100 Cases in Clinical Medicine

ANSWER 99

The flow–volume curve shows the same low flow throughout the whole volume of the vital
capacity. It is similar in both inspiration and expiration as shown in the flow volume loop
(Fig. 99.2). This situation is typical of a rigid large-airway obstruction. It is not reversible
with bronchodilator therapy. The spirometry trace of volume against time in such cases
shows a straight line of the same reduced flow right up to the vital capacity. These findings
are typical of a narrowing in a larger airway. On examination, this airway narrowing
is likely to produce a single monophonic wheeze which may be heard over a wide area of
the chest.

Flow Volume

Figure 99.3Flow–volume loop: intrathoracic

narrowing.

Flow Volume

Figure 99.4Flow–volume loop: extrathoracic

narrowing.

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The situation may easily be confused with asthma if the peak flow and the wheezing

are accepted uncritically. In asthma, the spirometry will show a reduced FEV 1 but the

flow rate (and therefore the slope of the line relating volume and time) will vary. The

wheezing in asthma comes from many narrowed airways of different calibre and

mass, and the wheezes are often described as polyphonic.

Differential diagnosis of rigid large-airway obstruction

The fixed flow in inspiration and expiration in this case suggest a rigid large-airway nar-
rowing. If the narrowing can vary a little with pressure changes, then the pattern will
depend on the site of the narrowing (Figs 99.3 and 99.4). If it is outside the thoracic cage,
as in a laryngeal lesion, it will be more evident on inspiration. If the site is intrathoracic,
the flow limitation will be greater in expiration. Large-airway narrowing can be caused
by inflammatory conditions such as tuberculosis or Wegener’s granulomatosis, damage
from prolonged endotracheal intubation or by extrinsic pressure such as a retrosternal
goitre. However, the commonest cause is a carcinoma of a large airway.