Pulse oximetry
- Pulse oximeters provide a safe, reliable and non-invasive method of continuous arterial oxygen saturation monitoring. They are standard monitors during anaesthesia and in the ICU.Haemoglobin saturation is measured from the absorption of light emitted from a probe placed on a digit or ear lobe.
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Limitations and errors:
- pulse oximetry is accurate above 90% oxyhaemoglobin saturation, but much less so below 70%, and inaccurate below 50%. Pulse oximeters estimate arterial haemoglobin oxygen saturation (SaO2) and not arterial oxygen tension (PaO2). Because of the shape of the oxygen-dissociation curve, large changes in PaO2 may occur at the extremes of the curve with minimal change in SaO2. They have a response time of 10–30 s, depending on the signal averaging time, circulation time and the site of the probe.
- Increase in the non-pulsatile component of light absorption through nail varnish or dirt increases error. Skin colour does not influence accuracy. Excess ambient light, infrared heaters and diathermy also interfere with accuracy. Hyperbilirubinaemia does not affect accuracy, nor anaemia unless the haematocrit is below 10%. Low peripheral perfusion due to low cardiac output or vasoconstriction interfere with accurate detection of arterial pulsations, as do atrial fibrillation or other arrhythmias.
- The probe contains two high intensity, monochromatic, light- emitting diodes, one emitting red light (660 nm) and the second infrared (940 nm) on one side and a photodetector on the other to measure the amount of light transmitted through the finger.
- The saturation of haemoglobin is calculated from the absorption at the two different wavelengths. The diodes are switched on in sequence with a pause with both diodes off.
- This pause allows the photocell and microprocessor to compensate for any ambient light. By analysing the pulsatile changes in light absorption, the absorption by venous blood and tissue is deducted and arterial saturation measured.
- The measurements are plotted against a standard calibration curve, determined by direct measurements of the arterial oxygen saturation of normal resting healthy volunteers.
pH electrode:
Blood Gas Analyzer
Blood ph measurement
- If a glass membrane separates two solutions of different hydrogen ion concentration a potential difference develops that is proportional to the hydrogen ion gradient between the two.
- A potentiometric electrode is designed to measure the potential between the sample and a buffer solution.
- Some blood gas analysers incorporate a co-oximeter that directly measures the various forms of haemoglobin including oxyhaemoglobin, total haemoglobin, carboxyhaemoglobin and methaemoglobin.
- The actual bicarbonate, standard bicarbonate, and base excess are calculated from the pH and pCO2 using the Siggard-Anderson nomogram derived from a series of in vitro experiments relating pH, pCO2 and bicarbonate.
- Blood gas analysers report a wide range of results, but the only parameters directly measured are the partial pressures of oxygen (pO2) and carbon dioxide (pCO2) and blood pH.
- The haemoglobin saturation (HbO2%) is calculated from the pO2 using the oxygen-dissociation curve and assumes a normal P50 and that there are no abnormal forms of haemoglobin.
The polarographic (Clark) oxygen electrode
Practical precautions:
Measures the oxygen partial pressure in a blood or gas sample. A platinum cathode and a silver/silver chloride anode are placed in a sodium chloride electrolyte solution, and a voltage of 700 mv is applied (Figure 1). The following reactions occur.
- At the cathode: O2 + 2H2O + 4e– = 4OH–.
- In the electrolyte: NaCl + OH– = NaOH + Cl–.
- At the anode: Ag + Cl– = AgCl + e–.
Electrons are taken up at the cathode and the current generated is proportional to oxygen tension. A membrane separates the electrode from blood, preventing deposition of protein but allowing the oxygen tension in the blood to equilibrate with the electrolyte solution. The electrode is kept at a constant temperature of 37°C and regular checks of the membrane are required to ensure it is not perforated or coated in proteins. Sampling two gas mixtures of known oxygen tension allows calibration.
A heparinized, freshly drawn, bubblefree, arterial blood sample is required. Heparin is acidic and if too much is present, the measured pCO2 and calculated bicarbonate are spuriously reduced. Delay in measurement allows continued metabolism by the erythrocytes and reduces pH and pO2 and increases pCO2. Keeping the specimen on ice allows accurate measurement to be delayed for up to 1 hour. Air bubbles introduce error and cause a fall in pCO2 and an increase in pO2.
- A measuring silver/silver chloride electrode is encased in a bulb of special pH-sensitive glass and contains a buffer solution that maintains a constant pH.
- This glass electrode is placed in the blood sample and a potential difference is generated across the glass, which is proportional to the difference in hydrogen ion concentration. The potential is measured between a reference electrode (in contact with the blood via a semi-permeable membrane) and the measuring electrode. Both electrodes must be kept at 37°C, clean and calibrated with buffer solutions of known pH.
- The assessment of respiratory function and metabolic state with blood gas analysis, combined with continuous monitoring from pulse oximetry and capnography is routinely performed in patients during anaesthesia, in resuscitation and in the critically ill.
- The Severinghaus or carbon dioxide electrode is a modified pH electrode in contact with sodium bicarbonate solution and separated from the blood specimen by a rubber or Teflon semipermeable membrane. Carbon dioxide, but not hydrogen ions, diffuses from the blood sample across the membrane into the sodium bicarbonate solution, producing hydrogen ions and a change in pH.
- Hydrogen ions are produced in proportion to the pCO2 and are measured by the pH-sensitive glass electrode. As with the pH electrode, the Severinghaus electrode must be maintained at 37°C, be calibrated with gases of known pCO2 and the integrity of the membrane is essential. Because diffusion of the CO2 into the electrolyte solution is required the response time is slow at 2–3 minutes.
Measurement of blood PCO2 and PO2