Chest
Clinical Significance of Pulmonary Function TestsUpper Airway Obstruction
Section snippets
PHYSIOLOGY
The airway may be divided functionally into the following three levels: first, peripheral airways of 2 mm diameter or less; second, larger or major airways from 2 mm diameter up to the main carina; and third, the upper airways which include the trachea, larynx, pharynx, and nose or mouth. The effects of any obstruction of the upper airways will depend on several variables which include (a) the size of the airway at the site of the obstruction, (b) the location of the obstruction, (c) the nature
LABORATORY FEATURES
In normal subjects during a forced expiration from total lung capacity (TLC), the maximum airflow is achieved during the first 25 percent of the vital capacity (VC) and is directly dependent on effort and inversely on resistance. In the remaining 75 percent of the VC, flow is limited in such a way that an increase in effort with its associated increase of pleural pressure does not result in increased flow. The precise mechanism of flow limitation over the lower 75 percent of the VC is the
FLOW VOLUME LOOP
Miller and Hyatt3 suggested that the flow volume plot of a forced expiratory and inspiratory vital capacity maneuver would be particularly helpful in categorizing UAO. They simulated major airway obstruction by having normal subjects breathe through fixed resistances and found that flow rate increased to a certain level early in both inspiration and expiration and then plateaued. The plateau was reached at lower flow rates as the resistances were progressively increased (Fig 4).
The effect of a
SPIROMETRY
A simple plot of a FVC maneuver of volume vs time, the timed spirogram, will also give some clues to the presence of upper airway obstruction.3 Figure 6 shows the spirogram and flow volume loop from a patient with an intrathoracic variable obstruction (a paratracheal tumor). The linear portion of the spirogram indicates constant flow which is equivalent to the plateau (ie, constant flow) of the flow volume loop. For comparison, a spirogram from a normal subject is shown on the right.
Rotman et al
MIXED UPPER AND LOWER AIRWAY OBSTRUCTION
The combination of CAO and UAO may arise in several circumstances, for instance, tracheal stenosis occurring after tracheal intubation for an episode of respiratory failure in a patient with chronic airflow obstruction (CAO). Using principles outlined below, the presence of UAO in patients with lower airflow limitation may be distinguished.
In the upper airways, flow is turbulent and depends on the density of the gas, whereas in peripheral airways, flow is laminar and independent of gas density.5
DISTRIBUTION OF VENTILATION AND BRONCHODILATOR RESPONSE
In the presence of UAO without peripheral airway obstruction, tests of distribution of ventilation such as the nitrogen washout are normal.8 A marked elevation in airway resistance unresponsive to bronchodilators is also often seen in patients with UAO. The combination of a normal nitrogen washout and elevated airway resistance in a patient complaining of wheezing or dyspnea should lead one immediately to suspect upper airway obstruction as the cause.
OTHER LABORATORY FEATURES
Resistance in any tube can be measured by dividing the pressure drop along the tube by the flow. In man, resistance across the upper airway is about 1 cm H2O/L/sec, whereas most patients with symptomatic UAO are found to have a resistance between 4 and 15 cmH2O/L/sec while breathing quietly. The resistance of the upper airway is not constant and with increasing flow (such as during exercise), there is a disproportionate rise in the resistance, probably explaining the dyspnea on exertion of
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