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.

The role of skin and muscle resistance vessels in reflexes mediated by the baroreceptor system.

The role of skin and muscle vascular beds in baroreceptor-mediated alterations of peripheral vascular resistance was evaluated in six normal subjects in whom the skin circulation in one forearm was temporarily suppressed by epinephrine iontophoresis. Baroreceptor activity was enhanced by application of negative pressure to the neck (neck suction) and inhibited by application of lower body negative pressure. Forearm blood flow was measured simultaneously in both arms with strain gauge plethysmographs. Since blood flow in the treated arm consisted entirely of muscle flow, skin flow was calculated from the difference between total forearm flow in the intact arm and muscle flow in the treated arm. Vascular resistances were calculated as the ratio of mean arterial pressure to the blood flow of each vascular bed. During neck suction, mean arterial pressure decreased from an average of 89 to 75 mm of Hg (P < 0.005), heart rate decreased from an average of 60 to 55 beats/min (P < 0.005), and total skin and muscle flows remained essentially unchanged. Cutaneous vascular resistance decreased from an average of 75 to 49 mm of Hg/ml per 100 g per min (P < 0.05), muscle vascular resistance from 68 to 51 (P < 0.005), and total forearm vascular resistance from 36 to 24 (P < 0.025). During lower body negative pressure, heart rate increased from an average of 59 to 69 beats/min (P < 0.005), mean arterial pressure did not change significantly, and significant decreases occurred in forearm blood flow from 5.4 to 2.7 ml/100 g per min, in skin blood flow from 3.1 to 1.4, and in muscle blood flow from 2.3 to 1.3. Cutaneous vascular resistance increased from an average of 47 to 110 mm of Hg/ml per 100 g per min (P < 0.05), muscle vascular resistance from 43 to 72 (P < 0.005), and total forearm vascular resistance from 20 to 38 (P < 0.001). These results demonstrate that both the skin and muscle resistance vessels participate in reflex changes initiated by alterations in baroreceptor activity.

Impairment of autonomically mediated heart rate control in patients with cardiac dysfunction.

Patients with cardiac disorders have defective parasympathetic control of heart rate. To evaluate the possibility of similar changes in sympathetic control of heart rate, we compared reflex chronotropic responses to 80 degree upright tilt and nitroglycerin-induced hypotension in 31 cardiac patients and 7 normal individuals before and after partial parasympathetic blockade with atropine. Tilting revealed an attenuation of the normal heart rate increase in patients; the magnitude of this defect was greatest in patients with more severe symptoms (class III) and evidence of left ventricular dysfunction (the heart rate increase averaged 25 plus or minus 3 beats/min in normal subjects, 12 plus or minus 2 beats/min in class I-II patients, and 7 plus or minus 1 beats/min in class III patients). Class III symptoms due to mechanical causes (mitral stenosis), however, were not associated with this defect. A marked reduction in heart rate rise with hypotension was seen only in those class III patients without mitral stenosis (0.4 plus or minus 0.1 beats min-minus 1 mm Hg-minus 1 vs. 3.0 plus or minus 0.5 beats min-minus 1 mm Hg-minus 1 in normal subjects). This abnormality also persisted after atropine administration, thus confirming a defect in the sympathetic as well as the parasympathetic component of baroreceptor-mediated reflex heart rate control in patients with cardiac dysfunction. Infusions of isoproterenol produced equivalent rises in heart rate in patients and normal individuals, excluding a reduction in beta-receptor responsiveness as a cause of impaired sympathetic influence. Norepinephrine depletion, however, is a well-recognized concomitant of cardiac failure. It is possible that the reduction in sympathetically mediated heart rate responses results in part from depletion of the sympathetic neurotransmitter.