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

The values of cerebrovascular pressure reactivity and brain tissue oxygen pressure reactivity in experimental anhepatic liver failure

BACKGROUND

We investigated in a porcine model of anhepatic acute liver failure (ALF), the value of two parameters describing cerebrovascular autoregulatory capacity, pressure reactivity index (PRx) and brain tissue oxygen pressure reactivity (ORx), regarding their power to predict the development of intracranial hypertension.

METHODS

In six pigs, hepatectomy was performed. Only one animal was sham operated. All animals received neuromonitoring including arterial blood pressure, intracranial pressure (ICP), and brain tissue partial oxygen pressure (P(br)O(2)). The average time of neuromonitoring was 31.0 h. Cerebral perfusion pressures (CPP), cerebrovascular pressure reactivity index (PRx) and brain tissue oxygen reactivity index (ORx) were calculated.

RESULTS

Perioperative disturbance of AR improved within 4 h after surgery. From 6 to 16 h post hepatectomy, ICP did slowly increase by 4 mmHg from baseline; CPP remained stable around 40 mmHg. PRx and ORx, however, indicated in this period a progressive loss of AR, reflected in a decrease of P(br)O(2) despite unchanged CPP. Beyond 16 h, ICP rose quickly. At CPP levels below 35 mmHg, P(br)O(2) fell to ischemic levels.

CONCLUSIONS

The loss of cerebrovascular autoregulatory capacity, indicated by a rise of PRx and ORx precedes the final crisis of uncontrollable intracranial hypertension in this animal model by hours. During this phase cerebral blood flow, as reflected in tissue oxygenation, deteriorates despite unchanged CPP. Monitoring of AR during ALF therefore seems to carry the power to identify a risk for development of critical CBF and intracranial hypertension.

The brain in acute liver failure. A tortuous path from hyperammonemia to cerebral edema

Acute liver failure (ALF) is a condition with an unfavourable prognosis. Multiorgan failure and circulatory collapse are frequent causes of death, but cerebral edema and intracranial hypertension (ICH) are also common complications with a high risk of fatal outcome. The underlying pathogenesis has been extensively studied and although the development of cerebral edema and ICH is of a complex and multifactorial nature, it is well established that ammonia plays a pivotal role. This review will focus on the effects of hyperammonemia on neurotransmission, mitochondrial function, oxidative stress, inflammation and regulation of cerebral blood flow. Finally, potential therapeutic targets and future perspectives are briefly discussed.

Cerebral blood flow autoregulation in experimental liver failure

Patients with acute liver failure (ALF) display impairment of cerebral blood flow (CBF) autoregulation, which may contribute to the development of fatal intracranial hypertension, but the pathophysiological mechanism remains unclear. In this study, we examined whether loss of liver mass causes impairment of CBF autoregulation. Four rat models were chosen, each representing different aspects of ALF: galactosamine (GlN) intoxication represented liver necrosis, 90% hepatectomy (PHx90) represented reduction in liver mass, portacaval anastomosis (PCA) represented shunting of blood/toxins into the systemic circulation thus mimicking intrahepatic shunting in ALF, PCA+NH(3) provided information about the additional effects of hyperammonemia Rats were intubated and sedated with pentobarbital. We measured CBF with laser Doppler, intracranial pressure (ICP) was measured in the fossa posterior and registered with a pressure transducer, brain water was measured using the wet-to-dry method, and cerebral glutamine/glutamate was measured enzymatically. The CBF autoregulatory index in both the GlN and PHx90 groups differed significantly from the control group. Conversely, CBF autoregulation was intact in the PCA and PCA+NH(3) groups despite high arterial ammonia, high cerebral glutamine concentration, and increased CBF and ICP. Increased water content of the brainstem or cerebellum was not associated with defective CBF autoregulation. In conclusion, impairment of CBF autoregulation is not caused by brain edema/high ICP. Nor does portacaval shunting or hyperammonemia impair autoregulation. Rather, massive liver necrosis and reduced liver mass are associated with loss of CBF autoregulation.

Hyperammonemia acts synergistically with lipopolysaccharide in inducing changes in cerebral hemodynamics in rats anaesthetised with pentobarbital

BACKGROUND/AIMS

The aim was to determine the effect of ammonia (NH(3)) and lipopolysaccharide (LPS) alone or in combination, on cerebral blood flow (CBF) and intracranial pressure (ICP) in the rat. Since amiloride-sensitive-ion-pathways in the blood-brain barrier (BBB) modulate CBF, we also aimed to test if Na(+)/H(+)-inhibitors could prevent this possible synergism between NH(3) and LPS.

METHODS

In experiment A, four groups of rats received ammonium acetate (140 micromol/kg/min) or saline, each of them associated with either vehicle or LPS (2 mg/kg). In experiments B and C, rats received similar treatments after having received amiloride (30 mg/kg) or 5-(N-methyl-N-isobutyl)-amiloride (MIA, 5 mg/kg). Plasma tumor-necrosis-factor-alpha (TNF-alpha), ICP (via a cisterna magna catheter) and CBF (by laser-Doppler flowmetry) were measured.

RESULTS

An increase in ICP and CBF within 60 min was observed only in rats that received NH(3) together with LPS as compared to any other group (P<0.01), which could be prevented by amiloride (P<0.05), but not by MIA. Both amiloride and MIA decreased the plasma TNF-alpha concentration.

CONCLUSIONS

In rats anaesthetised with pentobarbital NH(3) infusion aggravates a LPS induced rise in ICP and induces an increase in CBF less clearly seen with LPS alone. This effect is prevented by the non-specific Na(+)/H(+) inhibitor amiloride, but not by MIA, a specific inhibitor of Na(+)/H(+) exchanger. Thus, the synergistic effect of NH(3) and LPS seems mediated by other amiloride-sensitive-ion-pathways in the BBB than the Na(+)/H(+) exchanger.