Page 76 - Encyclopedia of Nursing Research

CEREBRAL ISCHEMIA n 43

status refers to the amount of information the complexities of the phenomenon, to test
(variances and covariances) available com- theoretical models specifying causal flow,
pared with the number of parameters that and to separate the effects of one variable C
are to be estimated. If the amount of infor- on another into direct and indirect effects.
mation equals the number of parameters to Although causal modeling cannot be used
be estimated, the model is “just identified.” If to establish causality, it provides information
the amount of information exceeds the num- on the strength and direction of the hypoth-
ber of parameters to be estimated, the model esized effects. Thus, causal modeling enables
is “overidentified.” In both cases, a unique investigators to explore the process by which
solution for the parameters can be found. one variable might affect another and to iden-
With the use of standard conventions, recur- tify possible points for intervention.
sive models are almost always overidentified.
When the amount of information is less than JoAnne M. Youngblut
the number of parameters to be estimated,
the model is “underidentified” or “unidenti-
fied,” and a unique solution is not possible.
nonrecursive models are underidentified Cerebral isCheMia
unless instrumental latent variables (a latent
variable for each path that has a direct effect
on one of the two latent variables in the recip- Cerebral ischemia is defined as inadequate
rocal causation relationship but only an indi- blood flow to the brain to meet meta-
rect effect on the other latent variable) can be bolic and nutritive needs of the brain tis-
specified. sue (Edvinsson, MacKenzie, & McCulloch,
Causal models can be analyzed with 1993). The severity of ischemia depends on
standard multiple regression procedures or the severity and duration of the reduction in
structural equation analysis programs, such cerebral blood flow (CBF) adversely affecting
as LISREL or EQS (see Structural Equation various functional and metabolic processes
Modeling). Multiple regression is appropriate as CBF decreases (Heiss & Rosner, 1983). The
when each concept is measured with only one brain stores no oxygen and little glucose and
empirical indicator. Path coefficients (stan- is thus dependent on a constant supply of
dardized regression coefficients, β ) are esti- oxygen and glucose from the blood.
mated by regressing each endogenous variable Cerebral ischemia may be focal or
on the variables that are hypothesized to have global, depending on whether a part of the
a direct effect on it. The fit of the model is cal- brain or the entire brain is ischemic. Focal
culated by comparing total possible explained cerebral ischemia occurs when a major cere-
variance for the just identified model with bral artery becomes occluded or constricted
the total explained variance of the proposed from arterial spasm, emboli, or thrombo-
overidentified model. Data requirements for sis. Global ischemia occurs from an overall
path analysis are the same as those for mul- decrease in CBF, for example, after cardiac
tiple regression: (a) interval or near-interval arrest. Global oxygen deprivation of the
data for the dependent measure; (b) interval, brain may also occur as a result of asphyxia,
near-interval, or dummy-coded, effect-coded, anemia, hypoxia, or near drowning. nurses
or orthogonally coded categorical data for the are responsible for identifying individuals
independent measures; and (c) 5 to 10 cases at risk for focal or global cerebral ischemia.
per independent variable. Assumptions of nursing assessment of early symptoms of
multiple regression must be met. cerebral ischemia can allow for intervention
In summary, causal modeling tech- and minimize the probability of permanent
niques provide a way to more fully represent damage.