Question 2

Outline the physiological consequences of therapeutic hypothermia at 32 degrees Celsius

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College Answer

It is well documented and often stressed that the Primary Exam is focused upon the basic  sciences that underpin clinical Intensive Care. It will examine a candidates understanding of, for example, the physiology associated with a clinical circumstance. Many candidates  discussed why we use therapeutic hypothermia after cardiac arrest rather than outline the physiological consequences of hypothermia at 32 degrees Celsius, for which they would not have scored any marks. A good answer was expected to outline changes in metabolism as well as specific organ responses such as cardiovascular (e.g. bradycardia; vasoconstriction; decreased cardiac output, etc), respiratory (decreased minute volume; haemoglobin-oxygen dissociation curve moves left; increased anatomical dead space; diminished HPV; increased pulmonary vascular resistance, etc), renal (e.g. diuresis, changes to GFR, etc) as well as other organs. Again candidates failed to synthesize a coherent and detailed answer.

Discussion

It is impossible to retort directly to the examiners without knowing the content of the candidate's answers here, but one must counter their last comment by reminding them of all the times the college failed to synthesize a coherent question. 

The Physiological Consequences of
Mild Hypothermia (32-35° C)

Endocrine and metabolic consequences

  • Decreased metabolism and oxygen consumption

  • Decreased carbohydrate metabolism and hyperglycaemia

  • Decreased drug metabolism and clearance

  • Essentially unchanged electrolytes

Haematological consequences

  • Increased hematocrit and blood viscosity

  • Neutropenia and thrombocytopenia

  • Coagulopathy and platelet dysfunction

Respiratory consequences

  • Decreased respiratory rate and medullary sensitivity to CO2
     

Acid-base changes: alkalosis and hypocapnea

  • Rise of pH with falling body temperature

  • Fall of PCO2 with falling body temperature

  • Increased oxygen solubility and O2-haemoglobin affinity

Pharmacological consequences

  • Delayed absorption
  • Decreased drug metabolism, especially hepatic metabolism
  • Delayed hepatic and renal clearance
  • Poorer affinity of receptors (eg. for catecholamines)

Cardiovascular consequences

  • Decreased cardiac output and bradycardia

  • QT prolongation and the J wave

  • Arrhythmias - classically AF and VF

  • Resistance to defibrillation

  • Vasoconstriction

Renal consequences

  • "Cold diuresis" due to decreased vasopressin synthesis

Central nervous system effects

  • Confusion and decreased level of consciousness

  • Shivering

  • Increased seizure threshold

Immunological consequences

  • Decreased granulocyte and monocyte activity

References

Wong, K. C. "Physiology and pharmacology of hypothermia." Western Journal of Medicine 138.2 (1983): 227.

Polderman, Kees H. "Application of therapeutic hypothermia in the intensive care unit." Intensive care medicine 30.5 (2004): 757-769.

Polderman, Kees H. "Mechanisms of action, physiological effects, and complications of hypothermia." Critical care medicine 37.7 (2009): S186-S202.