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A 21 year old man is brought in by his father with a one week history of vomiting. He has not been able to keep any food down. He has been diagnosed with Hashimoto’s thyroiditis by his local doctor 4 months previously. Today his blood pressure is 90/48 and pulse rate 104. These are his venous blood gas results:
pH 7.31
pCO2 39
pO2 25
HCO3- 19
BE -7
Na 108
K 6.2
Cl 72
Glc 3.3

SOLUTION

Is it acidosis or alkalosis?     ACIDOSIS
What is the primary cause? Given the low HCO3 and the not so high pCO2 it is METABOLIC ACIDOSIS.
Is there adequate compensation?
Expected pCO2 is [(1.5xHCO3) +8]+2  i.e.., [(~30) + 8]+2 = 38-39 so there is adequate compensation
What is the Anion Gap?
Na -(Cl + HCO3) = 108-(72 + 19) = 17 so raised anion gap metabolic acidosis. (reference is 8-16)
Is there any other process going on? Look at the delta gap.
change in AG/change in HCO3 = 17-12(use 12 as the expected AG)/24-19(24 is the expected HCO3)
= 5/5 = 1 so this is a pure anion gap metabolic acidosis. ( Given what I say below, I might have also expected a normal anion gap metabolic acidosis)
If we comment on the Na and K:
The Na is very low and the corrected K is (6.2 + 0.5) 6.7. This is indicative of Addison’s disease.
The patient was initially treated with 1L of normal saline stat. There was also a concern that there may have been an adrenal crisis occurring and he was given hydrocortisone IV, after which the blood pressure increase. He was admitted for slow correction of his hyponatremia and further investigation of his underlying condition.

 

A 68 yo woman with known COPD, on home oxygen, presents a little drowsy. Her initial Co2 on venous gases was 84mmHg. She is placed on a short course of CPAP and is soon sitting out of bed eating sandwiches. The medical registrar looks at the venous gases and doesn’t want to take the patient to the ward because of the CO2. What do you think?
pH 7.25
pCO2 74 mmHg
pO2 17 mmHg
HCO3 33 mmol/L

SOLUTION

Solving gases like this is relatively simple.
FIRST THING TO SAY IS: This patient should not be going to ICU anyway, as she is on home oxygen. The only question to ask is; is she OKAY to leave the ED and go to the ward?
I say yes! My suspicion in this patient, is that she normally is compensated for a high CO2. Lets see if that is correct.
Using the 7 Step System for gas interpretation( we only need the first 2 steps here):
Step 1: Acidosis or Alkalosis?This is an ACIDOSIS
What is it due to primarily? Given that the CO2 is very raised in comparison to the HCO3, this is a RESPIRATORY ACIDOSIS
Step 2: Is there adequate compensation?I use the 1234 rule for calculating compensation in respiratory primary causes.
In the acute phase we would expect the HCO3 to increase by 1 mmol/L for every 10mmHg rise in CO2(above 45)
Let’s take the baseline of the HCO3 as 24 we know that CO2 has risen by 29- lets say 30mmHg.
In acute compensation the HCO3 would be expected to be 24(baseline to use) + (3 x 1) = 27 
In chronic compensation we would expect the HCO3 to increase by 3mmol/L for every 10mmHg increase in CO2
Therefore the HCO3 would increase by 24 + (3 x 3) =33
The bicarb is in fact 33mmol/L. This indicates that this patient may normally be sitting at around this CO2 level. That’s why she is sitting out of bed, looking better and eating sandwiches. She can go to the ward.

 

A 68 yo woman with known COPD, on home oxygen, presents a little drowsy. Her initial Co2 on venous gases was 84mmHg. She is placed on a short course of CPAP and is soon sitting out of bed eating sandwiches. The medical registrar looks at the venous gases and doesn’t want to take the patient to the ward because of the CO2. What do you think?
pH 7.25
pCO2 74 mmHg
pO2 17 mmHg
HCO3 33 mmol/L

SOLUTION

Solving gases like this is relatively simple.
FIRST THING TO SAY IS: This patient should not be going to ICU anyway, as she is on home oxygen. The only question to ask is; is she OKAY to leave the ED and go to the ward?
I say yes! My suspicion in this patient, is that she normally is compensated for a high CO2. Lets see if that is correct.
Using the 7 Step System for gas interpretation( we only need the first 2 steps here):
Step 1: Acidosis or Alkalosis?This is an ACIDOSIS
What is it due to primarily? Given that the CO2 is very raised in comparison to the HCO3, this is a RESPIRATORY ACIDOSIS
Step 2: Is there adequate compensation?I use the 1234 rule for calculating compensation in respiratory primary causes.
In the acute phase we would expect the HCO3 to increase by 1 mmol/L for every 10mmHg rise in CO2(above 45)
Let’s take the baseline of the HCO3 as 24 we know that CO2 has risen by 29- lets say 30mmHg.
In acute compensation the HCO3 would be expected to be 24(baseline to use) + (3 x 1) = 27 
In chronic compensation we would expect the HCO3 to increase by 3mmol/L for every 10mmHg increase in CO2
Therefore the HCO3 would increase by 24 + (3 x 3) =33
The bicarb is in fact 33mmol/L. This indicates that this patient may normally be sitting at around this CO2 level. That’s why she is sitting out of bed, looking better and eating sandwiches. She can go to the ward.

 

A 65 year old patient with a history of COAD and CCF presents to your department complaining of difficulty breathing.

HR                   88        /min
BP                    135/99 mmHg
RR                   28        /min
Sats                100%   40% O2
T                      38.2     oC
Cap refill         2          s
FiO2                0.40
pH                  7.22
PO2                 182     mmHg
PCO2              80       mmHg
HCO3            34       mmol/L
BE                  +10
Na+                 137     mmol/L
K+                    4.1      mmol/L
Cl                   117     mmol/L
Urea                8.5      mmol/L
Creatinine       95       umol/L
Lactate            1.6      mmol/L

Question 1.
What are the major acid base disturbances present?

Question 2.
What is this patient’s resting CO2? Justify your answer.

Question 3.
Calculate this patient’s Aa gradient.

SOLUTION

Question 1. What are the major acid base disturbances present?Must include:

– Acute on chronic respiratory acidosis

– Metabolic compensation/alkalosis

Question 2. What is this patient’s resting CO2? Justify your answer.

Must include:

60mmHg. Assuming that:

  1. the rise in HCO3- is equal to the chronic portion of compensation and the acute portion of compensation
  2. chronic compensation of 4mmol HCO3 per 10mmHg CO2
  3. acute compensation of 1mmol HCO3 per 10mmHg Co2

Therefore we can assume if the resting CO2 is:

– 50mmHg, the chronic compensation element will be 4mmol/L HCO3-, meaning the acute fraction is 3mmol/L, yielding a change in HCO3 of 7mmol/L

– 60mmHg, , the chronic compensation element will be 8mmol/L HCO3-, meaning the acute fraction is 2mmol/L, yielding a change in HCO3 of 10mmol/L. This is the correct answer and implies a resting CO2 of 60mmHg.

Question 3. Calculate this patient’s Aa gradient.

Must include:

300 – 1.25 x 80 – 180- = 300 – 280 = 20mmHg.

This is within normal limits for this patient.

Posted in ,

Dr Peter Kas

Emergency Physician, Educator. Key Interests: Resuscitation, Airway, Emergency Cardiology, Clinical Examination. Creator resus.com.au.

67 Comments

  1. Chandan on 11/09/2019 at 3:59 pm

    Learnt a lot from the above case scenarios.Thanks a lot

  2. Chandan on 11/09/2019 at 4:00 pm

    Learnt a lot from the above case scenarios.Thanks a lot.Clarified so many things

  3. Alan C on 08/10/2020 at 4:27 pm

    Hello,

    Thanks so much for this, it’s helped alot.

    My quick query is regarding the use of the estimation of FiO2 40% = roughly 300 in calculating an AA gradient.

    For the example “65 year old patient with a history of COAD and CCF presents to your department complaining of difficulty breathing” the answer changes from ~5 to ~20 (as 0.4 x 713 = 285)

    I’m just wondering when it comes to the written exam if they’ll be accepting of the range of approaches / worked examples? My concern is in another question this difference might move an answer from “within normal limits” to abnormal.

    But knowing timing is everything in these exams I’m keen to keep any calculations as brief as possible!

    • Dr Peter Kas on 07/11/2020 at 5:28 pm

      Thanks for the question. They will accept the range. You will find that there will be few line balls. It will be clear-cut. So rounding up, will be fine. Good luck.

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