Whole-Body Vibration Exposure from Incubators in the Neonatal Care Setting: A Review

Margaret McCallig, Department of Environmental Science, Institute of Technology, Ireland, Email: McCallig.Margaret@itsligo.ie

Received: 28-Jan-2021 Published: 18-Feb-2021

Introduction

Although neonatal intensive care and hospital trans- fer is a crucial part of neonatal care it is not without risk to the patient. Several studies have identified whole body vibration (WBV) emission values well in excess of the European occupational exposure action and limit values for adults exposed to WBV in the workplace setting[1-5, 6-15]. Furthermore, WBV ex- posure of these magnitudes is categorized at the up- per end of the ISO 2631:1997 Comfort Rating Scale. Despite the growing recognition of WBV as a risk to the neonate during hospital transport, to date, little data exists around the risk of WBV in-situ in the hos- pital incubator setting. In this Review, previous stud- ies which involved a live neonate and / or neonatal mannequin were included. There exists a wide vari- ation across study design, sampling population and hypotheses among existing studies. This Review aims to summarize up-to-date information about WBV exposure in the neonatal setting from an inter-hos- pital, and intra-hospital perspective. A summary of the study design, sampling strategy, findings and key recommendations is included in (Table 1).

Definition of a Neonate

The World Health Organisation (WHO) defines preterm birth as babies born alive before 37 weeks of pregnancy are completed [16]. WHO estimate that approximately 1 in 10, or 15 million babies are born prematurely worldwide each year [16]. There are sub-categories of preterm birth, based on gestational age: Extremely preterm (<28 weeks); Very preterm (28 to <32 weeks); Moderate to late preterm (32 to
<37 weeks). A systematic review and modelling anal- ysis of data on preterm birth in databases of national civil registration and vital statistics, supplemented with population-representative surveys and research studies on a global level was conducted on data re- corded in 2014 [17]. The study reports that an esti- mated 10•6% of livebirths worldwide were preterm in 2014. Estimated pre-term birth rates and propor- tion of global pre-term births were reported for re- gions based on United Nation Standard Country or Area Codes for Statistical Use. These rates were re- ported respectively as follows: Asia (10.4%; 52.9%), Europe (8.7%; 4.7%), Latin America and the Carib- bean (9.8%; 7.2%), North America (11.2%; 3.3%), North Africa (13.4%; 5.2%), Oceania (10%; 0.4%) and Sub-Sahara Africa (12%; 28.2%). The Irish Neo- natal Health Alliance (INHA) states that ‘globally over 15 million infants are born too early, too small and too sick each year: that’s one in 10 babies. From an Irish perspective the figure stands around 4,500 and that equates to one baby born prematurely every 116 minutes’ [18]. According to the Central Statistics Of- fice (CSO), 67295 births were registered in Ireland in 2014 [19]. This equates to a preterm birth percent- age in Ireland of approximately 6.7% in 2014, which is lower than the estimated preterm births for Europe as published previously [20].

Whole Body Vibration (WBV) Legislation

The European Directive 2002/44/EC on the mini- mum requirements regarding the exposure of work- ers to the risks arising from physical agents (vibra- tion) is applicable to occupational exposures to WBV and hand-arm vibration (HAV). The Directive was transposed into Irish law by the Safety Health and Welfare at Work (General Application) Regulations 2007 (SI 299/2007) [21]. WBV is defined in the legis- lation as ‘the mechanical vibration that, when trans- mitted to the whole body, entails risks to the safety and health of employees, in particular lower-back morbidity and trauma of the spine [20,22]. The Direc- tive defines exposure action values (EAV) and expo- sure limit values (ELV) for WBV (Table 1), based on a standardised eight-hour reference period, simulating a typical workday. (Table 2)

The legislation places an obligation on the employer to risk assess, and if necessary, measure the levels of exposure to mechanical vibration. The results of the risk assessment must be recorded. The risk assess- ment is required to be updated at regular intervals, particularly if there have been significant changes which could deem it insufficient or inadequate. To the author’s knowledge, no specific exposure action or limit values with regards WBV exposure to infants and children in a hospital setting exists.

Figure 1. NIOSH Hierarchy of Control Principles

From an occupational perspective, where the risk as- sessment results in WBV emission values in excess of the legal limit values, a programme of control mea- sures to eliminate or reduce the exposure so far as is reasonably practicable is required. The National Institute for Occupational Safety and Health (NIOSH) developed the ‘Hierarchy of Control’ as a means of determining how to implement feasible and effective control solutions.

The Hierarchy of Control is a top-down approach to the management of risk. The application of the Hier- archy of Control requires consideration of the head- ings in the order shown in (Table 3), and not simply selecting the most convenient control measure to im- plement [23].

Neonatal incubator compliance falls under two sig- nificant pieces of European legislation when it comes to their design and specification; Machinery Directive 2006/42/EC [24] and Medical Device Regulations 2017/745 [25].

Annex I of the Machinery Directive 2006/42/EC de- tails the Essential Health and Safety Requirements (EHSR’s) relating to the design and construction of machinery, and specifically states with respect to vibration; ‘Machinery must be designed and construct- ed in such a way that risks resulting from vibrations produced by the machinery are reduced to the low- est level, taking account of technical progress and the availability of means of reducing vibration, in partic- ular at source.’ Furthermore, Annex I calls for instruc- tions to be provided relating to the installation and assembly of machinery to reduce vibration emissions. The accompanying instructions must give the follow- ing information concerning vibrations transmitted by the machinery to the whole body:

• the highest root-mean-square (rms) value of weighted acceleration to which the whole body is subjected, if it exceeds 0.5 m/s2. Where this value does not exceed 0.5 m/s2, this must be mentioned.

These values must be either those actually measured for the machinery in question or those established on the basis of measurements taken for technically com- parable machinery which is representative of the ma- chinery to be produced. Any uncertainty surrounding the measurements, the operating conditions or the measurement code must be described [24].

Neonatal incubator systems are generally classified as a Class II medical device. Annex 1 of the Medical De- vice Regulations 2017/745 outlines the general safe- ty and performance requirements of a medical device. Manufacturers are required to establish, implement, document and maintain a risk management system. This is an iterative process which involves identifying and analyzing known and foreseeable hazards, evalu- ating their risk during intended use and eliminating the hazards where possible. With regards vibration, the Regulations state ‘Devices shall be designed and manufactured in such a way as to reduce to the low- est possible level the risks arising from vibration generated by the devices, taking account of technical progress and of the means available for limiting vi- brations, particularly at source, unless the vibrations are part of the specified performance’ [25].

Whole Body Vibration (WBV) Exposure

It is almost impossible for any person to avoid vibra- tion exposure in today’s world. Vibration exposure can occur at work, commuting between home and work, and in leisure activities. The effects of whole- body vibration on the human body have been a sub- ject of research since the early 20th century [26]. It is well known that any form of transportation will ex- pose humans to some degree of mechanical vibration, or more specifically, WBV. The detrimental effects of WBV and their effect on humans has been researched and documented across the world [26]. WBV expo- sure can result in large variations between subjects with respect to biological effects. Most often, it is the lumbar spine and the connected nervous system that may be affected by WBV exposure [27]. Other stud- ies have highlighted the neck-shoulder, the gastroin- testinal system, the female reproductive organs, the peripheral veins, and the cochleo-vestibular system are also assumed to be affected by WBV [28-30]. The effects are complex and depend at least on the vibra- tion amplitude, direction, frequency, duration and to which part of the body it is directed. A major part of previous studies on WBV has been about measuring and analysing the vibration exposure levels concern- ing health for various work environments, machinery and equipment.

Vibration has deleterious effects on pregnant women and foetuses. There are studies presenting evidence of the detrimental effects of whole-body vibration on pregnancy. One such study found “that the higher risk of premature birth and menstruation disorders can be attributed to long-term exposure to whole body vi- bration, and no safe exposure limits can be established to avoid the enhanced risk to the woman’s health in the prenatal period” [31]. Furthermore, qualitative studies have indicated that pregnant women exposed to WBV on private (car) and public (tram) transport can result in similar complaints, such as spine aches, abdominal complaints, dizziness & headaches [28].

There is often a requirement to transport neonates across various hospital departments (i.e. birthing suite to NICU), as well as inter-hospital transportation for the purposes of specialised neonatal care. Many physical stressors exist in the neonatal care settings, both within hospital and during inter-hos- pital transfer. Exposure to WBV is a physical hazard of concern with potential to cause harm to the neo- nate. Several studies have measured, analysed, and some have attempted to predict the long-term health effects of WBV exposure on the neonate. Primary studies investigating the level of vibration that neo- nates were exposed to during transport between hos- pitals were first undertaken in the 1980’s. The first published study measured the vibration produced by vertical acceleration of the infant and recorded the time spent by neonates in land-based transit [12]. A further study was similar in observational design but included transfers completed by road and air [11].

WBV Measurement Methodology

International Standards Organisation (ISO) ‘ISO 2361-1:1997 Mechanical vibration and shock – Eval- uation of human exposure to whole-body vibration’ is the only globally recognised Standard for the assess- ment of WBV. The Standard states “There can be large variations between subjects with respect to biological effects. WBV may cause sensations (e.g. discomfort or annoyance), influence human performance capability or present a health and safety risk [32]. Clause 7 and Annex B concern the effects of periodic, random and transient vibration on the health of persons ‘in nor- mal health’ exposed to WBV during travel, work and leisure time. WBV in respect of Clause 7 is measured as acceleration and is reported as root mean square (RMS) acceleration, with the highest level of vibra- tion during a timed sample as the peak acceleration. Several previous studies on neonatal WBV exposure use this value to report emission values [1–5]. The vi- bration dose value (VDV), the fourth power vibration dose method, a more sensitive evaluation approach, may also be utilised where peaks are to be expected in the dataset. An American based study reported VDV emissions as the data obtained during an ambu- lance ride with occasional shocks [8]. Annex B of ISO 2631:1997 indicates ‘Health Guidance Caution Zones’ which takes account of the frequency-weighted vi- bration acceleration and the expected daily exposure, from an occupational perspective.

To the author’s knowledge, no International standards exist for the measurement of infant exposure to WBV. Several previous studies have used ISO 2631:1997 guidelines for adults’ exposure of WBV with respect to health as a guideline for infants and neonates in the hospital and inter-hospital transfer settings. A number of these studies [6,7] have compared WBV emission values to the ‘comfort reactions to vibration environments’ published in ISO 2631:1997 as presented in (Table 2).

For measurements, transducers shall be located to indicate the vibration at the interface between the human body and the source of its vibrations [32]. ISO 2631:1997 sets out the basicentric axes of the adult human body in three positions – seated, standing and recumbent. The standard further details specific lo- cations for transducers to be placed for adults in the recumbent position, as under the pelvis, the back and the head. There exists a wide variation when locating transducers in previous studies; from placement on the head and incubator frame [5], under the mattress [9,10], on the head and incubator base [4], on the in- cubator frame [2] , and on the incubator tray only[3].

ISO 2631:1997 requires that the duration of WBV measurements be reported. Whilst no specific guid- ance is provided on the length of measurements, the standard states ‘the duration of measurement shall be sufficient to ensure reasonable statistical preci- sion and to ensure that the vibration is typical of the exposures which are being assessed’ [32]. Similar to the transducer location requirement, there exists large variations between measurement durations in previous studies. Examples of convenience sampling durations varied from 34 hours [1] to 10 hours [2] for road transfers but were more aligned for air transfers at 2 hours [2] , 127 minutes [6] and mean transport time of 2 hours 30 minutes across 16 air journeys [10].

Factors that Affect WBV Emissions Reported

(Table 3) summarizes previous study design and hypotheses on neonatal WBV exposure. Whilst all studies applied the ISO 2631:1997 approach to mea- surement, there exists a broad variation between the parameters reported in the respective methodolo- gies.

Sample sizes varied significantly across the studies in- cluded in this review; from 1 to 1028. The methodol- ogy adopted by researchers influenced the significant difference; some chose to analyze complete journeys [6], and others defined criteria for replicate samples based on comparison of ancillary equipment such as mattresses[4,5,14]. The populations varied from neo- nate only [2,6,9-12], mannequin only [4,5,7,14,15], or a combination of both [1]. Research time constraints and ethical approval were the cited barriers to use of neonates. The focus of some studies was on the trans- port system and the potential impact of design chang- es on vibration emissions, rather than the impact on the safety and comfort of the neonate [3,8].

Inter-hospital neonatal transport occurs when patients in neonatal units are transported to other neo- natal or paediatric units for on-going care. Modes of transport include road (ambulance) and air (fixed wing or rotary wing). Neonates requiring transport of either form may already be in a compromised health condition. Comparisons of air and road travel hypothesised the misconception that air travel would emit higher levels of WBV. Air transfers have been found to emit higher levels of WBV, when compared to road travel due to the shock and dynamic events that occur during road transport. Road transfers are subject to more ‘impulsive’ events compared to air transfer [2]. However, WBV emission levels were also found to the contrary; road travel exposes the neo- nate to higher levels of WBV. Transport via air emits more stable vibration levels, even during take-off and landing compared to road transfer. This would indi- cate that air transfer is more comfortable than the alternative road option when compared to (Table 2)
[6] Efforts to minimize vibration emissions during transport result in moderate success [7,11,33]. Road classification and vehicle speed were analysed fur- ther to determine their influence on vibration mag- nitude during neonatal transfers. Similar studies in the construction sector reported that terrain type is a significant predictor of vibration magnitude across a range of mobile machinery and plant [34]. Explora- tion of road transfer WBV emissions have shown that as speed increases a continual upward trend in vibra- tion is noted, however, road type appears to have a weak influence on vibration [1].

Studies to reduce WBV and the effect on back pain in mobile plant and machinery have utilised suspen- sion applications [35] and damping systems [36] as effective means of reducing the health effects of WBV. Acceptable levels of WBV have not been defined in lit- erature or legislation for neonates. Simulated scenar- ios have been designed and analysed for the in-hospi- tal setting, which focus on the influence of vibration isolation components of the neonatal incubator sys- tem, such as latching mechanisms and varying wheel types. The study involves an empty incubator being pushed over a selected route within the hospital set- ting, as varying speed and configurations of isolation components. Vibrations increase substantially if any part of the shock suppression system is malfunction- ing and emphasises the importance of a preventative maintenance programme for neonatal incubators [3]. Comparative studies of different design of neo- natal transport systems further support the need for vibration suppression systems to be in place. Brac- ing mechanisms on neonatal incubator frames emit significantly lower vibration than those systems not diagonally braced, particularly when traversing floor surfaces, for example, entering and exiting an elevator [5].

Investigations of the effect of different mattress types have yielded various results. The first such study took place in the early 1990’s. Mattresses are typically cat- egorised as sponge, foam, air or gel-filled; the design of which have evolved over the years, making it dif- ficult to compare results across studies. In general findings show that gel-filled mattresses are more effi- cient in decreasing the vibration magnitude emitted, however limitations of these studies outline the use of mannequins only [4,15]. Interestingly the lower the weight of the mannequin reduced the efficiency of the gel-filled mattress’ attenuation [4]. In-hospi- tal transfers from delivery rooms to neonatal units demonstrated that air-foam mattress experienced less impulsive vibration compared to the standard mattress [14]. Combinations of mattress type have been analysed in observational studies, yielding im- proved attenuation of vibration with a gel mattress placed on top of an air mattress [7]. In contrast, re- sults have been found to be inconsistent, therefore making it difficult to recommend a specific type of mattress over another [5].

Conclusion

Important questions remain regarding the exposure of neonates to whole body vibration, particularly in the hospital setting. The majority of the literature has limitations with regards sample sizes, use of neonates versus use of mannequins and transport modes. How- ever, what is clear is that there exists an urgent need to determine the exposure of neonates from incubators in situ in the hospital setting. Neonates may spend a number of days/weeks in the neonatal incubator set- ting, therefore analysis of the WBV exposure will aid in determining the exposure in relation to legislative action and limit values, as well as the Comfort Scale Rating of ISO 2361:1997. Recommendations thus far are focused on improved design of incubator systems with a view to dampening vibration sources and sub- sequent risk. A better understanding of the preventa- tive maintenance requirements and ancillary equip- ment specifications of mattress and incubator frames is required in response to the ever-evolving design of neonatal incubators.

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Citation: Margaret McCallig. Whole-Body Vibration Exposure from Incubatorsin the Neonatal Care Setting: A Review. J Environ Occup Health. 2021; 11(2):37-46

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