The contribution of muscle strength to symptom intensity and work capacity was examined in normal individuals and patients with cardiorespiratory disorders. Respiratory muscle strengths (maximal inspiratory and expiratory pressures) and peripheral muscle strengths (leg extension, leg flexion, seated bench press, and seated row) were measured in 4,617 subjects referred for clinical exercise testing. Subjects then rated the intensity of leg effort, discomfort with breathing (dyspnea), and chest pain (Borg scale) during an incremental exercise task (100 kpm/min each minute) to capacity on a cycle ergometer. Subjects were classified into groups on the basis of pulmonary function, drug therapy for cardiac disorders, and the presence of chest pain during exercise with electrocardiographic changes indicative of myocardial ischemia. Respiratory and peripheral muscle strengths, normalized for differences in age, sex, and height, were significantly reduced in patients with cardiorespiratory disorders compared with normal individuals. Muscle strength was a significant contributor to symptom intensity and work capacity in both health and disease; a two-fold increase in muscle strength was associated with a 25 to 30% decrease in the intensity of both leg effort and dyspnea and a 1.4- to 1.6-fold increase in work capacity. These results emphasize the need for an integrative approach in the assessment and therapeutic management of exercise intolerance, which considers the contribution of muscle weakness to excessive symptoms and reduced work capacity, in addition to the contribution of ventilatory, gas exchange, and circulatory impairments.
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.
By the addition of externally added elastic loads at both functional residual capacity (FRC) and increased lung volume, increased respiratory muscle effort, tension, and breathlessness were induced in normal subjects. The magnitude of each of these sensations was estimated using the psychophysical technique of category scaling (Med. Sci. Sports Exercise 14: 377-381, 1982). The tidal volume, inspiratory time, and breathing frequency were kept constant to avoid variability in sensation due to these factors. The perceived magnitude of effort and breathlessness increased significantly as the inspiratory pressure and lung volume increased (P less than 0.05). The magnitude of perceived tension increased as the inspiratory pressure increased (P less than 0.05) but not as lung volume increased. To validate these results, the subjects estimated the perceived magnitude of a series of static inspiratory occlusion pressures at both lung volumes using open-magnitude scaling and sensory matching. The perceived magnitude of effort increased significantly as the pressure increased and as the lung volume increased (P less than 0.05). To match the perceived effort required to produce the target pressures at FRC, the subjects reproduced pressures. These were not significantly different. However, to match the effort required to produce the target pressures at increased lung volume, the pressures reproduced at FRC were significantly greater (P less than 0.05). The results suggest that the sensations of breathlessness and effort are psychophysically the same, whereas tension is perceived by a different sensory mechanism.
The aim of this study was to establish the perceived magnitude of dyspnea (discomfort associated with breathing) and leg effort experienced by normal subjects during a standardized incremental exercise test to maximal capacity; 460 normal subjects (297 male and 163 female 20 to 70 yr of age) were studied. The perceptual magnitude of both symptoms was rated using simple descriptive phrases (slight, moderate, maximal) tagged to numbers from zero to 10 on the Borg scale, which is an interval scale with ratio properties. Leg effort and dyspnea increased with power output, were higher in women than in men (p less than 0.0001), increased with advancing age (p less than 0.0001), and declined as height increased (p less than 0.0001). Leg effort = 4.82 + 0.007 kpm/min + 1.05 sex + 0.04 age - 0.055 Ht (r = 0.78; SD, 1.80). Dyspnea = 4.96 + 0.006 kpm/min + 0.96 sex + 0.04 age - 0.05 Ht (r = 0.74; SD, 1.80) (m = 1; f = 2). With power output expressed as a percentage of maximal power output (%MPO) both symptoms increased in an alinear manner. Effort = 0.0014 * %MPO1.86 (r = 0.86; SD, 1.50). Dyspnea = 0.0016 * %MPO1.79 (r = 0.81; SD, 1.57). Sex, age, or stature did not contribute to the rating of effort or dyspnea when power output was normalized in this way.
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