ABGs

Question 1

What is a normal anion gap?

Answer 1

12 +/ - 4

Question 2

How do you calculate the delta gap

Answer 2

Change Anion gap/Change HCO3-

(AG - 12)/ (24 - HCO3)

Question 3

How do you correct the anion gap corrected for albumin?

Answer 3

every 4g/L decrease albumin will decrease AG by 1

the normal anion gap depends on serum phosphate and serum albumin

albumin is the major unmeasured anion and contributes almost the whole of the value of the anion gap.
every 1g/L decrease in albumin will decrease anion gap by 0.25 mmoles
a normally high anion gap acidosis in a patient with hypoalbuminaemia may appear as a normal anion gap acidosis.
this is particularly relevant in ICU patients where lower albumin levels are common
also significantly deranged phosphate will alter AG too

Question 4

How do you correct sodium for glucose?

Answer 4

= Na + (glucose - 5)/3

Question 5

What does delta gap of 0.4 to 0.8 mean?

Answer 5

Normal anion gap acidosis +
High anion gap acidosis

Question 6

What does a delta gap of <0.4 mean?

Answer 6

Pure normal AG metabolic acidosis

Question 7

What does a delta gap of 0.8 - 2 mean?

Answer 7

Pure high anion gap acidosis

Question 8

What is the age adjusted A-a gradient?

Answer 8

Age/4 + 4

Question 9

What does a delta gap of > 2 mean?

Answer 9

Metabolic acidosis +
Metabolic alkalosis

Question 10

What is the expected pCO2 in a metabolic acidosis?

Answer 10

1.5 x HCO3 + 8 (+/- 2)

Question 11

What is the expected pCO2 in metabolic alkalosis?

Answer 11

0.7 x HCO3 + 20 (+/- 5)

Question 12

What is the expected K+ in acidosis?

Answer 12

5 + (0.5 x1 for every 0.1 unit change below 7.4)

K+ exchanges with H+ intracellularly to correct for acidosis. More K+ will become extracellular

Question 13

What is the expected K+ in alkalosis?

Answer 13

5 - (0.5 x 1 for every 0.1 unit change above 7.4)

Question 14

What does an elevated Aa gradient suggest?

Answer 14

VQ mismatch
- APO
- LRTI
- PE
- Aspiration
- ARDS
- membrane disease

Question 15

Explain the 1,2,3,4,5 rule for interpreting respiratory acidosis and alkalosis

Answer 15

3 is reminded to that this is correction for HCO3

Question 16

What is the “saturation gap”

Answer 16

Difference between paO2 on ABG sats from a pulse oximeter.
If it is greater than 5%, the hemoglobin may be abnormal:
- carbon monoxide poisoning
- methemoglobinemia
- sulfhemoglobinemia

sats 85% = 50
Sats 92% = 64
Sats 99% = 100

Question 17

What are the three broad causes of lactate elevation and give one example of each

Answer 17

Type A: inadequate oxygen delivery
–hypotension

Type B1: Diseases
- hepatic failure

Type B2: Toxins
- salicylates

Question 18

Name toxins that cause a HAGMA

Answer 18

• salicylates
• metformin
• salbutamol
• adrenaline
• ethanol
• toxic alcohol e.g methanol
• iron

Question 19

Name the three most common causes of a NAGMA

Answer 19

1) Normal saline (hyperchloraemia)
2) diarrhoea
3) renal disease (CKD, RTA)

consider pseudo NAGMA if albumin low.

Question 20

why is it vital to consider if someone is volume deplete or volume overloaded when figuring out the cause of metabolic alkalosis?

Answer 20

Volume deplete: stimulates aldosterone secretion –> retain sodium, retain HCO3. Renal loss of K+ and H+

Volume overloaded: primary hyperaldosteronsim
–> retain sodium, retain HCO3. Renal loss of K+ and H+

Question 21

A pt is volume deplete and has a metabolic alkalosis. Name 3 causes

Answer 21

• diuretic use
• vomiting/NG suctioning
• laxative abuse

• Gitelman or Barter syndrome (salt wasting renal disease)
• villous adenoma (secretory diarrhoea)
• cystic fibrosis (excess sodium chloride loss
• milk alkali syndrome, excess calium intake stimulates diuresis

Question 22

A pt is volume overloaded and has a metabolic alkalosis. Name 3 causes

Answer 22

• Hyeraldosteronism (Conn’s syndrome)
• Excess ACTH (pituitary adenoma, ectopic)
• High dose corticosteroid

• Liccorice ingestion
• Liddle syndrome

Question 23

What are three broad categories for respiratory acidosis?

Answer 23

1) Inadequate alveolar ventilation e.g COPD
2) Excess CO2 production e.g thyroid storm
3) Increased CO2 intake e.g rebreathing CO2

Question 24

Name 3 causes for inadequate alveolar ventilation causing respiratory acidosis

Answer 24

• central respiratory depression
• neuromuscular disease
• lung/chest wall defects
• airway obsturction/increased dead space e.g COPD
• inadequate mechanical ventilation

Question 25

Respiratory alkalosis can be though of as due to two broad reasons. What are they?

Answer 25

1) Central causes e.g head injury 2) Pulmonary causes e.g hypoxia ## Footnote Both can cause hyperventilation

Question 26

Name 5 central causes of respiratory acidosis

Answer 26

- head injury - CVA - anxiety/pain - progesterone (pregnancy) - Cytokines during sepsis

Question 27

Name 5 pulmonary causes of respiratory alkalosis

Answer 27

- hypoxia - PE - pneumonia - asthma - pulmonary oedema