Interpretation of lung function and ABG Flashcards
The normal flow volume loop, changes in spirometry in restrictive and obstructive pathology, interpretation of an ABG, acid-base disturbance and compensation
Normal flow-volume loop
Flow-volume loop in obstructive small airways disease
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
Grading of severity of obstruction
Flow-volume loop in restrictive airways disease
Shape/morphology of the loop is preserved but amplitude is reduced. NB spirometry can only suggest restriction - need confirmation by measuring lung volumes
Flow-volume loop in fixed large airway obstruction e.g. circumferential invading tumours of trachea, tracheal stenosis following prolonged intubation
Limitation to both inspiration and expiration - resulting in plateauing of both the expiratory and inspiratory limbs of the loop
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
Flow-volume loop in variable obstruction of large airways where the obstruction is extra-thoracic
Why perform an ABG?
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
Step-wise approach to ABG interpretation (with regards to assessing causes of hypoxia, SOB)
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
Validating an ABG sample
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
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
A-a at FiO2 0.21 at sea level = (150- (1.25PaCO2)) - Pa O2
Normal ~ (Age in years/4) + 4
When does respiratory acidosis occur?
When ventilation is inadequate to eliminate PCO2 at the same rate it is produced by the tissues
Compensation rules for respiratory acidosis
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
When does respiratory alkalosis occur?
When ventilation occurs at a level that eliminates CO2 in excess of that produced by metabolism
Compensation rules for respiratory alkalosis
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
When does metabolic alkalosis occur?
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)
Compensation rules for metabolic alkalosis
NB: all respiratory compensation is acute
Via hypoventilation, PaCO2 increases by 0.7mmHg for each mmol/L decrease in [HCO3-]
When does metabolic acidosis occur?
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
Compensation rules for metabolic acidosis
Occurs via hyperventilation
PaCO2 decreases by 1.2mmHg for each 1 mmol/L decrease in [HCO3-]
Winter’s Formula - what is it used for?
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
Winter’s formula
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
Normal anion gap calculation
([Na+] + [K=]) - ([Cl-] +[HCO3-]) = ~11 (largely attributable to the presence of albumin which is neg charged)
What is the anion gap calculation used for?
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
Examples of NAGMA
Diarrhoea
Renal wasting
Chloride excess
Examples of a HAGMA
Ketones
ETOH
Methanol
Uraemia
Ethylene glycol (antifreeze)
Isoniazid
Salicylates