Cardiopulmonary bypass management

Physiology - Cardiovascular

Goals of Cardiopulmonary Bypass

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]

Alpha stat

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 stat

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

Temperature

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

Rewarming to 34C (as opposed to 37C) may benefit “high risk” patients [Nathan et al, 2004; Nathan et al, 2001]

Hematocrit

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]

Glucose

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)

Summary

  • 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

Sources

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    Br J Anaesth;1992 Jul;69(1):51-7

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    J Thorac Cardiovasc Surg. 1996 Apr;111(4):900-1

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    J Thorac Cardiovasc Surg. 1997 Mar;113(3):619-20

    [PubMed: 8642829]

    Anesth Analg;2003 Mar;96(3):644-50, table of contents

    [PubMed: 12598237]

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2009

Year asked