How does BiPAP (Bilevel Positive Airway Pressure) function and what is the purpose of adjusting its parameters?

How BiPAP Functions and Parameter Purposes

Fundamental Mechanism of Action

BiPAP operates by delivering two independently adjustable pressure levels—IPAP (inspiratory positive airway pressure) during inspiration and EPAP (expiratory positive airway pressure) during expiration—which together reduce respiratory muscle effort, increase tidal volume, and maintain airway patency. 1

The pressure differential (pressure support) between IPAP and EPAP directly augments tidal volume and improves ventilation by reducing the patient's intrinsic work of breathing 1. EPAP serves a dual purpose: it keeps upper airways open (particularly important in obstructive sleep apnea) and counteracts intrinsic PEEP in obstructive lung disease, making it easier to initiate inspiration 1.

Core Physiologic Effects of Each Parameter

EPAP (Expiratory Positive Airway Pressure)

• Reduces inspiratory threshold load by counteracting intrinsic PEEP in patients with obstructive lung disease, decreasing the effort required to trigger a breath 1
• Prevents alveolar collapse by maintaining positive alveolar pressure at end-expiration, improving functional residual capacity and reducing work needed to re-expand collapsed lung units 1
• Promotes lung recruitment by improving ventilation in previously collapsed areas, enhancing gas exchange efficiency and oxygenation 1
• Maintains upper airway patency during expiration, particularly critical in sleep-disordered breathing 1

IPAP (Inspiratory Positive Airway Pressure)

• Provides ventilatory support during inspiration, directly augmenting tidal volume and reducing respiratory muscle work 1, 2
• The pressure support (IPAP minus EPAP) determines the degree of ventilatory assistance—higher pressure support increases tidal volume and reduces patient effort 1, 2

Ventilatory Modes and Their Clinical Applications

Spontaneous (S) Mode

• Patient-triggered breathing where the patient determines respiratory frequency and time spent at IPAP, with the machine responding to respiratory efforts 2
• Best for patients with intact respiratory drive who can reliably trigger breaths 2

Spontaneous-Timed (ST) Mode

• Provides backup respiratory rate to ensure minimal ventilation if the patient fails to initiate sufficient breaths 2
• Recommended for patients with inconsistent respiratory drive or those at risk of hypoventilation during sleep 2
• Typical inspiratory time corresponding to 30-40% of the respiratory cycle is 1.2 to 1.6 seconds 3

Timed (T) Mode

• Delivers controlled ventilation at a set respiratory frequency with fixed inspiratory time 2
• Rarely used in sleep centers but indicated for patients unable to synchronize with the device or those with consistently feeble respiratory efforts during sleep 3

Practical Parameter Selection Algorithm

Initial Settings

• Start with IPAP = 8 cm H₂O and EPAP = 4 cm H₂O as minimum baseline pressures 1, 2
• Typical pressure differential ranges from 4-6 cm H₂O for most patients 4
• For volume-targeted BiPAP, set EPAP = 4 cm H₂O, IPAP min = EPAP + 4 cm H₂O, and IPAP max = 25-30 cm H₂O, with target tidal volume approximately 8 mL/kg ideal body weight 3

Titration Strategy

• Increase IPAP by 2 cm H₂O increments to eliminate apneas, hypopneas, and respiratory effort-related arousals 5
• Increase EPAP by 1 cm H₂O increments to eliminate obstructive events and optimize comfort 5
• Target SpO₂ 90-94% during titration, with some guidelines recommending ≥92% for patients with strong respiratory drive 4
• Manual titration during attended polysomnography is the gold standard for determining optimal pressure settings 4, 2

Disease-Specific Adjustments

For Obstructive Airway Disease (COPD):

• Use lower %IPAP time and I:E ratio to allow sufficient expiratory time, as expiratory airflow is reduced 3
• Typical EPAP levels range from 3-5 cm H₂O, though levels >5 cm H₂O are rarely tolerated despite intrinsic PEEP potentially reaching 10-15 cm H₂O 1
• Caution: One study found BiPAP may increase work of breathing in spontaneously breathing COPD patients compared to pressure support ventilation, with higher PTP, WOB, and PEEPi during BiPAP 6

For Restrictive Lung Disease:

• Use greater %IPAP time (higher I:E ratio) to allow longer inspiratory time for adequate lung expansion 3

For Neuromuscular Disease:

• Consider ST or T mode with backup rate support for patients with poor respiratory drive 4
• Higher pressure support may be needed to compensate for respiratory muscle weakness 7

Critical Pitfalls and How to Avoid Them

Excessive EPAP

• Can cause gastric distension or paradoxically increase work of breathing 1
• In cardiovascular compromise, apply EPAP cautiously as positive intrathoracic pressure reduces venous return 1
• Monitor for aerophagia (air swallowing), which can be exacerbated by high pressures 4

Inadequate Inspiratory Time

• As respiratory rate increases, inspiratory time must decrease to maintain adequate I:E ratio 3
• Default maximum IPAP duration of 3 seconds exists on some devices—ensure this doesn't unduly delay IPAP to EPAP transition 3

Interface and Leak Issues

• Ensure proper mask fitting as interface problems commonly cause treatment failure 1
• Allow acclimatization to low pressures before titration to improve tolerance 1
• Intentional leak increases with higher IPAP or EPAP, which reduces effective FiO₂ when supplemental oxygen is used 3

Supplemental Oxygen Delivery

• Connect oxygen via 3-orifice "T" connector between device outlet and hose for optimal mixing 3
• Effective FiO₂ falls as IPAP or EPAP increases due to higher intentional leak, requiring higher oxygen flow rates 3
• Target slightly higher SpO₂ (90-94%) than minimum 88% to account for oximetry overestimation in some circumstances 3

Monitoring for Treatment Efficacy

• Monitor airflow, tidal volume, leaks, and delivered pressure to evaluate treatment effectiveness 2
• For hypercapnic patients, monitor blood gases for improvement in PaCO₂ and PaO₂ 4
• Assess symptom improvement and treatment adherence at regular follow-up visits 4
• In acute respiratory failure, close monitoring is essential to prevent delay in necessary intubation if BiPAP fails within 1-2 hours 4