We tested the hypothesis that differences in sympathetic reflex responses to head-up tilt (HUT) between males (n = 9) and females (n = 8) were associated with decrements in postural vasomotor responses in women. Muscle sympathetic nerve activity (MSNA; microneurography), heart rate, stroke volume (SV; Doppler), and blood pressure (Finapres) were measured during a progressive HUT protocol (5 min at each of supine, 20 degrees, 40 degrees, and 60 degrees ). MSNA and hemodynamic responses were also measured during the cold pressor test (CPT) to examine nonbaroreflex neurovascular control. SV was normalized to body surface area (SV(i)) to calculate the index of cardiac output (Q(i)), and total peripheral resistance (TPR). During HUT, heart rate increased more in females versus males (P < 0.001) and SV(i) and Q(i) decreased similarly in both groups. Mean arterial pressure (MAP) increased to a lesser extent in females versus males in the HUT (P < 0.01) but increases in TPR during HUT were similar. MSNA burst frequency was lower in females versus males in supine (P < 0.03) but increased similarly during HUT. Average amplitude/burst increased in 60 degrees HUT for males but not females. Both males and females demonstrated an increase in MAP as well as MSNA burst frequency, mean burst amplitude, and total MSNA during the CPT. However, compared with females, males demonstrated a greater neural response (DeltaTotal MSNA) due to a larger increase in mean burst amplitude (P < 0.05). Therefore, these data point to gender-specific autonomic responses to cardiovascular stress. The different MSNA response to postural stress between genders may contribute importantly to decrements in blood pressure control during HUT in females.

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Background-Blood flow limitation to exercising muscles engages the muscle reflex during exercise, evoking an increase in heart rate (HR), blood pressure (BP), and muscle sympathetic nerve activity (MSNA). Methods and Results-In the current study, we examined forearm flow and autonomic responses to ischemic handgrip in young and older subjects. We studied 6 younger subjects (mean age 23.5Ϯ2.2 years) and 7 older subjects (mean age 65.0Ϯ2.4 years). Subjects performed rhythmic handgrip (thirty 1-sec contractions/min) at 30% maximal voluntary contraction during six 1-minute stages: freely perfused exercise (E1) and exercise with forearm pressure of ϩ10, ϩ20, ϩ30, ϩ40, and ϩ50 mm Hg (E2 through E6). We measured HR, BP, MSNA, forearm flow velocity, forearm venous oxygen saturation, H ϩ , and lactate. Compared with E1, ischemic exercise (E2 through E6) increased HR, BP, and MSNA, reduced forearm velocity, lowered venous oxygen saturation, and raised venous lactate and H ϩ . Compared with the younger subjects, the older subjects had attenuated BP at E6, attenuated MSNA indices (%⌬bursts, bursts/100 heart beats and signal averaged MSNA), attenuated H ϩ at E6, a trend toward higher levels of oxygen saturation, and similar forearm velocity and HR responses. Key Words: aging Ⅲ exercise Ⅲ reflex Ⅲ blood flow D uring exercise, the sympathetic nervous system is activated. This helps redistribute blood flow to active muscle and aids in preventing blood pressure (BP) from falling. 1 Two neural systems contribute to sympathetic activation: central command, 2 a feed-forward process, and a muscle reflex termed the exercise pressor reflex. 3 The muscle reflex is engaged when mechanically or metabolically sensitive thin fiber afferents within contracting muscle increase their discharge. 4 During forearm exercise, the muscle reflex is engaged when the muscle fatigues and/or when a mismatch occurs between blood supply and metabolic demand. 5 In the present study, we examined the effects of aging on the exercise pressor reflex in humans. Despite the fact that this reflex is an important determinant of exercise flow regulation, little is known about the effects of aging on this reflex. The reflex is evoked by a muscle work/blood flow mismatch. Therefore, to engage the reflex, a paradigm was employed in which the level of work was kept constant as external impedance to muscle flow was progressively increased. We examined whether age affects the BP response to reflex engagement and if sympathetic nerve responses to reflex engagement is different in young and older subjects. The results of these studies support the concept that the muscle reflex becomes attenuated with age. Conclusions-Aging Methods SubjectsSix young (4 males, 2 females; mean age 23.5 years; mean body mass index 23.5) and 7 older subjects (4 males, 3 females; mean age 65.0; body mass index 26.2) were studied. All were normotensive non-smokers on no medications. Each signed an Institutional Review Board-approved consent. Forearm PressureSubjects performed handgrip in a sealed...

Atrial fibrillation is the most common arrhythmia worldwide with increasing frequency noted with age. Hyperthyroidism is a well-known cause of atrial fibrillation with a 16%–60% prevalence of atrial fibrillation in patients with known hyperthyroidism Ross et al. (2016). While hyperthyroidism as a causative factor of atrial fibrillation is well established, this literature review aims to answer several questions on this topic including:1. The relationship of atrial fibrillation to hyperthyroidism2. Atrial fibrillation as a predictor of hyperthyroidism3. The pathophysiology of thyrotoxic atrial fibrillation4. Subclinical hyperthyroidism and the relationship with atrial fibrillation5. Cardioversion and Catheter ablation of hyperthyroid patients with atrial fibrillation6. Thrombotic risk of hyperthyroid patients with atrial fibrillation7. Management of Thyrotoxic Atrial fibrillation8. Pharmacological rhythm control in patients with hyperthyroidism and atrial fibrillation9. Treatment of Hyperthyroidism to prevent atrial fibrillation10. Clinical Implications of Hyperthyroidism and Atrial Fibrillation