- Email: [email protected]
Single-vs double-walled incubators
330
Editorial correspondence
7"he Journal of Pediatrics August 1981
"neither (rat) pancreatic tissue nor juice contain a low molecular weight zinc-binding ligand." The possibility of a physiologic role of PA in pancreatic secretion and a contribution of PA from exogenous sources was suggested by our case, the human milk observations, and reports of decreased zinc levels in cystic fibrosis. However, we also suggested the possibility that PA functions at sites other than the intestines. This and related hypotheses 7 were based on the finding of zinc "dependency" in our case. ~ The PA story is complex and requires much further research.
Ingeborg Krieger, M~D. Childrens Hospital of MI 3901 Beaubien Blvd. Detroit, MI 48201 REFERENCES I.
Krieger I: Picolinic acid in the treatment of disorders requiring zinc supplementation, Nutr Rev 38:148, 1980. 2. Cash R, Krieger I, and Evans G: Treatment o f acrodermatiffs enteropathica (AE) with zinc picolinic acid, Pediatr Res 15:627, 1981 (abstr). 3. Suhadolnik R J, Stevens CO, Decker RH, Henderson LM, and Hankes LV: Species variation in the metabolism of 3-hydroxyanthranilate to pyridine carboxylic acids, J Biol Chem 228:973, 1957. 4. lkeda M, Tsuji H, Nakamura S, Ichiyama A, Nishizuka Y, and Hayaischi O: Studies on the biosynthesis of nicotinamide adenine dinucleotide, J Biol Chem 240:1395, 1965. 5. Hurley LS, and Lrnnerdal B: Picolinic acid as a zinc binding ligand in human milk: An unconvincing case, Pediatr Res 15:166, 1981. 6. Cousins RJ, Smith KT, Failla ML, and Markowitz LA: Origin o f low molecular weight zinc binding ligands in rat intestine, Life Sci 23:1819, 1978. 7. Krieger I: Acrodermatitis enteropathica and the relation to pellagra, Medical Hypotheses (in press).
Single- vs double-walled incubators To the Editor: The article by Yeh et aF investigating oxygen consumption (VO,2) and insensible water loss OWL) in single-walled incubators (SWI) versus double-walled incubators (DWI) was read with interest. The authors report a 30% reduction in IWL and a 17% reduction in "~O~ in the DWI compared with the SWI. They conclude that the DWI provides a "better" environment for premature infants and a 27% savings in energy expenditure. Unfortunately, the design of the study does not allow one to draw any conclusions about the specific effect o f the double wall on the observed changes in "VO~ or IWL. Although the authors state "the abdominal skin temperature was maintained by servocontrol at 36.5~ '' in each environment, the mean abdominal skin temperature was significantly higher in the DWI (35.75 vs 3 5 . 9 7 ~ < 0.05). Also, inner and outer incubator wall temperatures and the operative temperature were all significantly higher in the DWI. Infants in the SWI were not in a neutral
thermal environment, as evidenced by the relatively cool abdominal skin temperature (35.76~ and by the observation that resting VO~ decreased in the "warmer" environment provided by the DWI (6.42 vs 5.33 ml/kg/minute). In contrast, the infants in the DWI were in a heat-gaining environment as evidenced by the positive heat storage (0.19 kcal/kg/hour). Since heat storage is calculated from a change in body temperature, we can assume that body temperature was rising during the three-hour study period. By definition, then, infants in the DWI were also not in a neutral thermal environment. The authors, then, have compared the effects of a subneutral environment with those of a heat-gaining environment. Since thermoneutrality was not achieved in either environment, nothing can be inferred concerning the ability o f either of these incubators to provide a neutral thermal environment. Under the conditions of this study, the observed changes in X)O~ are predictable, since a heat gaining environment will result in a minimal rate of oxygen consumption unless the infant is overheated. Several investigators have assessed the "double wall effect" using plastic heat shields, -0-4 plastic blankets, ~ and the doublewalled incubator?. ' These studies demonstrate that a double wall completely enclosing the infant can reduce radiant heat loss by decreasing the skin-to-wall temperature gradient. A reduction in evaporative heat loss (IWL) has also been associated with the use of a double-walled enclosure. I agree with Yeh et al that the apparent reduction in air velocity in the DWI used in their study may partially explain the reduction in IWL. Changes in air velocity may also occur and account for some of the reduction in IWL observed with the use of a heat shield or plastic thermal blanket. Any decrease in air velocity associated with these devices is probably not related specifically to the double wall, but to the design or the interference with the forced air convection system. In studies where body temperature has been held constant, there has been no observed change in X?O2associated with the use of a double wall? -~ There may well be advantages of the double wall, but any future comparative studies should carefully control for body temperature and VO=, to provide a meaningful evaluation.
Robert A. Darnall, Jr., M.D. Department of Pediatrics University of VA Medical Center Charlottesville, VA 22908 REFERENCES
1. Yeh TF, Voora S, Lilien LD, Matwynshyn J, Srinivasan G, and Pildes RS: Oxygen consumption and insensible water loss in single-versus double-walled incubators, J PEDIATR 97:967, 1980. 2. Fanaroff AA, Wald M, C~uber HS, and Klaus MH: Insensible water loss in low birth weight infants, Pediatrics 50:246, 1972. 3. Bell EF, Weinstein MR, and Oh W: Heat balance in premature infants: comparative effects of convectively heated incubator and radiant warmer, with and without plastic shield, J PEDIATR 96:460, 1980.
Volume 99 Number 2
Editorial correspondence
33 1
Table Time (min)
Temperature Skin SWI DWI P Air SWI DWI P
35.65 • 0.29 35.84 • 0.21 NS
35.72 • 0.34 35.65 • 0.33 NS
35.81 • 0.27 35.73 -2-_0.31 NS
35.86 _+ 0.25 35.92 _+ 0.31 NS
35.90 • 0.23 36.03 • 0.38 NS
35.93 + 0.23 35.90 • 0.27 NS
35.86 • 0.2l 36.10 _+ 0.43 NS
35.32 _+ 1.29 34.60 + 0.84 NS
35.21 • 1.11 34.55 • 0.69 NS
35.28 +_ 1.23 34.85 _+ 0.48 NS
35.01 +_ 1.23 35.07 + 0.45 NS
35.42 _+ 0.83 34.97 • 0.69 NS
35.36 + 0.82 34.95 _+ 0284 NS
35.10 _+_ 1.21 34.94 _+ 0.69 NS
NS = Not significant: SWI = single-walled incubator; DWI = double-walled incubator.
4.
5.
6.
Marks KH, Bolan D, Maisels: The double wall effect on oxygen consumption (VO0 and m e a n skin temperature in premature infants in a forced convection incubator, Pediatr Res 14:605, 1980. Hoffman PL, and Whitfield JM. Comparative oxygen consumption in double wall (DW), single wall (SW), and single wall heat shield (SWHS) environments, Pediatr Res 14:600, 1980. Marks KH, Friedman Z, and Maisets M J: A simple device for reducing insensible w a t e r loss in low birth weight infants, Pediatrics 60:223, 1977.
Rep y To the Editor: The confusion arises mainly from the skin temperature data (Table I); the m e a n abdominal skin temperature was significantly higher in infants inside the double-walled incubator (DWI) than inside the single-walled incubator (SWI). The rectal temperature was measured only at the beginning (0 minutes) and at the end of the study (180 minutes). The rectal and skin temperatures listed in Table I were the m e a n values of temperatures recorded at the beginning and at the end of the study. However, skin and ambient temperatures were recorded every 30 minutes; the means are shown in the following Table. Skin and ambient air temperatures fluctuated during the study period in both the SW and D W incubators. There was no significant difference in skin and ambient air temperature between the SW and D W incubators at any time during the study. The m e a n skin temperature values calculated from the seven temperature recordings were also similar between the SW (35.82 + 0.31~ and D W (35.87 + 0.32~ incubators. The higher m e a n skin temperature in D W than that in SW incubator listed in the original article was misleading and should not have been used. We do not agree that from our data that one can assume that continuous heat gain or loss would occur during the study period. Nor can one assume that the infants were or were not in a neutral thermal environment. Heat storage calculated from temperature data obtained at the beginning and at the end of the study does not necessarily reflect the heat balance during that three-hour
study period, since heat balance derived from a fluctuating skin temperature would also fluctuate. Ideally, the skin and rectal temperature should be continuously recorded and then one would be able to calculate the ongoing heat gain or loss. Frequent rectal temperatures were not recorded because the probe caused increased stools and often irritated the infant. The m e a n ambient air temperature inside SW was 35.17~ and that o f D W incubator was 34.8~ The operative temperature inside SW was 32.9~ and that of D W was 34.4~ These temperatures were within or near the range of neutral thermal environment. Therefore, our interpretation of QO, and IWL in infants inside SW vs D W incubators is appropriate. T h e lower radiant heat loss and lower evaporative heat loss inside D W incubator m a y account for the lower QO2. Differences in minor activity, sleep pattern, or other nonthermal stress which cannot be grossly detected could also be responsible for the changes in
9o~. The reduction of QO._, noted in our D W incubator has not been observed by previous investigators using a plastic shieldr ' blanket? or double-walled incubator whose walls were not heated, 2- ~ possibly because of a difference in the heating system. The D W incubator used in our study is heated by transfer from a heated air stream which moves between the inner and outer walls. The circulating air is always heated to maintain a desired temperature level sensed by a skin sensor on the infant. Besides the specific effect of the double wall in reducing the radiant heat loss, the design of the walls also provides a m e c h a n i s m by which t h e heated air flow can be reduced to a m i n i m u m . We believe that the design of the D W incubator tested was better than that of SW incubator tested and m a y also account for the different results from previous studies.
T. F. Yeh, M.D. L. D. Lilien, M.D. J, Matwynshyn, M.S. R. S. Pildes, M.D. Division of Neonatology Cook County Children's Hospital & the University of Illinois College of Medicine 1835 West Harrison St. Chicago, 1L 60612
Editorial correspondence
7"he Journal of Pediatrics August 1981
"neither (rat) pancreatic tissue nor juice contain a low molecular weight zinc-binding ligand." The possibility of a physiologic role of PA in pancreatic secretion and a contribution of PA from exogenous sources was suggested by our case, the human milk observations, and reports of decreased zinc levels in cystic fibrosis. However, we also suggested the possibility that PA functions at sites other than the intestines. This and related hypotheses 7 were based on the finding of zinc "dependency" in our case. ~ The PA story is complex and requires much further research.
Ingeborg Krieger, M~D. Childrens Hospital of MI 3901 Beaubien Blvd. Detroit, MI 48201 REFERENCES I.
Krieger I: Picolinic acid in the treatment of disorders requiring zinc supplementation, Nutr Rev 38:148, 1980. 2. Cash R, Krieger I, and Evans G: Treatment o f acrodermatiffs enteropathica (AE) with zinc picolinic acid, Pediatr Res 15:627, 1981 (abstr). 3. Suhadolnik R J, Stevens CO, Decker RH, Henderson LM, and Hankes LV: Species variation in the metabolism of 3-hydroxyanthranilate to pyridine carboxylic acids, J Biol Chem 228:973, 1957. 4. lkeda M, Tsuji H, Nakamura S, Ichiyama A, Nishizuka Y, and Hayaischi O: Studies on the biosynthesis of nicotinamide adenine dinucleotide, J Biol Chem 240:1395, 1965. 5. Hurley LS, and Lrnnerdal B: Picolinic acid as a zinc binding ligand in human milk: An unconvincing case, Pediatr Res 15:166, 1981. 6. Cousins RJ, Smith KT, Failla ML, and Markowitz LA: Origin o f low molecular weight zinc binding ligands in rat intestine, Life Sci 23:1819, 1978. 7. Krieger I: Acrodermatitis enteropathica and the relation to pellagra, Medical Hypotheses (in press).
Single- vs double-walled incubators To the Editor: The article by Yeh et aF investigating oxygen consumption (VO,2) and insensible water loss OWL) in single-walled incubators (SWI) versus double-walled incubators (DWI) was read with interest. The authors report a 30% reduction in IWL and a 17% reduction in "~O~ in the DWI compared with the SWI. They conclude that the DWI provides a "better" environment for premature infants and a 27% savings in energy expenditure. Unfortunately, the design of the study does not allow one to draw any conclusions about the specific effect o f the double wall on the observed changes in "VO~ or IWL. Although the authors state "the abdominal skin temperature was maintained by servocontrol at 36.5~ '' in each environment, the mean abdominal skin temperature was significantly higher in the DWI (35.75 vs 3 5 . 9 7 ~ < 0.05). Also, inner and outer incubator wall temperatures and the operative temperature were all significantly higher in the DWI. Infants in the SWI were not in a neutral
thermal environment, as evidenced by the relatively cool abdominal skin temperature (35.76~ and by the observation that resting VO~ decreased in the "warmer" environment provided by the DWI (6.42 vs 5.33 ml/kg/minute). In contrast, the infants in the DWI were in a heat-gaining environment as evidenced by the positive heat storage (0.19 kcal/kg/hour). Since heat storage is calculated from a change in body temperature, we can assume that body temperature was rising during the three-hour study period. By definition, then, infants in the DWI were also not in a neutral thermal environment. The authors, then, have compared the effects of a subneutral environment with those of a heat-gaining environment. Since thermoneutrality was not achieved in either environment, nothing can be inferred concerning the ability o f either of these incubators to provide a neutral thermal environment. Under the conditions of this study, the observed changes in X)O~ are predictable, since a heat gaining environment will result in a minimal rate of oxygen consumption unless the infant is overheated. Several investigators have assessed the "double wall effect" using plastic heat shields, -0-4 plastic blankets, ~ and the doublewalled incubator?. ' These studies demonstrate that a double wall completely enclosing the infant can reduce radiant heat loss by decreasing the skin-to-wall temperature gradient. A reduction in evaporative heat loss (IWL) has also been associated with the use of a double-walled enclosure. I agree with Yeh et al that the apparent reduction in air velocity in the DWI used in their study may partially explain the reduction in IWL. Changes in air velocity may also occur and account for some of the reduction in IWL observed with the use of a heat shield or plastic thermal blanket. Any decrease in air velocity associated with these devices is probably not related specifically to the double wall, but to the design or the interference with the forced air convection system. In studies where body temperature has been held constant, there has been no observed change in X?O2associated with the use of a double wall? -~ There may well be advantages of the double wall, but any future comparative studies should carefully control for body temperature and VO=, to provide a meaningful evaluation.
Robert A. Darnall, Jr., M.D. Department of Pediatrics University of VA Medical Center Charlottesville, VA 22908 REFERENCES
1. Yeh TF, Voora S, Lilien LD, Matwynshyn J, Srinivasan G, and Pildes RS: Oxygen consumption and insensible water loss in single-versus double-walled incubators, J PEDIATR 97:967, 1980. 2. Fanaroff AA, Wald M, C~uber HS, and Klaus MH: Insensible water loss in low birth weight infants, Pediatrics 50:246, 1972. 3. Bell EF, Weinstein MR, and Oh W: Heat balance in premature infants: comparative effects of convectively heated incubator and radiant warmer, with and without plastic shield, J PEDIATR 96:460, 1980.
Volume 99 Number 2
Editorial correspondence
33 1
Table Time (min)
Temperature Skin SWI DWI P Air SWI DWI P
35.65 • 0.29 35.84 • 0.21 NS
35.72 • 0.34 35.65 • 0.33 NS
35.81 • 0.27 35.73 -2-_0.31 NS
35.86 _+ 0.25 35.92 _+ 0.31 NS
35.90 • 0.23 36.03 • 0.38 NS
35.93 + 0.23 35.90 • 0.27 NS
35.86 • 0.2l 36.10 _+ 0.43 NS
35.32 _+ 1.29 34.60 + 0.84 NS
35.21 • 1.11 34.55 • 0.69 NS
35.28 +_ 1.23 34.85 _+ 0.48 NS
35.01 +_ 1.23 35.07 + 0.45 NS
35.42 _+ 0.83 34.97 • 0.69 NS
35.36 + 0.82 34.95 _+ 0284 NS
35.10 _+_ 1.21 34.94 _+ 0.69 NS
NS = Not significant: SWI = single-walled incubator; DWI = double-walled incubator.
4.
5.
6.
Marks KH, Bolan D, Maisels: The double wall effect on oxygen consumption (VO0 and m e a n skin temperature in premature infants in a forced convection incubator, Pediatr Res 14:605, 1980. Hoffman PL, and Whitfield JM. Comparative oxygen consumption in double wall (DW), single wall (SW), and single wall heat shield (SWHS) environments, Pediatr Res 14:600, 1980. Marks KH, Friedman Z, and Maisets M J: A simple device for reducing insensible w a t e r loss in low birth weight infants, Pediatrics 60:223, 1977.
Rep y To the Editor: The confusion arises mainly from the skin temperature data (Table I); the m e a n abdominal skin temperature was significantly higher in infants inside the double-walled incubator (DWI) than inside the single-walled incubator (SWI). The rectal temperature was measured only at the beginning (0 minutes) and at the end of the study (180 minutes). The rectal and skin temperatures listed in Table I were the m e a n values of temperatures recorded at the beginning and at the end of the study. However, skin and ambient temperatures were recorded every 30 minutes; the means are shown in the following Table. Skin and ambient air temperatures fluctuated during the study period in both the SW and D W incubators. There was no significant difference in skin and ambient air temperature between the SW and D W incubators at any time during the study. The m e a n skin temperature values calculated from the seven temperature recordings were also similar between the SW (35.82 + 0.31~ and D W (35.87 + 0.32~ incubators. The higher m e a n skin temperature in D W than that in SW incubator listed in the original article was misleading and should not have been used. We do not agree that from our data that one can assume that continuous heat gain or loss would occur during the study period. Nor can one assume that the infants were or were not in a neutral thermal environment. Heat storage calculated from temperature data obtained at the beginning and at the end of the study does not necessarily reflect the heat balance during that three-hour
study period, since heat balance derived from a fluctuating skin temperature would also fluctuate. Ideally, the skin and rectal temperature should be continuously recorded and then one would be able to calculate the ongoing heat gain or loss. Frequent rectal temperatures were not recorded because the probe caused increased stools and often irritated the infant. The m e a n ambient air temperature inside SW was 35.17~ and that o f D W incubator was 34.8~ The operative temperature inside SW was 32.9~ and that of D W was 34.4~ These temperatures were within or near the range of neutral thermal environment. Therefore, our interpretation of QO, and IWL in infants inside SW vs D W incubators is appropriate. T h e lower radiant heat loss and lower evaporative heat loss inside D W incubator m a y account for the lower QO2. Differences in minor activity, sleep pattern, or other nonthermal stress which cannot be grossly detected could also be responsible for the changes in
9o~. The reduction of QO._, noted in our D W incubator has not been observed by previous investigators using a plastic shieldr ' blanket? or double-walled incubator whose walls were not heated, 2- ~ possibly because of a difference in the heating system. The D W incubator used in our study is heated by transfer from a heated air stream which moves between the inner and outer walls. The circulating air is always heated to maintain a desired temperature level sensed by a skin sensor on the infant. Besides the specific effect of the double wall in reducing the radiant heat loss, the design of the walls also provides a m e c h a n i s m by which t h e heated air flow can be reduced to a m i n i m u m . We believe that the design of the D W incubator tested was better than that of SW incubator tested and m a y also account for the different results from previous studies.
T. F. Yeh, M.D. L. D. Lilien, M.D. J, Matwynshyn, M.S. R. S. Pildes, M.D. Division of Neonatology Cook County Children's Hospital & the University of Illinois College of Medicine 1835 West Harrison St. Chicago, 1L 60612