To the Editor.
—The article by Heimbach and Crout in the Archives (129:430, 1972) found that the atropine-induced increase of heart rate was diminished in hyperthyroid subjects as compared to normals. They then approached the conclusion that tonic vagal inhibition is reduced in hyperthyroidism.This conclusion would be more nearly valid if they had ruled out, through appropriate controls, the initial value effect described by Wilder1: when an initial value is high (pulse for example), a drug or other influence which raises the value has a smaller than usual effect; conversely, when an initial value is low, an agent which decreases the value has a smaller than usual effect. This law seems intuitively obvious. However, it does not apply to all systems2 and does restrict certain conclusions and inferences from autonomic studies such as those of Heimbach and Crout.References
Hall WH. Initial Value Effect and Hyperthyroidism. Arch Intern Med. 1972;130(3):447. doi:10.1001/archinte.1972.03650030119026