Flow-Volume Loops

The finding of reduced peak flow, MVV, and FEV₁ without additional clinical evidence of chronic obstructive lung disease may indicate the presence of an obstructing lesion of the upper airway, larynx, or trachea. In some cases, this obstruction may be suspected by a careful history and physical examination, but in many instances, it may stimulate diffuse airway obstruction and may suggest a marked degree of lung dysfunction. The latter is most likely to occur in patients who are to undergo head and neck surgery and may present for operative procedures

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Figure 26-12 Maximum expiratory flow-volume curves before (solid line) and after (broken lines) induced bronchoconstriction. Flow is plotted against expired volume, which is expressed in liters (L), from total lung capacity (TLC) to residual volume (RV).

Figure 26-13 Partial expiratory flow-volume curves before (A) and after (B) bronchodilator treatment. Forced expiration is begun just above midpoint of vital capacity (VC). Flows are quantitated at 20% and 40% of VC above residual volume (RV). A reference maximum expiratory curve (C) is also inscribed by beginning exhalation from total lung capacity (TLC).

Flow-volume loops aid in detecting upper airway obstruction and may help to localize the site and identify the nature of the obstruction. Several characteristic patterns have been described. Perhaps the most common lesion is a fixed obstruction, such as a benign stricture resulting from tracheostomy or tracheal intubation. A tumor or mass such as a goiter may produce a similar picture, as would breathing through a fixed external resistance. No significant change in airway diameter occurs during inspiration or expiration. As a result, expiratory flows show a plateau of constant flow over the effort-dependent portion of the VC. Inspiratory flows show a similar plateau ( Fig. 26-15A ). Because both are reduced by nearly the same extent, the mid-VC ratio remains approximately 1.0.

Figure 26-14 Schematic representation of a maximum inspiratory (V̇₁ ) and expiratory (V̇_E ) flow-volume loop in a normal subject. The pleural pressure (PPL) associated with the maximum effort is plotted as a function of lung volume from total lung capacity (TLC) to residual volume (RV). V̇₁ and V̇_E at the midpoint (50%) of vital capacity are indicated by arrows.

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Figure 26-15 Maximum inspiratory and expiratory flow-volume curves (i.e., flow-volume loops) in four types of airway obstruction.

A lesion whose influence varies with the phase of respiration is called a variable obstruction. To understand the impact of such lesions on flow, it is necessary to characterize the behavior of the three basic partitions of the airway ( Table 26-4 ).

Variable extrathoracic obstructions (see Fig. 26-15B ) are most commonly associated with vocal cord paralysis, which is usually accompanied by inspiratory stridor. A similar pattern may be seen with marked pharyngeal muscle weakness in curarized volunteers^[17] and in patients with chronic neuromuscular disorders.^[18] The flow pattern occasionally has been identified in patients with severe obstructive sleep apnea. During forced inspiration, the negative transmural pressure inside the airway tends to collapse the airway with increasing effort and therefore reduces inspiratory flow. During expiration, the

TABLE 26-4 -- Airway partitioning and behavior

Upper (Extrathoracic)

Surrounding soft tissue unsupporting

Collapses during inspiration

Expands during expiration

Intrathoracic

Outer surface exposed to pleural pressure

Expands during inspiration

Collapses during expiration

Distal (Pulmonary)

Intimately related to lung tissue

Collapses as expiration proceeds

positive pressure within the upper airway tends to decrease the obstruction, and expiratory flow is reduced far less and may even be normal. The mid-VC ratio of expiratory to inspiratory flow is often greater than 2.0.

The other form of variable obstruction occurs intrathoracically and is usually caused by tumors of the trachea or major bronchi. During forced expiration, the high PPLS decrease airway diameter and may increase the obstruction. A plateau flow usually occurs during expiration when the compressed airway lumen assumes its minimal size at the area of the lesion (see Fig. 26-15C ). During inspiration, lowering of PPL surrounding the airway tends to decrease the obstruction, and the inspiratory portion of the flow-volume loop may be normal. The mid-VC ratio of expiratory flow is low, as in the case of diffuse airway obstruction (see Fig. 26-15D ). However, the shapes of the curves differ. Figure 26-15D , an example of diffuse or distal airway obstruction, exhibits abnormal decreased flow in the segment near the RV. Figure 26-15C demonstrates normal flow in this area.

The physiologic diagnosis of upper airway obstruction is sometimes difficult in patients with diffuse airway obstruction (e.g., chronic bronchitis, asthma). These conditions themselves produce significant abnormalities of the flow-volume loop (see Fig. 26-15D ), and flow-volume loops identify upper airway obstruction best in the absence of significant generalized airway disease.