ABG and Respiratory Failure

Question 1

Who needs a blood gas?

Answer 1

-ALL SICK PATIENTS
-Respiratory function
=hypoxia
=hypercapnia

-Metabolic disturbance
=Diabetic Ketoacidosis
=Sepsis
=Acidosis in renal failure
=Electrolytes

-Bleeding

Question 2

How to measure ABG

Answer 2

-Single arterial puncture technique
=Radial artery
=Femoral artery
=Brachial artery

-Measurement from in-dwelling arterial catheter or A-line
=Critical care only

Question 3

What does a blood gas measure?

Answer 3

-Hydrogen ion/pH
-PO2
-PCO2
-Bicarbonate
-Hb
-Electrolytes
-Lactate

Question 4

What are normal blood gas values at room air?

Answer 4

H+ 36-44 nmol/l
• PO2 12-15 kPa
• PCO2 4.4-6.1 kPa
• HCO3 21-27.5 mmol/l
• BE +2 to -2 mmol/l

Question 5

What does a blood gas tell us?

Answer 5

• Is there Respiratory Failure ?
– Severity?
– Type of respiratory failure?
• It there a metabolic disorder?
• Is it normal?

Question 6

What is respiratory failure?

Answer 6

Type 1:
-pO2 <8
-pCO2<6

Type 2
-pO2<8
-pCO2>6

Question 7

What causes respiratory failure and impaired gas exchange?

Answer 7

• Brain
=Respiratory depression (opiates)
• Thoracic Cage
=Extra-thoracic restriction (obesity / scoliosis)
=Neuromuscular (MND)
• Airways
=COPD/Asthma
• Alveolar
=Pneumonia / Oedema
• Interstitium
=Fibrosis
• Blood
=Anaemia

Question 8

Systems involved in acid base balance

Answer 8

-Neutralising
=Buffer systems
-Regulating
=Lungs
=Kidneys

Question 9

What are buffers?

Answer 9

-These systems work by neutralising the acid or base.
-They can tie up excess H+ temporarily.
-They only neutralise the H+, they can’t excrete them. H+ still have to be eliminated.
-If the H+ ions are not excreted the buffers will be exhausted.
-PREVENT EXTREME CHANGES IN FREE HYDROGEN ION CONCENTRATION

Question 10

Acid base balance equation

Answer 10

CARBONIC ACID-BICARBONATE BUFFER SYSTEM
CO2 + H2O ◄► H2CO3 ◄►H+ + HCO3-

Carbon Dioxide + Water (carbonic anhydrase)◄► Carbonic Acid◄►Hydrogen Ion + Bicarbonate

Question 11

Respiratory regulation in acid base balance

Answer 11

-Quick acting.
-Eliminates CO2
(Volatile acid)
-When PaCO2 increases, chemo receptors send messages to the respiratory centre in the brain to increase R.R. and CO2 is blown off
-If H+ is decreased respiratory rate decreased and Co2 is retained

Question 12

Renal regulation in acid base balance

Answer 12

-Very powerful acid-base regulator
-Slow acting – takes hours to days
-Eliminates all other acids other that CO2 (fixed acids not volatile acids)
-Regenerates and reabsorbs bicarbonate

Question 13

ABG interpretation

Answer 13

1. Acidosis/ alkalosis
2. Metabolic/ respiratory
3. Respiratory/ renal (metabolic) compensation

Question 14

Acidosis ABG

Answer 14

H+>44nmol/l

-HCO3<21 mmol/l (metabolic)
=PCO2<4.4kPa (respiratory compensation)

-PC02>6.1kPa (respiratory)
=HCO3>27.5mmols/l (renal compensation)

Question 15

Alkalosis ABG

Answer 15

H+<36nmol/l

-HCO3>27.5mmols/l (metabolic)
=PCO2>6.1kPa (respiratory compensation)

-PCO2<4.4kPa (respiratory)
=HCO3<21mmols/l (renal compensation)

Question 16

Causes of respiratory acidosis

Answer 16

• HYPOVENTILATION
• Due to
=Airways obstruction (COPD/Asthma)
=Inadequate chest expansion (neuromuscular disease)
=Decreased lung compliance (pulmonary oedema)
=Respiratory depression (opiates)
• Lungs cannot “blow off” CO2 so retain and
convert to H+

Question 17

Causes of metabolic acidosis

Answer 17

• Overproduction (DKA, lactic acidosis)
• Decreased elimination (renal failure)
• Increased ingestion of acid (e.g. drug overdose)
• Loss of bicarbonate (e.g. diarrhoea)

Question 18

Causes of respiratory alkalosis

Answer 18

-HYPERVENTILATION
• Hyperventilation syndromes e.g. anxiety
• Raised intracranial pressure
• Over-ventilation in ITU

Question 19

Causes of metabolic alkalosis

Answer 19

• Due to an excess loss of acid or net increase in
bicarbonate
=Excessive vomiting
=Diuretic therapy
=Hypokalaemia
=Ingestion of bicarbonate

Question 20

Important compensation example

Answer 20

The most important of these are metabolic compensation of type 2 respiratory failure and respiratory compensation of metabolic acidosis

Question 21

Why can compensation be difficult?

Answer 21

• Renal compensation of resp acidosis may take
days to develop
• Severe metabolic acidosis is not likely to be
compensated by the respiratory system
• Therefore treating the underlying illness is
important e.g. NIV for type 2 resp failure in
COPD or treatment of DKA