Degree of tracheal obstruction (%)
ΔMMRCa
Respondersb (%)
≦1
≧2
51–60
2
61–70
2
2
10/17 (58.8%)
71–80
5
6
81–90
2
9
11/13 (8406%)
91–100
2
Assessment of Lateral Airway Pressure
Analysis of the flow–volume curve could be used in defining the nature of the stenosis. However, flow–volume curves cannot identify the precise location of the lesion where airway resistance increases, nor can it immediately define the outcome of stenting.
With the use of airway catheters in dogs [13–15] and in human subjects [16–18], the FLS could be located by measuring lateral airway pressure (P lat) during induced flow limitation generated by either an increase in pleural pressure or a decrease in downstream pressure. Healthy subjects have relatively uniform pressure drop down of the bronchial tree during expiration. In patients with airway stenosis, the major pressure drop occurs across the stenosis. By measuring P lat on each side of the stenosis, we could detect the pressure difference between two sites (proximal and distal) of the stenotic segment [12].
After intubation, a double lumen airway catheter was inserted into the trachea during bronchoscopy. P lat was measured simultaneously at two points during spontaneous breathing with light general anesthesia before and after intervention. P lat at the two points was plotted on an oscilloscope (pressure–pressure (P–P) curve). The angle of the P–P curve was defined as the angle between the peak inspiratory and expiratory pressure points and the baseline of the angle. If the cross-sectional area (CSA) was small, then the angle was close to 0°; however, after intervention, the CSA significantly increased and the angle was close to 45°.
In healthy subjects, no pressure difference between the carina and trachea was observed (0.10 ± 0.22 cm H2O) during tidal breathing (Fig. 6.1a). The P–P curves were linear and the angle of the P–P curve was close to 45° (44.6 ± 0.98) (Fig. 6.1b).
Fig. 6.1
Typical patterns of lateral airway pressure (P lat) measurements during tidal breathing in a healthy subject. P lat is measured simultaneously at two points (upper trachea and carina). There are no pressure differences between the carina and upper trachea (a). (Blue, carina; red, upper trachea.) The angle of pressure–pressure (P–P) curve is defined as the angle between peak inspiratory and expiratory pressure points and the baseline of the angle. The P–P curves are linear and the angle of P–P curve is close to 45° (b)
In patients with tracheal obstruction , dyspnea scale, pressure difference, and the angle changed significantly beyond 50% obstruction (Fig. 6.2a, b). After stenting, the pressure difference disappeared and the angle was close to 45°. The degree of tracheal obstruction was significantly correlated with the pressure difference and the angle (r = 0.83, p < 0.0001, and r = −0.84, p < 0.0001, respectively) [12].
Fig. 6.2
Scatter plot of pressure difference and the angle of the pressure–pressure (P–P) curve versus the degree of tracheal obstruction. Blue diamonds show before intervention and red squares indicate after intervention in cases with fixed stenosis. Green triangles show before intervention and purple Xs indicate after intervention in cases with variable stenosis. Dotted line shows the threshold for 50% tracheal obstruction. The pressure difference (a) and the angle of P–P curves (b) are significantly correlated with the degree of tracheal obstruction. The pressure difference increased significantly above 50% obstruction (a). When the cross-sectional area was small, the angle of the P–P curve was close to 0°. After interventional bronchoscopy, the cross-sectional area increased and the angle of the P–P curve was close to 45° (b)
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