Betts, Aaron W.

The work and power of breathing (Wb & Pb, respectively) have been extensively studied in humans during exercise. Through the measurement of the Wb & Pb during exercise, we yield insight into the energetic demands of the respiratory muscles across a wide range of minute ventilations. It must be noted, however, that the methodological approach to quantifying Wb & Pb is very inconsistent in the literature. The 3 most popular methods of calculating Wb & Pb are: (1) the integrated pressure‐volume (PV) loop; (2) the Otis diagram; and (3) the Campbell diagram. Each of these methods are based on different theoretical assumptions and, as such, do not necessarily provide a consistent measure of Wb & Pb. A comparison of the Wb & Pb measured via these 3 methods during exercise is presently lacking. PURPOSE To compare values of the Wb & Pb obtained during incremental exercise as determined by the integrated PV, Otis and Campbell methods. METHODS Twenty‐three young, healthy adults (10 women; 21 ± 1 yrs) visited the laboratory on two separate occasions. The first occasion was used to obtain informed consent and to screen participants for normal pulmonary function (>85% age‐predicted). On the second visit, participants were instrumented with an esophageal balloon catheter to estimate variations in pleural pressure. Quasi‐static relaxation curves were obtained to measure chest wall recoil pressure. Participants then completed an incremental cycling protocol until volitional exhaustion. The total Wb & Pb were computed according to the numerical procedures for the integrated PV, Otis and Campbell methods. The Wb & Pb of each method were compared at standardized minute ventilations obtained during incremental. RESULTS The total Wb & Pb as determined via the Campbell method was systematically higher than the other two methods (integrated PV & Otis) at any given minute ventilation during exercise (Figure ; P < 0.05). Interestingly, the integrated PV method produced values for total Wb & Pb that were lower than those of the Otis method for minute ventilations below 80 L·min −1 , yet were higher at minute ventilations greater than 110 L·min −1 (Figure ; P <0.05). When expressed as a percentage of the values obtained via the Campbell diagram, it was clear that the Otis method constituted a decreasing proportion of total Wb & Pb as minute ventilation increased (Figure ; P <0.05), whereas the integrated PV method yielded values that represented an increasing proportion during exercise ( P <0.05). CONCLUSION The Campbell method produced the highest values at any given minute ventilation. Importantly, however, the Otis method tended to progressively underestimate the total Wb & Pb relative to the Campbell, while the opposite was true of the integrated PV method, as minute ventilation increased during exercise. Investigators must be cognizant of these differences when choosing a method to quantify the Wb & Pb during exercise. Support or Funding Information This research was funded by preliminary data funds provided by Northern Arizona University (JWD). TJC was supported by the Irene Diamond Fund/American Federation for Aging Research Postdoctoral Transition Award during the time of this study. The power of breathing (Pb) determined by the integrated pressure‐volume (PV), Otis and Campbell methods. Values represent means ± 95% confidence intervals. The left panel ( A ) represents the total Pb values obtained via the 3 different methods. The right panel ( B ) illustrates the percentage of the Campbell method's total Pb that is constituted by the integrated PV and Otis methods, separately. Only portions where no significant differences (NS) were observed have been illustrated. Figure 1

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 cmH 2 O). 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 = k 1 (flow) + k 2 (flow 2 ). 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 (P LUNG ) & 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 (P ES ). 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 P ES was taken to equal P LUNG . Second, alveolar pressure (P ALV ) was taken to equal P ES + P LUNG . 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 P LUNG , but P ALV 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 P LUNG did not differ between men and women, the differing location of EPP was the result of a greater P ES in men. This suggests that the larger airways present in men are more resistant to compression than those of women.

The respiratory muscle pressure (P mus ) waveform lends insight into the patterns of net inspiratory v expiratory muscle recruitment under a wide variety of conditions, including mechanical ventilation, spontaneous breathing, and during exercise. On this last point, inspection of the P mus waveform during incremental and constant work rate cycling reveals that inspiratory and expiratory muscle recruitment systematically increase with rising minute ventilations, and that the duration of post‐inspiratory inspiratory activity (PIIA) gradually disappears from low to high levels of respiratory motor output. In recent years, it has become increasingly evident that the work of breathing is systematically higher for women compared with men at a given minute ventilation during exercise. Yet, no study has examined the potential sex differences in the evolution of P mus (and its components; e.g., PIIA) during exercise. Thus, a comprehensive assessment of the P mus waveform during exercise between sexes is warranted. Purpose To compare the evolution of inspiratory and expiratory P mus between men and women during incremental cycling. Methods Thirteen men (21 ± yrs; 181 ± 5 cm; 82 ± 11 kg) and 10 women (21 ± 1 yrs; 165 ± 6 cm; 62 ± 6 kg) visited the laboratory on two separate occasions. The first occasion was used to obtain informed consent and to screen participants for normal pulmonary function (>85% age‐predicted). On the second visit, participants were instrumented with an esophageal balloon catheter to estimate variations in pleural pressure (P pl ). Quasi‐static relaxation curves were obtained to measure chest wall recoil pressure (P cw ). Participants then completed an incremental cycling protocol until volitional exhaustion. P mus was calculated at given minute ventilations during the exercise protocol as follows: P mus = P cw – P pl . Results The peak inspiratory P mus were significantly greater in females compared with men at a given minute ventilation during incremental cycling ( P < 0.05), yet peak expiratory P mus was similar between sexes. The average rates of change (slope) in P mus during inspiration & expiration were significantly faster in females than males at a given minute ventilation ( P < 0.05). Finally, the duration of PIIA (expressed as a fraction of expiratory time) gradually decreased with increasing minute ventilation during exercise ( P < 0.05). This reduction in PIIA was however similar between men and women. Conclusion The primary finding of this study was that the pattern of respiratory muscle recruitment was different between sexes during incremental cycling, insofar as women displayed systematically larger inspiratory P mus , and faster rates of net inspiratory & expiratory pressure‐development compared with men at matched ventilations. However, no differences in PIIA between sexes were observed. Support or Funding Information This research was funded by preliminary data funds provided by Northern Arizona University (JWD). TJC was supported by the Irene Diamond Fund/American Federation for Aging Research Postdoctoral Transition Award during the time of this study.