Hypothermia, hepatic oxygen supply-demand, and ischemia-reperfusion injury in pigs

Hepatic blood flow regulation but not oxygen extraction capability is impaired in prolonged experimental abdominal sepsis

Impaired oxygen utilization has been proposed to play a significant role in sepsis-induced liver dysfunction, but its magnitude and temporal course during prolonged resuscitation is controversial. The aim of this study is to evaluate the capability of the liver to increase oxygen extraction in sepsis during repeated acute portal vein blood flow reduction. Twenty anesthetized and mechanically ventilated pigs with hepatic hemodynamic monitoring were randomized to fecal peritonitis or controls (n = 10, each). After 8-h untreated sepsis, the animals were resuscitated for three days. The ability to increase hepatic O₂ extraction was evaluated by repeated, acute decreases in hepatic oxygen delivery (Do₂) via reduction of portal flow. Blood samples for liver function and liver biopsies were obtained repeatedly. Although liver function tests, ATP content, and Do₂ remained unaltered, there were signs of liver injury in blood samples and overt liver cell necrosis in biopsies. With acute portal vein occlusion, hepatic Do₂ decreased more in septic animals compared with controls [max. decrease: 1.66 ± 0.68 mL/min/kg in sepsis vs. 1.19 ± 0.42 mL/min/kg in controls; portal venous flow (Qpv) reduction-sepsis interaction: P = 0.028]. Hepatic arterial buffer response (HABR) was impaired but recovered after 3-day resuscitation, whereas hepatic oxygen extraction increased similarly during the procedures in both groups (max. increase: 0.27 ± 0.13 in sepsis vs. 0.18 ± 0.09 in controls; all P > 0.05). Our data indicate maintained capacity of the liver to acutely increase O₂ extraction, whereas blood flow regulation is transiently impaired with the potential to contribute to liver injury in sepsis.NEW & NOTEWORTHY The capacity to acutely increase hepatic O₂ extraction with portal flow reduction is maintained in sepsis with accompanying liver injury, but hepatic blood flow regulation is impaired.

Ubiquitin-proteasome system and oxidative stress in liver transplantation

A major issue in organ transplantation is the development of a protocol that can preserve organs under optimal conditions. Damage to organs is commonly a consequence of flow deprivation and oxygen starvation following the restoration of blood flow and reoxygenation. This is known as ischemia-reperfusion injury (IRI): a complex multifactorial process that causes cell damage. While the oxygen deprivation due to ischemia depletes cell energy, subsequent tissue oxygenation due to reperfusion induces many cascades, from reactive oxygen species production to apoptosis initiation. Autophagy has also been identified in the pathogenesis of IRI, although such alterations and their subsequent functional significance are controversial. Moreover, proteasome activation may be a relevant pathophysiological mechanism. Different strategies have been adopted to limit IRI damage, including the supplementation of commercial preservation media with pharmacological agents or additives. In this review, we focus on novel strategies related to the ubiquitin proteasome system and oxidative stress inhibition, which have been used to minimize damage in liver transplantation.

Subnormothermic Perfusion in the Isolated Rat Liver Preserves the Antioxidant Glutathione and Enhances the Function of the Ubiquitin Proteasome System

The reduction of oxidative stress is suggested to be one of the main mechanisms to explain the benefits of subnormothermic perfusion against ischemic liver damage. In this study we investigated the early cellular mechanisms induced in isolated rat livers after 15 min perfusion at temperatures ranging from normothermia (37°C) to subnormothermia (26°C and 22°C). Subnormothermic perfusion was found to maintain hepatic viability. Perfusion at 22°C raised reduced glutathione levels and the activity of glutathione reductase; however, lipid and protein oxidation still occurred as determined by malondialdehyde, 4-hydroxynonenal-protein adducts, and advanced oxidation protein products. In livers perfused at 22°C the lysosomal and ubiquitin proteasome system (UPS) were both activated. The 26S chymotrypsin-like (β5) proteasome activity was significantly increased in the 26°C (46%) and 22°C (42%) groups. The increased proteasome activity may be due to increased Rpt6 Ser120 phosphorylation, which is known to enhance 26S proteasome activity. Together, our results indicate that the early events produced by subnormothermic perfusion in the liver can induce oxidative stress concomitantly with antioxidant glutathione preservation and enhanced function of the lysosomal and UPS systems. Thus, a brief hypothermia could trigger antioxidant mechanisms and may be functioning as a preconditioning stimulus.

A mechanistic investigation of the oxygen fixation hypothesis and oxygen enhancement ratio

The presence of oxygen in tumours has substantial impact on treatment outcome; relative to anoxic regions, well-oxygenated cells respond better to radiotherapy by a factor 2.5-3. This increased radio-response is known as the oxygen enhancement ratio. The oxygen effect is most commonly explained by the oxygen fixation hypothesis, which postulates that radical-induced DNA damage can be permanently 'fixed' by molecular oxygen, rendering DNA damage irreparable. While this oxygen effect is important in both existing therapy and for future modalities such a radiation dose-painting, the majority of existing mathematical models for oxygen enhancement are empirical rather than based on the underlying physics and radiochemistry. Here we propose a model of oxygen-enhanced damage from physical first principles, investigating factors that might influence the cell kill. This is fitted to a range of experimental oxygen curves from literature and shown to describe them well, yielding a single robust term for oxygen interaction obtained. The model also reveals a small thermal dependency exists but that this is unlikely to be exploitable.

Prophylactic hypothermia and neuromuscular blockade to limit myocardial oxygen demand in a critically anemic Jehovah's Witness after emergency surgery†

Management of anemic patients refusing blood transfusion remains challenging. Concomitant coronary artery disease further complicates management. We sought to decrease the likelihood of cardiac events by employing hypothermia and neuromuscular blockade, in addition to limited phlebotomy, in a critically anemic Jehovah's Witness patient following emergent colectomy. The patient's hemoglobin concentrations were trended with serial blood gases. Neuromuscular blockade was instituted with cisatracurium, followed by hypothermia to a target of 32°C. The patient's lowest hemoglobin levels occurred on postoperative day 3 before beginning to rise. There were no postoperative cardiac events reported during the patient's course of stay. She recovered well with no evidence of anemia or cardiac events at 1-year follow-up. We conclude that targeted hypothermia with neuromuscular blockade, as an adjunct to accepted techniques, may be an alternative for critically anemic patients with coronary artery disease refusing blood transfusion.

Comparison of major hepatectomy performed under intermittent Pringle maneuver versus continuous Pringle maneuver coupled with in situ hypothermic perfusion

BACKGROUND

The Pringle maneuver (hepatic inflow occlusion), applied intermittently or continuously, carries the risk of inducing ischemic and reperfusion injury. The risk of damage is higher in the latter procedure. Studies have shown that continuous Pringle maneuver coupled with in situ hypothermic perfusion (CPM-HP) circumvents such adversity. However, reports comparing this technique with the intermittent Pringle maneuver (IPM) are lacking. We therefore report our experience with the use of CPM-HP and compare its outcome with that of IPM.

METHODS

We evaluated the outcome of similar sets of patients who had major hepatic resections performed under IPM and CPM-HP. Variables including short-term survival rate (>90 days), complications, operative time, transection time, intraoperative blood loss, postoperative liver functions, and postoperative hospital stay were used to compare the two groups.

RESULTS

Eighteen major hepatectomies were performed with CPM-HP and 16 with IPM. CPM-HP was safely performed in patients with chronic liver disease. Lowering the liver's temperature extends the clamping period to 140 min. Perioperative outcomes including operative time (383.9 ± 89.4 vs. 351.9 ± 70.3 min, p = 0.252), blood loss (225.6 ± 48.4 vs. 351.9 ± 70.3 ml, p = 0.057), postoperative hospital stay, morbidity rate, and the rate of liver functions following resections were comparable for the CPM-HP and IPM groups. There was no mortality. Parenchymal transection time was significantly longer in the CPM-HP group (104.1 ± 20.2 vs. 85.0 ± 15.4 min, p = 0.004)

CONCLUSION

Our findings did not show there to be a significant advantage of CPM-HP over IPM.

Local cooling provides muscle flaps protection from ischemia-reperfusion injury in the event of venous occlusion during the early reperfusion period

Clinicians often place patients in heated rooms following muscle flap transfers. We hypothesize that exposure of flaps to heated room temperatures could result in an unnecessary hyperthermic ischemic insult if the flaps were to be compromised by venous outflow obstruction, while exposure of elective flaps to local cooling during early perfusion may provide protection in the event of venous occlusion. The rat rectus femoris muscle flap was elevated and clamped for 1 h. The muscle was then exposed to various temperatures for 1 h of perfusion followed by complete venous occlusion for 3 h. Occlusion clamps were removed and flaps were allowed to reperfuse for 24 h. Flaps were assessed for muscle necrosis and edema. Venous occluded muscles demonstrated decreased muscle necrosis and edema in the locally cooled group (8.5 +/- 6.7%, 3.06 +/- 0.14; P < 0.001) compared to the room temperature group (76.2 +/- 23.0%, 3.73 +/- 0.13), and the local warming group (97.3 +/- 1.4%, 3.84 +/- 0.29) respectively. No difference was noted in muscle necrosis nor edema amongst non-ischemic muscles irrespective of temperature exposure. These results suggest a beneficial role for exposure of elective flaps to local cooling during the early perfusion period in order to provide protection from ischemia reperfusion injury in the event of a venous occlusion insult. The prophylactic exposure of flaps to local cooling is further supported by the lack of a harmful effect when flaps were not compromised by venous occlusion.

The relationship of systemic hemodynamics and oxygen consumption to early allograft failure after liver transplantation

The early postoperative hemodynamic data of 88 patients who underwent primary liver transplantation between July 1989 and October 1990 at the University Health Center of Pittsburgh were analyzed to establish the relationship of systemic hemodynamics and oxygen consumption to perioperative allograft function. The 15 patients whose allografts failed within the 1st month following transplantation were designated as group 1, while 73 patients who retained adequate graft function constituted group 2. Although the cardiac index and oxygen delivery did not differ significantly between the groups, group 1 consistently demonstrated a lower mean arterial pressure, oxygen consumption, arteriovenous oxygen content difference, and arterial ketone body ratio. The etiology of reduced oxygen consumption in group 1 patients is speculative, but the data support the notion that oxygen consumption is a useful, predictive indicator for liver allograft function after transplantation.