Effects of low humidity on small premature infants in servocontrol incubators. I. Decrease in rectal temperature

Variations in incubator temperature and humidity management: a survey of current practice

AIM

To describe and assess routine procedures and practices for incubator temperature and humidity management in France in 2009.

METHODS

A questionnaire was sent to all the 186 neonatal care units in France.

RESULTS

The questionnaire return rate was 86%. Seventy-five per cent of the units preferred skin servo-control to air temperature control in routine practice. Air temperature control was mainly used for infants with a gestational age of more than 28 weeks and aged over 7 days of life. In general, thermal management decisions did not depend on the infant's age but were based on a protocol applied specifically by each unit. All units humidified the incubator air, but there was a large difference between the lowest and highest reported humidity values (45% and 100% assumed to be a maximal value, respectively). More than 65% of the units used a fixed humidity value, rather than a variable, protocol-derived value.

CONCLUSION

We observed large variations in incubator temperature and humidity management approaches from one neonatal care unit to another. There is a need for more evidence to better inform practice. A task force should be formed to guide clinical practice.

The infant incubator in the neonatal intensive care unit: unresolved issues and future developments

Since the 19th century, devices termed incubators were developed to maintain thermal stability in low birth weight (LBW) and sick newborns, thus improving their chances of survival. Remarkable progress has been made in the production of infant incubators, which are currently highly technological devices. However, they still need to be improved in many aspects. Regarding the temperature and humidity control, future incubators should minimize heat loss from the neonate and eddies around him/her. An unresolved issue is exposure to high noise levels in the Neonatal Intensive Care Unit (NICU). Strategies aimed at modifying the behavior of NICU personnel, along with structural improvements in incubator design, are required to reduce noise exposure. Light environment should be taken into consideration in designing new models of incubators. In fact, ambient NICU illumination may cause visual pathway sequelae or possibly retinopathy of prematurity (ROP), while premature exposure to continuous lighting may adversely affect the rest-activity patterns of the newborn. Accordingly, both the use of incubator covers and circadian lighting in the NICU might attenuate these effects. The impact of electromagnetic fields (EMFs) on infant health is still unclear. However, future incubators should be designed to minimize the EMF exposure of the newborn.

Improved care and growth outcomes by using hybrid humidified incubators in very preterm infants

OBJECTIVE

To identify changes in temperature, fluid and electrolyte management, growth, and short-term outcome in extremely low birth weight (ELBW) infants nursed in humidified hybrid incubators (HI group) compared with a cohort of patients cared for in nonhumidified conventional incubators (CI group).

METHODS

Body temperature (BT), fluid and electrolyte balance, and growth velocity (GV) were collected retrospectively on 182 ELBW infants. The CI group included ELBW infants cared for with radiant warmers followed by an incubator without humidity. The HI group included ELBW infants cared for in the radiant warmer mode in a Giraffe OmniBed, followed by the incubator mode using high humidity.

RESULTS

The CI group included more multiple births (50.6%) than the HI group (35.8%; P < .05), but there was no difference in demographic characteristics. BT was similar during the first week. The HI group had less fluid intake, urine output, and insensible water loss, less maximum weight loss, and a lower incidence of hypernatremia during the first week than did the CI group (P < .05). The HI group also had a lower frequency of electrolyte sampling and packed red cell transfusion (P < .05), a higher incidence of hyponatremia on postnatal day 1 than the CI group (P < .05), and a higher GV than the CI group (15.2 +/- 5.0 vs 13.5 +/- 4.8 g/kg per day), especially among those with a birth weight of <or=749 g (P < .01). There was no difference in sepsis, necrotizing enterocolitis, intraventricular hemorrhage, and all bronchopulmonary dysplasia (BPD), but there was a decreased incidence of severe BPD (5.1% [HI] vs 16.4% [CI]; P < .05) and duration of assisted ventilation in the HI group compared with the CI group (15.5 +/- 2.1 vs 19.6 +/- 2.4 days, respectively; P = .068).

CONCLUSIONS

Use of a humidified hybrid incubator improved care for ELBW infants by making it possible to decrease fluid intake, improve electrolyte balance, and enhance GV without a disturbance of BT compared with conventional care. By adjusting fluid intake when using these devices, benefits may be enhanced and the risk of BPD and severe BPD may be reduced.

A clinical comparison of radiant warmer and incubator care for preterm infants from birth to 1800 grams

OBJECTIVE

The objective of this study was to compare radiant warmer and incubator care for preterm infants from birth with respect to temperature control and weight gain.

METHODS

Sixty preterm infants <33 weeks' gestation were randomized at birth to radiant warmer or incubator care. The initial goal was to maintain abdominal temperature at 36.8 degrees C in both groups and axillary temperature at 36.8 to 37.3 degrees C; air servocontrol was used for incubator infants. Infants in both groups received added humidity for 5 days if their weight was <1000 g and for 3 days if they weighed between 1000 and 1249 g. During a 3-hour period on days 1 to 7, recordings of abdominal, forehead, and foot temperatures were obtained. The percentage of the recording time during which the abdominal temperature was in the target range of between 36 degrees C and 37.5 degrees C was determined as an indicator of temperature control. Weight gain from birth to 1800 g was compared. Secondary outcomes included fluid balance and clinical events.

RESULTS

There were 30 infants in each group; 48 were <1500 g (of whom 17 were <1000 g). There were no significant differences in birth weight, gestation, gender, or illness severity scores in the 2 groups. Significant differences in temperature control were noted on day 1. Although admission temperatures were similar, lower abdominal temperatures were noted in the first 2 hours of life in the incubator group (medians were 36.6 degrees C and 35.9 degrees C in the radiant warmer and incubator groups, respectively). Similarly, mean abdominal temperatures during the 3-hour recording on day 1 were lower in the incubator group, and infants in this group spent a significantly greater percentage of the recording time with temperatures outside the target range (17.3% compared with 0.88%). Other temperature recordings from the forehead and foot were not significantly different in the groups. Fluid intakes were higher for infants under radiant warmer on days 2, 3, and 4, and the difference amounted to a mean of 12.8 mL/kg/d. Maximum sodium levels in the first week were similar in the 2 groups. Mean weight gain was 17.4 g/kg/d for the radiant warmer group and 17.1 g/kg/d for the incubator group; days to regain birth weight and length of hospital stay were not significantly different. Greater numbers of infants in the radiant warmer group required phototherapy, and adverse events (which included death, necrotizing enterocolitis, chronic lung disease, grade 3 or 4 intraventricular hemorrhage, periventricular leukomalacia, or retinopathy requiring laser treatment) were less frequent in the radiant warmer group (1 infant compared with 8 in the incubator group; relative risk 0.1; 95% confidence intervals: 0.01-0.82).

CONCLUSIONS

This study has shown differences in abdominal temperatures on day 1 and outcome, although the latter finding should be viewed with caution because of the sample size. The results indicate benefits for the initial use of the radiant warmer after birth. Although fluid requirements were higher in the radiant warmer group for days 2 through 4, the increased fluid volumes were given without apparent adverse effect.

Hypothyroidism delays fetal stratum corneum development in mice

The epidermal permeability barrier, required for terrestrial life, is localized to lipid-enriched lamellar membranes in the extracellular spaces of the stratum corneum (SC). Immaturity of the SC is a significant contributor to morbidity and mortality in premature infants. Previous studies have shown that supraphysiologic concentrations of thyroid hormone accelerate epidermis/SC ontogenesis. Here we studied SC development in Hyt/Hyt mice who are genetically hypothyroid due to a mutation in the TSH receptor. In control mice on d 18 of gestation (term 19.5 d), only focal areas displayed a mature SC membrane pattern. By 19 d of gestation there was a mature multilayered SC with lamellar unit structures filling the extracellular spaces similar to that seen in mature mice. In Hyt/Hyt mice SC development was delayed at both 18 and 19 d of gestation. In both strains of mice, within the first day after birth there were no differences in epidermal or SC appearance, and the SC was fully mature. These findings indicate that thyroid hormone plays a physiologic role during normal intrauterine development of the SC. However, normal SC maturation ultimately occurs, indicating that thyroid hormone is not absolutely essential. Previous studies have shown that glucocorticoids accelerate SC development in euthyroid rats, and in the present study we demonstrate that glucocorticoids also accelerate SC ontogenesis in euthyroid mice. In contrast, in Hyt/Hyt mice glucocorticoids did not accelerate or normalize SC development, indicating that the glucocorticoid effect on SC maturation requires a euthyroid state or that glucocorticoids act via thyroid hormone. These studies demonstrate that thyroid hormone status is an important regulator of fetal SC development.

Epidermal barrier ontogenesis: maturation in serum-free media and acceleration by glucocorticoids and thyroid hormone but not selected growth factors

Because the cutaneous permeability barrier develops late in gestation, prematurity may result in increased morbidity and mortality due to barrier incompetence. The purpose of the present study was to develop an in vitro model of barrier ontogenesis in order to identify those factors critical for fetal barrier formation. Skin explants from gestational day 17 fetal rats (term is 22 days) were incubated in hormone- and serum-free media. After 4 d in culture, a multi-layered stratum corneum (SC) developed that demonstrated a membrane pattern of fluorescence using the hydrophobic probe, nile red, and the deposition of mature lamellar unit structures throughout the SC interstices, ultrastructurally. Transepidermal water loss rates declined during explant culture such that after 4 d a competent barrier was present. Similarly, lanthanum permeation studies showed tracer penetration into all cell layers in 2-d explants, whereas it did not penetrate above the stratum granulosum in 4-d explants. Thus, the chronology of epidermal development in the explants precisely mirrored that observed in utero. Treatment with either 10 nM dexamethasone or 10 nM triiodothyronine accelerated SC development and barrier formation by 2 d. These results indicate that (i) the late events of fetal epidermal development progress in vitro under serum- and growth factor-free conditions, culminating in the formation of a functional barrier, and (ii) both dexamethasone and triiodothyronine accelerate barrier development.

Artifacts in the measurement of skin temperature under infant radiant warmers

All skin temperature probes measure, to some extent, operative temperature as well as skin temperature, and thus artifactually measure a temperature different from true skin temperature. To assess the magnitude and direction of these artifacts in the measurement of surface temperature in radiant warmers designed for human infants, the artifactual deviation of measured surface temperatures from mean surface temperature was determined under a short-wavelength warmer and a long-wavelength radiant warmer, using a copper ball as an experimental model. The measurements were made using both a disk-shaped thermistor and a tubular thermistor. All measurements were made near the top of the hemisphere of the ball facing the heating element of the warmer. In all cases, the average artifact was negative. That is, even on the surface of the ball near the radiant heat source, the surface temperature probes recorded an artifactually low temperature. In the analogous clinical setting, a somewhat larger negative artifact would be expected.

Infant incubators and radiant warmers

Incubators and radiant warmers are used to maintain the body temperature of newborn infants. This is best done so that the energy expended for metabolic heat production is minimized. The heat output of these devices is usually regulated by servocontrol to keep the skin temperature constant at a site on the abdomen where a thermistor probe is attached. In incubators, air temperature can also be controlled as an alternative to skin temperature servocontrol. Increased ambient humidity, heat shields and clothing have been used to decrease the evaporative or nonevaporative heat loss of infants in incubators under certain conditions. Double-walled incubators, by adding a second inner layer of Plexiglas, reduce radiant heat loss. They may also reduce total heat loss, but only if air temperature is controlled rather than skin temperature. The minimal oxygen consumption under a radiant warmer is the same or perhaps slightly higher than it is for the same infant in an incubator. Compared with incubators, the partition of body heat loss is quite different under radiant warmers. Radiant warmers increase convective and evaporative heat loss and insensible water loss but eliminate radiant heat loss or change it to net gain. A heat shield of thin polyethylene film can be used with a radiant warmer to reduce heat loss by convection and evaporation. The major advantage of the radiant warmer is the easy access it provides to critically-ill infants without disturbing the thermal environment. Its major disadvantage is the increase in insensible water loss produced by the radiant warmer. Most infants can be safely and adequately cared for in either incubator or radiant warmer bed.

Barrier properties of the newborn infant's skin

The barrier properties of the skin were examined in 223 studies in 70 newborn infants of 25 to 41 weeks' gestation, aged from 1 hour to 26 days. Percutaneous drug absorption was studied by observing the blanching response to solutions of 1% and 10% phenylephrine applied to a small area of abdominal skin. Skin water loss was measured at the same site using an evaporimeter. Infants of 37 weeks' gestation or more showed little or no drug absorption and had low skin water losses, indicating that their skin is an effective barrier. By contrast, infants of 32 weeks' gestation or less showed marked drug absorption and high skin water losses in the early neonatal period, indicating that their skin is defective as a barrier. Both drug absorption and water loss in these infants fell steadily; by about 2 weeks of age the skin of the most immature infants functioned like that of mature infants. The varying barrier properties can be explained by the poor development of the stratum corneum in the more premature infants at birth and its rapid maturation after birth. The trauma caused to the skin by use of adhesive tape and the fixation of transcutaneous oxygen electrodes resulted in increased drug absorption and water loss from the damaged area.

Effect of heat shielding on convective and evaporative heat losses and on radiant heat transfer in the premature infant

Ten premature infants nursed on servocontrolled radiant warmer beds were studied in three environments designed to alter one or more factors affecting heat transfer (convection, evaporation, and radiation). In the control environment, infants were nursed supine on an open warmer bed. The second environment (walled chamber) was designed to reduce convection and evaporation by placing plastic walls circumferentially around the bed. In the third environment convection and evaporation were minimized by covering infants with a plastic blanket. Air turbulence, insensible water loss, and radiant warmer power were measured in each environment. There was a significant reduction in mean air velocity in the walled chamber and under the plastic blanket when compared to the control environment. A parallel decrease in insensible water loss occurred. In contrast, radiant power demand was the same for control and walled environments, but decreased significantly when infants were covered by the plastic blanket. This study suggests that convection is an important factor influencing evaporation in neonates nursed under radiant warmers. The thin plastic blanket was the most effective shield, significantly reducing radiant power demand.

Minimal oxygen consumption in infants cared for under overhead radiant warmers compared with conventional incubators

Infants under radiant warmers have large increases in insensible water loss compared with infants in single wall incubators. To answer the question of whether or not a minimal rate of oxygen consumption could be achieved under overhead radiant warmers, we measured oxygen consumption, carbon dioxide production, and abdominal skin, cheek, rectal, thigh, and environmental temperature in ten healthy newborn infants in incubators and radiant warmers, using each infant as his/her own control. The minimal VO2 ranged from 4.41 to 8.87 and from 4.35 to 9.06 cc/kg/minute in the incubator and radiant warmer, respectively. The differences were clearly not significant (paired Student t-test, P greater than 0.60). There were no significant differences between the respiratory quotients, VCO2, or abdominal skin, check, rectal or environmental temperatures. These data support the hypothesis that a thermoneutral environment can be provided with a radiant warmer and imply that large increases in insensible water loss can occur without affecting minimal oxygen consumption.