Blood Gas Analysis and Acid-Base Balance Techniques & Osmotic Pressure Measurement of Biological Fluids

1. Blood Gas Analysis Technique:

1.1 Principle:

Blood gas analysis relies on the principle of selective electrode operation (the potential difference between two electrodes). There are three types of electrodes that operate based on different principles to directly determine the pH, pO2, and pCO2 values in blood. The software processes these values to calculate other parameters, such as:

• pH: This is based on the potential difference occurring in a special glass membrane that is sensitive to hydrogen ions (H+) when it comes into contact with two solutions with different H+ concentrations. The difference in potential corresponds to the difference in pH between the two solutions.
• pCO2: This is based on the principle of indirect pH quantification. The pCO2 value is determined using a bicarbonate solution (pHx) separated from the sample. This solution is identified by a glass selective membrane permeable to CO2 gas (Teflon membrane). Through this membrane, CO2 can diffuse from the sample into the bicarbonate solution, changing the H+ concentration and resulting in a change in the pH of the bicarbonate solution. This pH change directly corresponds to the diffusion of CO2 through the membrane.
• pO2: The Clark electrode measures the partial pressure of oxygen in blood. A platinum cathode and a silver/silver chloride anode are immersed in a sodium chloride electrolyte solution, with a potential of 700 mV applied. The electrons emitted from the cathode to the anode create a current in the electrolyte proportional to the pressure of O2.

1.2 Influencing Factors:

a) Patient Preparation Prior to Sampling:

• Incorrect patient, incorrect test tube.
• Incorrect amount of anticoagulant.
• Unstable respiratory condition of the patient.
• Insufficient arterial blood (AB) quantity.

b) Sampling Process:

• Mixing of AB and venous blood (VB).
• Air in blood.
• Incomplete mixing with Heparin.

c) Storage Process:

• Not refrigerated if tested within 15 minutes.
• Samples with high pCO2, high white blood cell (WBC), and high red blood cell (RBC) count need to be tested within 5 minutes.
• If delayed beyond 15 minutes, store in ice until analysis (45 minutes).

2. Osmotic Pressure Measurement of Biological Fluids Technique:

2.1 Principle:

The freezing point depression method is used to determine the osmotic pressure of biological fluids.

• Principle: The sample is cooled to -70°C, and freezing begins. During the breakdown of the ice crystals formed, the solution releases the heat of fusion. The sample reaches thermal equilibrium. This temperature is detected by a sensitive thermal probe. The degree of freezing point depression of the sample compared to pure water is directly proportional to the total dissolved molecules in the solution, or the osmotic pressure.

2.2 Influencing Factors:

• Factors that increase blood osmotic pressure:
  • Blood alcohol content.
  • Citrate anticoagulated plasma.
• Solid components in the sample (-> centrifuge before testing).

Note:

• Blood gas analysis and osmotic pressure measurement of biological fluids are important tests to assess a patient’s health status.
• It is essential to strictly adhere to the preparation, sampling, and storage steps to ensure accurate results.
• Influencing factors on the test results need to be controlled to the maximum extent possible.
• Always consult a doctor to understand the test results and receive appropriate treatment advice.