Effect of Lavender Essential Oil Topical Treatment on the Autonomic Nervous System in Human Subjects Without Olfactory Influence: A Pilot Study

Abstract

Objective/background

As part of a scientific study into the effects of aromatherapy, we investigated the effects of lavender essential oil (LEO) treatment on the autonomic nervous system in subjects for whom the sense of smell had been eliminated.

Methods

This study used a single-blinded cross-over design for verification. Heart rate variability was measured and effects on the autonomic nervous system were investigated.

Results and discussion

Although no significant differences were found, aromatherapy treatment with 1% LEO tended to increase parasympathetic nervous system activity. Further, when differences between values before and during aromatherapy treatment were compared, LEO treatment significantly increased parasympathetic nervous system activity. Given these findings, LEO appears to increase parasympathetic nervous system activity, even in the absence of a psychological effect due to an absence of olfactory stimulation.

Conclusion

The present results provide a scientific method for verifying the effects of aromatherapy and will aid in further elucidation of aromatherapy.

Introduction

Aromatherapy is thought to consist of a psychological effect due to olfactory stimulation and a pharmacological effect due to the actions of aroma compounds. The ambiguous psychological effects of aromatherapy are a factor contributing to ambiguity in the actual effects of aromatherapy. The understanding of the scientific basis for pharmacological effects from aromatherapy thus remains insufficient. ¹

This time, to clarify the effects of aromatherapy, we conducted aromatherapy treatment in subjects for whom the sense of smell had been blocked to verify the pharmacological effects. A single-blinded crossover design was used in this study to verify the results. The essential oil used in the study was lavender essential oil (LEO), which has already been reported to have anxiolytic and sedative effects in animal experiments and clinical research. LEO has been tested on animals, and reportedly produced effects such as stress reduction effects ² and anxiety suppression. ³

Regarding the mechanisms of action by which LEO exerts anxiolytic effects, animal experiments have similarly shown that serotonin neurotransmission is involved, but not gamma-aminobutyric acid A (GABA_A)/benzodiazepine neurotransmission. ⁴ LEO inhalation by stressed animals has been reported to affect mRNA and protein expression levels. ⁵ Further, olfactory impairment does not appear to impair the anxiolytic-like effects of LEO inhalation in mice. ⁶ As in animal experiments, the effects in humans are expected to involve pure pharmacological effects, but problems of species differences between animals and humans are difficult to control for.

Clinical research has shown that LEO relieves anxiety (based on a systematic review and meta-analysis of randomized controlled trials) ⁷ and exhibits sedative effects. ⁸ In those reports, the effects of LEO were considered to include psychological effects due to olfactory stimulation.

The present study examined clinical research methods to eliminate the problem of species differences. The measurement target focused on the relatively weak pharmacological effects of aromatherapy, and on the autonomic nervous system, which is known to be markedly affected by aromatherapy.

Olfactory stimulation by LEO has the effect of increasing parasympathetic nerve activity in rats and mice, ⁹ decreasing renal sympathetic nerve activity and blood pressure in mice ¹⁰ and decreasing renal sympathetic nerve activity and blood pressure in rats. ¹¹

In clinical studies, aromatherapy using LEO reportedly reduced premenstrual emotional symptoms, ¹² reduced autonomic nervous system activity, ¹³ and reduced autonomic nervous system activity. ¹⁴ As in many previous reports, those studies reported effects from aromatherapy that included psychological effects from olfactory stimulation.

On the other hand, other papers have reported that transdermal administration of linalool lowers systolic blood pressure and skin temperature, excluding psychological effects caused by olfactory stimulation. ¹⁵ We believe that eliminating olfactory stimulation and accumulating clinical research data that eliminates psychological effects will allow scientific verification of the effects of both LEO and aromatherapy on a broader basis.

Results

The aromatherapy treatments were performed by a female university student (therapist) under the supervision of a faculty member. Ten subjects were randomly selected from the therapist's friends. All subjects were female students from the same university (aged 22-23).

Heart rate variability was measured before and during aromatherapy treatment. Control (macadamia nut oil only) treatment showed tendencies for increased sympathetic nervous system activity (from 0.80 ± 0.12 to 0.89 ± 0.12, P = 0.627, Figure 1A) and decreased parasympathetic nervous system activity (from 46.3 ± 4.2 to 41.2 ± 2.8, P = 0.4015, Figure 2A) during aromatherapy treatment. On the other hand, with macadamia nut oil containing 1% LEO, no changes in sympathetic nervous system activity were observed during aromatherapy treatment (from 0.89 ± 0.12 to 0.89 ± 0.18, P = 0.9775, Figure 1B), although an upward trend in parasympathetic nervous system activity (from 39.0 ± 4.0 to 44.3 ± 3.9, P = 0.5229, Figure 2B) was observed. However, no significant differences were apparent.

Figure 1.

Changes in sympathetic nervous system activity before and during aromatherapy treatment. (A) Control (macadamia nut oil only). (B) LEO (macadamia nut oil with 1% LEO). Values represent mean ± standard error (n = 10).

Figure 2.

Changes in parasympathetic nervous system activity before and during aromatherapy treatment. (A) Control (macadamia nut oil only). (B) LEO (macadamia nut oil with 1% LEO). Values represent mean ± standard error (n = 10).

When heart rate was measured before and after aromatherapy treatment, a trend towards significance toward decreased heart rate was observed under both control and LEO conditions. However, significant differences were again lacking (Figure 3).

Figure 3.

Changes in heart rate before and after aromatherapy treatment. (A) Control (macadamia nut oil only). (B) LEO (macadamia nut oil with 1% LEO). Values represent mean ± standard error (n = 10).

Discussion

In general, it is thought that in aromatherapy aimed at relaxation, the parasympathetic nervous system becomes dominant. The LEO used in this study is a representative example of this. The results of this study showed that the sympathetic nervous system became dominant under control conditions. This suggests that because the subjects were selected from among the therapist's close friends, they became excited by lively conversation, which led to the sympathetic nervous system becoming dominant during aromatherapy. To keep the conditions constant, aromatherapy was conducted under similar conditions, such as having a pleasant conversation during aromatherapy treatment.

Since the values obtained indicated large individual differences in autonomic system activity, averaged differences may be difficult to interpret correctly. Figures 1 and 2 showed no significant differences due to large inter-individual differences. Therefore, to eliminate individual differences, we decided to clarify differences in values indicating autonomic nervous system activity before and during aromatherapy treatment for each individual. As a result, for values indicating autonomic nervous system activity, almost no changes were observed in the sympathetic nervous system (Control: 0.09 ± 0.13, LEO: 0.01 ± 0.12, Figure 4A), whereas significant differences were observed in the parasympathetic nervous system (Control: −5.2 ± 3.3, LEO: 5.3 ± 3.1, P = 0.0336, Figure 4B). This result suggests that increasing the number of subjects in future work will result in significant differences in values indicating autonomic nervous system activity. In other words, LEO aromatherapy is thought to result in parasympathetic nervous system predominance even in subjects for whom the sense of smell has been eliminated.

Figure 4.

Difference value obtained by subtracting the value before aromatherapy treatment from the value during aromatherapy treatment. (A) Sympathetic nervous system. (B) Parasympathetic nervous system. *P < 0.05 (n = 10).

On the other hand, no significant difference in results of aromatherapy treatments was observed using oils containing LEO. However, these results were obtained with nasal plugs in place to eliminate the sense of smell, and with a single session of aromatherapy lasting 10 min (limited conditions for clinical research). In actual aromatherapy, treatments lasting 10 min or more are performed multiple times, and olfactory stimulation is also present, so psychological effects are thought to be fully expressed. Considering the above, many people have accepted the effectiveness of aromatherapy and have been using this modality for a long time.

Furthermore, due to time constraints, this research has a limitation in that the number of subjects is small compared to clinical trials for general pharmaceuticals. Therefore, this study has to be considered as a pilot study for aromatherapy clinical research.

Conclusion

Even when the sense of smell was eliminated, thus presumably removing the psychological effect, aromatherapy with LEO tended to result in dominance of the parasympathetic nervous system. We believe that this result provides important evidence clarifying the pharmacological effects of aromatherapy. This clinical research excluded olfactory stimulation to clarify the clinical pharmacological effects of aromatherapy. In the future, such data on clear pharmacological effects will continue to be accumulated and will help to scientifically clarify the effects of aromatherapy.

Experimental Methods

Samples

Essential oil from lavender (Lavandula angustifolia) was purchased from Green Flask Co., Ltd. (Tokyo, Japan). Macadamia nut oil (Macadamia ternifolia) used as a solvent for control and LEO was purchased from Kaneda Aburaten (Tokyo, Japan). Linalool and linalyl acetate, as standard compounds for gas chromatography (GC), were purchased from Tokyo Kasei (Tokyo, Japan). Quantitative analysis of the components (direct comparison with standard compounds) performed using a GC flame ionization detector (GC-FID, GC-2010 Plus; Shimadzu Corporation, Kyoto, Japan) revealed linalyl acetate (32.3%) and linalool (20.4%) as the major constituents. The compounds were identified by direct comparison with standard compounds.

Measurement conditions were as follows. The capillary column used was a DB-5ms (30 m × 0.25 mm ID, 0.25 μm; Agilent Technologies, Tokyo, Japan), using ultra-high purity nitrogen (99.9999%) as the mobile phase. The column oven was held at 40 °C for 2 min, ramped to 200 °C at a rate of 5 °C/min, then held for 2 min. The injection port temperature was 250 °C.

Statement of Human Rights

All procedures in this study were conducted in accordance with the International University of Health and Welfare Ethics Review Committee (approval no. 22-Io-21) approved protocols.

Practitioner and Subjects

Aromatherapy was performed by a female university student under the guidance of teachers. Subjects were randomly selected from among the female university friends of the student performing the aromatherapy. Regarding the number of subjects, 10 healthy women (age, 22-23 years old) attending the university were selected, based on previous reports. ¹⁶

Experimental Procedure

Subjects provided fully informed consent. Subjects who consented to the study underwent a patch test using LEO to confirm the absence of allergic reactions after 30 min and after 48 h. Two types of samples were used: Control treatment contained only macadamia nut oil and LEO contained macadamia nut oil with 1% LEO. The concentration of LEO was set at 1% based on the concentration actually used in aromatherapy treatments. Each subject received aromatherapy with both samples, but the order application was randomized. In the single-blinded crossover study, one subject received 10 min of aromatherapy with 1 mL of each sample on either the left or right arm. A wash-out period of approximately 1 week was provided between each aromatherapy treatment (Figure 5). Heart rate measurements were performed before and after each aromatherapy treatment. Heart rate variability was measured before and during each aromatherapy treatment.

Figure 5.

Schematic diagram for single-blinded crossover study.

Details from before aromatherapy treatment to after aromatherapy treatment were as follows.

Heart rate measured (heart rate before aromatherapy treatment).

Heart rate variability from the ear pulse wave was measured while resting for 10 min (heart rate variability before aromatherapy treatment).

While administering aromatherapy treatment to subjects using control or LEO, heart rate fluctuations were measured for 10 min (heart rate fluctuations during aromatherapy treatment) from ear pulse waves.

Heart rate measured (heart rate after aromatherapy treatment).

One week after wash out, the same procedure was applied again, but this time the subject was treated using the oil (control or LEO) not used previously.

The subjects sat opposite the therapist and received an aromatherapy treatment (on one forearm only). And to minimize the burden on the subjects, heart rate variability was measured using ear pulse waves.

Method of Olfaction Exclusion for Subjects

Swim nose plugs were used to eliminate the sense of smell for aromatherapy treatments. Using the nasal plugs, subjects could no longer perceive the scent of LEO used in aromatherapy treatments. Subjects therefore received aromatherapy without being able to determine whether the control or LEO intervention was being applied.

Heart Rate Variability Analysis

Pulse waves were measured from the ear lobe of subjects for 10 min before aromatherapy and for 10 min during aromatherapy.

The equipment used was PowerLab, and the analysis software was Lab Chart. (Bio Research Center Co., Nagoya, Japan).

Each indicator was defined as follows: high frequency (HF), a value between 0.15 and 0.4 Hz; low frequency (LF), a value between 0.04 and 0.15 Hz; sympathetic nerve index calculated as LF/HF; and parasympathetic nerve index, taken as HF.

Heart Rate

The heart rate of subjects was measured before and after aromatherapy as the average of three measurements taken using an upper-arm blood pressure monitor (HEM-1040; OMRON Corporation, Kyoto, Japan).

Statistical Processing

Values are expressed as mean ± standard error of the mean. Analyses were performed using Excel Statistics (Social Information Service Co., Tokyo, Japan). Statistical significance was determined using two-tailed tests. Values of P < 0.05 from paired t-tests were considered significant.