Case Based Examples

Practice analysing some blood gas results

Case 1

A 32 year old female is brought to the emergency department due to a severe acute asthma attack. On examination she is tachypnoeic with a respiratory rate of 28 breaths/min, tachycardic with a heart rate of 140 beats/min, and is unable to complete sentences in one breath.

She is placed on oxygen at 15L/min through a non-rebreathe mask, and an arterial blood gas is taken.

ABG Results

Blood Gas Values
  pH 7.410   [ 7.350 - 7.450 ]
↓ pCO2 3.5 kPa [ 4.7 - 6.0 ]
↓ pO2 7.4 kPa [ 10.0 - 13.0 ]
Acid Base Status
  cHCO3-(P)c 23.9 mmol/L [ 22.0 - 28.0 ]
  cBase(B)c 0.8 mmol/L [ -2.0 - 2.0 ]

Questions

She has Type 1 respiratory failure (low PaO2 with normal/low PaCO2).

Her PaCO2 is low because she is tachypnoeic (RR 28/min), so she is blowing off more CO2 than normal.

A rough esimation for expected PaO2 is FiO2 - 10. Her FiO2 is 36% so we would expect her PaO2 to be around 26 kPa.

Case 2

A 74 year old man with known COPD is being assessed for home oxygen therapy. The results of his most recent arterial blood gas are shown below.

ABG Results

Blood Gas Values
↓ pH 7.320   [ 7.350 - 7.450 ]
↑ pCO2 7.1 kPa [ 4.7 - 6.0 ]
↓ pO2 8.0 kPa [ 10.0 - 13.0 ]
Acid Base Status
↑ cHCO3-(P)c 32.0 mmol/L [ 22.0 - 28.0 ]
  cBase(B)c -1.3 mmol/L [ -2.0 - 2.0 ]

Questions

This patient has Type 2 respiratory failure (low PaO2 with high PaCO2).

His pH is low due to carbon dioxide retention, resulting in an increased PaCO2. This shifts the buffer equation to produce more carbonic acid which dissociates to form H+ ions, lowering the pH.

This patient has a respiratory acidosis (low pH, high PaCO2) with metabolic compensation (increased HCO3-). The compensated increase in HCO3- takes days to occur, in keeping with his chronic COPD.

Case 3

A known type 1 diabetic is brought to the emergency department complaining of fatigue, nausea, and muscle aches. On questioning you discover they have had a flu-like illness for the past week and have been missing insulin doses due to limited oral intake.

On examination they are tachypnoeic (RR 30/min), and tachycardic (HR 118/min), with dry mucous membranes and a fruity smell on their breath.

ABG Results

Blood Gas Values
↓ pH 7.250   [ 7.350 - 7.450 ]
↓ pCO2 4.1 kPa [ 4.7 - 6.0 ]
  pO2 11.4 kPa [ 10.0 - 13.0 ]
Acid Base Status
↓ cHCO3-(P)c 16.9 mmol/L [ 22.0 - 28.0 ]
↓ cBase(B)c -8.4 mmol/L [ -2.0 - 2.0 ]
Electrolyte Values
  cK+ 3.9 mmol/L [ 3.5 - 5.3 ]
  cNa+ 138 mmol/L [ 135 - 145 ]
  cCa2+ 2.24 mmol/L [ 2.10 - 2.50 ]
  cCl- 102 mmol/L [ 95 - 108 ]
Metabolite Values
↑ cGlu 27.0 mmol/L [ 3.5 - 5.4 ]
↑ cLac 2.5 mmol/L [ 0.5 - 2.2 ]

Questions

The ABG shows a metabolic acidosis (low pH, low HCO3-) with respiratory compensation (low PaCO2).

The anion gap is calculated with the formula Na+ - (Cl- + HCO3-). Using the ABG values we get 138 - (102 + 16.9) = 19.1 mmol/L. The normal range for anion gap is 4-12 mmol/L so this patient has a high anion gap.

A high anion gap metabolic acidosis suggests the addition of acid (rather than the removal of bicarbonate). Given the history of diabetes with missed insulin doses and the fruity smell on the patients breath (suggestive of ketones) the most likely diagnosis is diabetic ketoacidosis.

Case 4

A 65 year old female is admitted to the acute medical assessment unit by her GP due to concerns about severe dehydration following 3 days of intractable vomiting thought to be caused by food poisoning.

On examination she is pale and lethargic with dry mucous membranes. Her heart rate is 145/min, with a respiratory rate of 7 breaths/min.

ABG Results

Blood Gas Values
↑ pH 7.540   [ 7.350 - 7.450 ]
↑ pCO2 8.6 kPa [ 4.7 - 6.0 ]
  pO2 10.4 kPa [ 10.0 - 13.0 ]
Acid Base Status
↑ cHCO3-(P)c 37.6 mmol/L [ 22.0 - 28.0 ]
↑ cBase(B)c 14 mmol/L [ -2.0 - 2.0 ]
Electrolyte Values
↓ cK+ 3.2 mmol/L [ 3.5 - 5.3 ]
↓ cNa+ 132 mmol/L [ 135 - 145 ]
  cCa2+ 2.12 mmol/L [ 2.10 - 2.50 ]
↓ cCl- 88 mmol/L [ 95 - 108 ]
Metabolite Values
  cGlu 3.7 mmol/L [ 3.5 - 5.4 ]
  cLac 0.8 mmol/L [ 0.5 - 2.2 ]

Questions

This ABG shows a metabolic alkalosis (high pH, high HCO3-) with respiratory compensation (high PaCO2).

In metabolic acidosis the body may retain carbon dioxide by reducing the respiratory rate, in an attempt to lower the pH.

Acid-base disorders alter potassium transport, so it is likely to fall in alkalosis (and rise in acidosis). Vomiting causes loss of hydrochloric acid (HCl) from the stomach, reducing the chloride ion concentration and resulting in a hypochloraemia.

Case 5

A 54 year old male is admitted to the vascular ward waiting for a peripheral arterial bypass graft for right lower limb ischaemia. During the morning ward round he complains of severe pain in his right foot that started overnight.

On examination he is tachypnoeic (RR 32/min) and tachycardic (HR 120/min). His right foot is pale and his first three toes are black. No pulses are palpable below the right knee.

His admission ABG taken 48 hours ago is normal, and his observations from the previous day were also normal. A repeat ABG is taken, and the results are shown below.

ABG Results

Blood Gas Values
↑ pH 7.480   [ 7.350 - 7.450 ]
↓ pCO2 2.9 kPa [ 4.7 - 6.0 ]
  pO2 12.0 kPa [ 10.0 - 13.0 ]
Acid Base Status
↓ cHCO3-(P)c 19.8 mmol/L [ 22.0 - 28.0 ]
↓ cBase(B)c -2.3 mmol/L [ -2.0 - 2.0 ]
Electrolyte Values
  cK+ 5.2 mmol/L [ 3.5 - 5.3 ]
  cNa+ 141 mmol/L [ 135 - 145 ]
  cCa2+ 2.28 mmol/L [ 2.10 - 2.50 ]
  cCl- 97 mmol/L [ 95 - 108 ]
Metabolite Values
  cGlu 3.7 mmol/L [ 3.5 - 5.4 ]
↑ cLac 12.3 mmol/L [ 0.5 - 2.2 ]

Questions

The only value on the ABG consistent with an alkalaemia is the low PaCO2. The patient is tachypnoeic (likely due to severe pain) which would explain the low PaCO2.

The patient has a lactate of 12.3 which is high. This is likely to result in a lactic acidosis, which would lower the bicarbonate as it tries to mop up excess H+ ions.

This pattern of results (high pH, low PaCO2, low bicarbonate) could be consistent with a respiratory alkalosis with metabolic compensation, however we know his admission ABG and observations were normal and metabolic compensation takes days to occur. In the context of this scenario, the results therefore do not fit with a single acid-base disorder.

This ABG is showing a mixed picture: this patient has a primary respiratory alkalosis and also a primary metabolic acidosis.