Biol. Neonate 26: 337-347 (1975)

Effects of Low Humidity on Small Premature Infants in Servocontrol Incubators 1. Decrease in Rectal Temperature'

Teertharaj K. Belgaumkar and Kenneth E. Scot; Grace Maternity Hospital, and Departments of Paediatrics and Obstetrics and Gynaecology, Dalhousie University, Halifax, N.S.

Key Words. Servocontrolled incubator • Low humidity • Drop in body temperature Abstract. 19 small premature infants in servocontrol incubators, whose abdominal skin temperature was 36.0 t 0.3 °C, were subjected to alternate high- and low-humidity environ­ ments. With low humidity, rectal temperature dropped significantly below abdominal skin temperature. Skin was the predominant site of evaporative heat loss. The temperature was lower on naked skin than on an area covered by adhesive tape. Thus, servocontrol with low humidity increases evaporative heat loss and engenders a cycle of events that results in paradoxical body temperature decrease as the incubator temperature increases.

In 1933 Blackfan and Yaglou (3) reported increased survival of small prema­ ture infants nursed in a high humidity, and attributed the increase to the hu­ midity per se. In later, controlled studies, however, Silverman et al. (7—9) showed that the increase was in fact due to the concomitant increase in body temperature, and that moderately low humidity little influences the survival of infants with normal temperature. Since then, servocontrol incubators with vari­ able humidity have become widely used; in some nurseries all premature infants are reared in low humidity, to prevent skin maceration and bacterial overgrowth. The relative humidity in servoincubators is inconsistent (2): the higher the gas flow the greater the difficulty in maintaining adequate humidity, necessi­ tating accessory humidifiers. Also, there is apparent discrepancy in temperature control; for example, many small, severely ill, premature infants have prolonged low rectal temperature despite adequate skin temperature (6).

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' Supported by grant No. 602-7-142 from the Department of National Health and Welfare, Ottawa.

Belgaumkar ¡Scott

338

To determine the effects of low humidity on their physiological parameters, we studied small premature neonates nursed in alternating low and high hu­ midity but with constant abdominal skin temperature. In this first part of the study we recorded body temperatures at various sites.

Subjects and Methods 19 newborn premature infants (table I) admitted to the intensive care nursery were studied during the first 4 days of life. The only criterion for selection was the availability of monitoring equipment and personnel. There were 13 males and 6 females. Gestational ageranged from 24 to 35 weeks, and weight from 775 to 1,965 g. Studies were commenced at age 4 - 54 h.

Table I. Clinical details of the 19 premature infants studied Studies

Infant No.

Weeks of Weight gestation g

Diagnosis

I 2

32 28

1,275 800

3 4 5 6 7 8

32 32 26 32 26 26

1,435 1,310 1,280 1,965 1,090 1,275

II I, II I, II I, II

9 10 ir 12 13 14

33 24 32 31 35 26

1,090 1,005 1,342 1,455 1,640 775

I, II

15

31

1,300

I, II, III I, III

16 17

28 26

1,255 915

I, III I, III

18 19

25 32

890 1,430

hyperbilirubinemia; apneic spells severe asphyxia neonatorum; RDS; apneic spells; PDA healthy severe asphyxia neonatorum; severe RDS mild RDS mild RDS mild RDS; apneic spells severe asphyxia neonatorum; severe RDS; apneic spells: PDA severe asphyxia neonatorum, severe RDS mild RDS severe RDS severe RDS severe asphyxia neonatorum severe asphyxia neonatorum; severe RDS; apneic spells; PDA severe asphyxia neonatorum; RDS: apneic spells apneic spells severe asphyxia neonatorum; RDS; hyperbilirubinemia; apneic spells apneic spells healthy

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I = Effect of humidity; II = site of heat loss; III = effect of adhesive tape; RDS = respiratory distress syndrome; PDA = patent ductus arteriosus. 1 This infant was being ventilated by positive pressure respirator.

Humidity and Body Temperature

339

Fig. 1. Monitoring body temperature: 1, dry- and wet-bulb hygrometer; 2, servocontrol incubator; 3, thermistor probes; 4, electronic thermometer; 5, preamplifier, and 6, recorder.

Temperature The incubator’s servocontrol probe and electronic thermometer varied 0.1-0.4 °C from readings with a standard Hg thermometer (partial IMM model; Fisher Scientific Co., Montreal, Que.). Three recording thermistor probes and thermal equipment (Y.S. Instru­ ment Co., Yellow Springs, Ohio, USA), calibrated over the range 28-45 °C with the same standard thermometer, measured temperatures at midincubator position (air temp, probe, No. 405), 3 -4 cm deep in the rectum (general purpose, No. 401), and from abdominal skin (surface temp.. No. 409) adjacent to the servocontrol probe. A 7 X 2.5 cm strip of adhesive tape (Hy-Tape; Surgical Hosiery Corp., New York, USA) secured the servocontrol and recording abdominal skin probes between the xiphisternum and umbilicus. The 3 thermis­ tors sensing the incubator air temperature (IT), abdominal skin temperature (AST), and rectal temperature (RT) were sampled by a scanning electronic thermometer (model 47, Y.S. Instrument Co.); the signals were amplified, and were recorded on a two-channel paper recorder (model 350-1500 low level preamplifier with No. 350-2 sub-plug-in unit and No. 296 recorder; Sanborn Co., Waltham, Mass., USA), as shown in figure 1. Effect o f humidity (18 infants). Because of his clinical condition, one infant was excluded from these experiments. The infants were placed initially in high humidity (> 70 %), which then was alternated with low humidity (< 35 %). Each phase of humidity lasted at least 2 h. Relative humidity in the incubators was mostly 8 5 -90% (range 70-98% ) or 30 -35 % (range 25-40% ). The infants were studied during a total of 54 high-humidity (HH) and 41 low-humidity (LH) phases. IT, RT, and AST were recorded every 60 sec throughout, including transitional periods. Site o f heat loss (6 infants). Breathing gas humidity (BH) and incubator humidity (IH) were controlled independently, to determine whether the predominant site of heat loss was

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Three incubators were used (Isolette model C-86, Airshields Inc., Hatboro, Penn., USA), in one area of the nursery. Abdominal skin temperature was servocontrolled at 36.0 t 0.3 °C, and studies were carried out (fig. 1) after the general environment had stabilized. Relative humidity in the incubator, measured by dry- and wet-bulb hygrometer and varied by the humidity control, was maintained by accessory humidifiers when necessary; the high airflow rate resulted in the desired humidity within 15-30 min.

Belgaumkar/Scott

340

Fig. 2. Method of recording breathing gas humidity (BH) and incubator humidity (IH) separately: 1, compressed gas tank; 2, gas-warming coil; 3, head bag; 4, hygrometer measuring BH; 5, hygrometer measuring IH, and 6, thermistor probes.

the respiratory tract or skin surface. Low BH could not be instituted in 2 infants, who were receiving assisted ventilation (60-80 % humidity). The other 4 inhaled gas warmed by passage through coiled plastic tubing in the IH chamber, then fed into a plastic bag sur­ rounding the infant’s head (fig. 2). The intrabag temperature was measured by an air probe (model 405), sampled by electronic telethermometer every 80 sec, and recorded. By varying the amount of warm wet gauze where the gas entered the bag, relative humidity within the bag was controlled at 20-35 % for low BH and at 65-80 % for high BH. The effects of various combinations of high and low BH and IH were related to body temperature. Effect o f adhesive tape (4 infants). The effect on the temperature of skin thus covered was tested, using Hy-Tape. Two thermistor skin probes (model 409) were taped side by side on the anterior thigh surface: one, for recording covered skin temperature (CST), was covered by a 5 X 2.5 cm strip of tape; the other, recording naked skin temperature (NST), was secured by two thin strips (5 X 0.25 cm) that crossed on the probe head. The average duration of experiments was 5 h for high humidity and 4 h for low humidity. IT, AST, RT, NST and CST were recorded every 100 sec throughout, including transitional periods. Data Processing We tabulated the temperatures from each site every 15 min, excluding transitional phases (IH, 36-69 %). For each infant, temperatures were averaged for each site during each phase, and a pairwise t test for 95 % confidence intervals was applied to evaluate differences in body temperature during high and low humidities.

The abdominal skin temperature was 36.0 ± 0.3 °C, except for three 15- to 20-min periods of 35.5-35.6 °C (infants No. 2 and 15) during low humidity.

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Results

Humidity and Body Temperature

341

Effects o f Humidity With low humidity, mean RT fell below mean AST, even though AST remained stable (table II). Mean RT was higher than AST in 78 % of high-humidity experiments but in only 4 % of low-humidity experiments. In 16 of the 18 infants thus tested, RT fell below AST in a significantly greater proportion of experiments during LH than during HH (p < 0.001). In all 18 infants, mean RT was higher during high than during low humidity (fig, 3); t testing showed this RT difference to be + 0.99 ± 0.5 °C. The dynamic changes in body temperature of one infant in response to IH variation that are shown in figure 4 are represen­ tative of the findings in all these infants.

Table II. Variation in mean rectal temperature (RT) during high-humidity (HH) and low-humidity (LH) experiments, with abdominal skin temperature maintained at 36 ± 0.3 °C LH (< 35 %)

n

duration mean RT, °C h

1 2 3 4 5 6 7 8 9 10 11' 12 13 14 15 16 17 18 19

3 3 4 2 2 2 4 2 2 2 (1) 4 2 2 4 4 2 7 3

8 34 9 8 6 6 49 4 4 7 (2) 10 17 5 45 9 12 33 7

Total

54

36.5 36.6 36.9 37.0 36.8 36.9 36.4 36.5 36.4 37.2 36.8 36.8 36.7 36.2 36.3 36.8 36.1 36.8 37.1

n

2 2 4 1 2 1

3 1 1 1 -

3 1 2 3 4 2 6 2 41

Number of experiments in which mean RT was u> O n

HH (> 70 %)

< 35.7 °c

LH

HH

0 0 0 0 1 0 0 0

0 0 2 0 1 0 1 0 0 0

0 0 0 0 0 0 0 0 0 0

2 2 2 0 1 1 2 1 1

-

-

-

-

1

0 1 1 1 1 2 0

0 0 0 1 1 0 2 0

0 0 1 0 0 1 0 0

3 1 2 2 2 2 4 2

10

8

2

30

> 36.3 °C

36 ±

duration mean RT, °C h

HH

HH

7 29 10 4 10 2 25 2 5 5

2 2 4 2 2 2 3 2 2 2 (1) 3 2 0 3 3 0 5 3

-

7 5 6 19 11 8 29 4

35.6 35.6 35.8 36.4 35.8 35.5 35.5 35.0 35.2 36.8 -

3S.6 35.6 35.0 35.2 35.9 35.2 35.7 35.5

42

LH

0 0 0 1 0 0 0 0 0 1 (0) 0 0 0 0 1 0 0 0 3

1 1

-

O

Infant No.

LH

0

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1 Data of this infant excluded, because his clinical condition precluded high-humidity experiments.

342

Belgaumkar/Scott

37.2-1

3 6 .9 -

è?

3 6 .6 -

uT

• «•— infant no. 10 o • ♦— infant na 4 oo oooo infant no. 10 o o oo oo • —— infant no. ;

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

19 small premature infants in servocontrol incubators, whose abdominal skin temperature was 36.0 +/- 0.3 degrees C, were subjected to alternate high- ...
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