ABGs pt 2 Flashcards
What are the types of acid/base disorders in a simplified view?
What are the main causes of metabolic acidosis with an anion gap?
“MUDPILES” DKA, lactic acidosis, renal failure and rhabdomyolysis, solicit
What is the main cause of metabolic acidosis with a non-gap?
Diarrhea
What are the main causes of acute respiratory acidosis?
CNS depression, airway obstruction, pulmonary edema, pneumonia, Hemo or pneumothorax, neuromuscular
What are the main causes of metabolic alkalosis?
Vomiting
What are the main causes of respiratory alkalosis?
Anxiety, salicylates, sepsis
On a BMP or CMP, CO2 and anion gap can give some info about acid/base status?
High CO2: Respiratory acidosis OR Metabolic alkalosis
Low CO2: respiratory alkalosis OR metabolic acidosis
Metabolic acidosis → normal vs high anion gap.
Is an ABG or a VBG more accurate at assessing acid based balance and identifying disorders?
ABG
What is the first step in interpreting ABGs?
Assess the pH and determine if it is acidic or alkaline
Look at the PCO2 (Respiratory) and Bicarb (Metabolic) to determine primary disorder
What is the second step in interpreting ABGs?
Assess for compensation
Uncompensated: PH is abnormal. PCO2 and Bicarb → one normal, one abnormal. The disorder is acute in the body has not adequately started to try and fix the disorder.
Partially compensated: PH is abnormal. Both PCO2 and Bicarb are abnormal and in opposite directions. The body is in the process of compensating for the disorder.
Fully compensated: PH returned to normal. Both PCO2, HCO3 are abnormal in opposite directions. The body has had a chance to bring PHA to normal by counterbalancing whatever is causing the problem.
What is the third step in interpreting ABGs?
Evaluate oxygen level:
PaO2 80-100 mmHg
Low PaO2 is due to either a ventilation issue or a gas exchange issue
If cause of hypoxia isn’t clear, A-a gradient can help
What is the A-a gradient?
A = PAO2; calculated value [(FiO2 x 713) – (PaCO2/0.8)]
a = PaO2
(A-a) should be ~ 10
Increased A-a gradient = gas exchange issue
Normal A-a gradient = ventilation issue
What method of ABG analysis is taught in med school?
Rules of thumb
What method of ABG analysis will we learn?
The 3 step method, but it is considered less accurate and doesn’t fully assess respiratory disorders
What is the 3 step method?
Assess primary disorder
Calculate the anion gap
Calculate the excess anion gap
Step 1 of the 3 step method is?
Assess the primary disorder
Step 2 of the 3 step method is?
Anion gap= = Na+ – (Cl- + HCO3-)
Need to either measure electrolytes with ABG OR get BMP at same time as ABG!!!
Expected AG = 12 +/- 4
If anion gap > 20, there is a high anion gap metabolic acidosis regardless of pH or serum HCO3- concentration
But then we must correct for hypoalbuminemia- has to have been within two days
How do we correct for hypoalbuminemia
Correction of the anion gap with hypoalbuminemia.
Lower albumin levels are suspected of nephrotic syndrome, malnutrition, liver disease severe illness
What is the formula for the corrected anion gap?
Corrected anion gap = Calculated AG + [2.5 * (normal albumin–actual albumin)]
Once a patient is below 2, they look anasarcic and very squishy
Step 3 of the 3 step method is?
Calculate excess anion gap
Excess AG= (calculated AG – normal AG of 12) + measured HCO3-
1.If the sum is >30, there is a metabolic alkalosis
2.If the sum is <23, there is a nonanion gap metabolic acidosis
Between these two numbers, means theres nothing else going on.
Exccess anion gap is basically the Delta Delta equation from the rules of thumb method rearranged
Can you solve this problem?
pH 7.50
pCO2 20 mmHg
HCO3- 16 mmol/L
Na+ 140 mEq/L
K+ 4.0 mEq/L
Cl- 103 mEq/L
Partially compensated respiratory alkalosis w/ HAGMA
Can you solve this problem?
pH 7.20
pCO2 17 mmHg
HCO3- 10 mmol/L
Na+ 142 mEq/L
K+ 4.7 mEq/L
Cl- 111 mEq/L
Partially compensated metabolic acidosis w/ HAGMA and NAGMA
What kind of patient could have Partially compensated metabolic acidosis w/ HAGMA and NAGMA?
DKA + Diarrhea
Uremia + saline infusion
Sepsis
Toxin ingestion + diarrhea
Lactic acidosis + RTA
What are some other things we can see on ABG?
Lactate
Hemoglobin
Carboxyhemoglobin (form carbon monoxide poisoning) and methemoglobin
Ionized calcium (active form, other 50% of the time, it rides on albumin
Base excess:
Volume of base above or below expected amount for pH 7.4
> +2 – suggests metabolic alkalosis
< -2 – suggests metabolic acidosis
When not to use a VBG?
Can’t use VBG to assess O2 status!!
Not useful with hypotensive, critically ill patients (higher variability between VBG and ABG)
What is a VBG?
Alternative to ABGs that doesnt stick the radial artery
Compared to ABG: pH will be slightly lower on VBG (-0.035 difference, more CO2 means more acidic) and pCO2 will be slightly higher on VBG (2-5 mmHg)
When to use a VBG?
Abd pain - if acidosis, differential should include ischemia ( worried about acute mesenteric ischemia)
Pure acid/base disorders- ex DKA (might get one every hr)
Concern for hypercapnia (COPD)- VBG pCO2 > 45 corresponds to ABG pCO2 > 45 (will need urgent treatment and bipap)
What two disorders can we not have at the same time?
We CANNOT have two respiratory disorders at the same time.
