Hepatic and gastrointestinal oxygen and lactate metabolism during low cardiac output in lambs

RESUSCITATION WITH 21% OR 100% OXYGEN IS EQUALLY EFFECTIVE IN RESTORING PERFUSION AND OXYGEN METABOLISM IN THE LIVER OF HYPOXIC NEWBORN PIGLETS

The differential effects of the use of high or low oxygen levels during resuscitation on the neonatal liver are unknown. We compared the hepatic hemodynamics and oxygen metabolism in hypoxic newborn piglets resuscitated with 21% or 100% oxygen. Twenty-seven piglets (age, 1-3 days; weight, 1.5-2.0 kg) were acutely instrumented to measure cardiac output, hepatic artery, and portal venous blood flows (hepatic artery flow index [HAFI] and portal venous flow index [PVFI], respectively). The animals underwent 2 h of hypoxia (fraction of inspired oxygen, 0.10-0.15), then reoxygenation with 21% (n = 9) or 100% (n = 9) oxygen for 1 h, then 1 h with 21% oxygen. The controls (n = 9) were sham-operated without hypoxia-reoxygenation. Oxygen transport and plasma lactate concentrations were studied. Hypoxic animals had hypotension and decreased cardiac index with metabolic acidosis (mean pH, 7.00-7.02; P < 0.05 vs. controls). The PVFI and the total hepatic blood flow (THFI = PVFI + HAFI), despite the absence of significant change in HAFI, decreased to 16 +/- 2 mL/min/kg and 19 +/- 3 mL/min/kg, respectively (versus 24 +/- 2 mL/min/kg and 28 +/- 2 mL/min/kg of controls; P < 0.05). Fifteen minutes after reoxygenation, the cardiac index improved, PVFI recovered, HAFI was maintained, and THFI was not different between the groups. The hepatic oxygen consumption decreased (59%; P < 0.05) and the extraction increased (89%; P < 0.001) during hypoxia. Similarly, on reoxygenation, the hepatic oxygen consumption improved; however, extraction decreased versus controls on 100% but not on 21% oxygen (P < 0.05). The plasma lactate concentrations increased in both groups with hypoxia and were not different during reoxygenation between the group administered with 21% oxygen and the group administered with 100% oxygen. The hypoxic neonatal liver has reduced hepatic blood flow but has relatively preserved HAFI, and oxygen consumption recovered similarly on reoxygenation with 21% and 100% oxygen. The increased oxygen extraction during hypoxia normalized in 21% but reduced in 100% reoxygenation, with no differences in plasma lactate concentrations.

Transhepatic lactate gradient in relation to liver ischemia/reperfusion injury during major hepatectomies

Hepatectomies performed under selective hepatic vascular exclusion are associated with a series of events culminating in ischemia/reperfusion injury, a state that shares common characteristics with situations known to result in global or regional hyperlactatemia. Accordingly, we sought to determine whether lactate is released by the liver during hepatic resections performed under blood flow deprivation and what relation this has to a possible systemic hyperlactatemic state. After ethical approval, 14 consecutive patients with resectable liver tumors subjected to hepatectomy under inflow and outflow occlusion of the liver were studied. Lactate concentrations were assessed in simultaneously drawn arterial, portal venous, and hepatic venous blood before liver dissection and 50 minutes postreperfusion. Moreover, the transhepatic lactate gradient (hepatic vein - portal vein) was calculated to see if there was net production or consumption of lactate. Before hepatic dissection, the transhepatic lactate gradient was negative, suggesting consumption by the liver. Fifty minutes after reperfusion, this gradient became significantly positive, demonstrating release of lactate by the liver (0.12 +/- 0.31 vs. -0.38 +/- 0.30 mmol/L, P < 0.05). The magnitude of lactate release correlated with systemic arterial lactate levels at the same time point (r(2) = 0.63, P < 0.001). A weaker but significant correlation was demonstrated between the transhepatic lactate gradient postreperfusion and systemic arterial lactate levels 24 hours postoperatively (r(2) = 0.41, P = 0.013). A strong correlation between the transhepatic lactate gradient postreperfusion and peak postoperative aspartate aminotransferase values was also demonstrated (r(2) = 0.73, P < 0.001). The liver becomes a net producer of lactate in hepatectomies performed under blood flow deprivation. This lactate release can explain some of the systemic hyperlactatemia seen in this context and relates to the extent of ischemia/reperfusion injury.

Splanchnic vasoregulation during mesenteric ischemia and reperfusion in pigs

We evaluated the hepatic arterial buffer response (HABR) to portal vein (PV) occlusion during 2 h of reduced superior mesenteric arterial blood flow (median 2 mL min(-1) kg(-1), range of 1-3 mL min(-1) kg(-1)) and 1 h of reperfusion in seven pigs and in seven controls. In animals with reduced mesenteric blood flow, celiac trunk blood flow (Qtr) increased during mesenteric hypoperfusion from 4 +/- 1 mL min(-1) kg(-1) (mean +/- SD) to 16 +/- 3 mL min(-1) kg(-1) (P = 0.028), and hepatic arterial blood flow (Qha) increased from 2 +/- 1 to 10 +/- 4 mL min(-1) kg(-1) (P= 0.018). The extra-hepatic fraction of Qtr (Qtr-Qha) also increased (P = 0.028). In controls, Qtr and Qha also increased, but to lower levels. At baseline, acute PV occlusion increased Qha by 5.0 +/- 2.8 mL min(-1) kg(-1) (P < 0.001), whereas Qtr-Qha decreased by 1.6 +/- 1.6 mL min(-1) kg(-1) (P = 0.007). After 120 min of reduced mesenteric blood flow, the HABR was exhausted (change in Qha to PV occlusion of 0.7 +/- 1.6 mL min(-1) kg(-1) [P= 0.27]). The efficacy of the HABR was also reduced in controls animals. Despite increased cardiac output, all flows from the celiac trunk decreased during reperfusion (P = 0.028) and the HABR partially recovered. We conclude that reduced mesenteric perfusion impairs the HABR, which recovers only partially after reperfusion. The distribution of the increased celiac trunk flow secondary to PV occlusion ranges from increased HABR and decreased non-hepatic blood flow (a steal) to decreased hepatic arterial blood flow and increased non-hepatic blood flow (an inverse steal).

Liver and intestinal lactate metabolism in patients with acute hepatic failure undergoing liver transplantation

OBJECTIVE

To determine the relative contribution of the gastrointestinal tract and the liver in lactate metabolism in patients with acute liver failure (ALF) and the effect of liver transplantation on this. We hypothesized that the liver and gut are net producers of lactate in ALF and that this is reversed after liver transplantation.

SETTING

A university-affiliated specialist liver transplant operating theater.

SUBJECTS

Eleven patients with ALF undergoing liver transplantation.

MEASUREMENTS AND INTERVENTIONS

After ethical approval, 11 patients with ALF listed for orthotopic hepatic transplantation were studied. Whole blood was analyzed for lactate concentration from radial artery (RA) catheter, portal vein (PV), and hepatic vein (HV) during the dissection phase and was repeated postreperfusion of the liver graft. Gradients across the gut and the liver were calculated to see if there was net production or consumption.

RESULTS

HV lactate was significantly higher than arterial (p =.028) in patients with ALF before liver transplantation, suggesting splanchnic production of lactate. Total splanchnic lactate gradient (HV-RA) is positive in ALF. Both the gut (PV-RA) and the liver (HV-PV) were net producers of lactate. After liver transplantation, hepatic venous lactate falls below arterial levels but not significantly. The gradient across the gut (PV-RA) remained positive, but the transhepatic gradient (HV-PV) became significantly negative, showing consumption by the graft (p =.021). The magnitude of lactate consumption after transplantation correlated positively with portal venous lactate concentration (p =.029) and inversely with graft cold ischemic time (p =.007).

CONCLUSION

The liver is a net producer of lactate in patients with ALF and an elevated whole blood lactate. After liver transplantation, the graft becomes a consumer of lactate as shown by the negative lactate gradient. The degree of consumption is dependent on portal venous lactate concentration and cold ischemic time.

Effects of systemic arterial hypoperfusion on splanchnic hemodynamics and hepatic arterial buffer response in pigs

The hepatic arterial buffer response (HABR) tends to maintain liver blood flow under conditions of low mesenteric perfusion. We hypothesized that systemic hypoperfusion impairs the HABR. In 12 pigs, aortic blood flow was reduced by cardiac tamponade to 50 ml. kg(-1). min(-1) for 1 h (short-term tamponade) and further to 30 ml. kg(-1). min(-1) for another hour (prolonged tamponade). Twelve pigs without tamponade served as controls. Portal venous blood flow decreased from 17 +/- 3 (baseline) to 6 +/- 4 ml. kg(-1). min(-1) (prolonged tamponade; P = 0.012) and did not change in controls, whereas hepatic arterial blood flow decreased from 2 +/- 1 (baseline) to 1 +/- 1 ml. kg(-1). min(-1) (prolonged tamponade; P = 0.050) and increased from 2 +/- 1 to 4 +/- 2 ml. kg(-1). min(-1) in controls (P = 0.002). The change in hepatic arterial conductance (DeltaC(ha)) during acute portal vein occlusion decreased from 0.1 +/- 0.05 (baseline) to 0 +/- 0.01 ml. kg(-1). min(-1). mmHg(-1) (prolonged tamponade; P = 0.043). In controls, DeltaC(ha) did not change. Hepatic lactate extraction decreased, but hepatic release of glutathione S-transferase A did not change during cardiac tamponade. In conclusion, during low systemic perfusion, the HABR is exhausted and hepatic function is impaired without signs of cellular damage.

Cerebral and intestinal perfusion and metabolism in normocythemic hyperviscous hypoxic newborn pigs

We studied the effects of hypoxia on cerebral cortical and intestinal perfusion and metabolism in normocythemic hyperviscous newborn pigs. Seven pigs were made hyperviscous by an injection of cryoprecipitate, increasing viscosity from 5.8 +/- 0.9 to 9.0 +/- 1. 2 (SD) cycles/s. Six normoviscous pigs received 0.9% NaCl. Reducing the inspired O(2) decreased the arterial O(2) content (Ca(O(2))) from 9.5 +/- 1.6 to 3.6 +/- 1.3 ml O(2)/100 ml. Increases in brain and decreases in gastrointestinal blood flow at the lower Ca(O(2)) values were similar between the groups. During hypoxia, blood flow to stomach, distal intestinal mucosa, and large intestines was lower (-50, -23, and -28%, respectively) in the hyperviscous than normoviscous group. At the lower Ca(O(2)) values, cerebral cortical vascular resistance decreased in both groups and intestinal vascular resistance increased (+257%) in the hyperviscous but not in the normoviscous group. During hypoxia, systemic oxygen delivery decreased, extraction increased, and uptake did not change; cerebral cortical O(2) delivery, extraction, and uptake did not change; and intestinal O(2) delivery decreased, extraction increased, and uptake did not change in both groups. Our study demonstrated that 1) during hypoxia, increases in systemic O(2) extraction compensated for decreases in delivery and systemic uptake did not change; vasodilation sustained cerebral cortical O(2) delivery and preserved metabolism; increases in intestinal oxygen extraction offset decreases in delivery and uptake was preserved; and 2) nonpolycythemic hyperviscosity did not have a major influence on cardiovascular or metabolic responses to hypoxia, except for modest effects on intestinal resistance and perfusion to certain gastrointestinal regions. We conclude that, under normocythemic conditions, a moderate increase in viscosity does not have a major impact on hemodynamic or metabolic adjustments to hypoxia in newborn pigs.

Reductions in cardiac output in hypoxic young pigs: systemic and regional perfusion and oxygen metabolism

We tested the hypotheses that, in hypoxic young pigs, reductions in cardiac output restrict systemic oxygen transport to a greater extent than does hypoxia alone and that compensatory responses to this restriction are more effective in higher than in lower priority vasculatures. To study this, 10- to 14-day-old instrumented awake hypoxic (arterial oxygen tension = 39 Torr) pigs were exposed to reduced venous return by inflation of a right atrial balloon-tipped catheter. Blood flow was measured with radionuclide-labeled microspheres, and oxygen metabolism was determined with arterial and venous oxygen contents from appropriate vessels. Hypoxia resulted in a reduction in oxygen tension; increases in cardiac output and perfusion to brain (72% over baseline), heart, adrenal glands, and liver without reductions to other organs except for the spleen; reductions in systemic and intestinal oxygen delivery; and increases in systemic and intestinal oxygen extraction without changes in systemic, cerebral, or intestinal oxygen uptake. During hypoxia, decreasing venous return was associated with increases in arterial lactic acid concentration and central venous pressure; attenuation of the hypoxia-related increase in cardiac output; sustained increases in brain (72% over baseline) and heart perfusion; reductions in lung (bronchial artery), pancreatic, renal, splenic, and intestinal (-50% below baseline) perfusion; decreases in systemic and gastrointestinal oxygen delivery; sustained increases in systemic and intestinal oxygen extraction; and decreases in intestinal oxygen uptake, without changes in cerebral oxygen metabolism. We conclude that when venous return to the heart is reduced in hypoxic young pigs, the hypoxia-related increase in cardiac output was attenuated and the relative reduction in cardiac output was associated with preserved cerebral oxygen uptake and compromised intestinal oxygen uptake. Regional responses to hypoxia combined with relative reductions in cardiac output differ from that of hypoxia alone, with the greatest effects on lower priority organs such as the gastrointestinal tract.