Temperature Dependence of Cerebral Blood Flow for Isolated Regions of the Brain During Selective Cerebral Perfusion in Pigs

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Temperature Dependence of Cerebral Blood Flow for Isolated Regions of the Brain During Selective Cerebral Perfusion in Pigs Justus T. Strauch, MD, Peter L. Haldenwang, MD, Katharina Müllem, Miriam Schmalz, Oliver Liakopoulos, MD, Hildegard Christ, Jürgen H. Fischer, MD, and Thorsten Wahlers, MD Departments of Cardiothoracic Surgery, Experimental Medicine, and Biomathematics, University Hospital of Cologne, Cologne, Germany

Background. Hypothermic circulatory arrest (HCA) and antegrade selective cerebral perfusion (ASCP) are utilized for cerebral protection during aortic surgery. However, no consensus exists regarding optimal ASCP-temperature showing a tendency toward higher values during the last years. This study investigates regional changes of cerebral blood flow (CBF) during ASCP at two temperatures. Methods. In this blinded study, 20 pigs (35 to 37 kg) were randomized to two groups. Animals were cooled to 10 minutes of HCA followed by 60 minutes of ASCP. Afterward the animals were perfused at 25°C and 30°C according to the study group. Fluorescent microspheres were injected at seven time points during the experiment to calculate total and regional CBF. Hemodynamics, cerebrovascular resistance (CVR) and cerebral metabolic rate of oxygen (CMRO2) were assessed. Tissue samples from the cortex, cerebellum, hippocampus, and pons were taken for microsphere count. Results. The CBF and CMRO2 decreased significantly (p < 0.002) during cooling in both groups; it was signif-

icantly higher throughout ASCP in the 30°C versus the 25°C group (p ⴝ 0.0001). These findings were similar among all brain regions, certainly at different levels. The CBF increased significantly (p ⴝ 0.002) during the early period of ASCP for analyzed regions and decreased significantly (p ⴝ 0.034) below baseline after 60 minutes of ASCP, reaching critical levels in the hippocampus and neocortex. The hippocampus turned out to have the lowest CBF, while the pons showed the highest CBF. Thirty minutes and more ASCP provides less CBF compared with baseline values at both temperatures. Conclusions. Antegrade selective cerebral perfusion improves CBF in all regions of the brain for a limited time. Our study characterizes the brain specific hierarchy of blood flow during ASCP. These dynamics are highly relevant for clinical strategies of perfusion.

P

evidence of embolization) an irreversible global ischemic insult [1, 2]. Various strategies have been used to improve protection of the brain during the mandatory interruption of normal antegrade perfusion required for aortic arch surgery, with the hope of lowering the morbidity and mortality of these operations. Hypothermic circulatory arrest (HCA) and antegrade hypothermic selective cerebral perfusion (ASCP) are among the most successful strategies, frequently used in combination [3– 8]. Experimental studies showed that, even at relatively low temperatures brain metabolism can still remain as high as 40% of baseline levels [9]. However, the physiology of ASCP after HCA of various duration is only beginning to be investigated. Little information is known about the exact cerebral blood flow (CBF) and metabolism during the period of ASCP at a stable nonpulsatile pump-flow rate, a stable mean arterial pressure (MAP), and a constant temperature, even though there is a tendency toward higher perfusion temperatures over the last years [10, 11]. Nothing is known about changes in CBF for different regions of the brain.

rotecting the brain from damage during replacement of the ascending aorta and aortic arch is still one of the major challenges in aortic surgery. Cerebral injury after aortic surgery has two major causes. Embolic stroke, when it occurs, is likely to result in a permanent focal deficit; although it has become less common with increasing experience, it is still a disastrous form of cerebral insult. A more frequent problem after aortic surgery is mild global cerebral injury. Such injury is clinically apparent as the syndrome known as transient neurologic dysfunction (TND). Transient neurologic dysfunction is thought to be a consequence of inadequate cerebral protection during the mandatory interval of interrupted antegrade cerebral perfusion and in particular can be caused by different regions of the brain, resulting in a wide variety of clinical symptoms. In rare instances, a very prolonged operation may produce (even without

Accepted for publication July 10, 2009. Address correspondence to Dr Strauch, University Hospital of Cologne, Department of Cardiothoracic Surgery, Kerpener Strasse 62, Cologne, 50925, Germany; e-mail: [email protected].

© 2009 by The Society of Thoracic Surgeons Published by Elsevier Inc

(Ann Thorac Surg 2009;88:1506 –14) © 2009 by The Society of Thoracic Surgeons

0003-4975/09/$36.00 doi:10.1016/j.athoracsur.2009.07.013

We conducted a study in pigs that would clearly determine regional changes in brain perfusion under conditions of mild to moderate ASCP, as far as cerebral oxygen metabolism and cerebral vascular resistance. Validation for accurate measurement of regional blood flow using fluorescent microspheres has been demonstrated in the brain and other organs [12, 13].

Material and Methods Study Design Twenty female juvenile pigs 4 to 5 months of age, weighing 35 to 37 kg, were used for this experiment. Seven days prior to operation, pigs were delivered from a local farm specializing in laboratory animals. To ensure equal and consistent conditions for all animals involved in the study, the transport, housing, and operation took place in climate-controlled facilities at 22°C. The protocol for the study was reviewed and approved by the University of Cologne Ethic Committee and human care was provided in accordance with the “Principles of Laboratory Animal Care,” as formulated by the National Society for Medical Research and the “Guide for the Care and Use of Laboratory Animals” published by National Academy Press (NIH Publication No. 88-23, revised 1996). After cooling on CPB the animals were randomized to one of the two study groups: (1) group 30: 10 minutes HCA followed by 60 minutes SCP at 30°C (n ⫽ 10); (2) group 25: 10 minutes HCA followed by 60 minutes SCP at 25°C (n ⫽ 10). In this study, ASCP was preceded by a 10-minute interval of HCA because we believe that the ideal method for anastomosis of the cerebral vessels requires HCA, and because many previous studies suggest that 10 minutes is a safe duration for arrest of cerebral circulation without perfusion. A short interval of HCA is essential for any ASCP protocol, but some surgeons introduce catheters into the cerebral vessels during a much briefer arrest period. A randomization scheme was developed prior to the start of the protocol by an independent member of the Department of Biomathematics.

Perioperative Management and Anesthesia All animals received preoperative care in compliance with the guidelines of the North Rhine-Westphalian Chamber of Agriculture. The protocol for the experiment was approved by the University of Colognes local Ethic Committee. After pretreatment with intramuscular azaperone (2 mg/kg) and ketamine (15 to 20 mg/kg) to induce deep sedation, animals were anesthetized with intravenous (IV) propofol 1% (1 to 2 mg/hour), fentanyl (25 ␮g/kg/ hour), and midazolam (0.2 mg/kg/hour). After endotracheal intubation, pigs were ventilated mechanically with a fraction of inspired oxygen of 0.5. Paralysis was achieved with IV pancuronium (0.2 mg/kg/hour). The ventilation rate and the tidal volume were adjusted (Fabius, Dräger, Germany) to maintain the arterial Paco2 at about 35 to 40 mm Hg. Arterial oxygen tension was maintained above 100 mm Hg.

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Temperature probes were placed in the rectum and the brain through a small burr hole in the skull. A 14 gauge (G) arterial line was placed in the right brachial artery for pressure monitoring and arterial blood sampling (Blood Gas Analyzer: ABL Radiometer Copenhagen, DK, Denmark).

Intracranial Pressure (ICP), Sagittal Sinus Pressure, and Sagittal Sinus Oxygenation Sagittal sinus cannulation was performed before cannulation and heparinization for CPB. A midline scalp incision was made and the underlying periosteum removed to identify the coronal and sagittal sutures. A 3 mm cutting burr was used to remove the bone. A 24G catheter was inserted into the sagittal sinus to permit both sampling of cerebral venous blood and monitoring of cerebral venous pressure. An ICP pressure probe was connected to a transducer (Codman ICP Express; Johnson and Johnson Professional Inc, Raynham, MA).

Operative Technique The chest was opened through a small left thoracotomy in the fourth intercostal space. After opening the pericardium the heart and the great vessel were exposed. The ascending and descending aorta were dissected and vessel loops were placed to define the levels of clamping. After heparinization (300 IU/kg) the distal ascending aorta was cannulated with a 16F arterial cannula, and the right atrium with a single 26F cannula. Nonpulsatile CPB, using alpha-stat pH management, was initiated at a flow rate of 80 to 100 mL/kg per minute and then adjusted to maintain a minimum mean arterial pressure of 50 mm Hg. As injection-port for fluorescent microsphere injection, a 10F vent catheter was inserted through the left atrium. The CBP circuit included roller pumps (Stöckert Instr, München, Germany), cardiotomy reservoir, and a membrane oxygenator (VPCML Plus; Cobe Cardiovascular Inc, Arvada, CO) which was primed with previous citrate-treated blood from an animal donor, heparin (5,000 IU) and KCl (1.5 mq/kg). The pH was maintained, by means of alpha-stat principles, at 7.40 with an arterial Paco2 of 35 to 40 mm Hg and uncorrected for temperature. Hemoglobin level was maintained between 8 and 10 mg/dL. Once stable CPB was established, cooling for approximately 30 minutes to a brain temperature of 25°C, respectively, 30°C was undertaken (heat-exchanger: Biomedicus; Medtronic, Eden Prairie, MN) and the operating room temperature was maintained at 22°C to 24°C. Diastolic cardiac arrest was achieved by clamping the ascending aorta and adding 1 mEq/kg potassium chloride through a 14F cardioplegia-cannula into the aortic root. Myocardial protection was afforded by applying iced saline (⬃4°C) topically in the pericardium during the 80 minute interval of combined HCA and ASCP. The ASCP was installed after 10 minutes of HCA by clamping the descending aorta. Isolation of the aortic arch allows the perfusate to flow only in the bicarotid trunc and into the left subclavian artery. In this concept the subclavian arteries were included in the ASCP circuit

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Fig 1. Pigs brain left hemisphere with marked analyzed regions for calculating cerebral blood flow (CBF): (A) pons; (B) cerebellum; (C) cortex; and (D) hippocampus.

for arterial pressure monitoring and reference blood sample withdrawal for microsphere flow calculation. After 10 minutes HCA and 60 minutes ASCP, CPB was reinstituted by releasing the clamp in the descending aortic position. Core and surface rewarming were begun and continued to a brain temperature of approximately 35°C to 36°C. Care was taken to avoid a temperature difference between the perfusate and core temperature of no more than 10°C.

Cerebral Blood Flow The CBF was measured with fluorescent microspheres as described previously [12, 13]. In brief, approximately 2 million microspheres, 15 ⫾ 0.5 ␮m in diameter, in six different colors were injected and flushed with 5 mL of saline solution into a left ventricular catheter before CPB and into the aortic cannula during ASCP or CPB. Before injection, the fluorescently labeled microspheres, suspended in 10% dextran with 0.05% polyoxyethylene sorbitan monooleate, were mixed, sonicated, and vortexed. To allow calculation of absolute blood flow Fig 2. Protocol of the experiment, showing time points at which measurements cited in the tables and figures were taken. No microsphere injection for cerebral blood flow (CBF) calculation at time point 7 due to color overlap. (CPB ⫽ cardiopulmonary bypass; HCA ⫽ hypothermic circulatory arrest; SCP ⫽ selective cerebral perfusion.)

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rates, a reference blood sample was taken from the brachial artery (upper right limb) at a rate of 2.9 mL/ minute with a Harvard pump (Harvard Bioscience, Inc). The withdrawal of blood started 10 seconds prior to injection of the microspheres and continued for 110 seconds after the microsphere injection. Microspheres of 15 ␮m were selected to avoid nonentrapment, a risk in the use of smaller microspheres, and to avoid streaming and hemodynamic sequelae from occlusion of larger than capillary vessels, a risk in using larger microspheres. The animals were sacrificed 30 minutes after aortic decannulation with an intravenous injection of sodium pentobarbital (30 mg/kg) and saturated potassium chloride (6 mEq/kg). In all animals the brain was removed, the two hemispheres were cut in the middle, and the specimens were weighed. Tissue samples (0.6 to 1.2 g) from four different regions, the cortex, cerebellum, hippocampus, and pons, were taken for microsphere count (Fig 1). Thereafter, the microspheres were recovered from the brain tissue by sedimentation and from the blood by using a commercial protocol (NuFlow Extraction protocol 9507.2; Interactive Medical Technologies Ltd, Irvine, CA). Fluorescent analysis was carried out by the same company. Regional cerebral blood flow was then calculated from the intensity of fluorescence microspheres in blood and tissue samples using the following formula: CBF (mL ⫻ 100g⫺1 ⫻ min ⫺1) ⫽ (R ⫻ IT) ⁄ (IR ⫻ Wt) where R was the rate at which the reference blood sample was withdrawn (2.9 mL/min), IT was the fluorescence intensity of the tissue sample, IR was the fluorescence intensity of the blood sample, and Wt was the weight of the tissue sample (in grams).

Cerebral Metabolism Cerebral sagittal sinus and arterial samples were obtained simultaneously for calculation of cerebral oxygen extraction (arteriovenous oxygen content difference), sagit-

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Table 1. Temperature, Hemodynamic Values, and Blood Gas Variable BrainTemp (°C) SCP 30°C SCP 25°C MAP (mm Hg) SCP 30°C SCP 25°C Hematocrit (%) SCP 30°C SCP 25°C pHa SCP 30°C SCP 25°C Paco2 (mm Hg) SCP 30°C SCP 25°C Oxygen saturation SS (%) SCP 30°C SCP 25°C

Baseline

Cooling

5 Minute Perfusion

15 Minute Perfusion

25 Minute Perfusion

60 Minute Perfusion

30 Minutes After CPB

36.1 ⫾ 1.0 36.7 ⫾ 1.0

29.3 ⫾ 0.9 24.6 ⫾ 0.8

28.8 ⫾ 0.4 24.3 ⫾ 0.4

28.0 ⫾ 0.3 24.4 ⫾ 0.4

27.7 ⫾ 0.4 24.7 ⫾ 0.4

28.3 ⫾ 1.0 25.2 ⫾ 1.1

34.8 ⫾ 1.3 34.9 ⫾ 1.1

64 ⫾ 8 66 ⫾ 9

60 ⫾ 5 61 ⫾ 7

56 ⫾ 8 54 ⫾ 3

52 ⫾ 8 53 ⫾ 5

57 ⫾ 9 54 ⫾ 6

57 ⫾ 6 56 ⫾ 7

52 ⫾ 6 52 ⫾ 4

29 ⫾ 4 29 ⫾ 4

20 ⫾ 3 19 ⫾ 4

19 ⫾ 2 20 ⫾ 4

19 ⫾ 2 18 ⫾ 3

19 ⫾ 2 18 ⫾ 2

24 ⫾ 4 25 ⫾ 3

26 ⫾ 4 27 ⫾ 3

7.42 ⫾ 0.03 7.41 ⫾ 0.04

7.48 ⫾ 0.05 7.47 ⫾ 0.02

7.46 ⫾ 0.09 7.41 ⫾ 0.03

7.57 ⫾ 0.06 7.42 ⫾ 0.03

7.53 ⫾ 0.06 7.48 ⫾ 0.06

7.53 ⫾ 0.09 7.43 ⫾ 0.08

7.31 ⫾ 0.13 7.41 ⫾ 0.04

48.6 ⫾ 3.5 47.4 ⫾ 4.1

37.9 ⫾ 3.4 40.4 ⫾ 5.1

35.3 ⫾ 5.1 34.4 ⫾ 3.1

39.0 ⫾ 7.1 39.2 ⫾ 2.9

37.1 ⫾ 5.2 39.9 ⫾ 4.1

40.6 ⫾ 4.6 44.3 ⫾ 4.2

43.2 ⫾ 2.8 42.9 ⫾ 2.9

59.2 ⫾ 4.9 48.8 ⫾ 4.1

69.6 ⫾ 5.9 77.1 ⫾ 4.8

79.6 ⫾ 6.9 77.1 ⫾ 7.3

73.7 ⫾ 6.1 81.3 ⫾ 5.3

71.8 ⫾ 4.9 73.7 ⫾ 5.5

59.4 ⫾ 5.9 60.7 ⫾ 9.5

Not done Not done

All values are shown as mean ⫾ standard deviation. CPB ⫽ cardiopulmonary bypass; perfusion; SS ⫽ sagittal sinus.

MAP ⫽ mean arterial pressure;

Paco2 ⫽ partial pressure of carbon dioxide, arterial;

tal sinus oxygen saturation, and cerebral oxygen saturation extraction (arteriovenous oxygen saturation difference). Cerebral vascular resistance was calculated by using following equation, where MAP is the mean arterial pressure and MSSP is the mean sagittal sinus pressure: CVR (mm Hg ⫻ mL⫺1 ⫻ 100g⫺1 ⫻ min ⫺1) ⫽ MAP ⫺ MSSP ⁄ CBF The cerebral metabolic rate of oxygen (CMRO2) was determined as follows: CMRO2 (mL ⫻ 100g⫺1 ⫻ min ⫺1) ⫽ CBF ⫻ (arterial O2 content ⫺ sagittal sinus O2 content)/100

Study Protocol Hemodynamics as heart rate, central venous pressure (CVP), MAP, sagittal sinus pressure, as well as core and brain temperature, were monitored continuously (Omnicare 24C; Hewlett Packard, Böblingen, Germany). Additionally, ICP, arterial blood gases, hemoglobin, glucose, and lactate (ABL Radiometer Copenhagen, DK, Denmark), were recorded at all the seven time points as shown in Figure 2. Regional and total cerebral CBF, CVR, and CMRO2 were calculated at the first six measurement time points: at baseline, after reaching the temperature of 25°C or 30°C (coolest T°C), at 5 minutes, 15 minutes, 25 minutes, and 60 minutes of ASCP.

Statistical Methods Randomization was carried out by an independent party, with individual group allocation revealed at the onset of

SCP ⫽ selective cerebral

selective cerebral perfusion. Groups were compared separately at baseline, during CPB, and ASCP. The t test or the Mann-Whitney test as appropriate, were used for comparisons at baseline. When the data were consistent with normality and equal variance assumptions, the measurements during CPB and ASCP as well as those after CPB were compared using repeated measures, with tests for average differences between groups and for group-time interactions (change in the difference between groups over time). Otherwise the groups were compared separately at each time point using the Mann-Whitney or Fisher exact tests. We report p values unadjusted for multiple testing; their purpose is not for an exact global assessment but rather as a guide to help interpret the pattern of differences between groups at different times. Analyses were implemented with SPSS software (SPSS Inc, Chicago, IL).

Results Comparability of Experimental Groups As intended by the design of the study, no significant differences (one-way analysis of variance [ANOVA]: p ⬎ 0.2) of basic data such as animal weight or hemodynamic parameters were seen between the groups at baseline. Table 1 shows values of measured variables for each group at subsequent time points; arterial oxygen saturations were uniformly 99.6% or greater, and have therefore not been included. All 20 animals, if in the 25°C or in the 30°C group, reached the final measurement time point.

Cerebral Blood Flow

At baseline, CBF ranged between 46.2 ⫾ 6.8 and 51.5 ⫾ 6.9 mL/minute/100g, and did not differ significantly between

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Table 2. Global Cerebral Blood Flow, Cerebral Vascular Resistance, and Cerebral Oxygen Metabolism Variable CBF (mL/min/100g) SCP 30°C SCP 25°C CVR (mm Hg/mL/min/100g) SCP 30°C SCP 25°C CMRO2 (mL/min/10g) SCP 30°C SCP 25°C ICP (mm Hg) SCP 30°C SCP 25°C

Baseline

Cooling

5 Minute Perfusion

15 Minute Perfusion

25 Minute Perfusion

60 Minute Perfusion

51.5 ⫾ 6.9 46.2 ⫾ 6.8

30.8 ⫾ 4.4 28.7 ⫾ 3.8

77.1 ⫾ 5.7 65.1 ⫾ 5.1

55.7 ⫾ 4.7 50.8 ⫾ 4.9

55.3 ⫾ 4.2 44.4 ⫾ 1.7

37.8 ⫾ 4.1 28.5 ⫾ 3.2

1.1 ⫾ 0.4 1.2 ⫾ 0.3

1.7 ⫾ 0.7 2.3 ⫾ 0.9

0.8 ⫾ 0.2 0.7 ⫾ 0.2

1.0 ⫾ 0.5 0.7 ⫾ 0.2

0.8 ⫾ 0.3 1.0 ⫾ 0.2

2.0 ⫾ 0.9 2.1 ⫾ 0.6

2.10 ⫾ .98 2.20 ⫾ .38

1.12 ⫾ .17 0.63 ⫾ .19

0.93 ⫾ .16 0.64 ⫾ .12

1.08 ⫾ .14 0.19 ⫾ .08

1.46 ⫾ 1.0 0.56 ⫾ .49

1.47 ⫾ .47 0.51 ⫾ .46

6.7 ⫾ 1.4 6.8 ⫾ 1.3

5.2 ⫾ 1.4 6.0 ⫾ 1.3

8.8 ⫾ 2.0 7.5 ⫾ 1.6

8.3 ⫾ 1.7 7.7 ⫾ 1.3

7.0 ⫾ 1.4 6.3 ⫾ 1.5

7.0 ⫾ 1.4 6.6 ⫾ 1.1

Two-way ANOVA p Values p ⫽ 0.008 p ⬍ 0.001 p ⬍ 0.001 NS p ⫽ 0.03 NS NS NS p ⫽ 0.04 NS NS NS

All values are shown as mean ⫾ standard deviation; p values from ANOVA in right-hand column for changes from baseline during perfusion: (1 row) between groups, (2 row) between times, (3 row) group by time interaction. ANOVA ⫽ analysis of variance; CBF ⫽ cerebral blood flow; resistance; ICP ⫽ intracranial pressure; NS ⫽ not significant;

the two groups at a constant MAP. Cooling to 30°C, respectively 25°C, resulted in a significant decrease compared with baseline (p ⫽ 0.003) of CBF to values about 30% below baseline with 28.7 ⫾ 3.8 and 30.3 ⫾ 4.4 mL/minute/100g in both groups (Table 2; Fig 3). Immediately after installing ASCP after a 10 minute period of HCA, there was a rapid increase to values above in the group 25, reaching a value of 65.1 ⫾ 5.1 mL/minute/100g. At the same time point, 5 minutes after ASCP was established, the CBF in group 30 increased

Fig 3. Overall cerebral blood flow (CBF), cerebral vascular resistance (CVR), and cerebral metabolic rate of oxygen (CMRO2) at different time points during the experiment. All values are shown as mean ⫾ standard error. (SCP ⫽ selective cerebral perfusion.)

CMRO2 ⫽ cerebral metabolic rate of oxygen; SCP ⫽ selective cerebral perfusion.

CVR ⫽ cerebrovascular

significantly versus baseline (p ⫽ 0.04) and not significantly versus group 25 toward a value of 77.1 ⫾ 3.9 mL/minute/100g. The CBF remained stable at higher levels in group 30 over the whole ASCP period than in group 25, but showed a slow decrease after 15 minutes of ASCP (30°C 55.7 ⫾ 4.7 vs 25°C 50.8 ⫾ 4.9 mL/min/100g). Global CBF showed a minor increase in both groups after 25 minutes of perfusion, without significance between both groups and compared with baseline (30°C 55.3 ⫾ 4.2 vs 25°C 44.4 ⫾ 1.7 mL/min/100g). During persisting ASCP for 60 minutes at both temperatures, CBF decreased in both groups, reaching statistical significance versus baseline in group 25 (28.5 ⫾ 3.2 mL/min/100g), failing to reach statistical significance in group 30 (37.8 ⫾ 4.1 mL/ min/100g). This trend of CBF was similar among all regions of the brain (Fig 4), certainly at different baseline levels, with the highest levels for CBF in the cerebellum (with 60.0 ⫾ 4.5 mL/minute/100g) and in the cortex (with 56.3 ⫾ 3.6 mL/minute/100g), followed by the pons region with a CBF baseline value of 45.6 ⫾ 6.8 mL/minute/100g and the lowest CBF in the hippocampus (with 34.0 ⫾ 4.4 mL/min/ 100g) measured at approximately 60 mm Hg. The values for CBF for the six time points, listed for separated regions are shown in Figure 4. The pons area (Fig 4) showed the lowest change for CBF during cooling to both target temperatures. Flow was only reduced by between 20% and 30%. During the early period of ASCP at 5 minutes, CBF reached the highest level with 115.6 ⫾ 6.3 mL/minute/100g in group 30, marking the highest absolute value for CBF for the entire experiment, versus 95.5 ⫾ 6.5 mL/minute/100g in group 25, with values differing significantly (p ⫽ 0.002) from baseline, but not between the two groups. The CBF values after 15 and 25 minutes of ASCP in group 30 were with 78.8 ⫾ 5.2 and 88.0 ⫾ 6.3 mL/minute/100g and with 71.8 ⫾ 6.1 and 80.7 ⫾ 5.7 mL/minute/100g in group 25 significantly (p ⫽ 0.04) increased compared with baseline.

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Fig 4. Temperature comparison of cerebral blood flow in absolute values for the four investigated regions of the brain. All values are shown as mean ⫾ standard error. (RBF ⫽ regional blood flow; SCP ⫽ selective cerebral perfusion.)

After 60 minutes of ASCP, the CBF fell markedly in both groups and was 37.2 ⫾ 4.6 mL/minute/100g in group 30 lower than baseline and was 42.8 ⫾ 4.8 mL/minute/100g in group 25 close to baseline, reaching no statistical significance. The CBF for the cerebellum after cooling (Fig 4) resulted in a significant (p ⫽ 0.03) decrease to values approximately 35% below baseline in both groups. During ASCP, the CBF did show a similar behavior like the CBF in the pons, but with greater differences between both study groups. Five minutes after the start of ASCP, the CBF increased significantly (p ⫽ 0.04) in group 30 to a value of 100.8 ⫾ 7.9 mL/minute/100g versus 70.6 ⫾ 5.9 mL/minute/100g in group 25, marking the highest difference in CBF at this time point for the entire experiment. The CBF at 15 and 25 minutes of ASCP decreased and was close to baseline values for both groups. The CBF decreased significantly (p ⫽ 0.04) below baseline at 60 minutes of ASCP in both groups, with absolute values of 33.3 ⫾ 4.6 at 30°C versus 21.2 ⫾ 6.6 mL/minute/100g at 25°C. Analyzing the cortex (Fig 4), the CBF decreased significantly (p ⫽ 0.001) during cooling to 30°C and 25°C. The CBF was reduced by 50% at lowest temperature. After 5 minutes of ASCP, group 30 showed an increase in CBF with values (71.5 ⫾ 6.9 mL/min/100g) differing significantly (p ⫽ 0.04) from baseline and between groups (p ⫽ 0.04). Continuing SCP, a further decrease of CBF during the entire perfusion period was to observe, whereas group 30 showed a gradual diminution in CBF. After 60 minutes of ASCP, the CBF was significantly lower in both groups compared with baseline (17.6 ⫾ 3.9 vs 58.4 ⫾ 3.5 at 30°C and 10.1 ⫾ 3.9 vs 54.2 ⫾ 3.7 mL/min/100g at 25°C). The hippocampus (Fig 4), the area with the lowest baseline level for CBF, showed the slightest changes in CBF over the whole observation period. After a decrease of about 35% in CBF during cooling, five minutes after the start of ASCP, CBF increased significantly (p ⫽ 0.04) in group 30 to a value of 66.7 ⫾ 5.5 mL/minute/100g versus 47.8 ⫾ 8.5 mL/minute/100g in group 25, nevertheless marking the lowest absolute values of CBF at this early ASCP time point of well

perfusion for the entire experiment. With ongoing ASCP both groups showed a clear decrease in CBF with very similar values at 15, 25, and 60 minutes, failing to reach any statistical significance.

Cerebral Vascular Resistance (CVR) No differences in cerebral vascular resistance (CVR) at baseline were found between the two groups. The CVR increased rapidly during cooling in both groups, resulting in higher absolute values, with statistical significance (p ⫽ 0.04) in the group 25 (2.3 ⫾ 0.9 at 25°C vs 1.7 ⫾ 0.7 at 30°C) (Table 2; Fig 3). With the installation of ASCP, CVR decreased to values below baseline in both temperature groups at 5, 15, and 25 minutes but increased to values above baseline after 60 minutes of ASCP, again in both groups. So, neither the 30°C nor the 25°C group showed significant recovery of CVR toward baseline during the first time points of ASCP. Interestingly both groups showed statistically significant (p ⫽ 0.03) increase compared with baseline in CVR at the final microsphere injection time point 60 minutes after the start of ASCP (2.1 ⫾ 0.6 vs 1.2 ⫾ 0.3 at 25°C and 2.0 ⫾ 0.9 vs 1.1 ⫾ 0.4 at 30°C).

Cerebral Metabolism Cerebral oxygen metabolism data are displayed in Table 2. Cerebral oxygen consumption decreased with cooling, as expected, with differences reaching no statistical significance between the groups. During the very early ASCP interval (5 minutes), animals in both groups showed further decrease in oxygen consumption (Table 2; Fig 3). With ongoing ASCP, there was a progressive decline in oxygen consumption in the 25°C group but not in the 30°C group. After 15, 25, and 60 minutes of ASCP, CMRO2 increased further in the 30°C group and was statistically significant (p ⫽ 0.002 at 15 minutes; p ⫽ 0.03 at 25 minutes; p ⫽ 0.03 at 60 minutes) higher than in the 25°C group, still remaining below baseline values.

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Intracranial Pressure Cooling to target temperatures resulted in a clear decrease in intracranial pressure (ICP) (Table 2) in both groups. After the start of perfusion, all animals showed a slight increase in ICP to values above baseline in both groups. There were no significant differences between groups and all values were within the normal range.

Comment During the surgical treatment of the ascending aorta or aortic arch aneurysm special techniques are used to prevent cerebral ischemia or hypoperfusion. Currently short periods of HCA and ASCP, certainly at different temperatures and cannulation ways, are the main supportive methods used to protect the brain from dysfunction. It is known from previous animal studies [10, 14, 15, 16] that ASCP is advantageous in preventing ischemic damage to the brain in longer lasting operations for aortic arch repair, though no consensus is achieved regarding the temperature to apply. To make the study comparable for the clinical setting, ASCP was preceded by a 10 minute interval of HCA because we believe that the ideal method for anastomosis of the cerebral vessels requires HCA, and because many previous studies suggest that 20 to 30 minutes is the longest duration for safe arrest of cerebral circulation without perfusion [1]. Previous clinical studies have reported that ASCP under nonpulsatile circumstances is useful and safe [17, 18]. The purpose of this study in pigs was to characterize the potential regional distribution differences in CBF, cerebral metabolism, and cerebral vascular responses between animals undergoing HCA and ASCP at two different temperatures, commonly used in clinical practice. As expected, there was an overall reduction in CBF at 25°C and 30°C after 30 minutes of cooling. This is in accordance with theoretical predictions and previous experimental work [12]. With ASCP after 10 minutes of HCA, CBF decreased gradually over the 60 minutes ASCP interval to 50% of baseline. Thus, our calculation for CBF during ASCP is considerably the same as the 40% to 50% of baseline reported by Sakurada [19]. Regionally, some differences in patterns of CBF with ASCP were apparent. All investigated regions of the brain had 30% varying range of CBF at baseline and therefore a characteristic CBF value. The cerebellum showed the highest CBF rate at baseline and the second highest increase in CBF of almost 30% above baseline in the 30°C animals in the very early period after establishing ASCP. The CBF peak was 100.8 mL/minute/100g at 5 minutes of ASCP, representing a considerable flow reserve. This was also the time point during the entire experiment where the difference for CBF between both temperature groups was to observe. Further CBF values for the cerebellum over the perfusion period showed a constant decrease reaching critical CBF levels with less than 50% of baseline values after 60

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minutes; however, with absolute values higher than other analyzed regions at both temperatures. The cerebellum is one of the regions with the highest variability in CBF of all examined regions. Taking into consideration that the cell density is the highest in this brain area with up to 50% of all the neurons, with only 10% of the brains weight, observed CBF variations in the cerebellum may result clinically in frequently observed coordination and minute motor activity dysfunction. The cortex demonstrated the second highest CBF at baseline. Nevertheless, CBF was reduced by almost 50% after cooling to 25°C or 30°C, confirmed by low absolute values for this measurement time point. The following curve for CBF was very similar to the one we found for the cerebellum, certainly at different levels. After 5 minutes of ASCP (normally the period of luxury perfusion for all regions of the brain) the 30°C group showed an increase in CBF to values about 20% above baseline; however, CBF did not increase in the 25°C animals marking the only time point in the protocol where CBF was below baseline during ASCP. Continuing ASCP, a further decrease of CBF during the entire perfusion period was to observe, ending up with tremendous values for CBF at the end of the experiment at 60 minutes ASCP in this normally well-perfused area of the brain. This interesting finding threatens and should be taken as a marker where ASCP should be finished, certainly at these “high” temperatures like 30°C or 25°C, because particularly the cortex presents with primary motoric and primary sensory functions, the area with tremendous dysfunction when ischemia occurs [20]. Baseline values for CBF in the pons region were lower than those in the cerebellum and cortex. However, the decrease during cooling was only 30% in the 30°C group, respectively 20% in the 25°C group. Our study confirms that there is a kind of luxury perfusion in the early period of ASCP, marking the pons region with values for CBF twice as high as at baseline, the best perfused area of the pig’s brain. The 30°C animals showed the highest CBF at 5 minutes ASCP with a peak of 115.6 ⫾ 6.3 mL/minute/ 100g. The pons was the only region of the brain where the CBF values were above baseline for the entire ASCP interval; phenomena not observed in any other region. This may reflect its function as a transit and connection station for many nerve fibers mainly between the cerebellum and the cerebrum. Further, the origins of cranial nerves V, VI, and VII, as well as the nuclei pontis are located here. Symptoms resulting from ischemia in this area will be dominated by short time vertigo episodes [21]. We observed just a slight decrease in CBF in the hippocampus region during ASCP at both temperatures, whether the hippocampus had already the lowest baseline levels at normal body temperature. These findings confirm previous studies, making the hippocampus the most sensitive region for ischemia in the brain [22, 23]; otherwise a region where just minor CBF variability occurs. Furthermore the influence of temperature regarding CBF in this area was virtually not notable. Except for the CBF peak in the very early period of ASCP,

observed in all regions of the brain, both temperature groups demonstrated similar CBF values. This fact leads over to the function of this area, the consolidation of memory, meaning the ability to transform information from the short-term memory to the long-term memory. Typical dysfunction in this area will be clinically seen as antegrade amnesia [24, 25].

Conclusion This study revealed the following interesting findings: ASCP, even at temperatures as high as 30°C, provides significantly more flow to all sampled regions of the brain compared with 25°C, but flow gradually declines with prolonged ASCP. A safe period of ASCP is considered to achieve for 30 minutes, thereafter CBF declines to values clearly below baseline for all regions. However, every analyzed region of the brain appeared to have it’ own characteristics for cerebral blood flow and characteristics are highly relevant for different clinical symptoms after ASCP. Our data confirm that the areas be subject to the greatest CBF variations like the cerebellum or pons, ascertain largely the postoperatively seen neurologic symptoms like coordination and minute motor activity dysfunction. These data suggest that further investigation of the benefits of a large variety of existing forms of ASCP is warranted. This study was supported by the German Research Foundation (STR 695/1-1).

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