Auscultatory indirect measurement of blood pressure in dogs

An indirect method of measuring blood pressure (cuff plus stethoscope) was evaluated in 70 dogs weighing 15 to 30 kg (17.5 +/- 8.8 kg; mean +/- standard deviation). A cuff 12 cm wide was used. The measurements were most audible with the cuff on the upper foreleg of the dog and with the stethoscope placed in the medial epicondylar region just distal to the cuff. The cuff was inflated to greater than systolic pressure and allowed to deflate slowly. In 70 lightly sedated dogs, systolic blood pressures averaged 145 +/- 25 mmHg (mean +/- standard deviation) and diastolic blood pressures averaged 84 +/- 14 mmHg. Indirect measurements were compared to direct measurements (femoral arterial catheter). Systolic pressures obtained by this direct method averaged 138 +/- 29 mmHg (mean +/- standard deviation) and diastolic pressures averaged 84 +/- 17 mmHg. The correlation coefficient for systolic pressure was 0.96 and for diastolic pressure 0.97.     

The maximum oxygen consumption and aerobic scope of birds and mammals: getting to the heart of the matter

Resting or basal metabolic rates, compared across a wide range of organisms, scale with respect to body mass as approximately the 0.75 power. This relationship has recently been linked to the fractal geometry of the appropriate transport system or, in the case of birds and mammals, the blood vascular system. However, the structural features of the blood vascular system should more closely reflect maximal aerobic metabolic rates rather than submaximal function. Thus, the maximal aerobic metabolic rates of birds and mammals should also scale as approximately the 0.75 power. A review of the literature on maximal oxygen consumption and factorial aerobic scope (maximum oxygen consumption divided by basal metabolic rate) suggests that body mass influences the capacity of the cardiovascular system to raise metabolic rates above those at rest. The results show that the maximum sustainable metabolic rates of both birds and mammals are similar and scale as approximately the 0.88 +/- 0.02 power of body mass (and aerobic scope as approximately the 0.15 +/- 0.05 power), when the measurements are standardized with respect to the differences in relative heart mass and haemoglobin concentration between species. The maximum heart beat frequency of birds and mammals is predicted to scale as the -0.12 +/- 0.02 power of body mass, while that at rest should scale as -0.27 +/- 0.04.     

Effect of atrial natriuretic factor on arterial blood pressure and frequency of heart contraction in rats with genetically determined hypertension and in normotensive rats

An effect of peptide released by the heart atria of mammals and called the atrial natriuretic factor (ANF) on blood pressure and heart contractions was studied in rats with genetically determined arterial hypertension (SHR) and in normotensive Wistar-Kyoto rats (WKY). Nine male SHR rats and 11 male WKY rats, aged between 12 and 16 weeks, were given normal saline infusion for 30 minutes through implanted catheters to both ulnar vein and artery. Then, an infusion of ANF at the rate of 0.3 microgram/kg per hour followed for 35 minutes. An infusion of ANF produces significant decrease in the mean arterial blood pressure, systolic and diastolic pressures without significant effect on pulse pressure and heart contractions. AFN infusion with the same rate did not produce any significant differences in the arterial blood pressure and heart contractions in Wistar-Kyoto rats. The obtained results suggest that ANF may play a role in pathogenesis of the arterial blood hypertension.     

Allometric scaling of mammalian blood pressure: A comment on White and Seymour (2014)

White and Seymour examined the scaling of central arterial blood pressure against body mass in mammals ranging in size from a 30 g mouse to a 4080 kg elephant. Exponents in power functions fitted to each of three datasets (systolic, diastolic, and mean arterial pressure) were reported to be significantly greater than zero and indistinguishable from 0.33. The first of these outcomes would indicate that blood pressure increases with body size, whereas the second is consistent with the heart working against gravity to move blood to the head. Taken together, these results seemingly refute the notion that the cephalic circulation functions as an energy‐neutral siphon. However, the main findings by White and Seymour were presented in the form of graphs that distorted the relationships between the variables of interest. I use simple graphics to show that the data were unsuited from the outset for use in allometric analyses and that conclusions of the investigation are not well supported.     

Scaling the physiological effects of exposure to radiofrequency electromagnetic radiation: consequences of body size

We have demonstrated that a comparative analysis of the physiological effects of exposure of laboratory mammals to radiofrequency electromagnetic radiation (RFR) may be useful in predicting exposure thresholds for humans if the effect is assumed to be due only to heating of tissue. The threshold specific absorption rate (SAR) necessary to affect a thermoregulatory parameter shows an inverse and linear relationship to body mass. The inverse relationship between threshold SAR and body mass is attributed to a surface area: body mass relationship. In comparison to small mammals, relatively large mammals have a reduced capacity to dissipate an internal heat load passively, and are therefore physiologically more sensitive to RFR exposure. The threshold for a thermoregulatory response depends on the type of response measured, species, ambient temperature, etc. By extrapolation, it can be shown that a SAR of only 0.2-0.4 W/kg is required to promote a thermoregulatory response in a mammal with a body mass of 70 kg (e.g. weight of adult human). The specific absorption rate bioeffects data collected from laboratory mammals can be related by means of a simple power formula: threshold SAR (W/kg) = aMb, where M is body mass in kg, a is a constant and b is equal to approximately -0.5. Through this equation we have illustrated that a threshold SAR measured in a species weighing 100 g would be 10 times greater than that of a species weighing 10 000 g. Accordingly, a relatively low SAR that is physiologically ineffective in small mammals may be stressful to larger species.     

Scaling of Cardiovascular Physiology in Snakes

The elongate body form of snakes and the wide diversity of habitatsinto which they have radiated have affected the form and functionof the cardiovascular system. Heart position is strongly correlatedwith habitat. The heart is located 15–25% of the bodylength from the head in terrestrial and arboreal species, but25–45% in totally aquatic species. Semi-aquatic and fossorialspecies are intermediate. The viperids are exceptional, withgenerally more posterior hearts but arboreal species have heartscloser to the head. An anterior heartis favored when snakesclimb because it reduces the hydrostatic pressure of the bloodcolumn above the heart and tends to stabilize cephalic bloodpressure. In water, where hydrostatic bloodpressure is not aproblem, a more centrally located heart is favored because theheart does less work perfusing the body. In terrestrial species,head-heart distance increases linearly with body length andthe increased hydrostatic pressure is matched by higher restingarterial blood pressure in longer animals. Unlike mammals andbirds, snakes have blood pressures that increasewith body mass.The added stress on the ventricle wall in larger snakes is correlatedwith ventricles that are larger than predicted by other reptiles.Heart mass scales with body mass to the 0.95 power in snakesbut only 0.77–0.91 in other reptiles that are not as subjectto the hydrostatic effects of gravity. The spongy hearts ofreptiles do not conform well to the Principle of Laplace.     

Scaling of Na+,K+-ATPase molecular activity and membrane fatty acid composition in mammalian and avian hearts

We have examined Na(+),K(+)-ATPase molecular activity and membrane fatty acid composition in the heart of six mammalian and eight avian species ranging in size from 30 g in mice to 280 kg in cattle and 13 g in zebra finches to 35 kg in emus, respectively. Na(+),K(+)-ATPase activity scaled negatively with body mass in both mammals and birds. In small mammals, the elevated enzyme activity was related to allometric changes in both the concentration and molecular activity (turnover rate) of Na(+),K(+)-ATPase enzymes, while in small birds, higher Na(+),K(+)-ATPase activity appeared to result primarily from an increased molecular activity of individual enzymes. The unsaturation index of cardiac phospholipids scaled negatively with body mass in both groups, while a significant allometric increase in monounsaturate content was observed in the larger mammals and birds. In particular, the relative content of the highly polyunsaturated docosahexaenoic acid (22:6n-3) displayed the greatest variation, scaling negatively with body mass and varying greater than 40-fold in both mammals and birds. Membrane fatty acid profile was correlated with Na(+),K(+)-ATPase molecular activity in both mammals and birds, suggesting a potential association between membrane lipid composition and the activity of membrane-bound enzymes in the hearts of endotherms.     

Whole-body systemic transcapillary filtration rates, coefficients, and isogravimetric capillary pressures in Bufo marinus and Rana catesbeiana

Whole-body and organ-level transcapillary filtration rates and coefficients are virtually unexamined in ectothermal vertebrates. These filtration rates appear to be greater than in mammals when plasma volume shifts and lymphatic function are analyzed. Gravimetric techniques monitoring whole-body mass changes were used to estimate net systemic filtration in Bufo marinus and Rana catesbeiana while perfusing with low-protein Ringer's and manipulating venous pressure. Capillary pressures were estimated from arterial and venous pressures after measuring the venous to arterial resistance ratio of 0.23. The capillary filtration coefficient (CFC) for the two species was 25.2+/-1.47 mL min-1 kg-1 kPa-1. Isogravimetric capillary pressure (Pci), the pressure at which net fluid is neither filtered nor reabsorbed, was 1.12+/-0.054 kPa and was confirmed by an independent method. None of these variables showed a significant interspecific difference. The anuran CFC and Pci are significantly higher than those found using the same method on rats (7.6+/-2.04 mL min-1 kg-1 kPa-1 and 0.3+/-0.37 kPa, respectively) and those commonly reported in mammals. Despite the high CFC, the high Pci predicts that little net filtration will occur at resting in vivo capillary pressures.     

The influence of straining maneuvers on the pressor response during isometric exercise

Experiments were performed to determine to what extent increments in esophageal and abdominal pressure would have on arterial blood pressure during fatiguing isometric exercise. Arterial blood pressure was measured during handgrip and leg isometric exercise performed with both a free and occluded circulation to active muscles. Handgrip contractions were exerted at 33 and 70% MVC (maximum voluntary contraction) by 4 volunteers in a sitting position and calf muscle contractions at 50 and 70% MVC with the subjects in a kneeling position. Esophageal pressure measured at the peak of inspirations did not change during either handgrip or leg contractions but peak expiratory pressures increased progressively during both handgrip and leg contractions as fatigue occurred. These increments were independent of the tensions of the isometric contractions exerted. Intra-abdominal pressures measured at the peak of either inspiration or expiration did not change during inspiration with handgrip contractions but increased during expiration. During leg exercise, intraabdominal pressures increased during both inspiration and expiration, reaching peak levels at fatigue. The arterial blood pressure also reached peak levels at fatigue, independent of circulatory occlusion and tension exerted, averaging 18.5-20 kPa (140-150 mm Hg) for both handgrip and leg contractions. While blood pressure returned to resting levels following exercise with a free circulation, it declined by only 2.7-3.8 kPa after leg and handgrip exercise, respectively, during circulatory occlusion. These results indicate that straining maneuvers contribute 3.5 to 7.8 kPa to the change in blood pressure depending on body position.     

The characteristics of the alpha-adrenergic regulation of the cardiovascular system in middle-aged and elderly subjects]

97 practically healthy young adult, elderly and old people were examined to study the age-associated changes in alpha-adrenergic regulation of the cardiovascular system. The values of central and renal hemodynamics, systolic and diastolic cardiac functions, skin and bulbar microcirculation and arterial blood pressure were determined in dynamics after injection of the alpha-stimulator phenylephrine (0.15 mg/kg of body mass intramuscularly) or alpha-blocker prazosin (0.015 mg/kg of body mass orally). In old age alpha-adrenergic effects on the heart were found to decrease. In elderly and old people the changes of peripheric blood circulation prevail in the structure of hemodynamic shifts arising in stimulation and alpha-adrenergic receptors blockage.     

Re-evaluation of the allometry of wet thermal conductance for birds

Wet thermal conductance is an important thermoregulatory parameter for birds and mammals. It is generally calculated as C(wet) (ml O2 g(-1) h(-1) degrees C(-1)) = VO2/(T(b)-T(a)), where VO2 is metabolic rate measured in ml O2 g(-1) h(-1), T(b) is body and T(a) is ambient temperature measured in degrees C. Minimum C(wet) is measured at T(a) at or below the lower critical temperature (T(lc)) of the thermoneutral zone, and is strongly influenced by time of day (rest or activity phase) and body mass [J. Aschoff, Comp. Biochem. Physiol. 69A (1981) 611]. Allometric analyses indicate differences in C(wet) for passerine and non-passerine birds, in their rest and active phases (Aschoff, 1981). The allometric slope for non-passerine rest-phase (-0.583) is lower than that for non-passerine active-phase (-0.484), and passerine rest-phase (-0.461) and active-phase (-0.463), although none of these slopes are significantly different. This different-sloped relationship for non-passerine rest-phase C(wet) extrapolates to lower-than-expected values at high body mass, and so this allometric relationship may be inappropriate for predictive purposes. Consequently, we have reanalysed Aschoff's (1981) data, as well as more recent compilations, to determine a more useful allometric relationship for C(wet) of non-passerine rest-phase birds. Re-analyses of minimum thermal conductance data from Drent and Stonehouse [Comp. Biochem. Physiol. 40A (1971) 689], Aschoff (1981) and Gavrilov and Dolnik [Acta XVIII Congressus Internationalis Ornithologici Moscow (1982) 421] indicate that the most appropriate regressions for predicting C(wet) (ml O2 g(-1) h(-1) degrees C(-1)) of birds from body mass (M; g) are the pooled regressions for non-passerine and passerine birds, in the active (alpha) and resting (rho) phases, using data tabulated by Aschoff (1981): alpha, C(wet)=0.994M(-0.509); rho, C(wet)=0.702M(-0.519). C(wet) is approximately 40% higher in the active phase than the rest phase. Regressions of various data sets for C(wet) of birds and mammals indicate a similar slope of approximately -0.5 for the allometric relationship, but significantly higher elevations for mammals compared to birds. The approximately 50% higher C(wet) for mammals than birds indicates a better physical insulation for birds than mammals of the same body mass. The general scaling of C(wet) with M(-0.5) indicates that (T(b)-T(lc)) should scale with M(0.22), if mass-specific metabolic rate scales with M(-0.28) [Reynolds and Lee, Am. Nat. 147 (1996) 735]. The observed scaling for (T(b)-T(lc)) of M(0.183) (calculated from Gavrilov and Dolnik, 1985) is consistent with this expectation.     

Cardiopulmonary reference standards in the pregnant sheep (Ovis aries): a comparative study of ovine and human physiology in obstetrics.

1. Complete blood counts, respiratory rate, tidal volume, arterial and mixed venous blood gases; heart rate, cardiac output, and arterial, pulmonary artery, central venous and pulmonary wedge pressures were determined in 10 pregnant adult sheep of 42.7-65.7 kg body weight. 2. Arithmetic means, standard deviations, and coefficients of variation were calculated to develop reference values; in addition, the 95% confidence limits for ranges were established. 3. Comparison of derived data with that from non-pregnant sheep shows changes similar to those seen when examining pregnant and non-pregnant humans. 4. These results indicate the pregnant sheep is an acceptable model for human obstetrical research.     

A method for calculation of human systolic and diastolic blood pressures using an elastic reservoir theory of the systemic arterial system,and some clinical and physiological applications

Two equations have been developed that describe the interrelationship of the clinically measurable variables of the human systemic arterial system. An approximation method is given for their simultaneous solution for systolic and diastolic pressures in terms of heart rate, cardiac output, total peripheral resistance, and aortic distensibility. In this way, blood pressures were calculated for various clinically important and didactically useful situations. The effects on systolic and diastolic pressures due to changing either cardiac output or peripheral resistance or heart rate or aortic distensibility alone are shown. The effects on pulse pressure of varying cardiac output and peripheral resistance while holding mean arterial pressure constant are demonstrated. Compensatory mechanisms in hypertension and exercise are explored. Opinions and conclusions contained in this report are those of the author. They are not to be construed as necessarily reflecting the views or the endorsement of the Navy Department.     

Systemic methionine-enkephalin evokes cardiostimulatory responses in the human

The cardiovascular effects of bolus doses of methionine-enkephalin (Met5-ENK) (1 to 100 micrograms/kg) were studied in 9 subjects in whom, at cardiac catheterization for evaluation of chest pain, patent coronary arteries were found. Met5-ENK produced a simultaneous increase in blood pressure and heart rate beginning within 20 sec, reaching maximal values between 30 and 40 sec, and then terminating by 60 sec. Heart rate, systolic, diastolic, and mean arterial blood pressures increased significantly (p less than 0.0005); pulse pressure remained unchanged. Positive dose-effect relationships were observed for heart rate (p less than 0.002), systolic, diastolic, and mean arterial blood pressures (p less than 0.05). Naloxone (0.5 mg/kg), given to 4 subjects, prevented the heart rate and blood pressure changes associated with Met5-ENK administration, demonstrating that the cardiovascular changes were mediated by opiate receptors. Subjects also described cutaneous paresthesias which were not prevented by naloxone pretreatment. These data suggest a role for peripheral enkephalins in cardiovascular regulation.     

Evaluation of the cardiovascular function of older adult Rhesus monkeys by ultrasonography

OBJECTIVE: To evaluate the cardiovascular structure and function of older adult Rhesus monkey by utrasonography. METHODS: Sixteen monkeys aged from 17 to 20 years and weighing from 8.2 to 15.3 kg, six adults aged 7-8 years and weighing from 8.1 to 9.2 kg. All monkeys were determined to be free from hypertension, hyperglycaemia and cardiac disease. The normal values of index related to heart and blood vessels including structure, haemodynamics and systolic or diastolic function were detected by 2D, M-mode, pulsed Doppler and tissue Doppler echocardiography respectively under ketamine hydrochloride sedation. Meanwhile, blood pressures were also measured by electronic sphygmomanometer. Each monkey underwent repeated detections in 2 weeks and all data were analysed with statistical methods. RESULTS: Compared with young adult monkeys, the older's heart rate (HR), the left ventricular diastolic function and the compliance of big artery including right and left common carotid artery, bulbus caroticus, internal carotid artery and abdominal aorta were decreased and the associated indexes changed significantly (P < 0.05 or P < 0.01). Meanwhile, older monkeys exhibited significant increase in the aorta diameter (AO), amplitude of aortic wall (AAO), left atrial diameter (LAD), end diastolic volume of left ventricle (EDV), stroke volume (SV), left ventricular mass (LVM) (P < 0.05 vs. young adult monkeys); however, cardiac output (CO) only slightly increased but the difference did not reach the statistical significance (P = 0.418, P = 0.644 respectively). CONCLUSIONS: The present results demonstrated the profiles of cardiovascular function and structure in the older Rhesus monkeys. Older monkey is accompanied by diminished left ventricular diastolic function and big artery compliance. Ultrasonography provides a means to non-invasively evaluate the anatomy and function of the heart and blood vessel, and plays an increasingly important role in the drug evaluation against cardiovascular dysfunction.     

Development of maximum metabolic rate and pulmonary diffusing capacity in the superprecocial Australian Brush Turkey Alectura lathami: an allometric and morphometric study

The Australian Brush Turkey Alectura lathami is a member of the Megapodiidae, the mound-building birds that produce totally independent, "superprecocial" hatchlings. This study examined the post-hatching development of resting and maximal metabolic rates, and the morphometrically determined changes in pulmonary gas exchange anatomy, in chicks during 3.7 months of growth from hatchlings (122 g) to subadults (1.1 kg). Allometric equations of the form y=aM(b) related gas exchange variables (y) to body mass (M, g). Metabolic rates were measured with open-flow respirometry (mL O2 min(-1)) of chicks resting in the dark and running above the aerobic limit on a treadmill. Resting metabolic rate (RMR=0.02 M(0.99)) and maximal metabolic rate (MMR=0.05 M(1.07)) scaled with exponents significantly above those of interspecific allometries of adult birds. However MMR was below that expected for other species of adult birds in flapping flight, consistent with the Brush Turkey's ground-dwelling habits. Total lung volumes (mL) increased faster than isometrically (V(L)=0.0075 M(1.19)), as did the surface area (cm(2)) of the blood-gas barrier (S(t)=7.80 M(1.23)), but the data overlapped those of adult species. Harmonic mean thickness of the blood-gas barrier was independent of body size (mean tau(ht),=0.39 microm) and was about twice that expected for flying birds. Diffusing capacity (mL O2 min(-1) kPa(-1)) of the blood-gas tissue barrier increased faster than isometrically (Dto2=0.049 M(1.23)); in hatchling Brush Turkeys, it was about 30% expected for adult birds, but this difference disappeared when they became subadults. When compared to altricial Australian pelicans that hatch at similar body masses, superprecocial Brush Turkeys had higher MMR and higher Dto2 at the same body size. A parallel allometry between MMR and Dto2 in Brush Turkeys and pelicans is consistent with the concept of symmorphosis during development.     

Development of maximum metabolic rate and pulmonary diffusing capacity in the superprecocial Australian Brush Turkey Alectura lathami: an allometric and morphometric study.

The Australian Brush Turkey Alectura lathami is a member of the Megapodiidae, the mound-building birds that produce totally independent, "superprecocial" hatchlings. This study examined the post-hatching development of resting and maximal metabolic rates, and the morphometrically determined changes in pulmonary gas exchange anatomy, in chicks during 3.7 months of growth from hatchlings (122 g) to subadults (1.1 kg). Allometric equations of the form y=aM(b) related gas exchange variables (y) to body mass (M, g). Metabolic rates were measured with open-flow respirometry (mL O2 min(-1)) of chicks resting in the dark and running above the aerobic limit on a treadmill. Resting metabolic rate (RMR=0.02 M(0.99)) and maximal metabolic rate (MMR=0.05 M(1.07)) scaled with exponents significantly above those of interspecific allometries of adult birds. However MMR was below that expected for other species of adult birds in flapping flight, consistent with the Brush Turkey's ground-dwelling habits. Total lung volumes (mL) increased faster than isometrically (V(L)=0.0075 M(1.19)), as did the surface area (cm(2)) of the blood-gas barrier (S(t)=7.80 M(1.23)), but the data overlapped those of adult species. Harmonic mean thickness of the blood-gas barrier was independent of body size (mean tau(ht),=0.39 microm) and was about twice that expected for flying birds. Diffusing capacity (mL O2 min(-1) kPa(-1)) of the blood-gas tissue barrier increased faster than isometrically (Dto2=0.049 M(1.23)); in hatchling Brush Turkeys, it was about 30% expected for adult birds, but this difference disappeared when they became subadults. When compared to altricial Australian pelicans that hatch at similar body masses, superprecocial Brush Turkeys had higher MMR and higher Dto2 at the same body size. A parallel allometry between MMR and Dto2 in Brush Turkeys and pelicans is consistent with the concept of symmorphosis during development.     

Metabolism during flight in two species of bats, Phyllostomus hastatus and Pteropus gouldii.

The energetic cost of flight in a wind-tunnel was measured at various combinations of speed and flight angle from two species of bats whose body masses differ by almost an order of magnitude. The highest mean metabolic rate per unit body mass measured from P. hastatus (mean body mass, 0.093 kg) was 130.4 Wkg-1, and that for P. gouldii (mean body mass, 0.78 kg) was 69.6 Wkg-1. These highest metabolic rates, recorded from flying bats, are essentially the same as those predicted for flying birds of the same body masses, but are from 2.5 to 3.0 times greater than the highest metabolic rates of which similar-size exercising terrestrial mammals appear capable. The lowest mean rate of energy utilization per unit body mass P. hastatus required to sustain level flight was 94.2 Wkg-1 and that for P. gouldii was 53.4 Wkg-1. These data from flying bats together with comparable data for flying birds all fall along a straight line when plotted on double logarithmic coordinates as a function of body mass. Such data show that even the lowest metabolic requirements of bats and birds during level flight are about twice the highest metabolic capabilities of similar-size terrestrial mammals. Flying bats share with flying birds the ability to move substantially greater distance per unit energy consumed than walking or running mammals. Calculations show that P. hastatus requires only one-sixth the energy to cover a given distance as does the same-size terrestrial mammal, while P. gouldii requires one-fourth the energy of the same-size terrestrial mammal. An empirically derived equation is presented which enables one to make estimates of the metabolic rates of bats and birds during level flight in nature from body mass data alone. Metabolic data obtained in this study are compared with predictions calculated from an avian flight theory.     

An allometric analysis of the giraffe cardiovascular system

There has been co-evolution of a long neck and high blood pressure in giraffes. How the cardiovascular system (CVS) has adapted to produce a high blood pressure, and how it compares with other similar sized mammals largely is unknown. We have measured body mass and heart structure in 56 giraffes of both genders ranging in body mass from 18 kg to 1500 kg, and developed allometric equations that relate changes in heart dimensions to growth and to cardiovascular function. Predictions made from these equations match measurements made in giraffes. We have found that heart mass increases as body mass increases but it has a relative mass of 0.51 ± 0.7% of body mass which is the same as that in other mammals. The left ventricular and interventricular walls are hypertrophied and their thicknesses are linearly related to neck length. Systemic blood pressure increases as body mass and neck length increase and is twice that of mammals of the same body mass. Cardiac output is the same as, but peripheral resistance double that predicted for similar sized mammals. We have concluded that increasing hydrostatic pressure of the column of blood during neck elongation results in cardiac hypertrophy and concurrent hypertrophy of arteriole walls raising peripheral resistance, with an increase in blood pressure following.     

Diastolic pressure determines autonomic responses to pressure perturbation in humans

Arterial baroreceptors reflexly regulate sympathetic and heart rate responses to alteration of blood pressure. The primary mechanical determinant of arterial baroreceptor activity in humans remains unclear. We examined the influence of systolic, diastolic, pulse, and mean arterial pressures on efferent muscle sympathetic nerve activity (MSNA, microneurography) and heart rate responses during perturbation of arterial pressure in 10 normal human subjects [age 25 +/- 2 (SE) yr]. We directly measured arterial pressure, heart rate, and MSNA during intravenous vasodilator infusion (nitroprusside, 6 +/- 1 micrograms.kg-1.min-1, n = 6; or hydralazine, 16 +/- 2 mg, n = 4) while central venous pressure was held constant by simultaneous volume expansion. Changes in arterial pressures were compared with changes in heart rate and MSNA over 3-min periods of vasodilator infusion during which we observed increases in systolic and pulse pressures with simultaneous decreases in mean and diastolic pressures. During vasodilator infusion, there were increases in systolic (124.2 +/- 2.1 to 131.7 +/- 2.9 Torr, P less than 0.001) and pulse pressures (57.0 +/- 2.2 to 72.7 +/- 2.7 Torr, P less than 0.001) although mean arterial pressure fell (88.0 +/- 2.6 to 80.4 +/- 2.7 Torr, P less than 0.001) because of decreases in diastolic pressure (67.2 +/- 3.0 to 59.0 +/- 2.7 Torr, P less than 0.001). The changes in arterial pressures were accompanied by simultaneous increases in heart rate (66.4 +/- 3.0 to 92.6 +/- 4.8 beats/min, P less than 0.001) and MSNA (327 +/- 59 to 936 +/- 171 U, P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)