Pulse Oximeter: how useful is it for checking COVID-19 at home?

Pulse Oximeter is an over-the-couter medical device being sold by major ecommerce companies and pharmacists. Till March-April 2020 this was known only to physicians and those suffering from some specific ailments, but it is a household name now; if it was earlier being sold by a few companies, it is now being manufactured by thousands, many of them local.

What is pulse oximeter and how does it work?

Pulse oximeter is a small device that is put on a thin body part (e.g. finger tip) for a few seconds, and it measures the 'oxygen load' in the blood. The oxygen load is measured in percentage. If it is 95, that means the blood is carrying 95% of its oxygen-carrying capacity.

A count of 95 to 99 on the oximeter is considered good while a count below 94 shows that the blood is carrying less oxygen than is required. However, doctors consider other factors before they make a value judgement on the oximeter reading.

The oximeter is fitted on finger tip (in case of infants, their foot) like a clamp. There are two pulsers inside one side of the clip: one that emits infrared rays and the other emits red light. There is also a sensor that measures how much of the two rays has been absorbed by the blood.

What happens inside the body is that oxygen is carried by hemoglobin (a complex molecule), which is carried by red blood corpuscles in the blood. The oxygen load is called SpO2 (Saturation of peripheral Oxygen) in technical terminology. Depending upon how much the hemoglobin is oxygenated, it absorbs the two types of lights. The difference in absorption of these lights gives indication what percentage of hemoglobin molecules (and so the blood cells, and so the blood itself) have oxygen and what percentage are devoid of oxygen. A micro-processor (a mini-computer) inside the oximeter uses a set of formulas to convert the absorption values to a final number and sends that to the counter as SpO2 number.

How reliable is pulse oximeter?

Luckily, pulse oximeter is a rather reliable quick measuring instrument (unlike many electronic blood pressure measuring instruments), unless a fly-by-night operator has designed/ produced an inferior product to cut corners. It is advisable to buy the instrument from a known brand or one that guarantees against fault (and you are sure that the seller will honour the promise).

Since the tech behind oximeter is simple and the material used is inexpensive, the production cost is very low, a few dollars (or a few hundred rupees). However, there is a big range in retail prices - that depends on features, marketing strategy of the producer, agent commission, availability, etc.

Pulse oximeters available in the market vary in features but even the simplest of them also comes with a pulse rate monitor. The ones for home use are usually battery operated. The basic oximeter that gives the percentage oxygen load or SpO2 reading is good enough for home monitoring of COVID patients.

What does the oxygen load indicate? How is it useful in COVID?

First, let us understand that pulse oximeter is not a device to test whether a person has COVID or not. So, one need not buy it and be under false impression that because it gives good readings, one does not have COVID. The device is useful for monitoring patients who have been found to be COVID-positive. It may also be used by health workers for mass screening of population, in conjunction with other ways of testing (for example, if a person is having fever, occasional coughing and shortness of breath, a low SpO2 reading would mean that he is perhaps COVID-positive and needs immediate treatment).

Now let us dissect the number that one gets on the oximeter. As said above, what the oximeter measures is the percentage of oxygen in the blood. However, a good SpO2 number does not tell other issues that the patients might be facing. For example, a person with severe anemia may have a high SpO2 number but overall oxygen carried by his blood to the tissues would be less due to low number of hemoglobin molecules in the blood. Similarly, a lower than normal blood flow may hurt the patient though his SpO2 count may be good, because the low blood flow delivers lower amount of oxygen to tissues even if the blood is loaded with oxygen. Some medicines may also lead to defective readings of SpO2.

It has been found that different body parts (different fingers, toe, ear lobe etc) give different readings on pulse oximeter. The right middle finger and right thumb have been reported to be the most reliable. Movement of body (shivering/ shaking/ moving finger) is also found to affect oximeter reading. Too cold extremities may also give wrong reading. Nail pigment/ henna (mehdi) coloring of finger and bright light falling on the sensor are other common factors that interfere with oximeter's performance.

Other uses of pulse oximeter

Pulse oximeter is used in hospitals in a large number of health issues such as acute asthma or chronic obstructive pulmonary disease (COPD) or pneumonia. It is also used in many situations such as during and after surgeries, monitoring patients in ICU, measuring exercise tolerance, examining the effectiveness of oxygen therapy or ventilator, and so on. Its home use is also prevalent among climbers, those suffering from

The takeaways

• Do not buy an oximeter because your friend claims it to be a reliable monitor for COVID-19.
• Buy it only if  someone has been tested positive for COVID and is at home, and the doctor has asked you to buy it so that you can monitor the patient's blood oxygen levels.
• Do not buy a fancy one or one with features that only doctors need in hospital situations.
• If possible, buy one of a reputed brand. Buy from a reliable ecommerce company. When buying from a shop, make sure that the device will be replaced immediately if found defective.
• Take these measures while taking reading:
• Take reading using right hand's middle finger or thumb, without movement.
  
• Be sure that the finger is not too cold or has nail polish/ henna (mehdi).
  
• Let a strong light not fall on the oximeter's sensor from a side when taking reading.
  
• If the patient is shaking or shivering, hold his finger to steady it or take reading after the shivering has stopped. If the oximeter shows waveform, it will be zig-zag and not smooth if it detects movement; reading taken in this situation will be incorrect.