Acid-base Imbalances
Acid-base Imbalances
Defining Acid-base Imbalances
Normal serum pH ranges from 7.34-7.45. This is a tightly regulated range, and any disturbances lead to acid-base imbalances. The imbalances include
- Metabolic acidosis and alkalosis
- Respiratory acidosis and alkalosis
The situation is more complex as patients can have mixed disturbances involving both respiratory and metabolic disorders. The body will attempt to move towards a normal pH; however, in a mixed disorder, this is not possible.
Diagnosing Acid-base Imbalances
Arterial Blood Gases (ABGs)
ABGs are useful in monitor acid-base changes as well as compensatory changes.
Arterial Blood Gas Test
The ABG will report
- pH: 7.35-7.45
- PO2: 80-100 mm Hg
- PCO2: 35-45 mm Hg
- Oxygen saturation: > 94%
- Bicarbonate: 22-32 mEq/L
- Base deficit/excess: -2 – +2
Related Video: Understanding and Interpreting ABGs Part 1: Introduction to ABGs
A PCO2 of 40 is used for equations to determine predicted pH. The size of the discrepancy between predicted and measure pH can help determine the type of alkalosis or acidosis present.
To evaluate ABGs to determine acid-base imbalances:
- Is there acidosis (pH< 7.35), alkalosis (pH>7.45) or neither (pH= 7.35-7.45)? Is the patient hypercarbic (PCO2 > 45) or hypocarbic (PCO2 <35)? Increased PCO2 suggests respiratory acidosis, while low PCO2 suggests respiratory alkalosis. The effect of PCO2 will on the pH will help determine the underlying imbalance
- Subtract PCO2 from 40. Positive numbers indicate respiratory acidosis while negative numbers indicated respiratory alkalosis
- Multiply the last number (40-PCO2) by 0.008. Numbers are negative or positive
- Add the prior number [(40 – PCO2) x 0.008] to 7.4. This new number is the predicted pH based on the measured PCO2. Note that an increase in PCO2 over 40 mm Hg will decrease pH. The correlation is 1mm Hg PCO2 above 40 mm Hg will decrease pH by 0.008.
- Change in pH (from 7.4 baseline) = [(40 – PCO2) x 0.008]
Related Video: Understanding and Interpreting ABGs Part 3: Acid Base Introduction
Related Video: Understanding and Interpreting ABGs Part 4: Respiratory Alkalosis
Related Video: Understanding and Interpreting ABGs Part 5: Respiratory Acidosis
Metabolic acidosis occurs if the measured pH is greater than the calculated pH.
Metabolic alkalosis occurs if the measured pH is lower than the calculated pH.
Related Video: Understanding and Interpreting ABGs Part 7: Metabolic Acidosis
Note that in compensated abnormalities, the body can respond using increased or decreased ventilation to push the pH back to normal. For example, in a primary metabolic acidosis, respiratory compensation will lead to hyperventilation (or respiratory alkalosis) to try and blow off excess CO2. Another example is respiratory acidosis, in which the kidneys would metabolically compensate by retaining the bicarbonate to increase pH back to normal. Be aware that respiratory compensation is usually immediate, while metabolic compensations may take 8-48 hours to take effect. The compensation will stop if the pH is normalized.
Respiratory Acidosis compensated with metabolic alkalosis
This acid-base imbalance can occur due to chronic obstructive pulmonary diseases (COPD) in which the patient retains CO2. With increasing PCO2, there is respiratory acidosis. The kidneys will gradually compensate for this by reabsorbing HCO3 to oppose the acidosis. This is a metabolic alkalosis compensation.
Base Deficit or Excess
- The ABG also includes information about variations in the base. This number is usually -2 – +2. When the base deficit is increased (<-2), there is a metabolic acidosis
- When the base excess is increased (>+2), there is metabolic alkalosis.
Overcompensation
Notice that the physiologic mechanisms to compensate are unlikely to go too far and overcompensate, rather they approach normal pH. So if a patient in respiratory failure (primary respiratory acidosis) has a pH that is alkalotic, it is unlikely that it is due to compensatory metabolic alkalosis. Rather there is likely an associated primary metabolic alkalosis driving the pH higher than normal. Such a cause may be hypokalemic or hyperchloremic metabolic alkalosis due to furosemide treatment in a patient who did not get adequate supplementation with potassium chloride. This knowledge helps determine primary versus compensatory conditions.
The Anion Gap
This calculated number can determine the etiology of acid-base imbalance. The positive cations (potassium and sodium) should be about equal to the negative anions (bicarbonate and chloride). However, a gap remains due to unmeasured anions such as lactic acid and ketones. The anion gap indicates the difference between major cations and anions:
- Anion gap = Na – [(HCO3 + Cl)]
The normal range is 7+/- 4 mEq/L. Normal gap acid-base imbalances indicate that the imbalances are due to bicarbonate and chloride. For example, in diarrhea, metabolic acidosis is secondary to lost bicarbonate and compensated by retained chloride. This is a normal gap acidosis. However, if the anion gap is high (over 11 mEq/L), unmeasured anions are contributing to acidosis (i.e., ketones and lactic acid) or not enough compensation with chloride. Diabetic ketoacidosis is a well-known case of a high-gap acidosis.