Biomedical Engineering Reference
In-Depth Information
of physiological responses of cortical neurons so that the most
anesthesia-resistant properties can be readily identified”
(112)
and
studied under controlled settings. “Such maps should not lead
to false conclusion, however, that they define only the functions
carried out in a given cortical region, or that the sensory prop-
erties are reflected in exactly the same physiological manner as
in the awake [state]”
(112)
. Successful in vivo fMRI and neu-
rophysiologic studies depend on maintenance of normal physi-
ologic state and sustaining the appropriate depth of anesthesia.
Whether the subject is awake or anesthetized, optimal shimming
conditions for fMRI scans are always desirable. Below, we discuss
some insights into potential pitfalls and, where possible, how to
avoid them.
The well being of experimental animals, whether under anesthe-
sia or awake, should be monitored and, whenever possible, con-
trolled. Usually, the overall physiologic state of the subject should
be judged at three levels: Central nervous system (e.g., by EEG,
MUA), cardiovascular system (e.g., by blood pressure, heart rate),
and respiratory system (e.g., by pCO
2
, respiratory rate). In our
studies, all three components are controlled and measured to
assess the anesthetized state. To provide external ventilation for
the respiratory control, we used neuromuscular blocking agents
which do not act on the central nervous system. Furthermore, we
monitor analgesic level frequently. Blood pressure provides a con-
tinuous indicator of anesthesia/analgesia depth as mildly noxious
stimuli should not cause increases in the instantaneous pressure of
more than
4.2.1. Importance of
Physiological State
10%
(113)
. Additional doses of anesthetic agent(s)
must be applied if there are indications of discomfort.
Arterial blood samples drawn periodically to monitor pCO
2
and pH provide very important measures of systemic physiology.
In the rat, the pCO
2
is in the range 35-40 mmHg under optimal
steady-state conditions. High pCO
2
leads to a reduced pH and
dilatation of vessel walls in an attempt to increase flow and thereby
remove more of the potentially toxic acidic substances from tissue.
This situation will also affect the baseline CBF level and therefore
affect the BOLD response and its temporal dynamics. Low pCO
2
has the opposite effect. Both situations interfere with the tis-
sue's ability to adjust flow in response to activity changes, thereby
constituting a serious
caveat
to the interpretation of hyperemic
responses.
In freely breathing animals, blood gas tension is a crucial
factor in determining the physiological state of the anesthetized
state. Excluding olfactory studies, we mechanically ventilated
through a tracheotomy and controlled both the rate and vol-
ume of breathing. Although buffering mechanisms in the blood
exist to keep the pH within a narrow optimal range (
∼
7.4), they
are not able to do so indefinitely in a situation where breathing
∼
