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J Thorac Cardiovasc Surg 1996;111:490-492
© 1996 Mosby, Inc.
Letters to the Editor |
Second Department of Surgery
Yamagata University School of Medicine
Iida-Nishi, 990-23
Yamagata, Japana
To the Editor:
We appreciate the experimental research on the pharmacologic improvement of retrograde cerebral perfusion by Yoshimura and associates.
1 Brain edema, a topic of investigation because of the broad use of this type of perfusion, was noted in the article to be alleviated by addition of mannitol and an antivasospastic agent, with the reduction of cerebral vascular resistance. The data shown encourage clinical trials of pharmacologic support to reduce the morbidity after aortic arch procedures.
The perfusion method of Yoshimura and associates, 1 however, is extremely different from clinical or other experimental retrograde brain (cerebral) perfusion methods. Yoshimura and associates 1 perfused the dogs through the bilateral maxillary veins while draining blood from the right atrium instead of the aorta. It is not clear why they performed this curious veno-venous bypass perfusion as "retrograde cerebral perfusion."
Fig. 1 demonstrates how small the brain is compared with the total area perfused through bilateral maxillary veins. This well-developed venous network would steal the perfused blood from the brain. This shunt flow can be reduced when the pressures in the azygos vein and the superior vena cava are maintained high, as in our previous study. 2 Contrarily, drainage from the right atrium should magnify this shunt flow from the brain through the superior vena cava, azygos vein, and other veins. Then the intracranial sinuses should collapse, even if the maxillary vein pressure is maintained as shown in Yoshimura and associates' article. 1 Their veno-venous bypass perfusion therefore never establishes effective retrograde circulation in the brain. The low and decreasing intracranial pressures shown in the article are evidence of a perfusion failure because the intracranial pressure is known to increase as the perfusion pressure increases during retrograde brain perfusion. 3 Apparently, the brain edema shown in the article is caused by absence of circulation in the brain tissue but not by excess blood flow into the intracranial sinuses.
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4-6 At 20º C, cerebral metabolic rate for oxygen (CMRO2) is a constant value (0.35 to 0.55 ml · 100 gm-1· min-1) that does not increase even with "luxury" perfusion. A greater value than this was presented in the article as CMRO2 during the maxillary vein perfusion. Oxygen demand in the brain is a temperature-dependent value not affected by cerebral blood flow (CBF). CMRO2 does not exceed this value even if a retrograde perfusion gives the brain "luxury" blood flow. Retrograde brain perfusion should cause a large amount of nonfunctional shunt blood flow on the brain surface as well as in the extracranial circulation. This shunt flow should be included in the CBF value when derived by hydrogen clearance methods. 2,7 When laser photometry is used, the CBF overestimation can be reduced if the probe position is carefully selected with continuous recording of the value, as we have done. 2 The veno-venous bypass perfusion of Yoshimura and coworkers, 1 however, would have further magnified the shunt flow on the brain surface but extinguished the capillary blood flow in the brain tissue. There, the CBF would be replaced by this magnified shunt blood flow. The CBF value obtained with a fixed probe during the "retrograde cerebral perfusion" of Yoshimura and associates 1 is an extreme example of methodologic overestimation of CBF. Their extraordinary CMRO2 value was calculated with this unacceptable CBF value.
In addition, as Fig. 1 demonstrates, the CBF cannot be isolated when the brain is perfused retrogradely but not antegradely. 8 The brain weight was only 0.67% ± 0.09% of the body weight in 31 dogs used in our previous study. 6 Granted that the CBF value shown by Yoshimura and associates 1 was true, the total blood flow in the brain was less than 11 ml/min. It is estimated as 5% of total perfusion flow rate in this experiment. As Fig. 1 and the anatomy of cranial vessels in the dog show, a large amount of the blood should flow into the carotid artery from the skull and from the extracranial part, but surprisingly small amount should flow from the brain. Data in their experiment therefore did not reflect the metabolic rate of the brain at all.
We believe that the brain protection during retrograde brain perfusion should be physiologically examined as follows: (1) Effective retrograde circulation should be established in the brain experimentally beyond the venous valves with blockage of the flow escaping through the well-developed veno-venous shunt pathways. For this purpose, the superior vena cava and the upper part of azygos vein should be simultaneously perfused or at least ligated, and the blood must be drained from the aorta but never from the right atrium nor from the superior vena cava. Pressure should be simultaneously monitored in the superior vena cava, the sagittal sinus, and the aorta to clarify the true driving pressure. Then the true functional blood flow through the brain capillary bed and the CMRO2 should be examined during retrograde brain perfusion by some precise measurements. Sophisticated knowledge of this fundamental physiology will clarify the optimal condition for retrograde brain perfusion to protect the brain and reduce or eliminate brain edema. Thereafter, pharmacologic interventions should be tested, with correct baseline estimation of the brain protection by the retrograde brain perfusion itself.
References
This article has been cited by other articles:
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  N. Oshikiri, T Watanabe, H Saitou, Y Iijima, T Minowa, K Inui, and Y Shimazaki Retrograde cerebral perfusion: experimental approach to brain oedema Perfusion, July 1, 1999; 14(4): 257 - 262. [PDF]
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