During standing, POTS patients suffer from orthostatic intolerance, the occurrence of an assortment of bizarre symptoms when upright. When POTS patients are upright there is a reduction in the return of blood from the lower body to the heart and recirculated to the brain and other vital organs. When one assumes the up-right posture, gravity causes a shift in blood toward the lower limbs, placing a physiological challenge on the body’s ability to maintain normal return of blood to the heart via the veins. Normally, in healthy individuals, about 25% -30% of the body’s circulating blood is located in the mid-chest (heart and lungs). During standing upright, gravity will shift 2 – 3 cups of this blood from the mid-body into the pelvis and legs – about 1 cup into the pelvis and 2 cups into the legs. Most of this gravitational shift takes place almost immediately after standing.
The role of excessive venous pooling during standing has been confirmed by testing with radioactive tagged red blood cells. Radioactivity in the calf region has been compared within healthy individuals while lying down and standing upright. Investigators tag red blood cells (add radioactivity to red blood cells), re-inject them and measure their circulatory paths. In healthy individuals, immediately upon assuming an upright posture, radioactivity (measured from radioactive tagged red blood cells) within the calf increases greater than 2-fold. Among POTS patients, the radioactivity increases almost 3-fold upon standing upright. Thus, in POTS patients, even more blood may gravitate into the lower abdomen, pelvis, thighs and legs during standing upright.
This pooling was confirmed 70 years ago among non-POTS patients by measuring the pressure within the veins of the ankle during lying, sitting and standing. Venous pressure in the foot increased 5-fold when changing from lying to sitting and 8-fold after assuming a standing posture for 22 seconds. Walking on a treadmill resulted in a venous pressure of only 2-fold greater than during lying and one-quarter of that measured during standing (a 4-fold fall). Walking really does pump blood uphill.
All of these measurements confirm the important role of muscular contractions of the leg muscles in pumping blood upward. Because there is reduced return of blood to the heart when upright, there is a subsequent reduced volume of blood pumped with each subsequent heartbeat . Specifically, during standing in individuals with POTS, the cardiac output is significantly reduced. The more severe the reduction in cardiac output and cerebral blood flow, the greater the reduction in oxygenation of the brain and severity of symptoms.