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Volume Control (VC) Versus Volume Control Auto Flow (VC-AF)

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Assist control (AC) ventilation is traditionally broken down into two flavors – pressure-control (PC, an inspiratory pressure is set) and volume-control (VC, a target tidal volume in 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 which 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 stinted 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 exactly 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 start off with a standard “square waveform” VC mode (look at the blue flow-time curve) with tidal volume 550 cc, rate of 15, and 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 aforementioned parameters. 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).

Drop me a comment below with questions!

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