Additional  Vital Signs:  Pulse & SpO2

In addition to the 5 Basic Vital Signs we can add Pulse and Sp02 (Oxygen Saturation), but to do this we require equipment; the Pulse Oximeter.

Pulse Oximeter



Checking a pulse is hard.   On a fit, healthy casualty in low stress situations it can sometimes be easy to feel a nice juicy pulse but in reality, with a poorly casualty, and you’re having a bit of a moment and you are not in a nice, quiet training environment, don’t bother.

In the late 90’s several studies were conducted to assess the accuracy of lay people (1), First Aiders, Paramedics and even Doctors in assessing simply the presence of a pulse.   In one study, success rates were as low as 45% in identifying the presence of a pulse (2) and in another, junior doctors took an average of 18 seconds to identify a pulse (3).

It was for these reasons that, based on advice from the International Liaison Committee on Resuscitation, the United Kingdom Resuscitation Council and American Heart Association removed routinely checking for a pulse as a sign of life from First Aid training in the 2000 updates.

But checking a pulse does have value – as with all of the basic vital signs, knowing whether the casualty’s pulse rate is within a normal range can assure us; if the casualty’s pulse is outside of these ranges it can even point us toward a particular problem. 

  • If someone has been running around we would expect their pulse to rise.  We’d also expect them to be hot, red and breathing faster.
  • If they haven’t been running around, but they’re hot, red, fast breathing and with a fast pulse,  we may have a problem, this could indicate sepsis.  
  • If they’re a trauma casualty; hot, red and with a slow, strong pulse, this could indicate internal head injury.
  • If they’re a trauma casualty, cold and pale with a fast pulse, this could be hypovolaemic shock.

Just don’t bother trying to check it manually.  We’re going to use the Pulse Oximiter:  The Pulse Oximeter is a small diagnostic tool which is predominantly used to identify the casualty’s oxygen saturations but it will also show the casualty’s pulse.   With one of these, we don’t need to be wasting time reaching around the casualty, desperately feeling for a pulse.

Pulse Oximetry

Pulse oximetry measures the amount of oxygen being carried in your blood, as a percentage.  The measurement is taken at the finger using a Pulse Oximeter.  This measurement is known as the Sp02 – the saturation of peripheral oxygen – and it is an estimate of the Sa02 – the saturations of arterial oxygen.

The haemoglobin in your red blood cells carry the oxygen (with a small amount being dissolved in the blood).   Each Haemoglobin molecule can carry 4 oxygen molecules.  If all of your haemoglobin have four oxygen molecules bound to them, your blood would be 'saturated' with oxygen and you would have an SpO2 of 100%.

Most people don’t have oxygen saturations of 100% so a range of 95-99% is considered normal.

Anything below 95% is an indicator of hypoxia – a lack of oxygen perfusing the tissues.

A reduction in SpO2 is the most reliable sign of hypoxia in a casualty; cyanosis and other clinical signs are difficult to detect and often late in presenting (4-6).  Increased breathing rate is associated with hypoxia but, again, the evidence suggests that this link is not strong enough (or even present in all cases) to rely on as a sign of hypoxia.  (7-9).

The pulse oximeter is a rapid diagnostic tool that allows you to measure and monitor the casualty’s oxygenation levels.  Knowing the casualty’s Sp02 will also allow you to give an appropriate amount of oxygen if that is within your skillset.

A reduction of 3% or more SpO2, even if within normal saturations, is an indicator to performing fuller assessment of the patient (and the oximeter signal) because this may be the first evidence of an acute illness (10).



  • Poor peripheral circulation and hypothermia are the most common cause of false low readings 
  • Movement – including shivering - and poor placement can also affect the reading.
  • Black, Blue and Green nail varnish can present inaccurate readings.  Acrylic nails do not appear to interfere. (11)
  • Bright lights, especially fluorescent lights can affect the readings. (12)
  • Carbon Monoxide has an affinity to haemoglobin 210 greater than oxygen; as such a casualty with Carbon Monoxide poisoning may show falsely high Sp02 readings as the Pulse Oximiter interprets the heamoglobin to be saturated with oxygen when they are saturated with Carbon Monoxide.(13)


The Waveform

As well as the pulse and oxygen saturations, the third piece of information we can look at is the waveform.   This undulating waveform is representative of the casualty’s blood pressure – peaking at the systolic pressure (the contraction of the heart) and dropping to the diastolic pressure.   While the pulse oximeter does not measure blood pressure nor give numerical values, this visual display can present a graphic indication of the quality of their pulse.

A regular, repetitive waveform with obvious peaks and troughs is indicative of a good quality peripheral perfusion.  

A particular feature of the cardiac cycle waveform is the “dicrotic notch”.  This notch on the descending side of the waveform indicates the closing of the aortic valve and the end of the systolic phase – the pulse.  It is speculated that the position of the notch can indicate vasomotor control – it appears the dictoric notch descends towards the baseline during vasodilation and ascends towards the peak during vasoconstriction. (14)  

A shallow, weak waveform would mean there is poor peripheral perfusion which could could indicate poor cardiac output or simply the casualty does not have great circulation in their fingers.  An exaggerated notch in addition to a low waveform is indicative of poor cardiac output (15). 

Strong waveform with good dicrotic notch indicating good pulse pressure and perfusion.

Strong waveform with good dicrotic notch indicating good pulse pressure and perfusion.

Strong pulse pressure but without an obvious dicrotic notch indicating good pulse pressure and perfusion but increased arterial resistance.

Strong pulse pressure but without an obvious dicrotic notch indicating good pulse pressure and perfusion but increased arterial resistance.

Weaker waveform but with noticeable dicrotic notch indicating lower blood pressure but normal vascular resistance.

Weaker waveform but with noticeable dicrotic notch indicating lower blood pressure but normal vascular resistance.


A pulse oximeter is an essential item for the remote medic, at less than £20.00 and about the size of two boxes of matches there is no reason not to have one.   Adding this to your first aid kit and the ability to interpret its reading will give you an accurate, rapid indication of the casualty's state of health and sometimes help point towards particular issues.

As with all vital signs, we look at ALL OF THE VITAL SIGNS.   If a casualty has a resting pulse of 45bpm and all of their other vital signs are normal, you can probably assume they are big into cycling, swimming or some other cardio sport.   If the basic vital signs are pointing towards a problem, their pulse and SpO2 can confirm or challenge that suspicion you have.

Next Article:  Blood Pressure


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