You have commenced your shift in ICU and received clinical handover on Carl, a 53 year old male patient who was transferred to ICU from the ward two hours ago following a MET call for acute respiratory distress on a background of CAP. He is now intubated and ventilated on Synchronised Intermittent Mandatory Ventilation (SIMV) + Pressure Support (PS).
High pressure alarms can indicate a problem with the ventilator such as a kinked circuit/tubing, pooling of condensed water vapour; dislodgement of the ETT; mucous plug/secretions blocking the ETT; bronchospasm; coughing; decreased compliance. Carl may also be agitated and biting on the ETT.
There are a range of actions that should take place to investigate the cause of the alarms and initiate appropriate management. These are discussed further in the Mechanical Ventilation webinar.
If the high alarms are leading to a decreased minute ventilation due to loss of Tidal Volume, the mode of ventilation should be reviewed.
SIMV is volume control ventilation mode that delivers a set number of volume controlled breaths, as well as allowing the patient to take spontaneous breaths. These additional breaths are triggered when the airway pressure drops below the end-expiratory pressure. The tidal volume of the spontaneous breaths can be enhanced using pressure support (PS).
The breaths in SIMV mode are provided by triggered ventilation which allows for synchrony between the patient and the ventilator. If a breath hasn’t been triggered within a predetermined time period, a mandatory breath is automatically delivered. Mandatory breaths are supported to set a PIP/volume target whereas patient triggered breaths are supported to either the PS level or to PS + PEEP depending on the ventilator. The mandatory rate supports ventilation during apnoea.
Because the TV of the mandatory breath is pressure limited to <30cmH2O (as per the PIP alarm setting), decreased TVs with resultant low MVs may occur in patients like Carl who develop increased airway pressures, such as with bronchospasm or decreased lung compliance. In these instances a change from SIMV to Pressure Controlled Ventilation (PCV) may be considered.
PCV delivers a constant pressure during each breath to a set pressure limit meaning that the TV delivered may vary, as well as the MV. The controlled inspiratory pressures provide protection against barotrauma. The inspiratory pressure in PCV should be set to create a tidal volume of 6mL/kg in patients with high airway pressures. Other ventilator settings include Rate, PEEP, FiO2, Flow Trigger, rise time and I:E (set directly or by Tinsp).
As the TV with PCV will vary as the patient's lung compliance evolves, it will need to be closely monitored. The inspiratory pressure may need to be adjusted to ensure that the patient is not receiving too high or too low a TV.
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