Antihypertensive drugs and baroreceptor reflex control of heart rate and blood pressure

Comparability of Heart Rate Turbulence Methodology: 15 Intervals Suffice to Calculate Turbulence Slope – A Methodological Analysis Using PhysioNet Data of 1074 Patients

Heart rate turbulence (HRT) is a characteristic heart rate pattern triggered by a ventricular premature contraction (VPC). It can be used to assess autonomic function and health risk for various conditions, e.g., coronary artery disease or cardiomyopathy. While comparability is essential for scientific analysis, especially for research focusing on clinical application, the methodology of HRT still varies widely in the literature. Particularly, the ECG measurement and parameter calculation of HRT differs, including the calculation of turbulence slope (TS). In this article, we focus on common variations in the number of intervals after the VPC that are used to calculate TS (#TSRR) posing two questions: 1) Does a change in #TSRR introduce noticeable changes in HRT parameter values and classification? and 2) Do larger values of turbulence timing (TT) enabled by a larger #TSRR still represent distinct HRT? We compiled a free-access data set of 1,080 annotated long-term ECGs provided by Physionet. HRT parameter values and risk classes were determined both with #TSRR 15 and 20. A standard local tachogram was created by averaging the tachograms of only the files with the best heart rate variability values. The shape of this standard VPC sequence was compared to all VPC sequences grouped by their TT value using dynamic time warping (DTW) in order to identify HRT shapes. When calculated with different #TSRR, our results show only a little difference between the number of files with enough valid VPC sequences to calculate HRT (<1%) and files with different risk classes (5 and 6% for HRT0-2 and HRTA-C, respectively). In the DTW analysis, the difference between averaged sequences with a specific TT and the standard sequence increased with increasing TT. Our analysis suggests that HRT occurs in the early intervals after the VPC and TS calculated from late intervals reflects common heart rate variability rather than a distinct response to the VPC. Even though the differences in classification are marginal, this can lead to problems in clinical application and scientific research. Therefore, we recommend uniformly using #TSRR 15 in HRT analysis.

Impact of antihypertensive treatment on resiliency to clinical pain

OBJECTIVES

Although progress has been made in the regulation of hypertension over the past decades, the USA and some other countries have faced a significant rise in incidence of chronic pain management cases during the same period. Studies of the relationship between pain and blood pressure (BP) regulations propose that these two processes may be interconnected. Studies of effects of antihypertensive treatment on pain in general and its chronification have never been reviewed.

METHODS

A narrative review of respective studies with analysis of credibility of the findings was conducted.

RESULTS

Some studies have suggested that aggressive reduction of high BP may contribute to a return in pain symptoms and may require more aggressive, long-term pain management. Other studies propose that long-term antihypertensive medication could also increase the risk for new cases of chronic pain. Pain initiates a central neuroplastic resetting of the baroreceptor activation accounting for sustained increase of BP with an adaptive 'pain-killing' or maladaptive 'pain-complication' effect associated with pain chronification, and these mechanisms may be moderated by antihypertensive medications. However, different antihypertensive drugs and nondrug treatments may diversely affect pain mechanisms at different stages of treatments.

CONCLUSION

Uncontrollable reduction of high BP in some patients with hypertension could increase the risk for chronic pain incidence and its severity. Practical recommendations in BP control should be reconsidered to take into account patients' chronic pain. Further research is needed of moderation effects of different antihypertensive manipulations on pain to improve pain management in these patients.

Proof of Principle for a Novel Class of Antihypertensives That Target the Oxidative Activation of PKG Iα (Protein Kinase G Iα)

Arterial hypertension continues to be a major health burden. Development of new antihypertensive drugs that engage vasodilatory mechanisms not harnessed by available therapies offer therapeutic potential. Oxidants induce an interprotein disulfide in PKG Iα (protein kinase G Iα) at C42, which is associated with its targeting and activation, resulting in vasodilation and blood pressure lowering. Consequently, we developed an assay and screened for electrophilic drugs that activate PKG Iα by selectively targeting C42, as such compounds have potential as novel antihypertensives with a mechanism of action that differs from current therapies. In this way, a drug that we termed G1 was identified, which targets C42 of PKG Iα to induce vasodilation of isolated resistance blood vessels and blood pressure lowering in a mouse model of angiotensin II-induced hypertension. In contrast, these antihypertensive effects were deficient in angiotensin II-induced hypertensive C42S PKG Iα knockin mice. These transgenic mice were engineered to have the reactive cysteinyl thiol replaced with a hydroxyl so that it cannot react with endogenous vasodilatory oxidants or electrophiles such as drug G1. These studies, therefore, provide validation of PKG Iα C42 as the target of G1, as well as proof-of-principle for a new class of antihypertensive drugs that have potential for further development for clinical use in humans.

Rocuronium-induced neuromuscular block after long pretreatment of clonidine in rabbits

Background

Clonidine, an α-2 adrenergic agonist, is used in the perioperative period and in intensive care for the management of hypertension. The in vivo and in vitro effects of clonidine on the actions of nondepolarizing neuromuscular blocking drugs are conflicting. We evaluated the potency and time course of rocuronium-induced neuromuscular block after prolonged pretreatment with clonidine in rabbits.

Methods

Sixty rabbits were randomly assigned to three groups; control (C) group: normal saline 0.1 ml/kg daily subcutaneous for 6 weeks; S3 group: clonidine 4 µg/kg daily subcutaneous for 3 weeks; S6 group: clonidine 4 µg/kg daily subcutaneous for 6 weeks. The dose-response relations of rocuronium were tested in 30 rabbits (10 from each of the three groups) during ketamine-thiopental anesthesia, while the time course of rocuronium 0.6 mg/kg was examined in 10 rabbits each from the three groups.

Results

There was no difference in mean arterial pressure and pulse rate among the experimental groups. The calculated ED₅₀ for rocuronium decreased significantly from 64.1 µg/kg (C group) to 50.3 µg/kg (S3 group) and 47.8µg/kg (S6 group) (P < 0.001). There was no difference in the onset and the recovery times after rocuronium.

Conclusions

Rocuronium after pretreatment with clonidine for three or six weeks may have an increased effect, but no difference in the duration of action compared with control group.

Arterial stiffness and orthostatic blood pressure changes in untreated and treated hypertensive subjects

Carotid-femoral pulse wave velocity (PWV), an integrated marker of segmental aortic stiffness, was recently proposed as one of the underlying mechanisms inducing orthostatic hypotension in the elderly with marked arterial rigidity. We examined the relationship between PWV (Complior; Colson, Paris, France) and orthostatic blood pressure (BP) changes, measured repeatedly, over a wide range of age and arterial stiffness. Sixty-nine hypertensive subjects (age, 37 to 76 years; 39 untreated and 30 treated) were studied. BP, in both sitting and erect position, was measured at two occasions a few weeks apart, and in between PWV was assessed by means of pulse wave analysis. In untreated hypertensive subjects, the orthostatic alterations in systolic, but not in diastolic blood pressure (DBP), were inversely related to PWV, independently from age, gender, mean BP, and diabetes mellitus. The greater the aortic stiffness the larger was the systolic blood pressure (SBP) decrease during upraises. On the contrary, no such association was found between PWV and orthostatic changes of BP in treated hypertensive subjects. These results suggest the presence of a pathophysiological association between arterial stiffening and BP postural changes. Antihypertensive drug treatment, as well as other factors that have not been evaluated in the present study, might have modulated this association. However, it might be argued that a causal association between arterial stiffness - disturbed baroreflex sensitivity - postural BP changes, even in subjects without pronounced vascular aging or orthostatic hypotension, is implied.

[To understand blood pressure and heart rate variability]

OBJECTIVES

To review current data on the heart rate and blood pressure variability.

DATA SOURCES

Search through Medline databases of articles in french or english.

DATA SELECTION

Original articles and case reports were selected according to their quality and main advances. The articles were analysed in order to obtain current data about the methods of study and clinical application of blood pressure and heart rate variability.

DATA SYNTHESIS

Various regulatory systems in the cardiovascular system play crucial roles in controlling and assuring adequate perfusion of the peripheral tissues. Among them the baroreceptor reflex is the most important regulatory mechanism in the short-term control of the heart rate and blood pressure, and operates through the autonomic nervous system. The gain of the cardiac baroreflex further referred to, as baroreflex sensitivity is an interesting way to study this system. Unfortunately, with our current knowledge, it is not possible to predict the instantaneous output of the baroreceptor in response to instantaneous changes in input within a frequency range of physiological importance. The fast Fourier transform can describe variables as the sum of elementary oscillatory components and it has been established as practical clinical methods for detecting abnormalities in cardiovascular control. A time-frequency distribution provides an indication of how the spectral energy distribution varies with time and it is an interesting tool in non-stationary data. One of the major motivations behind spectral analysis is the hope that the combination of time-domain and frequency-domain analyses will provide dynamical informations about the relation between blood pressure and heart rate.

Down-regulation of basal Fos expression at nucleus tractus solitarii underlies restoration of baroreflex response after antihypertensive treatment in spontaneously hypertensive rats

Antihypertensive therapy not only normalizes the elevated blood pressure but also restores the reduced baroreceptor reflex response associated with hypertension, although the underlying mechanism is not fully understood. We assessed the hypothesis that a reversal of the enhanced basal Fos expression seen during hypertension in nucleus tractus solitarii, the terminal site of baroreceptor afferents, underlies the restoration of baroreceptor reflex sensitivity after antihypertensive treatment. Male adult spontaneously hypertensive or normotensive Wistar-Kyoto rats received for 3 weeks captopril (100 mg/kg/day) added to their drinking water. Evaluated subsequently under pentobarbital anesthesia, captopril-treated spontaneously hypertensive rats exhibited significantly lowered systolic blood pressure and restoration of the sensitivity in baroreceptor reflex control of heart rate to levels comparable with Wistar-Kyoto rats. Reverse transcription-polymerase chain reaction analysis and immunohistochemical evaluation revealed concomitant down-regulation of basal expression in nucleus tractus solitarii of c-fos gene at both mRNA and protein levels. Captopril treatment, on the other hand, elicited no discernible effect on systolic blood pressure, cardiac baroreceptor reflex sensitivity or basal expression of Fos protein at the nucleus tractus solitarii of normotensive Wistar-Kyoto rats. From these findings we suggest that a down-regulation of basal Fos expression in nucleus tractus solitarii may contribute to the restoration of baroreceptor reflex sensitivity in spontaneously hypertensive rats that received antihypertensive treatment such as captopril.

Changes in short-term variability of blood pressure and heart rate during the development of obesity-associated hypertension in high-fat fed dogs

OBJECTIVE

To investigate the nature and time course of autonomic nervous system changes elicited by a 21-week ad libitum high-fat diet (HFD) in dogs.

RESULTS

The HFD increased body weight (+22.0+/-2.8% at week 21) with an abdominal circumference gain significantly more elevated than the thoracic one. The increases in insulin and free fatty acid plasma levels were correlated with body weight changes. Systolic and diastolic blood pressures and heart rate significantly increased (+23+/-6, +28+/-5 and 19+/-9% respectively). Arterial hypertension was characterized by an increase in cardiac output (+22.3+/-7.7%), in left ventricular mass (+18.1+/-5.0% at week 21) and a decrease in spontaneous baroreflex efficiency (-55+/-6%). The time course of autonomic changes (using spectral analysis of systolic blood pressure and heart rate) showed the existence of time-dependent modifications, which were linked with food intake. The initial rise in arterial blood pressure during body weight increment (observed between the 1st and 8th week of HFD) was associated with a transient increase in the low frequency band of systolic blood pressure variability and noradrenaline plasma levels associated with a long-lasting decrease in the high frequency band of heart rate variability. Early changes in short-term variability were significantly correlated with free fatty acid plasma levels. In contrast, the steady-state of obesity-related hypertension was associated with a decreased high frequency band of heart rate variability, without significant changes in noradrenaline plasma levels.

CONCLUSIONS

This study shows that the HFD induces abdominal obesity, hyperinsulinaemia and arterial hypertension, with a left ventricular hypertrophy associated with a biphasic changes in autonomic activity: an early and long-lasting decrease in parasympathetic nervous system activity and an early but transient increase in sympathetic activity. The present data suggest that autonomic nervous system changes are dependent on the time course of obesity development.

Endogenous central cholinergic systems and baroreflex modulation in the conscious dog

Intracisternal atropine and mecamylamine failed to modify baroreflex sensitivity in dogs, suggesting a lack of cholinergic neurons integrated in baroreflex pathways. Conversely, neostigmine lowered baroreflex sensitivity suggesting a putative cholinergic modulation.

Arterial baroreceptor reflex and calcium antagonists

The tachycardia elicited by decreases in aortic pressure produced by various calcium antagonists, regardless of their chemical structures, is not always simply related to the reduction in pressure. Several experimental and clinical experiments have clearly demonstrated that some of these compounds interfer with the baroreceptor reflex control of heart rate. The voltage-dependent calcium channel blockade induced by these antagonists is probably not involved in the baroreceptor mechanotransduction, but central modulation of the baroreceptor reflex is probably an important site for action of these drugs at therapeutic levels. During chronic treatment of hypertension with calcium antagonists, a resetting of the reflex occurred with parallel shift of the set-point and of the baroreceptor reflex-response curve towards lower pressures, thus explaining the lack of change in heart rate despite a permanent reduction in blood pressure under these conditions. However, these effects of calcium antagonists on the baroreceptor reflex do not provide evidence that such interactions could play a role in their antihypertensive action during chronic therapy.

Plasma catecholamines and adrenoceptors after chronic sinoaortic denervation in dogs

Chronic sinoaortic denervation (SAD) performed by section of both carotid and aortic nerves induced a significant and sustained increase in blood pressure and heart rate in conscious dogs; under our experimental conditions, the values of systolic blood pressure and heart rate were never lower than 190 mmHg and 120 beats/min, respectively. The present long-term study (8 mo) investigated the time-course of plasma catecholamine levels and circulating blood cell adrenoceptor (leukocyte beta 2 and platelet alpha 2) number. Catecholamine plasma levels were highly correlated with the variations of leukocyte beta adrenoceptors (measured by [125I]cyanopindolol binding and characterized as a beta 2 receptor in dogs). These two parameters followed a biphasic pattern after SAD: during the first 2 mo, significant increases in noradrenaline and adrenaline levels were associated with a decrease in leukocyte beta 2 adrenoceptor number; from the 4th month, they slowly returned to normal values, although blood pressure remained elevated. By contrast, platelet alpha 2 adrenoceptor number (measured by [3H]yohimbine binding) did not follow this pattern since it remained lower than before SAD during the whole experiment. Finally, the development (but not the maintenance) of SAD-induced hypertension is associated with an increase in sympathetic tone. Only leukocyte beta 2 (and not platelet alpha 2) adrenoceptors are directly regulated by the endogenous levels of catecholamines.