How to choose and use a pulse oximeter?

By Cristiano Antonino On Mar 21, 2023

Before the COVID-19 pandemic, the pulse oximeter (or saturation meter) was only widely used by ambulance teams, resuscitators and pulmonologists

The spread of the coronavirus has increased the popularity of this medical device, and people’s knowledge of its function.

They are almost always used as ‘saturation meters’, although in reality they can tell a lot more.

In fact, the capabilities of a professional pulse oximeter are not limited to this: in the hands of an experienced person, this device can solve many problems.

First of all, let us recall what a pulse oximeter measures and displays

The ‘clip’ shaped sensor is placed (usually) on the patient’s finger, in the sensor an LED on one half of the body emits light, the other LED on the other half receives.

The patient’s finger is illuminated with light of two different wavelengths (red and infrared), which are absorbed or transmitted differently by the oxygen-containing haemoglobin ‘on itself’ (HbO 2 ), and the free oxygen-free haemoglobin (Hb).

Absorption is estimated during the pulse wave in the small arterioles of the finger, thus displaying the indicator of haemoglobin saturation with oxygen; as a percentage of total haemoglobin (saturation, SpO 2 = ..%) and pulse rate (pulse rate, PR).

The norm in a healthy person is Sp * O 2 = 96 – 99 %.

* Saturation on a pulse oximeter is designated Sp because it is ‘pulsatile’, peripheral; (in microarteries) measured by a pulse oximeter. Laboratory tests for haemogasanalysis also measure arterial blood saturation (SaO 2 ) and venous blood saturation (SvO 2 ).

On the pulse oximeter display of many models, it is also possible to view a real-time graphical representation of the filling (from the pulse wave) of the tissue under the sensor, the so-called plethysmogram – in the form of a ‘bar’ or sine curve, the plethysmogram provides additional diagnostic information to the physician.

The advantages of the device are that it is harmless for everyone (no ionising radiation), non-invasive (no need to take a drop of blood for analysis), starts working on the patient quickly and easily, and can work around the clock, rearranging the sensor on the fingers as required.

However, any pulse oximeter and pulse oximetry in general has disadvantages and limitations that do not allow the successful use of this method in all patients

These include:

1) Poor peripheral blood flow

– lack of perfusion where the sensor is installed: low blood pressure and shock, resuscitation, hypothermia and frostbite of the hands, atherosclerosis of the vessels in the extremities, need for frequent blood pressure (BP) measurements with the cuff clamped on the arm, etc. – Because of all these causes, the pulse wave and the signal on the sensor are poor, a reliable measurement is difficult or impossible.

Although some professional pulse oximeters have an ‘Incorrect Signal’ mode (‘we measure what we get, accuracy is not guaranteed’), in the case of low blood pressure and no normal blood flow under the sensor, we can monitor the patient via ECG and capnography channels.

Unfortunately, there are some critical patients in emergency medicine who cannot use pulse oximetry,

2) Nail” problems in receiving a signal on the fingers: indelible manicure on the nails, severe nail deformation with fungal infection, too small fingers in children, etc.

The essence is the same: the inability to obtain a normal signal for the device.

The problem can be solved: by turning the sensor on the finger 90 degrees, by installing the sensor in non-standard places, e.g. on the tip.

In children, even premature ones, it is usually possible to obtain a stable signal from an adult sensor mounted on the big toe.

Special sensors for children are only available for professional pulse oximeters in a complete set.

3) Noise dependence and immunity to “noise

When the patient moves (altered consciousness, psychomotor agitation, movement in a dream, children) or shakes during transport, the sensor can be dislodged and an unstable signal can be produced, triggering alarms.

Professional transport pulse oximeters for rescuers have special protection algorithms that allow short-lived interference to be ignored.

The indicators are averaged over the last 8-10 seconds, the interference is ignored and does not affect operation.

The disadvantage of this averaging is a certain delay in changing the readings of the actual relative change in the patient (a clear disappearance of the pulse from the initial rate of 100, in reality 100->0, will be shown as 100->80- >60->40->0), this must be taken into account during monitoring.

4) Problems with haemoglobin, latent hypoxia with normal SpO2 :

A) Haemoglobin deficiency (with anaemia, haemodilution)

There may be little haemoglobin in the body (anaemia, haemodilution), there is organ and tissue hypoxia, but all the haemoglobin present may be saturated with oxygen, SpO 2 = 99 % .

It should be remembered that the pulse oximeter does not show the entire oxygen content of the blood (CaO 2 ) and undissolved oxygen in the plasma (PO 2 ), i.e. the percentage of haemoglobin saturated with oxygen (SpO 2 ).

Although, of course, the main form of oxygen in the blood is haemoglobin, which is why pulse oximetry is so important and valuable.

B) Special Forms of Haemoglobin (by poisoning)

Haemoglobin bound to carbon monoxide (HbCO) is a strong, long-lived compound that in reality does not carry oxygen, but has light absorption characteristics very similar to normal oxyhaemoglobin (HbO 2 ) .

Pulse oximeters are constantly being improved, but at present, the creation of inexpensive mass pulse oximeters that distinguish between HbCO and HbO 2 is a matter of the future.

In the case of carbon monoxide poisoning during a fire, the patient may have severe and even critical hypoxia, but with a flushed face and falsely normal SpO 2 values, this should be taken into account during pulse oximetry in such patients.

Similar problems may occur with other types of dyshaemoglobinaemia, intravenous administration of radiopaque agents and dyes.

5) Covert hypoventilation with O2 inhalation

A patient with depression of consciousness (stroke, head injury, poisoning, coma), if receiving inhaled O2 , due to the excess oxygen received with each respiratory act (compared to 21% in atmospheric air), may have normal saturation indicators even at 5 -8 breaths per minute.

At the same time, an excess of carbon dioxide will accumulate in the body (oxygen concentration during FiO 2 inhalation does not affect CO 2 removal), respiratory acidosis will increase, cerebral oedema will increase due to hypercapnia and the indicators on the pulse oximeter may be normal.

Clinical assessment of respiration and capnography of the patient are required.

6) Discrepancy between perceived and actual heart rate: ‘silent’ beats

In the case of poor peripheral perfusion, as well as heart rhythm disturbances (atrial fibrillation, extrasystole) due to the difference in pulse wave power (pulse filling), ‘silent’ pulse beats may be ignored by the device and not taken into account when calculating the heart rate (HR, PR).

The actual heart rate (heart rate on the ECG or during auscultation of the heart) may be higher, this is the so-called. ‘pulse deficit’.

Depending on the internal algorithm of this device model and the difference in pulse filling in this patient, the extent of the deficit may be different and change.

In appropriate cases, simultaneous ECG monitoring is recommended.

There may be a reverse situation, with the so-called. “dichrotic pulse”: due to a decrease in vascular tone in this patient (due to infection, etc.), each pulse wave on the plethysmogram graph is seen as double (“with recoil”), and the device on the display may falsely double the PR values.

Objectives of pulse oximetry

1) Diagnostic, SpO 2 and PR (PR) measurement

2) Real-time patient monitoring

The purpose of diagnostics, e.g. measurement of SpO 2 and PR is certainly important and obvious, which is why pulse oximeters are now ubiquitous, however, miniature pocket-sized devices (simple ‘saturation meters’) do not allow for normal monitoring, a professional device is required to constantly monitor the patient.

Types of pulse oximeter and related equipment

Mini wireless pulse oximeters (screen on finger sensor)
Professional monitors (sensor-wire-case design with separate screen)
Pulse oximeter channel in a multifunction monitor or defibrillator
Mini Wireless Pulse Oximeters

Wireless pulse oximeters are very small, the display and control button (there is usually only one) are located at the top of the sensor housing, there are no wires or connections.

Because of their low cost and compactness, such devices are now widely used.

They are indeed convenient for a one-off measurement of saturation and heart rate, but have significant limitations and disadvantages for professional use and monitoring, e.g. in the conditions of an ambulance crew.

Advantages

Compact, does not take up much space in pockets and storage
Easy to use, no need to remember instructions

Disadvantages

Poor visualisation during monitoring: when the patient is on a stretcher, you have to constantly approach or lean towards the finger with the sensor, cheap pulse oximeters have a monochrome screen which is difficult to read from a distance (it is better to buy a colour one), you have to perceive or change an inverted image, incorrect perception of an image such as SpO 2 = 99 % instead of 66 %, PR=82 instead of SpO 2 =82 can have dangerous consequences.

The problem of poor visualisation cannot be underestimated.

Now it would never occur to anyone to watch a training film on a black-and-white TV with a 2″ diagonal screen: the material is better absorbed by a sufficiently large colour screen.

A clear image from a bright display on the wall of a rescue vehicle, visible in any light and at any distance, allows one not to be distracted from more important tasks when working with a patient in serious condition.

There are extensive and comprehensive features in the menu: adjustable alarm limits for each parameter, pulse volume and alarms, ignoring a bad signal, plethysmogram mode, etc., if there are alarms, they will sound and distract all the way through or switch off all at once.

Some imported cheap pulse oximeters, based on experience of use and laboratory testing, do not guarantee real accuracy.

It is important to weigh up the pros and cons before purchasing, based on the needs of your area.

The need to remove the batteries during long-term storage: if the pulse oximeter is used infrequently (e.g. in an ‘on-demand’ home first aid kit), the batteries inside the device leak and damage it, in long-term storage, the batteries must be removed and stored nearby, while the fragile plastic of the battery cover and its lock may not withstand repeated closing and opening of the compartment.

In a number of models there is no possibility of external power supply, the need to have a spare set of batteries nearby is a consequence of this.

To sum up: it is rational to use a wireless pulse oximeter as a pocket instrument for rapid diagnostics, the monitoring possibilities are extremely limited, it is really only possible to carry out simple bedside monitoring, e.g. monitoring the pulse during the intravenous administration of a beta-blocker.

It is advisable to have such a pulse oximeter for ambulance crews as a second backup.

Professional monitoring pulse oximeters

Such a pulse oximeter has a larger body and display, the sensor is separate and replaceable (adult, child), connected via a cable to the body of the device.

A liquid crystal display and/or touchscreen (as in a smartphone) instead of a seven-segment display (as in an electronic watch) is far from always necessary and optimal, of course it is modern and cost-effective, but it tolerates disinfection worse, may not respond clearly to finger pressure in medical gloves, consumes more electricity, is fragile if dropped, and significantly increases the price of the device.

Advantages

Convenience and clarity of display: a sensor on the finger, a wall-mounted device on a bracket or in front of the doctor’s eyes, a sufficiently large and clear image, quick decision-making during monitoring
Comprehensive functionality and advanced settings, which I will discuss separately and in detail below.
Measurement accuracy
The presence of external power supply (12V and 220V), which means the possibility of 24-hour uninterrupted use
The presence of a child sensor (could be an option)
Resistance to disinfection
Availability of service, testing and repair of domestic devices

Disadvantages

Less compact and portable
Expensive (good pulse oximeters of this type are not cheap, although their price is significantly lower than that of cardiographs and defibrillators, this is a professional technique for saving patients’ lives)
The need to train staff and master this model of the device (it is advisable to monitor patients with a new pulse oximeter in “all in a row” so that skills are stable in a really difficult case)

To summarise: a professional monitoring pulse oximeter is definitely necessary for all seriously ill patients for work and transport, due to its advanced functionality, in many cases it saves time and does not need to be connected to a multi-channel monitor, it can also be used for simple saturation and pulse diagnosis, but it is inferior to mini-pulse oximeters in terms of compactness and price.

Separately, we should dwell on the choice of display type (screen) of a professional pulse oximeter.

It would seem that the choice is obvious.

Just as push-button phones have long since given way to modern smartphones with a touchscreen LED display, modern medical devices should be the same.

Pulse oximeters with a display in the form of seven-segment numerical indicators are considered obsolete.

However, practice seems to show that in the specifics of the work of ambulance teams, the version of the device with an LED display has significant drawbacks that one must be aware of when choosing and working with it.

The disadvantages of the device with LED display are as follows:

Fragility: in practice, a device with a seven-segment display easily withstands falls (e.g. from a stretcher on the ground), a device with an LED display – ‘fell, then broke’.
Poor touchscreen response to pressure while wearing gloves: during the outbreak of COVID-19, the main work with a pulse oximeter is on patients with this infection, staff were dressed in protective suits, medical gloves are on their hands, often double or thickened. A touchscreen LED display of some models have responded badly or incorrectly to pressing the controls on the screen with fingers in such gloves, as the touchscreen is originally designed to be pressed with bare fingers;
Viewing angle and working in bright light conditions: the LED display must be of the highest quality, it must be visible in very bright sunlight (e.g. when the crew is working on the beach) and at an angle of almost ‘180 degrees’, a special light character must be selected. Practice shows that the LED screen does not always meet these requirements.
Resistance to intensive disinfection: the LED display and a device with this type of screen may not withstand ‘serious’ treatment with disinfectants;
Cost: the LED display is more expensive, significantly increasing the price of the device
Increased power consumption: the LED display requires more energy, which means either more weight and price due to a more powerful battery or a shorter battery life, which can create problems during emergency work during the COVID-19 pandemic (no time to charge)
Low maintainability: the LED display and the device with such a screen are less maintainable in service, replacement of the display is very expensive, practically not repaired.

For these reasons, on the job, many rescuers quietly opt for the pulse oximeter with a ‘classic’ type display on seven-segment numerical indicators (as on an electronic watch), despite its apparent obsolescence. Reliability in ‘battle’ is considered a priority.

The choice of saturation meter, therefore, must be adapted on the one hand to the needs presented by the area, and on the other to what the rescuer considers it to be ‘performing’ in relation to his or her daily practice.

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