Measuring preterm cumulative stressors within the NICU: The neonatal infant stressor scale☆
Introduction
There is mounting evidence that repeated stress, especially that occurring during the critical early period of infant development, has profound and long-lasting effects on several physiological systems, including the central nervous system [1]. Preterm infants are, of necessity, living and developing in an environment that is stressful in multiple ways. These multiple stressors occur at a time of exquisite central nervous system (CNS) vulnerability, immature regulation of the blood–brain barrier and rapid development of the neuronal architecture [2]. Immature physiological systems, and especially the CNS, are probably vulnerable to insult via that stress [3]. It is the accumulated stress, or the totality of large and small, multiple stressors occurring over time that we argue may affect preterm infant development and therefore needs to be measured and managed. Stress responses are essential for adaptation and the maintenance of homeostasis, yet over longer time intervals, when called upon frequently or for prolonged periods, they exact a “allostatic load” [4] or cost that can accelerate disease processes [5].
Stressors are physical, psychological and social events that result in excessive reactions and difficulty in coping — “…a disturbance of the dynamic equilibrium between an organism and its environment” [6]. Whilst painful experiences are probably always stressful, non-painful experiences also cause disequilibrium in preterm neonates [6]: reactions in preterm infants are found for painful procedures such as heel lance [7], ventilation and venous blood sampling [8] and also for presumably non-painful stimuli such as clustered care [6], position changes, administration of medicine through an intravenous line and withdrawing fluid through an umbilical catheter [9]. Similarly, full-term newborns produce elevated plasma cortisol in response to undressing, weighing and measuring [10]. In addition to acute stressors, aspects of preterm infants' living environment (e.g., excessive noise and light) are thought to impose chronic stress [11].
Medical and nursing procedures can be unrelenting for sick preterm infants. Examples include: more than 700 procedures over the hospitalisation [12]; more than 200 handling episodes within a 24-h period [13]; and 93 heel pricks within a two-week period [14]. Pain in preterm infants can induce increased heart rate and blood pressure, a fall in oxygen saturation, and an increase in intracranial pressure [15], and routine nursing events can induce hypoxaemic episodes, increases in stress hormones [16], loss of blood volumes [1] and prolonged drops in temperature [17].
Despite evidence that preterm infants are able to perceive pain by early gestation (via activation of the somatosensory cortex [18]) they do not always mount a clear, sustained, predictable and measurable response to painful (and presumably stressful) stimuli. The hypothalamic, pituitary and adrenal (HPA) axis is not fully developed, and preterm infants' behavioural and physiological responses to pain are erratic [19]. They can change as a function of several factors, including the number of previous painful experiences, [20], [21], gestational (GA) and post-conceptional age [22], gender, position [7] and prenatal exposure to betamethasone [23], [24]. Associations between observable reactions (such as facial expressions and sleep/wake behaviours — crying is not possible in a ventilated infant) and autonomic indicators of pain and stress (such as heart rate and oxygen saturation) are complex and form another difficulty in assessing the validity of stress reactions. In one study, and in response to heel lance, some preterm neonates had high behavioural and low autonomic reactions, some had the opposite, while the majority showed concordant reactions [25]. In addition, not all infants display the same behavioural and/or physiological responses to the same painful or stressful stimuli, and there is considerable overlap between stress responses and pain responses [7]. A lack of concordance sometimes found between behavioural and physiologic reactions (especially at younger GAs), the multiple pre-existing influences found to affect them, and the lack of reliable or consistent responses reveal the challenges to validly identifying stress in preterm infants. An objective and standard measure of acute and chronic stress that also measures cumulative stress would therefore be helpful.
The measurement of pain in infants has become prevalent, because of the critical need to manage it [26] yet the measurement of infant stress, which involves living conditions as well as painful and non-painful procedures, has not been routinely addressed to the same extent. Als [27] developed a comprehensive but time-consuming infant observation approach to help nurses pace infants' interventions and Modrcin-McCarthy et al. [28] developed a system whereby nurses note subtle changes in autonomic, motor and state behaviours to manage infants' stress. However the percentage of hospitals that use this level of intensive individual assessment still appears to be small and the routine tracking of the number and severity of stressors for any one baby is still rare.
Similarly, while physiological correlates of stress (e.g., skin conductance and blood and salivary cortisol) remain the most precise measure of infant stress, their ubiquitous use to keep track of cumulative stress, when there are so many potential stressors, is untenable. Even though more easily-used behavioral responses (to pain) have been validated using physiological means, recent surveys of Australian [29] and United Kingdom [30] nurseries found disappointing system levels of pain assessment, articulated policies and practices, and the routine use of pharmacological and non-pharmocological analgesic measures.
In order to address the problem of unmanaged infant stress in a different way, the aim of this study was to develop a standard cumulative measure of presumed stress to a range of stressors. NICU clinicians were surveyed about their distilled knowledge of infant stress to multiple events and conditions and their responses used to develop a cumulative stressor notation system. Doctors' and nurses' judgements of the relative infant stress reactions to common acute handling, nursing, medical and care procedures and to chronic living conditions were collated into a list of stressors including their perceived severity of each stressor. We also aimed to assess doctors' and nurses' own stress while performing the procedures.
Section snippets
Subjects and methods
The prototype of the Neonatal Infant Stressor Scale (NISS — see Table 2), participant information, informed consent form and letter of explanation were distributed with fortnightly pay details to each nurse (n = 250) and doctor (n = 40) employed in NICUs at two tertiary level hospitals. They were asked to complete the forms, place them in a sealed envelope and post to boxes that were available in several places around each hospital department. The instructions to participants were as follows: “
Discussion
This study indicates that medical and nursing staff who care for preterm infants perceive infant stress in response to most acute medical and care procedures and conditions. As would be expected, painful items such as vascular insertions, eye examinations, surgery, lumbar punctures and intubation were perceived to be the most stressful of all. However, almost all instances in which the infant is handled with the notable exceptions of most nutrition items at any gestational age were perceived to
Acknowledgements
Thanks to Melinda Hall, Dr Rod Hunt, Merilyn Bear and Connie Wong for their help in compiling the questionnaire items, and to all the staff of the Mercy Hospital for Women and Royal Women's Hospital in Melbourne Australia who took the time to complete the questionnaires.
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