When the diaphragm pushes down during inspiration, negative pressure in the pleural cavity is generated, this, in turn, creates negative pressure in the airways that suck air into the lungs. Normal respiratory physiology works as a negative pressure system. Mechanical ventilation has several effects on lung mechanics. Since having a patient on mechanical ventilation allows a practitioner to modify the patient’s ventilation and oxygenation, it has an important role in acute hypoxic and hypercapnic respiratory failure as well as in severe metabolic acidosis or alkalosis. In respiratory physiology, total compliance is a mix of lung and chest wall compliance as these two factors cannot be separated in a patient. Signs that weaning is not being tolerated include increased work of breathing or increased ETCO2/PaCO2 (provided there is no significant increase in WOB or ETCO2 then a blood gas is not required for each weaning step).Compliance: Change in volume divided by change in pressure. You don’t need to wait till a certain point in the patients admission to start to wean them and can start weaning straight away (I would encourage you to ask yourself ‘can I wean’ on every gas you review). If the patient is stable on PS CPAP ON 6/6 (peak pressure of 12) and there are no contraindications a trial of extubation can be considered. Wean the PS in steps of 2 till a pressure of 6 cmH2O is reached. When switching to ‘PS CPAP’, keep the pressure support and PEEP set the same as it was on the previous mode. Once you have reached a rate of 5 breaths per minute the next step is to switch the patient to PS CPAP (the patient will already be mostly on ‘PS CPAP’ as only 5 of their breaths will be ‘big breaths’ and all other breaths will be PS breaths. Provided the patient is triggering breaths at or above the set rate then all you will be doing is swapping a ‘big breath’ with a guaranteed Ti and tidal volume/Pressure for a smaller ‘Pressure Support’ breath. Īs soon as able wean the rate in steps of 5 breaths. It is important to note that the ‘PC above PEEP’ can’t be reduced below what is set for ‘PS above PEEP’, nor ‘PS above PEEP’ turned above what is set for ‘PC above PEEP’ (this makes sense as the ‘PC above PEEP’ are meant to be the ‘big breaths’ and the ‘PS above PEEP’ are meant to be the smaller support breaths. This mode also supports all breaths and any breaths the patient takes above the set SIMV rate will be supported with a Pressure Support breath, just like in ‘SIMV (PRVC) + Pressure Support’. This is why it is important to monitor the tidal volume delivered on the ventilator and make sure it is appropriate for the patient and to set tight limits on the minute volume, so the ventilator will alarm appropriately if this is reduced. ![]() pneumothorax or collapse, the delivered tidal volume will decrease. The tidal volume delivered is not guaranteed and will vary depending on the compliance of the lungs, so if lung compliance worsens e.g. ![]() The ventilator will then deliver this pressure for the duration of the inspired time, at the SIMV rate times per minute. Instead of setting a tidal volume you set ‘Pressure Control above PEEP’ (remember this is the pressure above PEEP, not the peak pressure i.e setting ‘Pressure Control above PEEP’ to 15 and ‘PEEP’ to 6 will result is a ‘Peak Pressure’ of 21).
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