Cerebral Oxygen Metabolism Measured During Hypothermic Circulatory Arrest

Cerebral oxygenation during warming after cardiopulmonary bypass

OBJECTIVES

To evaluate jugular venous oxygen saturation (SjVO2), measured with a fiberoptic oximetry catheter, and brain tissue oxygen saturation, measured by near-infrared spectroscopy (NIRSO2), as monitors of cerebral oxygenation during cardiopulmonary bypass surgery.

DESIGN

Prospective, clinical study.

SETTING

Operating room of a Veterans Administration Hospital.

PATIENTS

Nineteen patients undergoing moderate hypothermic cardiopulmonary bypass surgery.

INTERVENTIONS

SjvO2 and NIRSO2 were monitored in the patients during the surgical procedure.

MEASUREMENTS AND MAIN RESULTS

Moderate hypothermic cardiopulmonary bypass surgery had two distinct cerebral hemodynamic phases. While the patients were hypothermic, SjvO2 averaged 80 +/- 7% and none of the patients had an increase in cerebral lactate production. During the rewarming period, however, reductions in SjvO2 to < 50% occurred in 16 (84%) patients and increased cerebral anaerobic metabolism developed in 11 (58%) patients. SjvO2 during rewarming was dependent on mean arterial pressure, with 60 mm Hg appearing to be a critical value. Two other factors appeared to also contribute to the jugular desaturation, a low hematocrit and a rapid warming time. The SjvO2 catheter had excellent performance during the surgery. The average difference between paired measurements of SjvO2 by the catheter and in blood samples was -0.4 +/- 4.25%, and the correlation between the two measurements was highly significant (r2 = .93; p < .001). The NIRSO2 trended with the SjvO2 in most patients (r2 = .63; p < .001).

CONCLUSIONS

The study confirms other studies showing that jugular venous desaturation can occur during rewarming after cardiopulmonary bypass surgery. Presently, SjvO2 appears to be a better monitor of cerebral oxygenation than NIRSO2. However, NIRSO2 has promise as a noninvasive monitor of cerebral oxygenation if future developments allow more quantitative measurements of oxygen saturation.

Pulse oximetry--clinical implications and recent technical developments

The pulse oximeter has been shown to be a reliable monitor of arterial oxygen saturation and has therefore been recommended as mandatory monitoring for patients during anaesthesia and intensive care. In 1989 two review articles on pulse oximetry were published (1, 2) and two years ago Severinghaus and Kelleher summarized the literature between 1989 and October 1991 (3). Our aim is to focus the discussion on technical aspects and applications of pulse oximetry with special attention centered on recent developments. This review is consequently an update on pulse oximetry since the end of 1991, and the first on technically-based publications in the two last decades.

Clinical experience with transcranial cerebral oximetry

Transcranial cerebral oximetry based on the principle of near-infrared spectroscopy has been successfully used in a variety of neurosurgical conditions, primarily those associated with disturbed cerebral circulation. The non-invasive technique of cerebral oximetry provides valuable information about brain oxygenation in patients with cerebral ischemia (due to occlusion or stenosis of the internal carotid artery). It is also used in intraoperative monitoring of carotid endarterectomy and surgical procedures performed under deep hypothermia and circulatory arrest, during neuroendovascular procedures, and in critical care settings (in patients with arterial vasospasm and during the terminal period). This article describes our preliminary clinical experience with the use of this new technique and summarizes the current literature on clinical and experimental use of transcranial cerebral oximetry.

Cognition-activated low-frequency modulation of light absorption in human brain

Animal model studies indicate light-absorption changes of the exposed animal brain in response to visual stimulation. Here we report observations of red-light absorbance changes, attributable to repetitive blood concentration changes in response to stimulation in the human brain frontal region by a cognitive process. These responses are observed as low-frequency recurrence of changes by Fourier transform analysis and are attributed to blood concentration change stimulated by the increased metabolic rate of brain tissue in cognitive function. A simple, portable dual wavelength spectrophotometer was attached noninvasively to the human forehead to measure the low frequency and power spectra of fluctuations of absorbances attributed to variations of brain blood concentration in the frontal region. The responses are associated with brain activity in responses to problem solving of analogies presented visually that require an associative function in the frontal region. The method of subtraction of test -rest Fourier transforms minimizes the arterial pulse frequency contributions and identifies specific frequencies--for example, 0.8, 1.6, 1.8 Hz in 24 of 28 tests of nine individuals (85%). Tests in which no increased brain activity was elicited (rest-rest) showed small differences. It is concluded that low-frequency recurrences of brain activity linked to blood concentration increases can be detected in human subjects with an optical device of potentially for simplified tests of cognitive function in the 0- to 3-Hz region and with modifications for wider band recordings in localized tissue volumes by time-resolved spectroscopy.