CICM SAQ 2015.2 Q18

Question

  • a) List and briefly describe the different mechanisms by which an ICU ventilator may detect (and thus is triggered by) a spontaneous inspiratory effort. Include in your answer the utility and potential disadvantages of each mechanism. (60% marks)
  • b) Outline the mechanisms by which an ICU ventilator may cycle from inspiration to expiration.(40% marks)

Answer

Answer and interpretation

a) List and briefly describe the different mechanisms by which an ICU ventilator may detect (and thus is triggered by) a spontaneous inspiratory effort.

Pressure triggering

  • The ventilator triggers in response to a fall in pressure by a user defined value below set PEEP or CPAP.
  • Requires a respiratory muscle contraction against a static load (closed inspiratory limb) to generate a negative pressure below the threshold set value before fresh gas flow can occur. The imposed work of triggering is high, and may exceed the patient’s reserve, resulting in missed triggers. Working against a static load may cause patient distress. There is significant delay between the initiation of respiratory effort and the onset of any fresh gas flow.

Flow triggering

  • The ventilator triggers in response to a user defined change in flow during the expiratory phase. The exact mechanism is ventilator specific and differs between ventilator types.
  • Obviates some of the disadvantages of pressure triggering. A constant fresh gas flow is available for any inspiratory effort, eliminating patient effort against a static load. However there still remains a delay between inspiriatory effort and the onset of support. Auto triggering and cardiac triggering can occur if the flow is too sensitive.

Neural Assistance, (NAVA)

  • Specific to Maquet Servo ventilators, diaphragmatic EMG is detected by a specific nasogastric tube with an array of bipolar electrodes positioned across the oesophago-gastric junction when the tube is placed correctly.NAVA improves patient-ventilator synchrony when compared with commonly used PSV.
  • NAVA improves patient-ventilator synchrony when compared with commonly used PSV.Patients ventilated with NAVA do not experience the increased tidal volumes and reduced ventilatory frequency seen at higher levels of PSV.
  • Patients ventilated with NAVA do not experience the increased tidal volumes and reduced ventilatory frequency seen at higher levels of PSV.NAVA prevents dynamic hyperinflation which has been implicated as the major
  • NAVA prevents dynamic hyperinflation which has been implicated as the major factory in asynchrony.NAVA eliminates ‘wasted efforts’ where a patient makes
  • NAVA eliminates ‘wasted efforts’ where a patient makes inspiratory effort but fails to trigger the ventilator. Requires specific nasogastric tubeb)

b) Outline the mechanisms by which an ICU ventilator may cycle from inspiration to expiration.(40% marks)

Time cycled.

  • Once the time programmed for inspiration (inspiratory flow time plus inspiratory pause time) is completed, the ventilator automatically cycles to expiration. This occurs independent of any patient effort or other variables.

Flow cycled.

  • Once flow has decreased to a pre-determined minimum value, (eg 25% maximum flow rate), the ventilator cycles to expiration. In lungs with poor compliance, the cycling threshold will be reached more quickly, resulting in a shorter time for inspiration and a smaller tidal volume. Used more in spontaneous modes.

Pressure cycled.

  • Once a set pressure is reached, the ventilator will cycle to expiration. Non-compliant lungs will have smaller tidal volumes than compliant lungs. The most common application for this mode is as an alarm setting as a safety feature to prevent sustained or excessive high pressures.

Volume cycled

  • Once a set volume is reached, the ventilator will cycle to expiration (or inspiratory pause).

  • Pass rate: 77%
  • Highest mark: 8.5

Additional Examiners’ Comments:

  • Overall there was a lack of knowledge on the core topic of ventilator triggering and cycling and inadequate explanation of basic concepts. Some candidates confused pressure with volume and/or flow. Most answers were incomplete and few candidates scored well.

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Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also the Innovation Lead for the Australian Centre for Health Innovation at Alfred Health and Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of two amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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