Java程序辅导

Study Guide for Gas Exchange Computer Lab
This computer model of pulmonary gas exchange is written in Java, and is
available on the WWW at http://omie.med.jhmi.edu/LungModel4.2.  B cause of a
peculiarity in the Netscape browser’s handling of Java applets, on Macintosh
computers the model currently works only with Microsoft Explorer.
The model allows the user to specify eleven input variables that together
determine a full set of values for ventilatory and gas exchange output variables.  New
for this year is the ability to specify a two-compartment model that simulates V/Q
abnormality and its effect on arterial blood gases.
Using this model to simulate both normal and abnormal states provides you
with an opportunity to see the relationships involved in gas exchange and to test your
knowledge of pulmonary physiology.
Wilmot C. Ball, Jr., M.D.
May 2000
How to Use the Program
Input variables are listed on the left side of the window.  To change one or more
variables, select each value and replace it  by typing, then click “Calculate”.  If you
enter a value outside the allowable range, the box turns red and a message appears.
“Reset” restores the standard normal input values and zeros all outputs.  A
combination of input variables that causes arterial or mixed venous PO2 to be
unacceptably low, or a PCO2 that is too high will give an error message. Results are
displayed in the four groups of output boxes.
Systematically work through the problems below until you have answers that
you believe are correct.  Students should work two to a computer, discussing the
expected outcome of each calculation before it is run and interpreting results after each
step.  Faculty will be available in the laboratory to help with problems, and there will
be a summary and review session immediately after the lab.
This program describes the behavior of the lung as a gas exchange organ,
showing the effect on blood gases of changes in ventilation, inspired gas, cardiac
output and hemoglobin concentration, as well as impairment of pulmonary function.  It
deliberately excludes the effect of compensatory processes such as reflex changes in
ventilation, renal compensation for changes in pH, and metabolic responses to hypoxia,
all of which would confound the basic gas exchange relationships.
Input Variables
SYMBOL NAME NORMAL UNITS ALLOWABLE RANGE
VT Tidal volume 500.0 ml 200-4500
VD Dead space 150.0 ml 100-450
f Resp. rate 15.0 breaths/min. 5-40
FIO2 Inspired O2 0.21 fraction 0.12-1.0
Altitude Above sea level 0.0 Meters 0-15,000
VO2 O2 uptake 300.0 ml/min. 150-2500
R Resp exchange ratio 0.8 ratio 0.7-1.0
Hb Hemoglobin conc. 15.0 Gm/dl 5-22
QT Cardiac output 5.5 L/min. 2-20
QS/QT Shunt fraction 0.0 ratio 0-0.4
DLO2 O2 diffusing cap. 32.0 ml/min./mm Hg 5-100
Output Variables
These are self-explanatory.  All partial pressures are in mm Hg; contents are in
ml/dl; minute ventilation (VE) and alveolar ventilation (VA) are in L/min.
Effects of Ventilation on Blood Gas Measurements
Start by resetting the model to normal, and then click “Calculate.” Record the
normal values in the first column of Table I.
Table I  Ventilation
       Experiment #1        Experiment #2        Experiment #3
Normal Predicted* Measured Predicted Measured Predicted Measured
FIO2
VT
VD
f
VE
VA
PaCO2
PAO2
PaO2
Arterial pH ­,   ¯,  « ­,   ¯,  « ­,   ¯,  «
* NOTE: When arrows are shown in a Predicted box, circle the direction of change that you expect.
For all other variables, you should be able to calculate a Predicted result before using the model.
Refer to your lecture handouts for the necessary equations.
Experiment 1.  How do you expect these values to change if respiratory
frequency is reduced by half?  Record your expectations. Make the change, click
calculate, and compare results with your predictions.  What name is given to the
resulting blood gas abnormalities?  Is the fall in alveolar PO2 exactly what you
calculated in advance, given the observed rise in PCO2?  It is OK to use the approximate
alveolar air equation, but recognize that results for alveolar PO2 may be off by a few
mmHg.
Experiment 2.  What effects on PaCO2 and PaO2 would you expect if you leave
the respiratory frequency at half normal but also change inspired oxygen to 28%?  Use
the Experiment  #2 column of Table I to record your predictions, then make the change
and explain your results.
Experiment 3.  Reset the model to normal and calculate.  Study the normal
values again and decide how you would have to change tidal volume in order to make
the steady-state value of arterial PCO2 exactly half normal.  Test your prediction to see
if you were correct.
Diffusion
Reset to normal and click “Calculate.”  Record the listed values in table II.  Now
lower diffusing capacity (DL)  to 12. While you cannot calculate predicted results,
decide what trend you expect to see as DL falls further.  Now lower DL stepwise to 6
ml/min./mm Hg, watching PaO2 as you do this.  Also look for changes in PACO2 and
PaCO2.  At each step, record the A-a O2 difference.  With DL = 6, increase VO2,
respiratory rate and cardiac output by 50% to simulate mild exercise.
  Notice that arterial PO2 hardly changes at first, then falls quite rapidly, with a
real plunge when O2 uptake is increased. Then change inspired O2 to 30% and note the
effect.  Explain this result.
Table II  Diffusion
Normal Lower DL Exercise 30% O2
DLO2 32 12 10 8 6 6 6
O2 uptake 300 300 300 300 300 450 450
Resp rate 15 15 15 15 15 22.5 22.5
QT 5.5 5.5 5.5 5.5 5.5 8.25 8.25
PAO2
PaO2
A-a O2 diff
Shunt
Reset to normal, and record in Table III the listed variables for shunt values of
zero to 30% (.30) in steps as shown in the table.  In particular, note the changes in
arterial PO2 as you go.  With a shunt of 30% note the effect of increasing inspired O2 to
30%.  How does this compare with the effect of increasing inspired O2 when there is a
diffusion defect?  Why does the PaCO2 remain nearly constant throughout?
Table III   Shunt
Normal Increasingshunt 30% O2
Shunt 0 0.05 0.10 0.20 0.30 0.30
PaO2
PaCO2
V/Q Imbalance
The effects of V/Q imbalance on gas exchange can be simulated by a lung model
that has two or more parallel compartments with different V/Q ratios.  Pull down the
V/Q menu at the bottom of the input panel and select “V/Q Mismatch Balanced
Example”.  The resulting table in this location shows that V and Q are evenly
distributed in this new two-compartment model, and gives the resulting V/Q ratios.
Click “Reset” and then “Calculate”.  Results for end-capillary blood from the two
compartments are shown at the upper right and the arterial values at the lower right.
Verify that these are the same as the values in Table I, and record them in column 2 of
Table IV.
Table IV   V/Q imbalance
No Mismatch Example 2 Higher VT Higher FIO2 Example 3
VT 500 500 500 500
FIO2 0.21 0.21 0.21 0.30 0.21
PaCO2
PaO2
Select “V/Q Mismatch Example 2.”  The V/Q table now specifies that 10.5% of
the V and 50% of the Q supply the low V/Q compartment, while the rest go to the high
V/Q compartment.  Note how different the V/Q ratios are.  Before you calculate,
predict which of the end-capillary output variables will show a higher value in the low
V/Q compartment.  Then click “Calculate” and see if you were correct.  Notice the
abnormal arterial blood gas values, and enter these in column 3.  Output values at the
lower left under Riley Model tell us that these values are equivalent to those that would
be produced by a dead space of 37% (30% is the normal anatomic dead space, and 7%
is alveolar dead space), plus a shunt of 30%.  Why is the arterial PCO2 slightly elevated?
Try to restore PCO2  to normal by changing tidal volume, and record results in column
4.  Then reset to return to Example 2.
Change FIO2 to 0.30, calculate, and record your results in column 5.  Note the
magnitude of rise in arterial PO2.  If you compare the effect of 30% O2 here with that
seen in the last column of the Diffusion experiment, and also the last column of the
Shunt experiment, where a nearly identical “shunt” was present, how can you explain
the difference?  Incidentally, example 2 is consistent with moderately severe COPD
(Bronchitis and emphysema).
Select “V/Q Mismatch Example 3” and reset.   Note that ventilation is fairly well
matched but that almost all the blood flow goes to the low V/Q compartment.
Calculate and record the arterial blood values in column 6.  You will also see from
the last two variables for the Riley Model that there is the equivalent of a 22%
shunt, but that the dead space ratio is very large.  This model is an example of the V/Q
abnormality that may occur after a pulmonary embolus.
The final choice of “Custom V/Q Mismatch” allows the user to select any
combination of V and Q for the two compartments.  Note, however, that the model
may not reach a solution if extreme values are chosen.
Cardiac Output
If cardiac output falls with everything else held constant, how will blood gas
values change?  After you reset and calculate as before, record normal values and then
predict the direction of changes you will see.  Lower cardiac output in steps to 3.0
L/min. as shown in table V.  Explain what happened to arterial PCO2 and PO2, and also
what happened to mixed venous PO2 and saturation. How does the low mixed venous
PO2 affect tissue PO2?
Table V  Cardiac Output
Normal Predicted Lower output
QT 5.5 5.0 5.0 4.0 3.0
PaCO2 ­,   ¯,  « 
PaO2 ­,   ¯,  « 
PVO2 ­,   ¯,  « 
SvO2 ­,   ¯,  « 
Hemoglobin
How does anemia affect gas exchange?  Reduce blood hemoglobin concentration
as shown in table VI and record the effects on blood gas values.  The low mixed venous
O2 when Hgb falls to 7.5 suggests impaired tissue oxygenation, but in the intact human
this level of anemia is very well tolerated because of a compensatory physiologic
change.  Can you figure out what change this is and use the model to see if it would
improve the oxygenation of peripheral tissues?
Table VI   Hemoglobin
Normal Low hemoglobin Compensated*
Hb 15 10 7.5 7.5
PaCO2
PaO2
PvO2
SvO2
*Change _______________ to ___________.