BiPAP Therapy for Respiratory Disorders

Introduction to BiPAP

The American Academy of Sleep Medicine recommends BiPAP as a non-invasive form of ventilatory therapy that provides higher pressure during inspiration and lower pressure during expiration, allowing for effective ventilatory support while maintaining airway patency 1, 2

Principles of BiPAP Function

The American Academy of Sleep Medicine states that BiPAP utilizes two distinct pressure levels: inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP), which are adjusted independently to optimize ventilation 3, 4
The American Academy of Sleep Medicine notes that IPAP provides ventilatory support during inspiration, while EPAP maintains the airway open during expiration 5, 2
The difference between IPAP and EPAP (pressure support) increases tidal volume and helps improve ventilation 5, 4

Modes of BiPAP Operation

The American Academy of Sleep Medicine describes the spontaneous (S) mode, where the patient determines the time spent in IPAP and respiratory frequency, and the machine responds to the patient's respiratory efforts 6, 5
The American Academy of Sleep Medicine explains the spontaneous-timed (ST) mode, which provides a backup respiratory frequency to ensure minimal ventilation if the patient does not initiate sufficient breaths 6, 5
The American Academy of Sleep Medicine discusses the timed (T) mode, where the machine delivers IPAP/EPAP cycles at a set respiratory frequency with a fixed inspiratory time 6, 5

Components of the BiPAP System

The American Academy of Sleep Medicine lists the components of the BiPAP system, including a pressure generator (air pump) that supplies pressurized airflow, an interface (nasal, oral, or oronasal mask) that fits the face using a harness, a flexible tube connecting the device to the interface, and control systems to adjust pressure levels and monitor therapy 7

Typical Configuration and Settings

The American Academy of Sleep Medicine recommends initial minimum pressure settings for IPAP and EPAP, typically 8 cm H₂O and 4 cm H₂O, respectively, which are adjusted during titration to eliminate respiratory events such as apneas, hypopneas, and respiratory effort-related arousals (RERAs) 3, 7

Differences between BiPAP and CPAP

The American Academy of Sleep Medicine notes that CPAP provides a single continuous pressure throughout the respiratory cycle, whereas BiPAP offers two different pressure levels, which can be more comfortable for some patients who do not tolerate CPAP 3, 2
The American Academy of Sleep Medicine suggests considering a switch to BiPAP if the patient is uncomfortable or intolerant to high CPAP pressures or if respiratory events persist at 15 cm H₂O of CPAP 7

Primary Indications for BiPAP

The American Academy of Sleep Medicine recommends BiPAP for patients with obstructive sleep apnea who do not tolerate CPAP, patients with chronic alveolar hypoventilation, and patients with central sleep apnea who do not respond adequately to CPAP 7, 8, 5, 9

Important Considerations

The American Academy of Sleep Medicine emphasizes the importance of proper mask fitting and a period of acclimatization to low pressures before titration for successful treatment 5, 9
The American Academy of Sleep Medicine recommends polysomnography titration as the preferred method for determining effective levels of nocturnal ventilatory support 5, 9
The American Academy of Sleep Medicine highlights the importance of monitoring airflow, tidal volume, leaks, and delivered pressure to evaluate treatment efficacy 5, 9

CO2 Elimination on BiPAP

Mechanism of CO2 Elimination

The difference between IPAP and EPAP increases tidal volume, which is the primary mechanism for improving alveolar ventilation and CO2 clearance, with IPAP providing ventilatory support during inspiration by generating higher pressure that overcomes the impedance of the lungs and chest wall, delivering a larger breath than the patient could achieve spontaneously 10
The volume of air delivered is a function of the inspiratory pressure level, the inspiratory time duration, and the patient's lung/chest wall mechanics 10

EPAP and CO2 Rebreathing

EPAP maintains positive pressure throughout expiration, which actively flushes exhaled CO2 from the mask and distal ventilator tubing, preventing rebreathing of CO2-rich air 10
A minimum EPAP of 3-4 cm H2O is required to adequately vent exhaled air through the exhalation port in single-limb circuits 10, 11

Titration to Normalize CO2

The American Thoracic Society suggests targeting normalization of PaCO2 in patients with chronic hypercapnic COPD on long-term NIV, using higher inspiratory pressures and backup respiratory rates (high-intensity NIV) 12
Studies demonstrate that NIV specifically targeting CO2 clearance reduces PaCO2 by approximately 4.9 mm Hg compared to less aggressive settings 12
Transcutaneous CO2 monitoring during titration allows real-time adjustment of pressures to optimize ventilation 12
Daytime arterial blood gases can serve as surrogates for nocturnal CO2 changes when overnight monitoring is unavailable 12

Critical Technical Considerations

BiPAP systems typically use single-limb circuits with an integral exhalation port on the mask or an exhalation valve inserted near the mask 10
Initial settings typically start at IPAP 8-10 cm H2O and EPAP 4-5 cm H2O, with pressure support (IPAP-EPAP difference) of 4-6 cm H2O 11

Common Pitfalls

Inadequate EPAP (<3 cm H2O) allows CO2 rebreathing in the circuit, negating the ventilatory benefits of higher IPAP 10
Excessive leak around the mask interface reduces effective pressure delivery and tidal volume, impairing CO2 elimination 11
Overly aggressive titration attempting to normalize CO2 rapidly can trigger glottic closure and patient-ventilator asynchrony 12

Understanding Pressure Support and IPAP in Bi‑Level Positive Airway Pressure (BiPAP)

Definitions and Basic Relationships

Pressure support (PS) is defined as the difference between inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP): PS = IPAP − EPAP. This mathematical relationship clarifies that PS is not an independent setting but derives from the two pressure levels set on the device. [13][14]
IPAP represents the absolute pressure delivered during the inspiratory phase of BiPAP ventilation, providing the driving force for inhalation. [13][14]
EPAP is the lower pressure maintained during expiration, serving to keep the airway open and provide a baseline level of positive pressure. [13][14]

Mechanical Effects of the Settings

The IPAP‑EPAP difference (pressure support) directly augments tidal volume and improves overall ventilation, independent of the absolute magnitude of IPAP alone. [13][14]
EPAP performs several physiological functions: it maintains upper‑airway patency, recruits underventilated lung regions, offsets intrinsic PEEP, and actively flushes exhaled CO₂ from the circuit. 13

Practical Illustrations of Setting Interactions

When IPAP is set to 15 cm H₂O and EPAP to 5 cm H₂O, the resulting pressure support is 10 cm H₂O. This example shows how the PS value is calculated from the two pressures. 14
When IPAP is set to 20 cm H₂O and EPAP to 10 cm H₂O, the pressure support remains 10 cm H₂O, demonstrating that identical PS can be achieved with different absolute pressure levels. 14

Clinical Titration Guidance

During BiPAP titration, clinicians must adjust IPAP and EPAP independently: IPAP (and thus PS) is tuned to achieve adequate ventilation, while EPAP is optimized for airway patency and intrinsic PEEP effects. [13][14]
The magnitude of pressure support (IPAP − EPAP) determines the delivered tidal volume, emphasizing that increasing IPAP without a corresponding increase in EPAP does not automatically raise tidal volume. [13][14]

Distinguishing Ventilation Modes

Do not confuse “pressure support ventilation” (PSV) mode—where a single pressure support level is set above PEEP—with the pressure support value in BiPAP, which is derived from two separately set pressures (IPAP and EPAP). Recognizing this distinction prevents misapplication of settings across modes. [15][13]
In assisted spontaneous breathing (pressure support mode), a single pressure value is set, whereas in bi‑level support (BiPAP), both IPAP and EPAP are set independently, allowing separate control of inspiratory drive and expiratory airway pressure. [15][13]

Role of Bias Flow in Mechanical Ventilation

Physiological Functions of Bias Flow

Bias flow delivers a continuous stream of fresh gas that (1) maintains circuit pressure during spontaneous breathing and (2) actively flushes exhaled CO₂ to prevent re‑breathing. 16, 17
The flow is typically set at 2–3 times the patient’s minute ventilation, which prevents pressure drops in the circuit during inspiration, especially in single‑limb non‑invasive ventilation (NIV) systems. 16, 17
In bi‑level pressure‑support ventilation, continuous bias flow sustains the expiratory positive airway pressure (EPAP) and simultaneously vents exhaled gas through the exhaust port; inadequate bias flow compromises EPAP maintenance. 16, 17, 18
A minimum EPAP of 3–5 cm H₂O, maintained by bias flow, is required to adequately vent exhaled air and avoid CO₂ re‑breathing. 16, 17, 19

Clinical Settings and Flow Requirements

In acutely distressed patients with obstructive lung disease who exhibit high minute ventilation, rapid respiratory rates, and short inspiratory times, peak inspiratory flows can exceed 60 L/min, necessitating high bias‑flow rates to preserve circuit pressure. 17, 19, 20
For home‑use applications such as treatment of obstructive sleep apnea, lower bias‑flow rates are sufficient because minute ventilation and peak inspiratory flows are considerably lower. 19, 18, 20

Ventilator Triggering Mechanism

Bi‑level ventilators employ flow sensors that detect disturbances in the machine‑generated bias flow; this change signals the onset of a patient‑initiated breath and triggers delivery of the preset inspiratory pressure. 21

Complications and Pitfalls

Obstruction of the exhaust port (e.g., by secretions) impairs CO₂ clearance, leading to hypercapnia because bias flow cannot effectively flush CO₂ from the circuit; ensuring proper function of exhalation ports is essential. 17, 19
In patients who are tachypneic or anxious and experience worsening hypercapnia on NIV, insufficient bias flow or EPAP should be considered as a cause; standard EPAP levels (3–5 cm H₂O) may not fully prevent re‑breathing when respiratory frequency is markedly increased. 17, 19, 16

Bias Flow in Mechanical Ventilation

Physiological Role

Bias flow continuously flushes exhaled carbon dioxide from the ventilator circuit and mask dead space, thereby preventing CO₂ rebreathing and the development of hypercapnia in patients receiving non‑invasive ventilation. 22

Flow‑Matching Requirements

To ensure effective CO₂ clearance, the bias flow must be greater than the patient’s peak inspiratory flow during wash‑in procedures; when the flow is insufficient, a drop in marker‑gas concentration is observed during inspiration. 23

Specialized Testing Applications

In multiple‑breath washout testing for preschool children, bias flow is used to deliver a tracer‑gas mixture (e.g., 4 % SF₆, 21 % O₂, 75 % N₂) that enables assessment of pulmonary function. 23
For leak‑free washout during pulmonary function testing, pure O₂ can be delivered through large‑bore tubing connected to a T‑piece and flowmeter, utilizing bias flow to maintain a stable gas environment. 24