STUDY OF THE POSSIBLE ROLE OF CENTRAL RESPIRATORY DRIVE IN THE SENSATION OF DYSPNEA IN OBESE SUBJECTS

Abstract

Obesity is a chronic metabolic disorder characterized by an excess of body fat. In addition to the long list of adverse cardiovascular and metabolic consequences, obesity is associated with consistent alterations in static and dynamic respiratory mechanicaVmuscular function. These changes may be at least partly responsible for the increased perception of dyspnea that characterizes this condition.

Dyspnea results from a mismatch between central respiratory motor activity and incoming afferent information from receptors in the airways, lungs, and chest wall structures. Yet, the role of increased central respiratory drive activity in the pathogenesis of dyspnea in healthy obese subjects is not yet widely determined.

The aim of this study was to investigate the possible role of respiratory drive in the sensation of dyspnea in obese subjects. This was done through studying the relationship between the degree of dyspnea and ventilatory drive index in different grades of obesity.

The study was conducted on forty subjects divided into two groups. The control group included ten normal-weight healthy subjects (7 females and 3 males), while the second group included thirty obese healthy subjects (20 females and 10 males).

To all studied subjects, detailed history was taken including onset, and characteristics of dyspnea. Thorough clinical examination was performed with special emphasis on cardiopulmonary system. Anthropometric measurements included height, weight, waist and hip circumference. BMI [weight/ (height in m)2], and WHR (waist !hip circumference) were calculated and dyspnea was quantitatively assessed using the Baseline Dyspnea Index (BDI) questionnaire.

Using a computerized dry spirometer (Erich Jaeger GmbH D-97204 Hoechberg), lung volumes and capacities including: VCIN, VCEx, ERV, and IC were measured. Using the same device, pulmonary flow rates; FVC, FEV1, FEV1/FVC, PEF, FEFzs%, FEF so%, FEF75%, FEF25_ 75%, PIF, MIF, and FIF5o% were also measured. Resting ventilatory data included: VT, fR, v•E, MVV, and TI /Ttot• Inspiratory (PI,max) and expiratory (PE,max) muscle pressures were recorded together with ventilatory drive data including: mouth occlusion pressure at 100 ms. of tidal breathing (P0.1), P0. 1 I P1,max, mean inspiratory flow (VT/ T1), effective inspiratory impedance (P0. 1NT/T1), and Po.I,max• The two studied groups underwent an incremental exercise test, and the following data was recorded: PWC1so, wmax, HRmax, v•E,max, inspiratory capacity at maximal exercise (ICex), dyspnea index calculated as (V" E,max I MVV), dynamic hyperinflation index (DHI) calculated as [(ICrest - ICex I !Crest) I v•E,max], and post exercise inspiratory occlusive mouth pressure (Po.J). Modified Borg Scale was measured at HR=l50 beats/min, and at W=50 watt.

Obese subjects experienced more dyspnea and heightened ventilation at rest compared to non-obese, and the intensity of dyspnea sensation was positively correlated to their BMI. Spirometric data demonstrated neither obstructive nor restrictive pattern in the studied obese subjects. However, both ERV and MVV were significantly reduced in obese compared to control subjects, while IC was significantly increased. As regard inspiratory flow rates, PIF and FIFso%, both were lower in obese compared to non-obese subjects, meanwhile, the obese group showed significantly higher V"E and fR with reduced VT.