The epidemic of orthostatic tachycardia and orthostatic intolerance

Children With PANS May Manifest POTS

Objectives

Pediatric acute-onset neuropsychiatric syndrome (PANS) is characterized by an abrupt-onset of severe psychiatric symptoms including OCD, anxiety, cognitive difficulties, and sleep issues which is thought to be a post-infection brain inflammatory disorder. We observed postural orthostatic tachycardia syndrome (POTS) which resolved with immunomodulation in a patient with Pediatric acute-onset neuropsychiatric syndrome (PANS). Here, we aim to present a case of POTS and to examine the prevalence of (POTS) in our PANS cohort, and compare the clinical characteristics of patients with and without POTS.

Study Design

We conducted this cohort study of patients meeting PANS criteria who had at least three clinic visits during the study period. We included data from prospectively collected questionnaires and medical record review. We present a case followed by statistical comparisons within our cohort and a Kaplan-Meier analysis to determine the time-dependent risk of a POTS diagnosis.

Results

Our study included 204 patients: mean age of PANS onset was 8.6 years, male sex (60%), non-Hispanic White (78%). Evidence of POTS was observed in 19/204 patients (9%) with 5/19 having persistent POTS defined as persistent abnormal orthostatic vitals, persistent POTS symptoms, and/or continued need for pharmacotherapy for POTS symptoms for at least 6 months). In this PANS cohort, patients with POTS were more likely to have comorbid joint hypermobility (63 vs 37%, p = 0.04), chronic fatigue (42 vs 18%, p = 0.03), and a family history of chronic fatigue, POTS, palpitations and syncope. An unadjusted logistic regression model showed that a PANS flare (abrupt neuropsychiatric deterioration) was significantly associated with an exacerbation of POTS symptoms (OR 3.3, 95% CI 1.4–7.6, p < 0.01).

Conclusions

Our study describes a high prevalence of POTS in patients with PANS (compared to the general population) and supports an association between POTS presentation and PANS flare within our cohort.

Cardiovascular Autonomic Regulation, ETCO2 and the Heart Rate Response to the Tilt Table Test in Patients with Orthostatic Intolerance

Chronic orthostatic intolerance (COI) is defined by changes in heart rate (HR), blood pressure (BP), respiration, symptoms of cerebral hypoperfusion and sympathetic overactivation. Postural tachycardia syndrome (POTS) is the most common form of COI in young adults and is defined by an orthostatic increase in heart rate (HR) of ≥ 30 bpm in the absence of orthostatic hypotension. However, some patients referred for evaluation of COI symptoms do not meet the orthostatic HR response criterion of POTS despite debilitating symptoms. Such patients are ill defined, posing diagnostic and therapeutic challenges. This study explored the relationship among cardiovascular autonomic control, the orthostatic HR response, EtCO₂ and the severity of orthostatic symptoms and fatigue in patients referred for evaluation of COI. Patients (N = 108) performed standardized testing protocol of the Autonomic Reflex Screen and completed the Composite Autonomic Symptom Score (COMPASS-31) and the Fatigue Severity Scale (FSS). Greater severity of COI was associated with younger age, larger phase IV amplitude in the Valsalva maneuver and lower adrenal baroreflex sensitivity. Greater fatigue severity was associated with a larger reduction in ETCO₂ during 10 min of head-up tilt (HUT) and reduced low-frequency (LF) power of heart rate variability. This study suggests that hemodynamic changes associated with the baroreflex response and changes in EtCO₂ show a stronger association with the severity of orthostatic symptoms and fatigue than the overall orthostatic HR response in patients with COI.

Gravity Threshold and Dose Response Relationships: Health Benefits Using a Short Arm Human Centrifuge

Purpose

Increasing the level of gravity passively on a centrifuge, should be equal to or even more beneficial not only to astronauts living in a microgravity environment but also to patients confined to bed. Gravity therapy (GT) may have beneficial effects on numerous conditions, such as immobility due to neuromuscular disorders, balance disorders, stroke, sports injuries. However, the appropriate configuration for administering the Gz load remains to be determined.

Methods

To address these issues, we studied graded G-loads from 0.5 to 2.0g in 24 young healthy, male and female participants, trained on a short arm human centrifuge (SAHC) combined with mild activity exercise within 40–59% MHR, provided by an onboard bicycle ergometer. Hemodynamic parameters, as cardiac output (CO), stroke volume (SV), mean arterial pressure (MAP), systolic blood pressure (SBP), diastolic blood pressure (DBP), and heart rate (HR) were analyzed, as well as blood gas analysis. A one-way repeated measures ANOVA and pairwise comparisons were conducted with a level of significance p < 0.05.

Results

Significant changes in heart rate variability (HRV) and its spectral components (Class, Fmax, and VHF) were found in all g loads when compared to standing (p < 0.001), except in 1.7 and 2.0g. There were significant changes in CO, cardiac index (CI), and cardiac power (CP) (p < 0.001), and in MAP (p = 0.003) at different artificial gravity (AG) levels. Dose-response curves were determined based on statistically significant changes in cardiovascular parameters, as well as in identifying the optimal G level for training, as well as the optimal G level for training. There were statistically significant gender differences in Cardiac Output/CO (p = 0.002) and Cardiac Power/CP (p = 0.016) during the AG training as compared to standing. More specifically, these cardiovascular parameters were significantly higher for male than female participants. Also, there was a statistically significant (p = 0.022) gender by experimental condition interaction, since the high-frequency parameter of the heart rate variability was attenuated during AG training as compared to standing but only for the female participants (p = 0.004).

Conclusion

The comprehensive cardiovascular evaluation of the response to a range of graded AG loads, as compared to standing, in male and female subjects provides the dose-response framework that enables us to explore and validate the usefulness of the centrifuge as a medical device. It further allows its use in precisely selecting personalized gravity therapy (GT) as needed for treatment or rehabilitation of individuals confined to bed.

Supraventricular Tachycardia and Postural Orthostatic Tachycardia Syndrome Overlap: A Retrospective Study

Postural orthostatic tachycardia syndrome (POTS) and supraventricular tachycardia (SVT) are disease states with distinctive features but overlapping clinical manifestations. Currently, studies on the presence of underlying SVT in patients with POTS are lacking. This retrospective study analyzed 64 patients [mean age: 43 years; 41 (61%) women] who had a POTS diagnosis and were found to have concomitant SVT during rhythm monitoring from September 1, 2013 to September 30, 2019 at our Syncope and Autonomic Disorders Clinic. The outcomes assessed were changes in disease severity, frequency of symptoms, heart rate, and blood pressure between before and after SVT ablation. The most frequent types of SVT noted on the electrophysiologic study were atrioventricular nodal reentrant tachycardia (57.81%), atrial flutter (29.68%), atrioventricular reentrant tachycardia (9.37%), atrial tachycardia (1.56%), and junctional tachycardia (1.56%). After SVT ablation, all 64 patients experienced an improvement in symptoms. Palpitations and lightheadedness experienced the most improvement after the procedure (72% vs. 31%; p < 0.001 and 63% vs. 22%; p < 0.001, respectively). There was a significant improvement in the resting heart rate (81.1 ± 12.8 vs. 75.8 ± 15.6 bpm; p < 0.002), but the orthostatic tachycardia on standing persisted (93.6 ± 16.5 vs. 77.3 ± 19.8 bpm; p = 0.14). Underlying SVT in patients with POTS can be missed easily. A strong suspicion and long-term ambulatory cardiac rhythm monitoring can help in diagnosing the condition.

Impaired Endothelial Function in Patients With Postural Tachycardia Syndrome

The purpose of this study is to evaluate endothelial function in postural tachycardia syndrome (PoTS), a poorly understood chronic condition characterized by a state of consistent orthostatic tachycardia (delta heart rate ≥30 beats per minute) upon standing without orthostatic hypotension. Nineteen patients with PoTS and 9 healthy controls were studied after 3 days of a fixed, caffeine-free, normal sodium (150 milliequivalents/day) diet. All participants underwent autonomic function testing, including sinus arrhythmia, valsalva maneuver, hyperventilation, cold pressor, handgrip, and a standing test with catecholamine measurements, followed by endothelial function testing. We analyzed 3 measures of endothelial function: percent brachial flow-mediated dilation, digital pulsatile arterial tonometry, and postischemic percent leg blood flow. Flow-mediated dilation was significantly lower in patients with PoTS (6.23±3.54% for PoTS) than in healthy controls (10.6±4.37% for controls versus, P=0.014). PoTS and controls had similar digital pulsatile arterial tonometry (1.93±0.40 arbitrary units for controls versus 2.13±0.63 arbitrary units for PoTS). PoTS had similar but suggestive percent leg blood flow to controls (313±158% for PoTS versus 468±236% for controls, P=0.098). Patients with PoTS have significantly reduced flow-mediated dilation compared with healthy controls, suggesting that PoTS is characterized by endothelial dysfunction in conduit arteries. Registration: URL: https://www.clinicaltrials.gov. Unique identifier: NCT01308099.

Supine Parasympathetic Withdrawal and Upright Sympathetic Activation Underly Abnormalities of the Baroreflex in Postural Tachycardia Syndrome: Effects of Pyridostigmine and Digoxin

[Figure: see text].

Clinical Prognostic Factors in Pediatric Patients With Orthostatic Intolerance

Midodrine is widely used for orthostatic intolerance (OI); however, little is known about the prognostic factors of OI after midodrine treatment. We retrospectively reviewed electronic medical charts to investigate clinical prognostic factors of OI on 159 OI patients aged 7 to 18 years who were treated with midodrine at a children's hospital. Logistic regression was conducted to clarify predictors for improving symptoms at the first month of the treatment. Patients with orthostatic uncomfortable feeling or fainting were significantly more likely to improve symptoms at the first month of the treatment (odds ratio [OR], 3.48; 95% confidence interval [95%CI], 1.36-8.89), but patients with underweight were significantly less likely to improve symptoms (OR, 0.19; 95%CI, 0.06-0.56). Our results suggest that predictive factors for OI by midodrine treatments are orthostatic symptoms and underweight in pediatric patients. These findings are useful to develop further studies for OI treatments.

Splanchnic Venous Compression Enhances the Effects of ß-Blockade in the Treatment of Postural Tachycardia Syndrome

Background

Splanchnic venous pooling induced by upright posture triggers a compensatory increase in heart rate (HR), a response that is exaggerated in patients with postural tachycardia syndrome. To assess whether abdominal compression attenuates orthostatic tachycardia and improves symptoms, 18 postural tachycardia syndrome patients (32±2 years) were randomized to receive either abdominal compression (40 mm Hg applied with an inflatable binder ≈2 minutes before standing) or propranolol (20 mg) in a placebo‐controlled, crossover study.

Methods and Results

Systolic blood pressure, HR, and symptoms were assessed while seated and standing, before and 2 hours postdrug. As expected, propranolol decreased standing HR compared with placebo (81±2 versus 98±4 beats per minute; P<0.001) and was associated with lower standing systolic blood pressure (93±2 versus 100±2 mm Hg for placebo; P=0.002). Compression had no effect on standing HR (96±4 beats per minute) but increased standing systolic blood pressure compared with placebo and propranolol (106±2 mm Hg; P<0.01). Neither propranolol nor compression improved symptoms compared with placebo. In 16 patients we compared the combination of abdominal compression and propranolol with propranolol alone. The combination had no additional effect on standing HR (81±2 beats per minute for both interventions) but prevented the decrease in standing systolic blood pressure produced by propranolol (98±2 versus 93±2 mm Hg for propranolol; P=0.029), and significantly improved total symptom burden (−6±2 versus −1±2 for propranolol; P=0.041).

Conclusions

Splanchnic venous compression alone did not improve HR or symptoms but prevented the blood pressure decrease produced by propranolol. The combination was more effective in improving symptoms than either alone. Splanchnic venous compression can be a useful adjuvant therapy to propranolol in postural tachycardia syndrome.

Registration

URL: https://www.clini​caltr​ials.gov; Unique identifier: NCT00262470.

Functional capacity and quality of life in the postural tachycardia syndrome: A retrospective cross-sectional study

Background

Postural tachycardia syndrome (POTS) is a complex syndrome of orthostatic intolerance that significantly affects quality of life. The relationship between functional capacity, quality of life, and age remains poorly understood in this patient population. The purpose of this cross-sectional study was to assess the clinical and exercise characteristics of patients with POTS who underwent exercise stress testing as part of cardiac rehabilitation, and to evaluate the relationships between functional capacity with age and sex, as well as the relationship between functional capacity and quality of life.

Methods

We included 255 consecutive adult patients with the diagnosis of POTS, by tilt table testing, who underwent exercise stress testing between 2012 and 2017. Clinical and stress test data were obtained from electronic medical records.

Results

Of the 255 patients, 232 (91%) were women, with median age 33.5 years. Prevalence of traditional CAD risk factors was low (2% diabetes, 13% hypertension, 7% hyperlipidemia, and 26% smoking history). Mean resting BP was 114 ± 13 mmHg systolic, resting HR was 76 ± 13 bpm, METs achieved 9.2 ± 2.2, and 1 min HR recovery 32 ± 17 bpm. 113 patients (44%) had abnormal functional capacity for age and sex. When analyzed by age groups, younger POTS patients had increasingly lower than expected functional capacity (compared to predicted normals) than did older patients (ANOVA P = 0.0017). The SF-36 physical component of patients with abnormal functional capacity was significantly lower than those with normal functional capacity. (p = 0.006).

Conclusions

In this large cohort, patients with POTS were predominantly female (91%) and relatively young. The novel findings are that younger patients with POTS were more likely to have lower-than-average functional capacity for their age and sex compared to older patients, and that abnormal functional capacity was associated with lower quality of life by SF-36 physical component.

Ivabradine in Postural Orthostatic Tachycardia Syndrome: A Review of the Literature

Postural orthostatic tachycardia syndrome (POTS) is an autonomic disorder characterized by symptoms such as palpitations, dyspnea, chest discomfort, and lightheadedness affecting various systems. The pathophysiology of POTS is not completely understood due to a variety of symptoms showing that the disease is multifactorial. There is no approved uniform management strategy for POTS and hence, no drug has been approved by the United States (US) Food and Drug Administration (FDA) for it. Ivabradine is an FDA-approved drug for stable symptomatic heart failure (HF) and patients with an ejection fraction (EF) of ≤35%. Previous studies have depicted improvement in symptoms of POTS with the use of ivabradine. It is a selective inhibitor of funny sodium channels (I_f) in the sinoatrial (SA) node cells resulting in the prolongation of the slow diastolic depolarization (phase IV) and reduction in the heart rate (HR). Although beta-adrenoceptor blockers are commonly used to lower HR in patients with POTS, they are less ideal due to numerous adverse effects. This review aims to provide a comprehensive and up-to-date picture of all the studies and case reports that utilized ivabradine for the treatment of POTS along with a precise overview of epidemiology, pathophysiology, and types of POTS. To conclude, we recommend further research on the effectiveness of ivabradine in patients who experience symptoms of POTS. Other than stable chronic angina pectoris, its application in this setting has been proven to be effective and safe. Further evaluation by means of randomized control trials is required to encourage use of this HR-lowering agent in common disorders other than HF and stable angina, i.e. POTS.

Clinical Course of Transgender Adolescents with Complicated Postural Orthostatic Tachycardia Syndrome Undergoing Hormonal Therapy in Gender Transition: A Case Series

Purpose: Postural orthostatic tachycardia syndrome (POTS), an increasingly recognized dysautonomia, may affect as many as 3,000,000 Americans. Concurrently, prevalence estimates suggest 10% of individuals identify as lesbian, gay, bisexual, transgender, or questioning/queer. The preponderance of female POTS patients implies hormonal differences between natal sexes and their role in POTS. Transgender POTS patients using hormone therapies may offer further insight into the mechanism of POTS. There have been no previously published studies of transgender patients with POTS undergoing gender-affirming hormone therapy. Methods: We reviewed our electronic health record for clinical histories of transgender patients in our POTS Database. Results: Three patients who transitioned from female to male demonstrated clinical improvement of their POTS symptoms with the addition of testosterone therapy. Conclusion: We present our clinical experience of three transgender POTS patients who transitioned from female to male with hormone therapy, all of whom demonstrated clinical improvement with testosterone. This may give further insight into the pathophysiology of POTS. However, the authors do not endorse the use of hormone therapy as primary therapy for the symptoms of POTS.

The face of postural tachycardia syndrome - insights from a large cross-sectional online community-based survey

BACKGROUND

Patients with postural tachycardia syndrome (POTS) experience chronic symptoms of orthostatic intolerance. There are minimal data detailing the demographics, clinical features and clinical course of this condition. This online, community-based survey highlights patients' experience with POTS. It consists of the largest sample of POTS patients reported to date.

OBJECTIVES

To describe the demographics, past medical history, medications, treatments and diagnostic journey for patients living with POTS.

METHODS

Postural tachycardia syndrome patients completed an online, community-based, cross-sectional survey. Participants were excluded if they had not received a diagnosis of POTS from a physician. The questions focused on the patient experience and journey, rather than physiological responses.

RESULTS

The final analysis included 4835 participants. POTS predominantly affects white (93%) females (94%) of childbearing age, with approximately half developing symptoms in adolescence (mode 14 years). POTS is a chronic multisystem disorder involving a broad array of symptoms, with many patients diagnosed with comorbidities in addition to POTS. POTS patients often experience lengthy delays [median (interquartile range) 24 (6-72) months] and misdiagnosis, but the diagnostic delay is improving. POTS patients can present with a myriad of symptoms most commonly including lightheadedness (99%), tachycardia (97%), presyncope (94%), headache (94%) and difficulty concentrating (94%).

CONCLUSIONS

These data provide important insights into the background, clinical features and diagnostic journey of patients suffering from POTS. These data should serve as an essential step for moving forward with future studies aimed at early and accurate diagnoses of these patients leading to appropriate treatments for their symptoms.

ST-segment changes during tilt table testing for postural tachycardia syndrome: correlation with exercise stress test results

BACKGROUND

Prior studies have reported ECG (Electrocardiogram) changes during tilt table testing (TTT), specifically during repolarization with ST-segment and T-wave changes. The correlation with ischemic evaluation remains unclear. The purpose of this study was to analyze the prevalence of ST-segment changes during TTT in a young, otherwise healthy population of patients with postural tachycardia syndrome (POTS), and correlate them with exercise stress test results.

METHODS

Two hundred and fifty-five patients with POTS who underwent TTT and an exercise treadmill test (ETT) were analyzed.

RESULTS

Forty-five had ST-segment changes/depressions during TTT (91% female, average age 36 years). Of the 45, three had ST-segment depression during ETT; all three had negative exercise stress echocardiograms (ESEs). Two others had ST-segment depressions on ETT (but not TTT), with negative ESEs.

CONCLUSION

In a cohort of young, female, otherwise healthy patients with POTS, ST-segment changes occurred in a significant portion (18%) of patients during TTT. When evaluated with exercise stress testing, these patients had no evidence of underlying ischemia on ETT or ESE.

Postural orthostatic tachycardia syndrome: clinical presentation, aetiology and management

Postural orthostatic tachycardia syndrome (POTS) is a variant of cardiovascular autonomic disorder characterized by an excessive heart rate increase on standing and orthostatic intolerance. POTS affects younger individuals 15-45 years old with a distinct female predominance (≈80%). The prevalence ranges between 0.2% and 1.0% in developed countries. The onset of POTS is typically precipitated by immunological stressors such as viral infection, vaccination, trauma, pregnancy, surgery or psychosocial stress. The most common complaints are dizziness, weakness, rapid heartbeat and palpitation on standing. Moreover, patients often report physical deconditioning and reduced exercise capacity as well as headache, 'brain fog', dyspnoea, gastrointestinal disorders and musculoskeletal pain. The aetiology of POTS is largely unknown and three main hypotheses include an autoimmune disorder, abnormally increased sympathetic activity and catecholamine excess, and sympathetic denervation leading to central hypovolaemia and reflex tachycardia. The golden standard for POTS diagnosis is head-up tilt test with a non-invasive beat-to-beat haemodynamic monitoring. Although long-term prognosis of POTS is poorly explored, around 50% of patients spontaneously recover within 1-3 years. After the diagnosis has been established, patient should be thoroughly educated about non-pharmacological measures alleviating the symptoms. Exercise training may be very effective and counteract deconditioning. In more symptomatic patients, different drugs directed at controlling heart rate, increasing peripheral vasoconstriction and intravascular volume can be tested. However, the overall effects of pharmacological therapy are modest and the most affected patients remain handicapped. Future efforts should focus on better understanding of POTS pathophysiology and designing randomized controlled trials for selection of more effective therapy.

Effect of Food Intake on Hemodynamic Parameters during the Tilt-Table Test in Patients with Postural Orthostatic Tachycardia Syndrome

BACKGROUND AND PURPOSE

The aim of this study was to determine the effect of food intake on the heart rate (HR) in postural orthostatic tachycardia syndrome (POTS).

METHODS

The following five-phase protocol was applied to 41 subjects who had a sustained HR increment of ≥30 beats/min within 10 min of standing in an initial tilt-table test: 1) 10-min supine phase, 2) 10-min 70°-tilted phase, 3) ingestion of 400 mL of Nutridrink Multi Fibre®, 4) 45-min supine phase, and 5) 10-min 70°-tilted phase. Subjects were divided into four groups: A) difference in HR for standing vs. supine (ΔHR) before the meal of ≥30 beats/min (n=13), B) ΔHR <30 beats/min before the meal but ≥30 beats/min after the meal (n=12), and C) ΔHR <30 beats/min both before and after the meal (n=16). Group D consisted of 10 healthy subjects.

RESULTS

Before the meal, ΔHR was significantly higher in group A than in all of the other groups, and in group B than in group D (p<0.001). After the meal, ΔHR was significantly higher in groups A and B (p<0.001 and p<0.0001, respectively). When we pooled patients (according to their symptoms) from group A and B into a POTS group and from group C and D into a non-POTS group, an increase in HR of 25 beats/min before the meal had a sensitivity of 92.0% and a specificity of 80.8%. After the meal, an increase in HR of 30 beats/min had a sensitivity of 96.0% and a specificity of 96.2%.

CONCLUSIONS

Food intake can significantly alter the results of the tilt-table test and so should be taken into account when diagnosing POTS.

Impact of the Autonomic Nervous System on the Skeleton

It is from the discovery of leptin and the central nervous system as a regulator of bone remodeling that the presence of autonomic nerves within the skeleton transitioned from a mere histological observation to the mechanism whereby neurons of the central nervous system communicate with cells of the bone microenvironment and regulate bone homeostasis. This shift in paradigm sparked new preclinical and clinical investigations aimed at defining the contribution of sympathetic, parasympathetic, and sensory nerves to the process of bone development, bone mass accrual, bone remodeling, and cancer metastasis. The aim of this article is to review the data that led to the current understanding of the interactions between the autonomic and skeletal systems and to present a critical appraisal of the literature, bringing forth a schema that can put into physiological and clinical context the main genetic and pharmacological observations pointing to the existence of an autonomic control of skeletal homeostasis. The different types of nerves found in the skeleton, their functional interactions with bone cells, their impact on bone development, bone mass accrual and remodeling, and the possible clinical or pathophysiological relevance of these findings are discussed.

Postural tachycardia syndrome - Diagnosis, physiology, and prognosis

Postural tachycardia syndrome (POTS) is a heterogeneous clinical syndrome that has gained increasing interest over the past few decades due to its increasing prevalence and clinical impact on health-related quality of life. POTS is clinically characterized by sustained excessive tachycardia upon standing that occurs in the absence of significant orthostatic hypotension and other medical conditions and or medications, and with chronic symptoms of orthostatic intolerance. POTS represents one of the most common presentations of syncope and presyncope secondary to autonomic dysfunction in emergency rooms and in cardiology, neurology, and primary care clinics. The most sensitive method to detect POTS is a detailed medical history, physical examination with orthostatic vital signs or brief tilt table test, and a resting 12-lead electrocardiogram. Additional diagnostic testing may be warranted in selected patients based on clinical signs. While the precise etiology remains unknown, the orthostatic tachycardia in POTS is thought to reflect convergence of multiple pathophysiological processes, as a final common pathway. Based on this, POTS is often described as a clinical syndrome consisting of multiple heterogeneous disorders, with several underlying pathophysiological processes proposed in the literature including partial sympathetic neuropathy, hyperadrenergic state, hypovolemia, mast cell activation, deconditioning, and immune-mediated. These clinical features often overlap, however, making it difficult to categorize individual patients. Importantly, POTS is not associated with mortality, with many patients improving to some degree over time after diagnosis and proper treatment. This review will outline the current understanding of diagnosis, pathophysiology, and prognosis in POTS.

Hemodynamic characteristics of postural hyperventilation: POTS with hyperventilation versus panic versus voluntary hyperventilation

Upright hyperventilation occurs in ~25% of our patients with postural tachycardia syndrome (POTS). Poikilocapnic hyperventilation alone causes tachycardia. Here, we examined changes in respiration and hemodynamics comprising cardiac output (CO), systemic vascular resistance (SVR), and blood pressure (BP) measured during head-up tilt (HUT) in three groups: patients with POTS and hyperventilation (POTS-HV), patients with panic disorder who hyperventilate (Panic), and healthy controls performing voluntary upright hyperpnea (Voluntary-HV). Though all were comparably tachycardic during hyperventilation, POTS-HV manifested hyperpnea, decreased CO, increased SVR, and increased BP during HUT; Panic patients showed both hyperpnea and tachypnea, increased CO, and increased SVR as BP increased during HUT; and Voluntary-HV were hyperpneic by design and had increased CO, decreased SVR, and decreased BP during upright hyperventilation. Mechanisms of hyperventilation and hemodynamic changes differed among POTS-HV, Panic, and Voluntary-HV subjects. We hypothesize that the hyperventilation in POTS is caused by a mechanism involving peripheral chemoreflex sensitization by intermittent ischemic hypoxia. NEW & NOTEWORTHY Hyperventilation is common in postural tachycardia syndrome (POTS) and has distinctive cardiovascular characteristics when compared with hyperventilation in panic disorder or with voluntary hyperventilation. Hyperventilation in POTS is hyperpnea only, distinct from panic in which tachypnea also occurs. Cardiac output is decreased in POTS, whereas peripheral resistance and blood pressure (BP) are increased. This is distinct from voluntary hyperventilation where cardiac output is increased and resistance and BP are decreased and from panic where they are all increased.

Postural Hyperventilation as a Cause of Postural Tachycardia Syndrome: Increased Systemic Vascular Resistance and Decreased Cardiac Output When Upright in All Postural Tachycardia Syndrome Variants

BACKGROUND

Postural tachycardia syndrome (POTS) is a heterogeneous condition. We stratified patients previously evaluated for POTS on the basis of supine resting cardiac output (CO) or with the complaint of platypnea or "shortness of breath" during orthostasis. We hypothesize that postural hyperventilation is one cause of POTS and that hyperventilation-associated POTS occurs when initial reduction in CO is sufficiently large. We also propose that circulatory abnormalities normalize with restoration of CO2.

METHODS AND RESULTS

Fifty-eight enrollees with POTS were compared with 16 healthy volunteer controls. Low CO in POTS was defined by a resting supine CO <4 L/min. Patients with shortness of breath had hyperventilation with end tidal CO2 <30 Torr during head-up tilt table testing. There were no differences in height or weight between control patients and patients with POTS or differences between the POTS groups. Beat-to-beat blood pressure was measured by photoplethysmography, and CO was measured by ModelFlow. Systemic vascular resistance was defined as mean arterial blood pressure/CO. End tidal CO2 and cerebral blood flow velocity of the middle cerebral artery were only reduced during head-up tilt in the hyperventilation group, whereas blood pressure was increased compared with control. We corrected the reduced end tidal CO2 in hyperventilation by addition of exogenous CO2 into a rebreathing apparatus. With added CO2, heart rate, blood pressure, CO, and systemic vascular resistance in hyperventilation became similar to control.

CONCLUSIONS

We conclude that all POTS is related to decreased CO, decreased central blood volume, and increased systemic vascular resistance and that a variant of POTS is consequent to postural hyperventilation.

Comparison of Autonomic Control of Blood Pressure During Standing and Artificial Gravity Induced via Short-Arm Human Centrifuge

Autonomic control of blood pressure is essential toward maintenance of cerebral perfusion during standing, failure of which could lead to fainting. Long-term exposure to microgravity deteriorates autonomic control of blood pressure. Consequently, astronauts experience orthostatic intolerance on their return to gravitational environment. Ground-based studies suggest sporadic training in artificial hypergravity can mitigate spaceflight deconditioning. In this regard, short-arm human centrifuge (SAHC), capable of creating artificial hypergravity of different g-loads, provides an auspicious training tool. Here, we compare autonomic control of blood pressure during centrifugation creating 1-g and 2-g at feet with standing in natural gravity. Continuous blood pressure was acquired simultaneously from 13 healthy participants during supine baseline, standing, supine recovery, centrifugation of 1-g, and 2-g, from which heart rate (RR) and systolic blood pressure (SBP) were derived. The autonomic blood pressure regulation was assessed via spectral analysis of RR and SBP, spontaneous baroreflex sensitivity, and non-linear heart rate and blood pressure causality (RR↔SBP). While majority of these blood pressure regulatory indices were significantly different (p < 0.05) during standing and 2-g centrifugation compared to baseline, no change (p > 0.05) was observed in the same indices during 2-g centrifugation compared to standing. The findings of the study highlight the capability of artificial gravity (2-g at feet) created via SAHC toward evoking blood pressure regulatory controls analogous to standing, therefore, a potential utility toward mitigating deleterious effects of microgravity on cardiovascular performance and minimizing post-flight orthostatic intolerance in astronauts.

Cognitive and psychological issues in postural tachycardia syndrome

Postural tachycardia syndrome (POTS) is one of the most common forms of chronic orthostatic intolerance. In addition to orthostatic symptoms, many POTS patients report incapacitating cognitive dysfunction or "brain fog" even while lying down or seated. Consistent with these subjective reports, there is accruing objective evidence of specific cognitive difficulties in POTS, with studies showing mild to moderate cognitive impairment using standardized neuropsychological assessment batteries. The precise profile of cognitive dysfunction in POTS patients has been shown to vary among these studies potentially due to the neuropsychological tests used, postural position, comorbidities and length of illness, inclusion of adolescent versus adult patients, and sites of recruitment. The extent of the impact that this cognitive challenge has in patients justifies ongoing investigation and research into lifestyle and pharmacological treatments. Psychologically, patients face challenges congruent with many chronic illnesses, perhaps especially early in adjusting to the condition. POTS patients often exhibit mild to moderate depression symptoms as well as symptoms of anxiety disorders. Since even low levels of anxiety can exacerbate symptoms, and a high number of patients experience sub-clinical low mood and sleep disturbances, there is a likely role for psychotherapy in helping control adjustment-related issues, and possibly aberrant physiology, in POTS.

Special issue for the silver anniversary of Postural Tachycardia Syndrome

Postural Tachycardia Syndrome (POTS) is a multi-system disorder that can be associated with significant functional disability. Likely due to the multi-system nature of the disorder, it can be difficult for physicians caring for these patients with POTS to know enough about the management of problems that fall within so many different specialties. This special issue was written for these clinicians and their patients. We have brought together experts on POTS with different backgrounds to share their approach to many of the medical issues that confront patients with POTS. The result is this special POTS Issue of Autonomic Neurosciences that can serve as a "User's Guide to POTS".

Pediatric Disorders of Orthostatic Intolerance

Orthostatic intolerance (OI), having difficulty tolerating an upright posture because of symptoms or signs that abate when returned to supine, is common in pediatrics. For example, ∼40% of people faint during their lives, half of whom faint during adolescence, and the peak age for first faint is 15 years. Because of this, we describe the most common forms of OI in pediatrics and distinguish between chronic and acute OI. These common forms of OI include initial orthostatic hypotension (which is a frequently seen benign condition in youngsters), true orthostatic hypotension (both neurogenic and nonneurogenic), vasovagal syncope, and postural tachycardia syndrome. We also describe the influences of chronic bed rest and rapid weight loss as aggravating factors and causes of OI. Presenting signs and symptoms are discussed as well as patient evaluation and testing modalities. Putative causes of OI, such as gravitational and exercise deconditioning, immune-mediated disease, mast cell activation, and central hypovolemia, are described as well as frequent comorbidities, such as joint hypermobility, anxiety, and gastrointestinal issues. The medical management of OI is considered, which includes both nonpharmacologic and pharmacologic approaches. Finally, we discuss the prognosis and long-term implications of OI and indicate future directions for research and patient management.

The Influence of Gender and Anthropometry on Haemodynamic Status at Rest and in Response to Graded Incremental Head-Up Tilt in Young, Healthy Adults

The body's ability to rapidly and appropriately regulate blood pressure in response to changing physiological demand is a key feature of a healthy cardiovascular system. Passively tilting the body, thereby changing central blood volume, is a well-recognized and controlled method of evaluating this ability. However, such studies usually involve single tilt angles, or intermittent tilting separated by supine, resting periods; valuable information concerning the adaptive capacity of the regulatory systems involved is therefore currently lacking. Furthermore, despite increasing recognition that men and women differ in the magnitude of their haemodynamic response to such stimuli, little is known about the degree to which gender differences in body composition and anthropometry influence these regulatory pathways, or indeed if these differences are apparent in response to graded, incremental tilting. In the present study we measured, in 23 young, healthy adults (13 men, 10 women), the continuous beat-to-beat haemodynamic response to graded, incremental tilting (0°, 20°, 40°, 60°, and back to 40°) with each tilt angle lasting 16 min. On average, we observed increases in heart rate (+41%), blood pressure (+10%), and total peripheral resistance (+16%) in response to tilting. However, whilst men showed an immediate decrease in cardiac output upon tilting (−8.9%) cardiac output in women did not change significantly from supine values. Interestingly, the decrease in stroke volume observed in women was significantly less than that observed in men (−22 vs. −36%, p < 0.05); although the present study could not determine if this difference was due to gender per se or due to differences in body size (in particular height) between the two gender groups. Such disparities in the magnitude of autonomic response may indicate (in the case of our gradual incremental tilt procedure) a better buffering capacity to progressive changes in central blood volume in women; which warrants further investigation, particularly in light of the well-recognized differences in cardiovascular disease risk between men and women.

Baroreflex Sensitivity Predicts Short-Term Outcome of Postural Tachycardia Syndrome in Children

Objective

The study was designed to examine if baroreflex sensitivity (BRS) could predict the short-term outcome of postural tachycardia syndrome (POTS) in children.

Methods

Seventy-seven children subjects were included in the study. Among them, 45 children were in the POTS group and another 32 healthy children were in the control group. A ninety-day clinical follow-up was conducted and the symptom score before and after the follow-up was calculated for POTS patients by using POTS score system. Hemodynamics and continuous BRS monitoring were recorded by Finapres Medical System-FMS (FinometerPRO, FMS Company, Netherlands). According to the symptom score change during follow-up period, POTS patients were further divided into subgroup A (n = 24) with symptom score decreased by at least two points and subgroup B (n = 21) with symptom score decreased by less than two points. The predictive value of BRS in the short-term outcome of POTS in children was analyzed using receiver-operating characteristic (ROC) curve.

Results

BRS of POTS children was significantly higher than that of the healthy children (18.76±9.96 ms/mmHg vs 10±5.42 ms/mmHg, P<0.01). It was higher in subgroup B than that of subgroup A (24.7±9.9 ms/mmHg vs 13.5±6.6 ms/mmHg, P <0.01). BRS was positively correlated with HR change in POTS Group (r = 0.304, P <0.05). Area under curve (AUC) was 0.855 (95% of confidence interval 0.735–0.975), and BRS of 17.01 ms/mmHg as a cut-off value yielded the predictive sensitivity of 85.7% and specificity of 87.5%.

Conclusions

BRS is a useful index to predict the short-term outcome of POTS in children.

Orthostatic Circulatory Disorders: From Nosology to Nuts and Bolts

When patients complain of altered consciousness or discomfort in the upright posture, either relieved by recumbency or culminating in syncope, physicians may find themselves baffled. There is a wide variety of disorders that cause abnormal regulation of blood pressure and pulse rate in the upright posture. The aim of this focused review is 3-fold. First, to offer a classification (nosology) of these disorders; second, to illuminate the mechanisms that underlie them; and third, to assist the physician in the practical aspects of diagnosis of adult orthostatic hypotension, by extending clinical skills with readily available office technology.

Postural orthostatic tachycardia syndrome is associated with platelet storage pool deficiency

Mechanisms have been postulated to explain postural orthostatic tachycardia syndrome (POTS), however, the etiology of this often debilitating disorder remains unknown. We conducted a retrospective case-control study of 181 POTS patients who exhibited/reported bleeding symptoms for a specific platelet (PL) dysfunction disorder, delta granule storage pool deficiency (δ-SPD).Patients were included only if results of blood tests for δ-SPD were available. Electron microscopy was utilized to diagnose δ-SPD. An ELISA assay was used to determine serotonin (5HT) concentration in PLs and medical record review was employed to collect patients' clinical symptoms.The most common bleeding symptom was easy bruising (71%) but frequent nose bleeds, heavy menstrual bleeding, and a family history of bleeding were also commonly reported. Of the patients studied, 81% were diagnosed with δ-SPD. Our investigation of 5HT concentration extracted from PLs revealed significantly lower levels of 5HT in POTS patients when compared to that of control subjects. Our data suggest that patients with POTS have significant comorbidities including bleeding symptoms and/or family bleeding histories, and have diminished PL 5HT levels supporting the hypothesis that POTS is a low 5HT level disorder. While we describe a significant relationship with POTS and δ-SPD, this finding does not constitute an etiology for POTS.Our results establish an additional comorbidity frequently seen in POTS that could explain a number of disparate symptoms often affecting the severity of POTS.

Management of Postural Tachycardia Syndrome, Inappropriate Sinus Tachycardia and Vasovagal Syncope

Postural tachycardia syndrome (POTS), inappropriate sinus tachycardia (IST) and vasovagal syncope (VVS) are relatively common clinical syndromes that are seen by physicians in several disciplines. They are often not well recognised and are poorly understood by physicians, are associated with significant morbidity and cause significant frustration for both patients and their physicians. The 2015 Heart Rhythm Society Expert Consensus Statement on the Diagnosis and Treatment of Postural Tachycardia Syndrome, Inappropriate Sinus Tachycardia and Vasovagal Syncope provides physicians with an introduction to these disorders and initial recommendations on their investigation and treatment. Here we summarise the consensus statement to help physicians in the management of patients with these frequently distressing problems.

Postural Tachycardia Syndrome: Beyond Orthostatic Intolerance

Postural tachycardia syndrome (POTS) is a form of chronic orthostatic intolerance for which the hallmark physiological trait is an excessive increase in heart rate with assumption of upright posture. The orthostatic tachycardia occurs in the absence of orthostatic hypotension and is associated with a >6-month history of symptoms that are relieved by recumbence. The heart rate abnormality and orthostatic symptoms should not be caused by medications that impair autonomic regulation or by debilitating disorders that can cause tachycardia. POTS is a "final common pathway" for a number of overlapping pathophysiologies, including an autonomic neuropathy in the lower body, hypovolemia, elevated sympathetic tone, mast cell activation, deconditioning, and autoantibodies. Not only may patients be affected by more than one of these pathophysiologies but also the phenotype of POTS has similarities to a number of other disorders, e.g., chronic fatigue syndrome, Ehlers-Danlos syndrome, vasovagal syncope, and inappropriate sinus tachycardia. POTS can be treated with a combination of non-pharmacological approaches, a structured exercise training program, and often some pharmacological support.

Distinct neurohumoral biomarker profiles in children with hemodynamically defined orthostatic intolerance may predict treatment options

Studies of adults with orthostatic intolerance (OI) have revealed altered neurohumoral responses to orthostasis, which provide mechanistic insights into the dysregulation of blood pressure control. Similar studies in children with OI providing a thorough neurohumoral profile are lacking. The objective of the present study was to determine the cardiovascular and neurohumoral profile in adolescent subjects presenting with OI. Subjects at 10-18 yr of age were prospectively recruited if they exhibited two or more traditional OI symptoms and were referred for head-up tilt (HUT) testing. Circulating catecholamines, vasopressin, aldosterone, renin, and angiotensins were measured in the supine position and after 15 min of 70° tilt. Heart rate and blood pressure were continuously measured. Of the 48 patients, 30 patients had an abnormal tilt. Subjects with an abnormal tilt had lower systolic, diastolic, and mean arterial blood pressures during tilt, significantly higher levels of vasopressin during HUT, and relatively higher catecholamines and ANG II during HUT than subjects with a normal tilt. Distinct neurohumoral profiles were observed when OI subjects were placed into the following groups defined by the hemodynamic response: postural orthostatic tachycardia syndrome (POTS), orthostatic hypotension (OH), syncope, and POTS/syncope. Key characteristics included higher HUT-induced norepinephrine in POTS subjects, higher vasopressin in OH and syncope subjects, and higher supine and HUT aldosterone in OH subjects. In conclusion, children with OI and an abnormal response to tilt exhibit distinct neurohumoral profiles associated with the type of the hemodynamic response during orthostatic challenge. Elevated arginine vasopressin levels in syncope and OH groups are likely an exaggerated response to decreased blood flow not compensated by higher norepinephrine levels, as observed in POTS subjects. These different compensatory mechanisms support the role of measuring neurohumoral profiles toward the goal of selecting more focused and mechanistic-based treatment options for pediatric patients with OI.

Caloric restriction decreases orthostatic tolerance independently from 6° head-down bedrest

Astronauts consume fewer calories during spaceflight and return to earth with an increased risk of orthostatic intolerance. Whether a caloric deficiency modifies orthostatic responses is not understood. Thus, we determined the effects of a hypocaloric diet (25% caloric restriction) during 6° head down bedrest (an analog of spaceflight) on autonomic neural control during lower body negative pressure (LBNP). Nine healthy young men completed a randomized crossover bedrest study, consisting of four (2 weeks each) interventions (normocaloric bedrest, normocaloric ambulatory, hypocaloric bedrest, hypocaloric ambulatory), each separated by 5 months. Muscle sympathetic nerve activity (MSNA) was recorded at baseline following normocaloric and hypocaloric interventions. Heart rate (HR) and arterial pressure were recorded before, during, and after 3 consecutive stages (7 min each) of LBNP (-15, -30, -45 mmHg). Caloric and posture effects during LBNP were compared using two-way ANOVA with repeated measures. There was a strong trend toward reduced basal MSNA following caloric restriction alone (normcaloric vs. hypocaloric: 22±3 vs. 14±4 burst/min, p = 0.06). Compared to the normocaloric ambulatory, both bedrest and caloric restriction were associated with lower systolic blood pressure during LBNP (p<0.01); however, HR responses were directionally opposite (i.e., increase with bedrest, decrease with caloric restriction). Survival analysis revealed a significant reduction in orthostatic tolerance following caloric restriction (normocaloric finishers: 12/16; hypocaloric finishers: 6/16; χ2, p = 0.03). Caloric restriction modifies autonomic responses to LBNP, which may decrease orthostatic tolerance after spaceflight.

Differential Diagnosis of Vasovagal Syncope: Postural Orthostatic Tachycardia

Postural tachycardia syndrome (POTS) is a syndrome defined as an increase in heart rate (HR) of ≥30 bpm within 10 min of assuming an upright posture in the absence of orthostatic hypotension (decrease in blood pressure (BP) >20/10 mmHg). The prevalence of POTS has been estimated as 500,000–3,000,000 patients in the USA. Because it affects mainly women between 13 and 50 years of age, the impact on productivity at work and lifestyle can be devastating. Considerable clinical overlap exists between POTS and vasovagal syncope (VVS) with patients experiencing similar orthostatic symptoms related to insufficient adaptation to blood volume shifts during upright posture. Nevertheless, differences emerge during head up tilt table testing. In patients with POTS, during upright tilt, sympathetic tone increases, there is an early and sustained tachycardia, and patients complain of presyncope without frank syncope. In contrast, patients with VVS experience delayed symptoms and abrupt drops in BP and HR and are more likely to lose consciousness. Treatment of POTS includes exercise and medications directed at decreasing sympathetic tone or increasing blood volume.

Oscillatory cerebral blood flow is associated with impaired neurocognition and functional hyperemia in postural tachycardia syndrome during graded tilt

We hypothesize that upright cognitive impairment in patients with postural tachycardia syndrome (POTS) is caused by reduced cerebral blood flow (CBF). The CBF velocity (CBF(v)) measured by transcranial Doppler ultrasound decreased excessively during 70° tilt in a minority of patients with intermittent hyperpnea/hypocapnia. Incremental tilt showed no difference in mean CBF(v). But N-back memory tasking indicated progressive compromised memory, reduced functional hyperemia, and reduced neurovascular coupling. Orthostasis caused slow oscillations in CBF(v) linked to oscillations in arterial pressure in patients with POTS. We also hypothesize that oscillatory CBF(v) degrades neurovascular coupling. We performed 2-back testing when subjects were in supine position and during incremental tilts to 15°, 30°, 45°, and 60° in 11 patients with POTS and 9 controls. Oscillatory arterial pressure, oscillatory CBF(v), and neurovascular coupling were similar in supine position. The oscillatory arterial pressure increased by 31%, 45%, 67%, and 93% in patients with POTS during tilt and remained unchanged in the controls. Oscillatory CBF(v) increased by 61%, 82%, 161%, and 264% in patients with POTS during tilt and remained unchanged in the controls. Functional hyperemia decreased from 4.1% to 3.0%, 1.1%, 0.2%, and to 0.04% in patients with POTS, but it was unchanged at 4% in the controls. Percent correct N-back responses decreased from 78% to 33% in patients with POTS, whereas they remained at 89% in the controls. In patients with POTS, oscillatory CBF(v) was linearly correlated with functional hyperemia (r(2)=0.76). Increased oscillatory CBF is associated with reduced neurovascular coupling and diminished cognitive performance in patients with POTS.

Exercise in the postural orthostatic tachycardia syndrome

Patients with the Postural Orthostatic Tachycardia Syndrome (POTS) have orthostatic intolerance, as well as exercise intolerance. Peak oxygen uptake (VO2peak) is generally lower in these patients compared with healthy sedentary individuals, suggesting a lower physical fitness level. During acute exercise, POTS patients have an excessive increase in heart rate and reduced stroke volume for each level of absolute workload; however, when expressed at relative workload (%VO2peak), there is no difference in the heart rate response between patients and healthy individuals. The relationship between cardiac output and VO2 is similar between POTS patients and healthy individuals. Short-term (i.e., 3 months) exercise training increases cardiac size and mass, blood volume, and VO2peak in POTS patients. Exercise performance is improved after training. Specifically, stroke volume is greater and heart rate is lower at any given VO2 during exercise after training versus before training. Peak heart rate is the same but peak stroke volume and cardiac output are greater after training. Heart rate recovery from peak exercise is significantly faster after training, indicating an improvement in autonomic circulatory control. These results suggest that patients with POTS have no intrinsic abnormality of heart rate regulation during exercise. The tachycardia in POTS is due to a reduced stroke volume. Cardiac remodeling and blood volume expansion associated with exercise training increase physical fitness and improve exercise performance in these patients.

Phenylephrine alteration of cerebral blood flow during orthostasis: effect on n-back performance in chronic fatigue syndrome

Chronic fatigue syndrome (CFS) with orthostatic intolerance is characterized by neurocognitive deficits and impaired working memory, concentration, and information processing. In CFS, upright tilting [head-up tilt (HUT)] caused decreased cerebral blood flow velocity (CBFv) related to hyperventilation/hypocapnia and impaired cerebral autoregulation; increasing orthostatic stress resulted in decreased neurocognition. We loaded the baroreflex with phenylephrine to prevent hyperventilation and performed n-back neurocognition testing in 11 control subjects and 15 CFS patients. HUT caused a significant increase in heart rate (109.4 ± 3.9 vs. 77.2 ± 1.6 beats/min, P < 0.05) and respiratory rate (20.9 ± 1.7 vs. 14.2 ± 1.2 breaths/min, P < 0.05) and decrease in end-tidal CO2 (ETCO2; 42.8 ± 1.2 vs. 33.9 ± 1.1 Torr, P < 0.05) in CFS vs. control. HUT caused CBFv to decrease 8.7% in control subjects but fell 22.5% in CFS. In CFS, phenylephrine prevented the HUT-induced hyperventilation/hypocapnia and the significant drop in CBFv with HUT (-8.1% vs. -22.5% untreated). There was no difference in control subject n-back normalized response time (nRT) comparing supine to HUT (106.1 ± 6.9 vs. 97.6 ± 7.1 ms at n = 4), and no difference comparing control to CFS while supine (97.1 ± 7.1 vs 96.5 ± 3.9 ms at n = 4). However, HUT of CFS subjects caused a significant increase in nRT (148.0 ± 9.3 vs. 96.4 ± 6.0 ms at n = 4) compared with supine. Phenylephrine significantly reduced the HUT-induced increase in nRT in CFS to levels similar to supine (114.6 ± 7.1 vs. 114.6 ± 9.3 ms at n = 4). Compared with control subjects, CFS subjects are more sensitive both to orthostatic challenge and to baroreflex/chemoreflex-mediated interventions. Increasing blood pressure with phenylephrine can alter CBFv. In CFS subjects, mitigation of the HUT-induced CBFv decrease with phenylephrine has a beneficial effect on n-back outcome.

Pathophysiology of neurally mediated syncope: Role of cardiac output and total peripheral resistance

Syncope is a common clinical condition occurring even in otherwise healthy people without underlying cardiovascular disease. Neurally mediated syncope is by far the most common cause of syncope in individuals without any structural heart disease. Based on traditional wisdom, loss of sympathetic tone with relaxation of vascular smooth muscle is the key mechanism underlying the pathophysiology of syncope, especially in patients without an acute decrease in heart rate. However, this concept has recently been challenged. Some microneurographic studies indicate that sympathetic withdrawal may not always be a prerequisite even for the development of classic "vasodepressor" forms of syncope. Conversely, a decrease in cardiac output appears to be a determinant factor for syncope in most circumstances. This article reviews the relative contribution of cardiac output versus sympathetic vasoconstriction in neurally mediated syncope in otherwise healthy individuals. It is suggested that a moderate to severe fall in cardiac output with or without vasodilatation may contribute to syncope.

Altered oscillatory cerebral blood flow velocity and autoregulation in postural tachycardia syndrome

Decreased upright cerebral blood flow (CBF) with hyperpnea and hypocapnia is seen in a minority of patients with postural tachycardia syndrome (POTS). More often, CBF is not decreased despite upright neurocognitive dysfunction. This may result from time-dependent changes in CBF. We hypothesized that increased oscillations in CBF occurs in POTS (N = 12) compared to healthy controls (N = 9), and tested by measuring CBF velocity (CBFv) by transcranial Doppler ultrasound of the middle cerebral artery, mean arterial pressure (MAP) and related parameters, supine and during 70° upright tilt. Autospectra for mean CBFv and MAP, and transfer function analysis were obtained over the frequency range of 0.0078-0.4 Hz. Upright HR was increased in POTS (125 ± 8 vs. 86 ± 2 bpm), as was diastolic BP (74 ± 3 vs. 65 ± 3 mmHg) compared to control, while peripheral resistance, cardiac output, and mean CBFv increased similarly with tilt. Upright BP variability (BPV), low frequency (LF) power (0.04-0.13 Hz), and peak frequency of BPV were increased in POTS (24.3 ± 4.1, and 18.4 ± 4.1 mmHg(2)/Hz at 0.091 Hz vs. 11.8 ± 3.3, and 8.8 ± 2 mmHg(2)/Hz c at 0.071 Hz), as was upright overall CBFv variability, low frequency power and peak frequency of CBFv variability (29.3 ± 4.7, and 22.1 ± 2.7 [cm/s](2)/Hz at.092 Hz vs. 14.7 ± 2.6, and 6.7 ± 1.2 [cm/s](2)/Hz at 0.077Hz). Autospectra were sharply peaked in POTS. LF phase was decreased in POTS (-14 ± 4 vs. -25 ± 10 degrees) while upright. LF gain was increased (1.51 ± 0.09 vs. 0.86 ± 0.12 [cm/s]/ mmHg) while coherence was increased (0.96 ± 0.01 vs. 0.80 ± 0.04). Increased oscillatory BP in upright POTS patients is closely coupled to oscillatory CBFv over a narrow bandwidth corresponding to the Mayer wave frequency. Therefore combined increased oscillatory BP and increased LF gain markedly increases CBFv oscillations in a narrow bandwidth. This close coupling of CBF to MAP indicates impaired cerebral autoregulation that may underlie upright neurocognitive dysfunction in POTS.

Neuronal and hormonal perturbations in postural tachycardia syndrome

The Postural Tachycardia Syndrome (POTS) is the most common disorder seen in autonomic clinics. Cardinal hemodynamic feature of this chronic and debilitating disorder of orthostatic tolerance is an exaggerated orthostatic tachycardia (≥30 bpm increase in HR with standing) in the absence of orthostatic hypotension. There are multiple pathophysiological mechanisms that underlie POTS. Some patients with POTS have evidence of elevated sympathoneural tone. This hyperadrenergic state is likely a driver of the excessive orthostatic tachycardia. Another common pathophysiological mechanism in POTS is a hypovolemic state. Many POTS patients with a hypovolemic state have been found to have a perturbed renin-angiotensin-aldosterone profile. These include inappropriately low plasma renin activity and aldosterone levels with resultant inadequate renal sodium retention. Some POTS patients have also been found to have elevated plasma angiotensin II (Ang-II) levels, with some studies suggesting problems with decreased angiotensin converting enzyme 2 activity and decreased Ang-II degradation. An understanding of these pathophysiological mechanisms in POTS may lead to more rational treatment approaches that derive from these pathophysiological mechanisms.

Blunted cerebral blood flow velocity in response to a nitric oxide donor in postural tachycardia syndrome

Cognitive deficits are characteristic of postural tachycardia syndrome (POTS). Intact nitrergic nitric oxide (NO) is important to cerebral blood flow (CBF) regulation, neurovascular coupling, and cognitive efficacy. POTS patients often experience defective NO-mediated vasodilation caused by oxidative stress. We have previously shown dilation of the middle cerebral artery in response to a bolus administration of the NO donor sodium nitroprusside (SNP) in healthy volunteers. In the present study, we hypothesized a blunted middle cerebral artery response to SNP in POTS. We used combined transcranial Doppler-ultrasound to measure CBF velocity and near-infrared spectroscopy to measure cerebral hemoglobin oxygenation while subjects were in the supine position. The responses of 17 POTS patients were compared with 12 healthy control subjects (age: 14-28 yr). CBF velocity in POTS patients and control subjects were not different at baseline (75 ± 3 vs. 71 ± 2 cm/s, P = 0.31) and decreased to a lesser degree with SNP in POTS patients (to 71 ± 3 vs. 62 ± 2 cm/s, P = 0.02). Changes in total and oxygenated hemoglobin (8.83 ± 0.45 and 8.13 ± 0.48 μmol/kg tissue) were markedly reduced in POTS patients compared with control subjects (14.2 ± 1.4 and 13.6 ± 1.6 μmol/kg tissue), primarily due to increased venous efflux. The data indicate reduced cerebral oxygenation, blunting of cerebral arterial vasodilation, and heightened cerebral venodilation. We conclude, based on the present study outcomes, that decreased bioavailability of NO is apparent in the vascular beds, resulting in a downregulation of NO receptor sites, ultimately leading to blunted responses to exogenous NO.

Adolescent fatigue, POTS, and recovery: a guide for clinicians

Many teenagers who struggle with chronic fatigue have symptoms suggestive of autonomic dysfunction that may include lightheadedness, headaches, palpitations, nausea, and abdominal pain. Inadequate sleep habits and psychological conditions can contribute to fatigue, as can concurrent medical conditions. One type of autonomic dysfunction, postural orthostatic tachycardia syndrome, is increasingly being identified in adolescents with its constellation of fatigue, orthostatic intolerance, and excessive postural tachycardia (more than 40 beats/min). A family-based approach to care with support from a multidisciplinary team can diagnose, treat, educate, and encourage patients. Full recovery is possible with multi-faceted treatment. The daily treatment plan should consist of increased fluid and salt intake, aerobic exercise, and regular sleep and meal schedules; some medications can be helpful. Psychological support is critical and often includes biobehavioral strategies and cognitive-behavioral therapy to help with symptom management. More intensive recovery plans can be implemented when necessary.

Cardiac dysfunction and orthostatic intolerance in patients with myalgic encephalomyelitis and a small left ventricle

The etiology of chronic fatigue syndrome (CFS) is unknown. Myalgic encephalomyelitis (ME) has been recently postulated to be the cause of CFS. Orthostatic intolerance (OI) has been known as an important symptom in predicting quality of life in CFS patients. Cardiac function may be impaired in patients with ME. The presence or absence of OI was determined both symptomatically and by using a 10-min stand-up test in 40 ME patients. Left ventricular (LV) dimensions and function were determined echocardiographically in the ME patients compared to 40 control subjects. OI was noted in 35 (97%) of the 36 ME patients who could stand up quickly. The mean values for the cardiothoracic ratio, systemic systolic and diastolic pressures, LV end-diastolic diameter (EDD), LV end-systolic diameter, stroke volume index, cardiac index and LV mass index were all significantly smaller in the ME group than in the controls. Both a small LVEDD (<40 mm, 45 vs. 3%) and a low cardiac index (<2 l/ min/mm2, 53 vs. 8%) were significantly more common in the ME group than in the controls. Both heart rate and LV ejection fraction were similar between the groups. In conclusion, a small LV size with a low cardiac output was common in ME patients, in whom OI was extremely common. Cardiac dysfunction with a small heart appears to be related to the symptoms of ME.

Reduced cerebral blood flow with orthostasis precedes hypocapnic hyperpnea, sympathetic activation, and postural tachycardia syndrome

Hyperventilation and reduced cerebral blood flow velocity can occur in postural tachycardia syndrome (POTS). We studied orthostatically intolerant patients, with suspected POTS, with a chief complaint of upright dyspnea. On the basis of our observations of an immediate reduction of cerebral blood flow velocity with orthostasis, we hypothesize that the resulting ischemic hypoxia of the carotid body causes chemoreflex activation, hypocapnic hyperpnea, sympathetic activation, and increased heart rate and blood pressure in this subset of POTS. We compared 11 dyspneic POTS subjects with 10 healthy controls during a 70° head-up tilt. In POTS subjects during initial orthostasis before blood pressure recovery; central blood volume and mean arterial pressure were reduced (P<0.025), resulting in a significant (P<0.001) decrease in cerebral blood flow velocity, which temporally preceded (17±6 s; P<0.025) a progressive increase in minute ventilation and decrease in end tidal CO2 (P<0.05) when compared with controls. Sympathoexcitation, measured by muscle sympathetic nerve activity, was increased in POTS (P<0.01) and inversely proportional to end tidal CO2 and resulted in an increase in heart rate (P<0.001), total peripheral resistance (P<0.025), and a decrease in cardiac output (P<0.025). The decrease in cerebral blood flow velocity and mean arterial pressure during initial orthostasis was greater (P<0.025) in POTS. Our data suggest that exaggerated initial central hypovolemia during initial orthostatic hypotension in POTS results in reduced cerebral blood flow velocity and postural hypocapnic hyperpnea that perpetuates cerebral ischemia. We hypothesize that sustained hypocapnia and cerebral ischemia produce sympathoexcitation, tachycardia, and a statistically significant increase in blood pressure.