acid-base analysis pediatric critical care medicine emory university children’s healthcare of...
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Acid-Base Analysis
Pediatric Critical Care MedicineEmory University
Children’s Healthcare of Atlanta
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Sources of acids
H2O + dissolved CO2
H2CO3
Volatile acids Non-volatile acids
Inorganicacid
Organic
acid
Lactic
acid
Ketoacid
H+ + HCO3-
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Henderson-HasselbalchpH = 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
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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
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Compensation• Acute:
– Minutes
– Respiratory: PCO2 regulation
• Chronic– Hours to days– Renal: via regulation of bicarb excretion
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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
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Acidosis: Metabolic • Change in pH by increased in acid or decrease
in bicarb• Anion Gap Acidosis: MUD PILES
Methanol Paraldehyde
Uremia Iron, isoniazid (INH)Diabetic ketoacidosis Lactic acid
Ethanol, ethylene glycolSalicylates
• Non-Anion Gap Acidosis: USEDCARPUretorostomy Carbonic anhydrase inhibitors (acetazolamide)Small bowel fistula Adrenal insufficiencyExtra Chloride RTADiarrhea Pancreatic fistula
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Alkalosis: Respiratory• Decrease in PCO2 by hyperventilation
• Compensate by increase renal excretion of HCO3
-
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Alkalosis: Metabolic• Increase in H+ loss or increase in HCO3
-
• PaCO2 increase by 0.5-1/1 mEq/L of increase in HCO3
-
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Nomenclature
pH pCO2 [HCO3] BE
Uncompensated metab acidosis
N
Compensated metab acidosis
N
Uncompensated metab alkalosis
N
Compensated metab alkalosis
N
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Partial Pressure
Gas % Total Partial Pressure
Air at sea level 760
Oxygen 20.9% 159
Nitrogen 79.0% 600
Alveolar gas at sea level
Oxygen 13.3% 101
Nitrogen 75.2% 572
CO2 5.3% 40
Water 6.2% 47 CO2
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Atmosphere
pCO2 pO2
alv
extravascular fluid
cells
0 160
40 100
Capillary
45 97
~47
~47 <39
<54 ~5
>55 <1
systemiccirculation
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CellsECF
EndotheliumRBC
CO2
CO2
CO2
CO2
Dissolved CO2= pCO2
5%
30%
65%
CO2 + Hb= HbCO2
CO2 + H2O= HCO3 + H+
CarboxyHgb
Utilizescarbonicanhydrase
CO2 Transport
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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
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Hgb dissociation curve
%Sat
pO2
100
75
50
25
20 40
60 80 100
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Dissociation curve
0
20
40
60
80
100
120
0 20 40 60 80 100 120
% Sat
pO2
Shifts
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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
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Normal Oxygen Levels
FiO2 PaO2
0.30 >150
0.40 >200
0.50 >250
0.80 >400
1.0 >500
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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 = 187.40 - 0.18 = 7.22
IF PCO2 = 18:
40 -18 = 227.40 + 0.22 = 7.62
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Predicting metabolic component
• Determine ‘predicted’ pH• Determine difference between predicted and
actual pH• 2/3 of that value is the base excess/deficit
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Deficit examples
• If pH = 7.04, PCO2 = 76
Predicted pH = 7.227.22 - 7.40 = 0.18 18 x 2/3 = 12 deficit
• If pH = 7.47, PCO2 = 18
Predicted pH =7.627.62 - 7.47 = 0.15 15 x 2/3 = 10 excess
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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
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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
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Sources of blood acids
• INFORMATION
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Sources of blood acids
• INFORMATION