Effects of porta-systemic shunting and ammonia infusion on cerebral blood flow autoregulation in the rat

Background: Portacaval shunting of blood, hyperammonemia, and impaired cerebral blood flow (CBF) autoregulation are assumed to be involved in the development of high intracranial pressure (ICP) in liver failure. In this study, we determined whether CBF autoregulation is impaired by portacaval anastomosis and hyperammonemia. Methods: Four groups of pentobarbital-sedated and mechanically ventilated rats were investigated after construction of a portacaval anastomosis or following sham operation. Half of the rats received either infusion of ammonia (55 mu mol/kg/minute) or saline for 180 minutes. Arterial pressure and ICP was monitored, and lower limit of CBF autoregulation was determined. Results: Lower limit of autoregulation was preserved in all four groups of studied animals; vehicle lower limits were 40 +/- 2.3,40 +/- 2, 54 +/- 1, and 51 +/- 3 mmHg in sham-operated rats, sham rats receiving ammonia infusion, portacaval anastomosis-vehicle animals, and portacaval anastomosis-hyperammonemia animals, respectively. The lower limit of autoregulation was higher in portacaval anastomosis rats (p = 0.01) compared to the sham-operated rats. Hyperammonemia in portacaval anastomosis rats did not aggravate this. Conclusion: Portacaval anastomosis and hyperammonemia does not impair the lower limit of CBF autoregulation. However, shunting of portal blood to the systemic circulation shifts the lower limit of autoregulation to higher blood pressure values, making the brain more sensitive to episodes of arterial hypotension.