Use of nonlinear methods to assess effects of clonidine on blood pressure in spontaneously hypertensive rats

Blood pressure reduction induced by chronic intracerebroventricular or peroral clonidine administration in rats with salt-dependent or angiotensin II-dependent hypertension

The agonists of α2-adrenergic receptors such as clonidine, rilmenidine or monoxidine are known to lower blood pressure (BP) through a reduction of brain sympathetic outflow but their chronic antihypertensive effects in rats with low-renin or high-renin forms of experimental hypertension were not studied yet. Moreover, there is no comparison of mechanisms underlying BP reduction elicited by chronic peroral (po) or intracerebroventricular (icv) clonidine treatment. Male salt-sensitive Dahl rats fed 4% NaCl diet and Ren-2 transgenic rats were treated with clonidine administered either in the drinking fluid (0.5 mg/kg/day po) or as the infusion into lateral brain ventricle (0.1 mg/kg/day icv) for 4 weeks. Basal BP and the contributions of renin-angiotensin system (captopril 10 mg/kg iv) or sympathetic nervous system (pentolinium 5 mg/kg iv) to BP maintenance were determined in conscious cannulated rats at the end of the study. Both peroral and intracerebroventricular clonidine treatment lowered BP to the same extent in either rat model. However, in both models chronic clonidine treatment reduced sympathetic BP component only in rats treated intracerebroventricularly but not in perorally treated animals. In contrast, peroral clonidine treatment reduced angiotensin II-dependent vasoconstriction in Ren-2 transgenic rats, whereas it lowered residual blood pressure in Dahl rats. In conclusions, our results indicate different mechanisms of antihypertensive action of clonidine when administered centrally or systemically.

Heart rate variability as a measure of autonomic regulation of cardiac activity for assessing stress and welfare in farm animals -- a review

Measurement of heart rate variability (HRV) is a non-invasive technique that can be used to investigate the functioning of the autonomic nervous system, especially the balance between sympathetic and vagal activity. It has been proven to be very useful in humans for both research and clinical studies concerned with cardiovascular diseases, diabetic autonomic dysfunction, hypertension and psychiatric and psychological disorders. Over the past decade, HRV has been used increasingly in animal research to analyse changes in sympathovagal balance related to diseases, psychological and environmental stressors or individual characteristics such as temperament and coping strategies. This paper discusses current and past HRV research in farm animals. First, it describes how cardiac activity is regulated and the relationships between HRV, sympathovagal balance and stress and animal welfare. Then it proceeds to outline the types of equipment and methodological approaches that have been adapted and developed to measure inter-beats intervals (IBI) and estimate HRV in farm animals. Finally, it discusses experiments and conclusions derived from the measurement of HRV in pigs, cattle, horses, sheep, goats and poultry. Emphasis has been placed on deriving recommendations for future research investigating HRV, including approaches for measuring and analysing IBI data. Data from earlier research demonstrate that HRV is a promising approach for evaluating stress and emotional states in animals. It has the potential to contribute much to our understanding and assessment of the underlying neurophysiological processes of stress responses and different welfare states in farm animals.

Aging and nonlinear heart rate control in a healthy population

In recent years more studies are using nonlinear dynamics to describe cardiovascular control. Because of the large dispersion of physiological data, it is important to have large studies with both male and female participants to establish a range of physiological healthy values. This study investigated the effect of gender and age on nonlinear indexes. Nonlinear scaling properties were studied by using 1/f slope (where f is frequency), fractal dimension, and detrended fluctuation analysis short- and long-term correlations (DFAalpha(1) and DFAalpha(2), respectively). Nonlinear complexity was described with correlation dimension (CD), Lyapunov exponent (LE), and approximate entropy (ApEn). The population consisted of 135 women and 141 men (age, 18-71 yr). Twenty-four hour ECG recordings were obtained by using Holter monitoring. The recordings were split into daytime (8 AM-9 PM) and nighttime (11 PM-6 AM). A day-night variation was present in all nonlinear heart rate variability (HRV) indexes, except for the CD in the female population. During the night the percentage of CD values of surrogate data files differing from the CD value of the original data increased. All nonlinear indexes were significantly correlated with age. Deeper analysis per age category of 10 yr showed a stabilization in the age decline of the fractal dimension and ApEn at the age of > or =40 yr. The vagal pathways seemed to be more involved in the generation of nonlinear fluctuations. Higher nonlinear behavior was evident during the night. No clear difference between men and women was found in the nonlinear indexes. Nonlinear indexes decline with age. This can be related to the concept of decreasing autonomic modulation with advancing age.

Linear and nonlinear approaches to the analysis of R-R interval variability

Analysis techniques derived from linear and non-linear dynamics systems theory qualify and quantify physiological signal variability. Both clinicians and researchers use physiological signals in their scopes of practice. The clinician monitors patients with signal-analysis technology, and the researcher analyzes physiological data with signal-analysis techniques. Understanding the theoretical basis for analyzing physiological signals within one's scope of practice ensures proper interpretation of the relationship between physiolgical function and signal variability. This article explains the concepts of linear and nonlinear signal analysis and illustrates these concepts with descriptions of power spectrum analysis and recurrence quantification analysis. This article also briefly describes the relevance of these 2 techniques to R-to-R wave interval (i.e., heart rate variability) signal analysis and demonstrates their application to R-to-R wave interval data obtained from an isolated rat heart model.

Prediction of severity of mean arterial pressure elevation after tracheal intubation in hypertensive patients by preanesthetic recurrence quantification analysis of heart rates

BACKGROUND

We tried to determine whether the severity of additional mean arterial pressure (MAP) elevation after endotracheal intubation during general anesthesia could be predicted with preanesthetic recurrence quantification analysis of heart rates in hypertensive and normotensive patients.

METHODS

Twenty-four newly diagnosed hypertensive patients and 34 normotensive patients were included. We defined DeltaMAP = maximal mean MAP after endotracheal intubation - average MAP before general anesthesia. We classified each of the hypertensive and normotensive patients into mild (DeltaMAP </= 35 mmHg [median value of DeltaMAP in the hypertensive patients]) and severe group (DeltaMAP > 35 mmHg). Using preanesthetic RR interval data, we calculated classical linear and non-linear indices of RR interval dynamics, and performed recurrence quantification analysis to calculate three preanesthetic recurrence indices, percentage of recurrence (%REC), percentage of determinism (%DET), and maximal length of recurrence (Lmax).

RESULTS

In the hypertensive patients, all preanesthetic classical indices showed no significant difference between the mild and severe group and showed no significant linear correlation with the DeltaMAP. However, the Lmax was significantly higher in the severe than in the mild group (16.10 +/- 3.79 vs. 7.90 +/- 0.73, P < 0.005) and, moreover, linearly correlated with the DeltaMAP (r = 0.671, P = 0.001). In the normotensive patients, all preanesthetic classical and recurrence indices showed no significant difference between the two groups.

CONCLUSION

In hypertensive patients, the severity of additional MAP elevation after tracheal intubation during general anesthesia can be predicted by the preanesthetic recurrence quantification analysis of heart rates.

Hemodynamic and cardiac anti-hypertrophic actions of clonidine in Goldblatt one-kidney, one-clip rats

In congestive heart failure, the chronic sympathetic hyperactivity contributes to a poor prognosis. In this respect, clonidine, a centrally acting sympathoinhibitory drug, has previously been tested in clinical trials. The aim of the current study was to evaluate the effects of clonidine on morbidity and mortality in an experimental model of cardiac hypertrophy associated with hypertension, renal failure, and intense sympathetic activation. One-kidney, one-clip Goldblatt hypertensive rats were treated orally with clonidine (200 microg/kg/d) during 30 days and were compared with untreated rats and with sham-operated animals. Cardiac hemodynamics, left ventricular volume and elasticity, cardiac morphometry and histology, and renal function were evaluated. A survival study was also performed. Clonidine normalized cardiac function, ventricular stiffness, and prevented ventricular structural remodeling. Moreover, despite a marked renal function impairment, survival of the animals was increased in the clonidine group. The centrally acting sympathoinhibitory drug clonidine exhibited marked cardioprotective properties. This study emphasized the interest of evaluating drugs whose aim is to treat congestive heart failure, in an experimental model of cardiac and renal failure.

[Comparison of spectral analysis and non-linear analysis of EEG in patients with cognitive decline]

Twenty-five elder subjects were classified in two groups according to the MMS score and the cognitive evoked potentials. Normal subjects (n = 15) had mean MMS = 27.6 and mean P3 amplitude = 7.1 uV), while patients with cognitive decline (n = 10) had respective values of 18 (MMS) and 3.3 uV (P3). Spectral analysis and non-linear analysis of EEG (recurrence plots of dynamical systems) were performed and both showed statistically significant differences between groups for all the parameters analysed. Subjects' classification with discriminant analysis was slightly better using the non-linear parameters. The recurrence plot method applied to EEGs, gave similar results as the dimension of correlation (D2) calculation, and was in favour of a more constraint and less complex dynamics of brain activity associated with cognitive decline.

Heart rate variability: a noninvasive approach to measure stress in calves and cows

The aim of this study was to evaluate the usefulness of heart rate variability (HRV) and its specific parameters as a new approach to assess stress load in cattle. We recorded HRV in 52 calves in three groups and in 31 cows in two groups. In calves we divided Group 1 with no obvious stress load (n=18), Group 2 with external stress load (n=17), and Group 3 with internal stress load from sickness (n=17). In cattle we divided lactating cows (n=21) and nonlactating cows (n=10). HRV parameters were analyzed in the time domain and in the frequency domain. Moreover, we applied Recurrence Quantification Analysis (RQA) to quantify nonlinear components of HRV. In calves, linear HRV parameter decreased from Group 1 to Group 3 (P<.05). However, not a single parameter showed significant differences regarding all three groups. The value of all nonlinear measurements increased at the same time (P<.05). The only parameter that exhibited significant differences between all three groups was the longest diagonal line segment in the recurrence plot (L(MAX)) which is inversely related to the Lyapunov exponent. We did not find differences concerning the linear HRV parameters between the two groups in the cows. The nonlinear parameter Determinism showed significant higher values in lactating cows compared to nonlactating cows. The importance of particular HRV-parameters was tested by applying a discriminant analysis approach. In calves and cattle nonlinear parameters were most important to indicate the level of stress load on the animals. Based on the results we assume HRV to be a valuable physiological indicator for stress load in animals. Whereas linear parameters of HRV are supposed to be useful to separate qualitative different level of stress, nonlinear components of HRV distinguish quantitative different challenges for the animals.

Quantification of sympathetic and parasympathetic tones by nonlinear indexes in normotensive rats

Because the use of spectral powers of blood pressure (BP) and R-R interval (RR) in the low (LF) and high frequencies (HF) to quantify sympathetic and parasympathetic activities is still under debate, we questioned whether nonlinear methods may give better results. The BP signal was recorded for 30 min before and after intravenous injection of hexamethonium (20 mg/kg), atropine (0.5 mg/kg), atenolol (1 mg/kg), and prazosin (1 mg/kg) in conscious, normotensive Wistar-Kyoto rats. Three nonlinear indexes [percentage of recurrence, percentage of determinism, and length index (Lmax)] extracted from the recurrence plot method were used to analyze the BP signal. Sympathetic but not parasympathetic blockade reduced BP level and its LF component. RR increased and decreased after beta- and alpha-blockades, respectively. Hexamethonium increased HF, and atropine reduced LF, of RR. Sympathetic blockade and, in particular, alpha-sympathetic blockade increased nonlinear indexes of BP. In contrast, parasympathetic blockade by atropine increased nonlinear indexes of RR. These results suggest that, compared with spectral indexes, nonlinear indexes may be more specific markers of sympathetic and parasympathetic tones.