To the Editor:—
Eikermann et al. are to be commended for rekindling interest in the importance of upper airway obstruction with residual paralysis. 1Previous observations in partially paralyzed volunteers indicated that although expiratory muscle strength was more impaired than inspiratory strength, expiratory flows were relatively more well maintained than inspiratory flows. 2These decreased inspiratory flows could not be simply attributed to decreased effort or driving pressure. Rather, flows are reduced out of proportion to the diminished inspiratory muscle strength. The inspiratory flow patterns suggested a variable extrathoracic obstruction that was most likely the result of weakened airway abductor muscle activity during inspiration. Consequently, many patients who may demonstrate inspiratory muscle strength (maximal inspiratory pressure) ample for ventilation may still have diminished strength in muscles necessary for upper airway protection. 3
Also implicit in the Eikermann study 1is the relatively poor sensitivity of forced vital capacity and its inspiratory subdivision as indicators of neuromuscular block, or more specifically, respiratory muscle weakness. The relationship between vital capacity and respiratory muscle strength in supine partially curarized subjects is curvilinear. 4That is to say, relatively large decrements in respiratory pressure generation must occur before volume reductions result. Thus, the relative preservation of vital capacity does not indicate a similar preservation of respiratory muscle strength as is often assumed. Such assumptions appear to be valid in the rather clinically irrelevant upright seated position. 5In supine subjects, whose mechanics are likely to be similar to Eikermann et al. ’s semirecumbent subjects, 1the greater efficiency of the diaphragm and the diminished contribution of rib cage expansion provide better preservation of lung volume.
