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Questions and Answers
What is measured at the patient's actual temperature using the pH-stat method?
What is measured at the patient's actual temperature using the pH-stat method?
Which method is considered superior for managing blood gases in hypothermic patients?
Which method is considered superior for managing blood gases in hypothermic patients?
What is the goal for maintaining arterial carbon dioxide tension in both pH-stat and α-stat methods?
What is the goal for maintaining arterial carbon dioxide tension in both pH-stat and α-stat methods?
What change in pH occurs with a 1 mmHg change in PaCO2 above or below 40 mmHg?
What change in pH occurs with a 1 mmHg change in PaCO2 above or below 40 mmHg?
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How does a change in HCO-3 concentration affect pH?
How does a change in HCO-3 concentration affect pH?
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What does a pH level below 7.35 indicate?
What does a pH level below 7.35 indicate?
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What can happen if the pH-stat method is implemented incorrectly?
What can happen if the pH-stat method is implemented incorrectly?
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What is the threshold PaO2 level that necessitates the administration of supplemental oxygen?
What is the threshold PaO2 level that necessitates the administration of supplemental oxygen?
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How does arterial carbon dioxide tension and pH behave in a primary respiratory disorder?
How does arterial carbon dioxide tension and pH behave in a primary respiratory disorder?
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A high arterial carbon dioxide partial pressure (PaCO2) indicates which of the following conditions?
A high arterial carbon dioxide partial pressure (PaCO2) indicates which of the following conditions?
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What is the reference temperature for measurements taken using the α-stat method?
What is the reference temperature for measurements taken using the α-stat method?
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What does a low HCO3- level indicate?
What does a low HCO3- level indicate?
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What is the normal range for arterial carbon dioxide partial pressure (PaCO2)?
What is the normal range for arterial carbon dioxide partial pressure (PaCO2)?
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Base excess values are primarily used to assess which component of acid-base disorders?
Base excess values are primarily used to assess which component of acid-base disorders?
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Which calculation is used for determining the extracellular fluid base excess (BE(ecf))?
Which calculation is used for determining the extracellular fluid base excess (BE(ecf))?
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What condition is characterized by a high HCO3- level?
What condition is characterized by a high HCO3- level?
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Study Notes
Blood Gas Analysis
- Blood gas analyzers measure pH, partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2), and calculate bicarbonate concentration (HCO3-)
- Results are typically available within five minutes.
Blood Gas Analysis in Hypothermia
- Two methods are used to manage blood gases in hypothermic patients: pH-stat and alpha-stat.
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pH-stat method: Measures pH and other blood gas results at the patient's actual temperature.
- Goal is to maintain pH of 7.40 and PaCO2 of 5.3 kPa (40 mmHg) at the patient's actual temperature.
- Requires adding CO2 to the oxygenator.
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Alpha-stat method: Measures pH and other blood gas results at 37°C, regardless of the patient's actual temperature.
- Goal is to maintain PaCO2 of 5.3 kPa (40 mmHg) and pH of 7.40 when measured at 37°C.
- Alpha-stat method is preferred for optimal myocardial function.
- pH-stat method may lead to loss of autoregulation in the brain, increasing the risk of cerebral microembolisation and intracranial hypertension.
Blood Gas Interpretation Guidelines
- A 1 mmHg change in PaCO2 above or below 40 mmHg results in a 0.008 unit change in pH in the opposite direction.
- PaCO2 decreases by about 1 mmHg for every 1 mEq/L reduction in HCO3- below 24 mEq/L.
- A change in HCO3- of 10 mEq/L will result in a change in pH of approximately 0.15 pH units in the same direction.
Assessing Respiratory vs. Metabolic Disorders
- If PaCO2 and pH move in opposite directions (e.g., PaCO2 ↑ when pH < 7.4), it indicates a primary respiratory disorder.
- If PaCO2 and pH move in the same direction (e.g., PaCO2 ↑ when pH > 7.4 or PaCO2 ↓ when pH < 7.4), it indicates a primary metabolic disorder.
Blood Gas Parameters and Reference Ranges
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pH: 7.34-7.44.
- Values below 7.35 indicate acidemia, while values above 7.45 indicate alkalemia.
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PaO2: 10-13 kPa (75-100 mmHg).
- Low PaO2 indicates hypoxemia (abnormal oxygenation of blood).
- Supplemental oxygen is recommended for PaO2 below 60 mmHg.
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PaCO2: 4.7-6.0 kPa (35-45 mmHg).
- High PaCO2 (respiratory acidosis) indicates underventilation or hypermetabolic disorder.
- Low PaCO2 (respiratory alkalosis) indicates hyperventilation or overventilation.
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HCO3-: 22-26 mEq/L.
- Low HCO3- indicates metabolic acidosis.
- High HCO3- indicates metabolic alkalosis.
- SBCe: 21 to 27 mmol/L.
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Base Excess: -2 to +2 mmol/L.
- Base excess is used for assessment of the metabolic component of acid-base disorders.
- Positive values indicate metabolic alkalosis, while negative values indicate metabolic acidosis.
Calculations
- Two calculations are used for base excess: BE(ecf) and BE(b).
- BE(ecf) = [HCO3-] - 24.8 + 16.2 × (pH - 7.4)
- BE(b) = (1 - 0.014 × Hgb) × ([HCO3-] - 24.8 + (1.43 × Hgb + 7.7) × (pH - 7.4))
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