A detailed discussion of the pathophysiology
of dyspnoea and tachypnoea is not necessary in this book, but some understanding
of the reasons for changing respiratory pattern in disease can be useful.
The minute volume is the product of the
tidal volume and the number of breaths per minute and roughly equates with the
alveolar ventilation, once the anatomical and physiological dead-space ventilations
In order to maintain or increase minute
volume the respiratory rate or tidal volume can be changed to suit physiological
In respiratory diseases which reduce
the available surface area for gaseous exchange (pneumonia, alveolar oedema)
or restrict lung expansion (pleural effusion), the ability to increase tidal
volume is compromised and so respiratory rate increases.
In conditions which interfere with airflow
during inspiration (laryngeal paralysis), the reduction in minute volume is a
function of the restriction of tidal volume expansion. An increased inspiratory
effort is then used to attain the required tidal volume and this results in a
slow inspiratory dyspnoea.
With expiratory dyspnoea there is interference
with removal of the inspired volume from the lungs. As air becomes trapped in
the lungs, stretch receptors activate an additional expiratory effort close to
the end-expiratory point, which is heard as an expiratory dyspnoea. This form
of respiratory embarrassment is not a function of alveolar impairment, and is
typically seen with pulmonary neoplasia, tracheal collapse and feline asthma.
In conditions where there is loss of
a large part of the functional lung (e.g. pulmonary oedema, pneumonia), expiration
is rapid and of short duration and is a function of the increased elastic recoil
of the lung in such situations.
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