Acid-Base

Question 1

4 components of an ABG (w/ normal values)

Answer 1

PaO2: 80-100 mm Hg

PaCO2: 35-45 mm Hg

pHa: 7.35-7.45

HCO3: 22-26 mEq/L

Question 2

What can the body do to change pH?

How?

Answer 2

Change HCO3

Kidneys can move HCO3 into urine or kick H+ into urine

Question 3

How does vomiting or diarrhea affect the pH?

Answer 3

Vomitting - lose the acidic contents of stomach (alkalosis)

Diarrhea - lose the alkaline contents of the intestines (acidosis)

Question 4

What can a human do to directly change their pH?

Answer 4

Ingest too much Tums or acidic things

Question 5

What can metabolism do to affect the pH?

Answer 5

Use the H+ to make unplanned acids (lactic acid, ketoacids, etc.)

Question 6

ALL of these changes in HCO3 or H+ are referred to as ____ disturbances

Answer 6

Metabolic

Question 7

While it’s obvious that changing HCO3 changes pH, it’s LESS obvious that changing ___ also affects pH

Why is this the case?

Answer 7

CO2

Le Chatelier’s Principle on the equation

Question 8

Anything that alters how much CO2 moves from blood to alveoli is referred to as a _____ disturbance

Answer 8

Respiratory

Question 9

Step 1 of interpreting ABG readings

Answer 9

Look at pH

Determine if it’s normal, acidotic, or alkalotic

Question 10

Step 2 of determining ABG readings (after looking at pH)

How to differentiate between respiratory or metabolic disturbance?

Answer 10

Look at PaCO2 and HCO3

If CO2 and pH are changed in opposite directions –> respiratory

If HCO3 and pH are changed in the same direction –> metabolic

Question 11

If the pH is low, what are the 2 possible ABG readings to look for in terms of a cause?

Name of each one?

Answer 11

Low HCO3 (metabolic acidosis)

High CO2 (respiratory acidosis)

Question 12

In regards to the body compensating for a change in pH, what are the 2 RULES?

Answer 12

1. Compensation will NEVER get you back to normal

2. Compensation must be made by the OPPOSITE system (renal or respiratory)

Question 13

Where in the kidney are HCO3 or H+ changes able to be made?

How?

Answer 13

Distal convoluted tubule and collecting duct of nephron

Can secrete either one based on demand

Question 14

In respiratory acidosis, what will the DCT/CD nephron cells do to compensate?

Answer 14

Excrete H+ into urine, reabsorb HCO3 into blood

Question 15

In metabolic acidosis or alkalosis, how does the body know to compensate via respiration?

Answer 15

Peripheral chemoreceptors in the aortic arch and carotid body sensitive to H+ tell the brain

Question 16

In metabolic acidosis, how will the body respond?

Answer 16

Peripheral chemoreceptors sense increased H+

Tells body to breathe off MORE CO2 to compensate

Question 17

Step 3 of interpreting ABG readings (after determining the cause of the disturbance)

How to do this (generally)?

Answer 17

Determine if the body is compensating or not (i.e. acute or chronic problem)

Use math to predict what the values SHOULD be if the body was compensating

Question 18

Rule of thumb for Step 3 (acute or chronic?)

Answer 18

Look to OPPOSITE system (the one not causing the disturbance)

Question 19

In respiratory acidosis, what will the expected compensation be?

If problem is acute, ∆HCO3 = ?

If problem is chronic, ∆HCO3 = ?

Answer 19

Increased HCO3

Acute: ∆HCO3 = 1 mEq/L per 10 mmHg ∆CO2

Chronic: ∆HCO3 = 3-4 mEq/L per 10 mmHg ∆CO2

Question 20

How to calculate expected HCO3 for acute or chronic respiratory acidosis compensation?

Answer 20

Acute:
= 24 + ((PaCO2 - 40) / 10)

Chronic:
= 24 + 4*((PaCO2 - 40) / 10)

Question 21

How to calculate expected HCO3 for acute or chronic respiratory alkalosis compensation?

Answer 21

Acute:
= 24 - 2((40 - PaCO2) / 10)

Chronic:
= 24 - 5((40 - PaCO2) / 10)

Question 22

How to calculate expected PaCO2 for adequate compensation of metabolic acidosis?

Answer 22

= (1.5[HCO3] + 8) ± 2

Question 23

WHENEVER you identify a metabolic acidosis, you MUST calculate the ____?

How?

Answer 23

Anion gap

= [Na] - ([Cl] + HCO3])

Question 24

When, in my metabolic acidosis, my body is making some acid (ketoacid, lactic acid, etc.) that my ABG doesn’t account for, what RULE #1 will be broken?

Answer 24

The HCO3 will be eaten up trying to balance out the acid, so in my anion gap equation…

The positives and negatives will NOT balance themselves out

Question 25

Normal anion gap range

How will this change if an acid is being made? Why?

Answer 25

≤ 12

Increase – less HCO3 makes the equation more positive

Question 26

Pneumonic for unmeasured chemicals that could be causing the metabolic acidosis

Answer 26

Methanol
Uremia
Diabetic ketoacidosis / starvation / alcoholic       ketoacidosis*
Paraldehyde
Isoniazid (INH) / Iron
Lactic acid*
Ethylene glycol / ethanol
Salicylates (aspirin)*

Question 27

How to calculate expected PaCO2 for adequate respiratory compensation of metabolic alkalosis?

Answer 27

∆CO2 = (0.5 to 1.0)*∆HCO3