XI. Fluids and Electrolytes
A. Should the overnight deficit be replaced?
(hourly fluid maintenence x hours N.P.O.)
1. Probably not.
2. Exceptions include patients undergoing
carotid artery repair, aneurysm repairs
and patients with stenotic valvular
lesions such as aortic stenosis.
3. Also consider replacement if there are
clinical signs of hypovolemia, such as
a sagging BP in the face of what seems
to be inadequate anesthesia, an a-line
tracing baseline that varies with
respiration, falling urine output, etc.
4. Remember, it is better to run these pts.
dry, as a lower vascular volume often
helps to maintain a lower brain volume.
Remember the axiom: "If it isn't broken,
don't fix it!"
B. Urinout output should be replaced cc for cc?
C. Which crystalloid should be used?
1. The main factor governing fluid movement
into and out of the brain is osmolality.
Fluids should therefore be as isotonic as
possible. Ringers has an osmolality of
273, making it the most hypotonic of the
crystalloids normally used. Plasmalyte
(295) and Normal Saline (308) are better
choices.
D. When should colloid be used?
1. Unless otherwise indicated, I start
limiting crystalloid and giving primarily
colloid after 2 to 3 liters of the former.
2. Labile blood pressures at induction in a
known hypertensive patient often means
that the patient is hypovolemic. Colloid
is a good choice here.
E. When should blood loss be treated?
1. Put the suction cannisters where you can
easily touch them from the anesthesia
machine. Remember that a plastic drape
with a pouch in the bottom may be placed
around the head. Get up frequently and
walk around the OR table to assess blood
loss. Write down a number for EBL on the
OR record no matter how wild a guess it
is.
2. This has to be individualized to the
patient. Aneurysm patients, with their
propensity for vasospasm, should be more
aggresively transfused. Some neurosurgeons
do not want the Hct to fall below 30.
3. The fluid restriction and forced diuresis
that occurs in neuroanesthesia may cause
the Hct to remain stable or even rise in
the face of anemia. Consider this when
hemodynamic instability and increasing
lactate levels occur.
F. How can you judge if blood volume is low?
1. Maintain strict I&O records.
2. Lactate levels. A normal lactate is 1. A
rising lactate may indicate hypoperfusion.
3. Variation of blood pressure with
ventilation greater than 10 mm Hg.
4. CVP
a. A normal CVP is 4 to 8.
b. The CVP is usually employed as a trend
indicator to be considered with all
the other indicators. It is usually
not necessary to automatically give
fluid to maintain a given CVP but
strong consideration should be given
to doing so.
(1) The patient in the sitting
position will require maintenance
of the CVP to reduce risk of air
embolism.
(2) Aneurysm patients need their CVP
maintained.
c. The CVP is exquisitly sensitive to
transducer height. Before making a
change in table height or table
rotation, glance at the CVP. Then make
the table adjustment and a subsequent
adjustment to transducer height so
that CVP is the same as before the
table turn. Putting the patient in
Trendelenburgh establishes a new
value. Mark the OR record as to the
change in head up or down. If the CVP
takes a sudden jump, think about a
change in Trendelenburgh.
5. Urine output
a. If there is no urine output after
mannitol, suspect a kink in the Foley
catheter.
6. A rising serum Na: consider SIADH
7. Changes in HCT. A Hct that is constant in
the face of continuing blood loss
indicates a possible the need for fluids.
Another possibility is that the patient
was overhydrated earlier.
8. Excessive response to vasodilators
9. Hypoxemia, tachycardia, hypotension
10. Measuring the serum osmolality may assist
in fluid balance. However, after mannitol
is given, these numbers may not mean much.
G. Blood glucose
1. Glucose is kept under control in
neuroanesthesia.
2. Patients on steroids, as well as known
diabetics may have elevated glucose.
a. Treat with 5-10 units boluses of IV
regular insulin at 30-45 min intervals
with a goal of serum glucose under
200.
H. Potassium
1. Hypokalemia is a strong possibility when
forced diuresis is employed. It is not always
necessary to replace K+ when it is > 3.0.
Except when more losses are anticipated, the
kidneys do a very good job of adjusting the balance.
Strict replacement is important in certain
patients, such as those taking digoxin.
Brain Protection