Non-nutritive sucking in the healthy pre-term infant

Abstract

In the infant, sucking behaviour is one of the first coordinated muscular activities. It is under the control of the brainstem. In utero sucking is observed from 13 weeks’ gestation. The healthy full-term newborn infant has a very stable rhythm of alternating bursts of sucking with pauses in between. The non-nutritive sucking (NNS) patterns of 58 low-risk prematurely born infants (gestational ages between 26 and 35 weeks) were studied using a specially designed computer-based method that analyses and quantifies NNS. A total of 183 observations were analysed. The typical NNS pattern with bursts of sucking activity separated by quiescent periods was recorded in all infants studied and was already present before 30 weeks of gestation. A gradual change over time of their NNS was seen. With increased maturation the sucking activity, sucking frequency, amplitude and burst duration all increased, while the variability of the sucking frequency and the duration of the intervals between bursts declined. Post-menstrual age (PMA) was the dominant predictor of this result but gender, state of activity and weight also influenced it to some extent. Girls had more sucking activity and a higher sucking frequency than boys. State of activity affected the stability of the rhythm. The weight of the infant influenced both sucking activity and duration of separate bursts.

Introduction

Sucking is a fundamental behaviour in the newborn infant. It appears in the neonate in two different modes depending on whether it is nutritive or non-nutritive. Nutritive sucking is almost continuous while NNS is organized as a series of bursts separated by brief pauses [1], [2]. Also the rate is lower during nutritive sucking. Evidently we are dealing with different central organizations for sucking behaviour when speaking of the two modes [3]. In utero jaw movements are seen from week 11 and sucking and swallowing from week 13 [4]. In the prematurely born infant NNS-like activity has been described before 30 weeks of PMA [1], [5], [6]. From PMA 30 weeks and upwards, quantification of the sucking variables has shown that the ontogenesis of NNS is primarily dependent on maturation and little affected by post-natal age or feeding experience. With increased maturation the frequency of sucking within bursts increases, the number of bursts per min increases and the inter-burst variance of rate declines [1], [5]. Healthy, full-term infants can all elicit a very stable burst/pause NNS pattern [1], [7]. If non-nutritive sucking is not stimulated by pacifier or thumb sucking the ability for non-nutritive sucking in this very robust pattern disappears by 4–5 months of age. The burst/pause pattern is reported to again be present in some adult patients with degenerative cerebral diseases [3].

Non-nutritive sucking in the pre-term infant has been thought to lessen distress, decrease crying during painful activities and improve behavioural state, the states more optimal for feeding increase [8], [9], [10]. Accelerated maturation of sucking reflex and earlier readiness for bottle feeding is reported when pre-term neonates were given NNS opportunities during gavage feeding, the gastric motor function is stimulated and the digestion process facilitated, the weight gain becomes more rapid and discharge from hospital earlier [11], [12]. Oxygenation (TcPO₂) is found to increase and heart rate has been reported to be at times increased and at times reduced during NNS in pre-term infants [13], [14].

Previous studies on quantifying the NNS-rhythm have been hampered by the necessity for time-consuming manual analysis of recordings and are therefore based on small numbers of observations.

The aim of the present study is to characterize and quantify the NNS pattern in a strictly selected, large group of pre-term infants without medical problems by an automated system for analysis. The influence on the NNS-pattern by the infants’ gestational age, post-natal age, sucking experience, and maturation (post-menstrual age) as well as gender will be analysed.