In a respiratory function laboratory the effect of changes in resistance is commonly assessed by measuring flow-volume loops ( Fig 4). Both inspiratory and expiratory flow are recorded and the subject is asked to breathe in to total lung capacity from a mouthpiece. Expiration is normally performed in a forced manner back to residual volume and a loop is created. Several loops can be superimposed to obtain a representative example. It is important to note that volume recordings start from residual volume and increase as the trace is read from right to left. A variety of measurements can be read from the tracing, although it is as easy to assess the shape of the graph. For comparison a tracing of a patient with mild obstructive lung disease is shown. In absolute terms, residual volume (and functional residual capacity) are greater, indicating that residual gas is trapped in the lungs. The characteristic shape of the curve is offset to the left with scalloping of the expiratory curve. Peak and all other expiratory flows are reduced from normal, and the ratio of flow to volume indicates an obstructed picture.
Fig. 4 Flow-volume loops from residual volume to total lung capacity for a normal subject and a patient with chronic obstructive pulmonary disease (COPD). In the normal subject, airflow into the chest is fixed and the subject exhales forcibly, quickly achieving peak flow. This contrasts with the COPD patient's tracing which starts at an inflated lung volume (offset to the left), exceeds the total lung capacity of the normal subject, and cannot reach the same expiratory flow. Note the shallower gradient of the mid-expiratory section of the curve.
In intubated patients similar tracings are obtained but with important differences. Patients rarely, if ever, perform flow-volume loops over the total lung volume range. In addition, they do not perform forced maneuvers and tend to exhale passively. Nevertheless it is possible to measure flow-volume loops in patients over the tidal volume range.
A flow-volume loop for a spontaneously breathing patient is shown in Fig 5. The loop is smaller, and expiratory flow is limited and equal to that of inspiration. In addition, expiratory flow comes to a sudden abrupt end typical of obstructive lung disease.
Fig. 5 A flow-volume loop during tidal ventilation. Although the loop is smaller, the same basic components as in Fig 4 are present. However, in this example expiratory flow is passive and related to the recoil pressure of the lung and chest wall.
Expiratory flow is dependent on effort, the elastic recoil of the lung and chest wall, and the resistance of the airways. If the patient is relaxed the expiratory flow is dependent on airways resistance, although it is important to scrutinize the flow-volume loop to ensure consistent results.
Simple visual inspection of flow-volume loops can be an effective method of demonstrating bronchodilation. Further evidence that treatment has been successful include a reduction in auto-PEEP and an increase in flow for any given pressure difference.
The application of PEEP to patients who exhibit flow limitation has previously been avoided. Expiratory flow will only be decreased and hyperinflation worsened if the external PEEP exceeds the level of auto-PEEP. As auto-PEEP is difficult to measure in the spontaneously breathing patient, the effect of changes in external PEEP are unknown. By monitoring flow-volume loops, external PEEP can be applied until the flow-volume curves move along the volume axis (T.o.bin a..Qd L.o.d.§.t.o 19.89).
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