Paediatric blood pressure calculator

When to measure

It is widely advised to measure children's BP every 1–2 years from age 3. Any child with an unexplained illness or failure to thrive should have their BP measured routinely.

How to measure

For older children, an automatic oscillometry machine is convenient; if the result is possibly or definitely raised, repeat it with a sphygmomanometer and either a stethoscope or Doppler probe. For preterm infants, a sphygmomanometer with a Doppler probe over the radial (or posterior tibial/dorsalis pedis) artery is best — inflate the cuff to stop the pulsatile flow, then deflate slowly to measure the SBP when flow reappears.

Cuff size

The cuff length must be at least 80% of the upper arm's circumference (cuff width 40%). Ideally encircle the arm completely, but if it doesn't quite meet, don't leave a gap over the brachial artery. Apply the cuff closely but not too tightly before inflation.

Which limb

Conventionally the right upper arm, at the same level as the heart. The leg can be used but must be at the same level as the heart — useful in a sleeping child lying flat, though it will need a bigger cuff.

Keeping the child relaxed

This is crucial. Distract or involve the child in the process rather than asking them to sit still. Take the BP at least three times and disregard the first value; if it's high, or falling serially, keep measuring for as long as you reasonably can.

Adjusting for height

Readings from particularly tall or short children need special care: normal ranges run slightly higher in taller children and slightly lower in shorter ones. The effect is small and grows gradually with age — around ±1 mmHg at birth, rising to roughly ±4.5 mmHg by age 13, for children on the 5th and 95th height centiles. When you select Short or Tall below, this calculator shifts the whole set of centiles used — both for classifying the reading and on the chart itself — rather than changing the reading or just marking it.

Defining hypertension

BP readings vary from minute to minute and are influenced by many factors including emotion and activity. A diagnosis of hypertension should not be made from a single reading (unless it is extremely high), but from at least 3 sessions.

Acting on the result

• Elevated BP (90th to <95th centile, without evidence of end-organ damage): monitor 6-monthly to annually.
• Stage 1 hypertension (95th to <95th centile+12 mmHg): complete a third measurement session within 3 months; if sustained, make a sub-speciality referral.
• Stage 2 hypertension (≥95th centile+12 mmHg): if the value is within 18 mmHg of this threshold and the child isn't symptomatic, repeat it and refer for a sub-specialist opinion within 1 week.
• Stage 2 hypertension, ≥18 mmHg above threshold, or a symptomatic child at Stage 2: make an immediate emergency referral to a paediatric nephrologist or intensivist.

You can plot a patient's SBP over several visits on one chart, and carry the readings forward to their next appointment:

1. Enter the current reading as usual, then click + Add to series (above the result). Repeat this at each visit.
2. Every reading you've added plots together on the chart as a trend line, and lists in the Serial readings panel below the chart, oldest to newest.
3. Click Save readings to file to download a small file containing everything added so far.
4. At the patient's next visit, open the calculator again and click Load readings from file to bring those readings straight back onto the chart.

Readings only exist on your screen and in that saved file — nothing is stored in the browser or sent anywhere, so keep the file the same way you'd keep any other file containing patient information.

Each reading keeps the stature it was entered with, so its category stays correct even if that changes between visits — but the coloured bands on the chart always reflect the stature currently selected above, so a point from a visit recorded under a different stature can appear to sit slightly off the band it belongs to.