What are the differences between types of ventilation for patients with respiratory failure, such as acute respiratory distress syndrome (ARDS) and chronic obstructive pulmonary disease (COPD)?

Differences Between Types of Ventilation in Respiratory Failure

Non-Invasive Ventilation (NIV) vs. Invasive Mechanical Ventilation

For patients with COPD and acute hypercapnic respiratory failure, non-invasive ventilation (NIV) using bi-level pressure support should be the first-line approach, as it reduces mortality, intubation rates, and hospital length of stay compared to invasive ventilation. 1, 2

When to Use Non-Invasive Ventilation

NIV is specifically indicated for:

• COPD patients with respiratory acidosis (pH 7.25-7.35) where it demonstrates success rates of 80-85% in randomized controlled trials 1, 2
• Hypercapnic respiratory failure secondary to chest wall deformity or neuromuscular disease 1
• Cardiogenic pulmonary edema unresponsive to CPAP alone 1
• Weaning from tracheal intubation 1

When NIV is Contraindicated

NIV should NOT be used in patients with:

• Impaired consciousness or inability to cooperate 1
• Severe hypoxemia (PaO2/FiO2 < 200 mmHg) 1
• Copious respiratory secretions with high aspiration risk 1
• Respiratory arrest or cardiovascular instability 1
• Recent facial/gastroesophageal surgery or craniofacial trauma 1

Key Ventilation Modes Explained

Bi-Level Pressure Support (BiPAP/NIPPV)

This is the preferred NIV mode for COPD and acute respiratory failure:

• Delivers two pressure levels: Inspiratory Positive Airway Pressure (IPAP) provides ventilation, while Expiratory Positive Airway Pressure (EPAP) recruits underventilated lung and offsets intrinsic PEEP 1, 3
• Initial settings: IPAP 10-15 cmH₂O, EPAP 4-8 cmH₂O, backup rate 10-14 breaths/min 3, 4
• Most studies showing improved survival in COPD have used pressure-controlled bi-level devices 2

Continuous Positive Airway Pressure (CPAP)

CPAP is distinct from true ventilatory support:

• Primary indication is to correct hypoxemia, not hypercapnia 1
• Maintains constant pressure throughout the respiratory cycle without providing inspiratory assistance 1
• Useful in cardiogenic pulmonary edema but less effective than bi-level support for COPD exacerbations 1
• Conventionally not considered respiratory support despite reducing work of breathing 1

Assist-Control Mode (Invasive Ventilation)

When intubation becomes necessary:

• Provides complete ventilatory support and is appropriate immediately after intubation 1
• Every patient breath triggers full ventilator support, with machine-delivered breaths if patient effort is insufficient 1
• Initial mode of choice for invasive ventilation in COPD patients requiring intubation 4

Pressure Support Ventilation (PSV)

• Patient triggers both the start and end of each breath, determining respiratory frequency and timing 1
• Most physiological NIV mode and most commonly used 5
• Requires adequate respiratory drive; if patient fails to make respiratory effort, no assistance occurs (though backup rates are typically incorporated) 1

Synchronized Intermittent Mandatory Ventilation (SIMV)

• Combines mandatory machine breaths with patient-triggered breaths 1
• Patient-triggered breaths delay the next machine-determined breath to maintain synchronization 1
• Also called Spontaneous/Timed (S/T) mode on NIV machines 1, 3

Critical Differences in Management Between COPD and ARDS

COPD-Specific Considerations

• Target SpO₂ 88-92% to avoid worsening hypercapnia from excessive oxygen 1, 3, 2
• EPAP offsets intrinsic PEEP (auto-PEEP) which is critical in hyperinflated COPD patients 1, 3, 4
• Longer expiratory times required with I:E ratio of 1:2 or greater to prevent dynamic hyperinflation 3, 4
• NIV success rate is higher (80-85%) compared to other causes of respiratory failure 1, 2

ARDS-Specific Considerations

• Low tidal volume ventilation (6 ml/kg predicted body weight) is mandatory to prevent ventilator-induced lung injury 1, 4
• Target plateau pressure <30 cmH₂O to minimize barotrauma 4
• Higher PEEP levels may be needed to recruit collapsed alveoli and improve oxygenation 1
• NIV has limited role in severe ARDS; invasive ventilation is typically required earlier 1
• Prone positioning improves oxygenation in approximately 65% of ARDS patients 1

When to Escalate from NIV to Invasive Ventilation

Intubation should be considered when NIV fails, defined by:

• Worsening ABGs and/or pH within 1-2 hours of NIV initiation 1, 4
• Lack of improvement in ABGs and/or pH after 4 hours of NIV 1, 4
• Severe acidosis (pH <7.25) with hypercapnia (PaCO₂ >60 mmHg) 1, 4
• Life-threatening hypoxemia (PaO₂/FiO₂ <200 mmHg) 1, 4
• Tachypnea >35 breaths/min despite NIV 1, 4

Most trials showing positive NIV response demonstrate early improvement in PaO₂, pH, and PaCO₂ at 1 hour and certainly by 4-6 hours; lack of progress by this timeframe predicts NIV failure 3.

Equipment Differences

NIV-Specific Ventilators

• Bi-level pressure support ventilators are preferred as they are simpler, cheaper, more flexible, and validated in the majority of randomized trials 3, 2
• Must support inspiratory flows ≥60 L/min as distressed COPD patients may exceed this threshold 3
• Single-circuit systems with intentional leaks for exhalation, unlike ICU ventilators with separate inspiratory/expiratory circuits 1

ICU Ventilators for Invasive Ventilation

• Separate inspiratory and expiratory circuits prevent rebreathing and allow precise monitoring 1
• Full monitoring and alarm systems for pressure, volume, and flow 1
• Can deliver volume-controlled or pressure-controlled ventilation with multiple advanced modes 1

Common Pitfalls to Avoid

• Excessive oxygen therapy in COPD: Maintain strict SpO₂ target of 88-92% to prevent worsening respiratory acidosis 1, 3, 2
• Inadequate expiratory time: Ensure I:E ratio of 1:2 or greater to prevent dynamic hyperinflation and auto-PEEP 3, 4, 2
• Poor mask fit causing patient-ventilator asynchrony: Approximately 20-30% of NIV failures are due to interface problems 3
• Delayed intubation: Delaying invasive ventilation when NIV is clearly failing increases mortality 2
• Insufficient PEEP/EPAP: Inadequate levels lead to atelectasis and worsening V/Q mismatch 4, 2
• Excessive tidal volumes in invasive ventilation: Use lung-protective ventilation (6-8 ml/kg) to avoid ventilator-induced lung injury 4, 2