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Ann Thorac Surg 2005;80:1460-1467
© 2005 The Society of Thoracic Surgeons

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Original article: Cardiovascular

Specific Bypass Conditions Determine Safe Minimum Flow Rate

^a Department of Cardiovascular Surgery, Children's Hospital Boston, Harvard Medical School, Boston, Massachusetts
^b Department of Biostatistics, Children's Hospital Boston, Harvard Medical School, Boston, Massachusetts
^c Department of Pathology, Children's Hospital Boston, Harvard Medical School, Boston, Massachusetts
^d Department of Surgery, University Hospital of Oulu, Oulu, Finland

Accepted for publication April 5, 2005.

^_* Address reprint requests to Dr Jonas, Department of Cardiovascular Surgery, Children's National Medical Center, 111 Michigan Ave, Washington, DC20010 (Email: rjonas{at}cnmc.org).

BACKGROUND: The purpose of this study is to define a safe minimum flow rate for specific bypass conditions using continuous monitoring with near-infrared spectroscopy and direct observation of the cerebral microcirculation.

METHODS: Two series of experiments (n = 72 in each) were conducted in which piglets were cooled to a temperature of 15°, 25°, or 34°C on cardiopulmonary bypass with hematocrit 20% or 30%, pH-stat management in all, followed by 1 or 2 hours of reduced flow (10, 25, or 50 mL·kg^–1 ·min^–1). Animals in series one had a cranial window placed over the parietal cortex to evaluate the microcirculation with intravital microscopy. Plasma was labeled with fluorescein-isothiocyanate-dextran for assessment of functional capillary density (FCD) and microvascular diameter. In series two, near-infrared spectroscopy was utilized to detect tissue oxygenation index (TOI). Outcome measures included histologic and neurologic injury scores.

RESULTS: The TOI during low flow and FCD during rewarming and after weaning from cardiopulmonary bypass were associated with neurologic injury. Failure of FCD to return to baseline during rewarming predicted worse functional and histologic outcome (p < 0.001). Regression analysis indicated that temperature and low-flow rate were multivariable predictors of TOI and FCD during rewarming (p < 0.001).

CONCLUSIONS: Tissue oxygen index derived from near-infrared spectroscopy is a useful real-time monitor for detecting inadequate cerebral perfusion during cardiopulmonary bypass. Minimal safe pump flow rate varies according to the conditions of bypass: using pH stat management and with an hematocrit of either 20% or 30%, a flow rate as low as 10 mL·kg^–1 ·min^–1 is safe for as long as 2 hours at a temperature of 15°C. However, under the same conditions at 34°C, a flow rate of 10 mL·kg^–1 ·min^–1 is very likely to be associated with neurologic injury.

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				D. Wypij, R. A. Jonas, D. C. Bellinger, P. J. Del Nido, J. E. Mayer Jr., E. A. Bacha, J. M. Forbess, F. Pigula, P. C. Laussen, and J. W. Newburger The effect of hematocrit during hypothermic cardiopulmonary bypass in infant heart surgery: results from the combined Boston hematocrit trials. J. Thorac. Cardiovasc. Surg., February 1, 2008; 135(2): 355 - 360. [Abstract] [Full Text] [PDF]

				M. K. Heinemann Invited commentary Ann. Thorac. Surg., October 1, 2005; 80(4): 1467 - 1467. [Full Text] [PDF]