What is positive pressure ventilation (PPV) in patients with respiratory failure, including those with neuromuscular diseases, chronic obstructive pulmonary disease (COPD), and acute respiratory distress syndrome (ARDS)?

What is Positive Pressure Ventilation?

Positive pressure ventilation (PPV) is a method of mechanical respiratory support that delivers air into the lungs by applying positive pressure to the airways, either through an endotracheal tube (invasive) or through a mask interface (non-invasive), thereby assisting or replacing spontaneous breathing in patients with respiratory failure. 1

Fundamental Mechanism

• PPV works by generating positive pressure during inspiration that pushes air into the lungs, in contrast to normal breathing where negative intrathoracic pressure draws air in 1
• The positive pressure increases mean airway pressure, which recruits collapsed alveoli, maintains patency of open lung units, and improves oxygenation by optimizing ventilation-perfusion matching 1
• PPV can be delivered continuously throughout the respiratory cycle or cyclically with varying pressures during inspiration and expiration 1

Primary Delivery Methods

Non-Invasive Positive Pressure Ventilation (NPPV/NIV)

• NIV delivers positive pressure ventilation through nasal, facial, or oronasal masks without requiring endotracheal intubation, allowing patients to maintain airway defense mechanisms, continue eating and speaking, and participate in treatment decisions 1
• NIV reduces inspiratory work of breathing, decreases respiratory muscle fatigue, improves minute ventilation, and enhances gas exchange 2, 3
• The British Thoracic Society established that NIV has become the standard method for treating acute hypercapnic respiratory failure, particularly in COPD, with facilities for NIV recommended to be available 24 hours per day in all hospitals admitting such patients 1

Invasive Positive Pressure Ventilation

• Invasive PPV requires sedation, endotracheal intubation or tracheostomy, and delivers ventilation by bypassing the upper airway 1
• This method provides superior airway protection and direct access for suctioning secretions but carries substantial risks including tracheal injury, ventilator-associated pneumonia, and complications from sedation 4, 5

Common Modes of Positive Pressure Ventilation

Continuous Positive Airway Pressure (CPAP)

• CPAP delivers a single constant positive pressure throughout the entire respiratory cycle, maintaining the same pressure during both inspiration and expiration 1
• The American Thoracic Society recommends CPAP for cardiogenic pulmonary edema, chest wall trauma with hypoxemia, and diffuse pneumonia 6
• CPAP improves oxygenation by increasing functional residual capacity and preventing alveolar collapse, though it was not associated with reduced intubation rates in acute hypoxemic respiratory failure 1

Bilevel Positive Airway Pressure (BiPAP)

• BiPAP provides two distinct pressure levels: higher inspiratory positive airway pressure (IPAP) during inspiration and lower expiratory positive airway pressure (EPAP) during expiration 6
• BiPAP is more effective than CPAP for hypercapnic respiratory failure and better tolerated in patients who cannot tolerate high CPAP pressures 6
• The British Thoracic Society notes that BiPAP ventilators are simpler to use, cheaper, and more flexible than other ventilator types, and have been used in the majority of randomized controlled trials of NIV 6

Pressure Support Ventilation (PSV)

• The European Respiratory Society describes PSV as a mode where the patient's respiratory effort triggers the ventilator both on and off, with the patient determining respiratory frequency and timing of each breath 6
• This mode is particularly useful for patients who require ventilatory support but can initiate their own breaths 6

Assist/Control Ventilation (ACV)

• ACV delivers a preset number of mandatory breaths per minute in the absence of patient effort while allowing patient-triggered breaths 6
• This mode is preferable to pressure support for patients at risk of hypoventilation 6

Clinical Applications by Disease State

COPD with Acute Hypercapnic Respiratory Failure

• Moderate quality evidence demonstrates that NIV plus usual medical care significantly reduces the need for endotracheal intubation (RR 0.38), inhospital mortality (RR 0.53), and hospital length of stay (WMD -2.68 days) compared with usual medical care alone 4
• The British Thoracic Society specifically recommends NIV for COPD patients with respiratory acidosis pH 7.25-7.35 after initial medical treatment 1
• NIV is particularly effective when the mean pH is less than 7.35, with a typical district general hospital serving 250,000 people expecting to treat approximately 70 such patients per year 1

Neuromuscular Disease and Chest Wall Deformity

• NIV using nasal masks was first described for treating hypoventilation in patients with neuromuscular disease and has become the standard method for chronic hypercapnic respiratory failure caused by chest wall deformity, neuromuscular disease, or impaired central respiratory drive 1
• This application has largely replaced external negative pressure ventilation and rocking beds 1

Acute Respiratory Distress Syndrome (ARDS)

• In moderate ARDS, noninvasive support may be considered in selected cases: cognizant younger patients, those with SAPS II < 34, and patients with ARDS not caused by pneumonia 1
• Critical Care guidelines emphasize that patients receiving noninvasive support must be monitored closely as deterioration can occur abruptly, with positive responses usually evident soon after initiation 1
• When PaO2/FiO2 remains < 150 mmHg despite optimization, invasive mechanical ventilation with low tidal volume ventilation, prone positioning, and neuromuscular blockers should be considered 1

Weaning from Invasive Mechanical Ventilation

• Moderate quality evidence shows that weaning COPD patients from invasive ventilation using NIV results in significant reductions in ICU mortality (RR 0.47), nosocomial pneumonia (RR 0.14), and weaning failure compared with continued invasive ventilation 4
• This approach shortens the duration of endotracheal intubation, thereby reducing the risk of ventilator-associated pneumonia 4

Absolute Contraindications to NIV

The British Thoracic Society and other guidelines identify the following contraindications where invasive ventilation should be used instead 1, 6:

• Respiratory arrest or severe hemodynamic instability (hypotension, arrhythmias, myocardial infarction) 1, 6
• Impaired consciousness, somnolence, or inability to cooperate 1, 6
• Copious and/or viscous respiratory secretions with high aspiration risk 1, 6
• Recent facial or gastroesophageal surgery 1, 6
• Craniofacial trauma and/or fixed nasopharyngeal abnormality 1, 6
• Severe hypoxemia despite supplemental oxygen (though this is relative depending on the clinical context) 1

Critical Implementation Considerations

Tolerance and Compliance

• NPPV intolerance ranges from 5% to 29%, with higher tolerance generally observed in patients with more severe respiratory failure 4
• Compliance with NIV protocols decreases over time, even over short periods such as 3 days 4
• Common complications causing intolerance include facial and nasal skin damage, gastric distension with aspiration risk, sleeping disorders, and conjunctivitis 1

Monitoring Requirements

• The European Respiratory Society emphasizes that NIV requires the same level of supervision as conventional mechanical ventilation 1
• Patients must be monitored for worsening arterial blood gases and pH, with NIV considered failed if deterioration occurs within 1-2 hours or no improvement is seen after 4 hours 1, 7
• Pulse oximetry should maintain saturation approximately 88-90% (approximating PaO2 of 60 mmHg) with non-toxic oxygen concentrations ideally FiO2 < 0.60 1

When to Transition to Invasive Ventilation

Intubation should be considered when 1, 7:

• NIV failure with worsening ABGs and/or pH in 1-2 hours or lack of improvement after 4 hours 1, 7
• Severe acidosis (pH < 7.25) and hypercapnia (PaCO2 > 60 mmHg) 1, 7
• Life-threatening hypoxemia (PaO2/FiO2 < 200 mmHg) 1
• Tachypnea > 35 breaths/min despite NIV 1

Common Pitfalls to Avoid

• Never use NIV as a substitute for endotracheal intubation when invasive ventilation is clearly more appropriate, as delays in intubation may result in worse outcomes 1
• Avoid excessive oxygen therapy in COPD patients, which can worsen hypercapnia; maintain target saturation of 88-92% 7
• Do not apply NIV in patients with impaired consciousness or copious secretions, as this increases aspiration risk and failure rates 1, 6
• Ensure adequate expiratory time in COPD patients to prevent dynamic hyperinflation and auto-PEEP 7
• Recognize that terminology for ventilation modes varies significantly between ventilator manufacturers, potentially causing confusion in clinical practice 6