CICM SAQ 2015.2 Q18

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).