Robert L. Van Citters

Cardiovascular Performance of Alaska Sled Dogs during Exercise

Radiotelemetry was used to study regional blood flow distribution in Alaska sled dogs during cross-country runs. Doppler ultrasonic flowmeter transducers were chronically implanted on the coronary, renal, and mesenteric arteries, terminal abdominal aorta, and ascending aorta or pulmonary artery, and a miniature blood pressure gauge was installed in the aorta or carotid artery. Flow and pressure data telemetered from dogs running on the trail were received and recorded remotely. The heart rate, 40 to 60/min in sleeping dogs, increased to 80 to 100/min when the dogs were ambulatory and to 100 to 150/min when the dogs were excited before a race. Heart rate accelerated to 300/min at the start of exercise and commonly remained at that level throughout prolonged runs. Aortic blood pressure averaged 130/90 mm Hg at rest, but the systolic pressure often exceeded 300 mm Hg when the dogs were running. A transient drop in mean pressure occurred at the onset of running, but mean pressure during sustained exercise was practically identical to that at rest. Flow in the terminal aorta increased 9 to 12 times and coronary flow 5 to 6 times, but mesenteric and renal flows were unchanged during violent, prolonged exercise. These findings contrast with diminished visceral flows recorded in exercising humans and suggest that compensatory redistribution of flow is not a significant reserve mechanism in these animals during exercise.

Effects of Carotid Sinus Nerve Stimulation on the Coronary Circulation of the Conscious Dog

Control of the coronary circulation by the carotid sinus was studied in intact, unanesthetized dogs instrumented with Doppler ultrasonic flow probes on the left circumflex coronary artery, miniature pressure gauges in the aorta, and stimulating electrodes on the carotid sinus nerves. A radiofrequency pacemaker was used to stimulate the nerves in dogs at rest, during sleep, exercise, and after autonomic blockade. Thirty-second periods of stimulation in the resting conscious dog resulted in an average decrease in aortic pressure of 28%, an average decrease in mean coronary flow of 7%, while heart rate decreased by 13% at the beginning of stimulation and then returned to control levels. Mean and late diastolic coronary resistances decreased by an average of 22% from control. Similar results occurred with carotid sinus nerve stimulation during sleep and during treadmill exercise. Combined beta-receptor blockade with propranolol and atropine prevented the changes in heart rate with carotid sinus nerve stimulation but not the decrease in arterial pressure or the coronary dilatation. After alpha-receptor blockade with phenoxybenzamine or sympathetic blockade with guanethidine, coronary dilatation was not observed with carotid sinus nerve stimulation. Thus sympathetic constrictor tone is present in the resting conscious dog and the coronary dilatation observed with electrical stimulation of the carotid sinus nerves is due to a reduction in resting sympathetic constrictor tone.

Balance Between Right and Left Ventricular Output

The right ventricular ejection pattern is characterized by early onset, gradual rise to peak flow velocities in midsystole and gradual return to baseline. Left ventricular ejection begins very shortly after right ventricular systole, abruptly reaches peak flow, and diminishes during the remainder of the systolic interval, terminating with a brief, sharp, retrograde surge as the aortic valves close. The duration of ejection is shorter in the left ventricle than in the right. In healthy dogs, respiratory activity produces very slight fluctuations in right and left ventricular output that are almost in phase. In dogs with hydrothorax and pulmonary atelectasis, the right and left ventricular outputs fluctuate in greatly exaggerated fashion with each forced respiratory effort. Under such conditions, the changes in right and left ventricular stroke volume are nearly 180 Degrees out of phase. Pulmonary outflow resistance is increased and the right ventricular ejection pattern closely resembles those generally characteristic of the left ventricle. Assume that the erect posture causes cardio-acceleration and large changes in right and left ventricular stroke volume. which are essentially synchronous. Treadmill exercise with sudden and unexpected onset produces changes in the outputs of the right and left ventricles without obvious lead or lag of one ventricle over the other. These experiments do not support the concept that changes in “venous return” are dominant mechanisms inducing alterations in cardiac output.

Architecture of Small Arteries during Vasoconstriction

The cytoarchitectural changes which take place in the walls of small arteries (about 1 mm. O.D.) during vasoconstriction and vasodilation have been studied. Vessels were fixed while in their functional state by immersion in liquid isopentane at −170 C. and prepared for microscopic examination by freeze substitution. The walls of control vessels were thin in relation to the diameter of lumina (WT:L 1:30), indicating that they are more distended than they appear in routinely fixed sections. Vessels dilated with ACH had even thinner walls (WT:L 1:40). Vasoconstriction, induced by local application of epinephrine, increased the wall thickness and reduced the size of the lumen (WT:L 1:2) but in no case was the lumen obliterated in arteries of this size. Although an increase in the WT:L ratio is commonly employed as evidence of vascular hypertrophy, normal vessels which are fixed and sectioned while in a functional state of vasoconstriction may exhibit similar gross characteristics. Constricted vessels are characterized by progressive deformation of the internal elastic lamina, crowding of endothelial cells, and distortion of smooth muscle cells and their nuclei, particularly in the region immediately adjacent to the lumen. During intense vasoconstriction, the wall tension appears to be supported only by the outer layers of the vessel.

Blood flow and pressure in the giraffe carotid artery

1. 1. Carotid artery blood pressure and blood flow were measured and telemetered from wild giraffes ranging freely on the African plains. 2. 2. The blood pressure ranged between 260/160 mm Hg when the animal was lying flat, and 120/75 mm Hg when it was standing upright; dp/dt at the onset of systole was 1500 mm/sec. 3. 3. Peak systolic blood velocity measured at the same site was 60 cm/sec; during diastole velocity remained above 40 cm/sec. Calculated blood flow in the carotid artery ranged between 50 cm3/sec in the prone animal, and 35 cm3/sec in the standing. 4. 4. XY plots of flow and pressure produced open clockwise loops which varied in shape with activity and posture; such phase differences probably reflect mechanical properties peculiar to the giraffes arterial system.

Changes in Peripheral Blood Flow Distribution in Healthy Dogs

The instantaneous velocity and the integrated flow through a splanchnic artery (superior mesenteric or hepatogastric), the renal artery, and the terminal abdominal aorta were recorded continuously and simultaneously, as were the mean aortic pressure and heart rate, during a wide variety of spontaneous cardiovascular adjustments of healthy active dogs. These responses were compared with the effects of infusing vasoactive substances, such as isoproterenol, l-epinephrine, norepinephrine, acetyichol lie, and Pitressiii, and with P to stimulation of selected sites in the base of the brain. During sponta neous activity, the flow through the splanch nic and renal arteries usually varied in a direction opposite to that of changes in flow to the hindciuarters. The changes in mesen teric or renal flow were relatively slight, even during strenuous exercise that produced a substantial increase in flow to the hindciuar ters. Vasoactive substances tended to produce changes in the same direction in all three beds. Changes in flow distribution and in wave forms of the instantaneous flow patterns produced by vasoactive substances did not resemble those produced by spontaneous ad justment. 1-lowever, stimulation of selected diencephalic sites (e.g., 112 fields of Forel) produced changes in both flow distribution and flow velocity patteras that closely simu lated those observed during exercise in the same animals. Stimulation in the same re gions was previously reported to produce changes in left ventricular performance also closely resembling those noted during tread mill exercise.

A biopsychosocial model of medical student distress

Medical student distress was examined in two consecutive first-year classes (N=312) in September, before they interacted with the school regimen, and again in May before exams. Anxiety means were one SD above the normative mean for nonpatients at both times. The number of students reporting a significant level of depression doubled from September (N=36) to May (N=78). The correlation of distress in September and May was .40, indicating that for many students distress was enduring. A biopsychosocial model of initial distress explained more variance (36%) in the cross-validation sample than did any one variable alone. Distressed students had higher Type A scores. Also, anger held in was a risk factor for distress in students with a family history of cardiovascular disease (CVD). Students who hold anger in may experience prolonged stress which, coupled with a family history of CVD, could make them psychobiologically vulnerable to distress.

Coronary Disease in Spawning Steelhead Trout Salmo gairdnerii

Coronary degeneration was absent in young trout taken in fresh water and rare in immature fish at sea, but the incidence and severity were sharply greater in migrating fish and almost uniformly present in spawning fish. Several fish taken after they had reentered salt water after spawning had no lesions; lesions in fish taken during their second spawning migration were not cumulative. These facts suggest that the process is reversible.

Blood pressure responses of wild giraffes studied by radio telemetry.

Blood pressure was telemetered from transducers chronically implanted in the carotid arteries of two adult, wild, male giraffes captured and released near Kiboko, Kenya. Cerebral perfusion pressure ranged from 280/180 mm-Hg while the animal was lying with its head on the ground to 125/75 mm-Hg when it was standing erect; it varied between these levels during spontaneous activity such as walking, grazing, and running.

Occlusion of Lumina in Small Arterioles During Vasoconstriction

The microscopic appearance of small arterioles during epinephrine-induced vasoconstriction was studied in the dog mesentery. The tissues were quick-frozen in situ with liquid isopentane, freeze-substituted, and sectioned serially. Obliteration of the lumina of small arterioles during vasoconstriction was demonstrated in both random and serial sections. Endothelial cells, which were ordinarily flattened against the internal elastic membrane, were compressed and deformed during vasoconstriction, so that they protruded into the lumen and often coalesced in the center of the vessel to form a mechanical obstruction to blood flow. Vascular obliteration occurred commonly in very small arterioles, but was observed with diminishing frequency in progressively larger arterioles, and was not identified in vessels large enough to accommodate more than 10 endothelial cells in cross sections. In an ideal cylinder, tension is greatest at the innermost layer, and the gradient of tension across the wall falls off in a nonlinear fashion. These conditions probably do not hold in a functioning arteriole.