Question 7

Regarding pressure support ventilation in the invasively ventilated patient:
a) Outline the possible methods which could be used to initiate ventilator cycling from inspiration to expiration. (4 marks)
b) List three circumstances under which ventilator default settings for inspiratory to expiratory cycling may contribute to patient-ventilator dyssynchrony. For each circumstance explain the modifications needed to improve patient-ventilator interaction, and briefly explain the rationale for each modification. (6 marks)
 

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College Answer

Syllabus topic/section:

2.1.5 Respiratory Intensive Care – L1.

Aim:

To evaluate candidate knowledge of fundamental principles of ventilator operation.
To allow the candidate to display knowledge of advanced ventilation strategies to improve patient ventilator interaction.

Discussion:

Part (A) – The objective of the question was to assess knowledge regarding cycling in both conventional and newer pressure support modes (PAV and NAVA). Candidates scored well if they mentioned conventional PSV- Flow cycling – the expiration begins when the flow reaches a percentage of peak flow range from 1-70% and traditional set at 25%. IN NAVA what is set by the clinician is the pressure support applied for each millivolt of EMG activity. Inspiration ends at a specific percentage of the peak EMG activity. Candidates performed poorly if they did not mention the above.

Part (B) – Objective of this question was to assess understanding of how to set cycling in spontaneous mode to avoid asynchrony. Candidates did well if they could write about causes of premature and delayed cycling.
Candidates did poorly:

•    If they failed to demonstrate an understanding of asynchrony related to cycling.
•    Candidates were not specific in their answer to pressure support ventilation.
 

Discussion

a) Methods of cycling:

  • Time-cycled ventilation
    • Cycled after a set inspiratory time
    • Mainly used for sedated or paralysed patients, and is typical of mandatory modes
  • Flow-cycled ventilation
    • Cycled after expiratory flow reaches a threshold (typically, 15% or 30%)
    • Typical of spontaneous modes
  • Pressure-cycled ventilation
    • Cycling when a certain pressure is reached
    • Typical of early ventilators
  • Volume-cycled ventilation
    • Cycling when a certain volume is reached
    • Typical of early ventilators
  • Neurally adjusted ventilatory assist (NAVA): 
    • Cycling when a specific percentage of the peak diaphragmatic EMG activity is measured
  • Proportional assist ventilation (PAV):
    • Cycling when flow decreases to a specified value (usually 3 L/min)

b) List three circumstances under which ventilator default settings for inspiratory to expiratory cycling may contribute to patient-ventilator dyssynchrony. For each circumstance explain the modifications needed to improve patient-ventilator interaction, and briefly explain the rationale for each modification. (6 marks)

One might object that if the "objective of this question was to assess understanding of how to set cycling in spontaneous mode to avoid asynchrony", then perhaps a better way to word it would have been "explain how you would  set cycling in a spontaneous mode to avoid asynchrony".  

  • Early cycling off

    • The patient's tidal volumes are insufficient because inspiration is terminated too early

    • This results in patient's voluntary inspiratory flow at the end of inspiration, which increases the work of breathing

    • The solution is to decrease the flow cycling value and therefore prolong the inspiratory time, increasing the inspired volume

  • Delayed cycling off

    • The patient attempts to exhale before the inspiratory phase is finished

    • The flow cycling variable needs to be increased so that the ventilator terminates inspiration at a higher flow rate

  • Double triggering

    • A second breath is triggered shortly after the first

    • This is because the ventilator has not met the patient's demand for tidal volume.

    • The solution is to decrease the flow cycling value and therefore prolong the inspiratory time, increasing the inspired volume

References

Gentile, Michael A. "Cycling of the mechanical ventilator breath." Respiratory care 56.1 (2011): 52-60.

Kacmarek, Robert M. "Proportional assist ventilation and neurally adjusted ventilatory assist." Respiratory Care 56.2 (2011): 140-152.

De Oliveira, Bruno, et al. "Patient–Ventilator Dyssynchrony in Critically Ill Patients." Journal of Clinical Medicine 10.19 (2021): 4550.