104 Since the mid 1980s, there has been a tremendous increase in knowledge concerning illnesses that result from disturbances in the normal functioning of the autonomic nervous system. Initially, many of these investigations were principally focused on neurocardiogenic (or vasovagal) syncope, primarily as a consequence of the development of head upright tilt table testing as a method for uncovering a predisposition to the condition. During the course of these investigations, it became evident that a distant subgroup of patients experienced a related, yet distinct, autonomic disturbance that resulted in persistent orthostatic tachycardia and orthostatic intolerance.1 This disorder has come to be known as postural orthostatic tachycardia syndrome (POTS), and seems to consist of a heterogeneous group of disorders that share similar clinical characteristics. This chapter will present a review of the pathophysiology, diagnosis, and management of this disorder. In a healthy individual, close to 25% to 30% of the body’s blood volume is in the thorax while supine.2 Upon standing, the effect of gravity is to displace approximately 300 to 800 mL of blood downward to the abdomen and lower extremities. This is volume drop of 25% to 30% occurs in the first few moments of standing, resulting in a decline in venous return to the heart. As a result, the heart can pump only the blood that it receives, which produces a decline in stroke volume of approximately 40% and a decline in arterial blood pressure. The area around which these changes occur is known as the venous hydrostatic indifference point (HIP), and it represents the point in the vascular system where it is independent of position. The arterial HIP is near the level of the left ventricle, and the venous HIP is around the diaphragm. Adequate maintenance of cerebral perfusion during upright posture is the product of the interaction of several cardiovascular regulatory systems. The exact changes that occur with standing (an active process) differ somewhat from those seen during head-up tilt (a more passive process). Wieling and van Lieshout2 have described three phases of orthostatic response: (1) the initial response (in the first 30 seconds), (2) the early steady state alteration (at 1 to 2 minutes), and (3) the prolonged orthostatic period (after at least 5 minutes upright). By the middle of the nineteenth century, physicians began to report on a group of patients who had developed a disorder characterized by exercise intolerance, severe fatigue, and palpitations.3 These symptoms would often appear suddenly without a discernible cause such as prolonged immobility, blood loss, or dehydration. At the time of the American Civil War, DeCosta described patients suffering from postural tachycardia and orthostatic intolerance, a condition he called “irritable heart syndrome”.4 Around the time of the World War I, a condition referred to as neurocirculatory asthenia began to be reported. The most remarkable of these was a study by Thomas Lewis, who described a condition he called the “effort syndrome.”5 He stated that the fatigue was “an almost universal complaint” among these patients, as well as exercise intolerance in conjunction with symptoms such as palpitations, chest pain, syncope, and near syncope. In addition, Lewis reported that these patients demonstrated a significant postural tachycardia, with heart rates changing from 85 beats/min (bpm) supine to 120 bpm while upright. In some of these patients, there was a significant decline in blood pressure while upright, whereas others demonstrated only a modest decline.6 Lewis concluded that in these patients “the potential reservoir in the veins takes up the blood, the supply to the heart falls away, and the arterial pressure falls rapidly,” oftentimes accompanied by a compensatory tachycardia. Lewis further wrote that the reduction in blood flow “may be sufficient to produce cerebral anemia.” Additional reports appeared,7,8 and this condition was later elucidated by Schondorf and Low, who performed extensive evaluations of 16 patients who suffered from extreme fatigue, exercise intolerance, bowel hypomotility, and lightheadness.9 During head-upright tilt table testing, these individuals displayed distinctly abnormal cardiovascular responses to upright posture, with heart rate elevations to as high as 120 to 170 bpm within the first 2 to 5 minutes of upright tilt. Some of these patients became hypotensive, but the majority remained normotensive, and a small percentage became hypertensive. In describing the condition, they used the term “postural orthostatic tachycardia syndrome” (POTS).10 Later investigations have found that POTS is not a single entity; rather, it is a heterogeneous group of disorders with similar clinical characteristics.6,11,12 Some patients can be so severely affected that the regular activities of daily life such as housework, bathing, and eating can greatly exacerbate symptoms. Studies have shown that some patients with POTS can suffer from the same degree of functional impairment as patients with congestive heart failure or chronic obstructive pulmonary disease. Interestingly, the severity of the symptoms can be greater in patients with POTS than in those with more severe autonomic failure syndromes, such as pure autonomic failure. A grading system to classify the severity of orthostatic intolerance has been developed (similar to that used in congestive heart failure) as noted in Box 104-1.
Postural Orthostatic Tachycardia Syndrome
Autonomic Nervous System
Historical Perspective
Definitions
Postural Orthostatic Tachycardia Syndrome
