Understanding Acid/Base Balance

Marilyn F. BrownBroad Concepts:Body pH tolerated in very narrow range 7.35 – 7.45pH is defined as H+ ion concentration

The more hydrogen ions, the more ACIDICpH range roughly compatible with life is 6.8 – 7.8, but that represents a 10-fold difference in H+ ion concentration

The primary buffer is bicarbonate (HCO3-

CO2 + H20 = H2CO3 = H+ + HCO3-

LUNGS Kidneys

Carbonic Acid

Hypoventilation = increased CO2

Hypervention = decreased CO2

Requires adequate kidney function

So………

• pH = 7.35 – 7.45• Kidneys regulate the metabolic side of acid

base disturbances– Kidneys are slow

– Indicator is bicarbonate (HCO3-)

• Lungs regulate the respiratory side of acid base disturbances– Lungs are fast

– Indicator is carbon dioxide (CO2)• Carbon dioxide is POTENTIAL ACID

Above the pyloric sphincter is ACID

Below the pyloric sphincter is BASE

pH < 7.35

PaCO2 = 35 – 45 (normal)

HCO3 < 22

METABOLIC ACIDOSIS (base bicarbonate deficit)

Causes:

• lose base (diarrhea)

• gain acids

Metabolic Alkalosis (base bicarbonate excess)

pH > 7.45

PaCO2 = 35 – 45 (normal)

HCO3 > 26

Causes:

• Lose acid (vomiting or N/G)

• Add base

Respiratory Acidosis (carbonic acid excess)

pH < 7.35

PaCO2 > 45

HCO3 = 22 – 26 (normal)

Cause:

• Hypoventilation

•Poor O2 CO2 exchange

Respiratory Alkalosis (carbonic acid deficit)

pH > 7.45

PaCO2 < 35

HCO3 = 22 – 26 (normal)

Cause:

• Hyperventilation

• Blow off too much CO2

Practice Acid/Base SituationsPractice Acid/Base Situations

pH HCO3 PaCO2 Imbalance Common Causes

7.05 5 38

7.26 24 56

7.62 45 42

7.52 24 30

Don’t Peek!

• Try the problems first

• Objective: given a set of abg results including pH, PaCO2 and HCO3, student can correctly calculate the imbalance

• Will always be uncompensated in Med-Surg I

pH HCO3 PaCO2

7.05 5 38

7.26 24 56

7.62 45 42

7.52 24 30

Metabolic Acidosis

Respiratory Acidosis

Metabolic Alkalosis

Respiratory Alkalosis