Preventing perioperative neonatal hypothermia
Alan Jay Schwartz MD MSEd, Myron Yaster MD, and Lynne G. Maxwell MD
Preventing perioperative hypothermia in neonates has been an important element of pediatric anesthesia care for several decades. Today’s PAAD by Ramjist et al.1 is a single center trial designed to “minimize intra-operative hypothermia in neonates undergoing thoracic and abdominal surgery in the OR by using an overhead radiant warmer and a thermal hat.” Really? This is new? To be honest, there is nothing new surprising in this study and I wondered whether we should review it in the PAAD. However, this is such an important topic, I thought it would be a good opportunity to review the topic by reposting a previous PAAD that covered this issue much more comprehensively than this new article in the J Pediatric Surgery. I should also add that this topic is comprehensively covered in all of the pediatric anesthesia textbooks including the recently reviewed Litman’s Basics of Pediatthiric Anesthesia, 3rd Edition.2,3 Myron Yaster MD
References
1. Ramjist JK, Sutthatarn P, Elliott C, Lee KS, Fecteau A. Introduction of a Warming Bundle to Reduce Hypothermia in Neonatal Surgical Patients. J Pediatr Surg 2024;59(5):858-862. (In eng). DOI: 10.1016/j.jpedsurg.2024.01.037.
2. Yaster M. Litman’s Basics of Pediatric Anesthesia, 3rd Edition. Anesthesiology 2024;140(5):1055-1056. DOI: 10.1097/ALN.0000000000004889.
3. Jones J, Litman RS. Temperature regulation. In: Litman RS, Ambardekar A, eds. Litman's basics of pediatric anesthesia 3d edtion: Elsevier; 2022:103-107.
Remembering the Classics: Transepidermal (Evaporative) Water Loss Understanding The Physiology of Neonatal Heat Loss
This PAAD was originally posted on August 9. 2023
Remember when you were a kid and wore a yellow raincoat/slicker when you were waiting for the school bus in the rain? Do you also remember that you got as hot as hell while you were wearing it and when you took off your raincoat you were absolutely soaking wet from sweating in your impermeable raincoat? Although you didn’t know it at the time, this was your first science experiment demonstrating the power of evaporative heat loss into an impermeable environment. Over the next 2 days we will discuss both a classic and current article on heat loss and conservation and why it so important in pediatric anesthesia. Further, because this is the new season for residents and fellows, these articles and the discussion points they contain are perfect for intraoperative teaching. And while you’re at it, share the PAAD and encourage them to subscribe. Myron Yaster MD
Original article
Rowe MI, Taylor M. Transepidermal water loss in the infant surgical patient. J Pediatr Surg. 1981 Dec;16(6):878-81. doi: 10.1016/s0022-3468(81)80839-1. PMID: 7338769.
A familiar and repetitious surgeon’s refrain heard in a pediatric OR:
“Can you make the OR cooler?”
The anesthesiologist’s retort is always the same:
“The baby requests these warm surroundings!”
Why do our babies (and increasingly adults) require strict attention to the OR’s thermal conditions and the patient’s thermal homeostasis? Patient hypothermia is a proverbial problem in the pediatric OR. Neonates and small infants rapidly lose heat and body temperature because of their high body surface area to volume ratio, increased evaporative heat loss, and the absence of brown adipose tissue. In the OR, heat loss and resultant hypothermia is further increased because of open body cavities, radiation to distant walls, and the vasoplegic effects of general anesthesia.
The basis for understanding heat loss and conservation:1-3
Mechanisms of heat loss:
A. Evaporation-heat consumed when body water is converted to water vapor.
B. Radiation-transfer of body surface heat via infrared electromagnetic waves to environmental temperature
C. Convection-transfer of body heat to air molecules surrounding and moving by the body surface.
D. Conduction-transfer of body surface heat due to direct physical contact with a surface of different temperature
Think about how you, personally and as an anesthesiologist in the OR, behave to minimize heat loss for both you and your patient. Radiation-We turn the thermostat up to warm the environment and reduce the gradient (differential) between the body and the environment. Conduction-We warm surfaces to which we come in contact. Convection-We wear a sweater or jacket/cover patients to retain body heat in the mini-environment under the garment/drape and prevent air currents from moving that heat away from the body or conversely turn up heated air currents to warm the patient. Evaporation-We increase relative humidity surrounding our body surface and in respiratory environments (employ an HME) (Before HMEs, as residents in the 1970/80s we were taught to rinse the breathing circuit and not drain the residual water clinging to the walls and corrugations of the tubing). Subsequently, active heated humidification was used and was considered essential during the era of utilization of the Mapleson anesthesia circuit, which caused even greater loss of humidity/heat because of the high fresh gas flows necessary to eliminate CO2 (there was no CO2 absorber). Heated humidifiers were plagued by heated wires that sometimes burned through circuit tubing in the absence of adequate gas flow, and frequent temperature alarms. Studies at the time comparing active heated humidification to HME demonstrated similar ability to maintain humidity with better heat conservation of active humidification especially in hypothermic patients. HME has been associated with greater accumulation of dried airway secretions in the ETT in situations of prolonged ventilation, and is associated with a higher incidence of ventilator-associated pneumonia.4 In light of the aforementioned disadvantages of HME in the setting of prolonged ventilation, active humidification continues to be used in ventilator circuits in the critical care setting. Finally, we minimize exposed moist surfaces from which water can be converted to water vapor.
Among the 4 mechanisms of heat loss, evaporation can result in a monumental risk for developing hypothermia especially in neonates.1-3 Quantifying clinical evaporative heat loss and identifying a method to significantly reduce it would benefit the neonate in the perioperative period.
These goals were the focus of the article of this Remembering the Classics PAAD. In 1981, Rowe and Taylor, 2 pediatric surgeons, devised an elegantly simple study.1 Fifteen infants undergoing surgical procedures were cared for in differing environments. Each infant was evaluated in 5 successive environments, 1. exposed, 2. covered by and lying on cloth, 3. covered by plastic and lying on cloth, 4. covered by and lying on plastic and 5. placed in a plastic bag. Evaporative rates and ambient relative humidity were measured for each environment described. Skin, ambient, and rectal temperatures were documented. The data enabled calculation of transepithelial water loss. Placing a baby in a plastic bag resulted in the greatest reduction in transepithelial water loss.
In the July 5, 2023 PAAD, Remembering the Classics: The “leak” Test, I (AJS) suggested that our first pediatric anesthesia lesson was learning geometry in junior high school. I’ll now suggest that another pediatric anesthesia lesson revolved around learning chemistry and physics in high school. What factors about evaporative heat loss did physics and chemistry teach us in high school that explain this pediatric anesthesia risk?
Minimize neonatal heat loss based upon understanding the physics and chemistry of evaporative heat loss:
A. The amount of energy it takes to vaporize water is very high (2.4 kJ/g).1-3 Just think about how much heat is required when you boil water from a liquid to a gas.
B. The evaporation of water from a surface is dependent on energy that is absorbed away from the surface during the transition from a liquid to a gaseous state. This energy is called the latent heat of vaporization, and in the case of sweat, it has a value of 2.5 × 106 J/kg.3
C. “For every gram of water lost from the body by evaporation, 0.58 kcal of heat is lost.”5
D. Evaporation accounts for 10%-25% of body heat loss under conditions of thermal neutrality.2, 3
E. Average transepithelial water loss is 6 g/sq m/hr and is responsible for 18% of the heat loss in the full-term neonate.
F. Transepithelial evaporative water loss is increased (as high as 75 g/sq m/hr) in the more permeable skin of the preterm neonate. This could result in as much as 70 cal/kg/day heat loss (50% of the neonates’ caloric intake being lost).1
G. Evaporative heat loss occurs through skin, the respiratory system, and any exposed body surface, ie, pleura, peritoneum, dura, muscle.
H. When hypothermic stress occurs, the neonate’s metabolic activity (oxygen consumption) increases to reestablish core temperature (370 C). The body mechanisms to increase temperature are triggered within a narrow range of thermal deviation, ie, 370 + 0.2 degrees C. These include vasoconstriction to shield core temperature from body surface influences, shivering and brown fat metabolism.6
The take home/in the OR lessons the neonates want you to know!
Turn the room thermostat up.
Maintain the room’s relative humidity.
Maintain the breathing circuit’s relative humidity.
Maintain the relative humidity of exposed operative surfaces.
Minimize air currents in the room.
Cover the neonate with dry drapes; consider plastic covering to minimize evaporative loss.
Active warming: under sheet circulating water mattress or underbody forced air warming device. Radiant heat devices
Whenever I (MY) would place neonates into a plastic bag during transport to and from the OR and in the OR I would always get this horrified look from all who saw it. After explaining the tremendous heat conservation afforded by preventing evaporative heat loss I would conclude that “I’m a professional…don’t do this at home”! This is such an effective and inexpensive technique (how much does a plastic bag cost?) that many infants return to the NICU hyperthermic! What tricks and tips do you use in your practice? Let Myron know and he will post in Friday’s Reader response.
References
1. Rowe MI, Taylor M. Transepidermal water loss in the infant surgical patient. J Pediatr Surg. 1981 Dec; 16(6): 878-81. doi: 10.1016/s0022-3468(81)80839-1.PMID: 7338769
2. Engorn BM, Harvey H, Luginbuehl I, Bissonette B. Thermoregulation. In: Davis PJ, Cladis FP, eds. Smith’s Anesthesia for Infants and Children. 10th ed. chap 7. Elsevier; 2022: 158-178:
3. Romanovsky AA. The thermoregulation system and how it works. Handbook of clinical neurology. 2018; 156: 3-43. doi:10.1016/b978-0-444-63912-7.00001-1
4. McNulty G, Eyre L. Humidification in anaesthesia and critical care. BJA Education. 2014; 15(3): 131-135. doi:10.1093/bjaceaccp/mku022
5. Stolwijk JA, Hardy JD. Partitional calorimetric studies of responses of man to thermal transients. J Appl Physiol. May 1966; 21(3): 967-77. doi:10.1152/jappl.1966.21.3.967
6. Dawkins MJ, Scopes JW. Non-shivering thermogenesis and brown adipose tissue in the human new-born infant. Nature. 1965; 206(980): 201-2. doi:10.1038/206201b0