Altered resting and exercise respiratory physiology in obesity

Abstract

Obesity, particularly severe obesity, affects resting and exercise-related respiratory physiology. Severe obesity classically produces a restrictive ventilatory abnormality characterized by reduced expiratory reserve volume. Obstructive ventilatory abnormality may also be associated with abdominal obesity. Decreased peak work rates are usually seen among obese subjects in a setting of normal or decreased ventilatory reserve and normal cardiovascular response to exercise. Weight loss may reverse many adverse physiologic consequences of severe obesity on the respiratory system.

Figures

Figure 1

Effect of obesity on lung volumes - Expiratory reserve volume (ERV) is decreased in obesity. Functional residual capacity (FRC), the sum of ERV and residual volume, is usually reduced as well, often approaching residual volume (see arrow). The decline in FRC in obese subjects is primarily the result of reduced ERV. Total lung capacity (TLC, the sum of FRC and inspiratory capacity or IC) is usually preserved. Therefore, in order to compensate for the reduced FRC, inspiratory capacity (IC), the sum of inspiratory reserve volume and tidal volume, may be increased in severe obesity.

Figure 2

Expiratory reserve volume (ERV) decreases rapidly in an exponential relationship with increase in body mass index (BMI). The best-fit exponential regression equation for ERV is as follows: ERV = 587.8 exp(−0.083 X BMI) + 6.5. The r2 value for ERV was 0.49 (p < 0.01). Obtained with permission from Chest .

Figure 3

Vertical banded gastroplasty in a study by Thomas et al. was associated with a mean weight loss of 34.2 kg in 29 morbidly obese subjects. Resulting change in static lung volumes (expressed as change in percent predicted values from baseline) are summarized. Bar lines indicate one standard error of the mean (SEM). Greatest improvement in expiratory reserve volume (ERV); intermediate improvement in residual volume (RV) and functional residual capacity (FRC); and least improvement in total lung capacity (TLC) was seen following surgical weight loss. Obtained with permission from Thorax .

Figure 4

Expiratory flow volume curve (dashed line) from a woman with BMI=50 kg/m2. Solid line shows the predicted curve. Both TLC (79% predicted) and FRC (68% predicted) are reduced, but maximal flows are well preserved and the FEV1/FVC is normal. Nevertheless, expiratory flows at low lung volumes are reduced relative to the predicted values derived from the vital capacity. Obtained with permission from Cheryl Salome, Ph.D., Woolcock Institute of Research, University of Sydney.

Figure 5

Obesity displaces the V̇ o2 -work rate relationship upward by about 6 ml/min/kg of excess body weight, but the slope itself is unchanged (Adapted from Figure 4.5, Principles of Exercise Testing and Interpretation, Wasserman et al., Fourth edition, Lipincott Williams & Wilkins, Philadelphia, PA, 2005 – copyright permission obtained from Lipincott Williams & Wilkins).