Results from our laboratory have shown that there is a sex-specific difference in cardiac size and mass even in healthy humans and that women have a smaller (and therefore, less distensible) heart compared with men (
2,
54). It is possible that such a sex difference is exaggerated in POTS patients. This notion is supported by a recent study of Miwa and Fujita (
8) showing that a considerable number of
chronic fatigue syndrome patients had a small heart, as assessed by roentgenography and
echocardiography. POTS is a frequent finding in patients with chronic fatigue syndrome (
55). It is highly likely that the small heart contributes to the development of POTS and probably should be included in the genesis of this syndrome.
By using a
cardiac MRI technique, we assessed precisely the heart size and mass in patients with POTS and found that the heart was approximately 16% smaller in these patients and more than 2 SDs smaller than the true mean for healthy sedentary controls. In the famous children's book
How the Grinch Stole Christmasby Dr. Seuss (
56), subsequently popularized by the movie of the same name (2000), the main character had a heart that was “two sizes too small.” We suggest, then, that a more pathophysiologic name for POTS is the “Grinch syndrome,” emphasizing that a small heart is the primary abnormality and target for therapy. A small heart coupled with reduced blood volume contributes to the small stroke volume, ultimately resulting in reflex tachycardia during orthostasis in these patients.