FRCA Notes

Temperature Management During Cardiopulmonary Bypass

Resources

  • Induced hypothermia is used during cardiopulmonary bypass to reduce CMRO2 and prevent cerebral ischaemia or hypoxic injury
  • NB said induced hypothermia is not for myocardial protection; that comes from cardioplegia or cross-clamp-fibrillation

Advantages Disadvantages
↓ CMRO2/cerebral ischaemia ↑ blood viscosity ∴ embolic risk
↓ myocardial VO2 ↑ infection risk
↓ rate of neurotransmitter release & neuronal cell death Impaired wound healing
↓ inflammatory response Peripheral vasoconstriction
↓ BBB permeability Impairs liberation of oxygen from haemoglobin
Improved organ protection Bleeding/coagulopathy
Hyperglycaemia
Dysrhythmia
Metabolic acidosis


Methods

  • The primary method of cooling is the use of heat exchangers as part of the cardiopulmonary bypass circuit
  • Modern systems use hollow-fibre technology, with:
    • Multi-lumen passage of blood in one direction
    • Counter-current flow of fresh water, whose temperature is controlled by the perfusionist
  • High resistance necessitates upstream placement of the main bypass pump
  • The aim is typically to decrease core temperature by 0.5 - 1°C/minute

  • An additive method includes placing ice packs around the head during circulatory arrest to maximise cerebral cooling

Management during CPB

  • During CPB: the blood temperature at the oxygenator arterial outlet is a surrogate marker of cerebral temperature
    • One should keep this to <37°C to avoid hyperthermia
  • During re-warming: assume the oxygenator arterial outlet temperature is an underestimate marker of cerebral perfusate temperature (i.e. to reduce risk of hyperthermia)
  • During weaning from CPB (and after): use a NP temperature probe or PA catheter to measure temperature

Ranges

  • Ranges of hypothermia include:

    • Grade Temperature
    Mild 32 - 36°C
    Moderate 28 - 32°C
    Deep 15 - 28°C

Mild-moderate hypothermia

  • Mild or moderate hypothermia is suitably neuroprotective, but still associated with adverse effects
  • Mild hypothermia is usually suitable for coronary procedures
  • Mild-moderate hypothermia is usually required for valvular surgery

Deep hypothermic circulatory arrest


Cardiac indication Non-cardiac indication
Aortic arch surgery Repair of large cerebral aneurysms
Pulmonary endarterectomy Surgery on the thoracoabdominal aorta
Repair of complex congenital cardiac defects Resection of cerebral AVM
Vascular reconstruction during cardiac transplant Resection of tumours with vena caval invasion
  • Slow-onset, extensive cooling (18 - 20°C) appears to provide better neuroprotection
    • At 18°C, CMRO2 is only 10-15% of base level
    • At 7°C, it is 5% of normal
  • Lower temperature means longer duration of bypass and longer re-warming time, so is associated with increased complications
  • However it's the duration of DHCA which is the limiting factor, rather than the decrease in temperature per se

  • The duration of safe arrest is not definitively known, although incidence of significant neurological injury increases beyond 30 minutes
    • The evidence base for the safety of 40mins of DHCA is not robust
    • The evidence base suggests 60mins of DHCA is poorly tolerated
  • Equally, slow re-warming is advocated as rapid re-warming or hyperthermia worsen neurological outcomes

  • Cerebral circulatory monitoring is often employed
    • If significant alterations are noted, selective anterograde (carotid) or retrograde (jugular) perfusion can take place

  • The process of cooling using CPB is reversed to re-warm the patient
  • Rate of rewarming is ≤0.5°C/min
  • This gives times for gases to equilibrate and reduces the risk of gas bubble formation and subsequent gas embolus
  • Avoidance of cerebral hyperthermia helps reduce the risk of neurological inury

  • Adjunctive methods for re-warming include:
    • IV fluid warmers following cessation of CPB, which may help prevent rebound hypothermia
    • Forced air warmers, which reduce convective loss although they are not particularly efficient
    • Heated operating mattresses, which prevent conductive loss and are more efficient than forced air warmers

Safe re-warming

  • If oxygenator arterial outlet temperature is <30°C, maintain a maximum gradient between venous inflow and arterial outflow of 10°C

  • If oxygenator arterial outlet temperature is ≥30°C
    • Maintain a maximum gradient between venous inflow and arterial outflow of ≤4°C
    • Maintain a rate of rewarming is ≤0.5°C/min


Respiratory Cardiovascular Neurological Metabolic Renal GI Haematological Infection
Left-shift of oxyHb dissociation curve leads to tissue hypoxia Myocardial depression: ↓ CO and MAP ↓ CMRO2 7% for each 1°C Slowed enzymatic reactions ↓ Na+ & H2O re-absorption i.e. diuresis ↓ hepatic blood flow ↑ blood viscosity Impaired wound healing
Pulmonary oedema if severe Bradycardia, VF, asystole ↓ CPP and ICP Altered drug metabolism ↓ GFR leads to acidosis Impaired glucose metabolism ↓ platelet activation Immunnosuppression
Apnoea once <24°C J-waves (<30°C) Confusion <35°C Prolonged NMBA activity Hyopkalaemia Ileus ↑ platelet sequestration ↑ wound infection rate
Shivering artefact Loss of consciousness <30°C Hyperglycaemia Submucosal erosions & GIB Inhibits fibrinolysis
Vasoconstriction Earlier return of GI motility ↓ factor 11 and 12 function