ACID-BASE FOR BEGINNERS - Dr. Sam...
Transcript of ACID-BASE FOR BEGINNERS - Dr. Sam...
______________________________ ACID-BASE FOR BEGINNERS______________________________
By Dr Sam Gharbi MD CM FRCPC
Types of Acid-Base Abnormalities
Respiratory ¤ Acidosis ¤ Alkalosis
Metabolic ¤ Acidosis ¤ Alkalosis
Types of Acid-Base Abnormalities
Respiratory ¤ Acidosis - hypoventilation à high CO2 à low pH ¤ Alkalosis - hyperventilation à low CO2 à high pH
Metabolic ¤ Acidosis ¤ Alkalosis
Focus for today’s lecture:
Metabolic ¤ Acidosis ¤ Alkalosis
For Metabolic Acidosis, we will examine ¤ Definition ¤ Calculation ¤ Cause ¤ Management
Metabolic Alkalosis - Definition
In terms of physiology, metabolic alkalosis is due to either loss of H+ or an increase in bicarbonate.
H+ is an acid à if low, then alkalosis Bicarbonate is a base à if high, then alkalosis
In terms of chemistry, a pH >7.4 is considered alkalotic (some sources use pH>7.45)
1. Metabolic Acidosis - Definition
In terms of physiology, metabolic acidosis is an increase in hydrogen (H+) or a loss of bicarbonate (HCO3-)
In terms of chemistry, metabolic acidosis is defined as a pH of <7.4 (some sources will say pH<7.35)
Question
What blood test that is done routinely in most outpatients and inpatients is the first clue that there may be an acid-base abnormality?
2. Metabolic Acidosis - Approach
Step 1 = Recognizing there is a problem.
● Look at the daily bicarbonate levels in the serum (part of your electrolyte panel)
● If bicarbonate level is low, then it suggests underlying metabolic acidosis ● Low bicarbonate is <24 ● Significantly low <20 ● Critically low <12
Approach
Step 2 = Get an ABG
● Why? So as to get the complete picture, meaning the pH/
O2/CO2/HCO3
● In most situations the pH will tell you whether your bicarbonate is low because of a metabolic acidosis or a compensated respiratory alkalosis.
Approach
Step 3: Calculate the Anion Gap.
● What is the Anion Gap?
● The Anion Gap is the difference between the measured cations (positively charged ions) and anions (negatively charged ions) in the blood.
Step 3 – Calculate the AG
● How do I calculate the Anion Gap?
● A normal AG is <12
● Note: in hospitals, the computer lab system may present you with a calculated AG level. This is often wrong, as it may include Potassium in the calculation. Always calculate the AG manually!
AG = ( [Na+] ) - ( [Cl-]+[HCO3-] )
Step 3 – Calculate the AG
If the AG >12, then you have diagnosed an Anion Gap Metabolic Acidosis (AGMA)
If the AG <12, then you have diagnosed a Non-Anion Gap Metabolic Acidosis (NAGMA)
Why is this important? - Because the causes for AGMA & NAGMA are different
(more to come)
Step 3 – Calculate the AG
What is a normal Anion Gap in a person with normal albumin?
<12
How does the value for the normal Anion Gap change if the albumin is low?
10 : 3
AG & Albumin
● The expected AG is the same as the normal baseline AG you would have if albumin is within the range of normal.
● For every 10 unit decrease in albumin, the expected AG decreases by 3.
● For example, normal albumin is 40. If your patient’s albumin is 20, then the expected normal AG threshold is now 6 instead of 12.
● Therefore, a calculated anion gap of greater than 6 in your patient with an albumin of 20 would be considered high in this scenario.
AG & Albumin
But why does the calculated Anion Gap decrease in a patient with low albumin?
The answer is simple: Because the AG is primarily determined by the
negative charges on plasma proteins. What is the major negatively charged plasma protein? Albumin! So less albumin means a lower anion gap.
Step 4 – Calculate the delta/delta
What the heck is that?
= delta AG/ delta HCO3- = (calculated AG – expected AG)/ (24 – HCO3-)
Step 4 – delta/delta
Ok, but what’s the point to calculating this?
Delta/Delta Significance
1 to 2 AG Met. Acidosis
<1 AG Met Acidosis + Non-AG Met Acidosis
>2 AG Met Acidosis + Metabolic Alkalosis
Step 5 - Compensation
● How does compensation work in metabolic acidosis?
● Easy! For every decrease in 1 unit for bicarbonate, the CO2 on the ABG should also decrease by 1. If this is not the case, then there is another underlying acid-base phenomenon.
1 : 1
Step 5 - Compensation
● Can you ever compensate back to a normal pH? NO!
● If you have a normal pH but abnormal acid-base variables, then it usually means there are multiple acid-base disorders going on.
Summary Approach to Metabolic Acidosis
STEP 1 = Is the bicarbonate level low (<24)? STEP 2 = Get an ABG to see if pH low (<7.4) STEP 3 = Calculate the Anion Gap (normal <12) STEP 4 = Calculate the delta/delta STEP 5 = Evaluate for compensation
Review – Metabolic Acidosis
1.Definition – What is it? 2. Calculation – How do we calculate it? 3. Cause - What causes it? 4. Management – How do we treat it?
3. Causes for AGMA
1. Ketoacidosis 2. Lactic Acidosis 3. Renal Failure 4. Toxins
You choose your approach
Option 1 Option 2
What did the “P” stand for again??!
In my opinion, mudpiles is exactly that... a pile of mud.
Ketoacidosis
Lactic Acidosis
Renal Failure
Toxins
Ketoacidosis •DKA •Alcoholism •Starvation
Lactic Acidosis Type A = impairment in tissue oxygenation •Shock (4 subtypes) •Respiratory distress •Sepsis •Ischemic Bowel
Type B = no impairment in tissue oxygenation •Meds: Metformin, ASA, NRTI •Alcoholism •Cirrhosis •Malignancy
Renal Failure • Any cause (AKI or CKD)
Toxins •Methanol/Ethylene Glycol/Paraldehyde •Salicylates/Acetaminophen
Metabolic Acidosis - Workup (labs)
In addition to routine bloodwork, consider ordering:
● Random Glucose ● Urine Ketones ● Lactate ● Creatinine/BUN ● Tox screen if appropriate
Algorithm
Ok, so you’ve diagnosed the underlying acid-base disturbance.
You’ve determined the underlying cause of this acid base disturbance.
Now what?!
4. Metabolic Acidosis - Management
Rule # 1: Treat the underlying cause of the metabolic acidosis.
Other options to consider in addition to above: 1. Bicarbonate infusion 2. Hemodialysis
Goals of Treatment
The initial aim of therapy is to raise the systemic pH >7.20
This is a level at which the major consequences of severe acidemia should not be observed, although there is some uncertainty about the absolute benefits of achieving this level.
Consequences of acidosis: ¤ Cardiovascular – decreased CO & MAP, increase risk of arrhythmias ¤ Resp – Hyperventilation ¤ Neuro – confusion/decreased LOC
So when do I give a bicarbonate infusion?
Somewhat controversial.
Most physicians would limit the use of sodium bicarbonate infusion to patients with severe metabolic acidemia (arterial pH below 7.10, some sources say <7)
The general aim in therapy is to maintain the pH above 7.2 until the primary process can be reversed.
Cautions with sodium bicarbonate infusions
● The infusion of sodium bicarbonate can lead to a variety of problems (particularly as evidenced in studies with patients who have lactic acidosis):
● Fluid overload ● Post-recovery metabolic alkalosis (as the excess lactate is
converted back to bicarbonate) ● Hypernatremia
___________________________________________ * Stacpoole, PW. Lactic acidosis: The case against bicarbonate therapy. Ann Intern
Med 1986; 105:276.
Questions?