Optical assessment of the response in cerebral metabolism to mean arterial pressure during the transition onto cardiopulmonary bypass

When transitioning onto cardiopulmonary bypass (CPB) during cardiac surgery, blood flow to the brain is maintained by controlling the CPB flow rate and mean arterial pressure (MAP). CPB flow rates are based on patient body mass, and a MAP target of 60 mmHg is based on clinical experience and guidelines for CPB. However, studies have shown that up to 20% of the population has limited cerebral autoregulation and that conditions such as hypertension can exceed an individual's autoregulatory limits, leaving room for potential adverse cerebral events. Therefore, maintenance of adequate cerebral blood flow (CBF), oxygen delivery, and metabolism during surgery plays a critical role in reducing the risk of neurological complications. Given its sensitivity to tissue oxygen saturation (StO(2)), near-infrared spectroscopy (NIRS) is frequently used for intraoperative neuromonitoring modalities; however, StO(2) is not a direct marker of CBF, or the energy demands of brain tissue. CBF can be measured by diffuse correlation spectroscopy (DCS) and the unique absorption features of cytochrome c oxidase (oxCCO) offers a means of assessing oxygen metabolism. In this study, an in-house built hyperspectral NIRS/DCS system was used to continuously monitor changes in the redox state of oxCCO (Delta oxCCO), StO(2), and CBF in fifteen patients when transitioning onto CPB, with the purpose of evaluating the relationship between MAP on pump and brain blood flow and metabolism. Results demonstrated a nonsignificant Delta oxCCO (-0.13 +/- 0.12 mu M) in those patients with MAP > 70 mmHg, while a significant decrease in Delta oxCCO (-0.69 +/- 0.17 mu M) was found for patients for whom their MAP dropped to < 50 mmHg when placed on CPB. These results indicate that Delta oxCCO monitoring has the capability of providing real-time assessment of the effect of MAP on brain health during cardiac surgery, which could help reduce the incidence of cerebral complications.