Principles

Accurate blood gas analysis is crucial for assessing a patient’s respiratory and metabolic status. Understanding the analytical principles behind these measurements is key to ensuring reliable results

pH Measurement
Partial Pressure of Oxygen (Pa\(O_2\)) Measurement
Partial Pressure of Carbon Dioxide (Pa\(CO_2\)) Measurement
Bicarbonate (\(HCO_3^-\)) Measurement
Electrolyte Measurements
Calculated Parameters

pH Measurement

Principle: The measurement of pH in blood gas analysis is based on potentiometry, using a pH-sensitive electrode
Electrode: A pH electrode consists of two half-cells: a measuring electrode and a reference electrode
- Measuring Electrode: Contains a pH-sensitive glass membrane that selectively binds to hydrogen ions (\(H^+\))
- Reference Electrode: Provides a stable and known electrical potential
Process
1. The blood sample is introduced into the measuring chamber, where it comes into contact with the pH-sensitive glass membrane
2. The hydrogen ions in the sample interact with the glass membrane, creating a potential difference between the measuring electrode and the reference electrode
3. The potential difference is measured by a voltmeter, which is calibrated to display the pH value
Reactions
- The glass membrane of the measuring electrode selectively binds to \(H^+\) ions
- The potential difference generated is proportional to the pH of the sample
Detection: The pH is displayed on the instrument readout
Advantages: Accurate, precise, and rapid measurement of pH
Disadvantages: Requires careful calibration and maintenance of the electrodes
Temperature Correction: The pH is temperature dependent (as temperature goes up, pH goes down). Blood gas analyzers are temperature controlled, or they may be able to adjust the pH value

Partial Pressure of Oxygen (Pa\(O_2\)) Measurement

Principle: The measurement of Pa\(O_2\) in blood gas analysis is based on amperometry, using a Clark electrode
Electrode: The Clark electrode consists of a platinum cathode and a silver/silver chloride anode immersed in an electrolyte solution, separated from the sample by an oxygen-permeable membrane
Process
1. The blood sample is introduced into the measuring chamber, where oxygen diffuses through the membrane and is reduced at the platinum cathode
2. The reduction of oxygen generates a current that is proportional to the partial pressure of oxygen in the sample
3. The current is measured by an ammeter, which is calibrated to display the Pa\(O_2\) value
Reactions
- At the platinum cathode: \(O_2 + 2H_2O + 4e^- → 4OH^-\)
- At the silver anode: \(4Ag + 4Cl^- → 4AgCl + 4e^-\)
Detection: The Pa\(O_2\) is displayed on the instrument readout
Advantages: Accurate, precise, and rapid measurement of Pa\(O_2\)
Disadvantages: Requires careful calibration and maintenance of the electrode, can be affected by protein contamination
Temperature Correction: The Pa\(O_2\) is temperature dependent, and blood gas analyzers are temperature controlled

Partial Pressure of Carbon Dioxide (Pa\(CO_2\)) Measurement

Principle: The measurement of Pa\(CO_2\) in blood gas analysis is based on potentiometry, using a Severinghaus electrode
Electrode: The Severinghaus electrode consists of a pH-sensitive glass electrode surrounded by a bicarbonate electrolyte solution, separated from the sample by a carbon dioxide-permeable membrane
Process
1. The blood sample is introduced into the measuring chamber, where carbon dioxide diffuses through the membrane and equilibrates with the bicarbonate solution
2. The change in pH of the bicarbonate solution is measured by the pH-sensitive glass electrode, which is proportional to the partial pressure of carbon dioxide in the sample
3. The change in pH is measured by a voltmeter, which is calibrated to display the Pa\(CO_2\) value
Reactions
- \(CO_2\) diffuses through the membrane and reacts with \(H_2O\) to form \(H_2CO_3\)
- \(H_2CO_3\) dissociates into \(H^+\) and \(HCO_3^-\)
- \(CO_2 + H_2O ↔ H_2CO_3 ↔ H^+ + HCO_3^-\)
Detection: The Pa\(CO_2\) is displayed on the instrument readout
Advantages: Accurate, precise, and rapid measurement of Pa\(CO_2\)
Disadvantages: Requires careful calibration and maintenance of the electrode, can be affected by protein contamination

Bicarbonate (HCO3-) Measurement

Principle: Bicarbonate is not measured directly by blood gas analyzers. Instead, it is calculated using the Henderson-Hasselbalch equation, based on the measured values of pH and Pa\(CO_2\)
Henderson-Hasselbalch Equation
- \(pH = pKa + log \left( \frac{[HCO_3^-]}{[H_2CO_3]} \right)\)
- Since \([H_2CO_3]\) is proportional to \(PCO_2\), the equation can be written as:
- \(pH = pKa + log \left( \frac{[HCO_3^-]}{0.03 \times PCO_2} \right)\)
- Where:
  - pKa = 6.1 (dissociation constant of carbonic acid)
  - 0.03 = Solubility coefficient of \(CO_2\) in plasma
Calculation: The bicarbonate concentration (\([HCO_3^-]\)) is calculated from the measured pH and Pa\(CO_2\) using the rearranged Henderson-Hasselbalch equation:
- \([HCO_3^-] = 10^{(pH - 6.1)} \times 0.03 \times PCO_2\)
Advantages: Provides a clinically useful estimate of bicarbonate concentration
Disadvantages: Is subject to errors in pH and PCO2 measurements

Electrolyte Measurements

Principle: Some blood gas analyzers can also measure electrolytes (e.g., sodium, potassium, chloride, calcium) using ion-selective electrodes (ISEs)
Ion-Selective Electrodes (ISEs)
- ISEs are electrochemical sensors that respond selectively to specific ions
- Each ISE consists of a membrane that is selectively permeable to the ion of interest
- When the electrode is immersed in a solution containing the ion, a potential difference develops across the membrane
- The potential difference is proportional to the activity (concentration) of the ion in the sample
Process
1. The sample is introduced into the measuring chamber, where the ISE comes into contact with the sample
2. The ion of interest diffuses across the membrane, creating a potential difference
3. The potential difference is measured by a voltmeter, which is calibrated to display the ion concentration
Advantages: Rapid and accurate measurement of electrolyte concentrations
Disadvantages: Requires careful calibration and maintenance of the electrodes, can be affected by protein contamination and other interfering substances

Calculated Parameters

Base Excess (BE)
- Definition: The amount of acid or base needed to restore normal pH in a blood sample
- Calculation: Base excess is calculated by the blood gas analyzer using a complex equation that takes into account the pH, Pa\(CO_2\), and hemoglobin concentration
- Interpretation:
  - Positive Base Excess: Indicates an excess of base (alkalosis)
  - Negative Base Excess: Indicates a deficit of base (acidosis)
Anion Gap (AG)
- Definition: The difference between measured cations (\(Na^+\), \(K^+\)) and anions (\(Cl^-\), \(HCO_3^-\)) in serum or plasma
- Calculation: Anion Gap = [\(Na^+\)] + [\(K^+\)] - [\(Cl^-\)] - [\(HCO_3^-\)]
- Normal Range: 8-16 mEq/L (with \(K^+\)), 10-20 mEq/L (without \(K^+\))
- Clinical Significance: Used to assess metabolic acidosis

Key Terms

Potentiometry: A method to measure the electrical potential difference between two electrodes
Amperometry: A method to measure the electrical current flowing through an electrochemical cell
Clark Electrode: An electrode used to measure the partial pressure of oxygen (Pa\(O_2\))
Severinghaus Electrode: An electrode used to measure the partial pressure of carbon dioxide (Pa\(CO_2\))
Ion-Selective Electrode (ISE): An electrochemical sensor that responds selectively to specific ions
Henderson-Hasselbalch Equation: An equation that relates pH to the concentrations of bicarbonate and carbon dioxide
Base Excess (BE): A measure of the amount of acid or base needed to restore normal pH
Anion Gap (AG): The difference between measured cations and anions in serum or plasma
Temperature Coefficient: The amount of change in pH of a substance per degree change in temperature
Assayed Quality Control Material: Commercial reference material with known values for control and calibration of blood gas instruments

Procedures

Precautions