Blood Gas
DEFINITION OF TERMS:
- Acid: a compound that could donate a H+ ion
- Base: a compound that could accept a H+ ion
- Acid-Base Balance: a mechanism by which the pH of blood is maintained at 7.35-7.45 for homeostasis
- Buffer: a weak acid/base with its conjugate salt that resists changes in Ph
ACID BASE BALANCE
HENDERSON-HASSELBACH EQUATION
- Implicates the relationship between pH, and the two involved organs – lungs and kidneys
| FOUR BASIC ABNORMAL STATES |
||||||
| Imbalance | pH | pCO2 | H2CO3 | HCO3 | Primary compensation |
Seen in: |
| Respiratory Acidosis |
↓ | ↑ | ↑ | N | Kidneys retain bicarbonate & excrete hydrogen |
Pneumonia, emphysema |
| Respiratory Alkalosis |
↑ | ↓ | ↓ | N | Reverse of respiratory acidosis |
Hyperventilation, early salicylate poisoning |
| Metabolic Acidosis |
↓ | N | N | ↓ | Hyperventilate (CO2 blew off) |
Diabetic ketoacidosis, renal disease and prolonged diarrhea |
| Metabolic Alkalosis |
↑ | N | N | ↑ | Hypoventilation (CO2 retention) |
Vomiting, antacids, NaHCO3 infusion |
EVALUATING ACID-BASE DISORDERS
- Determine if the pH is high (alkalosis) or low (acidosis
- Compare pCO2 and HCO3 to normal values
- If pCO2 is opposite to pH = respiratory
- If HCO3 is in the same direction with pH = metabolic
- If pH is within normal range, full compensation has occurre
- if main compensatory mechanism has already occurred yet the pH is still out
of range, partial compensation happened.
NORMAL VALUES
- pH: 7.35 – 7.45
- pCO2: 35-45 mmHg
- pO2: 81-100 mmHg
- HCO3: 21-28 mEq/L
- TCO2: arterial (19-24 mmol/L); venous (22-26 mmol/L)
- H2CO3: 1.05-1.035 mmol/L
- O2 saturation: 94-100%
| COMMON SOURCES OF ERROR |
||||
| Error | pCO2 | pH | pO2 | Effect |
| Sample sitting at room temperature for more than 30 mins | ↑ | ↓ | ↓ | Acidosis |
| Bubbles in syringe, uncapped specimen | ↓ | ↑ | ↑ | Alkalosis |
| Hyperventilation | ↓ | ↑ | ↑ | Alkalosis |
| Specimen exposed to air | ↓ | ↑ | ↑ | Alkalosis |
