The thermal needs of preterm and high-risk infants are well documented.1–4 The NICU thermal environment must support 2 diverse populations, thermally fragile infants and mature adults including caregivers, parents, and other support personnel. To accommodate the thermal capabilities of these 2 groups, specialized equipment, such as incubators and radiant warmers, are used to create a microenvironment suited to the thermoregulatory capacity of preterm infants while maintaining room temperatures supporting adult thermal comfort. A substantial amount of information provides thermal care of preterm and high-risk infants; however, there is limited attention paid to the thermal characteristics of the NICU itself. Nurses in our NICU raised concerns about variability of room temperature, particularly in relation to seasonal outdoor temperature and weather conditions. The purpose of this clinical research was to examine the NICU thermal environment throughout a calendar year. Specific aims of the project included the following: (1) thermal mapping of the NICU, conducted in each of 4 seasons (winter, spring, summer, and autumn), which involved measurement of evaporative and radiation effects in addition to ambient air temperature; (2) description of thermal characteristics of the NICU; and (3) comparison of NICU temperatures by season.
Thermal care is central to reducing morbidity and mortality among preterm and high-risk infants. Management of the thermal environment is a primary nursing activity. In addition to the use of incubators and radiant warmers, a number of thermal aids, such as plastic wrap, insulated hats, and heated gel pads, represent an array of interventions to maintain infants within a thermal neutral zone.1,3,5 Although incubators and radiant warmers are powerful tools, infants remain susceptible to the influence of the NICU room environment.5 Radiant warmers provide radiant energy and typically are the only devices that directly warm the infant. Although radiant warmers increase heat gain by radiation, the infant under a radiant warmer experiences heat loss from conduction, convection, and evaporation. Incubators act to decrease heat loss by convection rather than directly heating the infant (unless incubator air temperature is higher than body temperature). Infants in incubators lose heat via conduction, convection, evaporation, and radiation. Whether a radiant warmer or an incubator is used, the room thermal environment directly influences these forms of infant heat loss.
Standards for NICU thermal environment specify the acceptable range of air temperature from 72°F to 76°F (22°C-26°C) and relative humidity at 30% to 60%.6 Typically, monitoring of NICU thermal environment involves only these 2 factors. Although both temperature and humidity are important, these measures do not provide a complete picture of the thermal environment because it relates to thermally challenged preterm and high-risk infants.
While the 4 channels of heat flux, convection, radiation, evaporation, and conduction, are highly interrelated, particular thermal measures are reflective of particular forms of heat flux. Convection, the transfer of heat between a solid surface and the air, is directly related to room temperature. Ambient air temperature is measured with a dry bulb thermometer and is synonymous with what is typically thought of as room temperature. Cool nursery air temperature promotes infant convective heat loss.
The effect of evaporation can be assessed with a wet bulb thermometer, which is basically a thermometer covered by a wet fabric sleeve. Evaporation from the wet surface causes heat loss, reducing the temperature reading. The amount of evaporation and hence the difference between dry bulb temperature and web bulb temperature is directly related to room humidity. Relative humidity is expressed as a percentage and is the ratio of water vapor in the air to the highest amount of vapor that could be obtained at a particular temperature. Given the nature of preterm infant immature skin, evaporation is a critical concern because both fluid loss and heat loss are accentuated by limited keratinization of the epithelial surface.7,8
A globe thermometer measures radiant temperature. The globe thermometer involves a black copper (or other highly conductive metal) ball surrounding a thermometer or thermal sensing device. Radiation is the exchange of electromagnetic energy or heat between 2 solid surfaces not in direct contact. The black copper ball is heated or cooled by this exchange of radiant energy with surrounding solid surfaces in the room. In direct sunshine, the globe thermometer temperature will be warmer than that in the surrounding air because of radiant heat gain. Similarly, when surrounded by cool solid surfaces, the globe thermometer temperature will be lower than air temperature because of radiant heat loss. Radiation is a particular concern for preterm infants in incubators because the infant body radiates heat to the incubator walls, and the incubator walls, in turn, radiate heat to surrounding solid surfaces; heat is also exchanged between incubator exterior walls and surrounding air by convection.
Conduction is the transfer of heat between 2 solid objects in contact with one another. In the NICU, cool surfaces, such as bedding or weight scale, increase infant heat loss. Solid surface temperatures, which could be directly monitored with temperature probes, are altered by the effects of convection and radiation within the nursery room.
A repeated-measure descriptive and exploratory design was used to map the thermal environment of the level III NICU located in Seattle, Washington. Thermal measurements were conducted in each patient care cubicle throughout the year period with measurements recorded on a day in each of 4 seasons (January, April, July, and October).
The 32-bed NICU, constructed in 1980, is arranged into 5 rooms (5-7 beds per room). Floor space of the NICU is roughly 8000 sq ft, with 3200 sq ft designated for patients. This averages approximately 100 sq ft per patient. Rooms 1, 2, and 3 are contiguous as are rooms 4 and 5, with open doorways adjoining the rooms. A wall of windows is oriented to the east in rooms 1, 2, and 3, and a wall of windows oriented to the west in rooms 4 and 5. In each room, the windows are approximately 32 in from the floor and extend across the entire length of the room. Windows are double-pane glass, with adjustable mini blinds inserted between the glass layers. The open-room design includes half-wall partitions, approximately 54 in high and extending approximately 59 in from the surrounding wall that divide the room into patient care cubicles. The unit is staffed by 90 nurses and median daily census is 29.
The heating and cooling system includes wall-mounted, forced air-heating units and overhead forced air vents. Airflow in patient care areas is controlled by a dedicated fan system that utilizes 100% outside air, with temperature regulated to specified setting. Air change occurs at a rate of 12 room exchanges per hour.
The NICU was assessed with a thermal environment monitor (QUESTemp° 34; Quest Technologies, Oconomowoc, Wisconsin) equipped with dry bulb, wet bulb, and black globe thermometers as well as measurement of relative humidity and calculation of heat index. The thermal monitor provided a calculation of wet bulb globe thermometer (WBGT) temperature index, a weighted combination of evaporation and radiation effects as well as heat index. Heat index incorporates humidity and ambient air temperature and provides a measure of how the thermal environment “feels” to a typical adult (eg, high humidity increases discomfort at high air temperature). The thermal environment monitor has an operating range of −5°C to 100°C. It provided temperature accuracy of +0.5°C and humidity accuracy of ±5%. Resolution was 0.1°C and humidity 1%.
On the day of recording, thermal monitors were placed in predetermined (and replicated) locations within each cubicle and allowed to stabilize. Values for temperatures and humidity were visually read from the monitor display screen and hand-entered into a data collection form. Readings were obtained twice within a minute period and averaged if different, a condition that occurred rarely. Recordings were performed at a similar time of day across the 4 seasonal assessments.
Daily outdoor low and high temperatures (°F), obtained from the National Weather Service for the Seattle area, were as follows: summer, 59°F, 78°F; autumn, 45°F, 54°F; winter, 24°F, 42°F; and spring, 35°F, 49°F. Mean values for dry bulb, wet bulb, and globe thermometers along with combined WBGT temperature are illustrated in Figure 1. Dry bulb and globe temperatures were warmest in winter and lowest in spring and summer. Wet bulb temperature (and hence combined WBGT temperature) was lowest in winter and highest in summer. Multivariate repeated-measure analysis of variance provided statistically significant season, room, and season by room interaction effects (season, F 15,13 = 2614.44, P ≤ .000; room, F 20,104 = 2.089, P = .009; room by season interaction, F 60,64 = 3.914, P ≤ .000). Differences in room temperatures were less than 2°F. Univariate analyses showed significant differences by season (P ≤ .000) in dry, wet, globe, and WBGT temperatures as well as humidity. Figure 2 illustrates the pronounced effect of humidity by season as well as wide variability of air temperature in winter compared with low variability in summer. The gradient between mean nursery dry bulb temperature and wet bulb temperature was 9.3°F in summer and 16.8°F in winter.
DISCUSSION AND CONCLUSION
Temperatures and humidity differed both by season and by nursery room. Differences between rooms were likely due to effects of external windows, airflow, and variation in the heating and cooling system. Although room temperatures differed somewhat, all nursery rooms exhibited the same general trends in seasonal effects. Previous research has illustrated daily and weekly rhythms in NICU temperature.9 While room air temperatures themselves were generally within the recommended range of 72°F to 76°F, the wet bulb thermometer temperatures indicate substantial cooling due to evaporation. Limitations of the project include measurement restricted to 4 days throughout the year and standardized to a consistent time of day. However, the failure to identify outlying values and similarity of measures within time of measure suggest that the measurement strategy provided a reasonable picture of the thermal environment.
In summary, multiroom NICU presents a complex thermal environment. Dry bulb or ambient room temperature alone does not depict overall effects of convection, conduction, radiation, and evaporation. Room temperature is not consistent throughout the NICU. Outdoor temperature and humidity are seasonally dependent and influence heating and cooling in the NICU, producing variation throughout the year. The effect of humidity on potential for evaporative heat loss is clearly evident. The findings from this descriptive study of 1 NICU suggest the need for nurses to more closely monitor room thermal characteristics. Practice suggestions involve the routine use of a thermal monitor to expand ongoing measurement to include dry bulb, wet bulb, and globe radiant temperatures. In addition, the NICU thermal environment should be assessed periodically throughout the year. Findings from such assessment may suggest modification of ambient air temperature and/or changes in room humidification level. Further adjustments may include using draperies over exterior windows, positioning of incubators in relation to air vents, and augmenting incubator humidity.
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Keywords:© 2010 National Association of Neonatal Nurses
evaporation; humidity; nursery; thermal environment