Reply to the Editor

Abstract

We appreciate the interest in our recently published report1Ono M. Brady K. Easley R.B. Brown C. Kraut M. Gottesman R.F. et al.Duration and magnitude of blood pressure below cerebral autoregulation threshold during cardiopulmonary bypass is associated with major morbidity and operative mortality.J Thorac Cardiovasc Surg. 2014; 147: 483-489Google Scholar by Dr Brassard in his letter to the Editor.2Brassard P. Is hypotension or tissue oxygenation responsible for morbidity and mortality following cardiopulmonary bypass?.J Thorac Cardiovasc Surg. 2014; 147: 1430Google Scholar The major theme of his letter brings up the age-old rhetorical question of whether flow or blood pressure is more important for organ perfusion during cardiopulmonary bypass. Physiologically, the body has many mechanisms to preferentially perfuse the brain at the expense of visceral organs, which was adroitly demonstrated in a study by Rhee et al.3Rhee C.J. Kibler K.K. Easley R.B. Andropoulos D.B. Czosnyka M. Smielewski P. et al.Renovascular reactivity measured by near-infrared spectroscopy.J Appl Physiol (1985). 2012; 113: 307-314Google Scholar In that study, renal perfusion was compromised by reduced blood flow at blood pressures greater than the lower limit of cerebral autoregulation. Thus, cerebral blood flow was preserved because the blood pressure was within the autoregulation range even when renal perfusion was reduced by 50%. These results further support the notion that during the normal conduct of cardiopulmonary bypass in which the blood flow is kept within a physiologic range, the blood pressure will be the most important determinant of cerebral perfusion. Dr Brassard2Brassard P. Is hypotension or tissue oxygenation responsible for morbidity and mortality following cardiopulmonary bypass?.J Thorac Cardiovasc Surg. 2014; 147: 1430Google Scholar cited studies by himself and others showing that the administration of phenylephrine and norepinephrine reduced the near-infrared spectroscopy–measured regional cerebral oxygen saturation (rScO2) but that this measurement was not affected by equivalent doses of ephedrine. We caution, however, in interpreting these data as an indication that the cerebral oxygen saturation has been compromised by the use of vasoconstrictors. Clinically available near-infrared spectroscopy monitors use algorithms to subtract light absorption from superficial tissue (eg, scalp, bone, pia) from deeper tissue to provide the rScO2 measurements.4Zheng F. Sheinberg R. Yee M. Ono M. Zheng Y. Hogue C. Cerebral near-infrared spectroscopy (NIRS) monitoring and neurologic outcomes in adult cardiac surgery patients: a systematic review.Anesth Analg. 2013; 116: 663-676Google Scholar The importance of extracranial contamination of rScO2 measurements has recently been highlighted in studies in which scalp ischemia induced by inflation of a circumferential cranial tourniquet affected the rScO2 readings.5Davie S.N. Grocott H.P. Impact of extracranial contamination on regional cerebral oxygen saturation: a comparison of three cerebral oximetry technologies.Anesthesiology. 2012; 116: 834-840Google Scholar Whether phenylephrine or norepinephrine induces vasoconstriction of the scalp blood vessels, rather than reduces the cerebral oxygenation per se, is an explanation of the lower rScO2 measurements when these drugs are given has not been excluded. Furthermore, whether this source of error in interpreting the rScO2 results has clinical importance when near-infrared spectroscopy is used as a trend monitor is unclear. More importantly, the use of rScO2 as a surrogate of cerebral blood flow for autoregulation monitoring analyzes the correlation of rScO2 with the mean arterial pressure at the low frequencies associated with autoregulation.6Brady K.M. Lee J.K. Kibler K.K. Smielewski P. Czosnyka M. Easley R.B. et al.Continuous time-domain analysis of cerebrovascular autoregulation using near-infrared spectroscopy.Stroke. 2007; 38: 2818-2825Google Scholar, 7Lee J.K. Brady K.M. Mytar J.O. Kibler K.K. Carter E.L. Hirsch K.G. et al.Cerebral blood flow and cerebrovascular autoregulation in a swine model of pediatric cardiac arrest and hypothermia.Crit Care Med. 2011; 39: 2337-2345Google Scholar, 8Brady K. Joshi B. Zweifel C. Smielewski P. Czosnyka M. Easley R.B. et al.Real-time continuous monitoring of cerebral blood flow autoregulation using near-infrared spectroscopy in patients undergoing cardiopulmonary bypass.Stroke. 2010; 41: 1951-1956Google Scholar These measurements are not dependent on the relative rScO2 values but, rather, on the relationship between the changes in rScO2 and blood pressure. Regardless, our ongoing randomized clinical trial of blood pressure management using autoregulation monitoring versus the standard of care of empiric blood pressure targets during cardiopulmonary bypass (clinicaltrials.gov registration NCT00769691) could shed light on whether “hypotension” versus its treatment with vasoconstrictors affects patient outcomes. (Our study was funded in part by grant R01HL092259 from the National Institutes of Health to Dr Hogue.) Is hypotension or tissue oxygenation responsible for morbidity and mortality after cardiopulmonary bypass?The Journal of Thoracic and Cardiovascular SurgeryVol. 147Issue 4PreviewI read with great interest the study by Ono et al1 in a recent issue of the Journal. The authors elegantly showed that the duration and magnitude of blood pressure less than the lower limit of cerebral autoregulation, characterized by a moving Pearson's correlation coefficient between the blood pressure and low-frequency near-infrared spectroscopy signals, was associated with morbidity and mortality after cardiopulmonary bypass. Full-Text PDF