Studies of stress in farm animals

Major changes have taken place in animal production over the last three decades. Housing conditions have changed dramatically over this period, and there has been a striking increase in production. Agricultural animals try to cope with these highly demanding conditions (stressors) using behavioural and physiological stress responses aiming to restore homeostasis. When these responses are not successful or when they are thwarted, typical behavioural and physiological symptoms of chronic stress occur. In this situation, the welfare of the animal is clearly at stake. Moreover, chronic stress may seriously affect the efficiency of animal production and the quality of the product. Ultimately, detailed knowledge of stress responses in agricultural animals will allow the formulation of housing and management requirements, including handling by humans, which will benefit welfare and health of farm animals as well as production efficiency. This paper briefly addresses some important issues regarding the study of stress and welfare in farm animals, and discusses some recently developed experimental methods as well as relevant results obtained in our laboratory. Originally presented at ECCP 97.     

Heart rate complexity: A novel approach to assessing cardiac stress reactivity

Correlation dimension (D2), a measure of heart rate (HR) complexity, has been shown to decrease in response to acute mental stress and relate to adverse cardiovascular health. However, the relationship between stress‐induced changes in D2 and HR has yet to be established. The present studies aimed to assess this relationship systematically while controlling for changes in respiration and autonomic activity. In Study 1 (N = 25) D2 decreased during stress and predicted HR reactivity even after adjusting for changes in respiration rate, and cardiac vagal tone. This result was replicated in Study 2 (N = 162) and extended by including a measure of cardiac sympathetic activity; correlation dimension remained an independent predictor of HR reactivity in a hierarchical linear model containing measures of cardiac parasympathetic and sympathetic activity and their interaction. These results suggest that correlation dimension may provide additional information regarding cardiac stress reactivity above that provided by traditional measures of cardiac autonomic function.     

Genetic influences on heart rate variability at rest and during stress

We tested whether the heritability of heart rate variability (HRV) under stress is different from rest and its dependency on ethnicity or gender. HRV indexed by root mean square of successive differences (RMSSD) and high-frequency (HF) power was measured at rest and during 3 stressors in 427 European and 308 African American twins. No ethnic or gender differences were found for any measures. There was a nonsignificant increase in heritability of RMSSD (from 0.48 to 0.58) and HF (from 0.50 to 0.58) under stress. Up to 81% and 60% of the heritabilities of RMSSD and HF under stress could be attributed to genes influencing rest levels. The heritabilities due to genes expressed under stress were 0.11 for RMSSD and 0.23 for HF. The findings suggest that, independent of ethnicity and gender, HRV regulation at rest and under stress is largely influenced by the same genes with a small but significant contribution of stress-specific genetic effects.     

Alteration of cardiac autonomic functions in patients with major depression: a study using heart rate variability measures

BACKGROUND: Depression is associated with greater cardiac morbidity and mortality. One of the contributory factors for this may be altered cardiac autonomic activity in depression. However, cardiac autonomic involvement in depression remains controversial because of methodological issues. In this study, alteration of cardiac autonomic functions was studied in drug-naive patients with major depression without co-morbidity. Heart rate variability, a sensitive measure of neurocardiac autonomic regulation was used in addition to conventional methods of measuring cardiac autonomic functions. METHODS: We recruited 40 patients suffering from major depression, diagnosed based on DSM-IV-TR criteria. Their cardiac autonomic functions were measured using both conventional and heart rate variability measures. These were compared with those of age- and gender-matched healthy controls. RESULTS: Patients with major depression showed significantly lesser Valsalva ratio, maximum/minimum ratio and greater sympathovagal balance than healthy controls indicating decreased parasympathetic and increased sympathetic activity. CONCLUSIONS: Depression is associated with alteration of cardiac autonomic tone towards decreased parasympathetic activity and an increased sympathetic activity. It is possible that a common neurobiological dysfunction contributes to both depression and cardiac autonomic changes in the illness.     

Heart rate variability in relation to cognition and behavior in neurodegenerative diseases: A systematic review and meta-analysis

Neurodegenerative diseases, which frequently present with neuropsychiatric symptoms related to prefrontal cortical dysfunction, can alter the integrity of the neural networks involved in central autonomic nervous system regulation, which is proposed to be indexed by heart rate variability (HRV). We systematically reviewed the characteristics, methodology and outcomes of 27 studies of HRV in relation to measures of cognition and behavior in neurodegenerative conditions, and assessed the strength of this relationship, cross-sectionally, across 18 studies. A significant, moderate effect was observed (r = 0.25), such that higher HRV was related to better cognitive and behavioral scores, which was not influenced by mean age or cognitive status. There was no evidence of small-study effects but we could not rule out publication bias, and other factors may have contributed to heterogeneity between studies. Our findings support the proposal that HRV may be a marker of self-regulatory processes in neurodegenerative conditions, and further research on this association is needed in relation to neuropsychiatric symptoms and alongside neuroimaging methods.     

Validation of a cardiac monitor for measuring heart rate variability in adult female pigs: accuracy, artefacts and editing

Autonomic regulation of cardiac activity during stress has not been clearly defined in farm animals. In part, this is due to the limited availability of affordable ambulatory cardiac monitors capable of accurately monitoring and storing large amounts of data that meet the criteria necessary for heart rate variability analysis. Our objectives were to measure the accuracy of a 24-h Polar RR monitor using gold standard ECG, to examine and categorise any occurring anomalies and to ascertain their impact on the outcome of heart rate variability analysis. Five 1-year-old female pigs (gilts) were socially isolated from their pen mates and cardiac activity was simultaneously measured using two systems, a 24-h Polar RR Recorder and a Telemetric ECG system. The Polar data were manually assessed both against and in isolation of the ECG data to identify anomalous beats, which were then assigned to one of five identified error categories. The anomalies in the Polar data were corrected and statistical comparisons were performed among the three data sets to evaluate the effects of anomalies on heart rate variability analysis. Bland-Altman analysis was used to measure the level of agreement among the ECG, Uncorrected Polar and Corrected Polar data. No anomalies or ectopies were found in the ECG data but 46 anomalies (0.81% of total interbeat intervals [IBI]) were found in the Polar Uncorrected data. Manual identification and editing procedures reduced this error to 0.018%. Most mean heart rate and IBI parameters were unaffected by error (P>.05). Standard deviation (S.D.) and root mean square of successive differences (RMSSD) were 45% and 50% higher when anomalies were present in the data. Artefacts affected the magnitude of the frequency domain indices and overestimated total and parasympathetic activity and underestimated sympathetic activity. The mean difference between ECG and Uncorrected Polar data was 1.36 ms (limits of agreement -69.03 to 71.74 ms). This was greatly improved to 0.36 ms (limits of agreement -5.37 to 6.10 ms) after editing. Overall, even a small proportion of error biased the outcome of heart rate variability analysis. This bias was greatly reduced by correcting the anomalous beats. Bland-Altman analysis demonstrated that when there was error present in the Polar data, it could not be used interchangeably with the ECG data. However, if there were no anomalies present in the data or if they were classified and corrected using the approach in this study, then the two systems could be used interchangeably.     

Heart rate variability: a review

Heart rate variability (HRV) is a reliable reflection of the many physiological factors modulating the normal rhythm of the heart. In fact, they provide a powerful means of observing the interplay between the sympathetic and parasympathetic nervous systems. It shows that the structure generating the signal is not only simply linear, but also involves nonlinear contributions. Heart rate (HR) is a nonstationary signal; its variation may contain indicators of current disease, or warnings about impending cardiac diseases. The indicators may be present at all times or may occur at random—during certain intervals of the day. It is strenuous and time consuming to study and pinpoint abnormalities in voluminous data collected over several hours. Hence, HR variation analysis (instantaneous HR against time axis) has become a popular noninvasive tool for assessing the activities of the autonomic nervous system. Computer based analytical tools for in-depth study of data over daylong intervals can be very useful in diagnostics. Therefore, the HRV signal parameters, extracted and analyzed using computers, are highly useful in diagnostics. In this paper, we have discussed the various applications of HRV and different linear, frequency domain, wavelet domain, nonlinear techniques used for the analysis of the HRV.     

Heart rate and heart rate variability during a novel object test and a handling test in young horses

Forty-one Dutch Warmblood immature horses were used in a study to quantify temperamental traits on the basis of heart rate (HR) and heart rate variability (HRV) measures. Half of the horses received additional training from the age of 5 months onwards; the other half did not. Horses were tested at 9, 10, 21 and 22 months of age in a novel object and a handling test. During the tests, mean HR and two heart variability indices, e.g. standard deviation of beat-to-beat intervals (SDRR) and root mean square of successive beat-to-beat differences (rMSSD), were calculated and expressed as response values to baseline measures. In both tests, horses showed at all ages a significant increase in mean HR and decrease in HRV measures, which suggests a marked shift of the balance of the autonomic nervous system towards a sympathetic dominance. In the novel object test, this shift was more pronounced in horses that had not been trained. Furthermore, statistical analysis showed that the increase in mean HR could not be entirely explained by the physical activity. The additional increase in HR, the nonmotor HR, was more pronounced in the untrained horses compared to the trained. Hence, it is suggested that this nonmotor HR might be due to the level of emotionality. HR variables showed consistency between years, as well as within the second year. These tests bring about a HR response in horses, part of which may indicate a higher level of emotionality; and horses show individual consistency of these HR variables over ages. Therefore, it is concluded that mean HR and HRV measures used with these tests quantify certain aspects of a horse's temperament.     

Identification of low and high frequency ranges for heart rate variability and blood pressure variability analyses using pharmacological autonomic blockade with atropine and propranolol in swine

Understanding autonomic nervous system functioning, which mediates behavioral and physiological responses to stress, offers great potential for assessing farm animal stress and welfare. Evaluation of heart rate variability (HRV) and blood pressure variability (BPV), using time and frequency domain analyses may provide a sensitive and reliable measure of affective states and stress-mediated changes in sympathetic and parasympathetic tones. The aim of this research was to define low (LF) and high frequency (HF) power spectral ranges using pharmacological autonomic blockade, and to examine HRV and BPV parameter changes in response to atropine and propranolol in swine. Ten, 13-week old, barrows (n = 6) and gilts (n = 4) underwent surgery to place an intra-cardiac electrode and a blood pressure catheter attached to a biotelemetric transmitter; pigs had a 3-week recovery period prior to data collection. Each pig was subjected to administration of 4 intravenous (i.v.) drug treatments: a control treatment, 3 mL of saline, and 3 blockade treatments; 0.1 mg/kg of atropine, 1.0 mg/kg of propranolol, and .1 mg/kg of atropine together with 1.0 mg/kg of propranolol. All treatments were delivered by injection in the jugular vein with a minimum of 48 h between individual treatments. Behavior, ECG and blood pressure data were recorded continuously for a total of 1 h, from 30 min pre-injection to 30 min post-injection. For data analyses, two 512-beat intervals were selected for each treatment while the pig was lying and inactive. The first interval was selected from the pre-injection period (baseline), and the second was selected between 10 and 30 min post-injection. Time and frequency domain (power spectral density) analyses were performed on each data interval. Subsequent, LF and HF bands from the power spectral densities were defined based on general linear and regression analyses. The HRV and BPV were computed with a covariate (baseline) factorial analysis of treatment by sex interaction, and day of injection, with mixed models and Tukey's post-hoc tests. The best-fit range for LF was 0.0-0.09 Hz, and HF was 0.09-2.0 Hz (r²: 0.41 and 0.43, respectively). Propranolol and saline injections led to a greater overall total power and overall higher inter-beat interval, HF and LF power. Atropine led to a dominant sympathovagal balance of the cardiac activity in pigs. In addition, atropine led to an increase in LF power of both systolic and diastolic blood pressures in gilts suggesting vagal tone mediation of BPV. The understanding of autonomic regulation of HRV and BPV in domestic swine facilitates our ability to detect and quantify stress responses, and broadens its application in assessing farm animal welfare.     

Older people's cardiac responses as indicators of stress in familiar and unfamiliar environments

Little is known about older people's physiological and emotional responses to environmental triggers. We examined this by estimating cardiac stress from heart rate variability (HRV). Thirty-eight participants were monitored while observing environmental scenarios at familiar and unfamiliar locations. Image scenarios included pedestrian and driving scenes in a random order. HRV indices including heart rate (HR), QT variability index (QTVI) and Total HRV Power (TP) were quantified. Familiar locations were associated with higher HR (p<.0005) and lower TP (p=.005) than unfamiliar locations, suggesting they were more stressful. HRV responses to pedestrian and driving scenarios indicated that stress was not influenced by either the type of image scenario or the order of image presentation. There were no gender-related differences in cardiac responses. HRV is a useful surrogate of cardiac stress when assessing older people's responses to their environments.     

Sex differences in cardiac autonomic function of depressed young adults

Background

Cardiac autonomic dysfunction has been proposed as an important contributing factor to the increased cardiovascular risk observed in major depression (MDD). However, the evidence regarding alterations in heart rate variability (HRV) in otherwise healthy depressed subjects has been inconclusive.

Methods

A case–control study in 50 treatment-naïve young adults with a first MDD episode without comorbid psychiatric disorders and 50 healthy control subjects was conducted. Time- and frequency-domain indexes of HRV were determined at baseline supine and after 5-min of orthostatic stress at 60°.

Results

There were no significant differences in the time- or frequency-domain variables of HRV between depressed patients and controls. However, a random-effect ANOVA model showed that during orthostatic stress depressed men had a reduced HRV and decreased parasympathetic activity compared to control subjects, while no differences were found between depressed women and controls.

Conclusion

These results suggest a sex-dependent relationship between major depression and cardiac autonomic dysfunction and provide one potential explanation for sex differences in the association of depressive symptoms with cardiovascular morbidity.     

Heart rate variability during dynamic exercise in elderly males and females

It has been proposed that cardiac control is altered in the elderly. Power spectral analysis of heart rate variability (HRV) was performed on 12 male and 11 female elderly subjects (mean age 74 years) while at rest in supine and sitting positions, and at steady states during 5 min of exercise (35–95% peak oxygen consumption, V˙O_2peak). There were no differences in power, measured as a percentage of the total of the high frequency peak (HF, centred at about 0.25 Hz; 13% in males vs 12% in females), low frequency peak (LF, centred at 0.09 Hz; 25% in males and 22% in females), and very low frequency component (VLF, at 0.03 Hz; 66% in males and 69% in females) between body positions at rest. There was no difference in spectral power between male and female subjects. Total power decreased as a function of oxygen consumption during exercise, LF% did not change up to about 14 ml · kg⁻¹ · min⁻¹ (40% and 80% V˙O_2peak in males and females, respectively), then decreased towards minimal values in both genders. HF% power and central frequency increased linearly with metabolic demand, reaching higher values in male subjects than in female subjects at V˙O_2peak, while VLF% remained unchanged. Thus, the power spectra components of HRV did not reflect the changes in autonomic activity that occur at increasing exercise intensities, confirming previous findings in young subjects, and indicated similar responses in both genders. Accepted: 30 November 1999     

Heart rate as an indicator of the intensity of physical activity in human adolescents

The aims of this study were, in a group of adolescents, firstly to identify the absolute heart rates (HR) and the percentages of maximal heart rates (HR_max) corresponding to 40%, 60% and 80% of peak oxygen uptake ( ), secondly to identify absolute and relative ( ) oxygen uptakes ( ) corresponding to HR of 120, 140 and 160 beats·min^–1, and thirdly to examine a possible effect of fatness and fitness on the relationship between HR and . The subjects were 127 (60 boys, 67 girls) adolescents with a mean age of 14.8 (SD 0.3) years. The HR and were measured by means of an incremental exercise test to exhaustion. Linear regressions were performed for the and relationships using absolute and relative (%HR_max, ) data for each individual. From these regressions, target HR and were computed. Average target HR corresponding to 40%, 60% and 80% of were: 119 (SD 9), 145 (SD 9), 171 (SD 8), and 120 (SD 10), 146 (SD 8), 172 (SD 8) beats·min^–1 for boys and girls, respectively. Average corresponding to HR of 120, 140 and 160 beats·min^–1 were: 22 (SD 5), 30 (SD 5), 38 (SD 6) and 18 (SD 4), 24 (SD 4), 31 (SD 4) mlO₂·kg^–1·min^–1for boys and girls, respectively. An analysis of covariance showed a significant fitness effect (P<0.001) for predicted at all HR studied. The results suggest that the use of absolute HR to define exercise intensity levels when assessing young people's physical activity using HR monitoring detracts from the validity of the interpretation of the data. Electronic Publication     

Heart rate variability in dairy cows-influences of breed and milking system

Heart rate variability parameters in the time, frequency and nonlinear domains were investigated in two breeds of dairy cows (Austrian Simmental and Brown Swiss) milked either in an automatic milking system with partially forced cow traffic or in a herringbone milking parlour. Recordings were made of 24 cows (six of each breed and milking system) during lying, standing idle, and standing being milked, and analysed with linear mixed effects models taking the covariates time of day, live body weight, milk yield, stage of lactation and stage of pregnancy into account. Heart rate and nonlinear deterministic shares were higher, and heart rate variability in the time and frequency domains was lower, later in the day, in cows with higher body weight and in Simmental compared to Brown Swiss cows. Differences in the linear and nonlinear domains during lying indicated an increased level of chronic stress in cows in the automatic milking system with partially forced cow traffic, compared to cows milked in the herringbone milking parlour. No effects of milking system were found during milking, indicating that the stressor in the automatic milking system was not the milking process itself.     

Nonlinear relationship between electrodermal activity and heart rate variability in patients with acute schizophrenia

We investigated to what degree tonic skin conductance levels (SCL) and cardiac autonomic dysfunction are interrelated in schizophrenia. Heart rate variability (HRV) and SCL were simultaneously assessed in 18 unmedicated patients and 18 controls matched for age, sex, weight, and smoking habits. For comparison to prior studies, phasic sympathetic skin responses (SPR) were also recorded. Compared to controls, patients had prolonged SPR latency and reduced SPR amplitude with a right-greater-than-left asymmetry, which was inversely correlated with positive symptoms. An autonomic imbalance was reflected in linear and nonlinear measures of HRV and increased SCL. Patients showed a stronger nonlinear association between SCL and heart rate than controls. HRV and SCL findings were strongly affected by group differences in breathing rate. Stronger HRV-SCL coupling in patients may suggest augmented sympathetic modulation in schizophrenia.     

Effect of magnitopuncture on sympathetic and parasympathetic nerve activities in healthy drivers--assessment by power spectrum analysis of heart rate variability

The object of this study was to assess the effects of magnitopuncture applied to Dazhui (DU14) point and Neiguan (PC6) points on sympathetic and parasympathetic nerve activities by power spectrum analysis of heart rate variability in healthy drivers during simulated driving. Using power spectrum analysis, the low frequency (LF) and high frequency (HF) components of heart rate variability can be calculated reflecting the sympathetic and parasympathetic activity. The 40 healthy male subjects were randomly divided into two groups: A (study group) and B (control group). All subjects were required to be well rested before the experiment. The subjects of both groups were required to perform a simulated driving task for 3 h. During the driving, magnitopunctures were applied to the DU14 and PC6 points for A while the subject performed the task for 2.5 h, and for B magnitopunctures were applied to non-acupuncture points which were 1.5 cm away from the two acupuncture points respectively over the same time. Subjective response to a questionnaire was obtained after the simulated task in the two groups. At the end of the driving task the LF component in normalized units (NU) had decreased significantly (P<0.05) indicating a reduced sympathetic nerve activity and the HF component (NU) increased significantly (P<0.05) indicating a increased parasympathetic nerve activity for A compared with pre-stimulation while for B no significant differences were observed. There were significant group differences in LF (NU), HF (NU) and LF:HF at the end of the driving task (P<0.05). It was concluded that a modulating effect of magnitopuncture on sympathetic and parasympathetic nerve activities in healthy subjects was associated with the acupuncture points. The findings represent physiological evidence that magnitopuncture may reduce mental fatigue in healthy drivers. Electronic Publication     

Non-contact monitoring of heart rate variability using medical radar for the evaluation of dynamic changes in autonomic nervous activity during a head-up tilt test

Abstract

Electrocardiography (ECG) is a mandatory standard for monitoring electrical activity of the heart in many clinical settings such as intensive care and emergency units. However, in situations wherein the skin is damaged, such as acute burn injuries, it is impossible to efficiently attach electrodes to the skin. In this study, we developed a non-contact cardiac monitoring system using a 24-GHz medical radar for directly measuring the beat-to-beat heart mechanical activity at a distance from a subject. The heart rate variability (HRV) was analysed using an autoregressive model (AR) from the measured beat-to-beat intervals during a head-up tilt test. To investigate the feasibility of the proposed system, we compared medical radar and ECG recording by using Lin’s correlation coefficient and Bland–Altman analysis, which showed a negligible mean difference from the substantial agreement of Lin’s correlation coefficient of 0.9 between the radar and ECG. The non-contact radar clearly monitored dynamic changes in HRV indices induced by the head-up tilt test. This type of non-contact HRV-sensing technique as an alternative approach has significant potential for advancing personal healthcare in both clinical and out-of-hospital settings.     

Heart rate variability and pain: associations of two interrelated homeostatic processes

Between-person variability in pain sensitivity remains poorly understood. Given a conceptualization of pain as a homeostatic emotion, we hypothesized inverse associations between measures of resting heart rate variability (HRV), an index of autonomic regulation of heart rate that has been linked to emotionality, and sensitivity to subsequently administered thermal pain. Resting electrocardiography was collected, and frequency-domain measures of HRV were derived through spectral analysis. Fifty-nine right-handed participants provided ratings of pain intensity and unpleasantness following exposure to 4 degrees C thermal pain stimulation, and indicated their thresholds for barely noticeable and moderate pain during three exposures to decreasing temperature. Greater low-frequency HRV was associated with lower ratings of 4 degrees C pain unpleasantness and higher thresholds for barely noticeable and moderate pain. High-frequency HRV was unrelated to measures of pain sensitivity. Findings suggest pain sensitivity is influenced by characteristics of a central homeostatic system also involved in emotion.     

Autonomic cardiac control. II. Noninvasive indices and basal response as revealed by autonomic blockades

Heart period, systolic time intervals, low and high frequency heart period variability, blood pressure, and respiration were measured in female subjects under three drug conditions (saline, atropine sulfate, metoprolol) while sitting and standing on three consecutive days. Following preinfusion baseline recordings, saline, metoprolol (14 mg), or atropine sulfate (2 mg) was infused for 15 min (by using a double-blind procedure). Recordings were taken during a postinfusion baseline and in response to an orthostatic stressor (standing versus sitting postures). At the end of the metoprolol session, atropine sulfate was infused and responses were monitored during the post-infusion (i.e., double blockade) baseline and during orthostatic stressor. Analyses of the blockade data revealed that the preejection period (PEP) reflected sympathetic but not vagal influences on the heart, and high frequency (HF, 0.12–0.40 Hz) heart rate variability (respiratory sinus arrhythmia) reflected vagal but not sympathetic influences on the heart. No other measure provided a specific index of the tonic sympathetic or vagal activation of the heart. Postinfusion PEP under saline predicted individual differences in postinfusion cardiac sympathetic activation, whereas postinfusion heart period (but not HF variability) under saline predicted individual differences in postinfusion cardiac vagal activation.