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HEIDI NELSON, RN, BSN KAREN THOMAS, RN, PHD MARIBETH STEIN, RN, MN ZZWWUl Hooding Egects of Incubators Objective: To determine the effect of covering infant incubators on incubator wall and air temperatures, as well as on infant temperatures. Design: A within-subject ABA design, in which blankets covering the incubators were removed for a 30-minute period and then replaced. Setting: A neonatal intensive-care unit. Participants: Eight medically stable infants (gestational age, 28-33 weeks; birth weight, 913- 1,947 g; and postnatal age, 2-39 days). Interventions: Incubator wall and air temperatures as well as infant temperatures were measured during three study conditions: incubators covered (30 minutes), uncovered (60 minutes), and recovered (30 minutes). Main outcome measures: Incubator air and wall temperature; infant temperature. Results: All incubator walls decreased in temperature after being uncovered; the decrease ranged from 0.6-2.2OC. Conclusion: Although infants maintained relatively stable body temperatures during the uncovered period, the energy cost to their thermoregulatory eforts is unknown. Accepted: February 1992 ncreasing emphasis on developmental care has resulted in nursing interventions aimed at reduc- ing the negative impact of the neonatal intensive-care unit environment (Als et al., 1986; Lawhon, 1986; Van- denBerg, 1990). One such intervention is the cover- ing of infant incubators with blankets to reduce noxious light stimulation, a process known as “hood- ing.” In the neonatal intensive-care unit serving as the setting for this clinical study, hooding incubators is standard practice. Nurses reported that using blanket coverings significantly reduced the temperature set- ting needed on the incubators. Based on these obser- vations, a clinical project was devised to systematically examine the effect of covering incubators. The pur- pose of this study, therefore, was to determine changes in incubator wall temperature, incubator air temperature, and infant temperature occurring as a re- sult of covering the incubators with blankets. The solid Plexiglas surface of the incubator loses heat to the room through convection and radiation (LeBlanc, 1987). The gradients driving this heat loss are (a) the difference between the incubator surface and the surrounding air temperature and (b) the dif- ference between the incubator surface and the surrounding solid surface temperatures. Infant body temperature is directly influenced by incubator wall temperature. The incubator wall, which is typically cooler than the incubator air, is a source of convective heat loss, which reduces the incubator air tempera- ture. In addition, the infant loses heat by radiating en- ergy to the incubator wall. The incubator’s thermal environment is not described completely by air tem- perature alone. The operative temperature includes the combined effects of the incubator air and wall tem- peratures and is calculated as (Hey & Mount, 1967) Top = O.G(wal1 temperature) + 0.4(air tempera- ture). A decrease in wall temperature increases the in- fant’s heat loss, necessitating thermoregulatory activ- ity within the infant. The infant’s responses can in- clude vasoconstriction, postural change, and cues to caregivers signaling thermal discomfort. If these re- sponses are not adequate to compensate for the heat loss, the infant increases heat-producing activities. As the infant’s metabolic rate rises, so does the infant’s oxygen consumption. Thus, calories and oxygen that might support physiologic functions or growth are ex- pended for thermoregulation (Sauer, Dane, & Visser, 1984). In infants less than 28-30 weeks’ gestational age, the ability to increase oxygen consumption in re- sponse to cold is not mature; hence, the infant’s body temperature will drop when the external environmen- tal temperature decreases. September/October 1992 JOGNN 377

Thermal Effects of Hooding Incubators

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Page 1: Thermal Effects of Hooding Incubators

H E I D I N E L S O N , R N , B S N

K A R E N T H O M A S , R N , P H D

M A R I B E T H S T E I N , R N , M N

ZZWWUl Hooding

Egects of Incubators

Objective: To determine the effect of covering infant incubators on incubator wall and air temperatures, as well as on infant temperatures. Design: A within-subject ABA design, in which blankets covering the incubators were removed for a 30-minute period and then replaced. Setting: A neonatal intensive-care unit. Participants: Eight medically stable infants (gestational age, 28-33 weeks; birth weight, 913- 1,947 g; and postnatal age, 2-39 days). Interventions: Incubator wall and air temperatures as well as infant temperatures were measured during three study conditions: incubators covered (30 minutes), uncovered (60 minutes), and recovered (30 minutes). Main outcome measures: Incubator air and wall temperature; infant temperature. Results: All incubator walls decreased in temperature after being uncovered; the decrease ranged from 0.6-2.2OC. Conclusion: Although infants maintained relatively stable body temperatures during the uncovered period, the energy cost to their thermoregulatory eforts is unknown.

Accepted: February 1992

ncreasing emphasis on developmental care has resulted in nursing interventions aimed at reduc-

ing the negative impact of the neonatal intensive-care unit environment (Als et al., 1986; Lawhon, 1986; Van- denBerg, 1990). One such intervention is the cover- ing of infant incubators with blankets to reduce noxious light stimulation, a process known as “hood- ing.” In the neonatal intensive-care unit serving as the setting for this clinical study, hooding incubators is standard practice. Nurses reported that using blanket coverings significantly reduced the temperature set- ting needed on the incubators. Based on these obser- vations, a clinical project was devised to systematically examine the effect of covering incubators. The pur- pose of this study, therefore, was to determine changes in incubator wall temperature, incubator air temperature, and infant temperature occurring as a re- sult of covering the incubators with blankets.

The solid Plexiglas surface of the incubator loses heat to the room through convection and radiation (LeBlanc, 1987). The gradients driving this heat loss are (a) the difference between the incubator surface and the surrounding air temperature and (b) the dif- ference between the incubator surface and the surrounding solid surface temperatures. Infant body temperature is directly influenced by incubator wall temperature. The incubator wall, which is typically cooler than the incubator air, is a source of convective heat loss, which reduces the incubator air tempera- ture. In addition, the infant loses heat by radiating en- ergy to the incubator wall. The incubator’s thermal environment is not described completely by air tem- perature alone. The operative temperature includes the combined effects of the incubator air and wall tem- peratures and is calculated as (Hey & Mount, 1967)

Top = O.G(wal1 temperature) + 0.4(air tempera- ture).

A decrease in wall temperature increases the in- fant’s heat loss, necessitating thermoregulatory activ- ity within the infant. The infant’s responses can in- clude vasoconstriction, postural change, and cues to caregivers signaling thermal discomfort. If these re- sponses are not adequate to compensate for the heat loss, the infant increases heat-producing activities. As the infant’s metabolic rate rises, so does the infant’s oxygen consumption. Thus, calories and oxygen that might support physiologic functions or growth are ex- pended for thermoregulation (Sauer, Dane, & Visser, 1984). In infants less than 28-30 weeks’ gestational age, the ability to increase oxygen consumption in re- sponse to cold is not mature; hence, the infant’s body temperature will drop when the external environmen- tal temperature decreases.

September/October 1992 J O G N N 377

Page 2: Thermal Effects of Hooding Incubators

C L I N I C A L S T U D I E S

In many nurseries, covering incubators to reduce stimulation from light and sound is becoming a standard practice.

A blanket placed over the incubator provides insu- lation and reduces heat loss from the incubator wall. The effect is similar to the reduction in heat loss pro- vided by heat shields or double-walled incubators (Bell, 1983). The amount of this reduction is depen- dent on the insulating capability of the material used. Factors influencing insulation include the thickness of the material, the nature of the surface (smooth versus irregular), and the ability to trap air.

Method

Design A within-subject, ABA design was used in the study (Kazdin, 1982). In an ABA design, data are collected during a baseline period (A). After introduction of an independent variable, data are collected during an ex- perimental period (B) . Then, the independent vari- able is withdrawn and data are collected again during baseline conditions (A). Incubator air and exterior wall temperatures, as well as infant abdominal skin temperatures, were measured at 2-minute intervals throughout the study period. Incubators were covered and allowed to stabilize before the study was begun. Data were recorded for 30 minutes in the covered condition (A). The incubators were then uncovered, and data were recorded for 60 minutes (B). Then, 30 minutes of data collection (A) followed re-covering the incubators. Subjects served as their own controls.

Subjects Eight infants, a convenience sample, were included in the study. Criteria for inclusion were no intracranial hemorrhage, no major congenital anomalies, housed in an incubator, medically stable with no health prob- lems other than those typically associated with prema- turity, weight less than 2,000 gat the time of the study, and less than 34 weeks' gestational age.

Instruments Air temperature was recorded from the incubator con- trol panels. The infants' temperatures were recorded from the incubator servocontrol probe taped to the infants' right abdominal areas and operated in monitor mode. Next, infant temperatures were read from the incubator control panels. Incubator wall temperatures were recorded using an electronic thermometer. A

thermocouple was taped to the incubator exterior, in the center of each hood. The resolution for all temper- ature measures was 0.1"C. The incubator servocontrol probe, the air temperature, and the electronic ther- mometer were calibrated against a precision thermom- eter certified by the National Institute of Standards and Technology. Correlation between the standard and these means of measurement was r > .95. The ambient temperature and humidity of the nursery were recorded at the beginning and end of each re- cording period. For the eight data recordings, room temperature was consistently 23OC and humidity ranged from 47-54%.

Procedure Recordings were performed between feeding pe- riods. All subjects were fed at 3-hour intervals. The incubators were covered with two cotton blankets after feedings. In the neonatal intensive-care unit serving as the setting for the study, all incubators are located adjacent to interior walls. Temperature probes were applied as described above. Before data collec- tion, the incubators were allowed to stabilize for 15 minutes. The infants' care was not altered during the study, and interruptions during recording were mini- mal. The few incubator openings did not produce any changes in incubator air temperatures. The subjects' parents gave permission for participation by signing informed consent forms. Data were graphed for indi- vidual subjects and inspected visually. Differences be- tween the covered and uncovered conditions were calculated.

Results and Discussion

A description of the sample is provided in Table 1. The eight subjects included four girls and four boys. Birth weights ranged from 913 to 1,947 g, and weight at the time of the study ranged from 1,140 to 1,940 g. Infants were between 28 and 33 weeks' gestational age and between 2 and 39 days postnatal age.

Generally, the incubator air temperatures re- mained stable for all infants in both the covered and the uncovered conditions (see Table 2). One incuba- tor exhibited no variation in air temperature; in the remaining incubators, air temperature variations ranged from 0.1OC to 0.4"C. Infant abdominal temper- atures were relatively stable and did not decrease with

The wall temperatures of covered incubators are warmer than those of uncovered incubators.

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Table 1. Infant Biograpbk Data

Birth weight Gestational Postnatal Weight Subject Gender (g) age (weeks) age (days) (g)

Fraternal twins.

913 1,232 1,947 1,784

1,927 1,328 1,503 1,795

31 28

33 33 31-32 30 31-32 31-32

25 39 10

2 16 14 18 20

1,140

1,493 1,860 1,730 1,940 1,250 1,393 1,842

uncovering of the incubator. Lower abdominal temper- atures typically were associated with the initial cov-

Nursing

46 minutes. For four subjects, upon re-covering the incubators, the wall temperatures did not always re- turn to preexisting levels within the 30-minUte data collection period. Figure 1 shows the data for one sub-

Blankets used to cover incubators reduce radiant heat loss.

ject.

Table 2. Temperature Changes (in O C) With Incubator Covered and Uncovered

Tag: TO,: Two& TWOIF Decrease Time to stabilize uncovered TwoN (minutes) Subject covered uncovered covered

1 28.1 28.0 27.4 26.5 0.9 34 2 27.4 27.3 26.8 25.9 0.9 32 3 31.5 31.5 28.7 27.3 1.4 26 4 33.4 33.4 31.3 29.1 2.2 32 5 29.9 29.9 27.9 26.6 1.3 46 6 29.9 29.9 28.2 27.3 0.9 24 7 30.3 30.3 28.0 27.2 0.8 32 8 27.6 27.3 26.4 25.8 0.6 38

a Incubator air temperature mode; mode is presented since minimal changes occurred Incubator wall temperature, before uncovering. Incubator wall temperature, lowest point after uncovering.

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C L I N I C A L S T U D I E S

Degrees Cdsius ""i-;. ................................. ^.._......^._......_.._.. 34

33

28' I 0 30 60 90 120

Minutes

Infant . incubator air - - - incubator wall -

Figure 1. Effect qthooding incubator. lnfirnt 4

cant, but for fragile preterm infants, and particularly for extremely low-birth-weight infants, it necessitates thermal readjustments to counteract alterations in the operative temperatures provided by the incubators. Hill and Rahimtulla (1965) have shown that a 2OC de- crease in an incubator's operative temperature may in- crease oxygen consumption 25% in a naked premature infant. Although infant and incubator air temperatures remain relatively stable during uncovering, the energy cost to the infant's thermoregulatory responses, in- cluding vasoconstriction, increased activity, and in- creased heat production, are unknown. A decrease in infant temperature would not be detected until these thermoregulatory responses became ineffective in maintaining body temperature. In past research, poor correlation between rectal temperature and oxygen consumption has been documented (Rutter, Brown, & Hull, 1978). Therefore, a stable body temperature may not reflect oxygen consumption adequately. Subtle changes in the thermal conditions surrounding the in- cubator may result in thermoregulatory activity by the infant, such as increased heat production, that is not detected in changes in infant body temperature. In future research, the long-term thermal consequences of covering incubators and the influence of unstable nursery temperatures should be examined.

A blanket covering the incubator hood effectively insulates, decreases stimulation from light, and

The added insulation provided by blankets covering incubators requires that thermal implications be considered.

deadens some sound stimuli. The thermal advantages of covering incubators may benefit extremely small preterm infants or any infant experiencing difficulty maintaining his or her temperature. Covering incuba- tors also may assist in cool nursery rooms, during trans- port, when incubators are placed adjacent to exterior windows, and during exposure to overhead drafts caused by heating and cooling vents. The sensory ef- fects of covering incubators include decreasing arousal and the effects of the energy demands of arousal.

The following guidelines are suggested when in- cubator hooding is instituted. First, monitor infants by a reliable monitoring system. Second, anticipate a pos- sible need to reduce incubator control settings, keep- ing in mind that the thermal effects of covering the incubator occur over time. Third, monitor thermal changes by recording infant and air temperatures every 30 minutes during the first 2 or more hours of covering, and adjust incubator settings accordingly. Incubators, servocontrolled to infant abdominal skin temperatures, should compensate during covering by reducing their heat production. If the incubator is operated manually, decrease the settings gradually to avoid overcompensation and thermal instability. Ob- serve the behavioral and thermal effects of incubator hooding. Fourth, once hooding is instituted, attempt to maintain consistent covering to support thermal equilibrium.

~ ~ ~~~

In future research, the energy cost to radiant beat loss should be included in study designs.

In summary, from a thermal standpoint, incubator hooding decreases heat loss from the incubator wall. Covering incubators to reduce visual stimulation does not affect thermal status adversely, provided that in- fant and incubator temperatures are monitored closely and incubator settings adjusted appropriately.

References

Als, H., Lawhon, G., Brown, E., Gibes, R., Duffy, F. H., McAnulty, G., & Bickman, J. G. (1986). Individualized behavioral and environmental care for the very low birth weight preterm infant at high risk for bronchopulmonary dysplasia: Neonatal intensive care unit and developmen- tal outcomes. Pediatrics, 78, 1123-1 132.

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Hooding Incubators

Bell, E. F. (1983). Infant incubators and radiant warmers. Early Human Development, 8, 351-375.

Hey, E. N., & Mount, L. E. (1967). Heat losses from babies in incubators. Archives of Disease in Childhood, 42, 75-84.

Hill, J. R., & Rahimtulla, K. A. (1965). Heat balance and the metabolic rate of newborn babies in relation to environ- mental temperature: The effect of age and of weight on basal metabolic rate. Journal of Physiology, 180, 239- 265.

Kazdin, A. E. (1982). Single-case research designs: Methods for clinical and applied settings. New York: Oxford Uni- versity.

Lawhon, G. (1986). Management of stress in premature in- fants. In D.J. Angelini, C. M. Knapp, & R. M. Gibes (Eds.), Perinatal/neonatal nursing (pp. 319-328). Bos- ton: Blackwell Scientific.

LeBlanc, M. H. (1987). The physics of thermal exchange between infants and their environment. Medical Instru- ments, 21, 11-15.

Rutter, N. , Brown, S. M., & Hull, D. (1978). Variations in the resting oxygen consumption of small babies. Archives of Disease in Childhood, 53, 850-854.

Sauer, P. J. J., Dane, H. J., & Visser, H. K. A. (1984). Longitu- dinal studies on metabolic rate, heat loss, and energy

cost of growth in low birth weight infants. Pediatric Re- search, 18, 254-259.

VandenBerg, K. A. (1990). Behaviorally supportive care for the extremely premature infant. In L. P. Gunderson & C. Kenner (Eds.), Care of the 24-25 week gestational age infant (small baby protocol) (pp. 129-157). Petaluma, CA: Neonatal Network.

Address for correspondence: Heidi Nelson, RN, BSN, Special Care Nursery, Swedish Hospital Medical Center, 747 Summit Ave., Seattle, WA 98104.

Heidi Nelson is a staff nurse 111 in the special-care nursery at Swedish Hospital Medical Center in Seattle.

Karen Thomas is an associate professor in the Department of Parent-Child Nursing at the University of Washington in Seattle.

Maribeth Stein is the nurse manager of the special-care nursery at Swedish Hospital Medical Center in Seattle. Ms. Stein is a member of NAACOG.

September/October 1992 J O G N N 381