ABG interpretation Flashcards
What is the normal pH of blood?
7.35 - 7.45
What is a normal PaCO2?
4.7 – 6.0 kPa
What is a normal PO2?
11 – 13 kPa
What is a normal HCO3-?
22-26 mmol/l
What is a normal base excess?
-2 to +2 mmol/l
When looking at an ABG, what question should be asked first?
Is this patient hypoxic?
What should the PaO2 be for a patient on room air?
> 10 kPa
What should the PaO2 be for a patient on oxygen therapy?
10kPa less than the % inspired concentration FiO2
What type of mask will give an accurate concentration of oxygen to the patient regardless of the oxygen flow rate?
venturi mask
What % oxygen is given with
a) 1L/min?
b) 2L/min?
c) 3L/min?
d) 4L/min?
a) 24%
b) 28%
c) 32%
d) 36%
Which PaO2 indicates severe hypoxiaemia and respiratory failure for a patient on room air?
<8 kPa
Which type of respiratory failure has hypercapnia?
type 2
Which type of respiratory failure has normocapnia?
type 1
What is type 1 respiratory failure the result of?
V/Q mismatch
What is V/Q mismatch?
a mismatch of ventilation to perfusion ie. the volume of air going into the lungs doesn’t correlate with the volume of blood flowing into the lungs
Give some causes of type 1 respiratory failure
pulmonary oedema
pulmonary embolism
bronchoconstriction
Why does type 2 respiratory failure occur?
alveolar hypoventilation
Why might alveolar hypoventilation occur?
- increased resistance due to airway obstruction eg. COPD
- reduced compliance of chest wall or lung tissue eg. fracture, obesity
- reduced muscle strength eg. MND, guillain-barre
- drugs acting on respiratory centre eg. opiates
An imbalance in what causes a respiratory driven acidosis/alkalosis?
CO2
An imbalance in what causes a metabolism driven acidosis/alkalosis?
HCO3-
What helps to rule the respiratory system in or out as the cause for the derangement in pH?
abnormal CO2
What does CO2 bind with H2O to form?
carbonic acid (H2CO3)
What happens to pH when CO2 is
a) retained?
b) blown off?
a) increase
b) decrease
What is bicarbonate?
a base that helps to mop up H+
Give the carbonic acid equation
CO2 + H20 ⇌ H2CO3 ⇌ HCO3- + H+
What happens to pH when there is
a) increased HCO3-?
b) decreased HCO3-?
a) increases
b) decreases
When the cause of the problem is respiratory, how does the body compensate?
adjusts HCO3-
When the cause of the problem is metabolic, how does the body compensate?
blows off or retains CO2 (changes ventilation)
In respiratory acidosis, what is the
a) pH?
b) CO2?
c) HCO3?
a) decreased
b) raised
c) normal
In respiratory acidosis with metabolic compensation, what is the
a) pH?
b) CO2?
c) HCO3?
a) decreased or normal
b) raised
c) increased
In respiratory alkalosis, what is the
a) pH?
b) CO2?
c) HCO3?
a) increased
b) decreased
c) normal
In respiratory alkalosis with metabolic compensation, what is the
a) pH?
b) CO2?
c) HCO3?
a) high or normal
b) low
c) low
In metabolic acidosis, what is the
a) pH?
b) CO2?
c) HCO3?
a) low
b) normal
c) low
In metabolic acidosis with respiratory compensation, what is the
a) pH?
b) CO2?
c) HCO3?
a) low or normal
b) low
c) low
In metabolic alkalosis, what is the
a) pH?
b) CO2?
c) HCO3?
a) high
b) normal
c) high
In metabolic alkalosis with respiratory compensation, what is the
a) pH?
b) CO2?
c) HCO3?
a) high
b) high
c) high
What does a high base excess indicate?
high HCO3-
so primary metabolic alkalosis or a compensated respiratory acidosis
What does a low base index indicate?
low HCO3-
so primary metabolic acidosis or a compensated respiratory alkalosis
How quickly does
a) respiratory compensation
b) metabolic compensation
take to occur?
a) quickly
b) few days
What is the a) pH b) CO2 c) HCO3- in a mixed respiratory and metabolic acidosis?
a) low
b) increased
c) decreased
What is the a) pH b) CO2 c) HCO3- in a mixed respiratory and metabolic alkalosis?
a) high
b) low
c) high
Give some causes of respiratory acidosis
- respiratory depression due to opiated
- guillain-barre
- asthma
- COPD
- iatrogenic (incorrect mechanical ventilation)
Give some causes of respiratory alkalosis
- anxiety attack
- pain
- hypoxia
- pulmonary embolism
- iatrogenic (excess mechanical ventilation)
What is the anion gap?
used to evaluate metabolic acidosis to see if its due to increased acid production or ingestion vs decreased acid excretion or loss of HCO3–
How is the anion gap calculated?
Na+ - (Cl- + HCO3-)
What is a normal anion gap?
4-12 mmol/L
What does an increased anion gap indicate
increased production or ingestion
Give causes of metabolic acidosis that cause an increased anion gap
diabetic ketoacidosis
lactic acidosis
aspirin overdose
Give causes of metabolic acidosis that cause a decreased anion gap
GI loss of HCO3- (eg. diarrhoea, ileostomy, proximal colostomy)
renal tubular acidosis
addison’s disease
What does a decreased anion gap indicate?
decreased acid excretion or loss of HCO3-
Give some causes of metabolic alkalosis
GI loss of H+ (vomiting, diarrhoea)
renal loss of H+ (e.g. loop and thiazide diuretics/heart failure/nephrotic syndrome/cirrhosis/Conn’s syndrome)
Iatrogenic (e.g. addition of alkali such as milk-alkali syndrome)
How does hyperventilation lead to perioral and peripheral paresthesia?
hypocalcaemia
Because a portion of both hydrogen ions and calcium are bound to serum albumin, when blood becomes alkalotic, the bound hydrogen ions dissociate from albumin, freeing up the albumin to bind with more calcium and thereby decreasing the freely ionized portion of total serum calcium leading to hypocalcaemia.
