MECHANICS OF BREATHING AND PEEP

What are the problems associated with airway collapse?

·        Airway collapse increases the work of breathing and leads to ventilation-perfusion mismatch

Collapsed or closed airways are:

1. A source of ventilation-perfusion mismatch: this leads to hypoxemia and hypercarbia.

2. Tremendously difficult to reinflate: leading to a huge increase in the work of breathing & oxygen consumption.

Have you ever tried to inflate a completely deflated balloon: it’s really difficult to inflate initially, then you feel a give and the rest is a cinch! Inflating alveoli is like this. A more appropriate approach would be to let the balloon (alveolus) deflate to just above the point where inflation becomes easy (the lower inflection point of the pressure-volume curve), then reinflation is much less work. This is the concept behind CPAP.

So the outcome is reduction in FRC, venous admixture and increased work of breathing: for cause include just about any reason why a patient may require ventilatory assistance (including endotracheal intubation).

In the diagram above you can see that ventilation-perfusion mismatches exist along a spectrum of either completely wasted ventilation or perfusion without ventilation. In most situations the V/Q mismatch exists somewhere between these two ends, usually in the low V/Q territory. It is important to understand that mismatches do not necessarily occur during both phases of the respiratory cycle - the main timing of mismatch is during expiration, when vulnerable alveoli collapse en masse, in acute lung injury. If you run the mouse over the picture below you can see how this occurs:

If the alveoli are particularly vulnerable to collapse in expiration, it is clear that the application of positive end expiratory pressure, which serves to splint open the alveoli, will not only reduce the work of breathing, but also prevent ventilation-perfusion mismatch. The objective of using PEEP is to restore Functional Residual Volume.

The cartoon below demonstrates what happens to vulnerable alveoli when PEEP is applied: run your mouse over the picture to see the effect of PEEP.

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