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  • 1. EDITORIALBlood Pressure During Cardiopulmonary Bypass: HowLow Is Too Low?Hilary P. Grocott, MD, FRCPC, FASEU nquestionably, major advancements in the care of cardiac surgical patients have been made in the past half century since cardiopulmonary bypass(CPB) brought about the advent of modern-day cardiac their ability to define the LLA was not perfect, it does provide a major step in moving this field forward. In the absence of any specific monitoring, the usual, albeit arbitrary, LLA has typically been depicted to besurgery. However, despite improvements in surgical tech- approximately 50 mm Hg. This popular concept, however,nique, mechanical advances in the conduct of CPB, and an has previously been argued to be in error.4 Accordingly, itever-evolving understanding of cardiovascular physiology has been contested that this 50 mm Hg LLA should not beand pharmacology, several fundamental questions in car- used. But, however elegant these arguments may be, withoutdiac anesthesia persist. The target arterial blood pressure more objective confirmation, we have largely procrastinatedduring CPB to maintain homeostasis during surgery is one in making any meaningful change in our consideration of it.of these questions. There have been several well-intentioned As such, it remains a repeatedly described threshold despiteattempts to address this issue, but most studies have been the tenuous data on which it was originally based.5,6 Thelimited by significant patient heterogeneity or other deficien- uncertainty of any specific blood pressure target based on thiscies in study design and statistical power.1 So, despite decades traditional LLA approach is further accentuated in the cardiacdebating about what is the optimal blood pressure during surgery population where hypertension is highly prevalent.CPB, the answer remains elusive. In the absence of real-time data on CBF responses to blood In this issue of Anesthesia & Analgesia, Joshi et al.2 have pressure, the conventional approach has been to use a higherreported a major advancement in our understanding of LLA target in patients with chronic hypertension, predicatedblood pressure management during CPB, thereby offering by the assumption that these patients have a rightward shift insome hope that we could soon have a more definitive their autoregulatory curve.7 However, the degree to which theapproach to defining this important physiologic parameter. LLA is shifted, if at all, has always been uncertain. As a result,In this sophisticated study, they utilized near-infrared anesthesiologists have been unable to reliably determinespectroscopy (NIRS) measurements of regional cerebral when an individual’s blood pressure may be too low.oxygen saturation (Scto2) to determine cerebral autoregu- The implication of what Joshi et al. describe is that welatory thresholds. By using cerebral oximetry to examine an have largely been inaccurate in our estimation of where theindividual’s Scto2 responses to various changes in blood LLA is, with there being a wide range to this threshold inpressure, they were able to define, in real-time, the specific most of our patients, regardless of their baseline bloodcerebral lower limit of autoregulation (LLA). By definition, pressure condition. Even in the absence of significant preexisting hypertension, interpatient heterogeneity alonea patient’s cerebral autoregulation is intact when there is a appears to question the validity of any arbitrary bloodpoor correlation between perfusion pressure and cerebral pressure above which patients should be maintained. Fur-blood flow (CBF), and is lost when CBF becomes pressure thermore, when superimposed on variable degrees of cere-passive. The LLA is a threshold when the mathematical brovascular disease, it is likely even more difficult to definecorrelation between CBF and blood pressure transitions hypotension with any unifying number.8from near zero to that approaching one. In their study, the The question we are then faced with is, Where do we gotransition was defined when the correlation was Ն0.4, from here? For example, will actually knowing, and inter-which is a generally well-accepted threshold.3 Although vening based on, the LLA make any difference to patient outcome? This will require considerable study to determineFrom the Departments of Anesthesia & Perioperative Medicine and Surgery,University of Manitoba, Winnipeg, Manitoba, Canada. what outcomes could be expected to be improved. BecauseAccepted for publication December 2, 2011. this finding intuitively has direct relevance to the brain, aThe author declares no conflicts of interest. logical principle effect of utilizing the LLA to target hemo-Reprints will not be available from the author. dynamic management would be to reduce neurologic dys-Address correspondence to Hilary P. Grocott, MD, FRCPC, FASE, Depart- function. An example of how this may have an impact hasment of Anesthesia & Perioperative Medicine, University of Manitoba, St. previously been described by these same investigators in anBoniface Hospital, CR3008-369 Tache Ave., Winnipeg, MB, Canada R2H2A6. Address e-mail to observational study that identified impaired autoregula-Copyright © 2012 International Anesthesia Research Society tion during rewarming from hypothermic bypass.9 TheyDOI: 10.1213/ANE.0b013e3182456fbb highlighted that those with impaired autoregulation may488 March 2012 • Volume 114 • Number 3
  • 2. Blood Pressure and Bypassbe at risk for other cerebral complications, such as stroke warning system, it should more appropriately be consid-and/or transient ischemic attacks. Thus, using LLA deter- ered the opposite of a “canary in a coal mine” (in effect,minations could identify high-risk patients and also open being the last to go) because it is highly likely that once theup the possibility of intervening to optimize CBF to attenu- brain desaturates, the other organs have long since beenate these adverse outcomes. exposed to compromised blood flow and oxygenation. It is not clear, however, as to the scope of what neurologic Thus, using the cerebral LLA as a target that might influ-outcomes (i.e., stroke, encephalopathy, delirium, cognitive ence other outcomes is a potential limitation to the datadysfunction) might be influenced by using a real-time LLA- presented by Joshi et al. However, it is not so much aguided assessment and intervention. A relationship between limitation in their study design per se, but in the potentiallower CPB pressure and increased stroke risk in cardiac clinical utilization of this information.surgical patients with severe aortic atherosclerosis has previ- One further caveat to this study relates to the limitationously been reported.10,11 It was postulated that the lower that the technology and the software that they used topressure in those patients exposed to excess atheromatous collect and analyze the LAA data are not commerciallyemboli increased the risk of cerebral ischemia secondary to a available. Although commercialization in some relatedreduction in blood flow in the pressure-dependent cerebral form is likely to occur, it will not be known how this mightcollateral vessels.12 However, it is unlikely that using a impact patient care until it is more widely available in aglobal cerebral LLA target would be expected to influence more user-friendly mode. Importantly, however, cerebralregional thromboembolic events per se or that NIRS wouldbe able to identify these relatively small at-risk regions oximetry, irrespective of whether specific NIRS-guidedwithin the brain. However, if aortic atherosclerosis is determination of autoregulation becomes available, is in-indicative of severe diffuse cerebrovascular disease, then it creasingly being used in the day-to-day management ofcould have an impact on overall stroke risk. That is, it cardiac and other surgical patients and has shown greatwould be far more likely to have an impact on the general promise.19,21 Its ease of application and the intuitive informa-sequelae of impaired CBF. For example, watershed strokes tion it provides in reflecting both cerebral oxygen supply and(secondary to global cerebral hypoperfusion) have previ- demand issues can be applied to multiple physiologic param-ously been shown to occur more frequently in patients eters other than just blood pressure. However, the ultimatemanaged during CPB with a lower (than their prebypass impact of this technology will need to be determined inbaseline) blood pressure.13 Maintaining a blood pressure larger-scale prospective controlled trials.well above the LLA might reduce these types of watershed The information presented by Joshi et al. shows great(hypoperfusion) stroke events. As for more subtle neuro- promise, but until we have more data related to its impactlogic deficits, there is also evidence that prolonged hypo- on patient outcomes, the question of “how low is too low”tension could also have an impact on delirium.14 Finding a blood pressure remains incompletely answered.the optimal pressure for patients has the theoretical poten-tial to mitigate all of these subtypes of neurologic compli- DISCLOSUREScations, but the evidence thus far, is weak. Name: Hilary P. Grocott, MD, FRCPC, FASE. Other non-neurologic outcomes could also be influenced Contribution: Hilary P. Grocott, MD, FRCPC, FASE, wrotewith this information. Murkin et al.,15 in an interventional and approved this manuscript.trial using NIRS to optimize Scto2 (with one interventionbeing blood pressure manipulation), described an improve- REFERENCESment in their major morbidity composite outcome. That 1. Murphy GS, Hessel EA II, Groom RC. Optimal perfusionstudy essentially described the brain as a potential sentinel during cardiopulmonary bypass: an evidence-based approach.or index organ serving to identify “at-risk” situations when Anesth Analg 2009;108:1394 – 417Scto2 was low.16,17 However, it is well known that there is 2. Joshi B, Ono M, Brown C, Brady K, Easley B, Yenokyan G,a hierarchy of organ blood flow, and what is an optimal Gottesman RF, Hogue CW. Predicting the limits of cerebralblood pressure (and flow) for the brain may not be the same autoregulation during cardiopulmonary bypass. Anesth Analg 2012;114:503–10for other major organs (such as the kidney or other splanch- 3. 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