Question 7

College answer

Neuro: Loss of cerebrovascular regulation, coma, loss of ocular reflexes
CVS: Decline in BP and cardiac output, VF (<28°C) bradycardia and asystole (<20°C)
Respiratory: Pulmonary oedema, apnoea
Renal: Oliguria
Musculoskeletal: Pseudo-rigor mortis (may appear dead)
Metabolic: Decreased metabolic rate, hyper or hypoglycaemia

Considerations in providing ACLS
Decision to start

May commence cardiac life support in an apparently “dead” hypothermic patient. Beware that very slow, irregular small volume pulse may be present and an unrecordable blood pressure. The brain can tolerate cardiac arrest for long periods at 18°C.

Rewarming

Patients need to be actively rewarmed while resuscitation is being continued.
Extra-corporeal support, not mandated, but can be mentioned
More emphasis on the continuing re-warming, an issue of priority, should state early
Temperature should be measured centrally
 
Physical difficulties

Hypothermia can cause stiffness of chest wall making ventilation and chest compression difficult – early use of mechanical devices as resuscitation attempts are likely to be prolonged.
Be aware that interventions (e.g. CPR, central line placement, endotracheal intubation) may precipitate arrhythmias

Medications

Consider withholding drugs (e.g. Adrenaline) until core temp > 30°C and then double the interval between giving the drugs (i.e. give adrenaline every 4th cycle compared with every 2nd cycle) until temperature 35°C
The hypothermic heart may be unresponsive to cardioactive drugs, electrical pacing and defibrillation. Arrhythmias
Arrhythmias other than VF tend to revert spontaneously as temperature rises. Bradycardia does not usually need treatment as it is physiological in severe hypothermia
VF therapy: unclear at which temp shocking should be first attempted. After 3 shocks if no response, consider delaying further attempts at defibrillation until temperature > 28-30°C

Examiners Comments:

In part a), candidates listed things that were not clinical signs e.g. ECG changes and ETCO2, and there was also a focus on the CVS aspect which showed a limited breadth of clinical signs, and hence limited marks. In part b), many candidates did not show a breadth of considerations, and focussed mainly on the rewarming in great depth, hence did not score well on this section. Also, candidates often listed their management rather than outline their considerations, so the aspects they discussed also often lacked depth.
 

Discussion

The cranky CICM examiners complained that, when asked for clinical signs, many of the candidates "listed things that were not clinical signs e.g. ECG changes and ETCO₂" And then they themselves had listed "loss of cerebrovascular regulation", "decline in ... cardiac output" and "decreased metabolic rate" in their model answer. Looking at their list, one might come to the conclusion that the question asked for the physiological consequences of hypothermia.

Anyway, the clinical signs are:

• Observation
  • Mottled appearance
  • Cyanosis
  • Pallor
  • No shivering
  • Cold oedematous skin
• Respiratory findings
  • Apnoea, hypoventilation or Cheyne-Stokes respiration
  • Creps on auscultation, suggestive of pulmonary oedema
• Cardiovascular findings
  • Sinus bradycardia, AF or asystole
  • Hypotension
  • Weak thready pulse, or altogther impalpable peripheral pulses
  • Poor capillary refill
• Neurological findings
  • Coma (unresponsiveness below 32°C)
  • Increased muscle tone
  • Sluggish deep tendon reflexes
  • Extensor plantar responses
  • Loss of cranial nerve reflexes (with fixed mid-dilated pupils)
• Gastrointestinal findings
  • Absent bowel sounds
  • Abdominal tenderness (hypothermia-induced pancreatitis)
• Renal findings
  • Decreased urine output

All of this comes from excellent reviews by Rosin et al (1964) and Aslam et al (2006). 

Now, as for the management of cardiac arrest in hypothermia. "Outline the considerations", judging by the wording of the model answer, appears to mean "discuss all the ways hypothermic arrest is different". Though the college question asks specifically about ACLS, the college answer also discusses some BLS material. The ideal reference for this would probably have to be the AHA "Special Circumstances" section from the 2015 cardiac arrest guidelines. Following the AHA's own structure:

• Prognostic implications of cardiac arrest with severe hypothermia:
  • Prognosis may be better than expected given the usually prolonged duration of CPR
  • A large percentage of survivors (~ 40%) have a good neurological outcome
• Changes to basic life support:
  • It may take longer than normal to detect signs of life (up to 1 minute)
  • CPR should ideally be performed mechanically (prolonged CPR is to be expected)
  • Intermittent CPR (stopping for 5 minutes every 5 minutes) is reasonable for prehospital and retrieval staff, particularly when interruption facilitates retrieval
  • Manual or mechanical ventilation may encounter poor lung compliance
• Changes to advanced life support
  • Do not defibrillate until core temperature is over 30°C.
  • If you decide to defibrillate and after three shocks the rhythm remains VF, withhold further attempts until core temperature is over 30°C.
  • Do not give adrenaline until core temperature is over 30°C.
  • After 30°C is achieved, double the interval between adrenaline doses until 35°C
  • Use a low-reading thermometer to record core body temperature
• Rewarming techniques
  • Rewarming is central to the success of resuscitation
  • Extracorporeal circuit rewarming is the ideal
  • Warmed fluids and peritoneal lavage is the next best option
  • External warming is least effective
  • Remember that intubation will produce a increase in the rate of cooling by interruption of shivering though paralysis and anaesthesia.
• Supportive post-arrest management
  • The temperature of approximately 32° to 34°C can be maintained after rewarming, according to standard post-cardiac arrest guidelines.
  • The AHA recommend that we "do not delay urgent interventions such as airway management and insertion of vascular catheters regardless of evidence of cardiac irritability"

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