Assist-control (AC) ventilation is traditionally categorized into two types: pressure-control (PC), where an inspiratory pressure is set, and volume-control (VC), where a target tidal volume is set. Both styles of ventilation allow parameters like the fraction of inspired oxygen (FiO2), respiratory rate (RR), and positive end-expiratory pressure (PEEP) to be adjusted as well. Newer ventilators have modes that try to minimize peak/plateau/mean airway pressures by adjusting HOW a breath is delivered.
If you model the alveolus as a balloon, then one can imagine the effort it takes to initially inflate this sac (ie, overcoming alveolar elastance/surface tension). Once it’s stented open, filling additional air is much easier. It follows that if a ventilator can provide high flows early in the inspiratory phase, it won’t be as laborious to finish filling the alveoli until the preset tidal volume is achieved. This is precisely what we see in hybrid modes like volume control AutoFlow (VC-AF). Unlike traditional VC modes where there’s a square waveform on the flow curve, VC-AF has a high peaking flow early on which quickly decelerates through the rest of inspiration. Almost universally, this leads to decreased peak/plateau pressures on the ventilator with no other settings changed.
In the video, I begin with a standard “square waveform” VC mode (refer to the blue flow-time curve), with a tidal volume of 550 cc, a rate of 15, and a PEEP of 5. Peak airway pressures were ~23 (pink number). Inspiratory flow was initiated, maintained (the “flat” portion of the square), and then cycled to expiration once the tidal volume was achieved. I simply changed the mode to VC-AF without changing any of the parameters above. VC-AF has a high peaking flow early on which quickly decelerates through the rest of inspiration. Notice how the peak pressures dropped to ~19 while maintaining a minute ventilation of ~8.4 L/min the entire time.
Morbid obesity, steep head-down positioning where the abdomen is pushed up into the chest limiting diaphragmatic excursion, and abdominal laparoscopic insufflation are just a few examples where we struggle with elevated airway pressures. In addition to tweaking other ventilator settings, consider a hybrid mode like VC-AF (also called VC+ or pressure-control with volume guaranteed depending on the ventilator).
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