Thermoregulation: Hypothermia and Hyperthermia




© Springer International Publishing AG 2017
Robert S. Holzman, Thomas J. Mancuso, Joseph P. Cravero and James A. DiNardo (eds.)Pediatric Anesthesiology Review10.1007/978-3-319-48448-8_47


47. Thermoregulation: Hypothermia and Hyperthermia



Joseph P. Cravero1, 2  


(1)
Department of Anesthesiology, Perioperative, and Pain Medicine, Boston Children’s Hospital, Boston, MA, USA

(2)
Harvard Medical School, Boston, MA, USA

 



 

Joseph P. Cravero

Email:


Keywords
HypothermiaConductive heat lossRadiant heat lossConvective heat lossHyperthermiaEvaporationThermoneutralityAcidosisHyperkalemia


You are taking care of a 4 kg, 5-month-old infant scheduled for a hernia repair.


Preoperative Considerations



Questions





  1. 1.


    What are the mechanisms of heat loss in humans? What are the mechanisms of heat generation? Is heat generation different for an infant vs. an older child or adult? What is “thermoneutrality”?

     

  2. 2.


    What effect does general anesthesia have on thermoregulation? Contrast this with the effect of regional anesthesia on thermoregulation.

     

  3. 3.


    How do you define hypothermia? Where would you measure the temperature? Describe the advantages and disadvantages of each site.

     


Answers





  1. 1.


    Heat loss can occur through several mechanisms:


    1. (a)


      Radiation – heat lost between objects that are not in contact. This could occur between the baby and the cool walls (or any other object) in the OR.

       

    2. (b)


      Convection – heat lost from a body to moving molecules such as air or liquid. The amount of heat lost depends on the temperature of the air and the speed of the air moving around the patient.

       

    3. (c)


      Evaporation – heat that is lost through the latent heat of vaporization of water or any other liquid evaporating on the patient. This can occur from the skin, a surgical incision, or from mucosal surfaces (which can be 30 % or more of heat loss if dry air is used for ventilation in an infant).

       

    4. (d)


      Conduction – the heat loss that occurs between objects that are in direct contact. The extent of this loss depends on temperature differences and the area of contact. Heat generation can occur through voluntary muscle activity, non-shivering thermogenesis, shivering (non-voluntary muscle activity), and dietary thermogenesis. Non-shivering thermogenesis occurs from metabolism of brown fat in infants and toddlers (perhaps up to 2 years of age). Brown fat contains an increased number of mitochondria and is therefore very effective at generating metabolic energy/heat. The effect is significantly decreased in children under anesthesia, whether this is with inhaled agents or those that have received fentanyl/propofol. Shivering is of minimal importance in maintaining body temperature in newborns and infants as the musculoskeletal system is immature and the muscle mass is limited. Young infants will shiver between 35 and 35.3 °C, but the effect is generally negligible in maintaining core temperature. Dietary thermogenesis is stimulated by nutrients (proteins and amino acids). An infusion of a small amount of amino acids under anesthesia may increase heat generation by up to fivefold in adult models. Infants have a larger surface area to body mass ratio and a higher thermal conductance (lose heat faster due to less fat and greater surface area.). The combination of faster heat loss and reduced ability to generate heat markedly predisposes the infant to hypothermia.

      Thermoneutrality or thermoneutral environment is the ambient temperature at which the oxygen demand is minimal and temperature regulation is accomplished by non-evaporative physical processes alone. For the adult, the neutral temperature is 28 °C, while for neonates and young infants, it is approximately 32 °C.

       

     

  2. 2.


    General anesthesia interferes with thermoregulation. This is due to many factors but includes redistribution of core heat to the periphery, a 30 % reduction in heat generation, inhibition of central thermoregulation, and increased exposure (depending on the procedure).

    Vasoconstriction and non-shivering thermogenesis are the only thermoregulatory responses that are active under anesthesia. Maximal vasoconstriction is similar in the awake and anesthetized patient, although the threshold for vasoconstriction is reduced under anesthesia. Non-shivering thermogenesis is profoundly inhibited under general anesthesia within 10–15 min of induction. Under regional anesthesia in older children and adults, central temperature regulation is preserved, but areas that are anesthetized cannot sense temperature and therefore have inappropriate redistribution of blood flow by vasodilation. In addition, there is no vasoconstriction in the blocked areas. With major neuraxial blockade, vasoconstriction may be lost in a large portion of dermatomes with the result being a large amount of heat redistribution, which can be as marked (or worse) than general anesthesia. Contrary to this finding is the fact that caudal block in infants does not affect the temperature for vasoconstriction.

     



  1. 3.


    Hypothermia can be mild (33.9–36 °C), moderate (32.2–33.8 °C), or severe (below 32.2 °C). Temperature may be measured from the tympanic membrane (by placing a probe in the auditory canal and sealing it to the external environment), nasopharynx (convenient but can be associated with nosebleeds or inaccuracy if an uncuffed tube is in place), esophagus (convenient but can be confounded by transmission of respiratory gas temperature if not in the distal third of the esophagus), axillary (very convenient but can be very inaccurate if not placed carefully), rectal (can cause trauma and may be inaccurate if embedded in stool or during laparotomy in an infant), bladder (accurate if urine output is copious, not widely available), and skin (wildly inaccurate depending on body area and vasoconstriction).

     


Intraoperative



Questions





  1. 1.


    What would be your strategies for preventing heat loss during this case?

     

  2. 2.


    After 1 h of surgery, the temperature is 34.5 °C. Outline the positive and negative effects of hypothermia on an infant.

     

  3. 3.


    You institute a series of interventions to warm the patient and 90 min later the temperature is 39 °C. What is your differential diagnosis? What do you do for this situation?

     

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Oct 9, 2017 | Posted by in Uncategorized | Comments Off on Thermoregulation: Hypothermia and Hyperthermia

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