MODULE FOUR short answer questions - ABG
saq 1.
A 50 year old female presents to your department after a heroin overdose. Her vital signs are:
HR 76 /min
BP 90/62 mmHg
RR 3 /min
O2 sats 2L 96% 2L via prongs
T 37.2 oC
An arterial blood gas is shown below.
FiO2 28%
pH 7.20
pO2 87 mmHg
PCO2 105 mmHg
HCO3- 30 mmol/L
BE +8
Question 1. What are the major acid base disturbances? (3 marks)
Must include:
- mild-moderate acidaemia
- respiratory acidosis
- (compensatory) metabolic alkalosis
Question 2. What is the primary acid-base disturbance? (1 mark)
Respiratory acidosis.
Question 3. What HCO3- levels would you expect as acute and chronic compensation for the primary disturbance? (2 marks)
Acutely 30mmol/L
Bicarbonate rises by 1 for each 10mmHg of CO2 over 40mmHg
Chronically 50mmol/L
Bicarbonate rises by 4 for each 10mmHg of CO2 over 40mmHg
Question 4. Is there a V/Q mismatch, or is the primary problem hypoventilation? Why? (4 marks)
There is no V/Q mismatch.
The Aa gradient is calculated as:
225mmHg - 87 - 105 x 1.25
= 225 - 87 - 131
= 225 - 228
= 7mmHg.
The expected Aa gradient for a patient this age is 50/4 + 4 = 16.5.
Therefore the Aa gradient is normal, and no V/Q mismatch exists.
***NOTE, one of our candidates asked why we used 225mmHg for 28% oxygen. This is a great question, and we have left our original answer and the update here for everyone's information. Thanks Ignatius!***
For the examination, where no calculators are allowed (yes, seriously) you need to have something that works well for calculating alveolar oxygen pressure.
If you remember at sea level that the first number in the alveolar gas equation is 150 (on 21% oxygen, you can calculate through the Aa gradient for different FiO2s).
Out of (call it intellectual laziness if you want!) Luke used to approximate Oxygen values as follows:
- 21% = 150 mmHg
- 28% (1.5 x 21%) = 225mmHg
- 40% (2 x 21%) = 300 mmHg
- 60% (3 x 21%) = 450 mmHg.
Of course, a better approximation for 28% oxygen is 1.3 x 21%, i.e. 200mmHg. Rarely will it make an examination difference (and never in clinical practice!!!!) but it's worth using the most accurate approximation.
HR 76 /min
BP 90/62 mmHg
RR 3 /min
O2 sats 2L 96% 2L via prongs
T 37.2 oC
An arterial blood gas is shown below.
FiO2 28%
pH 7.20
pO2 87 mmHg
PCO2 105 mmHg
HCO3- 30 mmol/L
BE +8
Question 1. What are the major acid base disturbances? (3 marks)
Must include:
- mild-moderate acidaemia
- respiratory acidosis
- (compensatory) metabolic alkalosis
Question 2. What is the primary acid-base disturbance? (1 mark)
Respiratory acidosis.
Question 3. What HCO3- levels would you expect as acute and chronic compensation for the primary disturbance? (2 marks)
Acutely 30mmol/L
Bicarbonate rises by 1 for each 10mmHg of CO2 over 40mmHg
Chronically 50mmol/L
Bicarbonate rises by 4 for each 10mmHg of CO2 over 40mmHg
Question 4. Is there a V/Q mismatch, or is the primary problem hypoventilation? Why? (4 marks)
There is no V/Q mismatch.
The Aa gradient is calculated as:
225mmHg - 87 - 105 x 1.25
= 225 - 87 - 131
= 225 - 228
= 7mmHg.
The expected Aa gradient for a patient this age is 50/4 + 4 = 16.5.
Therefore the Aa gradient is normal, and no V/Q mismatch exists.
***NOTE, one of our candidates asked why we used 225mmHg for 28% oxygen. This is a great question, and we have left our original answer and the update here for everyone's information. Thanks Ignatius!***
For the examination, where no calculators are allowed (yes, seriously) you need to have something that works well for calculating alveolar oxygen pressure.
If you remember at sea level that the first number in the alveolar gas equation is 150 (on 21% oxygen, you can calculate through the Aa gradient for different FiO2s).
Out of (call it intellectual laziness if you want!) Luke used to approximate Oxygen values as follows:
- 21% = 150 mmHg
- 28% (1.5 x 21%) = 225mmHg
- 40% (2 x 21%) = 300 mmHg
- 60% (3 x 21%) = 450 mmHg.
Of course, a better approximation for 28% oxygen is 1.3 x 21%, i.e. 200mmHg. Rarely will it make an examination difference (and never in clinical practice!!!!) but it's worth using the most accurate approximation.
saq 2.
A 21 year old man with a history of insulin dependent diabetes presents to the emergency department with an altered level of consciousness.
His vital signs are:
HR 115 /min
BP 105/73 mmHg
RR 30 /min
O2 sats 2L 99% RA
T 37.3 oC
An arterial blood gas is shown below.
FiO2 21%
pH 7.15
pO2 114 mmHg
PCO2 20 mmHg
HCO3- 8 mmol/L
BE -16
Na+ 140 mmol/L
K+ 6.0 mmol/L
Cl- 110 mmol/L
Question 1. What are the major acid base disturbances? (3 marks)
Moderate acidaemia
Metabolic acidosis
Compensatory respiratory alkalosis
Question 2. What is the primary disturbance? (1 mark)
Metabolic acidosis
Question 3. What CO2 level would you expect as compensation for the primary acid-base disturbance? (1 mark)
Expect CO2 = 20mmHg
Question 4. What is the anion gap? (1 mark)
AG = 22 (raised)
Question 5. Give 4 acids that are causes of a raised anion gap? (4 marks)
Lactic acid
Ketoacids
Uraemia
Salicylates
His vital signs are:
HR 115 /min
BP 105/73 mmHg
RR 30 /min
O2 sats 2L 99% RA
T 37.3 oC
An arterial blood gas is shown below.
FiO2 21%
pH 7.15
pO2 114 mmHg
PCO2 20 mmHg
HCO3- 8 mmol/L
BE -16
Na+ 140 mmol/L
K+ 6.0 mmol/L
Cl- 110 mmol/L
Question 1. What are the major acid base disturbances? (3 marks)
Moderate acidaemia
Metabolic acidosis
Compensatory respiratory alkalosis
Question 2. What is the primary disturbance? (1 mark)
Metabolic acidosis
Question 3. What CO2 level would you expect as compensation for the primary acid-base disturbance? (1 mark)
Expect CO2 = 20mmHg
Question 4. What is the anion gap? (1 mark)
AG = 22 (raised)
Question 5. Give 4 acids that are causes of a raised anion gap? (4 marks)
Lactic acid
Ketoacids
Uraemia
Salicylates
saq 3.
A 21 year old girl presents complaining of severe chest pain and shaking in the hands. She is acutely distressed, and states that the pain came on after an argument with her boyfriend.
Her vital signs are:
HR 128 /min
BP 109/79 mmHg
RR 38 /min
O2 sats 2L 99% RA
T 37.0 oC
An arterial blood gas is taken by the intern.
FiO2 21%
pH 7.55
PO2 115 mmHg
PCO2 22 mmHg
HCO3- 20 mmol/L
BE +4
Question 1. What are the major acid base disturbances? (3 marks)
Must include:
- mild-moderate alkalaemia
- respiratory alkalosis
- metabolic acidosis
Question 2. What is the primary acid-base disturbance? (1 mark)
Respiratory alkalosis.
Question 3. What HCO3- levels would you expect as acute and chronic compensation for the primary disturbance? (2 marks)
Acutely 20mmol/L
Bicarbonate falls by 2 for each 10mmHg of CO2 below 40mmHg
Chronically 14mmol/L
Bicarbonate falls by 5 for each 10mmHg of CO2 below 40mmHg
Question 4. What is the Aa gradient? (2 marks)
The Aa gradient is calculated as:
150mmHg - 115 - 22 x 1.25
=150 - 115 - 27.5
= 150 - 142.5
= 7.5mmHg.
The expected Aa gradient for a patient this age is 21/4 + 4 = 9.25.
Therefore the Aa gradient is normal, and no V/Q mismatch exists.
Question 5. What is the likely diagnosis? (2 marks)
Must include:
Acute anxiety disorder
Her vital signs are:
HR 128 /min
BP 109/79 mmHg
RR 38 /min
O2 sats 2L 99% RA
T 37.0 oC
An arterial blood gas is taken by the intern.
FiO2 21%
pH 7.55
PO2 115 mmHg
PCO2 22 mmHg
HCO3- 20 mmol/L
BE +4
Question 1. What are the major acid base disturbances? (3 marks)
Must include:
- mild-moderate alkalaemia
- respiratory alkalosis
- metabolic acidosis
Question 2. What is the primary acid-base disturbance? (1 mark)
Respiratory alkalosis.
Question 3. What HCO3- levels would you expect as acute and chronic compensation for the primary disturbance? (2 marks)
Acutely 20mmol/L
Bicarbonate falls by 2 for each 10mmHg of CO2 below 40mmHg
Chronically 14mmol/L
Bicarbonate falls by 5 for each 10mmHg of CO2 below 40mmHg
Question 4. What is the Aa gradient? (2 marks)
The Aa gradient is calculated as:
150mmHg - 115 - 22 x 1.25
=150 - 115 - 27.5
= 150 - 142.5
= 7.5mmHg.
The expected Aa gradient for a patient this age is 21/4 + 4 = 9.25.
Therefore the Aa gradient is normal, and no V/Q mismatch exists.
Question 5. What is the likely diagnosis? (2 marks)
Must include:
Acute anxiety disorder
saq 4.
A 61 year old female presents to your department with a distended abdomen and copious vomiting which has worsened for the past 12 hours. She is complaining of extreme thirst, and has had three vomits since she arrived in your department 45 minutes previously. She has a history of bowel obstructions due to adhesions from previous surgery.
HR 116 /min
BP 115/74 mmHg
RR 8 /min
O2 sats 2L 99% RA
T 37.3 oC
An arterial blood gas is shown below.
FiO2 21%
pH 7.55
pO2 94 mmHg
PCO2 45 mmHg
HCO3- 32 mmol/L
BE +8
Question 1. What are the major acid base disturbances? Indicate the primary derangement. (4 marks)
Must include: (1 mark each)
- moderate alkalaemia
- metabolic alkalosis(primary, for an extra mark)
- borderline respiratory acidosis
Question 2. What CO2 levels would you expect as compensation for the primary disturbance? (2 marks)
Expected CO2 in a metabolic alkalosis = 20 + 0.8x[HCO3-]
Therefore expect 20 + 0.8x32 ~45mmHg
Question 3. Why does the patient have their acid base disturbance?(1 mark)
Must include:
Significant HCl loss due to excessive vomiting
Question 4. What further investigations will you order?
Must include:
Abdominal imaging: either CT or XRAY (at a minimum)
UEC - check hypochloraemia and renal failure
Plus any from (0.5 marks each up to 1 mark):
- Lactate level
- Liver function
- Lipase
- Urinalysis
- FBC
HR 116 /min
BP 115/74 mmHg
RR 8 /min
O2 sats 2L 99% RA
T 37.3 oC
An arterial blood gas is shown below.
FiO2 21%
pH 7.55
pO2 94 mmHg
PCO2 45 mmHg
HCO3- 32 mmol/L
BE +8
Question 1. What are the major acid base disturbances? Indicate the primary derangement. (4 marks)
Must include: (1 mark each)
- moderate alkalaemia
- metabolic alkalosis(primary, for an extra mark)
- borderline respiratory acidosis
Question 2. What CO2 levels would you expect as compensation for the primary disturbance? (2 marks)
Expected CO2 in a metabolic alkalosis = 20 + 0.8x[HCO3-]
Therefore expect 20 + 0.8x32 ~45mmHg
Question 3. Why does the patient have their acid base disturbance?(1 mark)
Must include:
Significant HCl loss due to excessive vomiting
Question 4. What further investigations will you order?
Must include:
Abdominal imaging: either CT or XRAY (at a minimum)
UEC - check hypochloraemia and renal failure
Plus any from (0.5 marks each up to 1 mark):
- Lactate level
- Liver function
- Lipase
- Urinalysis
- FBC
saq 5.
A 52 year old smoker on home oxygen for COPD present feeling generally unwell. On further questioning he describes a fever and a cough for several days.
His vital signs are:
HR 123 /min
BP 98/71 mmHg
RR 42 /min
O2 sats 2L 89% 6L
T 39.6 oC
An arterial blood gas is taken.
FiO2 40%
pH 7.12
pO2 60 mmHg
PCO2 75 mmHg
HCO3- 16 mmol/L
BE -1
Na+ 145 mmol/L
Cl- 101 mmol/L
Lactate 5.4 mmol/L
Question 1. What are the major acid base disturbances? Indicate the primary derangement. (4 marks)
Must include: (1 mark each)
- severe acidaemia
- metabolic acidosis
- respiratory acidosis
Question 2. Which of these is the primary disturbance? (1 mark)
These are 2 independent processes representative of independent primary pathologies (infective COAD and sepsis).
Question 3. What is the anion gap? (1 mark)
AG = 145 - (101+14) = 30
Question 4. Is there a metabolic alkalosis present? Justify your answer.
Must include: (Score 2 marks for delta ratio calculation, and 2 marks for an explanation).
DELTA ratio = (change in anion gap)/(change in HCO3-)
= (30-12)/(24-16)
= 18/8
>2.0.
The high delta ratio implies a co-existing, or pre-existing metabolic alkalosis underneath the acidosis, in this case most likely because of chronic CO2 retention with consquent chronic metabolic alkalosis as compensation.
Would also accept as an explanation the idea that the fall in bicarbonate is discrepant with the base excess, implying a resting bicarbonate of >24.
However, without a delta ratio, or reference to base excess, candidates score NO MARKS.
His vital signs are:
HR 123 /min
BP 98/71 mmHg
RR 42 /min
O2 sats 2L 89% 6L
T 39.6 oC
An arterial blood gas is taken.
FiO2 40%
pH 7.12
pO2 60 mmHg
PCO2 75 mmHg
HCO3- 16 mmol/L
BE -1
Na+ 145 mmol/L
Cl- 101 mmol/L
Lactate 5.4 mmol/L
Question 1. What are the major acid base disturbances? Indicate the primary derangement. (4 marks)
Must include: (1 mark each)
- severe acidaemia
- metabolic acidosis
- respiratory acidosis
Question 2. Which of these is the primary disturbance? (1 mark)
These are 2 independent processes representative of independent primary pathologies (infective COAD and sepsis).
Question 3. What is the anion gap? (1 mark)
AG = 145 - (101+14) = 30
Question 4. Is there a metabolic alkalosis present? Justify your answer.
Must include: (Score 2 marks for delta ratio calculation, and 2 marks for an explanation).
DELTA ratio = (change in anion gap)/(change in HCO3-)
= (30-12)/(24-16)
= 18/8
>2.0.
The high delta ratio implies a co-existing, or pre-existing metabolic alkalosis underneath the acidosis, in this case most likely because of chronic CO2 retention with consquent chronic metabolic alkalosis as compensation.
Would also accept as an explanation the idea that the fall in bicarbonate is discrepant with the base excess, implying a resting bicarbonate of >24.
However, without a delta ratio, or reference to base excess, candidates score NO MARKS.