Acid-Base Analysis
Pediatric Critical Care Medicine
Emory University
Children’s Healthcare of Atlanta
Sources of acids
Volatile acids
H2O + dissolved CO2
H2CO3
Non-volatile acids
Inorganic
acid
Organic
acid
H+ + HCO3Lactic
acid
Keto
acid
Henderson-Hasselbalch
pH = pKa + log [A-]
[HA]
and
pH = pKa + log [HCO3-] = 6.1 + log [HCO3-]
s x PCO2
0.03 x PCO2
H+ + HCO3-
H2CO3
CO2 + H2O
Anion Gap
[Na+] = [CL- + HCO3-] ~ 10-15
Acid-Base States
• Acidosis: pH<7.35
– Metabolic: increased acid or decreased in bicarb
– Respiratory: increased PCO2
• Alkalosis: pH>7.45
– Metabolic: increased bicarb or loss of H+
– Respiratory: decreased PCO2
Compensation
• Acute:
– Minutes
– Respiratory: PCO2 regulation
• Chronic
– Hours to days
– Renal: via regulation of bicarb excretion
Acidosis: Respiratory
• Decrease PCO2 excretion via hypoventilation
– Respiratory etiology
– CNS pathology
– Intoxication
• pH decreases 0.08 unit/10 mmHg increase in PaCO2
• Bicarb and base excess are normal
Acidosis: Metabolic
• Change in pH by increased in acid or decrease in bicarb
• Anion Gap Acidosis: MUD PILES
Methanol
Paraldehyde
Uremia
Diabetic ketoacidosis
Iron, isoniazid (INH)
Lactic acid
Ethanol, ethylene glycol
Salicylates
• Non-Anion Gap Acidosis: USEDCARP
Uretorostomy
Small bowel fistula
Extra Chloride
Diarrhea
Carbonic anhydrase inhibitors (acetazolamide)
Adrenal insufficiency
RTA
Pancreatic fistula
Alkalosis: Respiratory
• Decrease in PCO2 by hyperventilation
• Compensate by increase renal excretion of HCO3-
Alkalosis: Metabolic
• Increase in H+ loss or increase in HCO3• PaCO2 increase by 0.5-1/1 mEq/L of increase in
HCO3-
Nomenclature
Uncompensated
metab acidosis
Compensated
metab acidosis
Uncompensated
metab alkalosis
Compensated
metab alkalosis
pH
pCO2
[HCO3]
BE

N


N




N


N



Partial Pressure
Gas
% Total
Air at sea level
Partial Pressure
760
Oxygen
20.9%
159
Nitrogen
79.0%
600
Alveolar gas at sea level
CO2
Oxygen
13.3%
101
Nitrogen
75.2%
572
CO2
5.3%
40
Water
6.2%
47
pCO2
pO2
0
Atmosphere
160
40
alv
100
45
systemic
circulation
Capillary
~47
97
~47
<39
<54
extravascular fluid
~5
>55
cells
<1
Endothelium
ECF
Cells
CO2
RBC
5%
30%
CO2
CO2
65%
Dissolved CO2
= pCO2
CO2 + Hb
= HbCO2
CO2 + H2O
= HCO3 + H+
CarboxyHgb
Utilizes
carbonic
anhydrase
CO2
CO2 Transport
Excretion of CO2
• Metabolic rate determines how much CO2 enters blood
• Lung function determines how much CO2 excreted
– minute ventilation
– alveolar perfusion
– blood CO2 content
Hgb dissociation curve
%
Sat
20
40
100
75
50
pO2
25
60
80
100
Dissociation curve
% Sat
120
100
Shifts
80
60
40
20
0
0
20
40
60
pO2
80
100
120
Alveolar oxygen equation
• Inspired oxygen = 760 x .21 = 160 torr
• Ideal alveolar oxygen =
PAO2
= [PB - PH2O] x FiO2 - [PaCO2/RQ]
= [760 - 47] x 0.21 - [40/0.8]
= [713] x 0.21 -[50]
= 100 torr or 100 mmHg
• If perfect equilibrium, then alveolar oxygen equals
arterial oxygen.
• ~5% shunt in normal lungs
Normal Oxygen Levels
FiO2
PaO2
0.30
>150
0.40
>200
0.50
>250
0.80
>400
1.0
>500
Predicting ‘respiratory part’ of pH
• Determine difference between PaCO2 and 40 torr, then
move decimal place left 2, ie:
IF PCO2 76:
76 - 40 = 36 x 1/2 = 18
7.40 - 0.18 = 7.22
IF PCO2 = 18:
40 -18 = 22
7.40 + 0.22 = 7.62
Predicting metabolic component
• Determine ‘predicted’ pH
• Determine difference between predicted and actual pH
• 2/3 of that value is the base excess/deficit
Deficit examples
• If pH = 7.04, PCO2 = 76
Predicted pH = 7.22
7.22 - 7.40 = 0.18 18 x 2/3 = 12 deficit
• If pH = 7.47, PCO2 = 18
Predicted pH =7.62
7.62 - 7.47 = 0.15 15 x 2/3 = 10 excess
Hypoxemia - etiology
• Decreased PAO2 (alveolar oxygen)
– Hypoventilation
– Breathing FiO2 <0.21
– Unde rventilated alveoli (low V/Q)
• Zero V/Q (true shunt)
• Decreased mixed venous oxygen content
– Increased metabolic rate
– Decreased cardiac output
– Decreased arterial oxygen content
Blood gases
• PaCO2: pH relationship
– For every 20 torr increase in PaCO2,
pH decreases by 0.10
– For every 10 torr decrease in PaCO2,
pH increases by 0.10
• PaCO2: plasma bicarbonate relationship
– PaCO2 increase of 10 torr results in bicarbonate
increasing by 1 mmol/L
– Acute PaCO2 decrease of 10 torr will decrease bicarb
by 2 mmol/L
Sources of blood acids
• INFORMATION
24
Sources of blood acids
• INFORMATION
25