Deep Hypothermic Circulatory Arrest

DHCA is a neuroprotective strategy used to ↓ risk of ischaemia when surgical requirements demand complete cessation of the circulation. DHCA involves controlled cooling via CBP:

All organs are of course at risk of ischaemia during circulatory arrest, but the brain both particularly vulnerable and important.

Inflow temperature is the temperature of blood returned by the CPB circuit, and is measured accurately by the machine.

Indications

  • Cardiac surgery
    • Aortic arch surgery
    • Pulmonary thromboendarterectomy
    • Complex congenital surgery
  • Neurological
    • Cerebral aneurysms
    • Intracranial AVM
  • Vena caval surgery
    • Carcinoma with caval invasion

Contraindications

Principles

  • Oxygen demand is a function of metabolic rate, and is strongly affected by hypothermia
    Cerebral metabolism ↓ by 6-7% for every 1°C ↓ in temperature.
  • Safe cerebral ischaemic time is proportional to temperature
    Most patients tolerate 30 minutes of DHCA at 18°C.
Brain Temperature vs. Safe DHCA Duration

Practice

Preparation

Should consider:

There is little evidence for cerebral oximetry, ice, and use of corticosteroids; most institutions will have their own policy.

There is poor correlation between rectal and pulmonary artery catheter temperature and brain temperature. Nasopharyngeal and tympanic temperature are the best surrogate.

  • Cerebral oximetry monitoring
  • Processed EEG monitoring
  • Two temperature probes
    • Brain temperature
      Nasopharynx.
    • Body temperature
      Bladder.
  • Ice for topical head cooling
    Delays brain rewarming during arrest, but does not contribute meaningfully to cooling.
  • Corticosteroids
    1mg/kg prednisolone, ideally 6-8 hours before surgery.

Cooling

  • Ensure adequate anticoagulation prior
  • Commence topical cooling of the head
  • Consider TIVA rather than volatile
    Avoid uncoupling CMRO2 from metabolism.
  • Monitor for VF
    This may result in AI and ventricular distention, which requires either:
    • Aortic cross-clamping and cardioplegia
    • Venting

Cold

Considerations:

  • Turn off all infusions, particularly vasopressors
    • ↓ Metabolism of infused drugs
    • No circulation of infused drugs
      Will pool in the SVC, resulting in a bolus when circulation resumes.
  • Acid-base measurement can use the pH or alpha-stat approach
    • pH-stat may be beneficial during cooling
      Adding CO2 to the CPB circuit may improve neurological outcomes through more rapid and uniform cerebral cooling.
  • Haemodilution to a haematocrit of 20% may improve microcirculatory flow

Alpha-stat vs. pH-stat is covered under Measurement Variability. In brief, alpha-stat warms a cold sample to 37°C; pH-stat adjusts the measured gas samples for the patient temperature which results in a lower partial pressure due to the ↑ in solubility.

  • Adequate storage for heparinised blood
    The patient is traditionally exsanguinated.
  • Additional return lines for cerebral perfusion

Cerebral Perfusion Strategies

Cerebral perfusion can prolong safe operating time or ↓ the degree of hypothermia required. Options include:

  • Selective anterograde cerebral perfusion
    Perfusion of neck vessels with 10-20mL/kg/min of blood, maintaining some cerebral blood flow.
    • Cannulation or grafting of one or both carotid arteries, or the axillary or brachiocephalic artery
    • Right sided perfusion can be monitored targeting a right radial arterial pressure of 50-70mmHg
    • NIRS is particularly helpful to note the presence of perfusion and an intact Circle of Willis, and identify any interruptions (e.g. kinking) to flow
  • Intermittent systemic perfusion
    Intermittent perfusion prolongs total duration of DHCA, but is not appropriate for all operations.
  • Retrograde cerebral perfusion
    Perfusion of the brain with oxygenated blood administered retrograde into the SVC.
    • Relies on the fact that the cerebral vasculature has no valves
    • Patchy cerebral perfusion

Retrograde cerebral perfusion likely operates mostly via the azygous system (the the IJV possess valves), whose tributaries cover (loosely) cerebral territories supplied by the posterior circulation.

Anterograde vs. Retrograde Cerebral Perfusion
Anterograde Retrograde
Advantages Disadvantages Advantages Disadvantages
  • Greater ↑ in safe arrest time
  • ↓ Hypothermia required
  • Maintains cerebral cooling
  • Better global oxygenation
  • Embolic risk
  • ↑ Surgical complexity
  • Requires intact Circle of Willis
  • Maintains cerebral cooling
  • ↑ In safe arrest time
    Less than ACP.
  • Variable oxygen delivery
  • Cerebral oedema
  • ↑ ICP

Rewarming

Prior to rewarming:

  • De-air
  • Restart infusions

During rewarming:

TEG is covered under Thromboelastography.

  • Consider cold reperfusion
    Restart CPB without rewarming, and continue for 10-20 minutes.
  • Slow rewarming to minimise tissue thermal gradients
    • <2C
  • Prevent secondary brain injury
    Avoid:
    • Hyperthermia
    • Hypoxaemia
    • Hypotension
    • Hypoperfusion
  • Perform thromboelastography
    Warmed sample can provide early identification of clotting factor deficiencies.

Complications

Profound hypothermia brings a raft of adverse effects:

  • C
    • Arrhythmia
      • Proportional to degree of hypothermia
      • Partially related to ↓ K+
    • ↑ Plasma viscosity
    • ↑ SVR
  • D
    • Cerebral vasoconstriction
    • ↑ Stroke risk
      5-10%. Associated with:
      • Age
      • DHCA time
      • Aortic atheroma
    • POCD
    • Reperfusion injury
    • Hyperthermic injury
  • E
    • Hypothermia
      Temperature commonly falls after rewarming, and patients may become significantly hypothermic during the post-bypass phase.
      • This is ↓ by a slower rate of rewarming
  • F
    • ↓ GFR
    • Metabolic acidosis
    • ↑ BSL
      ↓ Glucose metabolism.
  • H
    • Coagulopathy
      Major contributor to early and late mortality.
    • Thrombocytopaenia
  • Other
    • Altered pharmacokinetics

Key Studies

References

  1. Conolly S, Arrowsmith JE, Klein AA. Deep hypothermic circulatory arrest. Continuing Education in Anaesthesia Critical Care & Pain. 2010;10(5):138-142. doi:10.1093/bjaceaccp/mkq024
  2. Hogue CW, Arrowsmith JE. Deep hypothermic circulatory arrest. In: Mackay JH, Arrowsmith JE, eds. Core Topics in Cardiac Anesthesia. 2nd ed. Cambridge University Press; 2012:387-394. doi:10.1017/CBO9780511979095.066