PULSE OXIMETRY

Q.What is pulse oximetry?
A. Pulse-oximetry (PulseOx) is a non-invasive and a simple method to determine the percentage of oxygenated hemoglobin. Pulse-Oximeter is a machine that calculates the amount of oxygenated Hb. It consists of a probe attached to the patient's finger or ear lobe, which is connected to a computerised machine. The machine has a display monitor, which shows the oxygen saturation of Hb (SaO2), the pulse rate and in some instruments, graphical display of the blood flow past the probe. Audible alarms and signals are available in various tones to detect any abnormality, which a user can program accordingly.

Thus, an oximeter helps to detect hypoxia and beat to beat variation of SaO2 even before the patient becomes clinically cyanosed.

Q. How does pulse-ox work?
A. The probe emits light which consists of red and infrared rays (650nm and 800nm respectively). The oxygenated Hb absorbs the red light whereas the infrared is absorbed by desaturated hemoglobin. By calculating the percentage of each wavelength absorbed, the proportion of oxygenated hemoglobin is calibrated automatically. The SaO2 is calculated at every beat as it detects pulsatile blood flow more clearly. Thus, in cases of hypovolemia, shock or vasoconstriction, the pulse Ox may not be very accurate.

Q. In which condition is the pulse-ox not accurate?
A. The pulse ox may not be accurate in the following conditions :-
(i) Hypovolemia, shock, cardiac failure, severe hypotension, cold or peripheral vascular disease. This is due to reduction in pulsatile blood flow in the periphery due to vasoconstriction.
(ii) Tricuspid regurgitation with venous congestion may produce venous pulsations causing low reading in ear probes.
(iii) Badly positioned probe.
(iv) Bright overhead lights in theatre.
(v) Carbon monoxide poisoning as the pulse-ox can not distinguish between carboxyhemoglobin and oxygenated hemoglobin. Falsely elevated saturation is noted.
(vi) Low readings are seen in methemoglobinemia.
(vii) Probe on an edematous limb.

Q. Does jaundice, dark skin or anemia affect the pulse Ox readings?
A. No.

Q.When is pulse ox monitoring required?
A. Pulse oximeters are used to monitor SaO2 and pulse rate during surgery under general anesthesia. They are also used during mechanical ventilation to guide and assess whether a patient’s oxygen therapy adequate. Thus, they are routinely used in the intensive care units.

They are also used to detect impending respiratory failure in a patient with neuro- muscular disease. In some hospitals, they are also used in the casualty and the wards.

Q. Does pulse-ox help to determine the CO2 level in the blood?
A. No, the pulse-ox can measure only the oxygen saturation of the blood. To measure the CO2 level in the blood, an instrument called capnograph is available.

Q. How useful is pulse-ox monitoring?
A. Pulse-oximeter is the greatest advance in patient monitoring. Its use in the intensive care unit and in the operation theatre is now indispensable.

Q. How accurate is pulse-ox monitoring?
A. The oximeters are calibrated at time of manufacturing and they automatically check their internal circuits when they are turned on. They are very accurate when the oxygen saturation ranges from 70 % to 100% but less accurate with SaO2 less than 70%. When the oxygen saturation (SaO2) is less than 90%, there is a marked fall in PaO2 and should be confirmed by an ABG.

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