Motor and sensory deficits are well-known consequences of spinal cord injury (SCI). During the last decade, a significant number of experimental and clinical studies have focused on the investigation of autonomic dysfunction and cardiovascular control following SCI. Numerous clinical reports have suggested that unstable blood pressure control in individuals with SCI could be responsible for their increased cardiovascular mortality. The aim of this review is to outline the incidence and pathophysiological mechanisms underlying the orthostatic hypotension that commonly occurs following SCI. We describe the clinical abnormalities of blood pressure control following SCI, with particular emphasis upon orthostatic hypotension. Possible mechanisms underlying orthostatic hypotension in SCI, such as changes in sympathetic activity, altered baroreflex function, the lack of skeletal muscle pumping activity, cardiovascular deconditioning and altered salt and water balance will be discussed. Possible alterations in cerebral autoregulation following SCI, and the impact of these changes upon cerebral perfusion are also examined. Finally, the management of orthostatic hypotension will be considered.

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Orthostatic hypotension following spinal cord injury: understanding clinical pathophysiology

Orthostatic intolerance is common when astronauts return to Earth: after brief spaceflight, up to two-thirds are unable to remain standing for 10 min. Previous research suggests that susceptible individuals are unable to increase their systemic vascular resistance and plasma noradrenaline concentrations above pre-flight upright levels. In this study, we tested the hypothesis that adaptation to the microgravity of space impairs sympathetic neural responses to upright posture on Earth. We studied six astronauts approximately 72 and 23 days before and on landing day after the 16 day Neurolab space shuttle mission. We measured heart rate, arterial pressure and cardiac output, and calculated stroke volume and total peripheral resistance, during supine rest and 10 min of 60 deg upright tilt. Muscle sympathetic nerve activity was recorded in five subjects, as a direct measure of sympathetic nervous system responses. As in previous studies, mean (+/- S.E.M.) stroke volume was lower (46 +/- 5 vs. 76 +/- 3 ml, P = 0.017) and heart rate was higher (93 +/- 1 vs. 74 +/- 4 beats min(-1), P = 0.002) during tilt after spaceflight than before spaceflight. Total peripheral resistance during tilt post flight was higher in some, but not all astronauts (1674 +/- 256 vs. 1372 +/- 62 dynes s cm(-5), P = 0.32). No crew member exhibited orthostatic hypotension or presyncopal symptoms during the 10 min of postflight tilting. Muscle sympathetic nerve activity was higher post flight in all subjects, in supine (27 +/- 4 vs. 17 +/- 2 bursts min(-1), P = 0.04) and tilted (46 +/- 4 vs. 38 +/- 3 bursts min(-1), P = 0.01) positions. A strong (r(2) = 0.91-1.00) linear correlation between left ventricular stroke volume and muscle sympathetic nerve activity suggested that sympathetic responses were appropriate for the haemodynamic challenge of upright tilt and were unaffected by spaceflight. We conclude that after 16 days of spaceflight, muscle sympathetic nerve responses to upright tilt are normal.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.2001.012575

Human muscle sympathetic neural and haemodynamic responses to tilt following spaceflight.

Background— Orthostatic intolerance is characterized by postural tachycardia syndrome (POTS) with exaggerated tachycardia, orthostatic symptoms, and “pooling” (which comprises acrocyanosis and dependent edema when upright). My colleagues and I tested the hypothesis that pooling results from increased venous compliance in POTS patients. Methods and Results— Fifteen patients aged 13 to 19 years were compared with 11 healthy, age-matched controls. The POTS group was divided into patients with high venous pressure (Pv>20 mm Hg) and normal Pv on the basis of resting supine Pv obtained in previous work. Subjects were studied using strain gauge plethysmography to measure blood flow, Pv, and the venous compliance volume-pressure relation while supine and during incremental head-up tilt testing at −10°, 0°, 20°, and 35°. Volume-pressure relations of controls and POTS patients with normal Pv and high Pv were not different and were unchanged by orthostasis. Supine leg peripheral resistance was greater than control r...

Pooling in Chronic Orthostatic Intolerance Arterial Vasoconstrictive but not Venous Compliance Defects

Syncope or near-syncope is a not uncommon effect of gravitational or other stresses and it occurs when cerebral blood flow falls to below about half the normal value. It is not necessarily abnormal, and individuals who are usually asymptomatic show the same reaction if a stress is sufficiently great to result in hypotension. Blood pressure is regulated mainly by baroreceptor reflexes by their control of vascular resistance and heart rate. The ability to vasoconstrict powerfully is important in resisting syncope; heart rate responses are of much less physiological significance. The intriguing unanswered question is what suddenly changes vasoconstriction and tachycardia to vasodilatation and bradycardia. It is now known not to be due to stimulation of cardiac receptors and some cerebral signal is more probable. People are more likely to faint when upright, motionless, warm, following meals, dehydrated or emotionally stressed, and these factors may be involved in some reflex syncopes including micturition and defaecation syncopes. Plasma volume is of considerable importance and increasing this by interventions such as salt loading, exercise training, and even sleeping with the bed head raised can often be of clinical benefit.

Pathophysiology of syncope.

Background— We tested the hypothesis that individual variability in orthostatic tolerance is dependent on the degree of neural and vasomotor reserve available for vasoconstriction. Methods and Results— Muscle sympathetic nerve activity (MSNA) and hemodynamics were measured in 12 healthy young volunteers during 60° head-up tilt (HUT), followed by a cold pressor test (CPT) in HUT. Orthostatic tolerance was determined by progressive lower-body negative pressure (LBNP) to presyncope. The same protocols were performed randomly in normovolemic and hypovolemic conditions. We found that mean arterial pressure increased and stroke volume decreased, whereas heart rate (HR), MSNA, and total peripheral resistance (TPR) increased during HUT (all P<0.01). Application of the CPT in HUT did not increase HR or decrease stroke volume further but elevated mean arterial pressure (P<0.01) and increased MSNA and TPR in some subjects. There was a positive correlation between the time to presyncope from −50 mm Hg LBNP (equivalen...

Vasoconstrictor reserve and sympathetic neural control of orthostasis

Foreword Introduction 1. Accentuation in Theory 2. Accentuation in Practice 3. The Notation of Accents and Dynamics 4. Articulation 5. Articulation and Expression 6. The Notation of Articulation and Phrasing 7. String Bowing 8. Tempo 9. Alla Breve 10. Tempo Terms 11. Tempo Modification 12. Embellishment, Ornamentation, and Improvisation 13. Appoggiaturas, Trills, Turns, and Related Ornaments 14. Vibrato 15. Portamento 16. The fermata Recitative Arpeggiation The Variable Dot and Other Aspects of Rhythmic Flexibility Heavy and Light Performance Index

Classical and Romantic Performing Practice 1750-1900

The aim of this study was to compare the changes in forearm vascular resistance that occurred during orthostatic stress in asymptomatic volunteer subjects with those in patients with posturally related syncope. The authors hoped firstly that it would indicate the importance of vasoconstriction in the maintenance of blood pressure, and secondly that it might have diagnostic value if there were differences between symptomatic patients and asymptomatic volunteers. Twelve volunteers and 67 patients with unexplained syncope were classified as early or late fainters, based on their endurance of a test of combined head-up tilting and lower-body suction. Responses of vascular resistance were assessed from the ratio of arterial blood pressure (Finapres) to brachial artery blood velocity (Doppler). Changes in vascular resistance were greater in volunteers at all stages of the procedure than in patients. There was, however, no significant difference between the responses of early and late-fainting volunteers. These results demonstrate the importance of vasoconstriction in the resistance to posturally related syncope, and they indicate that assessments of responses of vascular resistance may improve the accuracy of the diagnosis.

Forearm vascular responses during orthostatic stress in control subjects and patients with posturally related syncope.

Orthostatic stress causes, in addition to venous pooling, a loss of plasma fluid from capillaries to the dependent tissues. The rate of this loss may be one of the factors determining orthostatic tolerance. In this study we assessed the use of a multichannel impedance plethysmograph for determining changes in volume in the calf, thigh, and abdominal segments, in asymptomatic volunteers and in patients shown to have poor tolerance to orthostatic stress. Impedance plethysmography showed, for leg segments, that following head-up tilt there was an initial rapid change in volume followed after 2 to 4 minutes by an almost linear change. Results from the abdominal segment were more variable. The rate of change of leg (thigh + calf) volume was significantly correlated with the estimated loss of plasma volume derived from the changes in the concentration of plasma protein, using Evans Blue dye as the marker. Comparison of results of leg filtration rates between patients and volunteers indicated that some of the patients had abnormally high filtration rates and suggests that impedance plethysmography may have a role in assessing the possible reasons for orthostatic intolerance.

Assessment of capillary fluid shifts during orthostatic stress in normal subjects and subjects with orthostatic intolerance.

The highly complimentary reviews of Spohr's performances in Vienna between 1812 and 1815 make no reference to anything unfamiliar or controversial in his bowing style, and his performances of the chamber music of the three great Classical masters were greatly admired. It is clear that attitudes towards bowing styles changed radically during the nineteenth century. Whereas vibrato was much more sparingly used in the nineteenth century than it is today, portamento was freely employed as an expressive device. The only important respect in which the nineteenth-century violin differed from the modern one was its stringing. The connection between this form of accent and vibrato is a particularly strong one, but there can be no doubt that vibrato and accentuation of all kinds were closely linked in nineteenth-century violin playing. By the time the Joachim–Moser Violinschule was published the current was already running strongly against the concept of sound of which they were advocates.

Bowing Styles, Vibrato and Portamento in Nineteenth-Century Violin Playing

A good performance of Beethoven's Sonata op.17 (18oo) played on a natural (hand-stopped) horn and a piano of the period provides a quite different aural experience from one played on modern instruments. To hear Beethoven's symphonies played with the same degree of authenticity (i.e. on valveless brass instruments, on woodwind instruments without Theobald Boehm's bores and keywork, and on string instruments with gut strings played with pre-Tourte bows) would be no less revealing in sound quality, but the practical difficulties of assembling and equipping such an orchestra are almost insuperable.'

Clive Brown

Papers

Classical and Romantic Performing Practice 1750-1900

Forearm vascular responses during orthostatic stress in control subjects and patients with posturally related syncope.

Assessment of capillary fluid shifts during orthostatic stress in normal subjects and subjects with orthostatic intolerance.

Bowing Styles, Vibrato and Portamento in Nineteenth-Century Violin Playing

Historical performance, metronome marks and tempo in Beethoven's symphonies