A 53-year-old man is being evaluated for dyspnea. The graph on the right shows how the patient's work of breathing varies with respiratory rate, whereas the graph on left represents the work of breathing versus respiratory rate in a normal individual. The minute ventilation is constant and equal in both graphs.
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Which of the following is the most likely diagnosis in this patient?
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Work of breathing (WOB) is the energy expended during respiration, and comprises the work required to overcome both elastic and airflow resistance:
Elastic resistance is the opposition to lung expansion caused by the intrinsic elastic properties of the lungs and chest wall; it is increased at higher tidal volumes (elastic recoil increases at high lung volumes). Restrictive lung diseases (eg, pulmonary fibrosis, severe obesity) are characterized by higher than normal elastic resistance. Interstitial fibrosis increases lung stiffness whereas severe obesity increases chest wall stiffness.
Airflow resistance is the opposition to airflow created by limited airway diameter and turbulent airflow; it is increased at higher respiratory rates (due to faster airflow with increased turbulence) and at low lung volumes (due to reduced airway diameter). Obstructive lung diseases (eg, asthma, chronic obstructive pulmonary disease) increase airflow resistance due to bronchoconstriction and/or airway collapse.
To reduce the WOB, the tidal volume and respiratory rate are optimized by the respiratory control centers. In patients with restrictive lung disease, the WOB is minimized when the tidal volume is low. Therefore, to compensate for low lung volumes, rapid, shallow breathing is favored (as in this patient). In contrast, in patients who have obstructive lung disease, the WOB is minimized with lower respiratory rates and higher tidal volumes (ie, relatively slow, deep breathing) (Choice C).
(Choice A) Alpha-1 antitrypsin deficiency causes panacinar emphysema, a type of chronic obstructive pulmonary disease. This causes increased airflow resistance, and slow, deep breaths will be favored to minimize the WOB.
(Choice B) Anxiety disorders (eg, panic attacks) are occasionally associated with rapid, shallow breathing; however, the lungs function normally and, therefore, the WOB versus the respiratory rate curve is normal. Because the breathing rate is higher than the normal optimal rate, the work done against airflow resistance (and total work) is increased.
(Choice D) A fixed upper airway obstruction (eg, caused by a large goiter) leads to increased air flow resistance and favors slow, deep breaths to minimize the WOB.
Educational objective:
The work of breathing is minimized in patients with increased elastic resistance (eg, pulmonary fibrosis) when their respiratory rate is high and tidal volume is low (fast, shallow breaths). In contrast, patients with diseases that increase airflow resistance (eg, asthma, chronic obstructive pulmonary disease) breathe at a lower respiratory rate and higher tidal volume (slow, deep breaths) to minimize the work of breathing.