Deep hypothermic circulatory arrest

Rationale:

Circulatory arrest	· Allow surgery not possible on CP bypass o e.g. aortic dissection -> repair of aortic arch + branches
Deep hypothermia	· Cerebral protection o e.g. at 20°C : brain ischaemic time >30 mins (cf. 3 mins)

Effects of hypothermia:

Metabolic rate	· ↓temp 1°C = ↓metabolic rate 7% o ↓Rate of enzymatic reactions o ↓ATP hydrolysis o ↓Oxidative phosphorylation o ↓Krebs cycle activity o ↓Glycolysis (-> hyperglycaemia) · ↓Drug metabolism -> prolonged effect
Acid-base	· ↑pH 0.015 per 1°C (Rosenthal equation). Multiple factors: o ↓Water dissociation into H^+ and OH^- o ↓Metabolic rate -> ↓VCO2 o ↑CO2 solubility -> ↓partial pressure · N.B. ↑pH ∝ ↑pH of ICF ∝ pH of neutrality o Favours alpha stat management: i.e. allow pH to increase § Optimal acid-base buffering in ICF § Trapping of metabolic intermediates in ICF § Preserved function of enzymes and ion channels § Preserved CBF autoregulation o Alternative: pH stat management § Luxury cerebral perfusion § Rapid homogenous cooling
Cardiovascular	· Electrical: o Slowed conduction: bradycardia, prolonged QT, (J wave) o Arrhythmias: AF, VF (arrest at 28°C) o Resistance to defibrillation · Other: o ↓Metabolic rate -> ↓demand -> ↓preload -> ↓cardiac output o Vasoconstriction -> ↑SVR and ↑PVR
Respiratory	· Respiratory depression: o ↓RR o ↓Brainstem sensitivity to CO2 · Reduced O2 uptake (and ↑SvO2) o ↓Metabolic rate -> ↓VO2 o Left shift OHDC -> ↓tissue oxygenation
Neurological	· Peripheral: o Shivering (if not paralysed) o Hyporeflexia · Central: o ↓GCS o ↓MAC, ↓propofol Cp50 o ↓Metabolic rate = cerebral protection o ↓Seizures
Renal	· ↓ADH synthesis -> cold diuresis
Haematological	· ↑Viscosity · Coagulopathy: o <35°C platelet dysfunction o <33°C factor dysfunction
Immunological	· ↓WBC activity -> ↑susceptibility to infection (if <36°C)

Feedback welcome at ketaminenightmares@gmail.com