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Acid-Base Analysis. Pediatric Critical Care Medicine Emory University Children’s Healthcare of Atlanta. Sources of acids. Non-volatile acids. Volatile acids. Inorganic acid. Organic acid. H 2 O + dissolved CO 2. H + + HCO 3 -. H 2 CO 3. Keto acid. Lactic acid. - PowerPoint PPT Presentation
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Acid-Base Analysis
Pediatric Critical Care MedicineEmory University
Children’s Healthcare of Atlanta
Sources of acids
H2O + dissolved CO2
H2CO3
Volatile acids Non-volatile acids
Inorganicacid
Organic
acid
Lactic
acid
Ketoacid
H+ + HCO3-
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
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 ParaldehydeUremia 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
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 pH pCO2 [HCO3] BE
Uncompensated metab acidosis
N
Compensated metab acidosis
N
Uncompensated metab alkalosis
N
Compensated metab alkalosis
N
Partial PressureGas % 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
Atmosphere
pCO2 pO2
alv
extravascular fluid
cells
0 160
40 100
Capillary
45 97
~47
~47 <39
<54 ~5
>55 <1
systemiccirculation
CellsECF
EndotheliumRBC
CO2
CO2
CO2
CO2
Dissolved CO2= pCO2
5%
30%
65%
CO2 + Hb= HbCO2
CO2 + H2O= HCO3 + H+
CarboxyHgb
Utilizescarbonicanhydrase
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
pO2
100
75
50
25
20 40
60 80 100
Dissociation curve
0
20
40
60
80
100
120
0 20 40 60 80 100 120
% Sat
pO2
Shifts
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 = 187.40 - 0.18 = 7.22
IF PCO2 = 18:40 -18 = 227.40 + 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 = 7.227.22 - 7.40 = 0.18 18 x 2/3 = 12 deficit
• If pH = 7.47, PCO2 = 18Predicted pH =7.627.62 - 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
24
Sources of blood acids• INFORMATION
25
Sources of blood acids• INFORMATION