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How is blood pressure measured?

Having mentioned previously what blood pressure reading corresponds to low blood pressure, it’s essential to appreciate how blood pressure is measured and how the numbers can vary in any individual. Blood pressure is a measurement of the pressure in your arteries during the active and resting phases of each heartbeat. Your arteries are the blood vessels that carry blood away from the heart and have thick, elastic walls – this places the blood inside arteries under higher pressure. Blood pressure is measured in millimetres of mercury (mmHg), a unit of pressure. The two numbers correspond to the systolic pressure and the diastolic pressure:

  • Systolic pressure is the amount of pressure produced by your heart when actively pumping blood through your arteries.
  • Diastolic pressure is the amount of pressure in your arteries when your heart is at rest between beats.

Typical normal blood pressure readings range from 90/60 mmHg to 140/90 mmHg, above which are considered high blood pressure or hypertension.

Everyone’s blood pressure varies throughout the day depending on a variety of factors, including:

  • Your body position (i.e. standing up vs sitting or lying down)
  • Time of day (blood pressure usually lowest at night, rising sharply when you wake up)
  • Physical activity
  • What you eat or drink
  • Any medications you may take
  • Your physical condition and stress levels

What are the symptoms of low blood pressure?

  • Light headedness or dizziness, especially if you stand up too quickly
  • Feeling sick (nausea)
  • Blurred vision
  • Generally feeling weak
  • Confusion
  • Fainting

What causes low blood pressure?

Although blood pressure varies between individuals, and some people may have naturally lower blood pressure, there are other reasons why someone might develop low blood pressure, ranging from dehydration to serious medical disorders. Low blood pressure can signal an underlying problem if it drops suddenly or is accompanied by the above symptoms.

Medical conditions that can cause low blood pressure include:

  • Pregnancy
  • Heart problems, such as bradycardia (slow heart rate), heart valve problems and heart attacks
  • Blood loss
  • Severe infections (septicaemia)
  • Severe allergic reactions (anaphylaxis)
  • Lack of nutrients in your diet (a lack of vitamin B12, folate and iron can reduce the number of red blood cells your body produces, causing anaemia and reducing blood pressure)

Additionally, some medications can cause low blood pressure, such as:

  • Medications that treat high blood pressure, including diuretics (e.g. furosemide and bendrofluazide), beta-blockers (e.g. bisoprolol and atenolol), ACE-inhibitors (e.g. rampril and perindopril), angiotensin-receptor blocker (e.g. losartan and cadensartan) and calcium channel blockers (e.g. amlodipine and diltiazem)
  • Alpha-blockers, such as prazosin
  • Drugs used to treat Parkinson’s disease, such as pramipexole or levodopa
  • Some antidepressants, such as doxepin and imipramine
  • Drugs used to treat erectile dysfunction, such as sildenafil or tadalafil (this is particularly true if taken alongside the heart medication, nitroglycerin)

Are all cases of low blood pressure the same?

No, low blood pressure can be classified into several different categories, depending on their causes. Some types of low blood pressure include:

  • Orthostatic or Postural Hypotension, which is a sudden drop in blood pressure when you stand up from a sitting position or after lying down. This type can occur for various reasons, including dehydration, prolonged bed rest, pregnancy, diabetes, heart problems, skin burns, high temperatures, large varicose veins and certain neurological disorders.
  • Postprandial Hypotension, which is a drop in blood pressure that occurs one to two hours after eating. This type is more likely to affect older people, those with high blood pressure, or autonomic nervous system disorders such as Parkinson’s disease.
  • Neurally-Mediated Hypotension, which is low blood pressure caused by faulty brain signals. This disorder, which causes blood pressure to drop after standing up for long periods, primarily affects young adults and children.
  • Multiple System Atrophy with Orthostatic Hypotension. Also known as Shy-Drager Syndrome, this rare disorder has many symptoms similar to Parkinson’s. It causes progressive damage to the autonomic nervous system, which controls involuntary actions such as blood pressure, heart rate, breathing and digestion.

 

Written by Dr Sundeep Kalra, Consultant Cardiologist at OneWelbeck Heart Health.