Effect of Olfactory Stimulation with Breast Milk on Pain Responses to Heel Stick in Premature Infants: A Randomized Controlled Trial

Abstract

Purpose:

This study aimed to evaluate the effect of breast milk olfactory stimulation on pain response in premature infants during heel stick tests.

Methods:

A quasi-experimental randomized controlled trial was conducted in the neonatal intensive care unit of a tertiary-level hospital in Seoul, Korea. A breast milk group was provided olfactory stimulation with breast milk 30 seconds before the start of heel stick sampling, and a control group underwent heel stick sampling without olfactory stimulation. To evaluate infants' pain response, the heel stick procedure was video recorded, and pain response was assessed separately by two blinded research assistants using the Premature Infant Pain Scale (PIPS) through behavioral response, heart rate on physiological monitoring, and blood oxygen saturation on pulse oximetry through the video recording.

Results:

Forty-eight premature infants (gestational age, within 28–37 weeks, n = 24; control) were included in this study. The pain scores of the breast milk group were significantly lower than those of the control group (z = −2.04, p = 0.021).

Conclusion:

Breast milk odor may reduce pain in premature infants during heel stick tests.

Introduction

Most premature infants are admitted to neonatal intensive care units (NICUs) due to physiological immaturity.¹ Pain information is transmitted to the initial infant contralateral somatosensory cortex from week 25.² Considering that pain can be recognized from 25 weeks of gestation, most infants admitted to the NICU evidently feel and suffer from great pain during the treatment process. Newborns can experience pain for many reasons, especially as a result of invasive procedures.³ The most frequently performed pain-causing behavior in the NICU is the heel stick, which is performed an average of 16 times per patient, per week.⁴

Upon experiencing pain, infants show behavior patterns such as frowning or crying, increased heart rate and blood pressure, and decreased oxygen saturation.⁵ Stressful events can damage the neonatal brain through a complexity of events including free radical (FR) generation.⁶ Even common routine procedures can be potentially harmful for the newborn if they provoke a high level of pain, and a routine heel stick procedure can generate an increase in potentially harmful FR in neonatal blood.⁶

Repeated pain increases one's sensitivity to tissue damage and reduces the threshold for pain.⁷ Poor control of pain during frequent treatment can lead to long-term physiological and behavioral cognitive side effects in premature infants, including attention deficit disorder, decreased visual-perceptual ability, or visual–motor integration.⁷ Therefore, medical staff must make an effort to alleviate pain, even if the pain is not great. Pain management in the NICU is both an ethical and medical obligation; however, despite international guidelines, pain intervention is difficult to achieve.⁸

Infant calming methods can be divided into two main groups: pharmacological and nonpharmacological. Although painkillers can be used to control pain, they require additional mechanical ventilation, which can lead to other side effects such as delays in feeding or impaired brain growth, poor socialization, and short-term memory loss.⁹ Nonpharmacological methods are suitable to counteract procedural pain because of their short-term efficacy and good tolerance, and are, therefore, increasingly being recommended for pain prevention and pain management.¹⁰

Various methods have been proposed as nonpharmacological interventions for pain relief for premature infants, such as breastfeeding, use of non-nutritive sucking, skin contact with mothers such as kangaroo care, and oral glucose application.^11,12 Recent studies have reported that pain can be reduced through olfactory stimulation. These studies show that breast milk causes physiological changes such as increased cerebral blood flow or cerebral cortex activation in the orbital cortex.^13,14 In addition, breast milk odor influences autonomic nervous system changes such as in heart rate and respiration. Previous studies show that familiar odors such as those of breast milk and vanilla affect responses to pain in premature infants.^15–17

However, the methods and time required for breast milk olfactory stimulation have differed among studies. Therefore, in this study, the breast milk olfactory stimulation method was simplified. If it can be confirmed that pain is reduced through a simplified method, this could establish stronger evidence to guide clinical practices.

We aimed to provide breast milk olfactory stimulation as a method that can be easily applied in clinical practice and to evaluate the effect of olfactory stimulation with breast milk on pain response in premature infants.

Materials and Methods

Design

This study was a randomized controlled study conducted with premature infants who were hospitalized in the NICU at a tertiary hospital with 60 beds in Seoul, Korea, from October 2020 to April 2021.

Sample

The participants of this study were preterm infants with a gestation period of 28 weeks or more, and <37 weeks old. Infants who were admitted to the NICU, whose mothers were able to breastfeed and whose guardians agreed to participate in the study, were included. Infants were excluded if they had been administered drugs (sedatives, diuretics, hormones, and stimulants) that could have affected their pain response within 48 hours before heel stick; had left the invasive ventilation system within 48 hours, which could have affected physiological responses; and had congenital anomalies, genetic diseases, convulsions, chronic cardiopulmonary insufficiency, and intracranial hemorrhage.

Using the software G*power 3.1.9.2, the number of samples that fit a t-test with an effect size of 0.80, a significance level of 0.05 for a one-tailed test, and a power of 80% was at least 21 for each group. Considering a dropout rate of 10%, 48 premature infants were included in the sample (24 in each group).

Initially, 48 premature infants were included in the study. The experimental and control groups were randomly assigned. A randomization sequence was created using the RANDBETWEEN statement in Microsoft Excel 2016 (Microsoft, Redmond, WA), with a 1:1 allocation using random block sizes of six. Two premature infants from the experimental group and two from the control group were excluded from data analysis. As a result, the final number of newborns in each group was 22, for a final total of 44 premature infants who were included in the analysis. The Consolidated Standards of Reporting Trials (CONSORT) flow diagram of the study procedure is shown in Figure 1.

FIG. 1.

CONSORT diagram of this study. CONSORT, Consolidated Standards of Reporting Trial.

The design of the study is not appropriate for a double-blind study since the intervention and pain score assessment were performed by the same researcher. The nurse who assessed the Premature Infant Pain Scale (PIPS) used in this study did not know which participant was assigned to which group.

Instruments

Premature Infant Pain Scale

To assess the pain response of premature infants in this study, the PIPS developed by Kim et al.¹⁸ was used based on the Premature Infant Pain Profile of Stevens et al.¹⁹ and the Neonatal Infant Pain Scale of Lawrence et al.²⁰ The PIPS measures the severity of pain by gestational age, changes in vital signs, and differences in behavioral patterns. This tool consists of four items: corrected age, increase in heart rate, decrease in oxygen saturation, and change in behavioral state after pain stimulation. The difference in the amount of behavioral change is measured based on the degree of change related to the described basic behavior. Each item is evaluated on a scale of 0–3 points. The overall score obtainable ranges from 0 to 12 points, as the sum of each item, with higher scores indicating more severe pain (Table 1).

Table 1.

Premature Infant Pain Scale

	Indicator	0	1	2	3
Observe baseline before pain		HR_		SpO_{2_}
Observe pain response for 30 seconds after pain	Corrected age	>36 + 0 weeks	32–35⁺⁶	28–31⁺⁶	<28 weeks
	Max. HR	Increase 0–4/min	Increase 5–14/min	Increase 15–24/min	Increase 25/min above
	Min. SPO₂	Decrease 1–2%	Decrease 3–4%	Decrease 5–7%	Decrease 8% above
	Activity	No crying/no facial movement or intermittent limb movements or relaxation	Intermittent moaning or some facial movements or movement to avoid pain	Sobbing/intermittent crying or making a slight grimace or intermittent muscle rigidity	Crying/making a grimace or continuous muscle rigidity (stiff limb extension or flexion) or struggling
		Total score

Pain level: 0–3 points = no pain–mild pain, 4–6: mild–moderate pain, >7: moderate–severe pain.

Physiological monitoring

To assess variability in infants' physiological responses, infants' heart rate and blood oxygen saturation were assessed by physiological monitoring using a Philips IntelliVue MX850 bedside patient monitor. We used three-lead electrocardiogram (ECG) electrodes, an Ambu BlueSensor (BR-50-A), and a Nellcor pulse oximetry saturation sensor. Infants' heart rate was measured based on the ECG and confirmed when it was the same as the infants' pulse rate. The sensors were continuously evaluated to verify whether they were functioning correctly and were replaced when they malfunctioned.

Implementation of the study

Before the heel stick blood sampling procedure

The infants' guardians were informed about the purpose and methodology of the study, and their consent for participation was obtained. Mothers of the infants in the experimental group brought breast milk in a breast milk storage pack, 0.2 mL of which was placed into a sterilized syringe and stored in a breast milk refrigerator.

We used three-lead ECG electrodes, which were placed on infants' right arm, left arm, and left leg. A saturation sensor was placed on the right or left ankle of the premature infants, and heart rate and SpO₂ level were continuously recorded before and after sampling.

Breast milk olfactory stimulation in the experimental group

Milk was warmed for 2 minutes in a breast milk heater set to 70°C before the heel stick. Wearing disposable gloves, one research nurse involved in this process then soaked a cotton swab of ∼1.3 × 0.3 cm attached to the end of an 8 cm long stick in the warmed breast milk. The milk was gently applied to the infants' nose by moving the cotton swab slowly from one nostril to the other in 1 second. This was repeated three times. The exposure time was 30 seconds.

In this study, the breast milk olfactory stimulation performed was modified from the study of Bartocci et al.¹⁴ In that study, the substance to be tested was soaked into a cotton bud (∼1.5 × 0.5 cm) attached to the end of a 20 cm long stick, and it was found that Hb changes by near-infrared spectroscopy can assess olfactory responsiveness in 30 seconds from infants.

Taking the heel stick sample

Heel sticks were performed between 8:00 am and 10:00 am on all infants by one research nurse. The room temperature was set to 23–26°C.

Heel sticks were performed 1 hour after lactation. Before sampling, the infants were placed in a supine position, which was maintained in a stable state without nursing treatment or stimulation until the heel stick. The research nurse disinfected the heel, punctured the capillaries of the heel once with an automatic dedicated needle, squeezed for 2 seconds to collect samples, and collected blood for 8 seconds. After repeating this procedure three times, a disinfected gauze was applied to the heel. After sampling, the crying infants were swaddled or patted to calm them down.

Video recording

The heel stick procedure was videotaped. The recording shows the monitoring of premature infants' oxygen saturation and heart rate, as well as their behavioral response. After installing the video recorder at a location where the whole body could be seen, the recording began 5 seconds before the beginning of the heel stick and continued for 30 seconds after.

Pain response analysis

The pain response evaluation through video was conducted by two trained research assistants. The study assistants were two nurse practitioners with 16 and 10 years of clinical experience working in the NICU, and the difference between the two evaluators was minimized through training on PIPS measurement methods before beginning the study. The intraclass correlation coefficient (ICC) was calculated to determine the inter-rater reliability of the PIPS; the ICC value was 0.92. Heart rate and oxygen saturation were recorded as the difference between the highest and lowest values among the values shown in the patient monitoring device. Behavioral status was evaluated as the most severe degree of crying, facial expression change, arm movement, and leg movement of premature infants during nursing practice.

In the video analysis process, there were two infants whose facial expressions or behaviors could not be analyzed well because they moved away from the camera, and two infants whose changes in vital signs could not be accurately identified. Therefore, these patients were excluded from the study.

Ethical considerations

The ethics committee of Samsung Medical Center approved the study (SMC IRB No. 2020-03-114), and written consent was obtained from all the infants' parents/guardians.

Data analysis

SPSS for Windows (Version 23.0; SPSS, Chicago, IL) was used for statistical analyses. Numbers, percentage values, mean ± standard deviation (minimum–maximum), Shapiro–Wilk test, t-test, Mann–Whitney U test, chi-square test, and Fisher's exact test were used to assess the data. Results were evaluated at a 95% confidence interval, and statistical significance was set at p < 0.05.

Results

Table 2 shows the demographic characteristics of the infants in both groups. It can be seen that there are no significant differences among the two groups in demographic characteristics. The preterm infants in both groups were observed to be similar in terms of gender, gestational age, birth weight, corrected age, and body weight during the heel stick procedure.

Table 2.

Demographic Characteristics of Infants in the Breast Milk Odor and Control Group

Group	Gender		Gestational age (weeks)	Birth weight (g)	Corrected age at heel stick (weeks)	Body weight at heel stick (g)
Group	Girl	Boy	Gestational age (weeks)	Birth weight (g)	Corrected age at heel stick (weeks)	Body weight at heel stick (g)
Experimental group (n = 22)	10 (45.5)	12 (54.5)	33.31 ± 2.62^a	1914.55 ± 482.88^a	35.07 (33.86–35.96)^b	1994.55 ± 546.95^a
Control group (n = 22)	15 (68.2)	7 (31.8)	33.20 ± 2.03^a	1816.82 ± 554.59^a	35.07 (33.29–36.25)^b	1858.64 ± 503.50^a
p	0.128^c		0.884^d	0.536^d	0.879^e	0.396^d

Mean ± standard deviation.

Median (IQR).

Chi-squared test.

Student's t-test.

Mann–Whitney U test.

IQR, interquartile range.

The pain response scores of the two groups significantly differed (z = 2.04, p = 0.021), with 3.25 points for the experimental group and 4.75 points for the control group Table 3. It was confirmed that the experimental group with olfactory stimulation had a lower pain response score than the control group with no olfactory stimulation.

Table 3.

Infants' Premature Infant Pain Scale Scores After Sampling

Characteristics	Experimental group (n = 22)	Control group (n = 22)	z	p
Characteristics	Median (IQR)	Median (IQR)	z	p
PIPS	3.25 (2.88–5.63)^a	4.75 (4.00–6.00)^a	−2.04	0.021^b

Median (IQR).

Mann–Whitney U test.

PIPS, Premature Infant Pain Scale.

Discussion

The results of this study confirmed that olfactory stimulation of breast milk reduces the pain response in premature infants. In previous studies, olfactory stimulation with maternal breast milk was shown to be effective in calming the physiological responses of preterm infants undergoing painful procedures.

In premature infants, breast milk odor can decrease the variability of their heart rate and blood oxygen saturation during and after venipuncture.¹⁵ Other studies found that neonates exposed to their own mother's milk odor had a significantly lower pain score during venipuncture and a reduced crying duration after venipuncture, compared with the control group.^11,21

Similar results were observed in term infants. The pain response score was low during olfactory stimulation of breast milk before the heel stick, and the fluctuations in heart rate and oxygen saturation were significantly low.^16,17 In addition, the concentration of salivary cortisol was slightly increased, and crying, facial frowning, and exercise status indicators were reduced.^17,22

Although heel sticks have a short duration of high-intensity pain, it is necessary to actively apply nonpharmacological pain relief interventions as it is a nursing practice that is repeated daily or periodically.⁴ Observing pain reaction from venipuncture in premature infants, and heel stick in full-term infants, previous studies noted that heel sticks in premature infants caused a higher intensity of pain than venipuncture.²³ Since this study confirmed that pain was reduced in the heel stick, which caused higher intensity pain, it is thought that breast milk olfactory stimulation can be utilized as a nonpharmacological pain relief intervention in other procedures that cause lower intensity pain.

In this study, 0.2 cc of breast milk was gently applied under infants' nose using a cotton swab 30 seconds before the heel stick. Breast milk olfactory stimulation has been applied using various methods. In previous studies, 5 cc of breast milk was placed in a diffuser from 3 minutes before the painful procedure,¹¹ 5 minutes before in a glass bottle,¹⁶ and 1 minute before in a plastic bottle²⁴ to 5 minutes before using a sterile cotton swab^15,21,22 or filter paper.^1,17 The importance of this study lies in its confirmation of the usefulness of breast milk olfactory stimulation with a short application time and a small amount of breast milk.

Maternal breast milk causes no harm or stress to the infant; moreover, olfactory stimulation with maternal breast milk is natural, inexpensive, and easy to implement in the NICU for preterm infants.²⁵ In addition, if breast milk is used for purposes other than breastfeeding, parents can be comforted by the thought that they are helping their baby through the treatment process, even if their baby is fasting.

Conclusion

This study aimed to determine the effect of olfactory stimulation with breast milk on pain responses to heel sticks in premature infants. The results confirmed that breast milk olfactory stimulation was able to reduce the pain response. This study is meaningful in that it provides a basis for reducing premature infant pain through breast milk olfactory stimulation, which is a nonpharmacological intervention in the clinical field.

Footnotes

Authors' Contributions

J.H.K. and S.J.C. conceived and designed the study. J.H.K. contributed to data acquisition. All authors contributed to analysis and interpretation of the data, drafted the article and figures, made critical revisions, and approved the final version of the article.

Disclosure Statement

No competing financial interests exist.

Funding Information

This research was funded by Seoul Nurses Association Hanmaeum Scholarship in 2018.

References

Badiee

, Asghari

, Mohammadizadeh

. The calming effect of maternal breast milk odor on premature infants. Pediatr Neonatol, 2013; 54:322–325.

Slater

, Cantarella

, Gallella

, et al. Cortical pain responses in human infants. J Neurosci, 2006; 26:3662–3666.

Cakirli

, Acikgoz

. A randomized controlled trial: The effect of own mother's breast milk odor and another mother's breast milk odor on pain level of newborn infants. Breastfeed Med, 2021; 16:75–81.

Courtois

, Droutman

, Magny

, et al. Epidemiology and neonatal pain management of heelsticks in intensive care units: EPIPPAIN 2, a prospective observational study. Int J Nurs Stud, 2016; 59:79–88.

Grunau

, Linhares

, Holsti

, et al. Does prone or supine position influence pain responses in preterm infants at 32 weeks gestational age?. Clin J Pain, 2004; 20:76–82.

Bellieni

, Iantorno

, Perrone

, et al. Even routine painful procedures can be harmful for the newborn. Pain, 2009; 147:128–131.

Hall

, Anand

. Pain management in newborns. Clin Perinatol, 2014; 41:895–924.

Committee on Fetus and Newborn and Section on Anesthesiology and Pain Medicine. Prevention and management of procedural pain in the neonate: An update. Pediatrics, 2016; 137:e20154271.

Ferguson

, Ward

, Paule

, et al. A pilot study of preemptive morphine analgesia in preterm neonates: Effects on head circumference, social behavior, and response latencies in early childhood. Neurotoxicol Teratol, 2012; 34:47–55.

10.

Cignacco

, Hamers

, Stoffel

, et al. The efficacy of non-pharmacological interventions in the management of procedural pain in preterm and term neonates. A systematic literature review. Eur J Pain, 2007; 11:139–152.

11.

Baudesson de Chanville

, Brevaut-Malaty

, Garbi

, et al. Analgesic effect of maternal human milk odor on premature neonates: A randomized controlled trial. J Hum Lact, 2017; 33:300–308.

12.

Johnston

, Campbell-Yeo

, Disher

, et al. Skin-to-skin care for procedural pain in neonates. Cochrane Database Syst Rev, 2017; 2:CD008435.

13.

Aoyama

, Toshima

, Saito

, et al. Maternal breast milk odour induces frontal lobe activation in neonates: A NIRS study. Early Hum Dev, 2010; 86:541–545.

14.

Bartocci

, Winberg

, Ruggiero

, et al. Activation of olfactory cortex in newborn infants after odor stimulation: A functional near-infrared spectroscopy study. Pediatr Res, 2000; 48:18–23.

15.

Neshat

, Jebreili

, Seyyedrasouli

, et al. Effects of breast milk and vanilla odors on premature neonate's heart rate and blood oxygen saturation during and after venipuncture. Pediatr Neonatol, 2016; 57:225–231.

16.

Akcan

, Polat

. Comparative effect of the smells of amniotic fluid, breast milk, and lavender on newborns' pain during heel lance. Breastfeed Med, 2016; 11:309–314.

17.

Tasci

, Kuzlu Ayyildiz

. The calming effect of maternal breast milk odor on term infant: A randomized controlled trial. Breastfeed Med, 2020; 15:724–730.

18.

Kim

M-S

, Kim

M-J

, Ham

E-H

, et al. Development of a Premature Infant Pain Scale (PIPS). J Korean Acad Fundam Nurs, 2006; 13:510.

19.

Stevens

, Johnston

, Petryshen

, et al. Premature infant pain profile: Development and initial validation. Clin J Pain, 1996; 12:13–22.

20.

Lawrence

, Alcock

, McGrath

, et al. The development of a tool to assess neonatal pain. Neonat Netw, 1993; 12:59–66.

21.

Jebreili

, Neshat

, Seyyedrasouli

, et al. Comparison of breastmilk odor and vanilla odor on mitigating premature infants' response to pain during and after venipuncture. Breastfeed Med, 2015; 10:362–365.

22.

Rattaz

, Goubet

, Bullinger

. The calming effect of a familiar odor on full-term newborns. J Dev Behav Pediatr, 2005; 26:86–92.

23.

Shah

, Ohlsson

. Venepuncture versus heel lance for blood sampling in term neonates. Cochrane Database Syst Rev, 2011; 2011:Cd001452.

24.

Nishitani

, Miyamura

, Tagawa

, et al. The calming effect of a maternal breast milk odor on the human newborn infant. Neurosci Res, 2009; 63:66–71.

25.

Lee

, Ra

. Olfactory stimulation of preterm infants with breast milk. Clin Nurs Res, 2021; 30:1183–1192.