Brain-death-induced cardiac contractile dysfunction: studies of possible neurohormonal and blood-borne mediators

The aim of these studies was to investigate the mechanism underlying the haemodynamic changes associated with brain death. The initial series of studies were to assess whether these changes involved some blood-borne factor. When control rats (n = 6) were exsanguinated whilst being simultaneously transfused with blood from rats that had been brain-dead for 60 min, their haemodynamic function did not deteriorate. Likewise, when brain-dead rats (n = 6) were exsanguinated and transfused with blood from control rats there was no improvement of haemodynamic function. The absence of any blood-borne factor was further confirmed in studies in which isolated hearts from control rats were perfused with blood from a support rat which had been brain-dead for 15 min (n = 6/group). The brain-death-induced haemodynamic changes in the support rat (mean arterial pressure increased from 98 +/- 6 to 176 +/- 9 mm Hg after 30 s and then fell to 44 +/- 5 mm Hg after 5 min) were not associated with changes in cardiac function of the perfused heart (left ventricular developed pressure was 146 +/- 4 mmHg before the induction of brain death and 147 +/- 4 and 151 +/- 7 mm Hg at 30 s and 5 min, respectively, after the induction). In further in vivo studies, we assessed the involvement of the autonomic nervous system in brain-death-induced haemodynamic instability. We achieved this by employing beta-adrenoreceptor blockade or bilateral vagotomy (n = 6/group); propranolol (1 mg/kg given as a bolus 6 min before brain death followed by 0.5 mg/kg/h continuous infusion) abolished the early transient tachycardia and positive inotropic response to brain death but did not alter the subsequent deterioration of function (mean arterial pressure fell from 75 +/- 7 mmHg before brain death to 49 +/- 5 mmHg after 30 min). Bilateral vagotomy had no effect on the functional changes induced by brain death. The effect of catecholamine depletion was then investigated; 6-hydroxydopamine (given over 15 days) depleted myocardial norepinephrine content by approximately 90% (from 2.3 +/- 0.1 to 0.3 +/- 0.1 nmol/g wet wt; P < 0.05). Depletion of cardiac catecholamines reduced brain-death-induced mortality to zero but did not affect cardiac dysfunction. Finally, we used L-NAME and naloxone in an attempt to identify roles for nitric oxide and endogenous opioid peptides but were again unable to influence the cardiac events. In conclusion, the initial transient hyperdynamic response induced by brain death appears to be mediated through cardiac innervation and can be inhibited by beta-adrenoreceptor blockade. However, the autonomic nervous system, nitric oxide, endogenous opioid peptides and blood-borne factors do not appear to be involved in the subsequent deterioration of cardiac function.