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 predicting the response to various treatment options. The tests range from those that are simple and can be done at the bedside to those that are only available in specialist centres.

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, but has the disadvantage of being effort dependent and therefore it may be affected by abdominal or thoracic wound pain.

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)

  age <70 years.