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Ann Thorac Surg 1997;64:432-436
© 1997 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Evaluation of Brain Oxygenation During Selective Cerebral Perfusion by Near-Infrared Spectroscopy

First Department of Surgery and Critical Care Medical Center, Yamaguchi University School of Medicine, Ube, Japan

Accepted for publication February 17, 1997.

	Abstract

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Background. Although selective cerebral perfusion (SCP) has been used for cerebral protection in aortic arch operations, the appropriate perfusion conditions of SCP are unclear.

Methods. We used near-infrared spectroscopy, which evaluates brain ischemia noninvasively and continuously, to determine whether perfusion with SCP (core temperature, 20°C; flow rate, 10 mL · kg^-1 · min^-1) was acceptable in terms of oxyhemoglobin and deoxyhemoglobin in patients having SCP for aortic arch operations (SCP group, n = 6) versus patients having cardiopulmonary bypass (CPB) for coronary artery bypass grafting (CPB group, n = 6).

Results. There were no significant differences in age (65 ± 10 versus 63 ± 12 years), CPB time (199 ± 67 versus 199 ± 52 minutes), changes in hematocrit (-12.9% ± 3.7% versus -12.5% ± 6.0%), lowest blood pressure (43 ± 7 versus 45 ± 10 mm Hg), or highest central venous pressure (8 ± 2 versus 9 ± 4 mm Hg) between the SCP and CPB groups. In the SCP group, the maximum decrease in oxyhemoglobin level and the maximum increase in deoxyhemoglobin level were -5.0 to -11.4 µmol/L and -0.1 to 3.9 µmol/L, respectively; in the CPB group, the respective changes were -3.2 to -14.2 µmol/L and -0.4 to 3.6 µmol/L. Changes of oxyhemoglobin and deoxyhemoglobin levels in the SCP group were almost within the range of those in the CPB group. There were no brain complications in either group.

Conclusions. As described here, SCP is acceptable and safe for brain protection in aortic arch procedures.

	Introduction

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
During operations for aortic arch aneurysm, selective cerebral perfusion (SCP) has been used for cerebral protection. However, it is unclear what temperature is best, what flow rate is adequate, and whether pH-stat or alpha-stat blood gas management is superior. Our method of SCP specifies a deep body temperature of 20°C, perfusion rate of 10 mL · kg^-1 · min^-1, and alpha-static blood gas management. We compared the cerebral oxygenation by near-infrared spectroscopy (NIRS) during SCP with that during standard cardiopulmonary bypass (CPB). Near infrared spectroscopy has been reported as a useful noninvasive monitor to detect cerebral oxygen states [1]. If our method is appropriate, then changes in oxyhemoglobin and deoxyhemoglobin during SCP should be within the range of those during CPB.

	Material and Methods

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Six patients having coronary artery bypass grafting (CPB group) and 6 patients having operations on the aortic arch (SCP group) between May 1994 and July 1996 were examined by NIRS (NIRO-500; Hamamatsu Photonics Inc., Hamamatsu, Japan). Two probes, one a light source produced by laser and the other a receiver, were positioned 4.5 cm apart from each other in the right frontal region. They were covered with a black band to exclude outside light.

Near-infrared light waves have distinct characteristics. They are detectable as they pass through the cranium and are absorbed by oxyhemoglobin, deoxyhemoglobin, and cytochrome aa3 in the brain. The absorption curves of oxyhemoglobin, deoxyhemoglobin, and oxygenated minus deoxygenated cytochrome aa3 are already known [2]. Because this apparatus emits four wavelengths—775, 825, 850, and 904 nm—we can obtain the formula for each wavelength and derive the contents of oxyhemoglobin, deoxyhemoglobin, and oxygenated minus deoxygenated cytochrome aa3 by resolving the four formulas according to the linear least-squares curve-fitting method [3]. Zero values for oxyhemoglobin and deoxyhemoglobin were set just before CPB because the measured value is a relative value, although the change in each variable is an absolute value.

Cardiopulmonary bypass was established with arterial cannulation through the ascending aorta and venous cannulation through the right atrium to the superior and inferior venae cavae. The priming volume was about 2,200 mL of crystalloid fluid unless the calculated hematocrit fell below 20%; otherwise blood was primed in the pump reservoir. In the SCP group, CPB was established in the same fashion except that arterial cannulation was through the right or left femoral artery. Arterial cannulations in the SCP group were added from the right axillary artery and left common carotid artery (cases 1, 3, and 6) or from both axillary arteries and the left common carotid artery (cases 2, 4, and 5). The cannulas for SCP were connected by a Y-shaped connector. In both groups, the total bypass flow rate was 60 to 70 mL · kg^-1 · min^-1. After stabilization during CPB (cases 1, 2, 5, and 6) or at the same time as CPB initiation (cases 3 and 4), SCP was started at a rate of 10 mL · kg^-1 · min^-1 by a single roller pump with the same reservoir of CPB. During weaning from CPB, a blood concentrator (BC60; Jostra, Hirrlingen, Germany) was used in all cases. Residual blood in the CPB reservoir was processed in a cell-saving device (Haemonetics, Braintree, MA) and returned to each patient after pump disconnection.

In both groups, we examined the changing patterns of oxyhemoglobin and deoxyhemoglobin, the maximum decrease in oxyhemoglobin, and the maximum increase in deoxyhemoglobin by NIRS. We also examined changes in hematocrit, lowest blood pressure, and highest central venous pressure, which are thought to influence NIRS, except at the time of aortic clamping and declamping. During CPB and SCP, blood pressure was monitored continuously and directly from the left superficial temporal artery (patients 1 and 2) or from the cannula tip inserted into the right axillary artery (patients 3, 4, 5, and 6).

	Results

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
The mean age of the patients (± standard deviation) was 65 ± 10 years (range, 52 to 77 years) in the SCP group and 63 ± 12 years (range, 40 to 72 years) in the CPB group. Cardiopulmonary bypass time and aortic cross-clamp time were 199 ± 67 and 103 ± 61 minutes in the SCP group, respectively; in the CPB group they were 199 ± 52 and 94 ± 16 minutes. There were no significant differences in age, CPB time, or aortic cross-clamp time between the CPB and SCP groups. Hematocrit before CPB was 33.0% ± 2.8% in the SCP group and 32.3% ± 5.5% in the CPB group. The change in hematocrit was 12.5% ± 6.0% in the CPB group and 11.4% ± 3.3% in the SCP group. The lowest blood pressure ranged from 28 to 57 mm Hg (45 ± 10 mm Hg) in the CPB group and from 32 to 50 mm Hg (43 ± 7 mm Hg) in the SCP group. The highest central venous pressure in the SCP group was 8 ± 2 mm Hg; in the CPB group it was 9 ± 4 mm Hg. There were no significant differences for changes in hematocrit, lowest blood pressure, or highest central venous pressure (Table 1). The lowest rectal temperature ranged from 25.2° to 29.3°C (27.4° ± 1.3°C) in the CPB group and from 19.1° to 20.3°C (19.7° ± 0.5°C) in the SCP group (p < 0.01).

View larger version (33K): [in this window] [in a new window]   Fig 1. . Changing patterns of oxyhemoglobin (Oxy-Hb) and deoxyhemoglobin (Deoxy-Hb) levels in the cardiopulmonary bypass (CPB) group. The oxyhemoglobin pattern was as follows: an initial decrease when CPB started, then a gradual slight increase to a plateau, followed by a slight decrease around the time of declamping, and then an increase again because of hemoconcentration. Deoxyhemoglobin level decreased slightly when CPB was started and then showed a plateau. At the end of CPB, deoxyhemoglobin level was normalized or slightly increased, except in case 2. Hemodilution was not performed in patient 4, who had chronic anemia due to nephrosis, so the initial change in this case was less than in the others. (AoX = aortic cross-clamping; D-AoX = declamping.)

View larger version (40K): [in this window] [in a new window]   Fig 2. . Changing patterns of oxyhemoglobin (Oxy-Hb) and deoxyhemoglobin (Deoxy-Hb) levels in the selective cerebral perfusion (SCP) group. An initial drop was also seen in the SCP group. The tendencies of deoxyhemoglobin changes in the SCP group were similar to those in the cardiopulmonary bypass (CPB) group. In case 5, SCP was applied twice to terminate bleeding at the distal aortic anastomosis. (AoX = aortic cross-clamping; D-AoX = declamping.)

View larger version (14K): [in this window] [in a new window]   Fig 3. . Comparison of the maximum decreases in oxyhemoglobin level (Oxy-Hb) and the maximum increases in deoxyhemoglobin level (Deoxy-Hb) between the cardiopulmonary bypass (CPB) and selective cerebral perfusion (SCP) groups. The maximum decrease in oxyhemoglobin level was -5.0 to -11.4 µmol/L in the SCP group and -3.2 to -14.2 µmol/L in the CPB group. These changes in the SCP group were within those in the CPB group. The maximum increase in deoxyhemoglobin level was -0.1 to 3.9 µmol/L in the SCP group and -0.4 to 3.6 µmol/L in the CPB group. These changes of deoxyhemoglobin level in the SCP group were similar to those in the CPB group.

	Comment

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Protection of the brain in aortic arch aneurysm operations is important for good results. There are three methods of protecting the brain during such procedures: circulatory arrest with deep hypothermia [4], SCP [5–7], and retrograde cerebral perfusion [8–10]. The limit of brain ischemic time in circulatory arrest with deep hypothermia (at body temperature 10° to 15°C) has been reported to be 60 minutes [4]. Recently introduced retrograde cerebral perfusion allows a relatively longer ischemic time than circulatory arrest [8], whereas SCP permits the longest clamping time of the branches of the aortic arch [7]. We have adopted SCP because this method is thought to be safer and more consistent than the other two. However, detailed techniques and perfusion conditions of SCP have not been established.

In 1972, Jöbsis [11] reported experiments showing that NIRS is useful in evaluating the oxygenation state of the brain. This technique is noninvasive and sensitive to ischemic changes of the brain [1, 11–13]. We can monitor the oxygenation level in the brain tissue by NIRS continuously and in real time. Because oxyhemoglobin and deoxyhemoglobin evaluation by NIRS is considered a good monitoring strategy during aortic arch aneurysm operations, we used NIRS to evaluate perfusion in SCP. Our purpose in this study was to evaluate whether the conditions of SCP (core temperature, 20°C; flow rate, 10 mL · kg^-1 · min^-1; alpha-static blood gas management) were appropriate. We also compared the changing patterns of NIRS during CPB and SCP.

Oxyhemoglobin and deoxyhemoglobin are influenced by the hemoglobin concentration, oxygen level of hemoglobin, blood supply to the brain, and oxygen consumption of the brain. From recent research, oxygen saturation states of the brain tissue by NIRS are similar to oxygen saturation of the internal jugular venous hemoglobin [14, 15]. Because in our setting, arterial blood was oxygenated in 100% oxygen saturation during CPB and SCP, oxyhemoglobin and deoxyhemoglobin are thought to be influenced by hematocrit, blood flow, blood pressure, superior vena caval pressure, and temperature. The ranges of change in hematocrit, lowest blood pressure, and highest central venous pressure in our groups were not different. Autoregulation of blood flow in the brain exists in mean arterial pressures between 60 and 150 mm Hg under physiologic conditions. Cerebral blood flow was not measured, but autoregulation of the brain circulation exists even at a body temperature of 20°C [16] and in low blood pressure during CPB [17, 18]. Therefore, the blood supply to the brain should be maintained. Although temperature influences the values of oxyhemoglobin and deoxyhemoglobin, it also influences brain metabolism. If brain metabolism decreases with temperature, oxygen consumption also decreases. It has been reported that on average, the cerebral metabolic rate for oxygen is reduced about 7% for each centigrade degree decrease in body temperature [19, 20] and that unfavorable oxyhemoglobin dissociation kinetics are not observed at temperatures of 30°C [21] and 20°C [22]. As regards the balance between the oxygen demand and the supply of the brain tissue, it is thought that oxyhemoglobin and deoxyhemoglobin are affected little by temperature. If the demand is larger than the supply, oxyhemoglobin level may decrease and deoxyhemoglobin level may increase.

The results of NIRS in the CPB group showed a particular pattern. The initial decrease in oxyhemoglobin level was thought to be due to hemodilution, because the case without hemodilution in the CPB group (case 4) was the only one that apparently did not show an initial decrease in oxyhemoglobin level. The slight decrease in oxyhemoglobin level around the time of aortic declamping might reflect the reduction of CPB flow. The following increase in oxyhemoglobin level was thought to be due to hemoconcentration because this tendency continued after CPB when blood was transfused. The deoxyhemoglobin level decreased slightly when CPB was started, because of hemodilution. At the end of CPB, the deoxyhemoglobin level was normalized or slightly increased by rewarming and hemoconcentration. An initial drop in oxyhemoglobin level was also recognized at the beginning of CPB in the SCP group, and the reduction in oxyhemoglobin level during SCP was not regular. Changes in the deoxyhemoglobin level during SCP were a mirror image of oxyhemoglobin changes, as in the CPB group.

Because CPB is used safely worldwide in open heart operations, we hypothesized that, if the conditions of SCP are appropriate, then changes in the indices of NIRS during SCP should be comparable to changes during CPB. The results showed that the oxyhemoglobin level in the brain tissue during SCP was within the level seen during CPB. The level of deoxyhemoglobin during SCP, except in 1 case, was also within the level in the CPB group. This exceptional case showed only a small excess over the highest maximum increase in deoxyhemoglobin level in the CPB group. Deoxyhemoglobin is influenced by several factors. If the deoxyhemoglobin level is elevated, two conditions must be considered: insufficient venous drainage or reduction of oxygen delivery to the brain tissue. Because even the highest central venous pressure during SCP in this case remained normal, venous drainage was not disturbed. Even if insufficient brain oxygenation may have occurred in this case, elevation of the deoxyhemoglobin level in this range is thought to be acceptable.

In conclusion, our cerebral perfusion conditions for SCP (core temperature, 20°C; flow rate, 10 mL · kg^-1 · min^-1; and alpha-static blood gas management) were demonstrated by NIRS to be acceptable and safe in aortic arch operations. Further research should identify the best conditions for SCP.

	Footnotes

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Address reprint requests to Dr Katoh, First Department of Surgery, Yamaguchi University School of Medicine, Kogushi 1144, Ube, 755, Japan.

	References

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 

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