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


Focus for today’s lecture:


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-] )


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)


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?

acid-base for beginners - dr. sam...

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