Continuous 24-hour assessment of the neural regulation of systemic arterial pressure and RR variabilities in ambulant subjects

In this study, we tested the hypothesis that the neural control of circulation in humans undergoes continuous but in part predictable changes throughout the day and night. Dynamic 24-hour recordings were obtained in two groups of ambulant subjects. In 18 hospitalized patients free to move, direct high-fidelity arterial pressures and electrocardiograms were recorded, and in an additional 28 nonhospitalized subjects, only electrocardiograms were obtained. Spectral analysis of systolic arterial pressure and of RR interval variabilities provided quantitative markers of sympathetic and vagal control of the sinus node and of sympathetic modulation of vasomotor tone. With this approach, the low-frequency (approximately 0.1 Hz) component of RR interval and systolic arterial pressure variabilities is considered a marker primarily of sympathetic activity, whereas the high-frequency (approximately 0.25 Hz) component of RR interval variability, related to respiration, seems to be a marker primarily of vagal activity. We observed a pronounced and consistent reduction in the markers of sympathetic activity and an increase in those of vagal activity during the night. In the invasive studies, while the subjects were still lying in bed after waking up, the markers of sympathetic activity rose rapidly and concomitantly with a simultaneous vagal withdrawal. Noninvasive studies confirmed the early morning rise of the markers of sympathetic activity and the circadian pattern of sympathovagal balance. These data indicate that the ominously increased rate of cardiovascular events in the morning hours may reflect the sudden rise of sympathetic activity and the reduction of vagal tone.

Circadian changes in vascular sympathetic activity in ambulant subjects

In 10 ambulant subjects we studied the circadian changes in sympathetic vasomotor control as assessed by the spectral power of the 0.1-Hz low-frequency component of systolic arterial pressure variability measured with a Millar phi 3F tip transducer. The low-frequency component was higher during the daytime, while the subjects were performing light physical activity, and lower during the night, thus paralleling the circadian systolic blood pressure pattern. However, the morning low-frequency rise preceded the blood pressure increase by about 3 h, suggesting that vasometer control and blood pressure control are at least partly related to different mechanisms.

Spectral analysis of R-R and arterial pressure variabilities to assess sympatho-vagal interaction during mental stress in humans

We tested the hypothesis that spectral analysis of the R-R interval and systolic arterial pressure variabilities allows assessment of the dynamic changes in neural control of the cardiovascular system in men undergoing mental stress testing. Mental arithmetic increased the low-frequency components of R-R and systolic arterial pressure, i.e. markers of sympathetic activity to the SA node and the vasculature, respectively; it also decreased the high frequency component of R-R variability, a marker of vagal activity. Spectral analysis of R-R and systolic arterial pressure variabilities may be used in the clinic to test the dynamic effects of mental stress on both sympathetic and vagal activities.

Compressed spectral arrays for the analysis of 24-hr heart rate variability signal: enhancement of parameters and data reduction

Heart rate variability signal in the form of an R-R interval tachogram is detected in Holter type 24-hr ECG recordings. Spectral analysis is carried out over consecutive nonoverlapping records, and the information is displayed in the form of a compressed spectral array through parametric techniques. The trends of spectral parameters such as low-frequency (LF) and high-frequency (HF) powers and central frequencies are also plotted, together with the classical mean R-R value and variance relative to each single spectrum. These parameters quantify the effect of sympatho-vagal balance on heart rate control during the 24-hr period and provide important elements for the diagnostic evaluation of various pathologies, like hypertension. A spectral compression algorithm which checks the position of the poles relative to LF and HF bands inside the unitary circle in the complex zeta-plane is also developed. Applications of this procedure are foreseen in the clinical evaluation of ambulant patients as well as in the study of physical and psychological stress.

Changes in autonomic regulation induced by physical training in mild hypertension

The adaptive effects of physical training on cardiovascular control mechanisms were studied in 11 subjects with mild hypertension. In these subjects we assessed the gain of the heart period-systolic arterial pressure relationship in the unfit and the fit state by using 1) an open loop approach, whereby the gain is expressed by the slope of the regression of heart period as a function of systolic arterial pressure, during a phenylephrine-induced pressure rise and 2) a closed loop approach with proper simplification, whereby the gain is expressed by the index alpha, obtained through simultaneous spectral analysis of the spontaneous variabilities of heart period and systolic arterial pressure. Both methods indicated that training significantly increased the gain of the relationship between heart period and systolic arterial pressure at rest and reduced arterial pressure and increased heart period significantly. This gain was drastically reduced during bicycle exercise both in the unfit and fit state. In a second group of normotensive (n = 7; systolic pressure, 133 +/- 3 mm Hg) and hypertensive (n = 7; systolic pressure, 180 +/- 10 mm Hg) subjects undergoing 24-hour diagnostic continuous electrocardiographic and high fidelity arterial pressure monitoring, the index alpha was significantly reduced in the hypertensive group at rest. Furthermore, when analyzed continuously over the entire 24-hour period, this index underwent minute-to-minute changes with lower values during the day and higher values during the night. We propose the index alpha as a quantitative indicator of the changes in the gain of baroreceptor mechanisms occurring with physical training in mild hypertension and during a 24-hour period in ambulatory subjects.

Sympathetic predominance in essential hypertension: a study employing spectral analysis of heart rate variability

In this study on 91 subjects we tested the hypothesis of an enhanced sympathetic activity in uncomplicated essential hypertension employing spectral analysis of heart rate variability. With this technique the tonic sympathetic and vagal activities and their changes are respectively assessed by the power of approximately 0.1 Hz (low frequency, LF) and approximately 0.25 Hz (respiratory linked, high frequency, HF) components of the spectrum of the beat by beat variability of RR interval. When comparing the 40 subjects with diastolic blood pressure consistently greater than 95 mmHg (hypertensives, Ht), with 35 age-matched controls (diastolic arterial pressure less than 90 mmHg, Nt), we observed that LF was greater and HF smaller in Ht as compared to Nt, thus suggesting an enhanced sympathetic activity and a reduce vagal activity in Ht. Additionally, passive tilt, which in Nt enhances LF [delta = 26 +/- 2 normalized units (nu)] and reduces HF (delta = -22 +/- 2, nu), produced smaller (P less than 0.05) changes in Ht (delta LF = 6.3 +/- 2.7 and delta HF = -7.5 +/- 2.3 nu). Furthermore, the values of LF at rest and the altered effects of tilt on LF and HF were significantly correlated with the degree of the hypertensive state. Chronic beta-adrenergic blockade (atenolol 100 mg once daily for 2 weeks, n = 13) reduced heart rate and blood pressure (from 162/103 to 136/88 mmHg) together with a significant diminution of LF and an increase of HF. Thus, spectral analysis of RR variability appears to be a convenient non-invasive technique to follow the progressive alterations in sympatho-vagal balance present in essential hypertension.

Spectral analysis of heart rate variability in the assessment of autonomic diabetic neuropathy

We studied heart rate variability in 49 uncomplicated diabetics (27 with insulin therapy; 22 with oral hypoglycemic agents) and in 40 age-matched controls. An automatic autoregressive algorithm was used to compute the power spectral density (PSD) of beat by beat RR variability derived from the surface ECG. The PSD contains two major components (a low frequency approximately 0.1 Hz (LF) and a high frequency, respiratory linked, approximately 0.25 Hz (HF] that provide, respectively, quantitative markers of sympathetic and vagal modulatory activities and of their balance. As compared to controls, in diabetics, besides a reduced RR variance at rest (2722 +/- 300 and 1436 +/- 241 ms2, respectively), we observed during passive tilt an altered response of spectral indices of sympathetic activation and vagal withdrawal, suggestive of a complex modification in the neural control activities. In addition, we compared this approach to the commonly used clinical tests score, and observed that the latter provides overall results similar to those obtained with spectral changes induced by tilt (r = 0.42; P less than 0.01). Of potential clinical importance is that the data obtained with spectral analysis appear more thoroughly quantifiable and do not require the active collaboration of the patients.

Parameter extraction from heart rate and arterial blood pressure variability signals in dogs for the validation of a physiological model

The paper describes an automatic procedure for improving the extraction of parameters in heart rate (HR) and arterial blood pressure (ABP) beat-to-beat variability signals. Auto- and cross-spectral analysis of such signals is carried out through parametric models and the distribution of the power of the spectra and the phase relationships are compared in various physiological situations induced in the dogs via drug infusion or surgical interventions which do influence the control mechanisms of HR and ABP. This "black-box" approach allows the obtention, directly from the processing of the above-mentioned signals, of the estimation of parameters relative to cardiovascular models, as the ones described by simple equations (Windkessel and Starling laws are introduced as examples). These parameters seem to validate significantly the capability of such models to describe the physiological interactions existing between the two considered signals. Applications may be foreseen both for research and clinical purposes.

Heart rate variability as an index of sympathovagal interaction after acute myocardial infarction

By analysis of spectral components of heart rate variability, sympathovagal interaction was assessed in patients after acute myocardial infarction (AMI). At 2 weeks after AMI (n = 70), the low-frequency component was significantly greater (69 +/- 2 vs 53 +/- 3 normalized units [NU], p less than 0.05) and the high-frequency component was significantly smaller (17 +/- 1 vs 35 +/- 3 NU) than in 26 age-matched control subjects. This difference was likely to reflect an alteration of sympathovagal regulatory outflows with a predominance of sympathetic activity. At 6 (n = 33) and 12 (n = 29) months after AMI, a progressive decrease in the low- (62 +/- 2 and 54 +/- 3 NU) and an increase in the high-frequency (23 +/- 2 and 30 +/- 2 NU) spectral components was observed, which suggested a normalization of sympathovagal interaction. An increase in sympathetic efferent activity induced by tilt did not further modify the low-frequency spectral component (78 +/- 3 vs 74 +/- 3 NU) in a subgroup of 24 patients at 2 weeks after AMI. Instead, 1 year after AMI, this maneuver was accompanied by an increase in the low-frequency component (77 +/- 3 vs 53 +/- 3 NU, p less than 0.05) of a magnitude similar to the one observed in control subjects (78 +/- 3 vs 53 +/- 3 NU). These data indicate that the sympathetic predominance that is detectable 2 weeks after AMI is followed by recovery of vagal tone and a normalization of sympathovagal interaction, not only during resting conditions, but also in response to a sympathetic stimulus.

Heart rate variability signal processing: a quantitative approach as an aid to diagnosis in cardiovascular pathologies

The heart rate variability (HRV) signal carries important information about the systems controlling heat rate and blood pressure, mainly elicited by autonomic nervous system (sympathetic and parasympathetic) controls. The present paper illustrates methods of HRV signal processing by using autoregressive (AR) modeling and power spectral density estimate. The information enhanced in this way seems to be particularly sensitive in discriminating various cardiovascular pathologies (hypertension, myocardial infarction, diabetic neuropathy, etc.). This method provides a simple non-invasive analysis, based on the processing of spontaneous oscillations in heart rate. Particular emphasis is directed to the algorithms used and to their direct application by using proper computerized techniques: only a few paradigmatical examples will be illustrated as preliminary results.

Effects of verapamil, nifedipine, and dilazep on left ventricular relaxation in the conscious dog

The cardiac systolic and diastolic effects of the two major calcium blockers, verapamil and nifedipine, were studied and compared with those produced by dilazep, a relatively new vasodilator with calcium blocking properties, in conscious instrumented dogs to avoid the complications of anaesthesia and recent surgery. Mean arterial pressure was reduced by nifedipine and dilazep but not by verapamil, whereas peak left ventricular pressure was reduced only by dilazep and verapamil. Consistent tachycardia occurred, the rate being highest with nifedipine and lowest with dilazep. Left ventricular dP/dt was unaffected by dilazep, reduced by verapamil, and increased by nifedipine; this increase was no longer observed after beta adrenergic blockade. Ventricular relaxation was assessed by calculating the time relaxation constant, tau. Verapamil increased tau significantly only after beta adrenergic blockade, whereas nifedipine and dilazep reduced it both before and after beta adrenergic blockade. These data suggest that reflex beta adrenergic mechanisms may modulate the effects of calcium blockade on both systolic and diastolic performance.

Spectral and cross-spectral analysis of heart rate and arterial blood pressure variability signals

A parametric method for autoregressive (AR) auto- and cross-spectral analysis is presented for the contemporaneous processing of heart rate and arterial blood pressure variability signals. In particular, the introduced bivariate spectral analysis (phase and coherence spectra) provides quantitative and objective means which are useful to measure the role played by the neural controlling systems (sympathetic and parasympathetic systems) on the cardiovascular signals under different pathophysiological conditions. Algorithmic aspects, connected to the way of processing discrete numerical series synchronized to single cardiac beats, are particularly stressed. Important applications are foreseen both in physiological studies and in clinical practice as an aid to the detection of various relevant cardiovascular pathologies such as hypertension and diabetes.

Power spectral analysis of heart rate and arterial pressure variabilities as a marker of sympatho-vagal interaction in man and conscious dog

In 57 normal subjects (age 20-60 years), we analyzed the spontaneous beat-to-beat oscillation in R-R interval during control recumbent position, 90 degrees upright tilt, controlled respiration (n = 16) and acute (n = 10) and chronic (n = 12) beta-adrenergic receptor blockade. Automatic computer analysis provided the autoregressive power spectral density, as well as the number and relative power of the individual components. The power spectral density of R-R interval variability contained two major components in power, a high frequency at approximately 0.25 Hz and a low frequency at approximately 0.1 Hz, with a normalized low frequency:high frequency ratio of 3.6 +/- 0.7. With tilt, the low-frequency component became largely predominant (90 +/- 1%) with a low frequency:high frequency ratio of 21 +/- 4. Acute beta-adrenergic receptor blockade (0.2 mg/kg IV propranolol) increased variance at rest and markedly blunted the increase in low frequency and low frequency:high frequency ratio induced by tilt. Chronic beta-adrenergic receptor blockade (0.6 mg/kg p.o. propranolol, t.i.d.), in addition, reduced low frequency and increased high frequency at rest, while limiting the low frequency:high frequency ratio increase produced by tilt. Controlled respiration produced at rest a marked increase in the high-frequency component, with a reduction of the low-frequency component and of the low frequency:high frequency ratio (0.7 +/- 0.1); during tilt, the increase in the low frequency:high frequency ratio (8.3 +/- 1.6) was significantly smaller. In seven additional subjects in whom direct high-fidelity arterial pressure was recorded, simultaneous R-R interval and arterial pressure variabilities were examined at rest and during tilt. Also, the power spectral density of arterial pressure variability contained two major components, with a relative low frequency:high frequency ratio at rest of 2.8 +/- 0.7, which became 17 +/- 5 with tilt. These power spectral density components were numerically similar to those observed in R-R variability. Thus, invasive and noninvasive studies provided similar results. More direct information on the role of cardiac sympathetic nerves on R-R and arterial pressure variabilities was derived from a group of experiments in conscious dogs before and after bilateral stellectomy. Under control conditions, high frequency was predominant and low frequency was very small or absent, owing to a predominant vagal tone. During a 9% decrease in arterial pressure obtained with IV nitroglycerin, there was a marked increase in low frequency, as a result of reflex sympathetic activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Neural control of vasomotor tone of large coronary arteries

In man, the occurrence of constrictions of large coronary arteries accompanied by transient myocardial ischemia is now well established. However, the role of neural factors involved in such coronary artery spasms is still a matter of conjecture. A consistent reduction (9 +/- 2%) of the diameter of the large coronary arteries can be obtained in the conscious dog with alpha-adrenergic receptor stimulation with methoxamine in spite of the concomitant pressor rise (65 +/- 5%). Smaller reductions in coronary diameter can be obtained with electrical efferent sympathetic stimulation in anesthetized dogs. The diameter of a conduit artery such as the aorta can be reduced (5%) by reflex increases in sympathetic efferent activity: therefore it is not unlikely that similar neural influences might be exerted on the coronary tree as well. In normal life, stressful situations, such as emotion or exercise, will be accompanied by a drastic increase in sympathetic drive to the heart, together with a marked increase in coronary flow. The latter will induce an endothelial mediated vasodilation; however the net effect on coronary size of these two potentially opposite mechanisms is as yet unexplored. In the laboratory, intracoronary bradykinin and regional myocardial ischemia initiate a reflex increase in sympathetic activity to the heart; in the clinics acute myocardial ischemia can be accompanied by signs of sympathetic overactivity. The extent to which such increases in sympathetic activity, could play a role in the control of coronary tone and hence in the pathophysiology of coronary artery disease, is still under investigation.

Continuous recording of direct high fidelity arterial pressure and electrocardiogram in ambulant patients

A system providing high quality direct arterial blood pressure recordings and electrocardiograms in ambulatory patients was devised using a modified commercially available Holter type magnetic tape recorder together with a microminiature Millar (3F) tip transducer. This system did not require a perfusion line and solved the major drawbacks of other available systems. Pressure and electrocardiographic data were fed directly from the playback unit into a minicomputer for automatic beat to beat waveform analysis. Thus the blood pressure and RR interval variability signals could be simultaneously analysed with autoregressive modelling techniques to provide a quantitative estimate of sympathovagal balance in ambulant patients. The system was reliable, simple, and safe to use.

Variability analysis of fetal heart rate signals as obtained from abdominal electrocardiographic recordings

The present paper introduces an original method of digital signal processing for an automatic analysis of non-invasive abdominal ECG recordings on pregnant women starting from the 25th week of gestation. The procedure has been implemented on a DEC-VAX 750 digital computer at the Department of Electrical Engineering, Polytechnic of Milano and the signals are recorded at the Department of Obstetrics and Gynecology "L. Mangiagalli", University of Milano, Italy. The experimental results presented in here are still preliminary as only few cases have been considered up to now (about 20) and the goal of the paper is mainly focused on the algorithmic aspects of the whole procedure implemented in the computer and on the approach of heart rate variability (HRV) signal analysis both in the mother and in the fetus. Abdominal ECG lead processing is illustrated starting from the step of maternal (M) and fetal (F) QRS recognitions through linear digital filtering (derivative and low-pass FIR filter, Weber-Cappellini window) and weighted averaging techniques synchronized with maternal QRS's. Figure 1 a shows the original abdominal lead; figure 1 b the filtered signal for MQRS recognitions; figure 2 a the template of maternal cardiac cycle as obtained after the averaging operation synchronized with the instants of MQRS occurrence. The subtraction of the template results in the abdominal lead shown in figure 1 c in which the contribution of MECG is practically entirely reduced even in the case of MQRS and FQRS overlapping.(ABSTRACT TRUNCATED AT 250 WORDS)

Simultaneous analysis of beat by beat systemic arterial pressure and heart rate variabilities in ambulatory patients

The analysis of power spectral density (PSD) or RR variability in the electrocardiogram (ECG) has suggested that, in the early phase of essential hypertension, sympatho-vagal interaction is characterized by a sympathetic predominance. Recently, we have developed a high fidelity, direct arterial pressure ambulatory recording system which allows a beat by beat computer analysis of arterial pressure and heart rate. A microminiature tip transducer (Millar, diameter 0.8 mm) is inserted percutaneously into the radial artery and connected to a Holter two-channel magnetic tape recorder. The tip transducer has a wide band pass (> 1 kHz), excellent stability (congruent to 2 mmHg/24 h) and does not require a perfusion line. The overall frequency response of the entire recording-reproducing system is better than 20 Hz (-3 dB). The ECG and pressure signals are analysed with automatic autoregressive modelling algorithms to provide a quantitative estimate of blood pressure and heart rate variability through the computation of the PSD. In seven hypertensive patients, systolic arterial pressure and variance were higher during the day (157 +/- 9 mmHg and 122 +/- 9 mmHg2) than during the night (122 +/- 4 mmHg and 30 +/- 3 mmHg2). The PSD of RR and of systolic arterial pressure consisted of a predominant low frequency peak (congruent to 0.09 cycles/beat) during the day, and two peaks at low and high (congruent to 0.25 cycles/beat) frequency during the night. While RR variance was similar during both day- and night-time, a predominant low frequency peak was observed during the day.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of alpha 2-adrenoceptor binding properties of imidazoline-like drugs, azoloazepine derivatives and beta-phenethylamine-like drugs in human platelet membranes

To characterize the agonist profile of alpha 2-adrenoceptor agonists (imidazoline-like drugs, azoloazepine derivatives, beta-phenethylamines-like drugs) on human platelets, the characteristics of alpha 2-adrenoceptors (KD, Bmax) have been evaluated and the affinity constants measured by displacement technique and computer-assisted analysis of the curves. Furthermore, since alpha 2-adrenoceptor agonists interact with the post-synaptic receptors in a calcium-operated channel, whether the effect of calcium-entry inhibitors (verapamil, nifedipine, diltiazem) is related to a competition with alpha 2-receptors has also been examined. By Scatchard analysis, it was calculated that in human platelets alpha 2-adrenoceptors have KD = 3.45 nM and Bmax = 247 fmol (mg protein)-1. As far as the potency is concerned, imidazoline-like drugs were the most potent agonists in human platelet alpha 2-adrenoceptors (guanabenz IC50 = 8.6 +/- 0.8 X 10(-8), B-HT 920 IC50 = 2.9 +/- 0.3 X 10(-7), (-)-adrenaline IC50 = 3.4 +/- 0.5 X 10(-7)). Among the calcium-entry inhibitors only verapamil antagonized [3H]rauwolscine binding: the effect was stereospecific, (-)-D 600 being more potent than (+)-D 600. Nifedipine and diltiazem did not affect alpha 2-receptor binding. It is concluded that human platelets alpha 2-receptors share the agonist potency profile of other tissues containing alpha 2-receptors (brain, pre-synaptic junction), and that among calcium-entry blockers only verapamil can antagonize alpha 2-agonists. Nifedipine and diltiazem do not appear to interact stereospecifically with alpha 2-adrenoceptors.

Analysis of the pressor sympathetic reflex produced by intracoronary injections of bradykinin in conscious dogs

The reflex hemodynamic effects of intracoronary bradykinin were tested in 20 conscious instrumented dogs. When the experiments were performed after full recovery from surgery and anesthesia, graded doses (10-300 ng/kg) of bradykinin always produced graded pressor responses, in the absence of any pain reaction. At the maximum pressor response obtained with 100 ng/kg, mean arterial pressure rose 28 +/- 3% from 89 +/- 4 mm Hg, left ventricular pressure 20 +/- 3% from 121 +/- 2 mm Hg, heart rate 30 +/- 4% from 88 +/- 5 beats/min, rate of change of left ventricular pressure 18 +/- 3% from 2812 +/- 65 mm Hg/sec (P less than 0.01). Higher doses of bradykinin did not produce greater responses. The magnitude of the response was similar when the injection was performed in either the left anterior descending (change in mean arterial pressure 29 +/- 3%) or circumflex (change in mean arterial pressure 27 +/- 2%) coronary artery. The reflex nature of the response was proved by its disappearance after appropriate pharmacological blockades; moreover, after vagotomy, the pressor rise was maintained, the heart rate response was reduced (change in heart rate 10 +/- 2%), and the inotropic response was enhanced (rate of change of left ventricular pressure 24 +/- 3%). This suggested that the afferent pathway of the pressor reflex was in the sympathetic nerves and that a subordinate vagal depressor reflex was also operative. No pain reaction was obtained even when injecting very large amounts (1000-2000 ng/kg) of bradykinin, which, instead, induced arterial hypotension.(ABSTRACT TRUNCATED AT 250 WORDS)

Power spectral density of heart rate variability as an index of sympatho-vagal interaction in normal and hypertensive subjects

Instantaneous heart rate reflects sympatho-vagal influences on pace-maker activity. Hence computer analysis of heart rate variability might provide a quantitative index of that interaction. The power spectral density (PSD) estimate of heart rate variability was obtained in normal controls and in uncomplicated hypertensives, both at rest and during a non-hypotensive sympathetic stimulus (tilting). In normal controls PSD shows three major peaks of frequencies P1 = 0.07, P2 = 0.12, P3 = 0.25 cycles/beat. P1, which is associated with sympathetic activity, represents only a minor portion of total variability at rest, while becoming predominant with tilting. P2 and P3 are associated with vagal activity, and represent the major part of variability at rest, while they are reduced by tilting. In hypertensive patients PSD is altered, as P1 is already predominant at rest and increases only slightly with tilting. Thus PSD of heart rate variability is capable of detecting an early alteration in sympatho-vagal balance of cardiac control present in uncomplicated hypertension.

Global versus regional myocardial ischaemia: differences in cardiovascular and sympathetic responses in cats

The cardiovascular and sympathetic responses to occlusions of the left main ("global" ischaemia) or distal left anterior descending ("regional" ischaemia) coronary artery were studied in 19 anaesthetised cats with chronic sinoaortic baroreceptor denervation. "Global" ischaemia, before vagotomy, resulted in a significant reduction of mean arterial pressure (MAP), left ventricular pressure (LVP), and LVdP/dtmax while sympathetic efferent impulse activity was significantly augmented during the initial 15 +/- 2 s of occlusion (early phase) and, vice versa inhibited during the subsequent 20 +/- 2 s of occlusion (late phase). Vagotomy did not modify the haemodynamic responses, however, a significant increase in sympathetic discharge was detectable during the whole occlusion period (early and late phases). "Regional" ischaemia, before vagotomy, resulted in a significant increase in sympathetic neural discharge and MAP, with no changes in left ventricular function. After vagotomy the occlusion elicited a significant increase in MAP, LVP, LVdP/dtmax and efferent sympathetic neural activity. These excitatory responses were abolished after the interruption of a large part of the cardiac sympathetic afferents. Thus coronary artery occlusion induced haemodynamic and sympathetic reflex responses that were dependent upon the interaction of opposite influences mediated by the simultaneous activation of cardiac vagal and sympathetic afferents. The extent of "ischaemic myocardium" represented a determinant factor for the prevailing type of neural response.

Cardiovascular reflexes mediated by sympathetic afferent fibers

In this paper the experimental evidence supporting the hypothesis that excitatory sympathetic reflexes may participate in the tonic control of the cardiovascular system is discussed. Positive feedback pressor sympathetic reflexes can be obtained with physiological distensions of the descending thoracic aorta in conscious dogs with all nerves intact in absence of any pain reaction. These excitatory reflexes interact with supraspinal regulatory mechanisms, inhibitory in nature. A massive excitation of cardiac sympathetic afferents, produced by intracoronary injections of bradykinin, also elicits a pressor reflex, without pain reactions. In the absence of anesthesia and recent surgery, the cardiovascular excitatory reflexes, subserved by sympathetic afferent fibers, can easily prevail. We suggest that negative and positive feedback mechanisms interact continuously to achieve the most adequate neural cardiovascular control.

The role of the sympathetic nervous system in congestive heart failure

Congestive heart failure is usually accompanied by cardiovascular signs of an increased sympathetic and a decreased parasympathetic efferent activity. A current hypothesis for these autonomic changes holds the baroreceptor mechanisms mainly responsible for this complex neural reflex pattern together with a decreased responsiveness of cardiac vagal afferent ffibers. An alternative hypothesis is proposed here. Afferent sympathetic fibers with sensory endings in the atria and in the pulmonary veins are progressively excited by volume load. In cats with a chronic spinal section at C8, breathing spontaneously, an infusion of saline induces a reflex tachycardia through a sympatho-sympathetic neural circuit. In chronic dogs with intact cardiovascular innervation, the stimulation of aortic or cardiac sympathetic afferent fibers elicits an excitatory sympathetic reflex leading to hypertension and tachycardia; in addition, the sensitivity of baroreflexes is markedly reduced. Therefore, in congestive heart failure, especially in the absence of hypotension, the reflex excitation of the sympathetic outflow and the inhibition of the vagal efferent activity directed to the heart could be due to reflex mechanisms mediated by sympathetic cardiovascular afferents.

A positive feedback sympathetic pressor reflex during stretch of the thoracic aorta in conscious dogs

The role of pressor sympathetic reflexes in circulatory control was investigated in conscious dogs. Animals were previously instrumented with a 6- to 8-cm rigid core cannula covered by an inflatable rubber cylinder in the thoracic aorta, a pressure catheter implanted in the aorta above the cannula, and a second catheter inserted into the aorta below the cannula through a femoral artery. Two piezoelectric crystals were positioned at opposing adventitial sites to measure aortic distension with ultrasound techniques. After recovery from surgery, the diameter of the aortic segment surrounding the cannula was increased by 9.6 +/- 0.4% from 16 +/- 1 mm by inflating the rubber cylinder, without obstructing blood flow. Mean aortic pressure rose 31 +/- 3% from 100 +/- 3 mm Hg and heart rate 20 +/- 3% from 91 +/- 3 beats/min (P less than 0.01). The pressor response was abolished by alpha-adrenergic blockade (phentolamine 1 mg/kg, iv). The heart rate response was reduced either by beta-blockade (propranolol 1 mg/kg, iv) or muscarinic blockade (atropine 0.2 mg/kg, iv) and abolished by their combination. During aortic stretch, the sensitivity of the baroreflex was reduced 57 +/- 7% from 18 +/- 2 msec/mm Hg (P less than 0.01). The pressor response was increased by 49 +/- 8% after bilateral carotid sinus nerve section and vagotomy. These excitatory reflex responses were obtained in absence of any pain reaction. Thus, in the conscious dog, aortic distension within physiological ranges induces a potent pressor sympathetic reflex with positive feedback characteristics. Such a pressor reflex not only occurs in the presence of functioning baroreflexes, but is also capable of reducing their sensitivity.

A sympathetic hypertensive reflex from the heart of conscious dogs

1. The aim of the present experiment was to study in conscious animals the effect of chemical stimulation of cardiac sensory innervation by bradykinin, a physiological substance known to activate both vagal and sympathetic cardiac sensory nerve endings, at doses devoid of systemic haemodynamic effects. 2. In conscious dogs with implanted catheters bradykinin (100 ng/kg) injected into a cannulated branch of the left coronary artery induced significant (P less than 0.01, n = 5) reflex increases in mean arterial pressure and heart rate as well as increases in left ventricular pressure, left ventricular dP/dt max. and coronary blood flow. 3. These changes were obtained in the absence of pain reactions. 4. The concept, derived from experiments on anaesthetized animals, that chemical stimulation of the intact sensory supply of the heart always elicits a cardiovascular depressor reflex mediated by cardiac vagal afferents has to be modified, as pressor sympathetic reflexes may occur after an appropriate stimulus to the fully innervated heart of conscious dogs.

Functions of afferents in cardiovascular sympathetic nerves

This paper contains data and discussion of the role of afferents in the sympathetic trunks from the cardiovascular complex that evoke autonomic reflex action. A description is given of these cardiovascular reflexes induced by afferents of the sympathetic trunks in neurally intact as well as spinal animals. A positive feedback hypothesis is also proposed.

Alpha adrenergic vasoconstriction and nitroglycerin vasodilation of large coronary arteries in the conscious dog

The effects of methoxamine and nitroglycerin on measurements of large vessel (left circumflex) coronary dimensions were examined in eight conscious dogs using an ultrasonic dimension gauge, and total coronary resistance was calculated from measurements of arterial pressure and coronary blood flow. Methoxamine (50 mug/kg per min), after transiently increasing left circumflex coronary dimensions, induced sustained reductions in left circumflex diameter (9+/-2%) and external (18+/-4%) and internal (27+/-5%) cross-sectional areas, at a time when mean arterial pressure rose by 65+/-5%, left ventricular dP/dt had decreased only slightly, and heart rate and mean coronary blood flow remained at control levels. Calculated large vessel and total coronary resistances rose similarly, i.e., by 108+/-29 and 92+/-14%, respectively. Methoxamine reduced coronary arterial wall stiffness from control at comparable stress levels, although at any common radius, wall stiffness was augmented substantially. Nitroglycerin (25 mug/kg) induced an initial decrease in coronary dimensions along with the fall in arterial pressure. However, left circumflex coronary dimensions then rose, reaching a maximum 5 min later at a time when left circumflex coronary blood flow was reduced and heart rate and left ventricular dP/dt were at control levels. At this time, significantly different effects were observed on large vessel coronary resistance, which fell by 18+/-2%, and on total coronary resistance, which rose by 11+/-4%. Thus, in the conscious dog, large coronary vessels not only react passively to changes in aortic pressure but also undergo substantial active changes. Alpha adrenergic stimulation is sufficiently powerful to reduce cross-sectional area, despite the opposing elevation of distending pressure.

A sympathetic pressor reflex in the conscious dog

1. The effects of mechanical activation of aortic sympathetic sensory fibres have been studied in conscious dogs. 2. A cannula covered by an inflatable rubber balloon, previously implanted in the descending thoracic aorta, was used to stretch the aortic walls without obstructing blood flow. 3. Aortic stretch significantly increased mean arterial blood pressure and heart rate from control values in the absence of pseudo-affective reactions. 4. This response indicates that positive feedback sympathetic reflexes are operative in the conscious, fully innervated animal.

Positive feedback sympathetic reflexes and hypertension

A pressor reflex that can be elicited from the thoracic aorta in conscious dogs is described. Distension of the aorta excites sympathetic afferent fibers and results in an increase in arterial blood pressure because of increased sympathetic outflow to the heart and blood vessels. The reflex center for this positive feedback mechanism is located in the spinal cord and, when the reflex is activated, it can modulate other negative feedback control systems.

Depressed responsiveness to vasoconstrictor and dilator agents and baroreflex sensitivity in conscious, newborn lambs

The effects of vasoconstrictors and vasodilators were compared in conscious, newborn lambs and adult sheep instrumented with electromagnetic flow probes on the ascending aorta and catheters in the thoracic aorta. Methoxamine, angiotensin II, norepinephrine, nitroglycerin and isoproterenol were administered intravenously to evaluate their effects on arterial pressure, cardiac output and systemic vascular resistance (SVR). The difference in response between adults and newborns was most apparent with methoxamine. Methoxamine, 400 micrograms/kg, i.v., which increased mean arterial pressure by 57 +/- 6% and SVR by 278 +/- 27% in newborn lambs, caused greater increases (p less than 0.01) of 81 +/- 8% and 1418 +/- 141%, respectively, in the adults. Responses also differed significantly between newborn and adult animals to norepinephrine, angiotensin II, nitroglycerin and isoproterenol. In a second group of animals in which smaller amounts of methoxamine and isoproterenol were injected directly into the terminal aorta, changes in terminal aortic flow and resistance were examined. Again, both vasoconstrictor and vasodilator responses were more marked in adults than in newborns. Finally, the sensitivity of the arterial baroreceptor reflex was evaluated by comparing the regression of pulse interval (PI) on systolic arterial pressure (SAP) after an intravenous dose of methoxamine in conscious, adult and newborn animals. The PI/SAP slopes in adult sheep, 45.4 +/- 3.5 msec/mm Hg, were significantly greater (p less than 0.01) than in newborn lambs, 11.7 +/- 2.2 msec/mm Hg.

Measurement of multiple simultaneous small dimensions and study of arterial pressure-dimension relations in conscious animals

This paper describes the development of several important modifications that were incorporated into the ultrasonic, transit-time dimension system in order to obtain multiple simultaneous, instantaneous, and continuous measurements of the external dimensions of the aorta and its major branches in conscious, unrestrained animals. At operation a pair of small piezoelectric crystals was sutured to arterial adventitia, and a miniature pressure gauge was implanted in the vessel at the same cross-sectional plane. After recovery from surgery, wall motion was not altered appreciably and scarring was minimal. This technique allows long-term monitoring of aortic pressure-dimension relations and is applicable for small (fetal and neonatal) as well as large (adult dogs and sheep) animals. When vessel wall thickness is measured, stress-radius analysis can be performed so as to compute vascular elastic stiffness as a function of stress. Moreover, the suitability for radiotelemetry of the pressure and dimension signals measured with this technique enables the study of these parameters in unrestrained animals, e.g., during spontaneous severe exercise.

Initial myocardial adjustments to brief periods of ischemia and reperfusion in the conscious dog

The changes in left ventricular (LV) dynamics induced by brief periods of ischemia (100 seconds) and subsequent reperfusion were analyzed in conscious dogs. Global LV ischemia, induced by partially occluding the left main coronary artery, reduced LV flow homogeneously and impaired LV function as reflected by decreases in LV stroke "work" (89 +/- 4% M +/- SE), systolic shortening (72 +/- 4%), velocity of shortening (56 +/- 6%), LV systolic pressure (34 +/- 5%), and dP/dt (59 +/- 6%). Regional LV ischemia, induced by occluding either the left circumflex or anterior descending coronary artery completely, reduced flow to the ischemic segment (82 +/- 3%) while decreasing segment work (96 +/- 5%), shortening (82 +/- 3%), and velocity of shortening (70 +/- 5%), with minimal depression of overall LV function. In both groups the extent of shortening was reduced more rapidly and greater (P less than 0.01) than shortening velocity. Moreover, with localized ischemia, segment work was reduced more (P less than 0.01) than shortening. With reperfusion, a transient overshoot in function above preischemic control levels was observed in both groups (global work increased by 60 +/- 12% and regional work by 28 +/- 4% above control). This overshoot was not dependent on adrenergic mechanisms, but was prevented by inhibiting reactive hyperemia. Thus myocardial ischemia induces a dissociation between extent and rate of myocardial shortening. A further dissociation between shortening and work is apparent with regional ischemia. After reperfusion there is a transient overshoot in function which appears to be dependent upon the associated reactive hyperemia.

Effects of nitroglycerin on cardiac function and regional blood flow distribution in conscious dogs

The effects of intravenous infusion of nitroglycerin (NTG), 8 and 32 microgram/kg.min for 7 min, and of sublingual NTG, 1.2 mg, were examined on direct and continuous measurements of systemic, coronary, and regional hemodynamics, left ventricular (LV) dimensions, pressures, and myocardial contractility in conscious dogs. NTG induced sustained reductions in LV dimensions and transient increases in heart rate and dP/dt, and decreases in mean arterial pressure. Initially NTG increased cardiac output and flows to the coronary, mesenteric, renal, and iliac beds, while systemic and regional vascular resistances fell. Later, cardiac output, cardiac work, and mesenteric and iliac flows fell significantly below control, and significant vasoconstriction in the systemic as well as mesenteric, iliac, and coronary beds was observed at a time when LV end-diastolic dimensions were still significantly reduced. Peripheral vasoconstriction was not observed with systemic NTG in deafferented dogs or when NTG, 1 microgram/kg.min, was infused intra-arterially into the iliac bed. Thus, systemic NTG induces a biphasic response consisting of initial arteriolar vasodilation followed by vasoconstriction in the mesenteric, iliac, coronary and systemic beds, which is presumably due to longer lasting effects on preload and to secondary reflex responses to the drug.

Hemodynamic effects of intravenous sodium nitroprusside in the conscious dog

The hemodynamic effects of 7 min i.v. sodium nitroprusside (NP) were studied in conscious dogs previously instrumented for measurement of arterial pressure, cardiac output, regional blood flow distribution, left ventricular (LV) pressure, and internal dimensions. Nitroprusside, 25 microgram/kg/min, reduced mean arterial pressure by 23 +/- 3%. Cardiac output increased initially by 39 +/- 7% and returned toward control by the end of the infusion. Regional blood flows increased initially; the relative rise was greatest in the coronary (+ 225 +/- 39%), intermediate in the mesenteric (+ 98 +/- 23%) and iliac (+ 38 +/- 6%), and least in the renal (+ 10 +/- 3%) bed. By the end of the infusion period the vasodilation was unchanged in the iliac bed, less intense in the coronary and mesenteric, while in the iliac bed, blood flow was reduced and resistance was actually increased by 33 +/- 11% above control. A generalized vasonconstriction ensued after cessation of infusion. In contrast, when the drug was administered intra-arterially to the iliac bed, arterial pressure did not fall and only iliac vasodilation was observed. Peak cardiac effects were characterized by increases in heart rate and LV dP/dt, along with marked reduction in LV end-systolic diameter (- 13 +/- 2%), and in end-diastolic diameter (-17 +/- 2%) and pressure. LV end-diastolic diameter fell even heart rate was maintained at a constant rate by pacing. Thus, in the conscious dog, NP reduced LV dimensions substantially, while inducing changes in peripheral beds. The differences in these effects depend on interactions between the direct effects of NP and the opposing effects of reflex adjustments which appear sufficiently powerful to result in net constriction of the iliac bed late during the infusion.