Mechanical Ventilator Modes and Clinical Indications
Primary Classification Framework
Mechanical ventilators are fundamentally classified by their control variable: pressure-targeted modes deliver a set inspiratory pressure with variable tidal volume, while volume-targeted modes deliver a set tidal volume with variable pressure. 1, 2
Pressure-Targeted Ventilation
- The operator sets inspiratory pressure; delivered tidal volume depends on lung compliance, airway resistance, and inspiratory time 1, 2
- Offers constant pressure delivery, automatic compensation for air leaks, and positive expiratory pressure throughout the cycle 1
- Avoids sudden uncomfortable pressure increases that occur with volume control 1
- Limits maximum airway pressure, reducing ventilator-induced lung injury risk 3
Volume-Targeted Ventilation
- The operator sets tidal volume and inspiratory time; the ventilator generates whatever pressure is necessary to deliver this volume 1, 2
- Guarantees consistent minute ventilation regardless of changing lung mechanics 4
- Airway pressure increases with reduced compliance or increased resistance, potentially increasing lung injury risk 3
- Rarely employed for non-invasive ventilation in the UK outside specialist centers 1
Controlled Mechanical Ventilation (CMV)
CMV provides full ventilatory support with preset breaths delivered at fixed intervals regardless of patient respiratory drive, indicated for patients lacking respiratory effort. 2, 5
Clinical Indications
- Severe neurological injury with absent respiratory drive 2
- Deep sedation or neuromuscular blockade 2
- Patients requiring complete ventilatory control 2
Key Characteristics
- No patient effort required; ventilator delivers all breaths 2, 5
- In pressure-control CMV, tidal volume varies with lung mechanics 2
- In volume-control CMV, tidal volume is fixed and pressure varies 2
Assist/Control Ventilation (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 during sleep. 2
Clinical Indications
- Initial mode for most mechanically ventilated patients 2
- Patients with variable respiratory drive requiring guaranteed minute ventilation 2
- Patients at risk of central apneas during sleep 1, 6
Key Characteristics
- Guarantees a preset number of mandatory breaths per minute 2, 5
- Patient-triggered breaths receive identical preset parameters as mandatory breaths 2
- A lock-out period prevents breath stacking but can cause dyssynchrony if set incorrectly 2
- Prevents central apneas better than pressure support ventilation 1, 6
Common Pitfalls
- Excessively long expiratory times create prolonged lock-out periods, reducing patient tolerance 2
- Incorrect lock-out settings cause patient-ventilator dyssynchrony 2
Synchronized Intermittent Mandatory Ventilation (SIMV)
The European Society of Intensive Care Medicine recommends SIMV as an effective mode that prevents central apneas during sleep due to the backup respiratory rate, making it preferable to pressure support for patients at risk of hypoventilation. 6
Clinical Indications
- Patients requiring guaranteed minute ventilation with some spontaneous breathing 2, 6
- Patients at risk of central apneas or hypoventilation 6
- Weaning from mechanical ventilation 2
Key Characteristics
- Provides preset mandatory breaths while allowing spontaneous breaths between them 2, 6
- Patient-triggered breaths delay the next mandatory breath to maintain synchrony 2
- Can be volume-controlled or pressure-controlled 1, 6
- Also known as "spontaneous/timed" (S/T) or IE mode on some devices 2
Common Pitfalls
- More complex than other modes, requiring understanding of synchronization windows 2
- Terminology varies among manufacturers, causing potential confusion 1, 2, 5
- Incorrectly set lock-out periods cause poor patient tolerance 2, 6
- Spontaneous breaths often deliver lower tidal volumes than mandatory breaths, leading to variable oxygen saturation 6
Pressure Support Ventilation (PSV)
PSV allows patient effort to initiate and terminate each breath with respiratory frequency and timing entirely patient-determined, but carries risk of apnea if the patient fails to generate effort. 2, 5
Clinical Indications
- Patients with adequate respiratory drive needing partial ventilatory support 2
- Weaning from mechanical ventilation 2
- Patients requiring comfortable, patient-synchronized ventilation 7
Key Characteristics
- Patient's respiratory effort triggers the ventilator both on and off 2, 5
- Most devices include backup rate of 6–8 breaths/min if patient becomes apneic 2
- No ventilation occurs if patient fails to generate effort 2
Critical Limitations
- High levels of PSV may cause sleep disruption from periodic breathing and central apneas 1
- Excessive pressure support delivers higher-than-needed alveolar minute ventilation, causing hyperventilation, hypocapnia, and subsequent apnea 1
- 54% of patients developed central apneas (53 ± 8 events/h) during PSV in one study, leading to arousals and major sleep fragmentation 1
- Sleep quantity and quality are poorest during PSV compared to assist-control modes 1
Common Pitfalls
- Adjust pressure support level to prevent hyperventilation during sleep, particularly in patients with chronic heart failure 1
- Avoid excessive assistance that lowers PaCO₂ below the apneic threshold 1
- Consider adding dead space or switching to assist-control mode if central apneas develop 1
Bi-Level Positive Airway Pressure (BiPAP/NIV)
The British Thoracic Society recommends bi-level pressure-support ventilators as a simple, cost-effective, and flexible option for non-invasive ventilation, used in most NIV randomized trials. 2
Clinical Indications
- Acute COPD exacerbation with respiratory acidosis (pH < 7.35) despite maximal medical therapy 2
- Acute or acute-on-chronic hypercapnic failure from chest-wall deformity or neuromuscular disease 2
- Decompensated obstructive sleep apnea with respiratory acidosis 2
- Cardiogenic pulmonary edema when CPAP alone is insufficient 2
Key Characteristics
- Combines pressure support with CPAP using two pressure levels: IPAP (inspiration) and EPAP (expiration) 2, 5
- IPAP provides ventilation during inspiration 2
- EPAP recruits underventilated lung and offsets intrinsic PEEP 2
- More effective than CPAP for hypercapnic respiratory failure 5
Interface Selection
- Use full-face mask initially in acute settings; switch to nasal mask after 24 hours as patient improves 2
- Proper mask fit is essential for comfort and effective ventilation 1, 2
- Apply barrier dressings from the start to reduce nasal-bridge ulceration risk 2
Common Pitfalls
- Approximately 20–30% of patients with acute respiratory failure cannot be managed with NIV due to mask-fit problems or asynchrony 1, 2
- Overtightening headgear to reduce leaks causes skin ulceration, especially over the nasal bridge 1, 2
- Avoid overtightening, which causes skin damage and reduces compliance 2
Contraindications
- Recent facial or upper airway surgery 5
- Facial abnormalities or fixed upper airway obstruction 5
- Vomiting patients 5
- Impaired consciousness 5
- Severe hypoxemia 5
- Copious respiratory secretions 5
Continuous Positive Airway Pressure (CPAP)
CPAP maintains constant positive airway pressure throughout the respiratory cycle, recruiting underventilated lung and improving oxygenation, but does not provide ventilatory support. 2, 5
Clinical Indications
- Cardiogenic pulmonary edema with persistent hypoxia despite optimal medical therapy 2, 5
- Chest-wall trauma with ongoing hypoxia despite adequate regional anesthesia and high-flow oxygen 2, 5
- Diffuse pneumonia with hypoxia resistant to high-flow oxygen 2, 5
- Obstructive sleep apnea without respiratory acidosis 2, 5
Key Characteristics
- Delivers single constant positive pressure throughout entire respiratory cycle 5
- Improves oxygenation and reduces inspiratory muscle work 2
- In COPD, offsets intrinsic PEEP to lower PaCO₂ 2
- Does not provide ventilatory support; only maintains positive pressure 2
Critical Requirements
- Requires high-flow capability (> 60 L/min) for distressed COPD patients with high minute ventilation 2
- Low-flow CPAP generators suitable for sleep apnea are inadequate for acute respiratory failure 2
Common Pitfalls
- In hyperinflated patients with airflow obstruction, further lung-volume increase may impair inspiratory muscle function 2
- Patients with chest-wall trauma on CPAP should be monitored in ICU due to pneumothorax risk 2
- CPAP trials for acute pneumonia should be limited to HDU/ICU settings because of high intubation risk 2
Minimum Ventilator Specifications for NIV
Essential Features
- Pressure-controlled capability 1
- Pressure capability ≥ 30 cmH₂O 1, 2
- Support inspiratory flows ≥ 60 L/min 1, 2
- Assist-control and bi-level pressure support modes 1
- Rate capability ≥ 40 breaths/min 1
- Sensitive flow triggers 1
- Disconnection alarm 1, 2
Desirable Features
- Short pressure rise time capability with adjustable settings 1, 2
- Adjustable inspiratory and expiratory triggers 1
- Adjustable inspiratory-expiratory ratio in assist-control mode 1
- Temporary alarm cancellation facility 1
- Internal battery with power for ≥ 1 hour 1
- Accessible control panel with cover or lock-out facility 1
Clinical Decision Algorithm for NIV
Before initiating NIV, determine whether the patient would require immediate intubation if NIV fails; this decision should be documented early and verified by senior medical staff. 1, 2
Immediate Intubation Indicated
- Absent airway reflexes 2
- Severe upper-airway obstruction 2
- High oxygenation requirements 2
- Hemodynamic instability 2
NIV Appropriate
- COPD exacerbation with pH < 7.35 2
- Cardiogenic pulmonary edema 2
- Neuromuscular disease with hypercapnia 2