Determinants of Exercise Intolerance in Postural Orthostatic Tachycardia Syndrome: A Systematic Review.

Context: Postural orthostatic tachycardia syndrome (POTS) is characterized by increased heart rate (HR) with standing and is associated with dizziness, palpitations, and exercise intolerance, with poorly understood mechanisms. Objective: To review the literature on cardiorespiratory fitness in POTS, and explore possible determinants of exercise intolerance. Design: Systematic review of studies assessing exercise capacity in POTS. Eligibility Criteria: Eligible studies were original prospective and retrospective cohort studies and randomized controlled trials investigating formal exercise assessments (maximal exercise duration with or without gas exchange oxygen consumption (VO2) measures) in patients with established POTS using standard criteria. Study Selection: A literature search revealed 199 unique studies, of which we identified 17 cohorts with 1321 subjects with POTS and 502 age- and sex-matched controls. Peak VO2 was measured in 15 studies and exercise hemodynamics (stroke volume (SV), cardiac output) was measured in 10 studies. Main Outcome Measures: Outcome measures were peak VO2, gas exchange parameters, and hemodynamics (i.e., HR, SV, cardiac output). Results: All studies described higher HR at standing and submaximal exercise, with earlier peak HR in POTS. Peak VO2 was reduced in 80% of studies, but only in 30% when matched for deconditioning. Seven of 10 studies (70%) reported reduced SV with standing/exercise versus controls, but only 10% showed reduced cardiac output. Intravenous fluids did not affect hemodynamics or peak VO2. Exercise training, as well as propranolol, improved peak VO2. Conclusion: Patients with POTS reach peak HR faster. Increased HR accompanies reduced SV, but cardiac output is generally maintained or increased, making it unlikely to cause symptoms. Reducing HR may improve exercise capacity in POTS by delaying peak HR and reducing symptoms.