GE HEALTHCARE E-PSMP Technical Manual

Contents

GE HEALTHCARE E-PSMP Technical Manual Manual pdf 17 page image

Hemodynamic Modules E-PRESTN, E-RESTN, E-PRETN, E-PSMP, E-PSM2 - 32062973-004These modules use the constant current method. The NTC-resistor is connected in series with anormal resistor and a constant voltage is applied across them. The temperature dependentvoltage can be detected at the junction of the resistors, thus producing the temperature signalfrom the patient. The signal is amplified by analog amplifiers and further processed by digitalelectronics.1.2.5 Pulse oximetryA pulse oximeter measures the light absorption of blood at two wavelengths, one in the nearinfrared (about 940 nm) and the other in the red region (about 660 nm) of the light spectrum.These wavelengths are emitted by LEDs in the SpO2 probe, the light is transmitted throughperipheral tissue and is finally detected by a PIN-diode opposite the LEDs in the probe. Thepulse oximeter derives the oxygen saturation (SpO2 ) using an empirically determinedrelationship between the relative absorption at the two wavelengths and the arterial oxygensaturation SaO 2 .In order to measure the arterial saturation accurately, pulse oximeters use the component oflight absorption giving variations synchronous with heart beat as primary information on thearterial saturation.A general limitation of pulse oximetry is that due to the use of only two wavelengths, only twohemoglobin species can be discriminated by the measurement.The modern pulse oximeters are empirically calibrated either against fractional saturationSaO 2frac;Formula 1or against functional saturation SaO2func;Formula 2Functional saturation is more insensitive to changes of carboxyhemoglobin andmethemoglobin concentrations in blood.The oxygen saturation percentage SpO 2 measured by the module is calibrated againstfunctional saturation SaO 2func. The advantage of this method is that the accuracy of SpO2measurement relative to SaO 2func can be maintained even at rather high concentrations ofcarboxyhemoglobin in blood. Independent of the calibration method, pulse oximeters are notable to correctly measure oxygen content of the arterial blood at elevated carboxyhemoglobinor methemoglobin levels.Plethysmographic pulse waveThe plethysmographic waveform is derived from the IR signal and reflects the blood pulsationat the measuring site. Thus the amplitude of the waveform represents the perfusion.SaO2frac HbO2HbO2 Hb Dyshemoglobin+ +-----------------------------------------------------------------------------=SaO2func HbO2HbO2 Hb+----------------------------=