1. Blood gas and acid base evaluation
Chapter 20

2. Step one Normal Ph 7.35 – 7.45 PCO2 35 – 45 mmHg ( 31 – 39 )
PO2 60 – 100 mmHg BE
HCO3 24
AG 8 – 16
Na , K , Cl
Ph = pKa + log X HCO3/PCO2 x Solubility
A-a gradient N < 20 mmHg
O2 – Hb dissociation scale

3. Step 1 Ph = pKa + log X HCO3/PCO2 x Solubility
CO2 + H2O – H2CO3 – H + HCO3
A-a gradient N < 20 mmHg
O2 – Hb dissociation scale

4. Step two Is there an OXYGENATION problem
PO2
Saturation
Must always be interpreted with FiO2
PF Ratio = PO2/FiO2
Normal > 400
ALI < 300
ARDS < 200

5. Step 2 Reason for oxygenation problem Work out A-a gradient
Decide if it is an intrinsic lung problem

6. Step 3 Is there a VENTILATION problem Look at PaCO2
<35 hyperventilation
>45 hyperventilation
Remember that ventilation always interpreted with pH in mind

7. Step 4
What is the pH
Normal
Compensated
Mixed
Alkalosis
Acidosis

8. Step 5 Does the PaCO2 FIT IN with the pH
Alkalosis + PaCO2 < 40 mmHg = Respiratory Alkalosis
Acidosis + PaCO2 > 40 mmHg = Respiratory acidosis
If the PaCO2 does not fit in with the ph , move to next step
If the PaCO2 fit in with Ph = PRIMARY ABNORMALITY

9. Step 6 Look at the HCO3 Does the SBC fit in with the pH
Alkalosis + SBC > 24 = Metabolic alkalosis
Acidosis + SBC < 24 = Metabolic acidosis
This will be the primary abnormality if the PaCO2 did not fit in with pH

10. But wait , there is more Remember that basic 4 abnormalities are
Resp acidosis
Metab acidosis
Resp alkalosis
Metab alkalosis
Uncompensated disorders
Compensated disorders
Mixed disorders
Chronic versus acute

11. Step 7 Determine the degree of COMPENSATION Rules apply ( tables )
Thus the body always tries to compensate for the primary abnormality
If compensation as expected = normal compensation
If compensation not as expected = mixed disorder

12. Step 8 Calculate the ANION GAP if metabolic acidosis
AG = Cations – Anions = 8-16 mEq/L = unmeasured anions
AG = (Sodium + Potasium) – (Choride + Bicarbonate)
Acidosis + AG > 16 Mm = High anion gap metabolic acidosis
Acidosis + AG normal = Low anion gap metabolic acidosis

13. Step 9 Look at the Haemotological variables Haematocrit Haemoglobin
Abnormal haemoglobins

14. Step 10 Look at the metabolites and electrolytes Lactate Glucose
Calcium
Sodium
Potassium
Magnesium

15. Intracellular pH 6.8 Neutral at body temp of 37 degrees celcius
Hydrogen = Hydroxyl
Dissociation of water and other electrlytes

16. Administration of 8.4% NaHCO3
Rather treat the cause
Ph < 7.1 metabolic acidosis
Acidific toxins etc to aid renal excretion
Hypernatraemia , hyperosmolality , fluid overload

17. Chloride/Sodium Ratio
> in hyperchloraemic metabolic acidosis
0.9% Saline , Voluven , Venofundin

18. Sodium Hyponatraemia Hypervolaemic hyponatraemia
Normovolaemic hyponatraemia
Hypovolaemic hyponatraemia
< 130 Mm – No elective surgery
< 120 Mm – Neurologic derangements
Central pontine demyelinization if corrected too fast

19. Sodium Hypernatraemia Hypervolaemic Normovolaemic Hypovolaemic
> 150 no elective surgery
Brain oedema if corrected too fast

20. Potassium Hypokalaemia < 3,5 Causes Symptoms and signs
Redistribution, Decreased intake, Increased losses
Symptoms and signs
Cardiovascular , musculoskeletal