The equal pressure point is greater in men than women near total lung capacity
Authors: Gideon Elizabeth A. , Coriell Catherine L. , Betts Aaron W. , Cross Troy J. , Duke Joseph W. 
Published: April 2020

The equal pressure point (EPP) states that expiratory flow limitation occurs at the precise location along the tracheobronchial tree where intraluminal and peribronchial pressures are equal (i.e., where the local transmural pressure is 0 cmH2O). The EPP may be determined numerically via the expiratory isovolume pressure flow (IVPF) curve. For instance, using data from a given isovolume curve, the EPP is determined as the point along the x‐axis at which pressure‐flow data fails to conform to Rohrer’s equation: driving pressure = k1(flow) + k2(flow2). The probability of incurring expiratory flow‐limitation can be greatly determined by the EPP such that a lower EPP at a given lung volume requires only minimal pressure development to create a flow‐limiting segment during expiration. The EPP is affected by two principal factors: lung elastic recoil (PLUNG) & airway geometry. Given recent evidence suggesting that airway geometry is different between men & women, we sought to determine whether the EPP is likewise affected by sex in healthy adults.

Purpose

Determine whether or not there is an effect of sex on the magnitude of the EPP.

Methods

To address this question, 11 men (23.1 ± 4.8 years) and 12 women (21.2 ± 1.0 years) completed a study requiring two visits to the laboratory. On the initial visit, we obtained written informed consent and conducted spirometry to ensure subjects met the inclusion criteria of normal (>85% of predicted) lung function. During the second visit, a balloon catheter was passed intranasally to the lower one‐third of the esophagus and remained there for the duration of the visit to measure esophageal pressure (PES). Subjects then performed a minimum of 10–15 graded vital capacity maneuvers from 100‐5% of maximal effort. Data were imported into Matlab and, using custom‐written code, multiple calculations were performed. First, the lowest effort vital capacity efforts were taken as quasi‐static expiratory deflations of the lung and the negative sign of PES was taken to equal PLUNG. Second, alveolar pressure (PALV) was taken to equal PES + PLUNG. Finally, the EPP was determined by iteratively fitting Rohrer’s equation to pressure‐flow data at a given iso‐volume. The pressure at which Rohrer’s equation no longer adequately fit the data (>5% error) was termed the EPP. To test for an effect of sex and lung volume, a two‐way ANOVA was computed. Pairwise comparisons were adjusted for multiple comparisons using the Tukey‐Sidak post hoc test.

Results

There was no effect of sex on PLUNG, but PALV was greater in men than women throughout the entire VC range. EPP was significantly greater in men than women during the initial half of VC (95‐55% of VC) (p < 0.05), but not the remaining half (p > 0.05).

Conclusion

Because PLUNG did not differ between men and women, the differing location of EPP was the result of a greater PES in men. This suggests that the larger airways present in men are more resistant to compression than those of women.