Exertional dyspnea and ventilation in hyperthyroidism

Dyspnea is recognized to be an important feature in patients with hyperthyroidism at rest and during exercise. However, its etiology is not well-understood. Since dyspnea is thought to be related to the perception of excessive ventilatory effort, we explored the possibility that dyspnea in these patients might be related to an inappropriate ventilatory response to the increased metabolic rate. We studied 11 hyperthyroid patients and 11 age- and sex-matched controls, performing spirometry, lung volumes, mouth pressure measurements, and incremental exercise test. Central drive was estimated by measuring P_0.1 and sensation of dyspnea by the Borg scale. We found that hyperthyroid patients (1) have higher ventilation than normal subjects during exercise even when corrected for VCO₂ levels; (2) this increased ventilation is secondary to increased central drive which is correlated to the T₃ria level (r=0.85, p<0.01); (3) hyperthyroid patients are more dyspneic than controls; and (4) the increased drive can be normalized by β-blockade. We conclude that the main ventilatory abnormality in hyperthyroid patients is an inappropriate increase in respiratory drive, possibly secondary to increased adrenergic stimulation.