Spinal cord-evoked potentials and muscle responses evoked by transcranial magnetic stimulation in 10 awake human subjects. (1/807)

Transcranial magnetic stimulation (TCMS) causes leg muscle contractions, but the neural structures in the brain that are activated by TCMS and their relationship to these leg muscle responses are not clearly understood. To elucidate this, we concomitantly recorded leg muscle responses and thoracic spinal cord-evoked potentials (SCEPs) after TCMS for the first time in 10 awake, neurologically intact human subjects. In this report we provide evidence of direct and indirect activation of corticospinal neurons after TCMS. In three subjects, SCEP threshold (T) stimulus intensities recruited both the D wave (direct activation of corticospinal neurons) and the first I wave (I1, indirect activation of corticospinal neurons). In one subject, the D, I1, and I2 waves were recruited simultaneously, and in another subject, the I1 and I2 waves were recruited simultaneously. In the remaining five subjects, only the I1 wave was recruited first. More waves were recruited as the stimulus intensity increased. The presence of D and I waves in all subjects at low stimulus intensities verified that TCMS directly and indirectly activated corticospinal neurons supplying the lower extremities. Leg muscle responses were usually contingent on the SCEP containing at least four waves (D, I1, I2, and I3).  (+info)

Effect of obesity and erect/supine posture on lateral cephalometry: relationship to sleep-disordered breathing. (2/807)

Craniofacial and upper airway anatomy, obesity and posture may all play a role in compromising upper airway patency in patients with the sleep apnoea/hypopnoea syndrome. The aim of this study was to investigate the relationship between obesity, facial structure and severity of sleep-disordered breathing using lateral cephalometric measurements and to assess the effect of body posture on cephalometric measurements of upper airway calibre variables in obese and non-obese subjects. Lateral cephalometry was carried out in erect and supine postures in 73 awake male subjects randomly selected from patients referred for polysomnography who had a wide range of apnoea/hypopnoea frequencies (1-131 events x h sleep(-1)). Subjects were divided into non-obese (body mass index (BMI) < 30 kg x m(-2); n=42) and obese (BMI > or = 30 kg x m(-2); n=31) groups. Significant but weak correlations were found between apnoea/hypopnoea index (AHI) and measurements reflecting upper airway dimensions: uvular protrusion-posterior pharyngeal wall (r=-0.26, p<0.05) and hyoid-posterior pharyngeal wall (r=0.26, p<0.05). Multiple regression using both upper airway dimensions improved the correlation to AHI (r=0.34, p=0.01). Obese subjects had greater hyoid-posterior pharyngeal wall distances than non-obese subjects, both erect (42+/-5 versus 39+/-4 mm, respectively (mean+/-SD) p<0.01) and supine (43+/-5 versus 40+/-4 mm, p<0.05). Skeletal craniofacial structure was similar in obese and non-obese subjects. In conclusion, measurements reflecting upper airway size were correlated with the severity of sleep-disordered breathing. Differences in upper airway size measurements between obese and non-obese subjects were independent of bony craniofacial structure.  (+info)

Mechanism for the posture-specific plasma volume increase after a single intense exercise protocol. (3/807)

To test the hypothesis that exercise-induced hypervolemia is a posture-dependent process, we measured plasma volume, plasma albumin content, and renal function in seven healthy subjects for 22 h after single upright (Up) or supine (Sup) intense (85% peak oxygen consumption rate) exercise. This posture was maintained for 5 h after exercise. Plasma volume decreased during exercise but returned to control levels by 5 h of recovery in both postures. By 22 h of recovery, plasma volume increased 2.4 +/- 0.8 ml/kg in Up but decreased 2.1 +/- 0.8 ml/kg in Sup. The plasma volume expansion in Up was accompanied by an increase in plasma albumin content (0.11 +/- 0.04 g/kg; P < 0.05). Plasma albumin content was unchanged in Sup. Urine volume and sodium clearance were lower in Up than Sup (P < 0.05) by 5 h of recovery. These data suggest that increased plasma albumin content contributes to the acute phase of exercise-induced hypervolemia. More importantly, the mechanism by which exercise influences the distribution of albumin between extra- and intravascular stores after exercise is altered by posture and is unknown. We speculate that factors associated with postural changes (e.g., central venous pressure) modify the increase in plasma albumin content and the plasma volume expansion after exercise.  (+info)

Brain activation during maintenance of standing postures in humans. (4/807)

The regulatory mechanism of bipedal standing in humans remains to be elucidated. We investigated neural substrates for maintaining standing postures in humans using PET with our mobile gantry PET system. Normal volunteers were instructed to adopt several postures: supine with eyes open toward a target; standing with feet together and eyes open or eyes closed; and standing on one foot or with two feet in a tandem relationship with eyes open toward the target. Compared with the supine posture, standing with feet together activated the cerebellar anterior lobe and the right visual cortex (Brodmann area 18/19), and standing on one foot increased cerebral blood flow in the cerebellar anterior vermis and the posterior lobe lateral cortex ipsilateral to the weight-bearing side. Standing in tandem was accompanied by activation within the visual association cortex, the anterior and posterior vermis as well as within the midbrain. Standing with eyes closed activated the prefrontal cortex (Brodmann area 8/9). Our findings confirmed that the cerebellar vermis efferent system plays an important role in maintenance of standing posture and suggested that the visual association cortex may subserve regulating postural equilibrium while standing.  (+info)

Abnormal norepinephrine clearance and adrenergic receptor sensitivity in idiopathic orthostatic intolerance. (5/807)

BACKGROUND: Chronic orthostatic intolerance (OI) is characterized by symptoms of inadequate cerebral perfusion with standing, in the absence of significant orthostatic hypotension. A heart rate increase of >/=30 bpm is typical. Possible underlying pathophysiologies include hypovolemia, partial dysautonomia, or a primary hyperadrenergic state. We tested the hypothesis that patients with OI have functional abnormalities in autonomic neurons regulating cardiovascular responses. METHODS AND RESULTS: Thirteen patients with chronic OI and 10 control subjects underwent a battery of autonomic tests. Systemic norepinephrine (NE) kinetics were determined with the patients supine and standing before and after tyramine administration. In addition, baroreflex sensitivity, hemodynamic responses to bolus injections of adrenergic agonists, and intrinsic heart rate were determined. Resting supine NE spillover and clearance were similar in both groups. With standing, patients had a greater decrease in NE clearance than control subjects (55+/-5% versus 30+/-7%, P<0.02). After tyramine, NE spillover did not change significantly in patients but increased 50+/-10% in control subjects (P<0.001). The dose of isoproterenol required to increase heart rate 25 bpm was lower in patients than in control subjects (0.5+/-0.05 versus 1.0+/-0.1 microg, P<0.005), and the dose of phenylephrine required to increase systolic blood pressure 25 mm Hg was lower in patients than control subjects (105+/-11 versus 210+/-12 microg, P<0.001). Baroreflex sensitivity was lower in patients (12+/-1 versus 18+/-2 ms/mm Hg, P<0.02), but the intrinsic heart rate was similar in both groups. CONCLUSIONS: The decreased NE clearance with standing, resistance to the NE-releasing effect of tyramine, and increased sensitivity to adrenergic agonists demonstrate dramatically disordered sympathetic cardiovascular regulation in patients with chronic OI.  (+info)

Incidence of bradycardia during recovery from spinal anaesthesia: influence of patient position. (6/807)

We administered 0.5% plain bupivacaine 4 ml intrathecally (L2-3 or L3-4) in three groups of 20 patients, according to the position in which they were nursed in the post-anaesthesia care unit (PACU): supine horizontal, 30 degrees Trendelenburg or hammock position (trunk and legs 30 degrees elevated). Patients were observed until anaesthesia descended to less than S1. The incidence of severe bradycardia (heart rate < 50 beat min-1) in the PACU was significantly higher in patients in the Trendelenburg position (60%) than in the horizontal (20%, P < 0.01) or hammock (10%, P < 0.005) position. After 90 min, following admission to the PACU, only patients in the hammock position did not have severe bradycardia. In this late phase, the incidence of severe bradycardia in the Trendelenburg group was 35% (P < 0.005) and 10% in patients in the supine horizontal position. In four patients, severe bradycardia first occurred later than 90 min after admission to the PACU. The latest occurrence of severe bradycardia was recorded 320 min after admission to the PACU. We conclude that for recovery from spinal anaesthesia, the Trendelenburg position should not be used and the hammock position is preferred.  (+info)

Pulmonary perfusion is more uniform in the prone than in the supine position: scintigraphy in healthy humans. (7/807)

The main purpose of this study was to find out whether the dominant dorsal lung perfusion while supine changes to a dominant ventral lung perfusion while prone. Regional distribution of pulmonary blood flow was determined in 10 healthy volunteers. The subjects were studied in both prone and supine positions with and without lung distension caused by 10 cmH2O of continuous positive airway pressure (CPAP). Radiolabeled macroaggregates of albumin, rapidly trapped by pulmonary capillaries in proportion to blood flow, were injected intravenously. Tomographic gamma camera examinations (single-photon-emission computed tomography) were performed after injections in the different positions. All data acquisitions were made with the subject in the supine position. CPAP enhanced perfusion differences along the gravitational axis, which was more pronounced in the supine than prone position. Diaphragmatic sections of the lung had a more uniform pulmonary blood flow distribution in the prone than supine position during both normal and CPAP breathing. It was concluded that the dominant dorsal lung perfusion observed when the subjects were supine was not changed into a dominant ventral lung perfusion when the subjects were prone. Lung perfusion was more uniformly distributed in the prone compared with in the supine position, a difference that was more marked during total lung distension (CPAP) than during normal breathing.  (+info)

Human muscle performance and PCr hydrolysis with varied inspired oxygen fractions: a 31P-MRS study. (8/807)

The purpose of this study was to use 31P-magnetic resonance spectroscopy to examine the relationships among muscle PCr hydrolysis, intracellular H+ concentration accumulation, and muscle performance during incremental exercise during the inspiration of gas mixtures containing different fractions of inspired O2 (FIO2). We hypothesized that lower FIO2 would result in a greater disruption of intracellular homeostasis at submaximal workloads and thereby initiate an earlier onset of fatigue. Six subjects performed plantar flexion exercise on three separate occasions with the only variable altered for each exercise bout being the FIO2 (either 0.1, 0.21, or 1.00 O2 in balance N2). Work rate was increased (1-W increments starting at 0 W) every 2 min until exhaustion. Time to exhaustion (and thereby workload achieved) was significantly (P < 0.05) greater as FIO2 was increased. Muscle phosphocreatine (PCr) concentration, Pi concentration, and pH at exhaustion were not significantly different among the three FIO2 conditions. However, muscle PCr concentration and pH were significantly reduced at identical submaximal workloads (and thereby equivalent rates of respiration) above 4-5 W during the lowest FIO2 condition compared with the other two FIO2 conditions. These results demonstrate that exhaustion during all FIO2 occurred when a particular intracellular environment was achieved and suggest that during the lowest FIO2 condition, the greater PCr hydrolysis and intracellular acidosis at submaximal workloads may have contributed to the significantly earlier time to exhaustion.  (+info)