Epidemic of Obesity. Mayo Clinic Health Letter, Medical Essay, 1997 . NHANES III, 28% of Overweight Adults, 27 < BMI < 31 ( ~ 19 Million). NHANES III, 36% of Obese, BMI > 31 ( ~ 22 Million). 33% of M-to-M Obese, 30 < BMI < 43. Prevalence of Breathlessness with Exertion
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NHANES III, 28% of Overweight Adults, 27 < BMI < 31 (~ 19 Million) NHANES III, 36% of Obese, BMI > 31 (~ 22 Million) 33% of M-to-M Obese, 30 < BMI < 43 Prevalence of Breathlessness with Exertion in Overweight & Obese Individuals 40 35 30 25 Percent (%) 20 15 10 5 0 Sin, etal, ArchInternMed, 1996
10 8 6 RPB (Borg scale 0-10) 4 2 0 Obese without Breathlessness (n=8, BMI 36+5) Obese with Breathlessness (n=8, BMI 37+4) Intensity of Breathlessness on Exertion After 6 min of cycling at 60 W *
. VO2 - Work Rate Relationship Woman 49 yr 163 cm 154 kg DOE . VO2 (Observed) Extreme Obesity 3 . VO2 (Predicted) 2 VO2 (L/min) . 1 0 0 20 40 60 80 100 120 Work Rate (W)
Women VO2 (%Predicted) . Obese Without & With Breathlessness Cardiovascular Exercise Capacity Women Men 120 80 40 0 Obese Lean Obese Lean
Summary • 1. Dyspnea on exertion is prevalent in mild-to-moderate obesity • 2. Shortness of breath on exertion does not appear to be associated with CV deconditioning • 3. There are significant obesity-related changes in respiratory mechanics at rest and during exercise in mild-to-moderate obesity • 4. Shortness of breath on exertion appears to be associated with an increased work of breathing and abdominal fat distribution • 5. Obesity-related changes in respiratory mechanics, O2 cost of breathing, and abdominal fat distribution appear to change respiratory muscle efferent and afferent signals and these changes give rise to the primary sensation of work or effort to breathe
Potential Mechanisms of Dyspnea The word ‘dyspnea’ subsumes a variety of unpleasant respiratory perceptions described by terms such as chest tightness, excessive breathing effort, and air hunger. At least three separable ‘qualities’ of uncomfortable breathing sensations have been identified in the laboratory termed ‘Effort or work,’ ‘Air hunger,’ and ‘Tightness.’ ‘Effort or work’ of breathing is perceived when the work of breathing is increased by high minute ventilation (rate or tidal volume) or in the lab by external impedance to inspiration. ‘Air hunger’ is the conscious perception of the urge to breathe. It is described as ‘not getting enough air,’ ‘uncomfortable urge to breathe,’ and is the sensation felt at the end of a long breath hold. Subjects often comment that intense air hunger is a threatening or frightening sensation. ‘Chest tightness’ is specific to asthmatic bronchoconstriction.
Study Details To examine the basic mechanism of breathing discomfort (dyspnea) in obesity, we will use a debriefing session and a modified dyspnea questionnaire of qualitative respiratory sensation descriptors to investigate the qualities of respiratory sensations and the mechanisms of breathing discomfort in obese subjects during exertion. We propose that the mechanism of this breathing discomfort is related to changes in respiratory muscle efferent and afferent signals associated with the increased oxygen cost of breathing, which is in turn associated with altered respiratory mechanics and fat distribution, and that these changes give rise to the primary sensation of work or effort to breathe rather than the sensations of air hunger or chest tightness. These techniques have not been attempted in obese subjects.
Potential Applications for Identifying Types of Respiratory Sensation • Establish a specific diagnosis (e.g., a pts selection of descriptors may direct diagnostic testing) • To determine quality of discomfort ask the pt to note two to three statements that best describe dyspnea (similar to asking for characteristics and qualities of chest) • In pt with two concurrent diseases, selected descriptors may help identify which condition is the cause of dyspnea (e.g., ‘tightness of asthma from ‘work’ of COPD) • Distinguish progression of underlying disease from CV deconditioning secondary to disease process • Descriptor questionnaire may also be used to evaluate mechanisms whereby a specific intervention relieves dyspnea (e.g., asthma tightness from airways as well as work effort of Raw) Mahler etal AJRCCM, 1996
Inspiratory Force Theoretical Effects of Chest Wall Obesity Lean Obese LOAD LOAD ?
Women Men 120 80 VO2 (%Predicted) 40 . 0 Lean Obese Lean Obese
500 Obese with breathlessness Obese without breathlessness 400 y = 3.5x + 0.21 R2 = 0.99 300 VO2 (mL/min) y = 2.0x + 0.19 R2 = 0.99 200 . Eucapnic Voluntary Hyperpnea 100 Rest 0 0 20 40 60 80 . VE (L/min)
. Figure 1: VO2 - Work Rate Relationship Plot response side-by-side to predicted normal response VO2 (L/min) Observed . 3 Considerably more information is learned from CPET about CV fxn and Gx when the external work is known - cycle is better for this reason Linear relationship, independent of age, sex, or Ht. Predicted maximal work rate and VO2 displayed 2 1 Predicted Response 0 0 20 40 60 80 100 120 Work Rate (W)
