MECHANICS OF BREATHING AND PEEP

Are there any problems associated with PEEP?

• ·The patient requires sufficient PEEP to prevent alveolar derecruitment, but not so much PEEP that alveolar overdistension, dead space ventilation and hypotension occurs.

The problem with PEEP is how much to give: insufficient PEEP is of little benefit, excessive PEEP can cause three distinct problems:

1. Alveolar overdistention: alveoli in non-dependent zones are less likely to collapse at end expiration, and tend reside on the upper part of the pressure volume curve, near full distension. Further expansion of the alveoli may cause damage (barotrauma).

2. Excessively high alveolar pressures may squash the blood vessels which surround the airspaces, causing an increase in dead space (wasted ventilation) and an unnecessary increase in the work of breathing.

3. Increased intrathoracic pressure as a result of PEEP/CPAP will reduce the pressure gradient along which blood returns to the heart (flow is always from zones of high pressure to those of low pressure, the negative intrathoracic pressure associated with inspiration enhances this effect). This reduces right ventricular preload, right ventricular output and ultimately cardiac output. This may lead to a reduction in blood pressure and pooling of blood in the abdomen and peripheries. Conversely, in severe heart failure this may be beneficial.

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