What are the pathophysiological changes affecting the cardiovascular system that occur after brain death?

Usually two distinct phases, though the first is not seen in all patients

1. Hyperdynamic phase (sympathetic overactivity)
   • Transient catecholamine 'storm' (particularly adrenaline and noradrenaline) leads to:
     • Intense arterial constriction and hypertension with tachycardia and increase in cardiac output
     • Cushing's reflex due to massive increase in afterload with secondary baroreceptor activity causing bradycardia (occurs in about one-third of patients
     • Imbalance between myocardial oxygen supply and demand, manifesting as:
       • Acute myocardial injury and ST segment change
       • Arrhythmias and heart blocks
       • Apical ballooning and generalised LV dysfunction
2. Cardiovascular collapse phase (sympathetic underactivity)
   • Hypotension results from:
     • Loss of sympathetic tone and profound vasodilatation
     • Myocardial depression and arrhythmias:
       • Depletion of high energy phosphate
       • Mitochondrial inhibition
       • Reduction in T3 production
       • Electrolyte disturbance
     • Hypovolaemia:
       • Diabetes insipidus
       • Osmotic diuresis (mannitol, hyperglycaemia)
       • Therapeutic fluid restriction
   • Asystolic cardiac arrest generally occurring within 72 hours