Hypocapnia Causing Cerebral Desaturation

Mechanism of Injury

• Cerebral vasculature vasoconstricts during a state of hypocapnia and vasodilates during hypercapnia. Consequently, the partial pressure of carbon dioxide is one of the most influential determinants of cerebral blood flow.([FOOTNOTE=Denault A, Deschamps A, Murkin JM. A proposed algorithm for the intraoperative use of cerebral near-infrared spectroscopy. Semin Cardiothorac Vasc Anesth. 2007;11(4):274-281.],[ANCHOR=View Abstract],[LINK=/content/mitg/websites/languagemasters/emea/en-gb/support/clinical-evidence.html?id=297618])
• The reactivity of the cerebral vasculature to alterations in carbon dioxide may be impaired by increasing time of cerebral desaturation.([FOOTNOTE=Kadoi Y, Kawauchi C, Kuroda M, et al. Association between cerebrovascular carbon dioxide reactivity and postoperative short-term and long-term cognitive dysfunction in patients with diabetes mellitus. Journal of anesthesia. 2011;25(5):641-647.],[ANCHOR=View Abstract],[LINK=/content/mitg/websites/languagemasters/emea/en-gb/support/clinical-evidence.html?id=883106])
• Carbon dioxide becomes more soluble during hypothermia, resulting in decreased partial pressure and alkalosis. Consequently, two strategies utilized for acid-base management during hypothermia have a significant influence on cerebral oxygen saturation during CPB :
  • pH-stat
    • Definition – Arterial PaCO2 is temperature adjusted to 40 mmHg as hypothermia in induced([FOOTNOTE=Abdul Aziz KA, Meduoye A. Is pH-stat or alpha-stat the best technique to follow in patients undergoing deep hypothermic circulatory arrest? Interact Cardiovasc Thorac Surg. 2010;10(2):271-282.],[ANCHOR=View Abstract],[LINK=/content/mitg/websites/languagemasters/emea/en-gb/support/clinical-evidence.html?id=883093])
    • Effect on cerebral perfusion –
      • Higher CO2 levels cause vasodilation and hyperperfusion⁴
      • Autoregulation is inhibited([FOOTNOTE=Halstead JC, Spielvogel D, Meier DM, et al. Optimal pH strategy for selective cerebral perfusion. European journal of cardio-thoracic surgery : official journal of the European Association for Cardio-thoracic Surgery. 2005;28(2):266-273; discussion 273.],[ANCHOR=View Abstract],[LINK=/content/mitg/websites/languagemasters/emea/en-gb/support/clinical-evidence.html?id=883103])
  • Alpha-stat
    • Definition – Arterial PaCO2 is maintained at 40 mmHg at 37 degrees and not corrected for induction of hypothermia³
    • Effect on cerebral perfusion –
      • Reduced CO2 levels result in vasoconstriction and reduced cerebral blood flow⁴
      • Autoreguation is maintained⁴
• Akca et al. demonstrated in constant flow rate cardiopulmonary bypass, hypercapnia positively influenced cerebral oxygen saturation as opposed to normocapnia.([FOOTNOTE=Akca O, Sessler DI, Delong D, Keijner R, Ganzel B, Doufas AG. Tissue oxygenation response to mild hypercapnia during cardiopulmonary bypass with constant pump output. British journal of anaesthesia. 2006;96(6):708-714.],[ANCHOR=View Abstract],[LINK=/content/mitg/websites/languagemasters/emea/en-gb/support/clinical-evidence.html?id=297503])

Associated Outcome

• A systematic review evaluating the effect of the two acid-base management strategies on intraoperative and postoperative outcome in patients undergoing deep hypothermic circulatory arrest suggested that
  • Pediatric patients had improved outcome with pH-stat management³
  • Adult patients had improved outcome with alpha-stat management³
• A randomized controlled trial evaluating the effect of the two acid-based management strategies determined that patients undergoing coronary artery bypass graft receiving alpha-stat management had less impairment of cerebral autoregulation and reduced postoperative cerebral dysfunction([FOOTNOTE=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. The Journal of thoracic and cardiovascular surgery. 1996;111(6):1267-1279.],[ANCHOR=View Abstract],[LINK=/content/mitg/websites/languagemasters/emea/en-gb/support/clinical-evidence.html?id=883117])