Interpretation of lung function and ABG

Question 1

Normal flow-volume loop

Answer 1

Question 2

Flow-volume loop in obstructive small airways disease

Answer 2

Initial ‘scalloping of the expiratory loop’ - while FVC (amplitude) is mostly retained - then as degree of obstruction progresses the amplitude falls due to dynamic airway collapse resulting in a fall in VC

Question 3

Grading of severity of obstruction

Answer 3

Question 4

Flow-volume loop in restrictive airways disease

Answer 4

Shape/morphology of the loop is preserved but amplitude is reduced. NB spirometry can only suggest restriction - need confirmation by measuring lung volumes

Question 5

Flow-volume loop in fixed large airway obstruction e.g. circumferential invading tumours of trachea, tracheal stenosis following prolonged intubation

Answer 5

Limitation to both inspiration and expiration - resulting in plateauing of both the expiratory and inspiratory limbs of the loop

Question 6

Flow-volume loop in variable obstruction of large airways (e.g. tracheal polyps, non-circumferential tumours with vocal cord paralysis) where the obstruction is intra-thoracic

Answer 6

Question 7

Flow-volume loop in variable obstruction of large airways where the obstruction is extra-thoracic

Answer 7

Question 8

Why perform an ABG?

Answer 8

1) To accurately assess causes of SOB or hypoxia
- by allowing calculation of Alveolar-arterial gradient i.e. A-a gradient, which is a global assessment of pulmonary gas exchange
- since normal O2 sats do NOT exclude a problem with ventilation!

2) Determine acid-base balance and assess for cause of acidaemia or alkalaemia

Question 9

Step-wise approach to ABG interpretation (with regards to assessing causes of hypoxia, SOB)

Answer 9

1) Understand the indication - why was the test performed?
2) Ensure accurate record of the FiO2 at time of testing
3) pH n = 7.35-7.45, outside these is acidaemia and alkalaemia
4) PaO2 n = 80-100mmHg
5) PaCO2 n = 35-45mmHg
6) HCO3- n = 24-25mmol/L
7) O2 Sats (Hb) n = >94% NB: check against finger sats, if gas value is significantly lower consider accidental venous sampling
8) Na, K, Cl
9) Others - gluc, lactate

Question 10

Validating an ABG sample

Answer 10

1) Calculate the hydrogen ion concentration using Henderson-Hasselbach equation

[H+] = 24 (PaCO2/ [HCO3-])

If the pH obtained differs substantially from the calculated value consider a sampling or analysis error and recollect

Question 11

Calculating A-a gradient (Alveolar-arterial gradient)

When an ABG is obtained at Fi02 RA i.e. 21% the A-a gradient can be used to assess the ability of the oxygen to transfer from the alveolus into the bloodstream effectively. So… if A-a gradient is normal this argues AGAINST any parenchymal lung disease and point towards hypoventilation as a cause of the hypoxia

Answer 11

A-a at FiO2 0.21 at sea level = (150- (1.25PaCO2)) - Pa O2

Normal ~ (Age in years/4) + 4

Question 12

When does respiratory acidosis occur?

Answer 12

When ventilation is inadequate to eliminate PCO2 at the same rate it is produced by the tissues

Question 13

Compensation rules for respiratory acidosis

Answer 13

Acute response (mins to hrs):
[HCO3-] increases by 1mmol/L for every 10mmHg rise in PaCO2

Chronic response:
[HCO3-] increased by 4mmol/L for every 10mmHg rise in PaCO2

Question 14

When does respiratory alkalosis occur?

Answer 14

When ventilation occurs at a level that eliminates CO2 in excess of that produced by metabolism

Question 15

Compensation rules for respiratory alkalosis

Answer 15

Acute response (mins to hrs):
[HCO3-] decreases by 2mmol/L for every 10mmHg rise in PaCO2

Chronic response:
[HCO3-] decreased by 5mmol/L for every 10mmHg rise in PaCO2

Question 16

When does metabolic alkalosis occur?

Answer 16

Through excessive loss of acidic fluid (e.g. prolonged vomiting), or excessive consumption of alkaline substances (e.g. Milk alkali syndrome via excessive ingestion calcium carbonate)

Question 17

Compensation rules for metabolic alkalosis

Answer 17

NB: all respiratory compensation is acute

Via hypoventilation, PaCO2 increases by 0.7mmHg for each mmol/L decrease in [HCO3-]

Question 18

When does metabolic acidosis occur?

Answer 18

Through excessive ACCUMULATION of acidic fluid
- can be endogenous e.g. increased [H+] associated with sepsis
- can be exogenous e.g. ethanol, methanol, salicylates

or

Through excessive LOSS of alkaline substances e.g. prolonged diarrhoea

Question 19

Compensation rules for metabolic acidosis

Answer 19

Occurs via hyperventilation

PaCO2 decreases by 1.2mmHg for each 1 mmol/L decrease in [HCO3-]

Question 20

Winter’s Formula - what is it used for?

Answer 20

Used in metabolic acidosis
Uses the patient’s HCO3- level to calculate what the appropriate respiratory compensation should be if this was a PURE metabolic acidosis - if it does not match with this, it means there might also be a primary respiratory issue i.e. acidosis or alkalosis going on

Question 21

Winter’s formula

Answer 21

Expected PaCO2 = (1.5x HCO3-) +8 +/-2

If:
Measured CO2 = expected = appropriate respiratory compensation is occurring

Measured CO2 > expected = inadequate compensation or concurret respiratory acidosis

Measured CO2 < expected = concurrent respiratory alkalosis

Question 22

Normal anion gap calculation

Answer 22

([Na+] + [K=]) - ([Cl-] +[HCO3-]) = ~11 (largely attributable to the presence of albumin which is neg charged)

Question 23

What is the anion gap calculation used for?

Answer 23

For analysing whether a metabolic acidosis is due to a HAGMA or NAGMA as specific conditions fall into either category and this is useful for diagnosing the CAUSE of a metabolic acidosis

Question 24

Examples of NAGMA

Answer 24

Diarrhoea
Renal wasting
Chloride excess

Question 25

Examples of a HAGMA

Answer 25

Ketones
ETOH
Methanol
Uraemia
Ethylene glycol (antifreeze)
Isoniazid
Salicylates