minimizing ventilator-induced lung injury. 1
Basic Ventilator Classification
Ventilators operate through three primary control mechanisms: 1
- Pressure-based ventilators: Set inspiratory pressure is delivered, with resulting tidal volume determined by lung compliance, airway resistance, and chest wall mechanics 1
- Volume-based ventilators: Set tidal volume is delivered, with the ventilator generating whatever pressure is necessary to achieve this volume 1, 2
- Flow-based ventilators: Control inspiratory flow patterns, offering more informative monitoring of leakage, airflow obstruction, and circuit problems 1
Critical Alarm Systems
All ventilators require comprehensive alarm systems to ensure patient safety: 1
- Low pressure alarms: Detect disconnection or excessive leakage preventing achievement of set pressure
- High pressure alarms: Warn of excessively high pressures on volume-controlled ventilators, indicating worsening lung compliance or obstruction
- Flow alarms: Most informative, detecting changing leakage, worsening airflow obstruction, or partially occluded tubing
Major Ventilation Modes: Mechanisms and Clinical Applications
1. Controlled Mechanical Ventilation (CMV)
CMV provides complete ventilatory support with no patient effort required, delivering preset breaths at fixed intervals regardless of patient respiratory drive. 1
Mechanism:
- Inflation pressure or tidal volume is preset, along with respiratory frequency and breath timing 1
- In pressure-controlled CMV: Resulting tidal volume depends on circuit resistance, airflow limitation, and thoracic compliance 1, 2
- In volume-controlled CMV: Tidal volume is fixed and pressure varies based on circuit compliance and thoracic mechanics 1, 2
When to Use:
- Immediately after intubation when complete ventilatory support is needed 2
- Patients with absent respiratory drive (severe brain injury, deep sedation, neuromuscular blockade) 1
- Severe respiratory failure requiring full ventilatory control 3
Advantages:
- Guarantees minute ventilation regardless of patient effort 1
- Prevents central apneas 2
- Provides immediate complete support in critical situations 2
Disadvantages:
- Requires heavy sedation and often neuromuscular blockade 4, 3
- Increases risk of delirium, atelectasis, and diaphragm dysfunction 3
- No physiological variability in breathing pattern 3
- May cause patient-ventilator dyssynchrony if patient attempts to breathe 4
Common Pitfalls:
- Never use actual body weight for tidal volume calculations—always use predicted body weight 2
- Maintain plateau pressure ≤30 cmH₂O to prevent ventilator-induced lung injury 2
- Target 6 mL/kg predicted body weight in ARDS and sepsis-induced respiratory failure 2
2. Assist/Control Ventilation (AC or ACV)
The American Thoracic Society recommends starting with volume-cycled Assist/Control Ventilation when initiating mechanical ventilation, as it provides complete ventilatory support immediately after intubation and prevents central apneas. 2
Mechanism:
- Delivers a preset number of mandatory breaths per minute in absence of patient effort 1, 2
- Patient can trigger additional breaths, but all breaths (mandatory and triggered) deliver identical preset parameters 1, 2
- Ventilator has a "lock-out" period to prevent breath stacking from excessive triggering 1
- Patient-triggered breaths delay the next machine-determined breath, creating synchronization 1
When to Use:
- First-line mode when initiating mechanical ventilation 2
- Patients with variable respiratory drive who need guaranteed minute ventilation 1
- Acute respiratory failure requiring full support with some patient effort 2
- Patients at risk for central apneas during sleep 5
Advantages:
- Guarantees minimum minute ventilation while allowing patient triggering 1, 2
- Prevents central apneas, providing better sleep quality than pressure support 5
- Every breath is fully supported, reducing work of breathing 2
Disadvantages:
- Can cause respiratory alkalosis if patient triggers excessively at high tidal volumes 6
- May cause patient-ventilator dyssynchrony if lock-out period is improperly set 1
- Long expiratory time settings can create long lock-out periods, reducing patient tolerance 1
Common Pitfalls:
- Do not hyperventilate patients—this causes cerebral vasoconstriction, hemodynamic instability, and increased mortality 2
- Setting excessively long expiratory times creates long lock-out periods and poor tolerance 1
- At high tidal volumes (12 mL/kg), patients often develop respiratory alkalosis 6
3. Synchronized Intermittent Mandatory Ventilation (SIMV)
SIMV synchronizes patient-triggered breaths with machine-delivered breaths, allowing spontaneous breathing between mandatory breaths while preventing central apneas. 1, 5
Mechanism:
- Delivers preset number of mandatory breaths per minute 1, 5
- Patient can take spontaneous breaths between mandatory breaths 5
- Patient-triggered breaths delay the next mandatory breath to maintain synchronization 1
- Can be volume-controlled or pressure-controlled 5
- Sometimes called spontaneous/timed (S/T) or IE mode on NIV machines 1
When to Use:
- Patients at risk of hypoventilation or central apneas during sleep 5
- Weaning from full ventilatory support 5
- Patients who need guaranteed backup rate but can initiate some spontaneous breaths 5
Advantages:
- Prevents central apneas due to backup respiratory rate, making it preferable to pressure support for at-risk patients 5
- Allows gradual transition from full support to spontaneous breathing 5
- Provides better sleep quality than pressure support ventilation 5
Disadvantages:
- More complex than other modes, requiring understanding of synchronization windows 1
- Spontaneous breaths between mandatory breaths may be unsupported, increasing work of breathing 5
- Terminology varies between manufacturers, causing potential confusion 1, 2
4. Pressure Support Ventilation (PSV)
In Pressure Support Ventilation, the patient's respiratory effort triggers the ventilator both on and off, with the patient determining respiratory frequency and timing of each breath. 1
Mechanism:
- Patient effort triggers inspiratory pressure delivery 1
- Patient effort also cycles the ventilator off at end-inspiration 1
- Respiratory frequency and breath timing entirely patient-determined 1
- Most manufacturers incorporate backup rate of 6-8 breaths/minute if patient becomes apneic 1
When to Use:
- Patients with adequate respiratory drive who need partial support 1
- Weaning from mechanical ventilation 4
- Patients who can maintain their own respiratory pattern 1
Advantages:
- Maintains spontaneous breathing, improving gas exchange and systemic blood flow 4
- Reduces sedation requirements compared to controlled modes 4
- Allows physiological variability in breathing pattern 3
- Decreases duration of mechanical ventilation and ICU length of stay 4
Disadvantages:
- No ventilation occurs if patient fails to make respiratory effort 1
- Provides monotonous pattern of support despite respiration being normally dynamic 3
- May not prevent central apneas during sleep as effectively as AC or SIMV 5
- Degree of support still determined by ventilator, not patient demand 3
Common Pitfalls:
- Relying on PSV in patients with unreliable respiratory drive risks apnea 1
- At moderate tidal volumes (8 mL/kg) and low flow rates, PSV may not reduce work of breathing as effectively as pressure-controlled modes 6
5. Bi-Level Pressure Support (BiPAP/NIV)
The British Thoracic Society recommends bi-level pressure support ventilators as simpler to use, cheaper, and more flexible than other types currently available, used in the majority of randomized controlled trials of NIV. 1, 7
Mechanism:
- Combines pressure support with CPAP using two pressure levels: 1, 7
- IPAP (Inspiratory Positive Airway Pressure): Provides ventilation during inspiration
- EPAP (Expiratory Positive Airway Pressure): Recruits underventilated lung and offsets intrinsic PEEP
- EPAP vents exhaled gas through exhaust port 1
- Ventilator automatically compensates for air leakage inevitable with non-invasive interfaces 7
- Flow sensors detect changes in bias flow to trigger breaths 7
When to Use:
- Acute exacerbation of COPD with respiratory acidosis (pH <7.35) persisting despite maximal medical treatment 1
- Acute or acute-on-chronic hypercapnic respiratory failure from chest wall deformity or neuromuscular disease 1
- Decompensated obstructive sleep apnea with respiratory acidosis 1
- Cardiogenic pulmonary edema when CPAP unsuccessful 1
Advantages:
- Overcomes intrinsic PEEP in COPD through EPAP, offsetting recoil pressure of overinflated lungs 7
- Improves oxygenation through lung recruitment via EPAP 7
- Simpler, cheaper, and more flexible than ICU ventilators 1, 7
- Automatically compensates for air leaks 7
Disadvantages:
- Significant rebreathing potential exists, especially at low EPAP (3-5 cmH₂O) and high respiratory rates 7
- Can paradoxically worsen hypercapnia in tachypneic, anxious patients 7
- Patient-ventilator asynchrony from undetected inspiratory effort or excessive air leakage 7
- EPAP levels >5 cmH₂O rarely tolerated despite intrinsic PEEP potentially reaching 10-15 cmH₂O in severe COPD 7
Common Pitfalls:
- In tachypneic patients who fail to improve, consider rebreathing as cause of worsening hypercapnia 7
- When asynchrony cannot be resolved, switch to timed or assist-control mode 7
- Approximately 20-30% of patients with acute respiratory failure cannot be managed by NIV due to mask fit or asynchrony 1
- Overtightening headgear to reduce leakage causes skin ulceration, particularly over nasal bridge 1
6. Continuous Positive Airway Pressure (CPAP)
CPAP is employed in patients with acute respiratory failure to correct hypoxaemia by maintaining constant positive pressure throughout the respiratory cycle, recruiting underventilated lung similar to PEEP. 1
Mechanism:
- Maintains constant positive pressure throughout respiratory cycle 1
- Permits higher inspired oxygen content than other supplementation methods 1
- Increases mean airway pressure and improves ventilation to collapsed lung areas 1
- Unloads inspiratory muscles and reduces inspiratory work 1
- In COPD, offsets intrinsic PEEP, reducing ventilatory work and potentially lowering PaCO₂ 1
When to Use:
- Cardiogenic pulmonary edema with persistent hypoxia despite maximal medical treatment 1
- Chest wall trauma with persistent hypoxia despite adequate regional anesthesia and high-flow oxygen 1
- Diffuse pneumonia with hypoxia resistant to high-flow oxygen 1
- Obstructive sleep apnea without respiratory acidosis 1
Advantages:
- Improves oxygenation through lung recruitment 1
- Reduces work of breathing by unloading inspiratory muscles 1
- In COPD, can reduce PaCO₂ by offsetting intrinsic PEEP 1
Disadvantages:
- In hyperinflated patients with airflow obstruction, further lung volume increase may adversely affect inspiratory muscle function 1
- Does not provide ventilatory support—only maintains positive pressure 1
- Requires high flow capability (>60 L/min) in distressed COPD patients with high minute ventilation 1
Common Pitfalls:
- Patients with chest wall trauma treated with CPAP should be monitored in ICU due to pneumothorax risk 1
- In acute pneumonia, trials of CPAP should only occur in HDU or ICU settings due to high intubation risk 1
- Low-flow CPAP generators adequate for sleep apnea are insufficient for acute respiratory failure 1
7. Proportional Assist Ventilation (PAV)
PAV independently adjusts both flow (to counter resistance) and volume (to counter compliance), potentially improving patient comfort and success with acute NIV. 1
Mechanism:
- Adjusts flow assistance to counter airway resistance 1
- Adjusts volume assistance to counter lung/chest wall compliance 1
- Patient determines pattern and depth of ventilation 3
- Support level adapts to changing patient demand 3
When to Use:
- Patients requiring ventilatory support with preserved respiratory drive 3
- When patient-ventilator asynchrony is problematic with other modes 3
Advantages:
- May improve patient comfort and compliance with NIV 1
- Decreases patient-ventilator asynchrony 3
- Normalizes work of breathing 3
- Allows patient control of breathing pattern 3
Disadvantages:
- Clear pattern of clinical benefit not yet demonstrated 3
- Challenges monitoring patients with dynamic hyperinflation (auto-PEEP) 3
- Difficult to use in obstructive lung disease and with air leaks 3
- Requires sophisticated understanding by operators 1
8. Airway Pressure Release Ventilation (APRV)
APRV maintains lung volume to limit intratidal recruitment/derecruitment while permitting spontaneous breathing throughout the entire respiratory cycle. 8
Mechanism:
- Maintains high continuous positive airway pressure with brief releases 8
- Permits spontaneous breathing at any point in respiratory cycle 8
- Increases end-expiratory lung volume to recruit collapsed lung, especially juxtadiaphragmatic regions 8
When to Use:
- Acute lung injury and ARDS in trauma patients 8
- When conventional modes fail to maintain adequate oxygenation 8
- Patients requiring lung recruitment while maintaining spontaneous breathing 8
Advantages:
- Improves arterial oxygenation and ventilation-perfusion matching 8
- Reduces shunt and dead space ventilation 8
- Improves cardiac output and oxygen delivery 8
- Lower airway pressures compared to conventional modes 8
- Improves patient comfort and reduces sedation requirements 8
- May reduce ventilator days 8
Disadvantages:
- More complex mode requiring specialized training 8
- Limited availability on older ventilators 8
- Optimal settings not yet standardized 8
Essential Ventilator Specifications for Hospital Use
Minimum Requirements for NIV Ventilators: 1
Essential Features:
- Pressure controlled capability
- Pressure capability ≥30 cmH₂O
- Support inspiratory flows ≥60 L/min
- Assist-control and bi-level pressure support modes
- Rate capability ≥40 breaths/min
- Sensitive flow triggers
- Disconnection alarm
Desirable Features:
- Short, adjustable pressure rise time
- Adjustable inspiratory and expiratory triggers
- Adjustable inspiratory-expiratory ratio in assist-control mode
- Temporary alarm cancellation facility
- Internal battery with ≥1 hour power
- Accessible control panel with lock-out facility
- Simple control knobs
- LED/LCD displays
Critical Decision-Making Algorithm
Step 1: Determine Need for Invasive vs. Non-Invasive Ventilation
Before commencing NIV, decide whether patient is candidate for intubation if NIV fails—document this decision immediately. 1
- Intubate immediately if: Absent airway reflexes, severe upper airway obstruction, high oxygenation requirements, hemodynamic instability 1
- Consider NIV if: COPD exacerbation with pH <7.35, cardiogenic pulmonary edema, neuromuscular disease with hypercapnia 1
Step 2: Select Initial Mode Based on Clinical Scenario
For invasive mechanical ventilation:
- Start with volume-cycled Assist/Control at 6 mL/kg predicted body weight 2
- Target plateau pressure ≤30 cmH₂O 2
- For ARDS: Add prone positioning >12 hours/day 2
- For post-cardiac arrest: Target normocapnia (PaCO₂ 40-45 mmHg) 2
For non-invasive ventilation:
- COPD with respiratory acidosis: Bi-level pressure support 1, 7
- Cardiogenic pulmonary edema: CPAP first, then bi-level if unsuccessful 1
- Neuromuscular disease/chest wall deformity: Bi-level pressure support 1
Step 3: Monitor and Adjust
- Measure arterial blood gases after short interval to reassess need for ventilation 1
- If patient-ventilator asynchrony develops on pressure support, switch to assist-control or SIMV 5, 7
- If rebreathing suspected on bi-level support (worsening hypercapnia despite ventilation), increase EPAP or switch modes 7
Interface Selection for NIV
In acute settings, use full-face mask initially, changing to nasal mask after 24 hours as patient improves. 1
- Selection of different sizes of nasal masks, full-face masks, and nasal pillows should be available 1
- Mask fit is critical for comfort and effective ventilation 1
- Use barrier dressing from outset to reduce nasal bridge ulceration risk 1
- Avoid overtightening headgear—this exacerbates skin damage and reduces compliance 1
Modern Considerations and Emerging Evidence
Spontaneous Breathing During Mechanical Ventilation
Recent evidence questions the utility of heavy sedation, muscle paralysis, and complete mechanical control of ventilation. 4
- Maintaining spontaneous ventilatory effort improves gas exchange and systemic blood flow 4
- Lowering sedation levels decreases duration of mechanical ventilation, ICU length of stay, and hospitalization costs 4
- Use techniques that maintain rather than suppress spontaneous ventilatory effort, especially in severe pulmonary dysfunction 4
Pressure vs. Volume Targeting
At moderate tidal volumes (8 mL/kg) and low flow rates, pressure-controlled ventilation reduces respiratory work rate more effectively than volume-controlled assist control. 6