Biomedical Engineering Reference
In-Depth Information
REMARK 500 0 LYS A 99 CG LYS A 99 CD 0.195
REMARK 500 0 HIS B 2 CB HIS B 2 CG 0.097
REMARK 500 0 GLU B 6 CB GLU B 6 CA 0.153
REMARK 500 0 LYS B 66 CG LYS B 66 CD 0.487
REMARK 500 0 LYS C 16 CD LYS C 16 CE 0.480
REMARK 500 0 LYS C 60 CE LYS C 60 NZ 0.199
REMARK 500 0 HIS D 2 CB HIS D 2 CG 0.112
REMARK 500 0 LYS D 144 CD LYS D 144 CE 0.405
Perspectives
During the design of a computer model, one of the major decisions is what perspective to use. The
three basic simulation perspectives are continuous, discrete, and hybrid discrete/continuous. These
perspectives, which differ in how the system states change with time and events, define the tools,
methods, and algorithms that should be used in the model coding phase of the modeling and
simulation process.
Continuous Simulation
The continuous simulation methods are most appropriate when what is of primary interest is the time-
varying nature of objects or processes in some real-world system. The variables in a continuous
model are assumed to vary continuously with advancing time. Because there is no instant of time
when the system is not in flux, continuous simulations are said to be time-driven. Behavior patterns
modeled as a mixture of differential and algebraic equations provide the basis for this simulation
perspective.
A differential equation defines a relationship between a continuous variable and its own rate of
change. To take an example from pharmacokinetics, consider the time-varying nature of the plasma
level of a drug ingested. Given the initial concentration of the drug in the body, the time since the
drug was ingested, and the rate at which the drug is absorbed in the gut, we can model the current
concentration of drug in the body with the following relationship:
In this equation, the fraction of the drug lost from the plasma per-unit time is represented by
KT
,
where
K
is a constant and
T
is time. The elimination of constant
K
is a function of the type of drug
administered, administration route, method of elimination or conversion, health of the patient, and
renal function. Drugs with a large number for
K
will be eliminated faster from the body than those
with a smaller number for
K
.
When this model of drug elimination is coded, the formula for drug plasma concentration becomes a
DO LOOP in which the value for
T
is incremented by an appropriate value,
dt
, with each loop.
Depending on what is being studied,
dt
might be 1 millisecond or 10 seconds. In pseudocode form,
