Appendix
2: Investigation of the respiratory system
Any patient undergoing general anaesthesia requires some assessment of
respiratory function. This is especially important in patients who are
undergoing thoracotomy and lung resection, and in patients with limited
pulmonary reserve undergoing any surgery.
There
is a range of lung function tests available to provide objective evidence of
pulmonary disease. These tests are useful in determining the functional capacity
of the patient and the severity of the disease, and in
The
simpler tests are the following (Table 47.8).
Peak expiratory flow
rate (PEFR)
This
is measured by a Wright peak flowmeter or a peak flow gauge. This is a reliable
and reproducible test,
Forced
expiratory volume in 1 second (FEy1)
This is the amount of air forcibly expired in 1 second. It is low in
obstructive lung disease and may be normal in restrictive lung disease.
Forced
vital capacity (FVC)
This is the volume of air forcibly displaced following maximal
inspiration to maximal expiration. The FEV1 and the FVC can be
measured using a Vitallograph and a ratio (FEV1/FVC) can be
calculated (Fig. 47.41). A low ratio indicates obstruction and the test should
be repeated after bronchodilarors. A normal ratio (FVC and FEV1 reduced
to the same extent) indicates a restrictive pathology.
Blood
gases
A simple noninvasive probe will measure the oxygen saturation of
haemoglobin but it should be borne in mind that the oxygen content of blood may
fall precipitously at saturations of less than 90 per cent (as an effect of the
oxygen dissociation curve). Arterial blood gases give a great deal of
information; this is summarised in Table 47.9. Changing trends in the data
provided by blood gas analysis are as important as the absolute values.
The
risks and benefits of lung surgery should be discussed with the patients in the
light of these tests. Theme are no absolute guidelines but, in general, a
patient with the following values should tolerate a major lung resection:
• FEV1 >1 litre;
• FVC >2 litres;
• normal carbon dioxide tension (Pco2)