ACID-BASE BALANCE
1. HENDERSON-HASSELBACH EQUATION
A. USED BY BLOOD GAS MACHINES
TO COMPUTE BICARBONATE
B.
pH = pKa + LOG
BASE/ACID SALT
C. pH
= pKa + LOG HC03/H2CO3
D. pKa = 6.1 FOR THE
BICARBONATE/CARBONIC ACID BUFFER PAIR
E. pKa IS THE VALUE OF THE pH IF
THERE WERE EQUAL
CONCENTRATIONS OF BASE AND ACID
F. NORMAL BICARB. IS 24 Meq/L
G. NORMAL H2C03 = PaCO2 X .03
= 40 X .03 = 1.2 Meq/L
H. PH = 6.1 + LOG
HC03/H2C03
= 6.1 + LOG 24/1.2
= 6.1 + LOG 20
= 6.1 + 1.3
= 7.4
2. BUFFER SYSTEM- PRESENCE OF A WEAK ACID AND ITS CONJUGATE BASE WHICH
WILL COMBINE WITH EXOGENOUS ACIDS AND BASES TO .DECREASE THE CHANGE IN THE pH
I.E. ADD HCl TO SOLUTION WITH pH OF 7.4 WITHOUT BUFFERS
pH MAY DROP TO 5.7 BUT ONLY DROPS TO 7.1 BECAUSE OF BUFFERING
SYSTEMS
A. MAJOR BUFFERING SYSTEM:-BICARBONATE/C02
1. ACTUALLY NOT GOOD
BECAUSE OF LOW pKa
2. HOWEVER, BECAUSE
PaC02 EASILY REMOVED BY LUNGS, MAKES IT A
GOOD BUFFER
3. CO2 + H20
--(CARBONIC ANHYDRASE)-- H2C03 --> H+ + HCO3-
4. ADD C02 THEN BICARB WILL BE PRODUCED
5. ADD BICARB. THEN C02
WILL BE PRODUCED
B. HGB, PLASMA PROTEINS AND PHOSPHATES
ALL DO SOME BUFFERING
4a. PREDICTED Pa02 (ON ROOM AIR) = 103 - (.42 X AGE)
I.E. 80YR. = 103 - (.42 X 80) = 69
PREDICTED A-a GRADIENT = (.42 X AGE) This
last formula is more useful
4b. HYPOXEMIA(MILD) - 60-75 UNLESS PREDICTED FOR AGE
MODERATE - 40- 59
SEVERE - < 40
CLINICALLY ONE TREATS LOW Pa02 IF < 55-60 TORR
ONLY TIMES WHEN JUSTIFIFIBLE TO KEEP Pa02 > 100:
1. HEAD INJURY
2. POST STROKE
3. FRESH MI
4. CARBON MONOXIDE POISONING
4c. CAUSES OF HYPOXEMIA
1. ALTITUDE
2. HYPOVENTILATION
3. LOW V/Q
4. ANATOMIC SHUNTING
5. CAPILLARY SHUNTING
DIFFERENTIAL DIAGNOSIS- ON ROOM AIR AT SEA LEVEL:
A. IF Pa02 + PaC02 > 140 THEN PT. NOT ON ROOM AIR
B. IF Pa02 + PaC02 BETWEEN 110 AND 140 THEN HYPOXEMIA
DUE TO HYPOVENTILATION
C. IF Pa02 + PaC02 < 110, THEN HYPOXEMIA DUE TO 3, 4, OR 5
ABOVE
4d. HYPOXIA
A. PT. CAN BE HYPOXIC AND NOT BE HYPOXEMIC AND VICE VERSA
B. SIGNS OF HYPOXIA
1. NO WAY TO ABSOLUTELY DETERMINE IF HYPOXIA
PRESENT
2. SIGNS: CONFUSION, DECREASED CARDIAC OUTPUT,
LOW MEAN
ARTERIAL PRESSURE(LESS THAN 60 MM HG), LOW Pv02 (< 25 TORR),
VERY LOW OR HIGH Ca-v02, SEVERE HYPOXEMIA(Pa02 < 40 TORR) LOW
HGB LEVELS (LESS THAN 10 GRAMS %) AND LACTIC ACIDOSIS
C. TYPES OF HYPOXIA
1. HYPOXEMIC HYPOXIA (Pa02 MUST BE LESS THAN 60)
RX- OXYGEN AND/OR PEEP
2. CIRCULATORY OR STAGNANT HYPOXIA
DIAGNOSIS-LOW CARDIAC OUTPUT, LOW MAP, HIGH
Ca-V02
RX- INOTROPIC AGENTS(i.e. DOBUTAMINE AND
DOPAMINE)
3. ANEMIC HYPOXIA
DIAGNOSIS-HGB LESS THAN 10 OR CARBON MONOXIDE
POISONING WITH CARBOXYHEMOGLOBIN LEVELS
> 20-30% AND LOW Ca-v02 (< 3.5 VOL %)
RX. ANEMIA-PACKED CELLS
CO POISONING-100% 02 AND/OR HYPERBARIC OXYGEN
4. HISTOTOXIC HYPOXIA (CYANIDE POISONING)
5. BICARBONATE VALUES
A. ACTUAL
1. CALCULATED BY BLOOD GAS MACHINE USING H-H
EQUATION
2. ACCORDING TO C02 + H20 REACTING TO MAKE
BICARB AND H+, ACTUAL
BICARB WILL
CHANGE WITH CHANGES IN C02
a. INCREASING C02(FOR EVERY 15 TORR > 40) WILL INCREASE BICARB
BY 1 MEQ. (ACTUAL PaC02 - 40)/15 = PREDICTED BICARB CHANGE
b. DECREASING PaCO2(FOR EVERY 5 TORR < 40) WILL DECREASE
BICARB. BY 1 MEQ.
(40 - ACTUAL PaCO2)/5 = PREDICTED BICARB CHANGE
c. THESE CHANGES HAVE NOTHING TO DO WITH COMPENSATION, IT'S
JUST CHEMISTRY
d. THESE STIOCHIOMETRIC CHANGES IN BICARB. MAKE LOOKING AT
ABSOLUTE VALUES OF BICARB. MISLEADING WHEN ATTEMPTING TO
INTERPRET METABOLIC COMPONENT OF BLOOD GAS
B. STANDARD BICARB.
1.
A CALCULATED VALUE TO CONTROL TO CONTROL FOR CHANGES IN
BICARB
VALUE
2. .
DESIGNED TO ALLOW YOU TO LOOK AT BICARB. LEVEL AS IF THE
PaC02
WAS 40 TORR
3. .
NORMALLY 22-26 Meq/L
C. BASE EXCESS-SIMILAR
TO STANDARD BICARB. IN INTENT
D. PROBLEMS WITH B AND C ABOVE IS THAT THEY'RE
BASED ON
IN VITRO OR TEST TUBE
CHANGES WHICH ARE NOT A PERFECT
PICTURE
OF WHAT HAPPENS IN VIVO
6. ACID-BASE DISORDERS
A. RESPIRATORY ACIDOSIS (ACUTE HYPOVENTILATION)
1. Causes
a. CNS depression
(narcotic or barbiturate OD)
b. Neuomuscular
diseases
c. Variety of severe
pulmonary diseases
d. fatigue
e. Cardiac arrest or
apnea
2. RX-Continuous Mechanical Ventilation
B. RESP. ALKALOSIS(ACUTE HYPERVENTILATION)
1. Causes
a.
HYPOXEMIA THE MOST COMMON CAUSE SEEN IN THE
HOSPITAL--THINK
OF THIS FIRST
b.
ANXIETY-HAVE PT. BREATHE IN PAPER BAG
c.
Pain
d.
Ventilator induced
e.
CNS disorders
2. TREAT CAUSE: REMEMBER-HYPOXEMIA THE MOST
COMMON CAUSE
IF CAUSED BY
ANXIETY-HAVE PT. BREATHE IN PAPER BAG
C. METABOLIC ACIDOSIS
1. Causes
a. Lactic acidosis secondary to anaerobic
metaoblism in profound hypoxia
b. DIABETIC KETOACIDOSIS,
c. INGESTION OF ACIDS (i.e. aspirin OD
d. diarrhea
e. renal disease
f. Methonol or ethylene glycol ingestion
2. DIFFERENTIATION OF CAUSE, USE ANION GAP
A. ANION GAP = (K + NA) - (CL + HC03)
=
144 - 129 = 15 (NORMALLY 12-18)
ANION GAP INCREASED IF
METABOLIC ACIDOSIS CAUSED BY INCREASE
IN
UNMEASURED ANIONS, (KETOACIDOSIS, LACTIC ACIDOSIS, AND
INGESTION
OF ACIDS
WILL HAVE A NORMAL
ANION GAP IS ACIDOSIS CAUSED BY A
LOSS OF BICARB(DIARRHEA
OR RENAL DISEASE)
B. LACTIC
ACIDOSIS- LOOK FOR SIGNS OF HYPOXIA
C. KETOACIDOSIS- LOOK AT BLOOD GLUCOSE
LEVELS(> 300)
D. RENAL DISEASE LOOK AT LOW URINE OUTPUT OR
HIGH BUN (> 25) OR
CREATININE
(> 1.5)
3. RX. SODIUM BICARBONATE IF pH < 7.20 ALSO RX CAUSES
D. METABOLIC ALKALOSIS
1. DIFFERENTIAL DIAGNOSIS
A. HYPOKALEMIA (K < 3.0 OR LASIX WITHOUT
TAKING K
B. LOSS OF STOMACH ACID-LOOK FOR PROLONGED
VOMITING OR NG TUBE
FOR MORE THAN A
FEW DAYS ATTACHED TO SUCTION)
C. MASSIVE DOSES OF STEROIDS
D. IATROGENIC (OVERADMINISTRATION OF BICARB.
2. RX. DIAMOX OR AMMONIUM CHLORIDE IF SEVERE, OTHERWISE RX. CAUSES
7. 1. ACUTE RESP. ACIDOSIS AND ALKALOSIS GIVEN IN #5
2. CHRONIC RESPIRATORY ACIDOSIS (ASSUMES MUST BY AT
LEAST 48 HOURS OLD
FOR SIGNIFICANT RENAL COMPENSATION TO OCCUR)
SINCE BICARB. SHOULD
INCREASE UP TO 4 MEQ FOR EVERY INCREASE IN
PaCO2 BY 10 TORR,
EXPECTED HCO3 = 4 X {(PaC02 - 40)/10} + 24
i.e. IF CHRONIC PaCO2 =
65 THEN:
EXPECTED
HCO3 = 4 X {(65 - 40)/10} + 24 = 4 X 2.5 + 24
=
34 Meq
3. SINCE THE LUNGS WILL COMPENSATE IMMEDIATELY FOR
METABOLIC PROBLEMS
THERE IS NO SUCH THING AS AN ACUTE OR CHRONIC
METABOLIC PROBLEM
3. MET. ACIDOSIS-
1. PRED. PaC02 = (1.5 X ACT. HC03) + 8
a. IF PaC02 IS AS PREDICTED, THEN MAXIMAL
COMPENSATION IS PRESENT
EVEN THOUGH pH IS
< 7.35.
b. IF PaC02 > PRED., THAN HAVE A RESP.
PROBLEM IN ADDITION TO A
METABOLIC
PROBLEM
c. IF PaC02 < PRED. THAN YOU HAVE A RESP.
ALK. IN ADDITION TO MET.
ACIDOSIS
4. MET. ALKALOSIS-PaC02 NEVER GOES
ABOVE 50 TO COMPENSATE FOR A MET.
ALKALOSIS
IF PaC02 > 50 TORR THAN A CHRONIC RESP. ACIDOSIS MUST BE PRESENT
8. NORMAL MIXED VENOUS GASES
A. TAKEN VIA SWAN-GANZ CATHETER IN PULMONARY ARTERY
Pv02 AND Sv02 DECREASES WITH LOW ARTERIAL 02
CONTENT OR LOW
CARDIAC OUTPUT
9. BICARBONATE ADMINISTRATION GIVEN IN MET. ACIDOSIS
A. GIVEN ONLY IF pH < 7.20
B. (BASE DEFICIT X 1/4 BODY WT. IN KG.)/2 = MEQ OF HCO3
C. MEQ OF HCO3/44 = NUMBER OF AMPS REQUIRED
10. A. HEPARIN IS ACIDOTIC AND IF TOO MUCH IS USED CAUSE MEASURED
VALUES TO BE
LOWER THAN ACTUAL
B. AIR BUBBLES HAVE ESSENTIALLY 0 PC02 AND 159 P02 SO:
1. MEASURED PH
> THAN ACTUAL
2. MEASURED PaCO2
< THAN ACTUAL
3. MEASURED Pa02
USUALLY > THAN ACTUAL UNLESS THE ACTUAL Pa02
IS
> 159