MECHANICS OF BREATHING AND PEEP

What is “Auto PEEP”?

• Auto-PEEP is gas trapped in alveoli at end expiration, due to inadequate time for expiration, bronchoconstriction or mucus plugging. It increased the work of breathing.

Auto-PEEP is caused by gas trapped in alveoli at end expiration. This gas is not in equilibrium with the atmosphere and it exerts a positive pressure, increasing the work of breathing,

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In patients with obstructive airways disease, increased bronchial tone leads to resistance to both inspiration and expiration. The greatest pathological problem occurs in the expiratory limb. The patient feels air hunger and so increased the velocity of inspiration in order to get air into the lungs, however, once there, this air has difficulty leaving, particularly if the expiratory phase is short (in a tachypneic patient). The result is gas trapping at the end of expiration and dynamic hyperinflation of the lungs.
The FRC is actually increased. We can do the same thing by ventilating a patient in a manner that prevents full deflation to FRC at the end of expiration. In addition, gas can become trapped if the girth of the connecting airways is narrowed, for example with mucus plugs, increasing the closing volume.
The problem with gas trapping or “auto PEEP” is that gas trapped in the airways exerts a positive pressure, and normal gas transit cannot be reestablished until there is a pressure gradient from the mouth to the alveoli. Thus the patient must generate a much higher negative inspiratory pressure to open up dynamically compressed airways (to make the pressure within negative with respect to atmospheric pressure).

If auto-PEEP occurs during mechanical ventilation, the amount of time given over to expiration needs to be lengthened: either by reducing the respiratory rate or the inspiratory time, or both.

Giving a patient who has auto-PEEP CPAP is a bad idea right?

• The increased work of breathing associated with auto-PEEP can be offloaded by applying CPAP to the trachea/mouth, and splinting open the connecting airways.

This is a huge misnomer. If a patient is generating auto PEEP, then the pressure in their airway is positive with respect to the outside atmosphere at the end of expiration. This is not good – because, in order for the alveoli (in which air is trapped) to participate in gas exchange, the pressure inside them must be dragged down below atmospheric, in order for air to flow down through the airway. This means that they have to use their inspiratory reserve muscles – a process that hugely increases the work of breathing. CPAP is a perfect solution for a patient in respiratory failure with gas trapping: by applying a positive pressure to the airway, through a mask or endotracheal tube, a pressure gradient is developed, which opens up the airway. The secret is to apply a CPAP, at a level that is higher than the auto-PEEP (waterfall effect).

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