4-6-26 Respiratory Management and Mean Airway Pressure

Patient Oxygenation and Ventilator Management

Overview of Oxygenation Issues

• Discussion on situations where a patient's oxygen content or hemoglobin levels are normal, but alveolar function is impaired (e.g., removal of functioning alveolar units).

• Example with the left lung not picking up oxygen, resulting in "trucks leaving empty" to signify ineffective gas exchange.

Importance of Mean Airway Pressure

• Increasing oxygen supply (FiO₂) is not the only solution; mean airway pressure must be increased as well.

• Mean Airway Pressure (MAP):

  • Not required for tests but denotes importance in managing ventilation.

  • Helps in recruiting collapsed alveoli and facilitating oxygen exchange.

  • Mean Airway Pressure (MAP) is the average pressure applied to the airways over the entire respiratory cycle—including inspiration and expiration. It’s one of the most important determinants of oxygenation on the ventilator.

  • MAP is shaped by five variables:

    • PIP — ↑ PIP increases MAP

    • PEEP — the strongest driver of MAP

    • Inspiratory time (Ti) — longer Ti = higher MAP

    • Respiratory rate (RR) — higher RR shortens Ttot → increases MAP

    • Waveform — square wave produces higher MAP than decelerating ramp

Understanding PEEP and MAP

• Positive End-Expiratory Pressure (PEEP):

  • Enhances mean airway pressure and plays a key role in re-expanding collapsed alveoli.

  • Mechanistically, it increases pressure in the chest throughout the respiratory cycle.

  • Incrementally can be adjusted:

    • Increasing from 5 to 10 cm H₂O PEEP significantly enhances MAP.

• Merely increasing FiO₂ does not address the issue of collapsed alveoli.

Pressure Waveform Analysis

• Graphical representation of pressure over time with values ranging from 0 to 40.

  • Illustration of a breath cycle with pressure support rising up to 30 cm H₂O with a baseline of 5 cm H₂O PEEP.

  • Respiratory rate impacts pressure but does not significantly increase MAP unless corresponding increments in PEEP occur.

Strategies for Improving Oxygenation

• If standard oxygenation measures are insufficient, consider:

  • Adjusting PEEP further to augment mean airway pressure effectively.

  • Using a longer inspiratory time to maintain elevated pressure longer than a typical cycle (e.g., increasing from 1 second to 1.5 seconds).

    • Note: This still only affects pressure for the duration of the inspiratory phase.

• Rather than pursuing volumetric adjustments or increased respiratory rates, focus on maintaining mean airway pressure through PEEP adjustment.

  • This is referred to as the "most bang for your buck" strategy in improving oxygenation issues.

Clinical Context and Considerations

• Considerations around patient positioning and volume management, especially with postoperative patients.

  • Discussion of common scenarios leading to atelectasis, especially in supine patients.

  • Assessment of the case: Left-sided collapse and potential atelectasis due to flat positioning during recovery s are emphasized, touching on patients who may not receive adequate lung volumes during mechanical ventilation.

• Individuals addressing ventilation strategies should be cognizant of the patient's position and historical treatments to tailor interventions effectively.