Blood pressure in the heat: why readings fluctuate in summer

It is 31 July, it is 34 degrees outside, and your blood pressure monitor shows 118 over 72. In February it read 138 over 88. Nothing is broken, and you have done nothing wrong: blood pressure is a value that responds to temperature. In most people it falls measurably in summer – and that is exactly what can become a problem for some of them.

Why blood pressure usually falls in the heat

The body has only a few ways of getting rid of heat. The most important of these is the skin. When it gets hot, the circulation widens the small vessels directly beneath the skin's surface so that more warm blood flows there and the heat can be released outwards.

For blood pressure, this means: the vascular system becomes wider while the blood volume stays the same – so the pressure falls. It is the same effect as in a pipe system in which the pipes are enlarged without adding more water.

Then there is the second factor: sweating. Over a hot day, an adult can easily lose one to two litres of fluid, and considerably more during physical work. If this is not replaced, the blood volume falls – and the pressure drops further.

Why it still rises in some people

The search query “does blood pressure rise or fall in the heat" is so common because both observations can be true.

When fluid loss becomes large and is not compensated, the body counteracts: the nervous system narrows the vessels and the pulse rises to keep the circulation stable. In some people the balance then tips the other way – blood pressure does not go down but up, often together with a markedly raised pulse.

Poor sleep on hot nights, physical exertion in the midday heat and switching between air-conditioned rooms and the street also shift the readings. If you want to know why two measurements taken shortly after each other are never identical anyway, you will find the explanation in our article Why your blood pressure is different every time you measure it.

The critical point: blood pressure medication in summer

For most healthy people, a lower blood pressure in summer is harmless. Things look different if you take medication that lowers blood pressure.

These medicines were adjusted for a body that was not yet working at 34 degrees. The heat lowers the pressure further, and the medication lowers it too – the two effects add up. Diuretics also increase the fluid loss that sweating already causes.

The result can be a blood pressure that is too low: dizziness when standing up, light-headedness and, in unfavourable cases, a fall.

And here is the most important sentence in this article: do not change your medication yourself. Not the dose, not the time you take it, and do not stop anything, not even “just for the hot days". Whether and how an adjustment makes sense is decided solely by your doctor – on the basis of readings that you bring along.

Warning signs you should not ignore

Please seek medical advice if any of the following signs occur in summer:

• Dizziness or blacking out when standing up that is new or becoming more frequent
• Light-headedness, confusion or unusual drowsiness
• Brief fainting – even if it was “only once"
• A racing or skipping heartbeat that is unfamiliar to you
• Systolic values that are persistently well below your usual range and accompanied by symptoms
• Very little or very dark urine over the day

Unconsciousness, confusion with hot, dry skin or a seizure is an emergency – call the emergency services on 112.

Measuring correctly on hot days

In summer in particular, measuring technique determines whether your numbers are usable:

• Do not measure straight out of the heat. If you come in from outside, first sit in a cooler room for ten minutes.
• Five minutes of rest before every measurement – without exception, otherwise you are measuring the last flight of stairs and not your blood pressure.
• Two measurements morning and evening, each one to two minutes apart, and write both down.
• Note down your pulse. In summer it is often the more meaningful number: a pressure that falls while the pulse rises is a different picture from a pressure that is simply calmly lower.
• Note the temperature. A short note such as “32 °C" next to the value makes the diary much more valuable for medical assessment.

We have compiled the most common measuring mistakes and how to avoid them in Measuring blood pressure correctly: 7 mistakes that distort your readings.

Drinking: how much is right – and who this does not apply to

Drinking enough is the most obvious measure in hot weather, and for most people it is the right one: spread throughout the day, before you get thirsty, preferably water or unsweetened tea.

There is, however, an important restriction. People with heart failure or kidney disease are often given a limited fluid intake by their doctor. For these people, “drink plenty" is not good advice but a risk. If a fluid restriction applies to you, stick to it and explicitly discuss how to handle the summer with your doctor.

Alcohol additionally widens the blood vessels and has a dehydrating effect – so on hot days it amplifies both effects.

What you can do concretely this week

1. Measure morning and evening for seven days and write down every value – including the inconvenient ones.
2. Add your pulse and the outside temperature.
3. Mark days with dizziness, light-headedness or a racing heart.
4. Take this sheet with you to your next appointment. Seven days of documented readings are worth considerably more for an assessment than a single measurement at the surgery.
5. Until then, do not change anything about your medication.

Heat is not a state of emergency for your blood pressure but a stress test. If you document carefully during these weeks, by the end of the summer you will have a much more precise picture of your circulation than before.

If you want to capture what happens at the moment you feel dizzy on a hot day: the BPER blood pressure event recorder records pressure and pulse over 60 seconds – beat by beat.