Physiology of thermoregulation

Human body temperature is tightly regulated to keep it at a inter threshold range of 36.5 to 37.0 degree centigrade

The thermoregulation is controlled by Thermal regulators(warm and cold)

                                                                                   ↓

                                                              Thermal receptors in the spinal cord and the brain stem

                                                                                    ↓

                                                               Preoptic nuclei in the anterior hypothalamus

                                                                                    ↓

                                                                Efferent response

                                                                                    ⇩

                                                                  ↙                              ↘             

                 Cold response.                                                                     Warm response  

                Vasoconstriction                                                                         vasodilation

                 Shivering                                                                                    Sweating

    

Measurement of core temperature

• Best estimate is the pulmonary artery catheter temperature estimation from pulmonary circulation
• Tympanic membrane temperature 
• Nasopharyngeal and esophageal temperature probe
• Infrared temperature measurement probes

Effect of anesthesia and thermoregulation

Both general and regional anesthesia results in mild to moderate core temperature reduction(1 to 3 deg centigrade )

1. First phase- There is rapid reduction in core temp. Of 1 to 1.5 deg cent within 30 to 45 min.of the anesthesia induction and this is due to vasodilatation  which causes a disturbance in core to peripheral temp gradient with distribution of heat to peripheral tissue
2. Second phase -There is a more gradual linear decline in core temp of 1 deg cent in the next 2 hours and this loss is primarily due to radiation,convection and evaporation
3. Third phase -There is a balance achieved by heat loss and metabolic heat production

• Consequences of hypothermia
• Cardiac arrythmias and ischaemia
• Increased pulmonary vascular resistance
• Left sided shift of the oxygen-haemoglobin dissociation curve
• Reversible coagulopathy
• Postoperative protein catabolism and stress response
• Altered mental status
• Impaired renal function
• Decreased drug metabolism
• Increased incidence of infection