Dynamic Spirometry And Maximum Flow Rationale and Scientific Basis

Airway resistance is normal in uncomplicated respiratory muscle weakness (14). Airway function may appear to be supernormal when volume-corrected indices such as FEV1/VC or specific airway conductance are used (2).

The maximum expiratory and maximum inspiratory flow-volume curves characteristically show a reduction in those flows that are most effort dependent, that is, maximum expiratory flow at large lung volumes (including peak expiratory flow) and maximum inspiratory flow at all lung volumes (2, 5) (Figure 3). The descending limb of the maximum expiratory flow-volume curve may suggest supernormal expiratory flow when this is related to absolute volume (2, 3). With severe expiratory weakness, an abrupt fall in maximum expiratory flow is seen immediately before RV is reached (1). In health the FEV1 is usually less than the forced inspiratory volume in 1 second. Reversal of this ratio is seen with upper (extrathoracic) airway obstruction, as well as in respiratory muscle weakness, and may give a pointer to these diagnoses during routine testing.

The effect of coughing can be visualized on the maximum expiratory flow-volume curve in healthy subjects as a transient flow exceeding the maximum achieved during forced expiration. The absence of such supramaximal flow transients during coughing presumably results in impaired clearance of airway secretions and is associated with more severe expiratory muscle weakness (15). Even with quadriplegia, however, some patients can generate an active positive pleural pressure in expiration (16). This can allow them to achieve the pressure required for flow limitation through most of expiration so that FEV1 may still be reliable as an index of airway function. Impaired maximal flow in some neuromuscular diseases may also reflect poor coordination of the respiratory muscles rather than decreased force per se.

Oscillations of maximum expiratory and/or inspiratory flow—the so-called sawtooth appearance—are seen particularly when the upper airway muscles are weak and in patients with extrapyramidal disorders (17) (Figure 4).

Methodology and Equipment

Recommendations and requirements for maximum flow-volume curves are covered in detail elsewhere (9, 10).

Advantages

Maximum flow-volume curves are easily performed, widely available, and economical. Peak expiratory flow can be obtained with simple portable devices.

Figure 1. Relation between static lung compliance and total lung capacity in 25 patients with chronic respiratory muscle weakness of varying severity. Dashed line is regression line. Reprinted by permission from Reference 5.

Disadvantages

Intersubject variability is greater than for VC. Reference values for VEmax at standard percentages of FVC may present problems of interpretation.

Applications

Visual inspection may suggest the likelihood of weakness.

The sawtooth appearance in an appropriate context may suggest weakness or dyscoordination of upper airway muscles. However, this appearance is nonspecific and is seen also in some subjects with obstructive sleep apnea, nonapneic snoring, and thermal injury of the upper airway.

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