Influence of lung volume on oxygen cost of resistive breathing

Abstract

We examined the relationship between the O2 cost of breathing (VO2 resp) and lung volume at constant load, ventilation, work rate, and pressure-time product in five trained normal subjects breathing through an inspiratory resistance at functional residual capacity (FRC) and when lung volume (VL) was increased to 37 +/- 2% (mean +/- SE) of inspiratory capacity (high VL). High VL was maintained using continuous positive airway pressure of 9 +/- 2 cmH2O and with the subjects coached to relax during expiration to minimize respiratory muscle activity. Six paired runs were performed in each subject at constant tidal volume (0.62 +/- 0.2 liters), frequency (23 +/- 1 breaths/min), inspiratory flow rate (0.45 +/- 0.1 l/s), and inspiratory muscle pressure (45 +/- 2% of maximum static pressure at FRC). VO2 resp increased from 109 +/- 15 ml/min at FRC by 41 +/- 11% at high VL (P less than 0.05). Thus the efficiency of breathing at high VL (3.9 +/- 0.2%) was less than that at FRC (5.2 +/- 0.3%, P less than 0.01). The decrease in inspiratory muscle efficiency at high VL may be due to changes in mechanical coupling, in the pattern of recruitment of the respiratory muscles, or in the intrinsic properties of the inspiratory muscles at shorter length. When the work of breathing at high VL was normalized for the decrease in maximum inspiratory muscle pressure with VL, efficiency at high VL (5.2 +/- 0.3%) did not differ from that at FRC (P less than 0.7), suggesting that the fall in efficiency may have been related to the fall in inspiratory muscle strength. During acute hyperinflation the decreased efficiency contributes to the increased O2 cost of breathing and may contribute to the diminished inspiratory muscle endurance.

Hyperinflation on Chest Radiograph as a Marker of Low Adherence to Positive Airway Pressure Therapy in the Overlap Syndrome

“…7 Furthermore, any reduction of intrinsic PEEP with COPD results in a marginal deflating effect, and the reductions in effort of breathing may occur at the expense of increases in end-expiratory lung volume, which may aggravate the hyperinflation, 7 which in turn limits the ability of the diaphragm to increase lung volume 8 and increases oxygen cost of breathing during resistive breathing. 9 It is noteworthy that our best cutoff of 2.45 cm is close to the 2.6-cm cutoff of Reich et al, 15 who found that a right diaphragmatic arc height of Յ 2.6 cm identified 68% of subjects with abnormal pulmonary function tests, suggesting COPD. Although other radiographic parameters such as lung length and retrosternal lucency are also correlated with a COPD diagnosis, we chose the height of the right diaphragmatic arc based on the results of Reich et al, 15 the greater dependence of other parameters (such as lung length) on the subject's morphometry (such as height), and studies showing diaphragm flattening as having a better correlation with postmortem changes in emphysema.…”

“…For instance, hyperinflation limits the ability of the diaphragm to increase lung volume 8 and can further increase the oxygen cost of breathing during resistive breathing. 9 These considerations are of particular relevance in the overlap syndrome, in which PAP, as indicated for sleep apnea, may be met with the same adherence issues as with NIV in COPD. Accordingly, patients with the overlap syndrome provide a unique opportunity to assess determinants of adherence to PAP therapy in COPD.…”

Respiratory muscles and weaning failure

“…Similar arguments may account for the smaller critical Pdi, if the diaphragm operates at shorter lengths during acute hyperinflation, when a given force requires much greater excitation [25]. At half inspiratory capacity, E can be reduced by as much as 50% [26]. To our knowledge, the efficiency of respiratory muscles has never been measured in patients who fail to wean.…”

Pulmonary gas exchange in diving

“…] resistance (37,98,106,117). The increase in ERV has the effect of increasing internal respiratory load by raising the elastic lung load (14), which allows more passive expiration but results in a requirement for higher negative inspiratory pressures (76). The increased elastic load induced by immersion augments the gas density-related decrease in MVV (129).…”

“…The increased work of breathing with negative P TR can be attributed to an increase in internal respiratory resistance due to compression of the extrathoracic airways (2). Positive P TR causes subjects to have a higher expiratory reserve volume and increased elastic recoil of the lungs (14). However, several studies have shown that the increased work of breathing caused by changes in P TR between ϩ10 and Ϫ20 cmH 2 O during exercise can cause dyspnea but does not translate into higher PET CO 2 (40,75,107), although in these studies it is possible that Pa CO 2 was underestimated by PET CO 2 .…”