Respiratory mechanics

One approach to characterizing the underlying pathology is the pressure-volume ( P-V) curve, from which several variables can be derived (Gattinoni etM: 1987).

The initial slope (Cstart) of the P-V curve, which is also called the initial compliance, gives an idea of the dimensions of the baby lung at atmospheric pressure. In an adult man, Cstart values of 20 cmH2O, 30 cmH2O, 40 cmH2O, etc. correspond to baby lung volumes of 20 per cent, 30 per cent, 40 per cent, etc. of the original healthy lung. The inflection point (or inflection zone) suggests the potential for recruitment (i.e. the presence of compression atelectasis) and indicates the pressure at which most of the recruitment may occur. The slope (Cin() of the P-V curve after the inflection point may provide an estimate of the amount of recruitment (the steeper the P-V curve, the greater the recruitment). Finally, the upper inflection point indicates the pressure at which stretching of the pulmonary units becomes the dominant phenomenon.

The P-V curve may then allow an initial estimate of the underlying pathology. The presence of an inflection point suggests compression atelectasis, whereas its absence implies consolidation or vessel alterations. Unfortunately, the P-V curve may be misleading as the inflection point may be absent for a number of reasons other than lack of recruitment.

1. Recruitment proportional to the applied pressure results in a linear P-V curve.

2. A linear P-V curve may be obtained when one lung is overstretched and the other is being recruited.

3. As the P-V curve of the respiratory system is obtained from the P-V curves of the lung and the chest wall, an inflection point on the chest wall may lead to erroneous interpretation of the P-V curve of the total respiratory system.

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