Changing Temperature Management for Cardiopulmonary Bypass

There have been a variety of changes in the cardiac surgical operating theater over the last several years, including minimally invasive surgical techniques, new surface coatings for the cardiopulmonary bypass (CPB) apparatus, and the use of various antifibrinolytics. However, from the standpoint of patient physiology and management, perhaps the most substantial change has been the move to higher CPB temperatures. Although "warm" bypass has always formed a part of cardiac surgical practice, typically for very short cases, the shift toward normothermia in many adult patients has been quite dramatic. In our own practice, we shifted from <10% of adult cases being performed at normothermia to nearly 50% between 1991 and 1993. Over the same period, many institutions in North America and Europe have undergone a similar shift. The academic community responded by examining the practice change, and a considerable body of investigation followed. Based on those reports, the practice has again been modified. In this review, I briefly present the historical background of normothermic CPB and the initial reports that triggered the change in CPB practice. I also discuss how the physiology of warm CPB differs from hypothermic bypass and examine the impact of systemic normothermia in the postoperative period. The studies on the effects of warm bypass on neurologic outcome and the rationale for "tepid" bypass are also reviewed. Finally, some practical recommendations for the intraoperative management of these patients are provided. Historical Background: Warm CPB During Early Cardiac Surgery The surgical records from the early days of extracorporeal circulation show that a large part of that practice was conducted at temperatures approximating normothermia [1,2]. The heat exchanger was not a fundamental part of early bypass circuits, and patients' temperatures drifted downward to mildly hypothermic levels [3]. External cooling was sometimes used, but most efforts in early cardiac surgery went to supporting patient temperature, rather than to inducing hypothermia [1,4]. In addition to bypass approximating normothermia, these early cases were conducted with a circuit prime that consisted largely of whole blood. This was necessitated by the large priming volumes of the early CPB circuits [1,3-5]. At temperatures of 35-37[degree sign]C and a sanguineous prime, the flows targeted during CPB could approximate the cardiac index in the anesthetized state, and the adequacy of perfusion was assessed by comparing physiologic variables during CPB with those of the nonbypass state [1,2,6,7]. The Rationale for and Practice of Systemic Hypothermia for Cardiac Surgery For theoretical and practical reasons, the introduction of hypothermia and hemodilution mostly occurred in tandem. Considerable morbidity was demonstrated with early CPB, and the effect of hypothermia in reducing O2 demand and in increasing ischemic tolerance was established [6,8]. In practice, it was argued that the reduction in metabolic rate associated with hypothermia would also allow for the use of an asanguineous prime and/or reduced CPB flows [9]. In fact, hypothermia required hemodilution because hypothermia with a whole blood prime resulted in hypertension during CPB. The introduction of hypothermia was also of practical importance because cardiac surgery was placing a tremendous strain on blood banking resources. It was typical for these cases to use whole fresh blood (<24 h old) in volumes of 1500-2500 mL as a circuit prime [3-5,10]. This blood use limited cardiac surgical capacity [10]. Hypothermia had become an established practice for adult cardiac surgery by the late 1960s and, at most institutions, constituted the largest part of the surgical practice until the reintroduction of warm CPB in 1991 and 1992. The Arguments for Warm Cardioplegia The introduction of systemic normothermia for CPB in the early 1990s was indirect. Studies that eventually resulted in systemic normothermia during CPB included investigations of alternate cardioplegia techniques. Specifically, the research group at the University of Toronto, extending some of the earlier observations in canines by Partington et al. [11] and Buckberg et al. [12], advocated the myocardial advantages of a warm continuous cardioplegic technique [13-15]. Clinical literature indicated advantages of a warm induction of cardioplegia [16], as well as a terminal "hot shot," that is, perfusion of the heart with warm aerobic cardioplegia before removing the aortic cross-clamp [17]. The Toronto group reasoned that warm continuous cardioplegia would extend these advantages [13-15]. First, hypothermia, by its effect on enzyme kinetics, was known to alter myocardial substrate use and to impair the generation of adenosine triphosphate [14,18,19]. Second, hypothermia was also known to shift the oxyhemoglobin dissociation curve leftward, potentially limiting O2 offloading in the ischemic myocardium [20]. Third, the greatest reduction in myocardial O2 consumption is achieved by diastolic arrest [12,21,22]. The additional reduction in maximal venous oxygen consumption (MVO2) provided by hypothermia was relatively small [12,21] and difficult to justify based on its potentially harmful effects on myocardial contractility [23-25]. It was hypothesized that the MVO2 of the heart arrested in diastole could be met by warm blood cardioplegia provided continuously [15,18,22,26]. Fourth, the addition of continuous cardioplegia had the potential to eliminate or attenuate the reperfusion injury thought to be associated with intermittent cardioplegia techniques [26,27]. The Toronto Results of Warm Cardioplegia and CPB Clinical reports from Toronto with this new technique showed very favorable results [24,28]. In those studies, warm continuous blood cardioplegia was delivered from antegrade or antegrade and retrograde cannula. Because blood cardioplegia is composed of a high potassium solution mixed in a greater volume with blood shunted from the oxygenator, the cardioplegia and systemic temperatures in the initial reports were similar, although the major rationale for systemic normothermia was to allow for prompt separation from CPB. Although systemic normothermia represented a major shift in practice, the practical physiologic impact of systemic normothermia was not addressed until a the warm continuous cardioplegia technique and systemic normothermia, the Toronto group in myocardial outcome in bypass patients to a historical of patients with the intermittent cardioplegia technique In the period, cardiac was in the warm The of myocardial was in the warm group in the cardiac occurred in of hypothermic technique patients of the warm technique and the for was in the warm group in the of patients the warm technique to this technique in patients with to myocardial showed similar advantages with a historical group At nearly the same et al. the effects of antegrade and retrograde normothermic with hypothermic cardioplegia on postoperative and The of patients surgery with normothermic cardioplegia and systemic normothermia were with a historical of patients CPB with hypothermic cardioplegia and a body of Although the aortic cross-clamp were in the normothermic was Although it not the hypothermic group also showed a higher for postoperative Finally, the normothermic group had a of and at Because of the physiologic for the practice and the myocardial the techniques advocated by the Toronto group were Because these results were with warm cardioplegia and warm systemic temperature, it was not the advantages were to the cardioplegia The of myocardial and systemic are not However, of the Toronto reports, as well as and physiologic studies, additional to higher body some studies on the warm cardioplegia technique its introduction et al. at warm and cardioplegia techniques in a of and largely the physiologic for the practice and studies in normothermic blood cardioplegia with systemic normothermia with blood or cardioplegia with systemic hypothermia Although the myocardial injury in not between the warm technique resulted in diastolic and an state studies in also indicated that continuous blood cardioplegia and metabolic the initial reports from Toronto, cardioplegic management has to It is difficult to with the warm antegrade technique intermittent warm results from of and retrograde in of the and and of the cardioplegia are also with a warm cardioplegic technique Finally, and studies of cardioplegia and that a of antegrade and retrograde techniques at temperatures be The of Systemic and O2 Systemic There are physiologic systemic normothermia of hypothermia, is during CPB. warm practice, the historical of the technique is The during CPB are mostly based on and reports from the and 1960s it was that the cardiac index in an anesthetized adult or a whole blood prime and these systemic O2 would be to that in the the introduction of hypothermia, it was that a reduction in whole body O2 demand would allow for reduced or However, normothermic bypass as it in the 1990s was largely hypothermic CPB, with the of higher temperatures. This from the early with warm because patients were and with flows approximating those in the circulation with a of these that it is reduced by on the of to the state nonbypass hemodilution is associated with in cardiac that whole body O2 be over a of Although the heart of the cardiac surgical patient not for hemodilution the physiologic to a reduction in is an in cardiac the during normothermic CPB with prime and a cardiac index of differs from the normothermic CPB as well as from the physiology of the Although in are well and whole body O2 consumption is by in oxygen the between systemic O2 and demand is by warm CPB. in mixed venous O2 are but to the O2 is However, as the and in to perfusion and have metabolic and to O2 the body as a and not an of oxygen at the of This is during warm CPB, in the between systemic O2 and demand is CPB flows are CPB, Systemic and during CPB is a of a variety of patient and and the to extracorporeal Although CPB not a in these normothermic CPB has a effect on Although not by during warm bypass is typically reduced in the of to Because of the to CPB is to aortic normothermic patients the same to CPB as hypothermic patients is to the systemic associated with Although the it a for during warm CPB. The is is the is is and is The for the body as a blood and is in cardiac in to the reduction in blood The effect of hemodilution on blood is well Over the of that patients during CPB, is an between and blood The effect of this change is well by et al. the patient priming volume of the and the that allow the of the change during bypass these variables The a 50% reduction in with a reduction in blood in the early of for of reduction the of bypass for a priming volume and initial of the with from in and during cardiopulmonary are that the of the patient during CPB. First, blood is a and its Second, is not during CPB, and be by However, that the in during CPB in in During hypothermic the to and in blood with hypothermia the reduction in with during hypothermic CPB, is typically present during the early and late of CPB the patient During warm the effect of hemodilution on blood and be demonstrated the of extracorporeal The effect of warm CPB on is in practice. the introduction of normothermic bypass in 1991 and reports on the of CPB this Although the has not been most reports indicated to hypothermic warm bypass patients of greater or higher flows to during CPB a of et al. demonstrated a in in patients at sign]C to those in patients at sign]C of during normothermic and hypothermic cardiopulmonary normothermic cardiopulmonary bypass required of to systemic perfusion between the with from the of of Surgery for during CPB with systemic normothermia be as a potential because large of have effects in bypass and nonbypass In fact, studies of are is as the for have or with the use of during CPB has also been to of the in perfusion with the that blood is or This is by the of et al. cardiac and whole body O2 consumption in patients on but it has not been those results are to the of CPB. the between and perfusion be on a is reduced during CPB. Warm CPB and of systemic O2 during CPB with hemodilution and of whole body O2 is reduced to the oxygen of as is to be to be studies during hypothermic CPB, perfusion is reduced or in The same is during normothermic CPB, in and blood are This is of physiologic because oxygen is also with the associated with warm CPB, the of et al. the effect of and perfusion on blood during normothermic CPB in and that and blood was as the was from to by in not blood also in a was in a by the same In that the effect on perfusion of or to during normothermic CPB was a of blood was greater was by rather than with an Although the not a with CPB flows of and mL that perfusion be with flows and the perfusion that are during warm CPB. these physiologic an in or morbidity has not been with warm CPB or the use of during these in small studies of relatively patients Clinical for Warm CPB The to it difficult to the effect of normothermic CPB on although new as The volume associated with CPB because CPB blood of and the to from to Hypothermia has provided some additional that is being during CPB because hypothermia is an it the and to hypothermic CPB, it has not been established or has an during normothermic CPB However, this be hypothermia blood and the of these and the historical with in cardiac a be for techniques. because and volume of effects be of with these during warm CPB The effect of CPB on is In the use of be to during warm CPB to the hypothermic practice. sign]C of hypothermia the of of and the to studies, hypothermia the of and required to a of by that in of these to hypothermic be with warm CPB. Although the reports from Toronto not that the normothermic technique required of to a this be the effect of hypothermia on and blood of patients bypass this and very between CPB and However, the was for bypass the was Although CPB was at higher bypass temperatures the and, of CPB, was greater than patients hypothermic CPB The that greater to is required in patients warm CPB. The that normothermic CPB is associated with bypass has been with some Although this is not a CPB be the is Because is an between CPB and morbidity and because the be a neurologic the of the has theoretical Although not an of the effect of CPB on is during hypothermic CPB. This is to of and a in use Although the of is by normothermia, a and use is during normothermic CPB, a that a to a the in demonstrated during CPB is largely of Cardioplegia and Cardiac reports on the warm technique advantages with to cardiac of the aortic cross-clamp The Warm a for in warm cardioplegia these of the This effect on was in the from et al. and by in the and is from the of et al. of in patients to normothermic or hypothermic cardioplegia during CPB with systemic temperatures of sign]C or In addition to the with the warm et al. in of patients in the group the The warm cardioplegia technique also reduced the of in the by et al. the normothermic cardioplegic technique sign]C cardioplegia and systemic body reduced at and at late in patients sign]C cardioplegia and CPB a historical of patients cardioplegia at sign]C and CPB at sign]C in the bypass as by continuous between myocardial by normothermic blood cardioplegia and hypothermic blood cardioplegia between the with from et al. continuous blood cardioplegia heart at for normothermic cardioplegia postoperative hypothermic CPB, normothermic CPB. from et al. to it is to because the warm cardioplegia technique to greater volumes of potassium levels during CPB In the by et al. patients warm cardioplegia a of of potassium during CPB for patients the hypothermic This resulted in higher in the warm However, is shifted in the period, and have been reports effects of normothermic CPB. and the CPB CPB an a variety of including and and in normothermic CPB or this is not and CPB has been to the of of and a and a and are also demonstrated CPB in patients CPB at sign]C During CPB at as and or not be In hypothermia and also hypothermic Although generation of also be by hypothermia in the by et al. not a in between hypothermic and normothermic CPB and are by hypothermia in but these or not be demonstrated in patients CPB. At studies have demonstrated an effect of CPB on levels of and However, et al. patients normothermic CPB, and et al. provided on bypass et al. and et al. levels in hypothermic sign]C and and normothermic and The greater in levels in normothermic patients h bypass but in et al. could not effect of bypass on although this was in In that and were reduced in the warm group to the hypothermic group at although the not at In et al. that was in a warm CPB group at and h Finally, et al. that of was by hypothermic and CPB, but were not these reports, for the effect of CPB on the In studies on the effect of hypothermia on have not been to is demonstrated Clinical studies have typically been small and and at Finally, these are not to CPB. are also associated with of major surgery CPB levels of and results in and are also in to CPB. Hypothermia to some of these but it is not this has a effect on the of the or a effect on This a of cases in bypass was performed of systemic hypothermia for the of hypothermia is indicated by the are with cases in the was performed normothermic In cases, of the patient was associated with a in blood at a of with from et al. The effect of body on during cardiopulmonary CPB and the potential of normothermic CPB is of of the warm intraoperative effect on myocardial and approximating systemic normothermia during CPB have fundamental effects on postoperative by cardiac surgery is well be from CPB with a of but in the be sign]C In addition to heat to the in the period, is that patients hypothermic CPB are not of at the of that a large of body This to and cooling in the postoperative hypothermia and heart rate During the initial to postoperative body is whole body oxygen and myocardial is triggered Although this has not been in cardiac surgical these variables are associated with an of cardiac myocardial and surgery Although hypothermia on in the is also normothermic CPB, is a that postoperative hypothermia is and body of research comparing postoperative in patients warm and CPB has not been but some literature that is a et al. changes in patients CPB at Over a period, sign]C and h on in the was sign]C et al. also higher temperatures for the postoperative in of CPB In small et al. demonstrated that was higher in the normothermic CPB group the postoperative but not extend observations this period. Because patient in the is by many in addition to bypass temperature, as body blood and temperature, it to be between hypothermic and warm CPB be these are be of greatest importance in the patients have capacity to The of Warm CPB in Early Because most for surgical a for systemic normothermia, patients warm CPB be earlier than hypothermic CPB and demand in the early also the of hypothermic patients to patients with warm CPB. Finally, the of reduced and cardiac are the postoperative period, this earlier of patients warm CPB. these a studies have the effect of CPB on investigations have a to or with the warm studies also et al. could not an effect of CPB on the O2 in patients et al. as well as and in and patients with to CPB at sign]C or that the was reduced in patients warm CPB. The same was for and However, these were not demonstrated h in the of these to is a in the advantages or of an or surgical management Although the to is based on as of on the surgical and of a of the It to be warm CPB or the associated with the early period. In the of a group or to be as a In some the same be of the volume of blood as a CPB and Hypothermia has well established effects on In hypothermia to the during CPB and In is an of hypothermia with of that warm CPB have but the outcome are not et al. in of patients CPB at sign]C or with and that was most in the hypothermic and that these patients showed the of blood was also in the hypothermic Finally, the effects of hypothermia on in the hypothermic group but was effect on in patients warm CPB. by the same group the effect of CPB on and and the to the warm group hypothermic patients demonstrated levels of the greater reduction in and greater and of and greater blood and levels of CPB cardiopulmonary between the with from the of of Surgery to examine the effect of bypass on postoperative was performed by et al. CPB at 35-37[degree sign]C or sign]C were for blood and over and postoperative Although of the patients not a of this is that was by Over the patients in the warm group not an had a blood than similar hypothermic CPB were by CPB temperature, in the patients not In those were in the normothermic group at and h Clinical studies not to examine blood have also this studies in blood between or reduced blood in the warm group but greater blood in the warm CPB of blood and blood from et al. of normothermic systemic perfusion during bypass a CPB and the Because of the of CPB with neurologic the surgical reintroduction of normothermic CPB was of to a variety of of injury be from the warm technique because of the of the effects of However, the initial reports from the Toronto group not an of with warm bypass the change in practice physiologic and outcome O2 and Warm CPB Although studies had physiologic variables during warm these studies were performed during the CPB rather than during continuous normothermic CPB. the introduction of normothermic CPB at the et al. O2 in patients to CPB at or sign]C and that venous occurred in than of normothermic CPB patients the of CPB but by of CPB. This from the demonstrated in hypothermic patients showed venous O2 during these et al. conducted a to the O2 during normothermic CPB. this demonstrated that during CPB at metabolic rate and oxygen were from the state a reduction in CPB was in blood for the reduction in O2 that oxygen was were difficult to with the of venous during early CPB However, of the indicated that sign]C of was required to normothermia during the early of CPB was in the early venous and from the standpoint of O2 normothermic CPB with hemodilution could be well in a patient or a of O2 and normothermic CPB in adult are from et al. bypass temperature, and oxygen in addition to oxygen being with hemodilution and reports that is during normothermic CPB The literature also that the to is In fact, physiologic during normothermic CPB that many of the changes to CPB are a of the change and CPB is conducted with normothermia and blood and oxygen at the are largely from the nonbypass state Warm and Although the warm CPB technique to over neurologic to the in large reports were The Warm from Toronto an outcome of patients CPB at sign]C or and the of at was However, in the same et al. from a of patients CPB at sign]C or sign]C during This between and be greater body is at the or Because the blood the aortic to the and because the has a high blood on a a of are delivered to the as and have blood flows and change has been to these be than during CPB could in a of The literature that be the most of ischemic neurologic hypothermia be the most a is the of temperature, or temperatures the greatest although the that these Hypothermia and of the in CPB management between the studies from and Toronto is to the of Although a of to the outcome between the institutions, the sign]C has been There is in the literature that small of hypothermia have effects on and in In a et al. the effect of small in and on the of ischemic Although not hypothermia had a effect on At ischemic was at ischemic injury was in additional and at injury was demonstrated At injury was most and et al. that the effect of hypothermia is for of the of on in a investigations also demonstrated the effects of small of in ischemic The is to management during sign]C of on in the and of of was performed in There was between and are were by of by not with from The effect of and hypothermia on and of with the outcome in the Toronto and studies, has been in warm bypass from normothermia to hypothermia or CPB. and The to CPB The from Toronto and be for the and of the studies and that the introduction of the warm CPB technique has a in practice. studies also as an of the prompt of the academic community to an of the neurologic effects of warm CPB. Based on the observations from and Toronto and the that the practice was were and management techniques were again modified. The shift to or systemic temperatures has an in many operating this many of the of higher temperatures be the be and considerable is for the of CPB at Although with the warm CPB technique institutions and studies are not for management several observations be the management of the patient warm CPB. First, the warm cardioplegia technique but with cardioplegia be and in large myocardial on and retrograde However, with antegrade and retrograde sometimes be difficult to This is of greatest in patients with or with The temperature, and of for warm cardioplegia to Second, patients warm CPB have a than patients surgery at temperatures. In the of with higher flows or a this in during CPB and in the period. patients also have greater Third, during CPB, the between systemic O2 and demand is as a of hemodilution in the of a in O2 This be in a higher O2 CPB flows be a higher sometimes be Fourth, the of warm CPB from that of hypothermic CPB. relatively greater be as a greater for the for be reduced in this patient in the postoperative period, and greater of patient and variables be and this allow for earlier although the are not Finally, the during CPB is or are the most practical to although it is that these and The for CPB is an that with the physiologic that the to at practical patient First, although the literature CPB at large has been to of CPB performed at this with those at or higher temperatures. the practice of CPB is of an Second, because it is difficult to whole body normothermia a CPB at sign]C is being by some this is it be this practice has effects on postoperative O2 or that potential neurologic Third, the on the of on and be as rather than the increasing of and patients with additional investigation is Fourth, and perhaps most it to be be to be for of The of CPB be a between allow to and neurologic and and to an management I surgical and for and over the last as our practice has