Ann Thorac Surg 2007;84:855-856
© 2007 The Society of Thoracic Surgeons
Original Articles: Cardiovascular
Invited commentary
Marshall L. Jacobs, MD
Section of Cardiothoracic Surgery, St. Christopher’s Hospital for Children, Erie Ave at Front St, Philadelphia, PA 19134-1095
(Email: marshall.jacobs{at}tenethealth.com).
Dahlbacka and associates [1] have once again made an important contribution to the ever increasing body of knowledge concerning the potential significance of arterial blood gas and pH management strategies during cardiopulmonary bypass. Specifically, their article reports the findings using a porcine experimental preparation using cardiopulmonary bypass with a 45-minute period of selective bi-carotid perfusion at moderate hypothermia (25°C) and randomization to either alpha-stat or pH-stat blood gas management. Measured end points included a variety of biochemical indices, as well as observations of the status of brain microvasculature using a cranial window and intravital microscopy to assess the size at various points in time of arterial and venous cerebrocortical microvessels as well as the nature of leukocyte endothelial interactions. The important observations included higher concentration of brain lactate at various time points in the alpha-stat group without elevation of brain glutamate in either group. At no time point was there a statistically significant difference between groups regarding the lactate-pyruvate ratio. Using intravital microscopy, the authors observed no significant differences in the leukocyte endothelial interactions between groups. No statistically significant differences were observed in the microvascular diameters between the groups either. Although some previous studies have shown that the pH-stat strategy is associated with higher cerebral oxygenation at the end of cooling and during early rewarming after a period of hypothermic circulatory arrest, the present study failed to show any statistically significant differences between pH management strategies in cerebral tissue oxygenation or behavior of the microcirculation during cardiopulmonary bypass and selective cerebral perfusion at 25 degrees C. Although the authors infer that the higher concentrations of brain lactate in the alpha-stat group suggest the possibility of anaerobic metabolism during selective cerebral perfusion with that pH strategy, they concede that any perceived advantage of the pH stat strategy in the setting of hypothermic circulatory arrest (a controversial position in itself) is absent in the setting of continuous cerebral perfusion at moderate hypothermia.
Why is this largely negative result important? For decades cardiopulmonary bypass and deep hypothermia with circulatory arrest (DHCA) has been used to facilitate operations on the thoracic aorta (particularly arch reconstructions and replacements) in the setting of aneurysmal disease and dissection, and for the repair of congenital cardiac anomalies in neonates and infants. Although improvements in technique and care have brought about considerable reduction in operative mortality in both clinical realms, neurologic morbidity remains a major consideration. In the past decade, many surgeons have moved away from DHCA as a primary strategy and embraced continuous selective cerebral perfusion of one form or another. The enthusiasm with which these strategies have been embraced is largely based upon the intuitive notion that "some perfusion is better than no perfusion." So far, studies of the efficacy of selective cerebral perfusion during replacement of arch aneurysms are encouraging, but inconsistent with respect to the efficacy and optimal conduct of these strategies. In the realm of neonatal aortic arch reconstruction, the few clinical studies to date have fallen short of demonstrating any advantage of selective cerebral perfusion over DHCA with respect to measures of neurodevelopmental outcome at 1 year. In fact, the primary inference has been that these techniques are probably as safe as limited periods of DHCA. However, continuous selective cerebral perfusion at various temperatures and various flow rates has already found wide acceptance in clinical practice.
Therefore the importance of the present study is that Dahlbacka and associates [1] have applied rigorous scientific inquiry to the question of regional cerebral perfusion, an operative strategy that has found wide acceptance largely on the basis of intuitive appeal rather than objective evidence. To achieve the ultimate goal of optimizing neurologic outcome, the biochemical, vascular, cellular, and functional effects of the many technical details of regional cerebral perfusion (including but not limited to pH management strategy, temperature, flow rate, hematocrit, and monitoring) must be studied in at least as much detail and depth as has characterized prior investigations of bypass itself and of hypothermic circulatory arrest. Ultimately, the ideal strategy may vary widely depending on the clinical substrate, and the principles and features that make for best practices in the management of aortic dissection extending into the arch will almost certainly be entirely different from those that apply to arch reconstruction in a neonate with hypoplastic left heart syndrome. This study is a good start. A great deal more investigation will be necessary to achieve the maximal potential benefit of techniques such as selective antegrade cerebral perfusion.
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References
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- Dahlbacka S, Alaoja H, Mäkelä J, et al. Effects of pH management during selective antegrade cerebral perfusion on cerebral microcirculation and metabolism: alpha-stat versus pH-stat Ann Thorac Surg 2007;84:847-856.[Abstract/Free Full Text]
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