The prevailing wisdom can sometimes be difficult to budge. And how can we do better for our ventilated patients?
Dr Peter Oziemski, intensive care director at Maroondah, outlines a way of helping patients make a smoother recovery from mechanical ventilation.
But what if some of our other accepted norms around mechanical ventilation are not the best we could be doing?
And what if we could be giving something akin to PEEP throughout most of the respiratory cycle?
Say hello to Airway Pressure Release Ventilation (APRV).
Previously held as a “rescue therapy” in ARDS, could this strategy effectively give recruitment manoeuvres and ARDSnet-style low-tidal-volume methods a run for their money? And could we be using it earlier to help a large proportion of our intubated patients?
The dogma to question here is this: just because our normal respiratory cycle has an I:E ratio of 1:2 or 1:3 does that really mean that we should mechanically ventilate people the same way by default?
Many critically unwell patients do not have normal lungs. Most are at risk of atelectasis, atelectatrauma, volutrauma and so forth. Low tidal volume research might have answered one therapeutic question – we are fairly sure that larger tidal volumes (10ml/kg) delivered in the usual mechanical ventilation way are harmful to susceptible lungs. But the questions of how best to deliver pressure and how to cycle the ventilator remain unresolved.
So how do we set up APRV?
We need to select a Bilevel Pressure-cycled mode – so far we don’t have an APRV button.
T(high) 5.5 sec (on the Hamilton choose a rate of 8 and the longest Inspiratory Time allowable)
T(low) 0.7 sec (range 0.4-0.8)
P(high) 26 mbar (can start with plateau pressure from standard mode)
P(low) 0 (release time will cause intrinsic PEEP)
FiO2 as low as possible
Note: flow is set in order to achieve a termination of the release phase at about 75-80% of the peak expiratory flow.
Sounds TOTALLY counterintuitive .. or does it?
Perhaps it’s just different from what we have all been teaching each other for a few years.
The extensive series of diverse cases presented by Dr Oziemski illustrated just how marked an improvement patients can make clinically and radiologically when their alveoli are given the pressure they need to function (ie to stay open).
This ventilation strategy allows for spontaneous breaths at any time in the cycle (tube compensation set to on, if needed). The feedback from patients who have been ventilated this way is reassuring – they find it comfortable, even preferable.
APRV is not suitable for patients with severe asthma (due to their high intrinsic PEEP). In all other patient groups there is theoretically the risk of volutrauma. However Dr Oziemski’s explanation that lower peak and plateau pressures can be achieved (P high 15-30) suggests that careful bedside titration plus structured review of CXRs (such as counting ribs to check for hyperinflation) will minimise these drawbacks.
Perhaps the days of SIMV set-and-forget mechanical ventilation are not quite numbered, yet. But the next time you intubate someone with pneumonia, or aspiration, or morbid obesity, or COAD …. contemplate whether APRV should be your first strategy..
A safe approach to intubation starts with understanding the key components of this potentially complex resuscitative procedure ..
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