Factors Contributing to Breathlessness

Abstract

<p>Breathlessness may be defined as the conscious awareness of respiratory muscle effort. As with any skeletal muscle it is to be expected that the sense of effort increases as the pressure generated by this muscle increases as well as the velocity and extent of shortening. The purpose of this study was; 1. to quantify the intensity of breathlessness during exercise and respiratory loading; 2. to isolate the contributions of inspiratory muscle pressure to breathlessness; 3. to see if extent of shortening, velocity of shortening, frequency (fb), and duty cycle (Ti/Ttot) contribute to the intensity of breathlessness independently. The intensity of inspiratory muscle pressure was quantified by measurement of mouth pressure (Pm) as well as the estimated esophageal pressure (Pes), the extent of shortening by tidal volume (Vt), and the velocity of shortening by inspiratory flow (Vi). Six normal subjects underwent eight incremental (100 kpm/min/min) 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 and elastances presented in random order. The resistances and elastances were selected to provide a wide range inspiratory pressures, tidal volumes, and flows. The inspiratory resistive loads (33, 57, 73 cm H2 O/1/s) were used mainly to vary the flow (functional velocity of shortening of inspiratory muscles). The inspiratory elastic loads (21, 41, 52 cm H2O/1) were used mainly to vary the tidal volume (functional extent of shortening). At rest and at the end of each min during exercise the subjects estimated the intensity of breathlessness (Y) by selecting a number ranging from 0-10 (Borg psychophysical scale), 0 indicating no appreciable breathlessness and 10 the maximum tolerable sensation.</p> <p>When the velocity was altered (resistive loading study) breathlessness was significantly related to inspiratory pressure (p<0.0001), peak inspiratory flow (p<0.0001), frequency of breathing (p<0.01) and duty cycle (p<0.01). When the extent of shortening was altered (elastic loading study) breathlessness was significantly related to inspiratory pressure (p<0.001), tidal volume (p<0.001), and frequency of breathing (p<0.001).</p> <p>The results indicated that the perceived magnitude of breathlessness is closely related to the pressure generated by the inspiratory muscles and the shortening pattern of these muscles as reflected in Vt, Vi, Fb, and Ti/Ttot. The results also indicated that the contribution of these factors to the intensity of breathlessness differs quantitatively between loaded and unloaded breathing. Thus, in normal unloaded breathing the velocity and degree of shortening are important factors contributing to breathlessness during exercise; with resistive loading the inspiratory pressure, the velocity, and the duty cycle are important; with elastic loading the inspiratory pressure, the extent of shortening, and the frequency are important.</p> <p>The major contributions of these studies were in quantifying the intensity of breathlessness, and defining both the factors contributing to breathlessness and the relative importance of each.</p>