Effects of Anesthetics on the pO2 of the Brain
Taie* S, Leichtweis S B, Liu KJ, Grinberg OY, El-Kadi H, Miyake M, Swartz HM
EPR Center for the Study of Viable Biological Systems, Department of
Diagnostic Radiology,
Dartmouth Medical School, HB7785, Hanover, NH, USA, 03755
*On leave from Kagawa Medical School
INTRODUCTION: The effects of anesthesia on tissue oxygenation are
very complex due to the many
different ways in which they can affect perfusion and metabolism. While very
useful data can be
obtained from measurements of pO2 within various compartments of the
vascular system, such
measurements do not necessarily provide accurate information on the pO2 in
the tissue. Electron
Paramagnetic Resonance (EPR) Oximetry has the potential for non-invasively
carrying out
repeated direct measurements of pO2 in tissues during the course of
anesthesia. We have
initiated a series of studies of the effects of different types of
anesthetics on the pO2 of the brain
in rats under similar physiologic conditions with systematic variation of
the oxygen in the
breathing gas.
METHODS: The experimental animals were male Sprague-Dawley rats
weighing 350-450 g with 6
rats per group with 4 different anesthetic agents: ketamine / xylazine (100
mg/10 mg/KG IM,
pentobarbital (80 mg/kg IP), isoflurane (1.5 MAC, 2.2%), and halothane (1.5
MAC, 1.5%).
Approximately 7 days prior to the experiments with the anesthetics oxygen
sensitive lithium
phthalocyanine crystals were placed directly via a spinal needle into the
brain at a depth of 3 mm
from the surface of the skull through 1 mm drilled holes located 4.5 mm from
the midline and 1
mm in front of the bregma, using stereotactic techniques. After an adequate
level of anesthesia
was achieved, a tracheostomy tube was placed and positive pressure
ventilation started with
continuous monitoring of inspiratory & expiratory O2 and CO2 and
concentration of inhalation
anesthetics. In addition a polyethylene arterial catheter was placed in the
left femoral artery for
continuous monitoring of BP and periodic blood gas measurements. Body
temperature was
maintained at 37o on a heated pad (temperature monitored by a rectal probe).
FIO2 was
maintained at 33 % during surgical manipulation and during the first 30
minutes of pO2
measurement in the brain. Continuous measurements on pO2 of the brain in
rats were obtained
during three or four 30 minute periods of constant gas perfusion during
which the ventilating gas
was, sequentially, 21%, 33%, 50%, (and in some, 100%) oxygen. The
ventilatory volume was
continuously adjusted to maintain the measured pCO2 at 35-40 torr.
RESULTS AND DISCUSSION: The different anesthetics resulted in very
different patterns of response of
the pO2 in the brain. The two inhalation anesthetics had similar
relationships between the
amount of oxygen in the breathing mixture and the pO2 in the brain, but the
absolute value was
much higher for isoflurane. The values of pO2 in the brain were lower for
the injectable
anesthetics and the pattern of response differed. These variations could not
be explained simply
on the basis of the arterial pO2, ventilation, blood pressure, or heart
rate.
CONCLUSIONS: The effects of anesthetics on the pO2 of the brain
cannot be directly and simply
related to conventional cardiovascular parameters. It can be measured
directly using EPR
oximetry. Different anesthetic agents can have considerably different
effects on the local pO2 in
the brain under what appear to be physiologically similar conditions.
Presented at the 26th annual ISOTT conference August, 1998 and published in
the
conference proceedings: S. Taie, S. Leichtweis, K.J. Liu, M. Miyake, O.
Grinberg, E. Demidenko,
and H. M. Swartz, “The Effects of Ketamine/xylazine and Pentobarbital
Anesthesia on Cerebral Tissue
Oxygen Tension, Blood Pressure, and Arterial Blood Gas in Rats,” Adv. Exp.
Med. Biol., 471:189-198
(1999).