Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 75

Ventilatory performance varies greatly with patient height, age and sex. Tables and prediction equations are available to help determine a patient’s ‘predicted normal value’. The patient’s test result may be compared with the mean reference value and the standard deviation of results obtained in the healthy population or (more commonly, though less usefully) may be expressed as a percentage of the population’s mean reference value. For example, a man with a vital capacity of 3.95 litres (predicted value 4.70 litres). His result could be expressed as 84% of the predicted mean and an injudicious interpretation of this might lead to the mistaken assumption that there is a 16% ‘disability’. The deficit may be due to lung disease but the standard deviation for vital capacity is about 500 mL. His value is only 1.5 standard deviations from the mean, and many normal individuals (of the same age and size) with no apparent lung disease will have values lower than this.

Pulmonary function tests should not be interpreted in isolation and should be considered in the context of all additional information concerning the patient.

In this chapter, we will look at:

 simple tests of ventilatory function

 tests of gas transfer

 arterial blood gases.