Resp Physiology Part 3 - ABG Flashcards
What is the Henderson-Hasselbalch equation?
pH = pKa + log10 ([HCO3-] / 0.03 x PaCO2)
What is the anion gap equation?
Anion Gap = (Na+ + K+) – (Cl- + HCO3-)
What is the significance of the anion gap?
Used to evaluate metabolic acidosis and identify unmeasured anions.
What are the two types of lactic acidosis?
Type A: Caused by hypoxia.
Type B: Caused by metabolic or toxic derangements.
Define base excess.
The amount of acid or base required to return blood pH to 7.4 at standard PaCO2.
What are common causes of respiratory acidosis?
Hypoventilation, obstructive lung diseases, and neuromuscular impairment.
What are common causes of respiratory alkalosis?
Hyperventilation, anxiety, and pain.
What are the normal arterial blood gas (ABG) parameters for pH and PaCO2?
pH: 7.35 - 7.45
PaCO2: 4.7 - 6.0 kPa.
What is the normal bicarbonate (HCO3-) range?
22 - 26 mmol/L.
What is the A-a gradient?
The difference between alveolar and arterial oxygen pressure, used to assess oxygen exchange.
What does a low pH with high PaCO2 indicate in ABG interpretation?
Respiratory acidosis.
What does a low pH with low HCO3- indicate?
Metabolic acidosis.
What does a high pH with low PaCO2 indicate?
Respiratory alkalosis.
How is metabolic alkalosis identified on ABG?
High pH and high HCO3- (>26 mmol/L).
What is the significance of PaO2 in ABG interpretation?
Determines oxygenation status; PaO2 <10 kPa on air indicates hypoxemia.
What is the definition of a buffer?
A solution that minimizes changes in pH when an acid or base is added.
What are examples of physiological buffers?
- Bicarbonate buffer system
- Haemoglobin
- Plasma proteins
What is the role of bicarbonate in acid-base balance?
Bicarbonate acts as a base in the carbonic acid-bicarbonate buffer system to regulate pH.
What is lactic acidosis, and how is it produced?
Lactic acidosis is caused by the accumulation of lactic acid, a product of anaerobic metabolism.
(Debated!)
What are the causes of Type A lactic acidosis?
Hypoxia, shock, severe anemia, or respiratory failure.
What are the causes of Type B lactic acidosis?
Liver failure, sepsis, medications (e.g., metformin), or malignancy.
What is the PaO2/FiO2 (P/F) ratio used for?
To assess the severity of hypoxemia, commonly in ARDS.
What is the P/F Ratio equation.
PaO2/FiO2
How is the P/F ratio severity scaled?
- Severe <100
- Moderate 101-200
- Mild 201-300
- Non ARDS >301
What is the clinical significance of the A-a gradient?
Helps differentiate hypoxemia due to V/Q mismatch, diffusion impairment, or shunting.
What does a high A-a gradient indicate?
Issues like pulmonary embolism, pneumonia, or ARDS.
What would a:
pH 7.19
PaCO2 10.2 kPa
HCO3 23.6 mmol/L
indicate?
Acute respiratory acidosis
In ABG interpretation, what does a PaO2 of 7.5 kPa with a pH of 7.10 suggest?
Hypoxemia and severe acidosis
What would a PaCO2 of 18 kPa and a pH of 7.10 in a COPD patient suggest?
Acute respiratory acidosis superimposed on chronic respiratory acidosis
Define “Base Excess” in ABG interpretation.
The amount of acid or base needed to return blood pH to 7.4 at standard PaCO2 and temperature.
What is the significance of hypoalbuminemia in acid-base disorders?
It reduces the sensitivity of the anion gap in detecting metabolic acidosis.
What is the normal range for the anion gap?
4 to 12 mmol/L.
What are the derived and measured values from an ABG machine?
- Derived: Bicarbonate.
- Measured: pH, PaO2, and PaCO2.
What are the key steps in ABG interpretation?
1) Assess patient’s condition.
2) Assess oxygenation (PaO2).
3) Check pH (acidosis or alkalosis).
4) Determine respiratory component (PaCO2).
5) Determine metabolic component (HCO3-).
What diagnosis is consistent with a pH of 6.89, PaCO2 of 2.48 kPa, and HCO3- of 4.7 mmol/L in a diabetic patient?
Severe diabetic ketoacidosis with partial respiratory compensation
How is metabolic acidosis compensated?
By hyperventilation, which reduces PaCO2 levels.
What does a normal PaO2 value indicate in a healthy individual breathing air?
PaO2 > 10 kPa (~75 mmHg).
What does a base excess value outside +/-2 mmol/L indicate?
A disturbance in the metabolic component of acid-base balance.
Why is the bicarbonate level important in ABG interpretation?
It reflects the metabolic component of acid-base balance.
How does hyperventilation affect ABG values?
Decreases PaCO2, leading to respiratory alkalosis.
What is the expected relationship between pH and PaCO2 in respiratory acid-base disorders?
- Respiratory acidosis: Decreased pH and increased PaCO2.
- Respiratory alkalosis: Increased pH and decreased PaCO2.
What are the clinical causes of metabolic acidosis with a normal anion gap?
- Diarrhea
- Renal tubular acidosis
- Early kidney injury
What are clinical causes of metabolic acidosis with an increased anion gap?
- Diabetic ketoacidosis
- Lactic acidosis
- Toxic ingestions (methanol, ethylene glycol)
How can chloride loss lead to alkalosis?
Loss of chloride (e.g., via vomiting) reduces acid equivalents, causing metabolic alkalosis.
What does a PaCO2 of 2.65 kPa with a pH of 7.62 indicate?
Severe respiratory alkalosis.
What is the likely diagnosis with pH 7.17, PaO2 8.2 kPa, and HCO3- 12 mmol/L?
Sepsis-induced metabolic acidosis.
What is the compensation for a metabolic alkalosis?
Hypoventilation to retain CO2 and lower pH.
What is the primary buffer system in the blood?
The carbonic acid-bicarbonate buffer system.
What role does hemoglobin play in buffering?
Hemoglobin buffers hydrogen ions released during oxygenation of blood.
(?)
Why are plasma proteins considered buffers?
Their amino acid residues can donate or accept hydrogen ions to stabilize pH.
What is the physiological role of lactate under normal conditions?
Lactate serves as an energy source during anaerobic metabolism.
How does Type A lactic acidosis differ from Type B?
- Type A: Hypoxic causes (e.g., shock, hypovolemia).
- Type B: Non-hypoxic causes (e.g., sepsis, liver dysfunction).
Why is lactate accumulation clinically significant?
It signifies tissue hypoxia or impaired metabolic pathways, often seen in critical illnesses.
How does hypoalbuminemia affect the anion gap?
It reduces the sensitivity of the anion gap in detecting metabolic acidosis.
(?)
What anion gap value suggests an unmeasured anion accumulation?
An anion gap >12 mmol/L.
What are common unmeasured anions in metabolic acidosis?
Lactate, ketones, or toxins like methanol and ethylene glycol.
How can you determine if an acid-base disorder is mixed?
If both pH and compensatory mechanisms are abnormal (e.g., acidosis with inappropriate PaCO2 response).
What does a PaO2 of <10 kPa on FiO2 >0.21 suggest?
Impaired oxygenation, likely due to V/Q mismatch or shunting.
What does a low PaCO2 with normal pH indicate?
A compensated metabolic acidosis or respiratory alkalosis.
