A study of the venomotor tone in a short intact venous segment of the forearm of man

Practical Aspects of Functional Magnetic Resonance Imaging in Children

Functional magnetic resonance imaging (fMRI) has become a broadly accepted presurgical mapping tool for pediatric populations with brain pathology. The aim of this article is to provide general guidelines on the pragmatic aspects of performing and processing fMRI, as well as interpreting its results across children of all age groups. Based on the author's accumulated experience of more than 20 years on this specific field, these guidelines consider many factors that include the particular physiology and anatomy of the child's brain, and how specific peculiarities may pose disadvantages or even certain advantages when performing fMRI procedures. The author carefully details the various challenges that the practitioner might face in dealing with limited volitional behavior and language comprehension of infants and small children and remedial strategies. The type and proper choice of task-based paradigms in keeping with the age and performance of the patient are discussed, as well as the appropriate selection and dosage of sedative agents and their inherent limitations. Recommendations about the scanner and settings for specific sequences are provided, as well as the required devices for appropriate stimulus delivery, response, and motion control. Practical aspects of fMRI postprocessing and quality control are discussed. Finally, given the relevance of resting-state-fMRI for use in noncooperative patients, a praxis-oriented guide to obtain, classify, and understand the spontaneous neural networks (utilizing independent component analysis) is also provided. The article concludes with a thorough discussion about the possible pitfalls at different stages of the fMRI process.

Role of the venous system in baroreceptor-mediated reflexes in man

Although baroreceptor stimulation produced by marked alterations in arterial pressure has been shown to produce reflex changes in venous tone in animals, the effects on venous tone in man of altering arterial pressure within the physiologic range have not been clear. In six subjects, venous tone did not change when mean arterial pressure was raised by 25-40 mm Hg, although heart rate fell reflexly by 40%. Venous tone remained constant in 10 subjects when arterial pressure was lowered. This contrasted to the sustained rise in forearm vascular resistance and the persistent tachycardia that occurred. However, 12 subjects continued to respond to these interventions by transient venoconstriction. To eliminate possible emotional influences on venous tone due to the experimental intervention, venous responses were studied before and during general anesthesia in five of these subjects. In contrast to the response before anesthesia, an equivalent fall in arterial pressure during anesthesia no longer evoked a venoconstrictor response. Venous reactivity and the baroreceptor reflex arc remained intact during anesthesia, since venous tone always rose after a deep inspiration, and tachycardia always accompanied the fall in arterial pressure. It is concluded that changes in arterial pressure in the physiologic range in man do not induce measurable reflex alterations in venous tone, and that the increases sometimes seen with decreases in arterial pressure appear to be due to extraneous psychic factors.

Diurnal variation in pulmonary artery pressure

Previous observations have suggested that pulmonary artery pressure rises during sleep, whereas systemic artery pressure falls. A system has been developed for careful and accurate recording of pulmonary arterial pressure, and applied it to two groups of subjects: patients with heart failure, and patients with chronic stable angina. The results have largely confirmed the nocturnal pressure rise in pulmonary arterial pressure. Detailed analysis strongly suggests that the same physiological mechanisms producing a fall in systemic pressure are responsible for the rise in pulmonary pressure. The precise mechanism remains to be elucidated.

Radionuclide assessment of peripheral intravascular capacity: a technique to measure intravascular volume changes in the capacitance circulation in man

Changes in the capacitance vasculature influence venous return and cardiac performance, so an understanding of the effects of pathophysiologic states on the human capacitance vasculature is necessary to understand integrated cardiovascular function in man. Techniques available to assess the capacitance vasculature in man, however, have limitations. We performed radionuclide imaging of the calf or forearm in 51 patients whose erythrocytes had been labeled in vivo with technetium-99m, basing our approach on the principle that counts from the radiolabeled intravascular space are proportional to blood volume. Two-minute or 15 second count acquisitions were obtained from the calf in 42 patients. Counts obtained at rest demonstrated little variation. With veno-occlusion at 15 and 30 mm Hg, counts increased 8 +/- 1% (+/- SEM) (p less than 0.001) and 28 +/- 2% (p less than 0.001), respectively. After 0.4 mg of sublingual nitroglycerin, counts increased 9 +/- 1% (p less than 0.001). With leg elevation, counts decreased 34 +/- 4% (p less than 0.001). Response patterns were similar with 2-minute and 15-second acquisitions. In nine patients who underwent forearm imaging (2-minute acquisitions), counts increased 14 +/- 2% (p less than 0.001) and 26 +/- 4% (p less than 0.001) at 15- and 30-mm Hg veno-occlusion and 15 +/- 3% (p less than 0.001) after nitroglycerin. Volume displacements, recorded simultaneously with a fluid-filled plethysmograph about the contralateral forearm, correlated linearly in all nine patients. Thus, gamma camera imaging of the radiolabeled peripheral intravascular space provides a quantitative and reliable assessment of peripheral vascular capacity in man. The technique could be used in conjunction with gated cardiac imaging in order to assess the interactions of peripheral vascular capacity and ventricular performance.

The role of the central nervous system in chronic congestive heart failure

Even though congestive heart failure is extremely common, the mechanisms responsible for the clinical manifestations remain a puzzle. The central and autonomic (sympathetic) nervous systems are responsible for a large part of the clinical manifestations. The role of the nervous system in CHF is discussed briefly. It is evident that there are many gaps in knowledge that remain concerning the role of the central, peripheral, and autonomic nervous systems in congestive heart failure. The peripheral vascular constriction, increase in venous tone and pressure, tachycardia, sweating, dermal pallor, and tension and anxiety in patients with CHF reflect generalized sympathetic nervous system activity and the influence of the central and autonomic nervous systems on the clinical syndrome of CHF.

Psychogenic and neurogenic effects on the intact forearm vein of man

By means of an elegant venous preparation the behavior of intact veins of man can be studied decisively. The superficial veins of man are extremely sensitive to the environment and the autonomic and central nervous systems. This report illustrates the response of an isolated venous segment of the forearm to various psychogenic and neurogenic stimuli, with change in tone of the smooth muscle of the venous segment being reflected by change in segmental venous pressure. It is shown that the superficial forearm veins can be conditioned and that they are extremely dynamic and certainly not passive conduits for returning blood to the heart.

Comparison of the reflex reactivity of skin and muscle veins in the human forearm

To determine the relative participation of skin and muscle capacitance beds of the forearm in venomotor reflexes, epinephrine iontophoresis was combined with forearm plethysmography so that the volume of muscle veins could be estimated simultaneously with the volume of cutaneous veins, at a constant venous pressure. With this technique not only are the cutaneous veins markedly constricted but they also are prevented from filling since skin blood flow is abolished. In 10 normal subjects, the venous volume in the elevated control forearm at a congesting pressure of 30 mm Hg (VV[30]) was 3.16 +/-0.30 SEM cc/100 cc, while in the iontophoresed arm it was 2.54 +/-0.31 cc/100 cc. Thus the forearm cutaneous VV[30] was 1.62 cc/100 cc. With a deep breath, ice to the forehead, and leg exercise, and cutaneous VV[30] decreased 19.8% (P < 0.01), 36.6% (P < 0.01), and 32.6% (P < 0.02), respectively, whereas the muscle VV[30] was not altered significantly. Similar results were observed using the isolated forearm technique and a deep muscle vein. The infusion of epinephrine intra-arterially did not decrease reflex venomotor reactivity until cutaneous blood flow was completely suppressed, indicating that the inability of the veins to react in the iontophoresed arm was not the result of epinephrine diffusion into the muscle bed. Thus, these results indicate that, in the forearm, only cutaneous veins participate in venomotor reflexes. Further, since the forearm is principally composed of skeletal muscle and the hand skin, an explanation is provided for the observation that veins of the forearm, studied as a whole, appear less reactive to stimuli than veins of the hand. An explanation also is provided for fainting which occurs during motionless standing despite intense venoconstriction, thereby emphasizing the importance of the skeletal muscle pump in the legs in preventing postural syncope.

Response of lymphatics of canine hind limb to sympathetic nerve stimulation

1. The changes in lymphatic pressure in a limb whose circulation was temporarily arrested with a pneumatic cuff have been studied.2. Stimulation of the lumbar sympathetic chain caused an increase in lymphatic pressure. It has been shown that this is a primary not a secondary phenomenon, due to an active lymphomotor mechanism.3. The increase of lymphatic tone is proportional to the rate of stimulation; peak values are reached between 5 and 9 impulses/sec.

Effect of Angiotensin II on the Intact Forearm Veins of Man

Angiotensin II was injected into an isolated venous segment in eighteen human subjects and administered intravenously into a systemic vein in five other subjects. Injections of angiotensin II into an intact isolated venous segment produced no constriction of the segment. Intravenous systemic infusion of angiotensin II produced a slight constriction of the isolated venous segment in three subjects (mean 44 mm H 2 O) and no constriction of the segment in two subjects. The constriction in the venous segment associated with the systemic intravenous administration of the drug could be interrupted by blocking the nerves supplying the venous segment. Thus, angiotensin II has no direct effect upon the forearm veins of man. The rise in segmental venous pressure following the intravenous infusion of angiotensin II appears to originate from reflexes located outside the venous system which influence tone of the superficial veins of the forearm of man.

Peripheral Venous and Arteriolar Responses to Infusions of Angiotensin in Normal and Hypertensive Subjects

The distensibility of the peripheral veins of the hypertensive subject is normal, that is, there is no evidence of peripheral venoconstriction, despite the excessive arteriolar constriction. The forearm veins of hypertensive patients respond to pooling of blood in the upright position to a lesser degree than do veins of normotensive individuals, indicating a lesser degree of smypathetic nervous activity in response to this stimulus. The peripheral veins of hypertensive subjects constrict normally in response to an infusion of norepinephrine, indicating that failure of venoconstriction in any circumstance cannot be attributed to failure of the smooth muscle of the veins.

Infusions of angiotensin produce constriction of the veins as well as the arterioles in normotensive individuals thus failing to mimic the naturally occurring state of essential hypertension. Infusions of angiotensin in hypertensive patients result in further arteriolar constriction and hypertension without concomitant venoconstriction. The results of these experiments suggest that peripheral venous responses of hypertensive subjects to angiotensin infusions may have been modified by previous exposure of these vessels to a material at least similar to angiotensin.