Stuart J. Weiss

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

BACKGROUNDnDeep 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.nnnMETHODSnIn 109 patients undergoing hypothermic circulatory arrest with neurophysiologic monitoring, five electrophysiologic events were selected for detailed study.nnnRESULTSnThe 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.nnnCONCLUSIONSnWith 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.

Deep hypothermic circulatory arrest: II. Changes in electroencephalogram and evoked potentials during rewarming.

BACKGROUNDnElectrophysiologic studies during rewarming after deep hypothermic circulatory arrest probe the state of the brain during this critical period and may provide insight into the neurological effects of circulatory arrest and the neurologic outcome.nnnMETHODSnElectroencephalogram (EEG) and evoked potentials were monitored during rewarming in 109 patients undergoing aortic surgery with hypothermic circulatory arrest.nnnRESULTSnThe sequence of neurophysiologic events during rewarming did not mirror the events during cooling. The evoked potentials recovered first followed by EEG burst-suppression and then continuous EEG. The time to recovery of the evoked potentials N20-P22 complex was significantly correlated with the time of circulatory arrest even in patients without postoperative neurologic deficits (r = 0.37, (p = 0.002). The nasopharyngeal temperatures at which continuous EEG activity and the N20-P22 complex returned were strongly correlated (r = 0.44, p = 0.0002; r = 0.41, p = 0.00003) with postoperative neurologic impairment. Specifically, the relative risk for postoperative neurologic impairment increased by a factor of 1.56 (95% CI 1.1 to 2.2) for every degree increase in temperature at which the EEG first became continuous.nnnCONCLUSIONSnNo trend toward shortened recovery times or improved neurologic outcome was noted with lower temperatures at circulatory arrest, indicating that the process of cooling to electrocerebral silence produced a relatively uniform degree of cerebral protection, independent of the actual nasopharyngeal temperature.

Intraoperative Echocardiographic Diagnosis of Inferior Vena Cava Stenosis After Cardiopulmonary Bypass

THE INFERIOR VENA CAVA (IVC) is responsible for about two thirds of the total venous return to the right atrium (RA). Obstruction of the IVC can be due to chronic disease or an acute insult. The etiology of chronic IVC obstruction includes congenital abnormalities, indirect compression of the IVC (retroperitoneal tumors, hepatic tumors, giant left atrium), or intravascular tumor extension (metastatic renal tumors). Chronic obstruction of the IVC is associated with findings of hepatic congestion, ascites, lower extremity edema, and venous thrombosis. The slow progression of disease results in collateral venous return to the superior vena cava (SVC) via the azygos, intrathoracic, and vertebral veins. In contrast, acute stenosis of the IVC can result in hepatic and renal dysfunction, severe hemodynamic instability, and cardiovascular collapse if left untreated. Acute IVC obstruction is rare and has been reported as a complication of operative procedures, including bicaval heart transplantation, orthotopic liver transplantation, mitral valve replacement, and extrinsic compression from implantation of a Nuss bar for repair of pectum excavatum. The authors describe a case of acute IVC stenosis at the RA junction that was diagnosed by intraoperative transesophageal echocardiography after cardiopulmonary bypass (CPB). The intraoperative echocardiographic findings included a localized area of narrowing at the IVC-RA cannulation site, turbulent flow into the RA, underfilled cardiac chambers with hyperdynamic biventricular activity, and dilated hepatic veins. The echocardiographic findings were correlated with invasive hemodynamic measurements, and a therapeutic intervention with return to CPB was performed. The authors recommend the routine examination of the IVC at the IVC-RA junction with color-flow Doppler and spectral Doppler interrogation of flow after cardiac surgery cases, especially in the case of a bicaval venous cannulation technique.

Chapter 10 – Intraoperative Echocardiography

Few areas in cardiac anesthesia have developed as rapidly as the field of intraoperative echocardiography. In the early 1980s, when transesophageal echocardiography (TEE) was first used in the operating room, its main application was the assessment of global and regional left ventricular (LV) function. Since that time there have been numerous technical advances: biplane and multiplane probes; multifrequency probes; enhanced scanning resolution; color flow, pulsed wave, and continuous wave Doppler; automatic edge detection; Doppler tissue imaging; three-dimensional (3D) reconstruction; and digital image processing. With these advances, the number of clinical applications of TEE has markedly increased. The common applications of TEE include (1) assessment of valvular anatomy and function, (2) evaluation of the thoracic aorta, (3) detection of intracardiac defects, (4) detection of intracardiac masses, (5) evaluation of pericardial effusions, (6) detection of intracardiac air and clots, and (7) assessment of biventricular systolic and diastolic function. In many of these evaluations, TEE is able to provide unique and critical information that was not previously available in the operating room (Box 10-1).