ABGs

Question 1

How can you determine whether an acid base disturbance is respiratory or metabolic?

Answer 1

If the pH and pCO2 are moving in the same direction (either both high or both low) then it is metabolic. If they move in opposite directions then it is a respiratory cause.

Question 2

Name some examples of how a metabolic acidosis occur?

Answer 2

DKA, lactic acidosis, aspirin overload or renal failure. Diarrhoea

Question 3

What are some causes of metabolic alkalosis?

Answer 3

GI losses of H+ (vomiting mainly, sometimes diarrhoea),
Renal loss of H+ ions (diuretics).

Question 4

Explain how sepsis affects ABGs

Answer 4

Sepsis can cause reduced end organ perfusion which will cause tissue hypoxia. This results in anaerobic respiration with accumulation of lactic acid. Resulting in lactic acidosis.

Question 5

Explain how DKA affects ABGs

Answer 5

Absence of insulin, causes reduced glucose into cells so there is release of FFA which are converted into ketone bodies which causes the blood to become more acidic.

Question 6

What are some causes of mixed respiratory and metabolic acidosis?

Answer 6

Cardiac arrest and multi-organ failure

Question 7

What are some causes of mixed respiratory and metabolic alkalosis?

Answer 7

Liver cirrhosis and diuretic use,
hyperemesis gravidarum,
excessive ventilation in COPD

Question 8

What are the normal ABG values?

Answer 8

H+ = 36-43mmol/L
pH = 7.35-7.45
PaCO2 = 4.6-6.0 kPa
PaO2 = 10.5-13.5 kPa
HCO3 = 23-30 mmol/L

Question 9

What is type 1 respiratory failure and some causes

Answer 9

Low O2 with normal/low CO2. Aim for O2 sats of 94-98%.
Aeitiology: Ventilation-perfusion mismatch. Smaller volume of air in lungs in comparison to blood perfusing lungs.
Causes: pneumonia, PE, pulmonary oedema, pulmonary fibrosis, asthma, upper airway obstruction

Question 10

What is type 2 respiratory failure and what are some causes?

Answer 10

Low O2 with a high CO2. Aim for O2 sats of 88-92%.
Aeitology: Alveolar hypoventilation - fail to oxygenate and blow off CO2
Causes include COPD, idiopathic lung disease neuromuscular disease, opioid toxicity, thoracic wall disease (rib fractures) and obesity hypoventilation.

Question 11

What is the normal anion gap and when is it useful?

Answer 11

It is useful in determining the cause of a metabolic acidosis Normal = 8-16mmol/L

Question 12

What are some causes of a raised anion gap metabolic acidosis?

Answer 12

• renal failure, DKA, lactic acidosis, toxins

Question 13

What are some cause of a normal anion gap metabolic acidosis?

Answer 13

Renal tubular acidosis, diarrhoea, carbonic anhydrase inhibitors and ureteric diversion

Question 14

What is the presentation of acute respiratory failure?

Answer 14

Fever and cough - think pneumonia
Stridor, cough and wheeze - think obstruction
Crackles - think pulmonary oedema or ARDS
Muscle weakness - think GBS, MG or myositis
Decreased GCS - think CNS depression eg, opioid overload

Question 15

What investigations should be done for suspected respiratory failure?

Answer 15

• ABG,
• Pulse oximetry
• Investigate for specific causes eg, D-dimer, FBC to look for high WCC, chest x-ray

Question 16

What is the management of acute respiratory failure in stable patients and unstable

Answer 16

Stable and conscious - Oxygen, treat underlying cause. 2nd line is non-invasive ventilation.
Stable and unconscious - Oxygen, treat underlying cause. 2nd line is ETT
Unstable - Oxygen therapy and ET intubation and mechanical ventilation

Question 17

What should you be thinking in someone with type 1 resp failure?

Answer 17

1. Are they breathing air with enough O2 in it?
2. Is there a problem the lung structure? Think chest wall abnormality, pleural effusions, reduced innervation, weak muscles
3. V/Q mismatch? Think reduced blood flow (PE) or reduced O2 flow - Bronchospasm, mucus plug, alveolar collapse or filled alveolar (pus or fluid)