Breathlessness during exercise with and without resistive loading

Abstract

The purpose of this study was to quantify the intensity of breathlessness associated with exercise and respiratory resistive loading, with the specific purpose of isolating the quantitative contributions of inspiratory pressure, length, velocity, and frequency of inspiratory muscle shortening and duty cycle to breathlessness. The intensity of inspiratory pressure was quantified by measurement of estimated esophageal pressure (Pes = pressure at the mouth plus lung pressure), the extent of shortening by tidal volume (VT), and the velocity of shortening by inspiratory flow rate (VI). Six normal subjects underwent five incremental (100 kpm X min-1 X min-1) exercise tests on a cycle ergometer to maximum capacity. The first and last test were unloaded and the intervening tests were performed with external added resistances of 33, 57, and 73 cm H2O X l-1 X s in random order. The resistances were selected to provide a range of pressures, tidal volumes, flow rates, and patterns of breathing. At rest and at the end of each minute during exercise the subjects estimated the intensity of breathlessness (psi) by selecting a number ranging from 0 to 10 (Borg rating scale, 0 indicating no appreciable breathlessness and 10 the maximum tolerable sensation). Breathlessness was significantly and independently related to Pes (P less than 0.0001), VI (P less than 0.0001), frequency of breathing (fb) (P less than 0.01), and duty cycle [ratio of inspiratory duration to total breath duration (TI/TT)] (P less than 0.01): psi = 0.11 Pes + 0.61 VI + 1.99 TI/TT + 0.04 fb - 2.60 (r = 0.83). The results suggest that peak pressure (tension), VI (velocity of inspiratory muscle shortening), TI/TT, and fb contribute independently and collectively to breathlessness. The perception of respiratory muscle effort is ideally suited to subserve this sensation. The neurophysiological mechanism purported is a conscious awareness of the intensity of the outgoing motor command by means of corollary discharge within the central nervous system.

An Official American Thoracic Society Statement: Update on the Mechanisms, Assessment, and Management of Dyspnea

“…Respiratory muscle afferents project to the cerebral cortex, and subjects report sensations localized to respiratory muscles when the work of breathing is high (49). Perceptions of work and effort probably arise through some combination of respiratory muscle afferents and perceived cortical motor command or corollary discharge (50). Similar mechanisms give rise to sensations of work and effort from exercising limb muscles (reviewed in References 42 and 51).…”

Best practice in nursing care of dyspnea: The 6th vital sign in individuals with COPD

“…There is a consensus within the research literature focused on the physiological elements of dyspnea that the degree of perceived breathlessness is proportional to respiratory effort; the greater the unsuccessful respiratory effort exerted by an individual, the greater the sensation of breathlessness experienced [28,[30][31][32][33][34][35] . Although the evidence suggests a relationship between respiratory effort, chemoreceptors and mechanoreceptors, the precise link to the physical mechanism of dyspnea remains unclear.…”

Inspiratory muscle training improves rowing performance

“…The increase in strength may have attenuated the development of fatigue by decreasing the proportion of the maximal force capacity required for each breath (16). Similarly, with greater inspiratory muscle strength, a smaller fraction of maximum tension is generated with each breath, and it has been suggested that this reduces the motor output to the respiratory muscles and decreases the perceived sense of respiratory effort (10). Even though we do not have measures of dyspnea during the 5000-m test, when asked to describe their feeling afterward most subjects said that either the onset of breathlessness was delayed, allowing a longer maintenance of the previous pace, or a higher pace was kept throughout the test with the same breathing effort.…”