Goals of Cardiopulmonary Bypass
- Provision of a Surgical Field
- Myocardial Protection
- DO2 and RCO2
- Temperature Management
- Glucose Management
- Prepare for Separation
Management of Physiology
Ideal perfusion pressure not known. Cerebral autoregulation from 50-150 mm Hg. Confounded by chronic HTN, advanced age, diabetes. May be as low as 20 mm Hg when hypothermic. Excess perfusion may increase the risk of embolism
Ideal pump flow also not known. 1.2 L/min/m2 probably adequate with Hgb 7.3 during hypothermic CPB, although the common range is 1.6 – 3.0 L/min/m2. SvO2 >= 65% used by many perfusionists. Pump flow does not equal regional flow [Slater et al, 2001]
Charge on imidazole ring of histidine. pH 7.40 (pCO2 = 40) at 37C, thus during hypothermia pCO2 is actually lower and pH is higher. Multiple independent, prospective randomized trials have shown that α-stat during moderate hypothermia produces better neurologic outcomes than using pH-stat [Stephan et al; Murkin et al, 1995; Patel et al, 1996]. ACC/AHA Class I Level A Recommendation for adults undergoing moderate hypothermic CPB
pH 7.40 at actual core body temperature, CO2 often added, vasodilation. Impairs autoregulatory responses to changes in BP. May increase the risk of cerebral embolization, according to one study of 52 patients based on jugular venous oximetry [Kiziltan et al, 2003]. Data in pediatrics are conflicting but probably favor pH stat management
Rationale: potential benefits of hypothermia originally thought to be related to decreased metabolism but now believed to be related to delayed release of excitatory amino acids
Hypothermic vs. Normothermic
Cochrane Database Syst Rev. 2001;(1):CD002138 (Updated in 2009 with no new conclusions). 19 studies and 4311 patients. Trend towards a reduction in non-fatal strokes in the hypothermic group (OR 0.68 [0.43, 1.05]). Trend towards an increase in non-stroke-related perioperative deaths in the hypothermic group (OR 1.46 [0.9, 2.37]). When pooling all “bad” outcomes (stroke, perioperative death, myocardial infarction, low output syndrome, intra aortic balloon pump use) there was no significant advantage of either hypothermia or normothermia (OR 1.07 (0.92, 1.24)). Methodological flaw – cross clamp goes on and off when warm (and this may be when many of the potential infarcts occur, due to release of atherosclerotic debris)
Rewarming to 34C (as opposed to 37C) may benefit “high risk” patients [Nathan et al, 2004; Nathan et al, 2001]
Low Hematocrit is associated with poor outcomes. 21%: renal failure [Karkouti et al, 2005]. Multivariate association with % creatinine change [Swaminathan et al, 2003]. Perioperative stroke [Karkouti et al, 2005] (“Each percent decrease in hematocrit was associated with a 10% increase in the odds of suffering perioperative stroke”)
Evidence from prospective trials? Small study (n = 54) showed no difference in outcomes [Von Heymann et al, 2006]
There IS good evidence-based medicine for this. 400 adults undergoing on-pump CABG randomized to 80-100 mg/dL vs. < 200 mg/dL, more deaths (4 vs. 0, p = 0.061) and stroke (8 vs. 1, p = 0.020) in the intensive group [Gandhi GY et. al. Annals of Internal Medicine 146: 233, 2007]. Conclusion: aggressive treatment of hyperglycemia in cardiac surgery patients does not improve mortality and in fact increases the incidence of stroke (p = 0.020) and death (p = 0.061)
- Pressure vs. Flow: no good data to guide management. Traditionally, keep flows above 1.6 L/min/m2 and SvO2 > 65%
- Alpha vs. pH Stat: alpha stat in adults (Class I Level A recc.), probably pH stat in children
- Temperature: Cochrane DB review shows no difference in overall outcomes. Rewarming at 34C (as opposed to 37C) may be prudent
- Hematocrit: association with poor outcomes, no good prospective data
- Glucose: aggressive glucose management (goal 80-100 mg/dL) leads to worse outcomes
- Slater JM, Orszulak TA, Cook DJ. Distribution and hierarchy of regional blood flow during hypothermic cardiopulmonary bypass. Ann Thorac Surg. 2001 Aug;72(2):542-7.
- Stephan H, Weyland A, Kazmaier S, Henze T, Menck S, Sonntag H. Acid-base management during hypothermic cardiopulmonary bypass does not affect cerebral metabolism but does affect blood flow and neurological outcome. Br J Anaesth. 1992 Jul;69(1):51-7.
- Murkin JM, Martzke JS, Buchan AM, Bentley C, Wong CJ. AA randomized study of the influence of perfusion technique and pH management strategy in 316 patients undergoing coronary artery bypass surgery. I. Mortality and cardiovascular morbidity. J Thorac Cardiovasc Surg. 1995 Aug;110(2):340-8.
- Patel RL, Turtle MR, Chambers DJ, James DN, Newman S, Venn GE. Alpha-stat acid-base regulation during cardiopulmonary bypass improves neuropsychologic outcome in patients undergoing coronary artery bypass grafting. J Thorac Cardiovasc Surg. 1996 Jun;111(6):1267-79.
- Kiziltan HT, Baltali M, Bilen A, Seydaoglu G, Incesoz M, Tasdelen A, Aslamaci S. Comparison of alpha-stat and pH-stat cardiopulmonary bypass in relation to jugular venous oxygen saturation and cerebral glucose-oxygen utilization. Anesth Analg. 2003 Mar;96(3):644-50, table of contents.
- Rees K, Beranek-Stanley M, Burke M, Ebrahim S. Hypothermia to reduce neurological damage following coronary artery bypass surgery. Cochrane Database Syst Rev. 2001;(1):CD002138.
- Nathan HJ, Parlea L, Dupuis JY, Hendry P, Williams KA, Rubens FD, Wells GA. Safety of deliberate intraoperative and postoperative hypothermia for patients undergoing coronary artery surgery: a randomized trial. J Thorac Cardiovasc Surg. 2004 May;127(5):1270-5.
- Nathan HJ, Wells GA, Munson JL, Wozny D. Neuroprotective effect of mild hypothermia in patients undergoing coronary artery surgery with cardiopulmonary bypass: a randomized trial. Circulation. 2001 Sep 18;104(12 Suppl 1):I85-91.
- Karkouti K, Beattie WS, Wijeysundera DN, Rao V, Chan C, Dattilo KM, Djaiani G, Ivanov J, Karski J, David TE. Hemodilution during cardiopulmonary bypass is an independent risk factor for acute renal failure in adult cardiac surgery. J Thorac Cardiovasc Surg. 2005 Feb;129(2):391-400.
- Swaminathan M, Phillips-Bute BG, Conlon PJ, Smith PK, Newman MF, Stafford-Smith M. The association of lowest hematocrit during cardiopulmonary bypass with acute renal injury after coronary artery bypass surgery. Ann Thorac Surg. 2003 Sep;76(3):784-91
- von Heymann C1, Sander M, Foer A, Heinemann A, Spiess B, Braun J, Krämer M, Grosse J, Dohmen P, Dushe S, Halle J, Konertz WF, Wernecke KD, Spies C. The impact of an hematocrit of 20% during normothermic cardiopulmonary bypass for elective low risk coronary artery bypass graft surgery on oxygen delivery and clinical outcome–a randomized controlled study [ISRCTN35655335]. Crit Care. 2006;10(2):R58.