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Questions and Answers
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?
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?
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How does a change in HCO-3 concentration affect pH?
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What does a pH level below 7.35 indicate?
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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?
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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?
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What is the reference temperature for measurements taken using the α-stat method?
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What does a low HCO3- level indicate?
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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?
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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?
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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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