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medsurg3
11/19/98

James Orsini

housekeeping items.
there 
are a lot of notes in the handout. the most important ones are the ones 
called Acid-Base balance fluid therapy.

history - 
Antoine someone, 
french guy, late 1700s, who identified Oxygen. He first described what 
happens when O2 is inhaled and CO2 is exhaled, during rest and during 
eating. this guy was such a revolutionary that he was killed during the 
French Revolution. Laborsier is his last name?

Acid base medicine:

pH 
is proportional to [base]/[acid]
[bicarb]/[CO2]

blood gas report:

PaO2 
52 (low)
PaCO2 62 (high)
pH 7.16 (low)
HCO3 20.5 (a tiny bit low)

these 
are all abnormal (this is a person, btw)
this is respiratory acidosis

pH 
- concentration of free H+ in a solution

PCO2 = the pressure exerted by 
CO2 in any gas mixture

plasma bicarb - concentration of bicarb in blood


BE: change in buffer base from its normal value

total buffer base: sum 
of all conjugate bases in 1 liter of whole blood

standard bicarb: the 
plasma bicarb concentration of whole blood eqiulibrated to a PCO2 of 40 
mmHg with Hb fully oxygenated at a temp of 39 C

acidemia: an abnormal 
state of blood in which pH is low
alkalemia: abnormal state of blood in 
which pH is high > 7.4

pH = pK + log[HCO3-]/[H2CO3}
henderson-hasselbach 
equation usually 6.1 + 1.3 = 7.4

remember the pK is just the pH of a 
substance in solution when half is dissociated and half is bound. 

blood 
pH -- [metabolic component]/[respiratory component]
metabolic component: 
plasma system --> [HCO3-]; whole blood system [BE]
respiratory component: 
plasma system --> PCO2, whole blood system PCO2

the buffer is in ECF and 
RBC --> bicarbonate is the measured one in ECF and in RBC, Hb, oxyHb, 
phosphates (org and inorg) are also buffers in the ECF and RBC.

buffers 
in general: the two important ones - plasma bicarb is 35% of the total 
buffer, and Hb+OxyHb are another 35% of buffering capacity. RBC bicarb is 
another 18%. the plasma proteins are only about 7%, the phosphates are 
about 5%.

an acid-base disorder can't be diagnosed with certainty from 
the plasma HCO3- alone. this is important. people often try to use one 
number, in isolation, for diagnostic purposes. you can't do that. you 
need to look at pH, PCO2, and 
[HCO3-]. you have to look at those three 
components.


aim of therapy: to correct the acid-base imbalance. 
therefore, it is important to establish the correct diagnosis.
remember - 
history is 75% of dx; signs are maybe another 20%...
signs of acidosis:


cardiovascular:
ventricular arrhythmias, shock
as pH goes down, risk of 
arrhythmia goes up. the more acidotic also the more likely to be 
hypovolemic, leading to shock. 
neurologic: these signs are seen with 
ketoacidosis, for example
depression, disorientation, coma
bone function: 
(young and chronic cases, such as renal acidosis)
growth retardation 

osteitis fibrosa

Metabolic acidosis: an abnormal physiological process 
characterized by:
1. a primary gain of strong acid by ECF - renal failure 
and failure to excrete H+, for example
2. primary loss of bicarbonate 
from ECF - parvo dog, horse with salmonella

the anion gap is helpful in 
the diagnosis of metabolic acidosis - if you have no blood gas machine 
this is particularly helpful. if you recall metabolic acidosis is the 
most common acid/base disturbance, you realize the importance.

serum or 
plasma electrolytes are divided into cations and anions.
several anions 
are not routinely measured.
those represent the normal anion gap of 10-20 
mEq/L.

unmeasured cation mEq/liter: K 4.5, Ca 5, Mg 1.5 = 11
protein 15, 
PO4 2 So4 1 organic acid 5 - 23 == unmeasured anion

increase in anion 
gap usually means that bicarbonate has been replaced with lactates, 
sulfates, or phosphates. 
lactates -- lactic acidosis, produced when 
there is lack of O2

clinical conditions causing increased anion gap:

abdominal crisis: horse with colic, dog with intussusception, etc.

diarrhea: parvo, salmonella
sepsis: premature foal, etc. 
shock
renal 
failure

Na+ + unmeasured cations = (Cl- + HCO3-) + unmeasured anions
so 
anion gap = Na+ - (Cl- + HCO3-) = UA-UC

metabolic acidosis secondary to 
a gain of acid will be associated with an increased anion gap - 
exception: addition of HCl to ECF.

metabolic acidosis with increased 
anion gap (normochloremic acidosis):

ketoacidosis
lactic acidosis
uremic 
acidosis
ingestion of toxins like ethylene glycol, methanol

metabolic 
acidosis caused by loss of base will be associated with normal anion gap 
- hyperchloremic acidosis:
renal tubular acidosis of aldosterone 
deficiency (when Na+ is retained, so is Cl-)
acidosis after respiratory 
alkalosis
intestinal loss of bicarb or organic acid anions
administration 
of a Cl continaining acid (HCl, NH4Cl)
acidosis due to shift of H+ from 
the cell

Respiratory acidosis: an abnormal process in which there is a 
primary decrease in rate of alveolar ventilation relative to rate of CO2 
production

metabolic alkalosis: abnormal physiological process in which 
there is:
1. a primary gain of strong base or primary loss of strong acid 
by ECF
2. primary gain of exogenous bicarbonate by the ECF

signs of 
alkalosis:
weakness (K+ derangement)
muscle tremor
vomiting 
(sequestration of gastric fluid)
tonic/clonic convulsions
synchronous 
diaphragmatic flutter 
lassitude
polyuria

respiratory alkalosis: unusual 
type of process in which there is a primary increase in the rate of 
alveolar ventilation relative to CO2 producton. 

so if there is 
hyperventilation, and individual is blowing off too much CO2, this can 
occur. imagine dog, HBC, in a lot of pain, may breathe very rapidly and 
develop respiratory alkalosis. tx of choice - fix the pain! breathing 
will normalize, problem will resolve.

your blood gas results are only as 
good as the sample you obtain. use the right technique and the right 
supplies.
put sample on ice if there will be a delay in processing

PaO2 
...torr
PaCO2...torr
pH

you need those things

Overview:
interpretation 
of ABG and pH
components of ABG reports
achieve desirable range

potentiate resuscitation

paO2 is the driving force of O2 into tissues. 
the higher it is, the higher the tissue concentration will be.
normally 
the paO2 is 80-100 torr (mmHg)
if you find paO2 < 80 torr, check the O2 
delivery system, check that you're delivering 100% O2. 

hypoxemia:
lung 
reasons:
endobronchial intubation w/failure to ventilate both lungs

aspiration and reduced surface area exposed to gas due to fluid in there

pulmonary edema
pneumothorax

acid/base balance: 
components: 

respiratory, metabolic

look at respiratory first, see what it means, 
then look at metabolic.

buffer systems:
function - maintain normality

primary buffers: sodium bicarbonate-carbonic acid system
H+ ion 
concentration - pH

pH: base/acid
ratio: 20/1
if you took value of bicarb 
at about 25 and of the carbonic acid at 1.2 you'd get about 20:1, giving 
a pH of 7.4


normal pH: mean 7.4, range 7.36-7.45
normal paCO2: mean 40 
torr, range 36-44 torr

if ventilation increases (hyperventilation 
occurs), paCO2 decreases
if ventilation decreases, paCO2 increases
paCO2 
is inversely proportional to the ventilation. the greater the ventilation 
the lower the CO2, the lower the ventilation, the higher the CO2.

so if 
you have paCO2 > 40 torr, think hypoventilation, which in pure form means 
respiratory acidosis.

if paCO2 < 40, hyperventilation is occuring, 
respiratory alkalosis

GOLDEN RULE I:

for every 10 torr change in paCO2, 
expect a pH change of .08 in the opposite direction (if paCO2 increases 
by 10, pH decreases by 0.08; if paCO2 decreases by 10, pH increases by 
0.08)

NORMAL
paCO2 40 torr = pH 7.4

Respiratory 
acidosis/hypoventilation: tx increase RR, blow off more CO2
paCO2 50 torr 
= pH 7.32

respiratory alkalosis/hyperventilation: tx decrease RR
paCO2 
30 torr = pH 7.48

so respiratory component: first you look at the paCO2. 
decide how far off 40 torr it is, above or below. calculate pH using rule 
I and see if that is the real pH, or if it is different. if it is 
different, there is probably also a metabolic component.

ABG data: 
clinical status of patient is essential for correct interpretation and 
therapy.

if calculated and measured pH are the same, it's a pure 
respiratory disorder. if it is not the same, there is also a metabolic 
component, it is a mixed disorder. 

Metabolic acidosis: the most common. 
after you look at respiratory part, look at this. 

golden rule II: pH 
change of 0.15 = base change of HCO3- 10 mEq/L

if pH is reduced by .15, 
HCO3- should be reduced by 10
if pH is increased by .15, HCO3- should be 
increased by 10

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example:

paCO2 52 torr, pH 7.3
paCO2 is 
increased by 12 --> respiratory acidosis --> hypoventilation
calculated 
pH 7.3
measured pH 7.3
difference - 0
no metabolic component --> pure 
respiratory acidosis

so remember, the calculated pH works because

for 
every increase in 10 torr paCO2, expect decrease in .08 pH

10/.08 = 12/x

10x = 12*.08 --> x = 0.1 so calculated pH = 7.4- 0.1 = 7.3

case 2
paCO2 
40 torr, pH 7.25
normal paCO2, no respiratory component
calculated pH 
7.40 (no change in paCO2)
measured pH 7.25
pH difference = -0.15
base 
deficit = 10 mEq/L --> metabolic acidosis
for every change in pH +/- 
0.15, expect BE/BD 10 mEq/L
pure metabolic acidosis

case 3

paCo2 50 
torr, pH 7.26
paCO2 increased by 10 --> respiratory acidosis, 
hypoventilation
calculated pH 7.32
measured pH 7.26
difference = -.06

base deficit = 4 mEq/l --> metabolic acidosis
respiratory acidosis and 
metabolic acidosis

.15/10 = .06/x
.15x = .6
x = .6/.15 = 4
since pH 
decreased, the 4 = base deficit

case 4
paCO2 32 torr, pH 7.26
paCo2 is 
down 8 torr --> respiratory alkalosis, hyperventilation
calculated pH 
7.46
measured pH 7.26
pH difference -0.20
base deficit = 13 mEq/L --> 
metabolic acidosis
respiratory alkalosis with metabolic acidosis
what's 
going on? well, pH is 7.26, so we have a primary metabolic acidosis, with 
some respiratory compensation. 

to differentiate b/w primary problem and 
compensatory component, look at the pH. if you have primary acidosis, pH 
is under 7.4, if pH is over 7.4 you have a primary alkalosis.

Base 
Deficit due to sodium bicarbonate loss:
ok, we just saw a primary 
metabolic acidosis with respiratory alkalosis...but pH is still not 
normal. metabolic acidosis is associated with gain of acid or loss of 
bicarbonate, right? so maybe there is a problem there. we have a base 
deficit of 13 mEq/L. how much bicarb do we have to give? 
the BD we 
measure is due to loss of bicarbonate.
the bicarb we measure is bicarb in 
ECF only, not the RBC or other bicarb.
ECF = intravascular fluid + 
interstitial fluid
ECF = 33% total body water = 25% total body weight

this confuses people. some use the denominator 3 and some use 4. 
don't 
sweat the small stuff; it's easier to take 25% of total body weight 
clinically. just divide weight by four.

base deficit - expressed loss of 
sodium bicarb in mEq/L (ECF)

look at respiratory acidosis - tx for this 
is to ventilate or supplement O2
metabolic acidosis: tx w/bicarbonate


golden rule III:

dose of bicarbonate:

base deficit (mEq/L) x patient 
weight (kg)
------------------------------------------  =  dose (mEq 
NaHCO3)
			4

case:
paCO2 52 torr, pH 7.17, weight 150 lb 70 kg
paCO2 
increased by 12 --> respiratory acidosis, hypoventilation
calculated pH 
7.3
measured pH 7.17
pH difference -0.13
base deficit 9 mEq/L --> 
metabolic acidosis
respiratory acidosis and metabolic acidosis
how do you 
determine primary problem? we know it is an acid problem. but it's both 
metabolic and respiratory. probably both are primary. patient may be 
shocky or very deeply anesthetized or something. 

tx: improve 
ventilation - use increased RR, or diuretic to get rid of pulmonary 
edema, or whatever. also tx BD with rule II

9 mEq/L x 70 kg / 4 = 160 
mEq NaHCO3
so you have to give this patient 160 mEq bicarb. however, you 
should only give 50% of that because when the pH is 7.20 or greater, the 
body will start to kick in with its own compensatory stuff. if you give 
too much you cause iatrogenic metabolic alkalosis. so give 80 mEq bicarb, 
then recheck. 

excess bicarb: what if you give too much?
metabolic 
alkalosis may ensue
hypokalemia
dysrhythmias
hyperosmolality
think about 
what happens at the cellular level. body tries to put H+ into ECF and 
move K+ into the cell. you could screw things up big time. 
because 
bicarb is hyperosmolar, you could screw up osmolality of blood also, 
affecting renal perfusions, renal excretion of fluid, etc. 

rule of 
thumb: give 50% of calculated bicarb dose, let sit 30-45 minutes to 
equilibrate, recheck ABG. 

daily water requirements: 20-30 ml/pound/day; 
20-30 liters/day per 1000 pounds 
(10-15 ml/kg?)

body weight (kg) * % 
dehydration = liters of water needed for replacement

when skin tents, 5% 
dehydration
sunken eyes 8-10% dehydration

remember to replace 
electrolytes - K+, etc - in patients who are not eating.


rule III:

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