Everything you need to know about blood pressure – but were afraid to ask

What is blood pressure exactly?

This is the pressure exerted by the blood flowing through your arteries. There are two levels that we look at, the systolic and diastolic — the top number is the systolic and the bottom number is the diastolic.

Let me explain the diastolic first. The metaphor I use is that you have a garden hose, the nozzle is switched off, you turn the tap on, fill the hose with water, and turn the tap off again so that there is a resting quiescent pressure inside the hose that is caused by the amount of water filling the hose. If you pinch the hose, you would feel the pressure against your fingers. This is the diastolic pressure.

Every time your heart beats, it expels approximately 70 mls of blood into your arterial tree. This sends a pressure wave of blood travelling down through all your arteries. Getting back to the metaphor, it’s like someone very quickly grabs that hose full of water again, turns on the tap on briefly, and then tightens the tap closed again. Image that pressure wave of water travelling down the hose. This gives you the top number, or systolic.

Why is it important?

There are very few diseases that give low blood pressure but certainly if you do have significantly low blood pressure, the body’s response is to make you faint so that your head is at least horizontal with the ground — the body is trying to preserve blood flow to the brain, a critically important organ. Whilst the brain is only 20% of our body weight, it uses 80% of the body’s oxygen.

Much more common is high blood pressure — typically values greater than 140/90. High blood pressure causes stress on the inner layers of the arteries which can cause a breech between cells, allowing toxic LDL cholesterol to sneak into the arterial wall and set-up the inflammatory action called plaque or fatty build-up, predisposing you to heart attack and stroke.

It is really important to note that stroke is much more driven by BP than cholesterol or other risk factors. It is therefore critically important in my view to ensure that your blood pressure is perfect, particularly as we age beyond 60 as age is the biggest risk factor for stroke and heart attacks.

Why do people have high blood pressure?

Rarely is high blood pressure a manifestation of another disease. Examples of such diseases include stenosis or narrowing of the kidney arteries, and the rare Cushing’s disease. Generally, in excess of 98% of people with high blood pressure, one doesn’t find a disease such as this. The current thinking is that there are probably microscopic changes within your kidneys that lead to hormonal dysregulation and hence hypertension.

The drivers for hypertension, in the absence of the rare diseases are:

  • high salt intake
  • obesity
  • excessive alcohol use (more than 200 mls of wine a day or 2 middies of beer or 60 mls of spirits)
  • daily use of certain medications, for example anti-inflammatories which can push up the blood pressure

What is the best way to treat high blood pressure?

The most important thing is to have other risk factors for a heart attack or stroke assessed in combination with the high blood pressure. Hypertension in isolation is not nearly as significant as hypertension in a diabetic smoker with high cholesterol. The older you are, the higher the risk.

The optimal way to treat high blood pressure is through lifestyle, by:

  • getting your diet and waist circumference right (under 94 cm in men and under 80 cm in women)
  • minimising your salt intake — an excellent site is www.saltmatters.org
  • minimising your alcohol intake if relevant
  • exercising

On occasions it’s important that as doctors we treat with medications which typically act on the hormonal influences of the kidney or directly on the arteries to relax them and dilate them — hence giving more room for the resting blood volume and lowering the blood pressure — using the hose analogy above.

In our next newsletter we will have some expert advice from Duncan Hunter on the lifestyle approaches that have the most powerful influence on reducing your blood pressure naturally — equivalent to taking a blood pressure lowering medication often.

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