Acid/base interpretation and disturbances Flashcards
What is an acid?
A molecule that donates a H+
What is a base?
A molecule that accepts a H+
What is a buffer?
A weak acid or base that helps protect against large changes in pH
What is the primary extracellular buffer?
Bicarbonate
What are the primary intracellular buffers?
- Phosphate
- Proteins
- Haemoglobin
What role does bone play in acid-base balance?
Bone acts as a buffer
What is the normal blood pH range?
7.35-7.45
At what pH levels may life become incompatible?
< 6.9 or > 7.65
What are the significant concerns for pH levels?
< 7.2 or > 7.5
What can severe acidosis cause?
- Cardiac arrhythmias
- Vasodilation leading to hypotension
- Decreased cardiac contractility
List the four basic types of acid-base disturbances.
- Metabolic acidosis
- Metabolic alkalosis
- Respiratory acidosis
- Respiratory alkalosis
What is metabolic acidosis?
A primary gain in acid or loss of base
What is metabolic alkalosis?
A primary gain in base or loss of acid
What causes respiratory acidosis?
Retention of CO2 due to inadequate alveolar ventilation
What causes respiratory alkalosis?
Removal of more CO2 by ventilation than is produced
What is the normal range for arterial PO2 (mm Hg)?
90-100
What is the normal range for arterial PCO2 (mm Hg)?
35-45
What is the normal range for HCO3- (mmol/L)?
20-24
What is the normal base excess (BE) value?
-4 to +4 mmol/L
What indicates respiratory acidosis?
Hypoventilation (increased CO2)
What indicates respiratory alkalosis?
Hyperventilation (decreased CO2)
What is base excess (BE)?
A calculated value reflecting the metabolic contribution to acid-base balance
More reliable reflection of metabolic component than HCO3- as it can also be effected by CO2
What indicates metabolic alkalosis in terms of BE?
BE > 4 mmol/L
What indicates metabolic acidosis in terms of BE?
BE < -4 mmol/L
What is a simple approach to acid-base analysis?
- Evaluate the pH
- Evaluate the respiratory component
- Evaluate the metabolic component
- Define the primary process
- Assess compensation
- Check oxygenation
What is compensation in acid-base disorders?
Homeostatic mechanisms attempting to return pH closer to normal
How quickly can the lungs compensate for acid-base disturbances?
Within minutes
How long does it take for the kidneys to compensate?
Starts after a few hours, maximum compensation in 4-5 days
What does the absence of expected compensation indicate?
An additional acid-base disturbance may be present
What characterizes respiratory acidosis?
Decreased blood pH and increased PCO2 due to hypoventilation
What are common causes of respiratory acidosis?
- Upper airway obstruction
- Diseases of the brainstem and brain
- Cervical spinal cord disease
- Chest wall disease
- Depression of the respiratory centre
- Neuromuscular disease
- Restrictive disease
What is the treatment approach for respiratory acidosis?
Correct the underlying problem, consider supplemental oxygen, and potentially emergency intubation
What characterizes respiratory alkalosis?
Increased blood pH and decreased PCO2 due to hyperventilation
What are common causes of respiratory alkalosis?
- Fear, pain, anxiety
- Drugs
- Decrease in arterial blood oxygen content
- Neurological disease
What is metabolic acidosis characterized by?
Decreased blood pH and more negative BE
What are common causes of metabolic acidosis?
- Addition of acid to the body
- Loss of base from the body
What is the anion gap (AG)?
The difference in the sum of commonly measured cations and anions
What is the formula for calculating the anion gap?
AG = (Na+ + K+) - (Cl- + HCO3-)
What does a normal anion gap indicate?
Electroneutrality is maintained by unmeasured anions and cations
What conditions are associated with an increased anion gap?
- Metabolic acidosis
- Ethylene glycol poisoning
- Ketoacidosis
- Uraemia
- Lactic acidosis
DUEL
When is sodium bicarbonate therapy considered?
In cases of refractory severe metabolic acidosis or cardiovascular compromise (hypotension with vasodilattion and arrthymias)
What are potential side effects of sodium bicarbonate therapy?
- Respiratory acidosis
- Alkalaemia
- Hypernatremia
- Hyperosmolarity
- Paradoxical CNS acidosis
- Hypokalaemia
- Ionised hypocalcaemia
What are the potential effects of sodium bicarbonate therapy?
Effects include:
* Hypernatremia ± volume overload
* Hyperosmolarity
* Paradoxical CNS acidosis
* Hypokalaemia
* Ionised hypocalcaemia
not to be used in patients with hypercapnia or hypernatraemia
How is the bicarbonate deficit calculated?
Bicarbonate deficit (mls) = BE x body weight (kg) x 0.3
BE stands for Base Excess.
What is the typical dose range for sodium bicarbonate administration?
Typically, ¼ to 1/3 of the calculated bicarbonate deficit is given.
This is often diluted in 5% dextrose or water for injection.
What monitoring should occur when administering sodium bicarbonate?
Careful monitoring of:
* pH
* HCO3-
* BE
* CO2
Monitoring helps to avoid complications.
What is metabolic alkalosis?
A condition manifesting as increased blood pH and concurrent positive BE or increased HCO3.
PCO2 is normal in acute metabolic alkalosis.
What causes metabolic alkalosis?
Causes include:
* Decrease in acid load from the body
* Addition of base to the body
* Compensatory response to chronic respiratory acidosis
esecially gi loss from vomit or renal loss from loop dieuretics
What gastrointestinal losses can lead to metabolic alkalosis?
Gastrointestinal losses can include:
* H+
* K+
* Cl- losses in vomit
Typically associated with gastrointestinal obstruction.
What is the treatment approach for metabolic alkalosis?
Treatment should aim at correcting the underlying cause.
This may include addressing gastrointestinal obstructions or adjusting diuretic therapy.
What fluid therapy is preferred for patients with GI losses of Cl-?
0.9% NaCl because of its high Cl- concentration, often with KCl supplementation.
This helps correct alkalosis more rapidly.
