Noninvasive Ventilation: CPAP and BiPAP

Introduction

• There is a range of therapeutic interventions for patients who develop respiratory distress. Supplemental oxygen delivery is the first step, but patients who fail to respond to initial oxygen therapy may require more advanced respiratory support, and escalation to NIV may be indicated.
• Respiratory failure remains one of the leading causes of ICU admission. In patients presenting with respiratory distress, NIV offers a rapid, accessible means of support, reduces the need for endotracheal intubation, lowers the risk of ventilator-associated complications, and improves outcomes in acute respiratory failure.
• NIV provides ventilatory assistance without the need for more invasive endotracheal intubation and can be delivered through several modalities, including HHFNC, CPAP, and BiPAP. Each has unique functions and ways to augment oxygenation, ventilation, or both.

Oxygenation

• Oxygenation in NIV is achieved by delivering supplemental oxygen and applying positive airway pressure to enhance alveolar recruitment and improve gas exchange, thereby correcting hypoxia. Oxygen delivery is titrated to maintain target arterial oxygen saturation (SaO₂) and peripheral oxygen saturation (SpO₂), typically aiming for SpO₂ greater than 90%.¹
• Using SaO₂, PaO₂, and SpO₂, clinicians can monitor how effectively oxygenation improves; however, the main mechanisms for augmenting oxygen delivery are through the fraction of inspired oxygen (FiO₂) and positive end-expiratory pressure (PEEP).
  • FiO₂ is the fraction of inspired oxygen, which is the actual percentage of oxygen a patient breathes in from the air, either atmospheric or supplemental. In atmospheric air at any altitude, FiO₂ is 0.21, corresponding to 21% oxygen being inhaled in the air. With a regular nasal cannula, the maximum FiO₂ achievable is approximately 40-45% at a rate of 6 L/min. Above these rates, there is a risk of discomfort and diminishing returns of FiO₂.²
  • Another factor that can influence oxygenation is PEEP, which is the positive pressure that remains in the airways at the end of the respiratory cycle and is used to overcome closing capacity, which is the lung volume at which airways collapse. This residual pressure from PEEP increases the lung’s functional residual capacity to a volume greater than the closing capacity, ensuring that the air maintained in the lungs keeps them inflated and prevents alveolar collapse, which helps reduce the work of breathing. PEEP can also augment oxygenation via the alveolar-capillary membrane. Because air flows from high to low pressure, PEEP maintains a greater pressure gradient, allowing oxygen to diffuse more readily.²

Ventilation

• Ventilation is the clearance of CO₂, a byproduct of metabolism and oxygenation, from the body. The main mechanism for augmenting ventilation is through minute ventilation, which is the product of respiratory rate and tidal volume.
• Respiratory rate refers to the number of breaths a patient takes per minute and helps regulate the CO₂ content in the blood. The inspiratory to expiratory ratio, or I:E ratio, refers to the time spent inhaling vs exhaling. In spontaneously breathing patients, this is usually set to 1:3 or 1:5. In ventilated patients, it is often set to 1:2 or 1:3, and higher for patients who require a longer exhalation (e.g., patients with chronic obstructive pulmonary disease exacerbations).³
• Tidal volume refers to the volume of air that moves in and out of the lungs with each breath. With supportive ventilation, the goal is to deliver a tidal volume that is large enough to maintain adequate ventilation but small enough to prevent lung barotrauma. The RENOVATE trial, which included patients with acute respiratory failure managed with NIV, specifically targeted tidal volumes of 6-9 mL/kg of ideal body weight. This range was chosen to balance adequate ventilation with minimization of ventilator-induced lung injury and barotrauma.⁴
• Minute ventilation, also known as total ventilation, is the volume of air that enters the lungs per minute. It is the product of respiratory rate and tidal volume; increasing either variable can improve minute ventilation and thus CO₂ removal.

Noninvasive Positive Pressure Ventilation (NIPPV)

• NIPPV describes the delivery of oxygen at either constant or variable pressures via different modalities. Most commonly, this is done with a face mask using BiPAP or CPAP. However, there are other interventions, such as HHFNC, that can be used to support hypoxic patients as well. These modalities can deliver 100% oxygen and are patient-triggered, meaning that the patient initiates each breath, and the ventilator subsequently delivers inspiratory pressure support.

CPAP

• CPAP provides a constant, fixed positive pressure throughout both inspiration and expiration. This constant pressure throughout the lungs helps splint the airways open (as opposed to the alveoli collapsing with every respiratory cycle) and reduces the work of breathing. It can be used in various clinical scenarios and with different flow rates. For example, it can be used with a low-flow generator for patients with obstructive sleep apnea (OSA) who require nocturnal CPAP. CPAP stops the soft tissue of the nasopharyngeal region from collapsing and closing off. It can also be used in a hospital setting with high flow settings to ensure that the flow rates delivered are able to sustain patients in respiratory distress.⁵

• Clinically, the primary role of CPAP is to augment oxygenation by adjusting PEEP and FiO2, as well as for patients with OSA and those recovering from anesthesia.

BiPAP

• BiPAP provides varying airway pressures during inspiration and expiration. The inspiratory airway pressure (IPAP) is higher than the expiratory airway pressure (EPAP). The IPAP augments oxygenation similarly to CPAP, but the lower EPAP helps recruit under-ventilated or collapsed alveoli for gas exchange and the removal of exhaled CO₂. Thus, BiPAP augments both oxygenation and ventilation, whereas CPAP primarily augments oxygenation. This allows BiPAP to be beneficial for patients with type II respiratory failure, which is characterized by the inability to adequately ventilate.⁵

• Clinically, BiPAP is more commonly used because it provides more control over both oxygenation and ventilation. BiPAP can also have additional modalities, such as average-volume assured pressure support (AVAPS).

AVAPS

• AVAPS is a mode on BiPAP that automatically adjusts the inspiratory pressure support to achieve a clinically defined target minute ventilation. By continuously monitoring the patient’s exhaled tidal volume and incrementally increasing or decreasing IPAP as needed to reach that target, AVAPS is designed to maintain more consistent ventilation despite changes in patient effort, respiratory mechanics, or sleep stage.⁶

Heated-High Flow Nasal Cannula

• A HHFNC allows the delivery of up to 60 L/minute of 100% oxygen (FiO₂ 1.0), helping address the limitations of low-flow nasal cannulation. With a simple nasal cannula, you can only achieve FiO₂ of 0.40-0.45, whereas non-rebreather masks allow 0.90-0.95 FiO₂ at rates up to 15L/min. HHFNC supports oxygenation by delivering large volumes of 100% oxygen.²

Adverse Effects and Contraindications

• The main adverse effects of NIPPV are primarily related to the interface itself and pressure delivery. The most common of these are skin symptoms (facial rash, nasal ulceration), eye symptoms (dry eyes, conjunctivitis), nose and mouth symptoms (nasal stuffiness, rhinorrhea, nosebleed, mucosal dryness), gastrointestinal symptoms (gastric distension, aerophagia), and device or mask intolerance (claustrophobia, discomfort, nonadherence).
• In patients with moderate to severe acute respiratory distress syndrome (ARDS), NIV should be used with caution due to concerns of ventilator-associated lung injury from volutrauma and barotrauma. Due to increased thoracic pressure, NIPPV can also have adverse cardiovascular effects, including decreased cardiac output and splanchnic blood flow.⁶ Serious adverse effects can include aspiration, hypotension, pneumothorax, and severe alkalosis.⁷
• Contraindications to NIPPV generally refer to scenarios where the patient is undergoing severe ARDS or instability that requires immediate intervention.

Other Considerations

Costs and Hospital Stay

• NIPPV is associated with lower hospital costs and shorter length of stay compared to invasive mechanical ventilation (IMV). Studies on chronic obstructive pulmonary disease (COPD) exacerbations have demonstrated a 19% reduction in length of stay (1.6 days shorter) and a 32% reduction in cost (average $5,673 less) with NIPPV. These benefits are primarily attributable to reduced rates of hospital-acquired pneumonia and other complications, such as tracheostomy, ventilator-associated pneumonia, and other device-related adverse events. Overall, NIPPV reduces resource utilization due to these complications and the need for prolonged intensive care compared to IMV.
• It is essential to note that the cost and length-of-stay benefits of NIPPV may be less pronounced in patients with a higher comorbidity burden. Individual patient considerations and clinical context are crucial for balancing economic and resource implications with outcomes.⁸

NIPPV Postextubation

• Although NIPPV can be used as an alternative to IMV, it can also facilitate early liberation from IMV, reducing hospital length of stay and mortality. NIPPV is also recommended immediately after extubation in high-risk populations, such as those with COPD, cardiac comorbidities, or obesity, to prevent extubation failure and reduce reintubation risk, especially when compared to conventional oxygen therapy or high-flow nasal cannula alone.⁵

NIV Mask Interfaces

• Oronasal masks are the most commonly used interface, securing both the nose and mouth with head straps. Although they allow higher pressures to be delivered, they can cause discomfort, facial ulceration, and air leaks, which may limit efficacy and patient tolerance.
• Total face masks exert no direct pressure on the nose, which can reduce skin breakdown, and have similar efficacy to oronasal masks at lower internal volumes. Similar to oronasal masks, they also have limitations in pressure delivery.
• Nasal masks and nasal pillows/prongs cover only the nose, which patients may find more tolerable, but have limited pressure delivery and higher air leak risk. Thus, they are primarily used in stable patients with sleep-disordered breathing.
• Helmet interfaces consist of a transport hood with soft collar that is fitted around the neck. These are generally better tolerated, allow for prolonged use, and enable delivery of higher PEEP with fewer facial pressure injuries and air leaks. However, high gas flows may be necessary to avoid collapse of the hood and avoid CO₂ re-breathing. This interface can also make accurate tidal volume measurements challenging.
• Overall, interface selection is highly individualized with the ultimate goal to balance efficacy, comfort, and compliance for patients.^1,9