The influence of end-expiratory lung volume on measurements of pharyngeal collapsibility

Abstract

Changes in end-expiratory lung volume (EELV) affect upper airway stability. The passive pharyngeal critical pressure (Pcrit), a measure of upper airway collapsibility, is determined using airway pressure drops. The EELV change during these drops has not been quantified and may differ between obese obstructive sleep apnea (OSA) patients and controls. Continuous positive airway pressure (CPAP)-treated OSA patients and controls were instrumented with an epiglottic catheter, magnetometers (to measure change in EELV), and a nasal mask/pneumotachograph. Subjects slept supine in a head-out plastic chamber in which the extrathoracic pressure could be lowered (to raise EELV) while on nasal CPAP. The magnitude of EELV change during Pcrit measurement (sudden reductions of CPAP for 3-5 breaths each minute) was assessed at baseline and with EELV increased approximately 500 ml. Fifteen OSA patients and 7 controls were studied. EELV change during Pcrit measurement was rapid and pressure dependent, but similar in OSA and control subjects (74 +/- 36 and 59 +/- 24 ml/cmH(2)O respectively, P = 0.33). Increased lung volume (mean +521 ml) decreased Pcrit by a similar amount in OSA and control subjects (-3.1 +/- 1.7 vs. -3.9 +/- 1.9 cmH(2)O, P = 0.31). Important lung volume changes occur during passive Pcrit measurement. However, on average, there is no difference in lung volume change for a given CPAP change between obese OSA subjects and controls. Changes in lung volume alter Pcrit substantially. This work supports a role for lung volume in the pathogenesis of OSA, and lung volume changes should be a consideration during assessment of pharyngeal mechanics.

Figures

Fig. 1.

Raw data illustrating that rib cage and abdominal anterior-posterior distance fall within the first breath of a pressure drop, and in a pressure-dependent manner. The holding pressure is reduced from 7 cmH2O by the amounts indicated for 3–5 breaths, until flow limitation is achieved. The average change in end-expiratory lung volume (ΔEELV) during the pressure drop, compared with the average volume at the holding pressure baseline, is shown. One minute of baseline breathing between drops is omitted.

Fig. 2.

Relative change in EELV that occurs during pharyngeal critical closing pressure (Pcrit) measurements in one obstructive sleep apnea (OSA) subject. Two Pcrit measurements made under baseline conditions (black diamond and black solid line; gray square and gray solid line), and one Pcrit measurement made at increased lung volume (open circle and dashed line). Each point represents one of the 3rd through 5th breaths of a pressure drop. The holding pressure did not change between conditions. Marked changes in pressure can cause substantial changes in EELV.

Fig. 3.

The change in EELV on a breath-to-breath basis during two different pressure drops (2 cmH2O, squares; 6 cmH2O, circles) averaged across all subjects and both baseline and increased lung volume conditions. EELV changes are means with SE bars. Most of the lung volume change has occurred by the 3rd breath. OSA subjects (□, ○, connected by dashed lines) and controls (■, ●, connected by solid line) had a similar decrease in lung volume for a given change in pressure. Statistically significant lung volume changes are indicated in the text of results.

Fig. 4.

Pcrit at baseline and at increased EELV in individual subjects with OSA (○) and controls (☐). The average lung volume increase for control subjects (■) was 453 ± 64 ml. OSA subjects (●) lung volumes were raised 552 ± 115 ml.

Fig. 5.

Raw data illustrating compromise in inspiratory and expiratory flow in a single subject during a pressure drop. With the holding pressure reduced from 10 cmH2O to 6 cmH2O there are ongoing flow-limited inspiratory efforts; however, there is also minimal expiratory flow. Rib cage anterior-posterior diameter does not drop, and abdominal anterior-posterior distance increases with each successive inspiration. Compare with Fig. 1 in which expiratory flow is preserved.