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)
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 inspiratory 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.
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 factory in asynchrony.
NAVA eliminates ‘wasted efforts’ where a patient makes inspiratory effort but fails to trigger the ventilator.
Requires specific nasogastric tube
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.
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
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.
Once a set volume is reached, the ventilator will cycle to expiration (or inspiratory pause).
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
Methods of triggering, their advantages and disadvantages:
|Pressure||triggered by a patient-generated drop in pressure, from PEEP.||
|Flow||Triggered by a patient-generated change in fresh gas flow though a circuit||
|NAVA||Triggered by a change in diaphragmatic EMG, detected by a properly positioned electrode array on a special NGT||
BANNER, MICHAEL J., PAUL B. BLANCH, and ROBERT R. KIRBY. "Imposed work of breathing and methods of triggering a demand-flow, continuous positive airway pressure system." Critical care medicine 21.2 (1993): 183-190.
Sassoon, Catherine SH. "Triggering of the ventilator in patient-ventilator interactions." Respiratory Care 56.1 (2011): 39-51.