Approximately 30% of surgical patients experience postoperative nausea and vomiting (PONV), and this number increases to 80% in higher risk populations [1]. PONV has a multifactorial etiology, which includes individual risk factors, as well as factors associated with the anesthetic agent and surgical procedure [2,3].

The prophylaxis and treatment of PONV is controversial as the currently available antiemetic medications are unable to prevent its occurrence. There is no consensus on whether these drugs should be prophylactically used in all patients or only in symptomatic patients [4]. However, as the risk factors for PONV have clearly been established, when present, the use of multimodal prophylaxis is justified [1,5].

Vomiting and nausea is comprised of elaborate mechanisms and distinct brain centers. The vomiting center, located in the brainstem, and the chemoreceptor area, located in the area postrema, are the two primary regions in the brain involved in generating the action of vomiting. The vomiting center is responsible for the actions of the smooth muscles and the striated muscles involved in vomiting. The chemoreceptor zone is sensitive to neurotransmitters that trigger the vomiting reflex, including dopaminergic and serotonergic substances. As such, dopamine and serotonin antagonists can have clinically useful applications in treating nausea and vomiting [6,7].

Nausea is triggered by an imbalance of modulation between sympathetic and parasympathetic stimuli and involves a broad central autonomic network. This network comprises brain regions such as the insula, anterior cingulate, the orbitofrontal, somatosensory and prefrontal cortexes, amygdala, putamen, and the dorsal pons and locus ceruleus [8,9].

As PONV is multifactorial, a multimodal approach has been adopted for its prevention and treatment. Nonpharmacologic techniques, such as acupuncture, may be used in conjunction with medications, especially in high-risk patients [10].

Acupuncture has been considered effective in the treatment of PONV when used before the induction of general anesthesia, as a single technique or combined with conventional antiemetics. The stimulation of the pericardium point 6 (PC6, Neiguam) is frequently used for treatment of PONV, anxiety, and motion sickness [11]. Needle insertion in the region of the PC6 point has been proven to be safe with no adverse effects and does not reach the level of the underlying muscle fascia [12]. Studies have shown that digital pressure on the PC6 point is also effective in the treatment of PONV [11,13].

Acupuncture may provide PONV prophylaxis via disruption of the serotoninergic mechanism, among other neurochemical substances, which desensitizes the chemoreceptor trigger zone [14]. This study was aimed at evaluating the action of acupuncture performed preoperatively at point PC6 on the incidence of PONV in patients undergoing open hysterectomy, in addition to analyzing their plasma serotonin values.

This is a prospective, randomized clinical trial, approved by the Institutional Ethics Committee (opinion number 56080016.0.0000.5411), and registered in the Brazilian Registry of Clinical Trials (Registro Brasileiro de Ensaios Clínicos - ReBEC) (Number RBR-9KWJGX). The study was conducted at the Teaching Hospital, Botucatu Medical School, São Paulo State University (Universidade Estadual Paulista - UNESP). All participants signed the informed consent form. The study was conducted in accordance with the CONSORT (Consolidated Standards of Reporting Trials) statement guidelines for clinical trials.

A total of 107 female patients, aged 18 to 65 years, with physical status I, II, and III according to the classification of the American Society of Anesthesiologists (ASA) [15], who were candidates for elective open hysterectomy surgery, were eligible to participate in the study. Patients who were apprehensive about acupuncture needles and those who used other antiemetics in the preoperative period were excluded.

1. Procedures

The patients were transferred to the preoperative unit 30 minutes before the surgery, where they underwent peripheral venipuncture for the infusion of fluids (Ringer’s solution with lactate, 6 ml.kg^–1.h^–1) and to administer medications. All patients remained in bed and were monitored by pulse oximetry, continuous cardioscopy, and non-invasive blood pressure measurement. At this time point, the first venous blood sample (5 ml) was collected for the measurement of serotonin levels (baseline).

Randomization was performed by drawing lots, and the allocation ratio was 1:1. An opaque sealed envelope containing the numbers 1 and 2, matching the acupuncture and control groups, respectively, were randomly drawn by the researcher in the preoperative unit before the surgical procedure. For the patients in the acupuncture group, acupuncture was performed with a needle at point PC6, in both arms, for 30 minutes.

As described in a study published by our research group, PC6 is a point on the anterior surface of the forearm between the palmaris longus and the carpi radialis flexor tendons. It extends for a distance two times wider than the distance between the thumb interphalangeal joint and the distal wrist crease (almost four centimeters from the distal wrist crease) [11]. The needle is inserted into the skin perpendicularly until a sensation of pain, distension, and, at times, electrical sensation radiates to the fingers [16-18]. A needle was inserted in the bilateral PC6 points and left in place for a period of 30 minutes. During this period, the needle was manually stimulated twice before being removed from the skin.

All the needling was performed by the same certified acupuncturist. The acupuncturist completed their 1,200 hours of training in 2016, and since then has practiced extensively prior to enrollment of the subjects in this study.

In the control group, patients remained lying down and no acupuncture was performed. After 30 minutes, a new blood sample was collected from subjects of both groups to measure the serotonin levels, and the patients were subsequently transferred to the operating room.

In the operating room, after a period of pre-oxygenation via face mask, anesthesia was induced with the injection of sufentanil (0.5 µg.kg^–1) and propofol (2-3 mg.kg^–1), until the loss of eyelid corneal reflex was observed. Neuromuscular function was monitored using the sequence of four stimuli (train-of-four) and rocuronium (0.6 mg.kg^–1) was administered to obtain neuromuscular blockade and tracheal intubation was performed. The maintenance of neuromuscular blockade was accomplished with subsequent doses (corresponding to 20% of the initial dose) of the neuromuscular blocker used in the anesthetic induction, based on observation of the second motor response following the train-of-four. Anesthesia was maintained with the use of isoflurane at an alveolar concentration of 0.5-0.7% in an inspired fraction of 40% oxygen and continuous infusion of remifentanil (0.1-0.3 µg.kg^–1.min^–1). At the end of the surgery, all patients underwent neuromuscular blockade reversal with sugammadex (2-4 mg.kg^–1), were extubated, and transferred to the post-anesthesia care unit.

Postoperative analgesia was administered intravenously using methadone after anesthetic induction (5 mg), and dipyrone (2 g) and tramadol (100 mg) at the end of surgery. Patients who reported pain in the post-anesthesia care unit, with a pain score > 3 according to the verbal analogue pain scale (0 to 10), were administered a dose of morphine (0.05 mg.kg^–1). For PONV prophylaxis, all patients received dexamethasone (8 mg) after anesthetic induction and ondansetron (8 mg) at the end of the surgical procedure [19]. Metoclopramide (10 mg), which could be repeated every 8 hours, was used as a rescue treatment for PONV.

The primary outcome was the incidence of nausea and vomiting, as answered by the subject to an objective question, within 24 hours postoperatively. The secondary outcome was the use of rescue doses of metoclopramide. This information was obtained via chart review of the patient’s medical records. Plasma serotonin levels were obtained as previously described.

Demographic variables (age, body mass index), ASA physical status, duration of the anesthetic procedure, smoking habits, previous history of nausea, vomiting, use of postoperative morphine, and motion sickness were also analyzed.

2. Statistics and sample size analysis

The incidence of PONV in high-risk patients is 70% [20]. With a treatment effect of 50%, α = 0.05, and statistical power of 90%, a total of 94 patients were required to power the study. In order to account for cases with possible modification of the initial surgical plan, eventual break of protocol and loss of cases, 15% of patients per group were added, totaling 108 patients (54 patients per group).

The statistical calculation was performed using the StataMP 12.1 software (StataCorp. 2011. Stata: Release 12. Statistical Software. College Station, TX: Stata Corp. LP). For qualitative variables, the chi-square test was used, and for quantitative variables, the Mann-Whitney test was used to compare groups. The Wilcoxon test was used to compare timepoints for non-normally distributed variables and Student’s t-test, for those with normal distribution. p-value < 0.05 was considered statistically significant. The analysis was performed as per protocol.

The study was conducted from April 2017 to October 2018, and a total of 108 patients were eligible. One patient refused to participate in the study, and ten patients were excluded from the analysis because of a change in the anesthetic technique due to a modification in the surgical strategy. After these patients were excluded, 97 participants were analyzed (Fig. 1).

Figure 1. Patient selection flowchart.

The groups were matched by age, body mass index, and risk factors for PONV. The mean age of included subjects was approximately 48 years old, as expected for patients undergoing hysterectomies. The median body mass index was 29.6 (control group) and 30.1 (acupuncture group), classified as overweight and obese, respectively. The number of smokers in each group was identical (Table 1).

Values represent mean ± standard deviation and mean and *frequency and percentage. Student’s t test: age, body mass index and duration of anesthesia. Chi-square test: physical status ASA (American Society of Anesthesiologists)..

&md=tbl&idx=1' data-target="#file-modal"">Table 1

Characteristics of patients and perioperative control variables.

	Groups		p-value
	Control (n = 48)	Acupuncture (n = 49)
Age (years)*	48.3 ± 10.9	48.6 ± 11.7		0.91
Body mass index (kg.m–2)*	29.6 ± 5.1	30.1 ± 4.6	0.61
Physical statuse ASA I, II, III	5 (10.4)	8 (16.3)	0.25
	40 (83.3)	34 (69.4)
	3 (6.3)	7 (14.3)
Smoker	16 (33.3)	16 (32.6)	0.94
Duration of anesthetic procedure (minutes)*	164 ± 51.1	180.6 ± 59.8	0.15

Values represent mean ± standard deviation and mean and *frequency and percentage. Student’s t test: age, body mass index and duration of anesthesia. Chi-square test: physical status ASA (American Society of Anesthesiologists)..

All patients in this clinical trial had at least four risk factors (women, surgery duration > 60 minutes, use of opioids, inhalation anesthesia) for PONV. Five risk factors (previous four + motion sickness) were observed in 37.5% and 40% of included subjects in the control and acupuncture groups, respectively. Six risk factors (previous five + PONV history) were present in 31.3% and 34.7% of patients in control and acupuncture groups, respectively. Morphine was used for postoperative pain in 95.8% and 98% of included patients in control and acupuncture groups, respectively (Fig. 2).

Figure 2. Number and percentage of patients with risk factors for postoperative nausea and vomiting and the use of morphine in the 24-hour observational period. *Presence of four risk factors: women, surgery duration > 60 minutes, use of opioids and inhalation anesthesia. ^#Presence of five risk factors: women, surgery duration > 60 minutes, use of opioids, inhalation anesthesia and motion sickness. ^&Presence of six risk factors: women, surgery duration > 60 minutes, use of opioids, inhalation anesthesia, motion sickness and PONV history. Values represent mean frequency and percentage.

The incidence of nausea was significantly reduced in the acupuncture group compared to the control group (p < 0.003). Similarly, the use of rescue medication (metoclopramide) was significantly lower in the acupuncture group compared to the control group (p < 0.006). No difference was found in the occurrence of vomiting between the two groups (Table 2).

Values represent frequency and percentage. Chi-square test..

&md=tbl&idx=2' data-target="#file-modal"">Table 2

Incidence of nausea or vomiting and the use of rescue medication (metoclopramide) in the postoperative period.

	Groups		p-value
	Control (n = 48)	Acupuncture (n = 49)
Nausea	14 (29.2)	3 (6.1)		0.003
Vomiting	2 (4.2)	2 (4.1)	0.98
Use of metoclopramide	16 (33.3)	5 (10.2)	0.006

Values represent frequency and percentage. Chi-square test..

The serotonin levels between the two groups, in the baseline and 30 minutes after acupuncture, did not differ (Table 3).

Values represent the median (1^st-3^rd quartiles). Mann-Whitney and Wilcoxon tests..

&md=tbl&idx=3' data-target="#file-modal"">Table 3

Serotonin plasma levels (ng/ml), classified by the time points and groups.

Moments	Groups		p-value
	Control (n = 48)	Acupuncture (n = 49)
Baseline	29.0 (0.40-77.6)	25.0 (0.30-63.3)		0.66
30 minutes after baseline (control group) or acupuncture	34.9 (0.30-67.6)	25.4 (0.30-58.1)	0.59
p-value	0.56	0.35

Values represent the median (1^st-3^rd quartiles). Mann-Whitney and Wilcoxon tests..

The acupuncture group had a lower incidence of postoperative nausea, however, the incidence of vomiting remained unchanged. The serotonin levels did not differ between the two groups or timepoints. As nausea and vomiting have different areas of the brain involved in their mechanisms [6,8,9], the use in both groups of ondansetron, an anti-serotoninergic drug, and dexamethasone, that has direct central action at the solitary tract nucleus and interaction with serotonin, may explain the results [1,19]. The low incidence of vomiting in both groups may reflect the prophylactic use of antiemetics drugs in all included subjects.

Mast cells have been reported to play an important role in the local response in acupoints. When an acupuncture needle is manually inserted into acupoints, there is deformation of collagen fiber and degranulation of mast cells which results in a local and systemic change of bioactive substances [21,22]. Serotonin, a substance released by mast cells, plays a pivotal role in the effects of the acupuncture, especially as an analgesic [21].

Although studies correlate increased serotonin plasma levels with acupuncture [14], its correlation with different measurement sites is not entirely clear. The cerebrospinal fluid has the best serotonin level correlation with brainstem concentrations, but sampling it is an invasive procedure. Levels of serotonin in the blood are less strongly correlated with levels in cerebrospinal fluid [23], and this may have influenced our results.

Cellular and molecular biological mechanisms of the acupoint initiation on the acupuncture effect have been proposed. Along with serotonin, histamine, adenosine, and beta-endorphin may also play important roles [22]. When the PC6 acupoint is infiltrated by local anesthetics, the antiemetic effects of its stimulation may be blocked. This fact suggests that nerve transduction also plays a role in the antiemetic effects of acupuncture [24].

All patients had at least four risk factors for PONV: women, surgery lasting more than 60 minutes, use of opioids, and use of inhalational anesthesia. According to the fourth consensus on PONV published in 2020, a previous history of PONV and no history of smoking, observed in both groups, are also considered risk factors. Thus, the patients included in this study can be considered at high risk for the occurrence of PONV [1,5,25]. The mean duration of the surgeries was longer than 60 minutes and isoflurane was the anesthetic of choice. These characteristics were also risk factors for the groups analyzed in this study [1].

Previous studies demonstrated that the correct choice of the acupuncture point, and the precise insertion of the needle in the exact location of the point, are associated with improved results of the technique. The sensation of pain, distension, and sometimes electrical sensation radiating to the fingers, are used to confirm the correct insertion of the needle [16-18]. In order to ensure efficacy of acupuncture, the correct points must be chosen, the technique has to be performed correctly, and the stimulation time must be followed. None of our subjects experienced bleeding or other signs or symptoms that would suggest nerve puncture. For this reason, the PC6 point can be considered safe, which is consistent with previously reported studies [26,27].

Though the mechanism by which PC6 point stimulation prevents the occurrence of PONV is not totally elucidated, there are studies that demonstrate its effect on the release of neurochemical substances that justify its use in the in the perioperative setting. Zhang et al. [28] investigated the role of acupuncture, performed for three days, in the prophylaxis of nausea and vomiting in patients undergoing chemotherapy. In this study, the role of serotonin and dopamine in nausea and vomiting was also analyzed. These substances were measured at two time points – before and after the acupuncture. The authors identified a decrease in the occurrence of nausea and the concentrations of these plasma amines. The present study did not find differences between the plasma serotonin dosages before and after acupuncture, or between the studied groups.

This is an inconsistent finding in the literature. Different populations, with different interventions with the use of acupuncture, in which the amines were dosed, have shown an increase or a decrease or, as the results of this study, no changes were found in the plasma levels of serotonin [28-30].

Some studies show that the use of acupuncture after anesthesia induction is ineffective for PONV prophylaxis or treatment [24]. The neurophysiological response to acupuncture stimulation is believed to be reduced when performed in patients anaesthetized with the use of propofol, for example [31]. Nevertheless, studies have shown the efficacy of acupuncture in PONV prophylaxis, in different surgeries and populations, when performed prior to induction [1,32]. For this reason, in this study, all needling was performed prior to anesthetic induction.

In a meta-analysis that included 4,558 patients and ten different acupuncture modalities, the stimulation of the PC6 point demonstrated an increased incidence in reducing nausea and vomiting and the need to use rescue medication. Its effectiveness can be comparable to that of prophylactic medications such as ondansetron and metoclopramide, among others. This efficacy was also demonstrated in pediatric patients [1].

The present study is corroborated by other studies that have shown that acupuncture is more effective in preventing nausea than vomiting [32]. In our study two antiemetics drugs were used since all patients were at high risk for the occurrence of PONV. The use of these drugs may have been a significant protective factor for the occurrence of vomiting in this sample.

Our study contributes to the body of evidence that acupuncture is a cheap, safe, and effective option for the prophylaxis of PONV in patients at high risk for these complications. However, it has some limitations, as the study population was composed only of women undergoing general anesthesia to perform hysterectomy using an open technique and performed in a single center. Generalization of results for other surgeries or genders is not advised. In addition, for ethical reasons, the use of two prophylactic antiemetics was mandatory, as this is a high-risk population for the occurrence of PONV. A placebo group was not included, since a systematic review on the effectiveness of acupuncture with needling at the PC6 point in preventing the occurrence of PONV suggests that there is no need to use this type of group for the testing of hypotheses [27].

The clinical results found in our study reinforces the already published data regarding the effectiveness of acupuncture at PC6 in preventing nausea [1,11,13]. The novelty presented in our results is related to the plasma serotonin levels that did not differ among the study groups.

Acupuncture performed at the PC6 point is effective as a complementary therapy to the use of antiemetics drugs in preventing nausea, but not vomiting in patients at high risk for PONV undergoing hysterectomy. The results presented could not confirm that serotonin is involved in the mechanism of action of acupuncture at point PC6 for the prevention of PONV. More studies are needed to understand the mechanism of action of acupuncture in PONV prophylaxis.

MCP received of the following financial support for the research, authorship, and/or publication of this article: Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.

The authors declare no conflict of interest.