Mechanical Ventilation: Bilevel Ventilation (Respiratory Therapy)

ALERT

Don appropriate personal protective equipment (PPE) based on the patient’s signs and symptoms and indications for isolation precautions.

Bilevel ventilation is not recommended in patients who require deep or heavy sedation.

Neuromuscular blockade should not be used with bilevel ventilation that is dependent on spontaneous breathing to meet the patient’s ventilatory needs.

OVERVIEW

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 Bilevel may use traditional inspiratory-to-expiratory (I:E) ratios, where the inspiratory portion of the ventilatory cycle is shorter than the expiratory portion. However, bilevel allows for prolonged inverse ratio ventilation (IRV), where the expiratory portion is shorter than the inspiratory portion of the ventilatory cycle.³ When IRV is used, bilevel conceptually applies airway pressure release ventilation (APRV) principles.⁵ 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:

• Time high (T high), which is the length of time at P high
• Time low (T low), which is the length of time at P low
• Time high-to-time low ratio (T high:T low), which is the ratio of time at P high to P low

The T high:T low ratio is known as the 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.

EDUCATION

• Provide developmentally and culturally appropriate education based on the desire for knowledge, readiness to learn, and overall neurologic and psychosocial state.
• Explain the need for ventilator changes to the patient and family.
• Encourage questions and answer them as they arise.

ASSESSMENT AND PREPARATION

Assessment

1. Perform hand hygiene before patient contact. Don appropriate PPE based on the patient’s need for isolation precautions or the risk of exposure to bodily fluids.
2. Introduce yourself to the patient.
3. Verify the correct patient using two identifiers.
4. Assess the patient’s level of consciousness and ability to understand and participate in decisions. Include the patient as much as possible in all decisions.
5. Assess the patient for indications for bilevel use and signs of ARDS.⁴
   1. Decreasing partial pressure of arterial oxygen/fraction of inspired oxygen (PaO₂/FIO₂) ratio
   2. Increasing plateau pressure, peak inspiratory pressure (PIP), or mean airway pressure (MAP)
   3. Bilateral lung infiltrates on a chest radiograph
6. Assess the patient’s hemodynamic and cardiorespiratory systems.

Preparation

1. Gather equipment, including the ventilator with bilevel or equivalent mode, circuit, humidification device, filters (if needed), and closed-suction device.
2. Before initiating the mechanical ventilator, check the system microprocessor or ventilation system. Perform a short self-test as appropriate.
   1. Verify compliance of the ventilator circuit with the humidification device and filters (if needed).
   2. Document the completed ventilation system test. Include pass or fail, date, initials or signature, and credentials.
3. Verify the authorized practitioner’s order for the initiation of mechanical ventilation.

PROCEDURE

1. Perform hand hygiene and don gloves. Don additional PPE based on the patient’s need for isolation precautions or the risk of exposure to bodily fluids.
2. Verify the correct patient using two identifiers.
3. Explain the procedure and ensure that the patient agrees to treatment.
4. Transition the patient to bilevel ventilation from conventional ventilation using the prescribed settings.

   Setting options, terminology, and abbreviations may be brand specific based on the trademarked mechanical ventilator specifications.⁷

5. Set P high for the higher pressure. The starting point is usually the plateau pressure from the volume-controlled mode or PIP from the pressure-controlled mode.²

   P high should be no higher than 30 cm H₂O.^2,5

6. Set P low for the lower pressure. The starting point is usually the optimal PEEP from the previous ventilation mode.¹
7. Set the frequency.
8. Determine which of the time-variable settings will be constant and locked if this option is available. Time-variable options include:
   1. T high is the length of time at P high.
   2. T low is the length of time at P low.
   3. T high:T low ratio is the length of time at P high in relationship to P low.

      There are three possible time-variable options available, but only one at a time can be set constant or locked in the bilevel mode.

9. Set the constant time and the lock icon to the closed position on the time-variable that is desired to remain constant, if this option is available.

   Locking one of the time-variable settings ensures that the time-variable remains constant when the set frequency is changed.

10. Set the inspiratory rise time (%) for patient comfort.
11. Set the patient sensitivity for flow-triggered or pressure-triggered spontaneous breaths.
12. Set the PS for both P low and P high, if desired.
    1. Verify spontaneous supported breaths at P low with the pressure-time waveform.

       PS is generally applied to P low and designed to support spontaneous breaths at P low.

    2. Verify spontaneous supported breaths at P high with the pressure-time waveform.

       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.

13. Set the FIO₂ for the desired PaO₂ or peripheral oxygen saturation (SpO₂) level as prescribed.
14. Adjust settings based on the patient’s release and spontaneous VT, SpO₂, end-tidal carbon dioxide (ETCO₂), arterial blood gas (ABG) values, expiratory flow pattern, and clinical status.

    MAP is best below 30 cm H₂O.^4,5

    1. To decrease partial pressure of carbon dioxide (PaCO₂):
       1. Decrease T high.

          Rationale: Less time at T high means more time at T low for exhalation and carbon dioxide elimination.

       2. Increase P high.

          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.

       3. Increase set frequency.

          Rationale: More release breaths means more time in the expiratory phase.

    2. To increase PaCO₂:
       1. Decrease set frequency.
       2. Increase T high.
       3. Decrease P high.

          Monitor oxygenation and avoid alveolar derecruitment.

    3. To increase PaO₂:
       1. Increase FIO₂.
       2. Increase T high.
       3. Increase P high.

          Rationale: Longer time at P high increases MAP, alveolar recruitment, and oxygenation.

       4. Decrease set frequency.

          Rationale: Less time at T low means less alveolar derecruitment time.

15. Remove PPE and perform hand hygiene.
16. Document the procedure in the patient’s record.

MONITORING AND CARE

1. Regularly perform a check of ventilator settings and measured parameters.
2. Ensure that all ventilator alarms are on and set appropriately for the patient’s individual ventilator settings.
3. Monitor the patient’s ventilator pressure-time and flow-time waveforms to evaluate spontaneous and delivered pressure and inspiratory and expiratory flow patterns.
4. Monitor the patient’s SpO₂, ETCO₂, and hemodynamic status.
5. Report any signs of increased MAP.
6. Assess the patient’s exhaled minute volume, release VT, and spontaneous VT.
7. Monitor the patient’s ABG values as needed.
8. Assess the patient’s overall comfort level.
9. To minimize alveolar derecruitment, consider using a closed-suction device to minimize the number of times the patient is disconnected from the ventilator.
10. Observe the patient for signs and symptoms of pain. If pain is suspected, report it to the authorized practitioner.

EXPECTED OUTCOMES

• Improved oxygenation
• Improved ventilation
• Improved patient-ventilator synchrony and comfort
• Alveolar recruitment
• Minimization for ventilator-induced lung injury
• Reduced sedation requirements

UNEXPECTED OUTCOMES

• Alveolar overdistention
• Alveolar derecruitment
• Worsening oxygenation
• Worsening ventilation

DOCUMENTATION

• P high
• P low
• T high
• T low
• FIO₂
• T high:T low ratio (set)
• I:E ratio (actual)
• PS
• Inspiratory rise time (%)
• Minute volume (total)
• Minute volume (spontaneous)
• Exhaled VT release breath
• Exhaled VT spontaneous breath
• Frequency (set)
• Respiratory rate (total)
• MAP
• SpO₂
• ETCO₂ (optional)
• Patient’s tolerance of procedure
• Education
• Unexpected outcomes and related interventions

REFERENCES

1. Cairo, J.M. (2020). Chapter 5: Selecting the ventilator and the mode. In Pilbeam’s mechanical ventilation: Physiological and clinical applications (7th ed., pp. 58-79). St. Louis: Elsevier.
2. Cairo, J.M. (2020). Chapter 23: Special techniques used in ventilatory support. In Pilbeam’s mechanical ventilation: Physiological and clinical applications (7th ed., pp. 474-502). St. Louis: Elsevier.
3. Davies, J. (2020). Chapter 4: Mechanical ventilators. In D.C. Shelledy, J.I. Peters (Eds.), Mechanical ventilation (3rd ed., pp. 155-280). Burlington, MA: Jones & Bartlett Learning.
4. Hess, D.R., Kacmarek, R.M. (2019). Chapter 8: Advanced modes of mechanical ventilation. In Essentials of mechanical ventilation (4th ed., pp. 73-86). New York: McGraw-Hill Education.
5. Holt, G.A., Habib, S.A., Shelledy, D.C. (2020). Chapter 3: Principles of mechanical ventilation. In D.C. Shelledy, J.I. Peters (Eds.), Mechanical ventilation (3rd ed., pp. 95-154). Burlington, MA: Jones & Bartlett Learning.
6. NIH NHLBI ARDS Clinical Network. (2008). Mechanical ventilation protocol summary. Retrieved on February 25, 2022, from http://www.ardsnet.org/files/ventilator_protocol_2008-07.pdf (classic reference)* (Level VII)
7. Shelledy, D.C., Peters, J.I. (2020). Chapter 6: Ventilator initiation. In D.C. Shelledy, J.I. Peters (Eds.), Mechanical ventilation (3rd ed., pp. 311-366). Burlington, MA: Jones & Bartlett Learning.

ADDITIONAL READINGS

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

*In these skills, a “classic” reference is a widely cited, standard work of established excellence that significantly affects current practice and may also represent the foundational research for practice.

Elsevier Skills Levels of Evidence

• Level I - Systematic review of all relevant randomized controlled trials
• Level II - At least one well-designed randomized controlled trial
• Level III - Well-designed controlled trials without randomization
• Level IV - Well-designed case-controlled or cohort studies
• Level V - Descriptive or qualitative studies
• Level VI - Single descriptive or qualitative study
• Level VII - Authority opinion or expert committee reports