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 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

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 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

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 = 18

Predicted 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

25

Sources of blood acids

• INFORMATION