The effect of hypothermia on neuronal viability following cardiopulmonary bypass and circulatory arrest in newborn piglets

Does supply meet demand? A comparison of perfusion strategies on cerebral metabolism in a neonatal swine model

OBJECTIVE

We aimed to determine the effects of selective antegrade cerebral perfusion compared with other perfusion strategies on indices of cerebral blood flow, oxygenation, cellular stress, and mitochondrial function.

METHODS

One-week-old piglets (n = 41) were assigned to 5 treatment groups. Thirty-eight were placed on cardiopulmonary bypass. Of these, 30 were cooled to 18°C and underwent deep hypothermic circulatory arrest (n = 10), underwent selective antegrade cerebral perfusion at 10 mL/kg/min (n = 10), or remained on continuous cardiopulmonary bypass (deep hypothermic cardiopulmonary bypass, n = 10) for 40 minutes. Other subjects remained on normothermic cardiopulmonary bypass (n = 8) or underwent sham surgery (n = 3). Novel, noninvasive optical measurements recorded cerebral blood flow, cerebral tissue oxyhemoglobin concentration, oxygen extraction fraction, total hemoglobin concentration, and cerebral metabolic rate of oxygen. Invasive measurements of cerebral microdialysis and cerebral blood flow were recorded. Cerebral mitochondrial respiration and reactive oxygen species generation were assessed after the piglets were killed.

RESULTS

During hypothermia, deep hypothermic circulatory arrest piglets experienced increases in oxygen extraction fraction (P < .001), indicating inadequate matching of oxygen supply and demand. Deep hypothermic cardiopulmonary bypass had higher cerebral blood flow (P = .046), oxyhemoglobin concentration (P = .019), and total hemoglobin concentration (P = .070) than selective antegrade cerebral perfusion, indicating greater oxygen delivery. Deep hypothermic circulatory arrest demonstrated worse mitochondrial function (P < .05), increased reactive oxygen species generation (P < .01), and increased markers of cellular stress (P < .01). Reactive oxygen species generation was increased in deep hypothermic cardiopulmonary bypass compared with selective antegrade cerebral perfusion (P < .05), but without significant microdialysis evidence of cerebral cellular stress.

CONCLUSIONS

Selective antegrade cerebral perfusion meets cerebral metabolic demand and mitigates cerebral mitochondrial reactive oxygen species generation. Excess oxygen delivery during deep hypothermia may have deleterious effects on cerebral mitochondria that may contribute to adverse neurologic outcomes. We describe noninvasive measurements that may help guide perfusion strategies.

Cerebral mitochondrial dysfunction associated with deep hypothermic circulatory arrest in neonatal swine

OBJECTIVES

Controversy remains regarding the use of deep hypothermic circulatory arrest (DHCA) in neonatal cardiac surgery. Alterations in cerebral mitochondrial bioenergetics are thought to contribute to ischaemia-reperfusion injury in DHCA. The purpose of this study was to compare cerebral mitochondrial bioenergetics for DHCA with deep hypothermic continuous perfusion using a neonatal swine model.

METHODS

Twenty-four piglets (mean weight 3.8 kg) were placed on cardiopulmonary bypass (CPB): 10 underwent 40-min DHCA, following cooling to 18°C, 10 underwent 40 min DHCA and 10 remained at deep hypothermia for 40 min; animals were subsequently rewarmed to normothermia. 4 remained on normothermic CPB throughout. Fresh brain tissue was harvested while on CPB and assessed for mitochondrial respiration and reactive oxygen species generation. Cerebral microdialysis samples were collected throughout the analysis.

RESULTS

DHCA animals had significantly decreased mitochondrial complex I respiration, maximal oxidative phosphorylation, respiratory control ratio and significantly increased mitochondrial reactive oxygen species (P < 0.05 for all). DHCA animals also had significantly increased cerebral microdialysis indicators of cerebral ischaemia (lactate/pyruvate ratio) and neuronal death (glycerol) during and after rewarming.

CONCLUSIONS

DHCA is associated with disruption of mitochondrial bioenergetics compared with deep hypothermic continuous perfusion. Preserving mitochondrial health may mitigate brain injury in cardiac surgical patients. Further studies are needed to better understand the mechanisms of neurological injury in neonatal cardiac surgery and correlate mitochondrial dysfunction with neurological outcomes.

A pilot and feasibility study of the plasma and tissue pharmacokinetics of cefazolin in an immature porcine model of pediatric cardiac surgery

BACKGROUND

Surgical site infection (SSI) prevention for children with congenital heart disease is imperative and methods to assess and evaluate the tissue concentrations of prophylactic antibiotics are important to help maximize these efforts.

AIM

The purposes of this study were to determine the plasma and tissue concentrations with standard of care, perioperative cefazolin dosing in an immature porcine model of pediatric cardiac surgery, and to determine the feasibility of this model.

METHODS

Piglets (3-5 days old) underwent either median sternotomy (MS) or cardiopulmonary bypass with deep hypothermic circulatory arrest (CPB + DHCA) and received standard of care prophylactic cefazolin for the procedures. Serial plasma and microdialysis sampling of the skeletal muscle and subcutaneous tissue adjacent to the surgical site was performed. Cefazolin concentrations were measured, noncompartmental pharmacokinetic analyses were performed, and tissue penetration of cefazolin was assessed.

RESULTS

Following the first intravenous dose, maximal cefazolin concentrations in the subcutaneous tissue and skeletal muscle were similar between groups with peak tissue concentrations 15-30 min after administration. After the second cefazolin dose given with the initiation of CPB, total plasma cefazolin concentrations remained relatively constant until the end of DHCA and then decreased while muscle- and subcutaneous-unbound cefazolin concentrations showed a second peak during or after rewarming. For the MS group, 60-67% of the intraoperative time showed subcutaneous and skeletal muscle concentrations of cefazolin >16 μg·ml(-1) while this percentage was 78-79% for the CPB + DHCA group. There was less tissue penetration of cefazolin in the group that underwent CBP + DHCA (P = 0.03).

CONCLUSIONS

The cefazolin dosing used in this study achieves plasma and tissue concentrations that should be effective against methicillin-sensitive Staphylococcus aureus but may not be effective against some gram-negative pathogens. The timing of the cefazolin administration prior to incision and a second dose given during cardiopulmonary bypass may be important factors for achieving goal tissue concentrations.

Granulocyte colony stimulating factor reduces brain injury in a cardiopulmonary bypass-circulatory arrest model of ischemia in a newborn piglet

Ischemic brain injury continues to be of major concern in patients undergoing cardiopulmonary bypass (CPB) surgery for congenital heart disease. Striatum and hippocampus are particularly vulnerable to injury during these processes. Our hypothesis is that the neuronal injury resulting from CPB and the associated circulatory arrest can be at least partly ameliorated by pre-treatment with granulocyte colony stimulating factor (G-CSF). Fourteen male newborn piglets were assigned to three groups: deep hypothermic circulatory arrest (DHCA), DHCA with G-CSF, and sham-operated. The first two groups were placed on CPB, cooled to 18 °C, subjected to 60 min of DHCA, re-warmed and recovered for 8-9 h. At the end of experiment, the brains were perfused, fixed and cut into 10 µm transverse sections. Apoptotic cells were visualized by in situ DNA fragmentation assay (TUNEL), with the density of injured cells expressed as a mean number ± SD per mm(2). The number of injured cells in the striatum and CA1 and CA3 regions of the hippocampus increased significantly following DHCA. In the striatum, the increase was from 0.46 ± 0.37 to 3.67 ± 1.57 (p = 0.002); in the CA1, from 0.11 ± 0.19 to 5.16 ± 1.57 (p = 0.001), and in the CA3, from 0.28 ± 0.25 to 2.98 ± 1.82 (p = 0.040). Injection of G-CSF prior to bypass significantly reduced the number of injured cells in the striatum and CA1 region, by 51 and 37 %, respectively. In the CA3 region, injured cell density did not differ between the G-CSF and control group. In a model of hypoxic brain insult associated with CPB, G-CSF significantly reduces neuronal injury in brain regions important for cognitive functions, suggesting it can significantly improve neurological outcomes from procedures requiring DHCA.

Therapeutic hypothermia for neuroprotection: history, mechanisms, risks, and clinical applications

The earliest recorded application of therapeutic hypothermia in medicine spans about 5000 years; however, its use has become widespread since 2002, following the demonstration of both safety and efficacy of regimens requiring only a mild (32°C-35°C) degree of cooling after cardiac arrest. We review the mechanisms by which hypothermia confers neuroprotection as well as its physiological effects by body system and its associated risks. With regard to clinical applications, we present evidence on the role of hypothermia in traumatic brain injury, intracranial pressure elevation, stroke, subarachnoid hemorrhage, spinal cord injury, hepatic encephalopathy, and neonatal peripartum encephalopathy. Based on the current knowledge and areas undergoing or in need of further exploration, we feel that therapeutic hypothermia holds promise in the treatment of patients with various forms of neurologic injury; however, additional quality studies are needed before its true role is fully known.

Ulinastatin as a neuroprotective and anti-inflammatory agent in infant piglets model undergoing surgery on hypothermic low-flow cardiopulmonary bypass

OBJECTIVE

Infants are potentially more susceptible to brain injury mediated via cell death attributed to cardiopulmonary bypass (CPB) especially with prolonged hypothermic low flow (HLF). We hypothesized that a human urinary protease inhibitor (ulinastatin), by its anti-inflammatory effect, would reduce central nervous system (CNS) injury during HLF.

METHODS

Fifteen general-type infant piglets were randomized to ulinastatin group (group U, n = 5), control group (group C, n = 5), and sham operation group (group S, n = 5). Routine CPB was established after median thoracotomy in group U and C under anesthesia. When the temperature of infant piglets dropped down to 25 °C, low-flow CPB (50 ml·kg(-1) ·min(-1) ) was instituted. After 120 min of aortic cross-clamping and 20- to 30-min rewarming, the aortic cross-clamp was removed and finally the piglet was weaned from CPB. Five thousand units per killogram of ulinastatin and equivalently normal saline were, respectively, given at the beginning of and at aortic declamping in group U and group C. group S just received sham median thoracotomy. Venous blood samples were taken immediately after anesthesia induction in all three groups, 5- and 120-min post CPB in both group U and C, respectively; plasma markers of inflammation and CNS injury were compared. Pathology results of hippocampus were observed by light microscopy.

RESULTS

Statistically significant differences between group C and U were noted in the expression of inflammatory markers such as IL-10, TNF-α and neuron-specific enolase at 120-min post CPB. Brain injuries were observed in both groups (index cases and controls) and were milder in group U.

CONCLUSIONS

In our study, HLF CPB on infant piglets resulted in brain injury, and ulinastatin might reduce the extent of such injury.

Facts and fiction: the impact of hypothermia on molecular mechanisms following major challenge

Numerous multiple trauma and surgical patients suffer from accidental hypothermia. While induced hypothermia is commonly used in elective cardiac surgery due to its protective effects, accidental hypothermia is associated with increased posttraumatic complications and even mortality in severely injured patients. This paper focuses on protective molecular mechanisms of hypothermia on apoptosis and the posttraumatic immune response. Although information regarding severe trauma is limited, there is evidence that induced hypothermia may have beneficial effects on the posttraumatic immune response as well as apoptosis in animal studies and certain clinical situations. However, more profound knowledge of mechanisms is necessary before randomized clinical trials in trauma patients can be initiated.

Effect of deep hypothermic circulatory arrest followed by low-flow cardiopulmonary bypass on brain metabolism in newborn piglets: comparison of pH-stat and α-stat management

OBJECTIVE

To compare the effects of pH-stat and α-stat management before deep hypothermic circulatory arrest followed by a period of low-flow (two rates) cardiopulmonary bypass on cortical oxygenation and selected regulatory proteins: Bax, Bcl-2, Caspase-3, and phospho-Akt.

DESIGN

Piglets were placed on cardiopulmonary bypass, cooled with pH-stat or α-stat management to 18 °C over 30 mins, subjected to 30-min deep hypothermic circulatory arrest and 1-hr low flow at 20 mL/kg/min (LF-20) or 50 mL/kg/min (LF-50), rewarmed to 37 °C, separated from cardiopulmonary bypass, and recovered for 6 hrs.

SUBJECTS

Newborn piglets, 2-5 days old, assigned randomly to experimental groups.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Cortical oxygen was measured by oxygen-dependent quenching of phosphorescence; proteins were measured by Western blots. The means from six experiments ± sem are presented as % of α-stat. Significance was determined by Student's t test. For LF-20, cortical oxygenation was similar for α-stat and pH-stat, whereas for LF-50, it was significantly better using pH-stat. For LF-20, the measured proteins were not different except for Bax in the cortex (214 ± 24%, p = .006) and hippocampus (118 ± 6%, p = .024) and Caspase 3 in striatum (126% ± 7%, p = .019). For LF-50, in pH-stat group: In cortex, Bax and Caspase-3 were lower (72 ± 8%, p = .001 and 72 ± 10%, p = .004, respectively) and pAkt was higher (138 ± 12%, p = .049). In hippocampus, Bcl-2 and Bax were not different but pAkt was higher (212 ± 37%, p = .005) and Caspase 3 was lower (84 ± 4%, p = .018). In striatum, Bax and pAkt did not differ, but Bcl-2 increased (146 ± 11%, p = .001) and Caspase-3 decreased (81 ± 11%, p = .042).

CONCLUSIONS

In this deep hypothermic circulatory arrest-LF model, when flow was 20 mL/kg/min, there was little difference between α-stat and pH-stat management. However, for LF-50, pH-stat management resulted in better cortical oxygenation during recovery and Bax, Bcl-2, pAk, and Caspase-3 changes were consistent with lesser activation of proapoptotic signaling with pH-stat than with α-stat.