Health rising blog post, by Cort Johnson, 6 December: Exercise Exposes New Types of Postural Orthostatic Tachycardia Syndrome (POTS)
Exercise has been used in many studies to understand chronic fatigue syndrome (ME/CFS), but nobody until recently has used exercise to try to understand POTS. Exercise is a particularly interesting tool in the case of POTS because exercise intolerance is often present, and because like with fibromyalgia, exercise has become a kind of go-to therapy for POTS.
In these two studies researchers at the Mayo Clinic in Rochester, New York exercised adolescent POTS and ME/CFS patients (in one of the studies) to exhaustion while measuring their heart rates, oxygen usage, anaerobic threshold, ventilation, gas exchange, etc. The hypothesis – POTS is a heterogeneous condition that is caused in several ways. The goal – to elucidate different subsets.
POTS is characterized by high heart rates upon standing which attempt to compensate for blood pooling in the lower body
- Exercise stress tests exposed types of POTS that tilt tests failed to reveal
- The high cardiac outputs and blood flows in hyperkinetic POTS patients attempt to compensate for a failure to vasoconstrict or tighten down their blood vessels when they stand.
- The overly vasoconstricted blood vessels in hypokinetic POTS patients attempt to compensate for reduced blood volume, low venous capacity and reduced blood flows to the heart
- The study suggested that deconditioning adds another burden to both adolescent POTS and ME/CFS patients.
Neuropathic POTS – caused by decreased vasoconstriction of the blood vessels in the legs and/or abdomen causing blood to pool in the lower body upon standing. Not associated with autoimmune issues according to a 2014 review. Treatment is focused on improving circulation with exercise and vasoconstricting drugs such as Midodrine, droxidropa and Mestinon (pyridostigmine)
Hyperadrenergic POTS – associated with increased sympathetic nervous system activity which can be caused in multiple ways. Treatment includes exercise, beta blockers and possibly angiotensin receptor blockers and droxidopa.
Hyperkinetic – The high cardiac output seen in the “hyperkinetic” POTS group attempts to compensate for problems constricting their blood vessels. During exercise our blood vessels should narrow in order to develop enough pressure (perfusion pressure) to force more blood into our tissues. In the hyperkinetic group, however, their blood vessels remain open; instead the group kicks their heart output up in order to produce the needed perfusion pressure.
That compensatory effort – like so many compensatory efforts in the body – is not entirely successful. These patients get lots of blood flowing through their systems but still get reduced oxygen extraction at the muscles. The authors noted that the reduced oxygen extraction could be due to metabolic issues but they believe is probably simply a blood flow problem. They characterized these patients’ muscles as starving in the land of plenty – and becoming fatigued because of it.
Unlike the hypokinetic group, these patients do not have problems with blood volume or preload.
The Hypokinetic Group – Hypokinetic POTS patients have the opposite problem; their low blood volume and decreased blood vessel capacity means they can’t increase their cardiac output. Instead, they tighten down their blood vessels in order to apply pressure.
In 2004 Stewart called this group of patients the “low-flow” group. Low blood volume clearly plays a major role. These patients’ low calf blood volumes, reduced venous capacitance and tightened down blood vessels left Stewart describing them as being “chronically vasoconstricted”. The “muscle pump” that’s supposed to kick in to keep their blood from draining into their lower body when they stood isn’t working either.
This study indicates that this group of POTS patients also has reduced stroke volume (cardiac output) due to reduced preload. Reduced preload – or the inability to fill the heart with enough blood to pump it out in normal amounts – is same problem that Systrom uncovered in his large exercise study of patients with unexplained exercise problems.
The only thing the body can do to combat a problem like this is to clamp down on the blood vessels in an attempt to build up enough pressure to get the blood to the tissues (e.g. perfusion pressure again).