Neurophysiologic effects of retrograde cerebral perfusion used for aortic reconstruction

Perioperative electroencephalography in cardiac surgery with hypothermic circulatory arrest: a narrative review

OBJECTIVES

Cardiac surgery with hypothermic circulatory arrest (HCA) is associated with neurological morbidity of variable severity and electroencephalography (EEG) is a sensitive proxy measure of brain injury. We conducted a narrative review of the literature to evaluate the role of perioperative EEG monitoring in cardiac surgery involving HCA.

METHODS

Medline, Embase, Central and LILACS databases were searched to identify studies utilizing perioperative EEG during surgery with HCA in all age groups, published since 1985 in any language. We aimed to compare EEG use with no use but due to the lack of comparative studies, we performed a narrative review of its utility. Two or more reviewers independently screened studies for eligibility and extracted data.

RESULTS

Fourty single-centre studies with a total of 3287 patients undergoing surgery were identified. Most were observational cohort studies (34, 85%) with only 1 directly comparing EEG use with no use. EEG continuity (18, 45%), seizures (15, 38%) and electrocerebral inactivity prior to circulatory arrest (15, 38%) were used to detect, monitor, prevent and prognose neurological injury. Neurological dysfunction was reported in almost all studies and occurred in 0–21% of patients. However, the heterogeneity of reported clinical and EEG outcome measures prevented meta-analysis.

CONCLUSIONS

EEG is used to detect cortical ischaemia and seizures and predict neurological abnormalities and may guide intraoperative cerebral protection. However, there is a lack of comparative data demonstrating the benefit of perioperative EEG monitoring. Use of a standardized methodology for performing EEG and reporting outcome metrics would facilitate the conduct of high-quality clinical trials.

Evoked Potentials during Cardiac and Major Vascular Operations

Somatosensory evoked potentials are widely used in spine surgery to prevent injury to the spinal cord. However, their application in cardiac and major vascular surgery is largely unappreciated. This paper will review the unique stresses placed on peripheral nerves, spinal cord, and brain during these operations. In addition, the potential benefits of perioperative somatosensory evoked potentials monitoring are described in detail.

Time to loss of brain function and activity during circulatory arrest

PURPOSE

Brain function during the dying process and around the time of cardiac arrest is poorly understood. To better inform the clinical physiology of the dying process and organ donation practices, we performed a scoping review of the literature to assess time to loss of brain function and activity after circulatory arrest.

MATERIALS AND METHODS

Medline and Embase databases were searched from inception to June 2014 for articles reporting the time interval to loss of brain function or activity after loss of systemic circulation.

RESULTS

Thirty-nine studies met selection criteria. Seven human studies and 10 animal studies reported that electroencephalography (EEG) activity is lost less than 30seconds after abrupt circulatory arrest. In the setting of existing brain injury, with progressive loss of oxygenated circulation, loss of EEG may occur before circulatory arrest. Cortical evoked potentials may persist for several minutes after loss of circulation.

CONCLUSION

The time required to lose brain function varied according to clinical context and method by which this function is measured. Most studies show that clinical loss of consciousness and loss of EEG activity occur within 30seconds after abrupt circulatory arrest and may occur before circulatory arrest after progressive hypoxia-ischemia. Prospective clinical studies are required to confirm these observations.

Neurocognitive monitoring and care during pediatric cardiopulmonary bypass-current and future directions

Neurologic injury in patients with congenital heart disease remains an important source of morbidity and mortality. Advances in surgical repair and perioperative management have resulted in longer life expectancies for these patients. Current practice and research must focus on identifying treatable risk factors for neurocognitive dysfunction, advancing methods for perioperative neuromonitoring, and refining treatment and care of the congenital heart patient with potential neurologic injury. Techniques for neuromonitoring and future directions will be discussed.

Clinical Predictors for Prolonged Intensive Care Unit Stay in Adults Undergoing Thoracic Aortic Surgery Requiring Deep Hypothermic Circulatory Arrest

OBJECTIVE

The purpose of this study was to describe clinical predictors for prolonged length of stay in the intensive care unit (PLOS-ICU) after adult thoracic aortic surgery requiring standardized deep hypothermic circulatory arrest (DHCA); and to determine the incidence of PLOS-ICU after DHCA, univariate predictors for PLOS-ICU, and multivariate predictors for PLOS-ICU.

STUDY DESIGN

A retrospective and observational study. PLOS-ICU was defined as longer than 5 days in the ICU.

STUDY SETTING

Cardiothoracic operating rooms and the ICU.

PARTICIPANTS

All adults requiring thoracic aortic repair with DHCA INTERVENTIONS: None.

MAIN RESULTS

The cohort size was 144. The incidence of PLOS-ICU was 27.8%. The mortality rate was 11.1%. Univariate predictors for PLOS-ICU were age, stroke, DHCA duration, vasopressor dependence >72 hours, mediastinal re-exploration for bleeding, and renal dysfunction. Multivariate predictors for PLOS-ICU were stroke, vasopressor dependence >72 hours, and renal dysfunction.

CONCLUSIONS

PLOS-ICU after DHCA is common. The identified multivariate predictors merit further hypothesis-driven research to enhance perioperative protection of the brain, kidney, and cardiovascular system.

Major Clinical Outcomes in Adults Undergoing Thoracic Aortic Surgery Requiring Deep Hypothermic Circulatory Arrest: Quantification of Organ-Based Perioperative Outcome and Detection of Opportunities for Perioperative Intervention

OBJECTIVE

The purpose of this study was to describe clinical outcome after adult thoracic aortic surgery requiring standardized deep hypothermic circulatory arrest (DHCA), to determine mortality and length of stay, neurologic outcome, cardiorespiratory outcome, and hemostatic and renal outcome after DHCA.

DESIGN

Retrospective and observational.

SETTING

Cardiothoracic operating rooms and intensive care unit (ICU).

PARTICIPANTS

All adults requiring thoracic aortic repair with DHCA.

INTERVENTIONS

None. The study was observational.

MAIN RESULTS

The cohort size was 110. All patients received an antifibrinolytic. The mortality rate was 8.2%. The mean length of stay was 6.8 days (ICU) and 14.0 days (hospital). The incidence of stroke was 8.1% and postoperative delirium was 10.9%. The rate of postoperative atrial fibrillation was 43.6%; 19.1% required postoperative mechanical ventilation longer than 72 hours. Chest tube drainage was 931 mL for the first 24 hours. Postoperative dialysis was required in 1.8% of patients. Renal dysfunction occurred in 40% to 50% of patients, depending on the definition.

CONCLUSIONS

The protocol for DHCA at the authors' institution is associated with superior or equivalent perioperative outcomes to those reported in the literature. This study identified the need for further quantification of the clinical outcomes after DHCA in order to prioritize outcome-based hypothesis-driven prospective intervention in DHCA.

Retrograde cerebral perfusion as a method of neuroprotection during thoracic aortic surgery

Retrograde cerebral perfusion is commonly used as an adjunct to hypothermic circulatory arrest to enhance cerebral protection during thoracic aortic surgery. This review summarizes a large number of studies that demonstrate a spectrum of beneficial, neutral, and detrimental effects of retrograde cerebral perfusion in humans and experimental animal models. It remains unclear whether retrograde cerebral perfusion provides effective cerebral perfusion, metabolic support, washout of embolic material, and improved neurological and neuropsychological outcome.

Deep hypothermic circulatory arrest: I. Effects of cooling on electroencephalogram and evoked potentials

BACKGROUND

Deep hypothermia is an important cerebral protectant and is critical in procedures requiring circulatory arrest. The purpose of this study was to determine the factors that influence the neurophysiologic changes during cooling before circulatory arrest, in particular the occurrence of electrocerebral silence.

METHODS

In 109 patients undergoing hypothermic circulatory arrest with neurophysiologic monitoring, five electrophysiologic events were selected for detailed study.

RESULTS

The mean nasopharyngeal temperature when periodic complexes appeared in the electroencephalogram after cooling was 29.6 degrees C +/- 3 degrees C, electroencephalogram burst-suppression appeared at 24.4 degrees C +/- 4 degrees C, and electrocerebral silence appeared at 17.8 degrees C +/- 4 degrees C. The N20-P22 complex of the somatosensory evoked response disappeared at 21.4 degrees C +/- 4 degrees C, and the somatosensory evoked response N13 wave disappeared at 17.3 degrees C +/- 4 degrees C. The temperatures of these various events were not significantly affected by any patient-specific or surgical variables, although the time to cool to electrocerebral silence was prolonged by high hemoglobin concentrations, low arterial partial pressure of carbon dioxide, and by slow cooling rates. Only 60% of patients demonstrated electrocerebral silence by either a nasopharyngeal temperature of 18 degrees C or a cooling time of 30 minutes.

CONCLUSIONS

With the high degree of interpatient variability in these neurophysiologic measures, the only absolute predictors of electrocerebral silence were nasopharyngeal temperature below 12.5 degrees C and cooling longer than 50 minutes.

Hematocrit, volume expander, temperature, and shear rate effects on blood viscosity

Our goal was to determine and predict the effects of temperature, shear rate, hematocrit, and different volume expanders on blood viscosity in conditions mimicking deep hypothermia for cardiac operations. Blood was obtained from six healthy adults. Dilutions were prepared to hematocrits of 35%, 30%, 22.5%, and 15% using plasma, 0.9% NaCl, 5% human albumin, and 6% hydroxyethyl starch. Viscosity was measured over a range of shear rates (4.5-450 s(-1)) and temperature (0 degrees -37 degrees C). A parametric expression for predicting blood viscosity based on the study variables was developed, and its agreement with measured values tested. Viscosity was higher at low shear rates and low temperatures, especially at temperatures less than 15 degrees C (P: < 0.016 for all conditions in comparison with 37 degrees C). Decreasing hematocrit, especially to less than 22.5%, decreased viscosity. Hemodilution with albumin or 0.9% NaCl decreased blood viscosity more than hemodilution with plasma or 6% hydroxyethyl starch (P: < 0.01 for all cases). The derived mathematical model for viscosity as a function of temperature, hematocrit, shear rate, and diluent predicted viscosity values that correlated well with the measured values in experimental samples (r(2) > 0.92, P: < 0.001).

IMPLICATIONS

A theoretical model for blood viscosity predicted independent effects of temperature, shear rate, and hemodilution on viscosity over a wide range of physiologic conditions, including thermal extremes of deep hypothermia in an experimental setting. Moderate hemodilution to a hematocrit of 22% decreased blood viscosity by 30%-50% at a blood temperature of 15 degrees C, suggesting the potential to improve microcirculatory perfusion during deep hypothermia.

Predictors of adverse outcome and transient neurological dysfunction after ascending aorta/hemiarch replacement

BACKGROUND

This study was undertaken to determine predictors of adverse outcome and transient neurological dysfunction after replacement of the ascending aorta with an open distal anastomosis.

METHODS

All 443 patients (300 male, median age 63) undergoing replacement of the ascending aorta with an open distal anastomosis between 1986 and 1998 were included in the analysis. The ascending aorta alone was replaced in 190 (42.9%); 253 (57.1%) also had proximal arch replacement. Median hypothermic circulatory arrest (HCA) time was 25 minutes (range 12 to 68). Either death or permanent neurological dysfunction were considered adverse outcome (AO).

RESULTS

Adverse outcome occurred in 11.5% (51 of 443) of patients overall: in 7.4% of elective (20 of 269) or urgent (4 of 54) operations, but in 17% (19 of 113) of emergencies. Multivariate analysis of the group as a whole revealed that significant (p < 0.05) independent preoperative predictors of AO were age greater than 60 [odds ratio (OR) 2.2], hemodynamic instability (OR 2.7), and dissection (OR 1.9). For the 435 operative survivors, procedural variables predictive of AO were contained rupture (OR 2.8) and HCA time (OR 1.03/min). When only the 271 elective patients were analyzed separately, the need for a concomitant procedure (p = 0.009, OR 3.6) and HCA time (p = 0.002, OR 1.06/min) were the only predictors of AO in multivariate analysis. Transient neurological dysfunction (TND) occurred in 86 of 392 patients (22%). Significant predictors of TND for all patients without AO were age (OR 1.06/y), HCA time (OR 1.04/min), coronary artery disease (OR 2.2), hemodynamic instability (OR 3.4), and acute operation (OR 2.2). Survival of discharged patients was 93% at 1 year and 83% at 5 years.

CONCLUSIONS

Early elective operation and shorter HCA time during ascending aorta/hemiarch surgery will reduce both AO and TND.

Oxygen delivery during retrograde cerebral perfusion in humans

Retrograde cerebral perfusion (RCP) potentially delivers metabolic substrate to the brain during surgery using hypothermic circulatory arrest (HCA). Serial measurements of O2 extraction ratio (OER), PCO2, and pH from the RCP inflow and outflow were used to determine the time course for O2 delivery in 28 adults undergoing aortic reconstruction using HCA with RCP. HCA was instituted after systemic cooling on cardiopulmonary bypass for 3 min after the electroencephalogram became isoelectric. RCP with oxygenated blood at 10 degrees C was administered at an internal jugular venous pressure of 20-25 mm Hg. Serial analyses of blood oxygen, carbon dioxide, pH, and hemoglobin concentration were made in samples from the RCP inflow (superior vena cava) and outflow (innominate and left carotid arteries) at different times after institution of RCP. Nineteen patients had no strokes, five patients had preoperative strokes, and four patients had intraoperative strokes. In the group of patients without strokes, HCA with RCP was initiated at a mean nasopharyngeal temperature of 14.3 degrees C with mean RCP flow rate of 220 mL/min, which lasted 19-70 min. OER increased over time to a maximal detected value of 0.66 and increased to 0.5 of its maximal detected value 15 min after initiation of HCA. The RCP inflow-outflow gradient for PCO2 (slope 0.73 mm Hg/min; P < 0.001) and pH (slope 0.007 U/min; P < 0.001) changed linearly over time after initiation of HCA. In the group of patients with preoperative or intraoperative strokes, the OER and the RCP inflow-outflow gradient for PCO2 changed significantly more slowly over time after HCA compared with the group of patients without strokes. During RCP, continued CO2 production and increased O2 extraction over time across the cerebral vascular bed suggest the presence of viable, but possibly ischemic tissue. Reduced cerebral metabolism in infarcted brain regions may explain the decreased rate of O2 extraction during RCP in patients with strokes.

IMPLICATIONS

Examining the time course of oxygen extraction, carbon dioxide production, and pH changes from the retrograde cerebral perfusate provided a means to assess metabolic activity during hypothermic circulatory arrest.