Respiratory Muscle Strength
All measurements of pulmonary function that require patient effort
(e.g., FVC, FEV1
peak flow, MVV) are influenced by the strength of the
respiratory muscles. The latter can be specifically evaluated by measurement of
maximum static respiratory pressures. The pressures are generated against an occluded
airway during a maximum forced inspiratory or expiratory effort and are usually measured
with aneroid gauges.[8]
Maximum
static inspiratory pressure (PImax) is
measured when inspiratory muscles are at their optimal length near RV ( Fig.
26-3
). Similarly, maximum static expiratory pressure
(PEmax) is measured when expiratory muscles are optimally
stretched after a full inspiration to near TLC. In young adult men, the PImax
is about -125 cm H2
O, and the PEmax is
about +200 cm H2
O.
The pressure measured at the mouth includes that generated by
the respiratory muscles and a portion of that resulting from the elastic recoil of
the respiratory system. The latter is essentially zero at functional residual capacity
(FRC). Pressures measured at FRC are less than at the extremes of lung volume (see
Fig. 26-3
), but unlike the
other values, they reflect solely the pressures developed by the respiratory muscles.
A PImax of -25 cm H2
O
or less indicates severe inability to take a deep breath, whereas a PEmax
of less than
Figure 26-3
Normal values for maximum static inspiratory pressure
(PImax) and expiratory pressure (PEmax),
measured at the mouth, are plotted as a function of lung volume from residual volume
(RV) to total lung capacity (TLC). FRC, functional residual capacity.
+40 cm H2
O suggests severely impaired coughing ability. Although these
pressures are not measured routinely in all pulmonary function laboratories, they
are particularly useful in the evaluation of patients with neuromuscular disorders.
For these patterns, the VC has long been used to indicate the severity of respiratory
muscle weakness and to predict respiratory failure. Measurements of respiratory
muscle strength can more readily identify patients in whom respiratory muscle weakness
is the prime cause of hypercapnic respiratory failure.[9]
