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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.

​

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

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
​

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
 

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.
back to week 6 assessment page

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