Acid-Base Analysis

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

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Sources of blood acids• INFORMATION

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Sources of blood acids• INFORMATION