Arterial Blood Gases laboratory interpretation

1. Lab interpretation – Blood
gases
Dr Jess Dorman
On Call Africa

2. Aims and Objectives
 Understand when ABGs/VBGs are indicated
 Systematically approach interpreting samples
 Understand what the pathology is that causes abnormalities on samples

3. ABGs/VBGs

4. Why?
 Hypoxia
 Unconsciousness
 Patients you are concerned about
 Seizures
 Immediate results of biochemical markers

5. A vs V
 Venous where possible except for PaO2/PaCO2/HCO3 where A is gold standard
 All research indicates venous is good enough for decision making
 Normal PCO2 on VBG negates need for ABG

6. Where?
 Radial artery
 Brachial artery
 Femoral artery
 Veins

7. How?
 Position of hand/wrist
 Palpate
 Clean
 Adjust if necessary

8. Values
 pH 7.35-7.45
 PCO2 4.5-6 or 35-45mmhg
 PO2 10-14 or 80-100mmhg
 HCO3 22-28
 BE -1-1
 Electrolytes (Na/K/Ca/Cl)
 Hb male vs female
 Glu >4
 Lac <2

9. How to read blood gases
 Biochemical equations
 4 disorders of pH
 Metabolic
 Acidosis/alkalosis
 Respiratory
 Acidosis/alkalosis
 Other abnormalities
 Low Hb
 High/low glucose
 High lactate

11. How to read blood gases
1. pH
1. Are they acidaemic of alkalaemic?
2. HCO3/BE
1. Is the cause metabolic/Is there metabolic disturbance?
2. If values don fit – is this compensation for something respiratory?
3. PCO2
1. Is any acidosis caused by raised CO2?
2. Is there any compensation for metabolic acidosis
4. PO2
1. Are they diffusing enough oxygen?
5. Lac
1. Is there evidence of hypoperfusion?
6. Other
1. Is there something else going on?

12. Causes of acidosis
 Metabolic (raised anion gap vs non-raised anion gap)
 MUDPILES
 Respiratory
 Hypercapnia

13. Anion Gap
 Helps evaluate acid-base disturbance
 Na – (Cl + HCO3)
 Beware albumin

14. Raised anion gap metabolic acidosis
 MUDPILES
 Methanol
 Uraemia
 DKA
 Paracetamol/Paraldehyde
 Iron
 Lactic acidosis
 Ethanol/ethylene glycol
 Salicylate

15. Normal anion gap metabolic acidosis
 Diarrhoea
 Renal tubular acidosis
 Adrenal insufficiency
 CKD
 Drugs – spironolactone, prostaglandin inhibitors, ciclopsorin

16. Causes of alkalosis
 Metabolic
 Vomiting
 Respiratory
 Hyperventilation

17. Respiratory failure
 Type 1
 Low PaO2
 Low or normal PaCO2
 Type 2
 Low PaO2
 High PaCO2
 Acidosis = decompensated = badness

18. Compensation

19. Compensation
 Partial vs full
 Metabolic compensation of chronic hypercapnia
 Respiratory compensation of acute metabolic acidosis (sepsis)

20. Other disorders
 Low Hb – chronic anaemia vs acute bleeding
 Note Hb can be normal in acute trauma body has not had time to compensate with increased
blood volume, showing lower hb)
 High Hb – polycythaemia
 Chronic hypoxia
 Glucose
 Low – give dextrose
 High – use in conjunction with other markers – DKA vs HHS vs hyperglycaemia
 Lactate
 Causes metabolic acidosis
 Indicates hypoperfusion/metabolic disturbance
 Raised in metformin accumulation

21. How to read blood gases
1. pH
1. Are they acidaemic of alkalaemic?
2. HCO3/BE
1. Is the cause metabolic/Is there metabolic disturbance?
2. If values don fit – is this compensation for something respiratory?
3. PCO2
1. Is any acidosis caused by raised CO2?
2. Is there any compensation for metabolic acidosis
4. PO2
1. Are they diffusing enough oxygen?
5. Lac
1. Is there evidence of hypoperfusion?
6. Other
1. Is there something else going on?

22. Examples

23. 56 year old female with shortness of breath, productive cough, fevers and crepitations
at her right base. HR 134, BP 90/60, RR 30, sats 92% on air, T 38.5.
A or V?
pH 7.28 (7.35-7.45)
PCO2 4.5 (4.5-6.0)
PO29.2 (10-14)
HCO3 17 (22-26)
BE -6.2 (-1-1)
Hb 14.7 (12-15)
Glu 7.0 (>4)
Lac 4.5 (<2)

24. 20 year old male. Presents with vomiting and lethargy for 5 days. HR 120, BP 104/86,
RR 26, sats 97% on air, T 37.1.
A or V?
pH 7.15 (7.35-7.45)
PCO2 4.0 (4.5-6.0)
PO28.9 (10-14)
HCO3 14 (22-26)
BE -8.2 (-1-1)
Hb 16 (13-16)
Glu 32 (>4)
Lac 3.7 (<2)

25. 67 year old male with COPD. Admitted with 1 week history of cough, today not eating
much and his wife thinks he is drowsy and confused. HR 112, BP 103/87, RR 32, SpO2
85% on air, T 37.4.
A or V?
pH 7.29 (7.35-7.45)
PCO2 8.1 (4.5-6.0)
PO28.8 (10-14)
HCO3 25 (22-26)
BE 4 (-1-1)
Hb 13.2 (12-14)
Glu 7.1 (>4)
Lac 1.8 (<2)

26. 26 year old male. Fell off his bike on the way to work. Complaining of pain to left
upper quadrant. HR 121, BP 98/63, RR 25, SpO2 96% on air, T 36.5.
A or V?
pH 7.35 (7.35-7.45)
PCO2 5.9 (4.5-6.0)
PO210 (10-14)
HCO3 23 (22-26)
BE -0.8 (-1-1)
Hb 8.7 (12-14)
Glu 7.6 (>4)
Lac 3.2 (<2)

27. 72 year old male with generalized abdominal pain and vomiting for 5 days. He has not
been eating and has had loose stools. HR 106, BP 110/75, RR 18, SpO2 95% on air, T
36.9.
A or V?
pH 7.31 (7.35-7.45)
PCO2 6.0 (4.5-6.0)
PO29.1 (10-14)
HCO3 20 (22-26)
BE -4 (-1-1)
Hb 11.1 (13-15)
Glu 7.1 (>4)
Lac 8.9 (<2)

28. In Summary…

29. How to read blood gases
1. pH
1. Are they acidaemic of alkalaemic?
2. HCO3/BE
1. Is the cause metabolic/Is there metabolic disturbance?
2. If values don fit – is this compensation for something respiratory?
3. PCO2
1. Is any acidosis caused by raised CO2?
2. Is there any compensation for metabolic acidosis
4. PO2
1. Are they diffusing enough oxygen?
5. Lac
1. Is there evidence of hypoperfusion?
6. Other
1. Is there something else going on?