Summary

Establishment of Deep Hypothermic Circulatory Arrest in Rats

Published: December 16, 2022
doi:

Summary

This protocol presents the establishment of deep hypothermic circulatory arrest in rats, which can be applied to investigate systemic inflammatory response syndrome, ischemia/reperfusion injury, oxidative stress, neuroinflammation, etc.

Abstract

Deep hypothermic circulatory arrest (DHCA) is routinely applied during surgeries for complex congenital heart disease and aortic arch disease. The present study aims to provide a method for establishing DHCA in rats. To evaluate the impact of the DHCA process on vital signs, a normal temperature cardiopulmonary bypass (CPB) rat model without circulatory arrest was used as a control. As expected, DHCA led to a significant decrease in body temperature and mean arterial blood pressure. The blood gas analysis indicated that DHCA increased lactic acid levels but did not influence the blood pH and the concentrations of hemoglobin, hematocrit, Na+, Cl, K+, and glucose. Furthermore, compared with the normal temperature CPB rats, the results of the transmission electron microscopy showed a mild increase in hippocampal autophagosomes in the DHCA rats.

Introduction

Deep hypothermic circulatory arrest (DHCA) has been used in cardiac surgery since 19531. DHCA involves reducing the patient's core temperature to profoundly hypothermic levels (15-22 °C) before globally interrupting the blood flow to the body2. The circulatory arrest can provide a relatively bloodless operating field. Deep hypothermia decreases the metabolism, especially in the brain and myocardium, which is an effective method of protection against ischemia3. DHCA is commonly applied during surgeries for complex congenital heart disease, aortic arch disease, and even renal or adrenal tumors with a vena cava thrombus4,5. Therefore, establishing DHCA animal models provides an important reference for the refinement of the procedure and the prevention of complications in clinical settings.

Although models can be established with canines6, rabbits7, and other animals, it is preferable to use rats because of their operability and low cost. The DHCA rat model was described for the first time in 2006 by Jungwirth et al.8. It was found that the duration of circulatory arrest had an impact on the neurologic outcomes. Since then, DHCA rat models have been investigated broadly. It has been clarified that DHCA could provoke systemic inflammatory response syndrome (SIRS)9. In subsequent studies, pharmacologists found that the DHCA-related neuroinflammation induced by SIRS could be attenuated by resveratrol10 and triptolide11. Our team also found that DHCA-related neuroinflammation could be attenuated by inhibiting the cold-inducible RNA-binding protein12. In the cardiovascular system, superoxide dismutase has a cardioprotective effect on ischemia/reperfusion (I/R) injuries during DHCA13. These results expanded the understanding of DHCA-related pathophysiologic processes and offered new directions for improving the outcomes of DHCA. However, the results regarding endotoxemia, oxidative stress, and autophagy after DHCA are inconclusive. DHCA uses the same operational technology as the cardiopulmonary bypass (CPB)14, but its management strategy is different, and the steps to generate DHCA differ across various teams8,9,10,11. The present study aims to provide a method for establishing the DHCA procedure in rats.

Protocol

The protocols underwent an institutional review and received approval from the Institutional Animal Care and Use Committee, Fuwai Hospital, Chinese Academy of Medical Sciences (FW-2021-0005). All the experimental procedures were performed in accordance with the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health.

NOTE: Male Sprague-Dawley rats (weight: 500-600 g, age: 12-14 weeks) were kept under standard laboratory conditions with free access to food and water. The rats were randomly allocated into two groups (n = 6, each group): the DHCA group, and the normal temperature CPB group (NtCPB group).

1. Preparatory work

  1. Sterilize the surgical instruments (forceps, scissors, micro-forceps, an electrocoagulator, a shaver, etc.) before the experiment (Figure 1).
  2. Ensure the availability of the consumables, which include 2-0 silk, a 16 G cannula (endotracheal catheter), a 22 G cannula, a homemade 16-G cannula (multi-orifice intravenous catheter), injection syringes, gauze, and tape.
    NOTE: For the homemade 16 G cannula, use a scalpel to cut two or three orifices of 2 mm diameter at the tip of the cannula, which will help to make the venous drainage smoother.
  3. Ensure the availability of sevoflurane, 2% lidocaine, saline, heparin (5 IU/mL, 250 IU/mL), epinephrine (40 µg/mL), norepinephrine (20 µg/mL), hydroxyethyl starch, and bicarbonate.
  4. Ensure the DHCA circuits contain a reservoir (modified from Murphy's dropper), a roller pump, a heat exchanger, a membrane oxygenator, connecting tubes, and a water tank (Figure 2). Connect the circuit, and mix 12 mL of hydroxyethyl starch with 1 mL of heparin sodium (250 IU) and 1 mL of saline. Prime the circuit with 14 mL of the priming solution with the roller pump gently rotating (10-40 mL/min).
    NOTE: The reservoir is remolded from a blood transfusion device with Murphy's dropper. The venous inflow part of the dropper remains at 10-15 cm, and the venous outlet part remains at 10 cm.

2. Anesthesia and cannulation

  1. Anesthetize the rats with 2%-3% sevoflurane, and then test for the lack of the conjunctival reflex and muscle relaxation after the rat loses consciousness.
    NOTE: The conjunctival reflex refers to the instant closure of the eyelid whenever the cornea is touched. Use a cotton swab to touch the cornea slightly. When the anesthesia depth is sufficient, the eyelids will not close.
  2. Perform endotracheal intubation with a 16 G cannula after the conjunctival reflex disappears and no muscular resistance is observed. Connect the tube to a ventilator, and set the parameters by clicking the buttons on the ventilator (tidal volume: 1.0-1.2 mL/100g, heart rate: 80 beats per minute [bpm], I:E = 1:1, inspired oxygen fraction: 60%).
  3. Put an electric heating blanket under the rat, and fix the rat with tape. Apply ophthalmic ointment to the eyes to prevent dryness. Shave the hair on the left inguinal region, right cervical region, and tail with a shaver. Then, disinfect the skin three times with iodine and alcohol.
  4. Check the depth of anesthesia before moving to the next steps. If the respiratory rate is higher than that set by the ventilator (80 bpm), or if there is muscle rigidity, then increase the output concentration of sevoflurane.
    NOTE: When the depth of anesthesia is adequate, the respiratory rhythm should be synchronized with the ventilator, and the muscles should be completely relaxed without tension. Check the depth of anesthesia every 30 min to ensure that the rat is not experiencing any return of consciousness throughout the procedure.
  5. Use a scalpel to cut the skin at the left inguinal region (approximately 1 cm), and dissect the muscle and tissue softly to expose the left femoral vein and artery. Separate the artery carefully.
  6. Cannulate a 22 G intravenous catheter into the left femoral artery. Ligate the artery and catheter with a 2-0 silk (at the region of cannulation). Use saline-containing heparin (5 UI/mL) to flush the cannula to avoid clotting. Connect the catheter with the pressure sensor to monitor the blood pressure.
  7. Cut the skin of the tail (approximately 1.5 cm), and then use a scalpel to cut the superficial fascia of the tail artery to expose the tail artery, which is in the middle of the surgical field.
  8. Cannulate the tail artery with a 22 G intravenous catheter. Ligate the artery and catheter with a 2-0 silk (at the region of cannulation). Use saline-containing heparin (5 UI/mL) to flush the catheter to avoid clotting.
    NOTE: When cannulating the intravenous catheter, the left hand holds the artery/vein with forceps, and the right hand pierces the artery/vein with the needle inside the catheter and then puts the cannula into the artery.
  9. Cut the skin on the right jugular vein (approximately 2 cm), and then separate the muscle and tissue to expose the vein. Insert a 16 G homemade multi-orifice intravenous catheter into the right external jugular vein, and put it into the right inferior vena cava or the right atrium carefully.
    NOTE: The left femoral vein and artery are under the surface of the left inguinal region. The vein is thicker than the artery, and the blood color of the arteries is bright red. The right jugular vein is in the middle of the right cervical region; when the skin is cut and the muscles are separated, the vein can be seen (approximately 0.3-0.4 cm wide). When the tip of the catheter touches the right atrium, the wave of blood pressure will fluctuate. Then, after pulling the catheter back a little bit, the tip of the catheter will be in the superior vena cava.
  10. Administer heparin sodium (500 IU/kg) via the right external vein. Cover each cannulated region with moist gauze to avoid contamination.
    ​NOTE: Put a box under the operating table to elevate it about 40 cm.

3. DHCA initiation

  1. Connect the DHCA circuit with the catheter in the tail artery first, and keep the pump flow rate at 1-2 mL/min. Then, connect the reservoir with the catheter in the right external jugular vein. Make sure there is always a blood level of about 1 cm in the reservoir.
  2. Turn on the water tank, and set the water temperature at 37 °C first.
  3. After the blood pressure is stable, gently increase the pump flow up to 80-100 mL/kg/min to pump the blood.

4. Cooling

  1. Set the room temperature to around 20 °C. Put ice cubes in disposable gloves, and then place them on the rat's head and sides. Adjust the temperature of the tank in real-time according to the rectal temperature of the rats.
  2. Collect 0.1 mL of blood from the left femoral artery, and place it on the blood gas machine for blood gas analysis. Change the relevant parameters of the ventilator appropriately according to the results of the blood gas analysis (e.g., PaCO2).
    ​NOTE: The heart rate and blood pressure may change, and the pump flow rate should be adjusted accordingly. The temperature gradient between the water tank and the rat needs to be less than 10 °C. Make sure the temperature can be reduced to 15-20 °C within 30 min. The normal range of PaCO2 is 35-45 mmHg. If the blood gas results show a lower PaCO2, one may decrease the tidal volume and vice versa.

5. Deep hypothermic circulatory arrest

  1. When the rectal temperature drops to 15-20 °C, change the disposable gloves (containing ice) to ensure the maintenance of deep hypothermia during the circulatory arrest.
  2. Stop the roller pump, keep the reservoir in contact with the environment, and drain the blood slowly from the external jugular vein to the reservoir.
  3. Pay attention to the blood pressure waveform. When the blood pressure and the heart beat rate are 0, stop the drainage, and keep the reservoir closed. Turn off the ventilator.
    ​NOTE: The duration of the circulatory arrest varies according to the purpose of the experiment.

6. Warm-up and reperfusion

  1. Remove all the disposable gloves, and increase the room temperature to 25 °C. Restore the membrane oxygenator ventilation while keeping the venous drainage tube clipping. Turn on the roller pump to make sure the blood in the reservoir slowly goes back to the rat's body.
  2. Turn on the ventilator. Once the blood level in the reservoir remains at 1 cm, loosen the drainage tube, and drain the blood from the right atrium to the reservoir slowly.
  3. Turn on the heating lamp, the heating pad, and the water tank. Set the temperature of the water tank to 25 °C firstly, and then adjust its outlet temperature in a timely manner according to the rectal temperature of the rat.
    NOTE: The heating lamp should be directed at the large blood vessels in the rat thoracic cavity, and it should be kept at a certain distance to avoid burning the tissues. Pay attention to the temperature difference between the outlet temperature and the rat's rectal temperature (<10 °C). If necessary, test the blood gas, and then adjust the ventilator parameters accordingly, and administer bicarbonate, electrolytes, etc.
  4. Remove the heating lamp after the rectal temperature returns to 34 °C.
    ​NOTE: This step, as a continuation of the rapid rewarming process, should be slow. At this stage, the equipment parameters of the sevoflurane vaporizer, mechanical ventilator, and roller pump can be restored to the levels at the beginning of the CPB.

7. Weaning off the CPB

  1. Slowly and gradually reduce the roller pump flow rate, and adjust the venous drainage speed until the flow rate reduces to 1 mL/min.
    NOTE: Each flow rate adjustment should be observed for 3-5 min.
  2. Keep the reservoir in contact with the environment (by taking the reservoir cap off). Infuse the remaining blood in the circuit with a flow rate of 1 mL/min.
  3. Stop the membrane oxygenation and the roller pump.
  4. Euthanize the rat after a period of mechanical ventilation under deep anesthesia.
    NOTE: This is a terminal procedure. The duration between weaning off the CPB and euthanasia varies according to the different study protocols. Remember to disinfect the wounds with iodine and alcohol and then cover each cannulated region with moist gauze to avoid contamination before euthanasia. Increase the output concentration of sevoflurane to increase the depth of anesthesia.

Representative Results

As the control group, the normal temperature CPB (NtCPB) rats without circulatory arrest showed a stable mean arterial blood pressure (MAP) and body temperature during the whole procedure, while the MAP of the DHCA rats decreased during the cardiac arrest (p < 0.01, Figure 3A). The temperature of the DHCA rats dropped quickly during the cooling phase and recovered gradually during the rewarming phase. When weaning the rats off the DHCA circuits, the temperature of the DHCA rats returned to normal (Figure 3B).

The effect of the DHCA process on rats was investigated by blood gas analysis. After the whole blood contact with the priming solution, the concentration of hemoglobin (Hb) was higher than 6 g/dL in both groups (Figure 4A). When weaning the rats off the DHCA circuit, the concentration increased to 9 g/dL because of the infusion of the remaining blood in the CPB circuit into the rat. Hematocrit (HCT) showed a similar tendency to Hb (Figure 4B). At the initiation of the CPB procedure, the differences in Hb and HCT may have been due to the different weights of the rats. The average weight of the DHCA rats was 571.1 g ± 7.254 g, while the average weight of the rats in the NtCPB group was 535.0 g ± 8.317g (p = 0.075). Although differences in Hb concentration would lead to differences in the ability of the blood to transport oxygen, the change trends of the two groups were the same, indicating that DHCA did not additionally influence the Hb concentration. After DHCA and reperfusion, the level of lactic acid increased quickly, and this was more pronounced in the DHCA group (Figure 4C). The pH decreased after the DHCA procedure, which was most likely the result of lactic acid accumulation (Figure 4D). During the entire experiment, the concentrations of Na+, Cl, K+, and glucose did not show significant differences at any time point (Figure 5). These results suggest that DHCA only caused increased lactic acid but did not influence the blood pH and the concentration of hemoglobin, hematocrit, Na+, Cl, K+, and glucose.

Autophagy is a process in which eukaryotic cells use lysosomes to degrade their cytoplasmic proteins and damaged organelles15. In physiological and some pathological conditions, a mild level of autophagy is essential for the maintenance of cellular homeostasis. However, excessive autophagy can lead to metabolic stress, the degradation of cell components, and even cell death16. In order to evaluate the impact of DHCA on neural autophagy, we used transmission electron microscopy and, surprisingly, found an increased number of autophagosomes in the hippocampi of the DHCA rats (Figure 6). Based on the bidirectional functions of autophagosomes, whether the increased autophagosomes play a neuroprotective and compensatory or a pathological role during DHCA still needs further research.

Figure 1
Figure 1: Surgical instruments used in the DHCA model. (a) Iodine, (b) injection syringes, (c) adhesive tape, (d) moist gauze, (e) forceps, (f) scissors, (g,h) micro-forceps, (i) an electrocoagulator, (j) a shaver, and (k) silk. Please click here to view a larger version of this figure.

Figure 2
Figure 2: Cardiopulmonary bypass circuit of the DHCA rat model. (A) a: Membrane oxygenator; b: Heat exchanger; c: Reservoir; d1: The tube attaching the roller pump (outer diameter [OD), 6 mm; inner diameter [ID], 4 mm; length, 15 cm); d2: The tube connecting the heat exchanger and membrane oxygenator (OD, 6 mm; ID 4 mm; length, 8 cm); d3: The artery outlet line (OD, 2.5 mm; ID, 1.5mm; length, 20 cm). (B) a: Reservoir; b: Membrane oxygenator; c: Heat exchanger; d: Roller pump. The yellow arrow shows the direction of blood flow. Please click here to view a larger version of this figure.

Figure 3
Figure 3: Vital signs of the DHCA rats and normal temperature CPB rats. (A) The mean artery pressure and (B) rectal temperature were continuously monitored throughout the procedure. Data are presented as mean ± standard error of the mean (SEM), n = 6 per group. DHCA = 30 min. The differences between the two groups at each time point were compared using an unpaired Student's t-test. Abbreviations: DHCA = deep hypothermic circulatory arrest; NtCPB = normal temperature cardiopulmonary bypass; MAP = mean arterial blood pressure. * p < 0.05, ** p < 0.01, *** p < 0.001; p > 0.05 not shown. Please click here to view a larger version of this figure.

Figure 4
Figure 4: The pH and the concentrations of hemoglobin, hematocrit, and lactic acid in rats. Artery blood samples for the analysis of (A) hemoglobin, (B) hematocrit, (C) lactic acid, (D) and pH were collected via the femoral artery at three time points: the initiation of CPB, before DHCA, and weaning off the CPB. DHCA = 30 min. Data are presented as mean ± SEM, n = 6 per group. The difference between the two groups at each time point was compared using an unpaired Student's t-test. Abbreviations: DHCA = deep hypothermic circulatory arrest; NtCPB = normal temperature cardiopulmonary bypass; Hb = hemoglobin; Hct = hematocrit; Lac = lactic acid. * p < 0.05. Please click here to view a larger version of this figure.

Figure 5
Figure 5: The concentration of Na+, Cl, K+, and glucose in rats. Artery blood samples for the analysis of (A) Na+, (B) Cl, (C) K+, and (D) glucose were collected via the femoral artery at three time points: the initiation of CPB, before DHCA, and weaning off the CPB. DHCA = 30 min. Data are presented as mean ± SEM, n = 6 per group. The differences between the two groups at each time point were compared using an unpaired Student's t-test. Abbreviations: DHCA = deep hypothermic circulatory arrest; NtCPB = normal temperature cardiopulmonary bypass; Glu = glucose. p > 0.05 not shown. Please click here to view a larger version of this figure.

Figure 6
Figure 6: Autophagosomes in the hippocampi of rats. The rats were euthanized 30 min after weaning off the CPB circuit, and the hippocampi were harvested immediately. Then, the hippocampi were fixed in glutaraldehyde for further transmission electron microscopy to investigate the expression of autophagosomes in the hippocampi of (A) NtCPB rats and (B) DHCA rats. DHCA = 30 min. Scale bars: 1 μm and 250 nm. The arrows point to autophagosomes. Abbreviations: DHCA = deep hypothermic circulatory arrest; NtCPB = normal temperature cardiopulmonary bypass. Please click here to view a larger version of this figure.

Discussion

Cannulation is the most fundamental procedure for establishing DHCA in rats. Before cannulation, soaking the artery with 0.5 mL of 2% lidocaine will make it easier to cannulate. After cannulation, heparinization with 500 IU/kg heparin via the external jugular vein is necessary to avoid microthrombus formation17. We have repeatedly found that this dose of heparin can achieve the goal of an activated clotting time (ACT) >480 s. The rewarming period is the most difficult part. It took more than 60 min for the temperature to rise from 18 °C to 34 °C in our experiment, while the rewarming period could be done in 30 min or 40 min in some other experiments18,19. Linardi et al. reported that a higher rewarming rate (45 min) increased the inflammatory response and could influence brain edema after DHCA20. Meanwhile, guidelines from The Society of Thoracic Surgeons, The Society of Cardiovascular Anesthesiologists, and The American Society of Extracorporeal Technology indicate that the temperature gradients during cooling or rewarming should not exceed 10 °C to avoid the generation of gaseous emboli and outgassing, respectively21.

During the rewarming period, the heart may have difficulty re-beating due to the low oxygen delivery or acidosis accumulated during cardiac arrest. Additionally, the heart may not respond to 10-20 µg of epinephrine. At this point, the pump flow rate should be increased, and sufficient perfusion pressure should be ensured. If refractory hypotension is still present when a sufficient blood volume is determined, norepinephrine (4 µg per time) can be administered to constrict the peripheral vessels, improve the diastolic pressure, and, thus, improve the coronary perfusion22.

There are some limitations of our experiment. Thoracotomy was not performed, so the nociceptive stimulus was different from that of clinical patients. Secondly, the cardioplegic solution was not used for cardioplegia. In our experiment, the cardiac arrest was induced by hypothermia and hypotension. The existing method reduces the damage from the thoracotomy, meaning it can be used to investigate the influence of hypothermia and ischemia on the organs.

This model can be applied to investigate the pathophysiological mechanisms of and pharmacological treatments for DHCA-induced SIRS, I/R injury, oxidative stress, neuroinflammation, neurobehavioral changes, etc.

Divulgazioni

The authors have nothing to disclose.

Acknowledgements

The authors thank Liang Zhang for helping to collect the video data during the experiment. This study was supported by the National Natural Science Foundation of China (Grant number: 82070479) and the Fundamental Research Funds for the Central Universities (Grant number: 3332022128).

Materials

Heat Exchanger Xi’an Xijing Medical Appliance Co., Ltd Animal-M
Membrane Oxygenator Dongguan Kewei Medical Instrument Co., Ltd. Micro-M
Monitor Chengdu Techman Co., Ltd BL-420s
Roller Pump Changzhou Prefluid Technology Co.,Ltd BL100
SD Rat HFK Bioscience Co.,Ltd. /
Sevoflurane Maruishi Pharmaceutical Co. Ltd H20150020
Shaver Hangzhou Huayuan Pet Products Co.,Ltd. /
Vaporizer SPACECABS /
Ventilator Shanghai Alcott Biotech Co., Ltd ALC-V8S
Water Tank Maquet Critical Care AB Jostra HCU20-600

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Citazione di questo articolo
Yan, W., Ji, B. Establishment of Deep Hypothermic Circulatory Arrest in Rats. J. Vis. Exp. (190), e63571, doi:10.3791/63571 (2022).

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