3.2 Vital Signs: Measurement Technique, Pulse Oximetry & Blood Pressure

Vital Signs: Measurement Technique, Pulse Oximetry & Blood Pressure

RCSI groups Observations as a topic area in its own right, listing vital signs, the INEWS chart, ISBAR, measuring blood pressure with an automatic monitor, using a pulse oximeter correctly, shift handover, and pages 48-53 of the INEWS guideline. It also publishes photographs of the observation equipment used at the OSCEs, including a contactless thermometer.

An observation set is only as good as the technique that produced it. A respiratory rate written from habit, a blood pressure taken with an undersized cuff or a saturation read through nail varnish do not merely produce a wrong number - they produce a wrong INEWS score, and therefore the wrong escalation.

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Respiratory Rate

The most sensitive and the most abused vital sign. Respiratory rate changes earlier than pulse or blood pressure in deterioration, and it is the parameter most often estimated.

• Count for a full 60 seconds, as the INEWS chart directs. Counting for 15 seconds and multiplying by four amplifies any error fourfold and misses irregular patterns entirely.
• Count without announcing it. People alter their breathing when they know it is being watched. Keep your fingers on the radial pulse and count respirations afterwards.
• Observe pattern and effort, not just rate: use of accessory muscles, pursed-lip breathing, tracheal tug, intercostal recession, ability to speak in full sentences, Kussmaul breathing in metabolic acidosis, Cheyne-Stokes respiration.
• Normal is 12-20 breaths per minute; 21-24 scores 2 on INEWS, 25 or more scores 3, and 8 or fewer scores 3.

Oxygen Saturation and Pulse Oximetry

A pulse oximeter estimates arterial oxygen saturation from the differential absorption of red and infrared light by oxygenated and deoxygenated haemoglobin.

Technique:

• Warm hands; remove nail varnish, gel nails and false nails.
• Position the probe correctly on the finger, with the light source over the nail bed.
• Wait for a stable trace or waveform and a pulse rate that matches the patient's palpated pulse. A number without a plausible pulse reading is not a reading.
• Record whether the patient is on air or on oxygen, and the flow rate and device. This is a separately scored INEWS parameter.

Where pulse oximetry misleads:

The last row is the clinically dangerous one. A COPD patient can have a comfortable saturation and a rising, life-threatening carbon dioxide level at the same time. Only an arterial blood gas measures ventilation.

Heart Rate and Pulse

The INEWS chart directs that the pulse is checked manually to ascertain rate, rhythm and quality - not simply read off a monitor.

• Palpate the radial pulse for 60 seconds where the rhythm is irregular; 30 seconds doubled is acceptable only when the rhythm is regular.
• Rhythm: regular, regularly irregular, or irregularly irregular. An irregularly irregular pulse suggests atrial fibrillation and is invisible to a machine-generated number.
• Quality: bounding, normal, weak or thready. A thready pulse with cool peripheries points to poor perfusion.
• Apical-radial deficit: where atrial fibrillation is suspected, a second nurse counts the apical rate while you count the radial; a deficit indicates ineffective ventricular contractions.
• RCSI's reading list includes palpating the pulses of the foot - relevant to neurovascular assessment, peripheral arterial disease and the safety check before applying anti-embolism stockings.
• INEWS scores a heart rate of 40 or below as 2, and separately mandates immediate medical review at that rate.

Blood Pressure

More points are lost to technique here than to interpretation.

Before measuring:

• The patient should be seated and rested for several minutes, back supported, legs uncrossed, feet flat.
• The arm is supported at heart level. An unsupported arm below heart level falsely raises the reading; above heart level falsely lowers it.
• Remove tight clothing from the upper arm rather than rolling a sleeve into a tourniquet.
• The patient should not talk during measurement.

Cuff selection - the classic error:

The bladder inside the cuff should encircle roughly 80% of the arm circumference and cover about two-thirds of the upper arm length. Position the cuff about 2-3 cm above the antecubital fossa with the artery marker over the brachial artery.

Interpretation and follow-up:

• Record which arm was used, and the patient's position.
• INEWS scores 111-249 mmHg systolic as 0, with 250 or above scoring 3 and progressively lower pressures scoring 1, 2 and 3. The chart carries a separate instruction that a systolic BP of 200 or above should be rechecked manually and reviewed by a doctor.
• Mean arterial pressure approximates to (systolic + 2 x diastolic) / 3; a MAP of at least 65 mmHg is the usual perfusion target in sepsis.
• Postural (orthostatic) hypotension: measure lying, then at 1 and 3 minutes standing. A fall of 20 mmHg systolic or 10 mmHg diastolic is significant and is a direct falls risk.
• RCSI's reading list includes 24-hour ambulatory blood pressure monitoring (ABPM), which it also names as a teaching-station example paired with hypertension. ABPM measures pressure over a full day and night, removes white-coat effect and shows whether the normal nocturnal dip is preserved.

Temperature

• Record the route used, because normal ranges differ: tympanic, oral, axillary or the contactless (temporal) thermometer RCSI photographs on its equipment page.
• INEWS scores 36.1-38.0 as 0, with 35.1-36.0 and 38.1-39.0 scoring 1, 39.1 or above scoring 2 and 35.0 or below scoring 3.
• Hypothermia matters as much as fever. Older and immunocompromised patients with sepsis are frequently normothermic or hypothermic; the absence of fever never excludes infection.

Level of Consciousness: ACVPU

Alert, new Confusion, responds to Voice, responds to Pain, Unresponsive. Anything other than Alert scores 3 on INEWS, including new confusion. Treat confusion as new unless a documented baseline says otherwise, and follow it with a formal cognitive screen such as the 4AT.

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Putting the Set Together

A complete INEWS observation set has seven scored parameters plus the concern row. A partial set produces a false score: omit the respiratory rate and you have removed the most sensitive parameter from the calculation.

At the station:

1. Wash hands, introduce yourself, confirm identity, explain and gain consent.
2. Take the observations in a fixed order so nothing is missed, counting the respiratory rate unobtrusively.
3. Record each value on the chart as you go, not from memory afterwards.
4. Total the score and say it aloud, then state the escalation tier it requires.
5. Escalate using ISBAR if triggered, document, and state when the next set is due under that tier.