Silent hypoxia is a potentially fatal condition where Covid-19 patients exhibit alarmingly low blood oxygen saturation levels without experiencing noticeable symptoms such as breathlessness. This hidden danger can be detected using a pulse oximeter, a non-invasive device that measures arterial blood oxygen saturation (SpO2) and pulse rate.
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Understanding Silent Hypoxia and the Role of Pulse Oximeters
By regularly using a pulse oximeter at home, individuals can monitor their oxygen levels and seek medical attention before the condition worsens. Although it is not a definitive diagnostic tool for Covid-19, the device has been in high demand. If a reading appears abnormal, it is advisable to consult a physician.
How Does a Pulse Oximeter Work?
A pulse oximeter determines oxygen saturation levels by using light absorption principles. The device comprises a probe with a light source on one side and a sensor on the other. When a finger is inserted into the probe, some light is absorbed while the rest passes through. The absorption level depends on the oxygenation of the blood, which the device then calculates.
Oxygen Saturation Guidelines (WHO)
| Oxygen Saturation (SpO2) Level | Condition |
|---|---|
| 95% – 100% | Normal |
| 94% | Requires monitoring |
| Below 90% | Medical emergency |
Additionally, the normal perfusion index (PI) ranges from 0.02% to 20%, indicating weak to strong pulse strength.
Accuracy of Pulse Oximeter Readings
No pulse oximeter is 100% accurate. A variation of ±2% is possible due to arterial blood gas fluctuations or mechanical inconsistencies. For instance, a reading of 96% could range between 94% and 98%. Several factors can affect the accuracy of readings.
Factors Affecting Pulse Oximeter Readings
| Factor | Effect on Reading |
| Nail varnish | Absorbs light, leading to incorrect readings |
| Henna pigmentation | Can alter light absorption, causing errors |
| User movement | Shaking or movement can result in fluctuations |
| Bright ambient light | Interferes with sensor accuracy |
| Poor perfusion | Weak pulse can produce unreliable readings |
| Carbon monoxide poisoning | May give falsely high readings |
For instance, smokers or individuals exposed to fire smoke may receive an inaccurate reading due to carbon monoxide binding to haemoglobin.
Best Finger for Pulse Oximeter Readings
The third finger (middle finger) of the dominant hand is considered the most reliable for pulse oximetry. The thumb of the dominant hand is the second-best option. However, the differences between fingers are minor, making the index finger a suitable alternative if necessary.
Should One Own a Pulse Oximeter During the Pandemic?
Dr Shahida Akhter, Senior Consultant at BIRDEM and Professor at Ibrahim Medical College, states:
“A pulse oximeter does not diagnose Covid-19. It only measures oxygen saturation levels in the blood, irrespective of the cause. However, monitoring oxygen levels at home is crucial as decreasing saturation can indicate the need for hospitalisation, especially for suspected Covid-19 patients. Thus, in current circumstances, having a pulse oximeter at home is advisable.”
While a pulse oximeter is a valuable tool for early detection of declining oxygen levels, individuals experiencing symptoms should always seek medical advice rather than relying solely on home monitoring.
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