Understanding ABGs

Question 1

Normal Ranges

Answer 1

pH: 7.35 – 7.45
PaCO2: 4.7 – 6.0 kPa || 35.2 – 45 mmHg
PaO2: 11 – 13 kPa || 82.5 – 97.5 mmHg
HCO3–: 22 – 26 mEq/L
Base excess (BE): -2 to +2 mmol/L

Question 2

PaO2 of Patient on O2 Therapy

Answer 2

PaO2 should be approximately 10kPa less than the % inspired concentration FiO2

Question 3

Hypoxaemia

Answer 3

Hypoxaemia: <10kPa PaO2 on air

Severe hypoxaemia/resp failure: <8kPa on air

Question 4

Type 1 Resp Failure

Answer 4

Hypoxaemia and normocapnia

Result of ventilation/perfusion (V/Q) mismatch

Question 5

Examples of V/Q Mismatch

Answer 5

Reduced ventilation w normal perfusion (pulm oedema, bronchoconstriction)
Reduced perfusion w normal ventilation (PE)

Question 6

Type 2 Resp Failure

Answer 6

Hypoxaemia and hypercapnia

Result of alveolar hypoventilation

Question 7

Causes of hypoventilation

Answer 7

Incr airway resistance due to obstruction (COPD)
Reduced lung tissue/chest wall compliance (pneumonia, rib fracture, obesity)
Reduced strength of resp muscles (MNR)
Drugs acting on resp centre reducing overall ventilation (opiates)

Question 8

Acidotic/Normal/Alkalotic

Answer 8

Acidotic: pH <7.35
Normal: pH 7.35 – 7.45
Alkalotic: pH >7.45

Question 9

Respiratory or Metabolic Change

Answer 9

Changes in pH are caused by an imbalance in the CO2 (respiratory) or HCO3– (metabolic)

Question 10

Compensation

Answer 10

The body can try and adjust other buffers (CO2/HCO3) to keep the pH within the normal range.

Question 11

Compensation in Respiratory

Answer 11

Adjust HCO3

Question 12

Compensation in Metabolic

Answer 12

Adjust CO2

Question 13

HCO3

Answer 13

Base which helps mop up acids (H+ ions).
When HCO3– is raised –> pH is increased as there are less free H+ ions (alkalosis).
When HCO3– is low –> pH is decreased as there are more free H+ ions (acidosis).

Question 14

Resp Acidosis

Answer 14

Decr pH
Incr CO2
Normal HCO3

Question 15

Resp Alkalosis

Answer 15

Incr pH
Decr CO2
Normal HCO3

Question 16

Resp acidosis w metabolic compensation

Answer 16

Decr/normal pH
Incr CO2
Incr HCO3

Question 17

Resp alkalosis w metabolic compensation

Answer 17

Incr/normal pH
Decr CO2
Decr HCO3

Question 18

Metabolic Acidosis

Answer 18

Decr pH
Decr HCO3
Normal CO2

Question 19

Metabolic Alkalosis

Answer 19

Incr pH
Incr HCO3
Normal CO2

Question 20

Metabolic acidosis w resp compensation

Answer 20

Decr pH
Decr HCO3
Decr CO2

Question 21

Metabolic alkalosis w resp compensation

Answer 21

Incr pH
Incr HCO3
Incr CO2

Question 22

Base excess

Answer 22

High BE: >+2mmol/L, primary metabolic alkalosis/comp resp acidosis
Low BE:

Question 23

Rate of Compensation

Answer 23

Resp compensation for metabolic disorder: quickly (by adjusting alveolar vent)
Metabolic compensation for resp disorder: few days (kidneys must adjust HCO3 production)

Question 24

Mixed Acidosis and Alkalosis

Answer 24

CO2 and HCO3 moving in opposite directions

Treatment is directed towards correcting each primary acid/base disturbance

Question 25

Causes of Resp Acidosis

Answer 25

Respiratory depression (e.g. opiates) Guillain-Barre: paralysis leads to an inability to adequately ventilate Asthma COPD Iatrogenic (incorrect mechanical ventilation settings)

Question 26

Causes of Resp Alkalosis

Answer 26

Anxiety (i.e. panic attack) Pain: causing an increased respiratory rate. Hypoxia: resulting in increased alveolar ventilation in an attempt to compensate. Pulmonary embolism Pneumothorax Iatrogenic (e.g. excessive mechanical ventilation)

Question 27

Causes of Metabolic Acidosis

Answer 27

Increased acid production or acid ingestion. | Decreased acid excretion or rate of gastrointestinal and renal HCO3– loss.

Question 28

High Anion Gap Metabolic Acidosis

Answer 28

Diabetic ketoacidosis Lactic acidosis Aspirin overdose Renal failure

Question 29

Normal Anion Gap Metabolic Acidosis

Answer 29

Gastrointestinal loss of HCO3– (e.g. diarrhoea, ileostomy, proximal colostomy) Renal tubular disease Addison’s disease

Question 30

Causes of Metabolic Alkalosis

Answer 30

Gastrointestinal loss of H+ ions (e.g. vomiting, diarrhoea) Renal loss of H+ ions (e.g. loop and thiazide diuretics, heart failure, nephrotic syndrome, cirrhosis, Conn’s syndrome) Iatrogenic (e.g. addition of excess alkali such as milk-alkali syndrome)

Question 31

Causes of Mixed Resp/Metabolic Acidosis

Answer 31

Cardiac arrest | Multi-organ failure

Question 32

Causes of Mixed Resp/Metabolic Alkalosis

Answer 32

Liver cirrhosis in addition to diuretic use Hyperemesis gravidarum Excessive ventilation in COPD