Lecture 17: Acid Base Disturbances DSA/Acid Base Physiology

Question 1

Normal ranges for ….
PaO2
PaCO2
Arterial pH
Arterial HCO3-

Answer 1

80 -100 mmHg
35-45 mmHg
7.35 - 7.45 mmHg
22 - 26 mEq/L

Question 2

How does HCO3- affect pH?

Answer 2

HCO3- is basic so…
Low HCO3- = acidosis/low pH
High HCO3 = alkalosis/high pH

Question 3

What can alter levels of HCO3- in body?

Answer 3

renal reabsorption/excretion
Vomiting and diarrhea causes loss of HCO3- and H+ secreted by GI tract
Anti-acids (increases HCO3-)

Question 4

What does HCO3- abnormalities indicate?

Answer 4

Metabolic disturbances (acidosis or alkalosis)

Question 5

How does PaCO2 affect pH?

Answer 5

CO2 is acidic (breaks down into H+) so…
Increase CO2 = acidosis/low pH
Decrease CO2 = alkalosis/high pH

Question 6

What controls CO2 levels in the blood?

Answer 6

Lungs - exhale excess CO2 when acidotic or keep CO2 when alkalotic

Question 7

What do CO2 abnormalities indicate?

Answer 7

Respiratory disturbances (acidosis or alkalosis)

Question 8

Steps to interpreting arterial blood gas

Answer 8

Check pH 
Decide cause (abnormal HCO3 means metabolic, abnormal PaCO2 means respiratory)

Question 9

What happens to HCO3- levels in respiratory disturbances?

What happens to PaCO2 levels in metabolic disturbances?

Answer 9

• same trend as PaCO2
• same trend as HCO3-

they change in the same direction, so you have to figure out which one makes sense given the pH

Question 10

How does the law of mass action relate to acid and base balance in the body?

Answer 10

Changes in CO2 reflects similar changes the HCO3- and vice versa

Same trend. If CO2 decreases, HCO3- also decreases (due to the carbonic anhydrase equation)

Question 11

If both CO2 and HCO3- change, how can you distinguish the cause of the acid/base disturbance?

Answer 11

Make sure the direction of the change is consistent with the pH change

Example: if acidotic - HCO3- has to decrease, and CO2 has to increase, not the other way around.

Question 12

Can PaCO2 and HCO3- change simultaneously?

Answer 12

Yes. Cases of both respiratory and metabolic disturbances happening simultaneously are possible.