What determines the cycling between inspiratory and expiratory pressures in a BiPAP (bilevel positive airway pressure) non‑invasive ventilation (NIV) machine?

What Cycles a BiPAP NIV Machine

BiPAP machines cycle between inspiratory (IPAP) and expiratory (EPAP) pressures primarily through flow-based triggering mechanisms that detect changes in patient inspiratory effort and inspiratory flow patterns to determine both the start and end of each breath. 1

Triggering Mechanisms

Inspiratory Triggering (IPAP Initiation)

• Bi-level ventilators employ flow sensors which detect a change in the machine-produced bias flow to initiate the inspiratory phase 1
• The ventilator senses patient inspiratory effort through changes in airflow patterns within the circuit 1
• Trigger sensitivity varies by device, with some ventilators showing poor trigger sensitivity when simulated inspiratory effort is small 1

Expiratory Cycling (IPAP Termination)

The cycling from IPAP to EPAP varies significantly by device and can be adjusted through different mechanisms:

• Flow-based cycling: The ventilator switches to expiration when inspiratory flow decreases to 20-80% of maximum inspiratory flow (Breas PV 401, Puritan Bennett) 1
• Time-based cycling: Inspiratory support is given for a predetermined set period (e.g., pressure setting of 20 cm H₂O for 1.2 seconds on Nippy 1 and 2, Sullivan VPAP) 1
• Manufacturer preset triggers: Some devices have preset cycling criteria that cannot be adjusted (Respironics BiPAP), while others allow only inspiratory trigger adjustment 1

Ventilation Modes and Their Cycling Characteristics

Spontaneous Mode (S)

• The patient determines both the respiratory rate and the time spent in IPAP (within device IPAP time limits) 2
• All breaths are patient-triggered and patient-cycled based on flow detection 2

Spontaneous-Timed Mode (ST)

• Provides a backup rate to ensure minimum respiratory rate when patient effort is insufficient 2
• If the patient fails to initiate an IPAP/EPAP cycle within a time window based on the backup rate, the device delivers a machine-triggered IPAP cycle for the set inspiratory time 2
• For example, with a backup rate of 10 breaths/min, if no spontaneous breath occurs within 6 seconds, the device provides a machine-triggered breath 2

Timed Mode (T)

• The NPPV device delivers IPAP/EPAP cycles at a set respiratory rate with a set inspiratory time, independent of patient effort 2
• This mode is particularly important in patients with advanced acute respiratory failure who may cease making spontaneous effort, or in neuromuscular disease where respiratory effort may be insufficient to trigger a breath 1

Critical Factors Affecting Cycling

Air Leaks

• Excessive leakage can impair both inspiratory triggering and expiratory cycling detection 1
• Patient-ventilator asynchrony may result from undetected inspiratory effort or delays in detecting the end of a breath, especially with excessive leakage 1
• Time-cycled expiratory triggers provide better patient-machine interaction than flow-cycled triggers when air leaks are present 3

Intrinsic PEEP (PEEPi)

• In patients with airflow limitation (especially COPD), PEEPi must be overcome by isometric respiratory muscle contraction before airflow can begin and trigger a breath 1
• EPAP helps offset intrinsic PEEP, thereby facilitating triggering and improving patient comfort 1
• Although PEEPi may reach 10-15 cm H₂O in severe COPD, EPAP levels >5 cm H₂O are rarely tolerated 1

Cycling Criteria Optimization

• In COPD patients, augmenting the cycling criterion from default settings (typically 20-30% peak inspiratory flow) to 40-50% improves patient-ventilator synchrony 4
• Low cycling criteria (10-20%) lead to severe expiratory cycle latency, increased intrinsic PEEP, and non-supported breaths 4
• Setting cycling to 50% of peak inspiratory flow achieves best synchronization in simulated COPD models 4

Common Pitfalls

• Delayed off-cycling in COPD patients causes the ventilator to continue delivering IPAP into neural expiration, leading to patient-ventilator dyssynchrony and potential NIV failure 4, 5
• Interface selection matters: Helmet interfaces increase expiratory cycle latency compared to face masks in almost all settings 4
• NIV mode versus invasive mode: Using the dedicated NIV mode on ventilators decreases expiratory cycle latency compared to invasive mode settings 4