Neuromuscular blockade should not be used with bilevel ventilation that requires spontaneous breathing to meet the patient’s ventilatory needs.
Bilevel ventilation is not recommended in patients who require deep or heavy sedation or have obstructive lung conditions that require longer expiratory time.
Bilevel is a pressure-controlled, time-triggered, time-cycled mode of ventilation that allows unrestricted, spontaneous breathing with or without pressure support (PS) throughout the entire ventilatory cycle.undefined#ref2">2 Bilevel is designed for invasive mechanical ventilation. Setting options, terminology, and abbreviations may be brand specific based on the mechanical ventilator specifications.2,4
Bilevel is a mechanical ventilation lung-protective strategy used to meet the acute respiratory distress syndrome (ARDS) management goals by maximizing alveolar recruitment, patient comfort, and patient-ventilator synchrony, while minimizing the risk of barotrauma and the need for heavy sedation.7 When inverse ratio ventilation (IRV) is used, bilevel conceptually applies airway pressure release ventilation (APRV) principles.5 APRV is a lung-protective strategy that helps to meet the goals of ARDS management and to diffuse pneumonia and atelectasis by maximizing alveolar recruitment while limiting the transalveolar pressure gradient and barotrauma. There are limited studies on bilevel, and the consensus among practitioners regarding initial settings is limited and primarily provided in the operator’s manual for mechanical ventilators that have bilevel or an equivalent mode.
Bilevel uses two set levels of pressure, usually referred to as positive end-expiratory pressure (PEEP), that are set by the respiratory therapist (RT). The higher level of pressure (P high) is set to support alveolar recruitment and oxygenation. The lower level of pressure (P low) is set to minimize alveolar derecruitment during a brief expiratory (release) phase.3 The difference between the two pressure levels determines the tidal volume (VT) delivered, where most of the ventilation and carbon dioxide removal occurs during the release from P high to P low (Figure 1). The difference between P high and P low can be adjusted to deliver a VT of 6 to 8 ml/kg in accordance with ARDS Network protocol.6 Again, conceptually APRV principles of ventilation are used.5
Bilevel uses a set frequency in conjunction with three time-variable options to determine the time at P high and P low. The set frequency may be referred to as the release rate or release breath, which means the number of times the ventilator releases the pressure from P high to P low in a 60-second time frame. Although there are three possible time-variable options available, only one of them is a set value that is locked constant. The other two time-variables are determined by the set frequency and the set time-variable that is locked constant. Three time-variable options include:
The T high:T low ratio is known as the inspiratory-to-expiratory (I:E) ratio in relationship to the total ventilatory cycle during conventional ventilation modes. The T high:T low value is often set to be inverse when using a bilevel ventilation mode.
PS may be used to augment spontaneous VT breathing. Depending on the brand of ventilator used, the PS is generally applied at P low. It is important for the RT to ensure that PS is set high enough to deliver pressure-supported breaths at P high if this support is desired.3 The mechanical ventilator operator’s manual that is specific to the brand being used should be consulted.
Patients who have obstructive lung disease or who require a longer expiratory time may not benefit from bilevel because of patient-ventilator asynchrony caused by auto-PEEP and IRV.
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Setting options, terminology, and abbreviations may be brand specific based on the trademarked mechanical ventilator specifications.7
Rationale: A higher transalveolar pressure recruits the lungs.
P high should not exceed 30 cm H2O to minimize the risk of ventilator-induced lung injury.2,5
There are three possible time-variable options available, but only one at a time can be set constant or locked in the bilevel mode.
Locking one of the time-variable settings ensures that the time-variable remains constant even when changes occur in the set frequency.
PS is generally applied to P low and designed to support spontaneous breaths at P low.
Consider the difference between P high and P low when setting PS for P high. Refer to the brand-specific mechanical ventilator manual for specific recommendations.
MAP is best below 30 cm H2O.4,5
Rationale: Less time at T high means more time at T low for exhalation and carbon dioxide elimination.
Rationale: The greater the difference between P high and P low, the larger the exhaled volume during the release breath for more carbon dioxide removal.
Rationale: More release breaths means more time in the expiratory phase.
Monitor oxygenation and avoid alveolar derecruitment.
Rationale: Longer time at P high increases MAP, alveolar recruitment, and oxygenation.
Rationale: Less time at T low means less alveolar derecruitment time.
Fredericks, A.S. and others. (2020). Airway pressure release ventilation: A review of the evidence, theoretical benefits, and alternative titration strategies. Clinical Medicine Insights: Circulatory, Respiratory and Pulmonary Medicine, 14, 1-9. doi:10.1177/1179548420903297
Gallagher, J.J. (2018). Alternative modes of mechanical ventilation. AACN Advanced Critical Care, 29(4), 396-404. doi:10.4037/aacnacc2018372
Clinical Review: Jennifer Elenbaas, MA, BS, RRT, AE-C
Published: August 2023
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