ABGs Interpretation Flashcards Preview

8. Respiratory > ABGs Interpretation > Flashcards

Flashcards in ABGs Interpretation Deck (19)
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1
Q

Describe the normal values on an ABG. Consider:

a) Normal [H+]
b) Normal pH
c) Normal pCO2
d) Normal pO2
e. Normal HCO3-

A

pH: 7.35-7.45

pCO2: 4.6-6.0

pO2: 10.5-13.5

HCO3-: 23-30 mmol/L

2
Q

What is the Henderson-Hasselbalch equation?

A

Relationship between [H+], pCO2 and [HCO3-]

pH = 6.1 + log[HCO3-] / (0.23 x pCO2)

3
Q

Describe the systematic approach to interpretation of ABGs. (6)

A
1. Look at [H+]
2, Look at pCO2
3. If pCO2 normal, look at HCO3-
4. Look at anion gap
5. Look for compensation
6. Look for osmolal gap
4
Q

When looking at [H+] on ABGs, what are you looking for?

What are the normal values?

A

Acidosis
Alkalosis

Normal: 36-43 nmol/L

5
Q

When looking at the pCO2 on ABGs, what are you looking for?

A

Whether H+ derangement is metabolic or respiratory

6
Q

In acidosis, what does:

a) High pCO2 indicate?
b) Low pCO2 indicate?

In alkalosis, what does:

a) High pCO2 indicate?
b) Low pCO2 indicate?

A

IN ACIDOSIS:
High pCO2 - respiratory acidosis
Low pCO2 - metabolic acidosis

IN ALKALOSIS:
High pCO2 - metabolic alkalosis
Low pCO2 - respiratory alkalosis

7
Q

In acidosis, what does:

a) High HCO3- indicate?
b) Low HCO3- indiacte?

In alkalosis, what does:

a) High HCO3- indicate?
b) Low HCO3- indicate?

A

IN ACIDOSIS:
High HCO3-: respiratory acidosis
Low HCO3-: metabolic acidosis

IN ALKALOSIS:
High HCO3-: metabolic alkalosis
Low HCO3-: respiratory alkalosis

8
Q

How do you calculate the anion gap?

What is the function?

A

Anion gap = [Na+] - ([Cl-] + [HCO3-])

Function: determines cause of metabolic acidosis

9
Q

What is the normal anion gap?

A

8-12 mmol/L

10
Q

List 5 causes of metabolic acidosis with a high anion gap.

A
Renal failure
Diabetic ketoacidosis
Lactic acidosis
Toxins (e.g. salicylate)
Inborn errors of metabolism
11
Q

List 4 causes of metabolic acidosis with a normal anion gap.

A

Renal tubular acidosis
Severe diarrhoea
Carbonic anhydrase inhibitors
Ureteric diversion

12
Q

In terms of compensation for acid-base disorders, what does each of the following indicate?

a) No compensation
b) Partial compensation
c) Complete compensation (i.e. normal H+)
d) Over-compensation

A

No compensation - acute H+ derangement

Partial compensation - acute H+ derangement

Complete compensation - chronic H+ derangement

Over-compensation - not possible; indicates mixed disorders (acidosis AND alkalosis)

13
Q

How fast do each of the 3 compensatory systems (buffers, respiratory and renal) act to compensate H+ derangement?

A

Buffers - immediate
Respiratory compensation - minutes-hours
Renal compensation - days

14
Q

How do you calculate the osmolal gap?

What other calculation is needed to do this?

A

Osmolal gap = (measured osmolality) - (calculated osmolality)

Calculated osmolality = 2([Na+] + [K+]) + urea + glucose

15
Q

What is the normal osmolal gap?

A

<10 mOsm/kg

16
Q

What is the normal serum osmolality?

A

275-295 mOsm/kg

17
Q

What does a raised osmolal gap indicate? (2)

A

Something else dissolved in the serum (a possible cause of metabolic acidosis)

Ethanol

18
Q

When would you suspect MIXED acid-base disorders when looking at ABGs? (3)

A

Inadequate or too extreme compensation (over-compensation not possible!)

If pCO2 and HCO3- are abnormal in opposite directions (i.e. one increased, one decreased)

If pH is normal but pCO2 or HCO3- is abnormal

19
Q

Apart from ABGs, which 2 additional biochemistry might you order in acid base disorders?

A
Lactate levels
Glucose levels (may indicate ketoacidosis)