Journal of Equine Veterinary Science 68 (2018) 46e50
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Original Research
Effect of Aromatherapy on Equine Heart Rate Variability Ann Linda Baldwin a, b, *, Isabelle Chea a a b
Department of Physiology, University Medical Center, University of Arizona, Tucson, AZ Laboratory for the Advances in Consciousness and Health, Department of Psychology, University of Arizona, Tucson, AZ
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 March 2018 Received in revised form 4 May 2018 Accepted 7 May 2018 Available online 26 May 2018
This study explored whether aromatherapy increases the parasympathetic component (root mean square of successive differences [RMSSD]) of heart rate variability (HRV) in horses. Horses are highly sensitive, easily stressed, have an acute sense of smell, and have been shown to benefit from aromatherapy. The protocol, performed on eight dressage horses, followed a crossover design with an experimental treatment of humidified essential oil (lavender, Lavandula angustifolia) and a control treatment of humidified air. Heart rate variability was recorded for 7 minutes before, during, and immediately after treatment, and 30 minutes after treatment. The protocol was repeated with chamomile, (Matricaria recutita). Lavender transiently increased RMSSD from 86.5 ± 14.2 ms (standard error of the mean [SEM]) to 104.3 ± 14.3 ms (P ¼ .020) and reduced percentage of very low frequency HRV oscillations from 81.2% ± 3.9 (SEM) to 72.1% ± 7.9 (P ¼ .039) immediately after treatment. Chamomile had variable effects, none of which reached significance. These results indicate that lavender is effective as a calming agent for horses. © 2018 Elsevier Inc. All rights reserved.
Keywords: Aromatherapy Heart rate variability Horse Equine Lavender Chamomile
1. Introduction The use of aromatherapy for medicinal purposes started over 6,000 years ago in ancient Egypt, the Far East, China, and Europe [1]. Lavender, one of the most popular medicinal herbs, has been used as an analgesic, antibacterial, antidepressant antispasmodic, circulatory stimulant, and a relaxant [2e6]; its medicinal properties have been noted in the British Pharmacopoeia for about 250 years. More recent studies indicate that lavender aromatherapy can produce a short-term relaxing effect on the autonomic nervous system [7,8]. Inhalation of lavender increases heart rate variability (HRV) and parasympathetic tone in midlife women with insomnia [9]. Chamomile (Matricaria recutita) is also one of the most ancient medicinal herbs, and inhalation of its vaporized essential oils is recommended to relieve anxiety and general depression [10]. Although little is known about effects of chamomile on horses, inhalation of chamomile vapor by rats can reduce stress-induced increases in plasma adrenocorticotropic hormone levels [11].
Animal welfare/ethical statement: All animals used in this study were housed to an exceptionally high standard and all research procedures adhered strictly to AAALAC International guidelines, the gold standard for animal care and use programs. Conflict of interest statement: Neither of the authors have any conflicts of interest. * Corresponding author at: Ann Linda Baldwin, Department of Physiology, University Medical Center, University of Arizona, 1501 N Campbell Ave, Tucson, AZ 85721 E-mail address:
[email protected] (A.L. Baldwin). https://doi.org/10.1016/j.jevs.2018.05.213 0737-0806/© 2018 Elsevier Inc. All rights reserved.
When an essential oil aromatic vapor is inhaled, the effector molecules bind to the receptors in the nasal cavity. Olfactory sensory neurons then transmit the signal to the olfactory bulb that filters and processes the signal. Mitral cells carry the output signals from the olfactory bulb to the olfactory cortex, allowing perception of the aroma [12,13]. Some mitral cells connect directly to the amygdala [14], a part of the limbic system that regulates emotion [15]. Through its connections to the cerebral cortex and thalamus, the amygdala also plays a crucial role in autonomic regulation and the stress response. In fact, the functional connectivity of the amygdala with the prefrontal cortex is directly related to HRV [16], the variation in time between heartbeats produced by changes in sympathetic and parasympathetic activities. A high HRV indicates that the heart is adaptable and responsive to second-by-second changes in the body's needs. It is therefore clinically relevant to examine the utility of aromatherapy in improving relaxation as shown by an increase in the parasympathetic component of HRV (root mean square of successive differences [RMSSD]). Aromatherapy may benefit horses because they are highly sensitive, easily stressed, and have an acute sense of smell. However, limited research has been performed on the use of aromatherapy to relax horses. In one study [17], horses were exposed to acute stress (an air horn being blown twice) followed by enforced inhalation of either humidified lavender essential oil or humidified air. In both cases, the horses' heart rate (HR) increased after stress but returned to normal values more quickly in those that inhaled lavender. In
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2.2. Participants
Table 1 Profile for each participating horse. Name
Age (y)
Sex
Breed
Pluto Moon Dewie Bianca Remi Anna Flash Daphne Major
29.5 15 18 18 17 8 14 7 16
Gelding Gelding Gelding Mare Gelding Mare Gelding Mare Gelding
Lipizzaner American Saddlebred Thoroughbred Azteca Appaloosa/American Paint Oldenberg Morgan Dutch Warmblood American Quarter horse
Nine riding horses stabled at a private boarding facility in Tucson AZ participated in this study. The sample size was chosen based on a previous equine aromatherapy study performed [17] in which a sample of seven (n ¼ 7) horses was sufficient to yield statistically significant changes in the HR. The horses varied in age and breed and the profile for each horse is listed in Table 1. 2.3. Design
another study [18], after horses had been hauled in a trailer, their salivary cortisol concentrations and HR had increased, but pretreatment with inhalation of lavender essential oil vapor only resulted in a nonsignificant decrease in HR. The cortisol data were unreliable due to the significant differences in the baseline cortisol concentrations between the control and treatment groups. In a third experiment [19], horses received either lavender oil in carrier or carrier alone, applied around the nostrils and were then subjected to 30 minutes of stress tests. Stress indicators such as HR, alert postures, and defecations were lower in the lavender oiletreated horses. Two of these previous reports [17,18] noted that some of the horses had a high baseline level of arousal even before the stressor was applied. Currently, there are no studies to determine a horse's response to aromatherapy in the absence of an imposed stressor. Such information would be useful for calming naturally highly strung horse breeds, such as Arabians and Thoroughbreds [20], and also individual horses that may be more prone to nervousness than others. In addition, in all three previous studies, HR was measured rather than HRV. Heart rate variability is a superior measure to HR because it provides information about the sympathetic and parasympathetic interaction over a particular time interval and directly monitors the strength of the calming parasympathetic response. The aim of this study was to investigate whether aromatherapy with lavender or chamomile would increase the parasympathetic component (RMSSD) of HRV in horses. If it is shown that aromatherapy increases RMSSD, this will provide clinical relevance for its usage to alleviate nervousness and anxiety horses and to make the riding experience more pleasant and safe. A portion of this study was submitted to the Honors College, University of Arizona by one of the authors (Isabelle Chea), in partial fulfillment of a Bachelors degree with Honors [21].
The experimental protocol followed a crossover design with an experimental treatment of humidified essential oil and a control treatment of humidified air (water). Every horse received the experimental and control treatment, separated by 1 week at the same time of day (2e4 pm), thus allowing for each horse to serve as its own control to account for possible confounding variables such as personality, breed, sex, and age. The first experimental treatment was lavender and then the experiment was repeated with chamomile. Every horse participated in both the lavender and the chamomile studies, except for two (Daphne and Major). Daphne had relocated and was unable to participate in the chamomile study; Major was recruited into the chamomile study as her replacement. 2.4. Experimental Measures 2.4.1. Heart Rate Variability Testing Procedure The horse was haltered and led into a familiar turnout paddock where the horse handler came to a stop. Although haltered, the horse was free to move around but generally stayed within an area about 2e3 meters in diameter. Heart rate variability was measured using the Polar Equine RS800CX Science Heart Rate Monitor (Polar, Warminster, PA, USA). The monitor consisted of an electrode transmitter that attached to a strap that was placed around the horse's girth and a wristwatch receiver that was worn by the investigator collecting data. Heart rate variability was recorded for 7 minutes each at baseline, during treatment, immediately after treatment, and a half hour after each treatment. The measurement duration of 7 minutes was chosen because pilot studies indicated this was the maximum time period that the horse could be compliant and attentive. The horse handler stood less than a meter away from the horse throughout the experiment, as did the investigator collecting HRV data. 2.5. Aromatherapy
2. Materials and Methods 2.1. Animal Subjects Approval This study was approved by the University of Arizona Institutional Animal Care and Use Committee.
The essential oils used in this study came from PlantLife Natural Body Care, San Clemente, CA. The company states that their products are 100% pure, are steam distilled, and are not manufactured with synthetics. In both studies, six drops of either 100% Lavandula angustifolia or 100% Chamomile (Matricaria recutita) essential oil
Table 2 Average mean and standard deviation for heart rate variability data in response to lavender. Baseline HR (bpm) SDNN (ms) RMSSD (ms) % VLF % LF % HF
36.7 173.1 86.5 81.2 12.4 6.4
± ± ± ± ± ±
Lavender 1.4 (SEM) 26.2 14.2 3.9 2.2 2.7
36.1 170.2 104.3 71.7 17.7 10.6
± ± ± ± ± ±
1.3 (SEM) 29.3 14.3* 5.5 3.0 4.4
Post 1 37.2 180.9 93.7 72.1 15.5 12.4
Post 2 ± ± ± ± ± ±
1.5 (SEM) 27.1 16.4 7.9* 3.2 7.3
40.0 139.8 83.1 76.1 12.6 11.2
± ± ± ± ± ±
2.6 (SEM) 16.4 14.2 6.0 2.4 5.2
Abbreviations: HF, high frequency; HR, heart rate; LF, low frequency; RMSSD, root mean square of successive differences; SDNN, standard deviation of the time intervals; SEM, standard error of the mean; VLF, very low frequency. Means and standard deviations of HRV parameters measured during lavender treatment. Statistical significance from baseline (*) was found in RMSSD during Lavender treatment and in VLF (%) during Post-1.
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Table 3 Average mean and standard deviation for heart rate variability data in response to water control. Baseline HR (bpm) SDNN (ms) RMSSD (ms) % VLF % LF % HF
36.5 136.7 89.5 73.1 19.4 7.3
± ± ± ± ± ±
Water 3.2 (SEM) 26.4 15.9 5.6 5.0 1.5
39.9 195.4 77.2 80.1 14.4 4.9
± ± ± ± ± ±
Post 1 2.9 (SEM) 23.1 18.3 6.7 4.8 1.9
33.4 158.6 90.5 75.2 17.6 7.2
Post 2 ± ± ± ± ± ±
2.7 (SEM) 24.0 19.0 6.5 4.2 2.8
36.8 111.3 71.5 67.2 22.1 10.7
± ± ± ± ± ±
2.8 (SEM) 23.8 18.3 7.1 4.0 3.4
Abbreviations: HF, high frequency; HR, heart rate; LF, low frequency; RMSSD, root mean square of successive differences; SDNN, standard deviation of the time intervals; SEM, standard error of the mean; VLF, very low frequency. Means and standard deviations of HRV parameters measured during water control (lavender study). No statistical significance from baseline was found for any parameter.
treatments or water control were placed onto the filter pad of an essential oil diffuser (SpaRoom Scentifier). PlantLife tested the specific lots of Lavandula angustifolia and Matricaria recutita that were used for this study. The major constituents of Lavandula angustifolia were linalool (29.24%), linalyl acetate (31.09%), terpinen-4-ol (5.67%), lavandulyl acetate (3.84%), and lavendulol (0.37%). The major constituents of Matricaria recutita were transbeta farnesene (26.19%), alpha farnesene (18.49%), germacrene D (8.8%), chemazulene (8.2%), alpha bisabolol oxide B (7.81%), and bisabolone oxide A (5.64%). The high percentage of chemazulene is indicative of a superior quality of oil. Six drops of essential oil were sufficient to produce a distinct odor from the diffuser without saturating the filter pad. Separate diffusers were used for essential oil treatments versus water control. The diffuser was held by the horse handler at about 3e5 cm from the horse's nose. Each horse was free to walk around, move its head, and to sniff the vapor from the diffuser. An effort was made by the horse handler to keep the diffuser in the vicinity of the horse's nose most of the time, but none of the horses were forced to sniff the vapor.
LF: low-frequency power, 0.04e0.15 Hz, degree of modulation of sympathetic and parasympathetic tone HF: high-frequency power, 0.15e0.4 Hz, degree of modulation of parasympathetic tone. SigmaStat software was used to perform analysis of variance (ANOVA) one-way repeated measures, followed by Holm-Sidak multiple comparison to test for statistically significant changes in each parameter from baseline. The data were compared for four categories: “Baseline,” in which the horse had not yet experienced the diffuser; “Treatment,” in which the horse was given either the water or the essential oil vapor; “Post 1,” which was the immediate observation of the horse after the diffuser had been removed; and “Post 2” in which the horse was given some time after experiencing the diffuser and its behavior was observed again. 3. Results 3.1. Lavender Study
2.6. Heart Rate Variability Data Analysis Extraction of the HRV data was accomplished using Polar ProTrainer Equine Edition software. Outlying data points (more than ± 2 standard deviation [SD] from the mean) were corrected by substituting the average of the numbers proceeding and following the outlier. Analysis of the data was performed using a freeware HRV program, http://kubios.uef.fi/, which allowed for the following parameters, defined below, to be analyzed. Definitions of heart rate variability parameters are as follows: SDNN: SD of the time intervals between successive heartbeats; reflects amplitude of HRV RMSSD: root mean square of successive differences between the interbeat intervals; indicator of parasympathetic tone VLF: very low frequency power,
