25 March, 2024

Blood Pressure Basics

Introduction:
Blood pressure (BP) is the ‘pressure’ that ‘blood’ exerts on the vessel (tubes that carry blood) wall.

There are 2 basic concepts to understand.

First, blood flow through the vessels is dynamic. It increases when the heart contracts, and decreases when the heart relaxes. Due to this, blood pressure also increases during heart contraction and vice versa. The peak is labelled as ‘systolic’ pressure (sBP) and the nadir is labelled as ‘diastolic’ (dBP).

Second, blood vessels are made up of a special type of muscle that enables them to expand and relax to accommodate changes in blood flow. This property is called vessel compliance, and it helps absorb some of the impact of the dynamic blood flow. If vessel compliance decreases (as with age), a similar blood flow change produces a higher blood pressure fluctuation. This explains why elderly people typically have high sBP and low dBP.

The basic physiology of blood pressure can be understood in this video.

Impact of blood pressure:
Blood needs to flow with a certain pressure to overcome the resistance of a body organ. A drop in pressure leads to decreased blood flow through an organ (e.g. Kidneys). Human organs underperform when the blood supply is reduced. To a certain extent, such organ dysfunction is reversible if optimal blood flow is promptly restored. But, if blood flow compromise is severe or prolonged, permanent damage to the organ may ensue. On the other hand, high blood pressure can cause damage in 2 ways. Immediate surge in BP leads to physical damage that percolates to the cellular level. Such damage is typically severe and may have a long-lasting effect. This is often a medical emergency and needs to be treated urgently. Long-term uncontrolled BP, even with a modest increase, leads to structural changes in organs (adapting to high pressures). These changes affect almost every organ, and the damage is permanent. Because the human body has significant reserves, organs with moderate damage may still function reasonably enough to not cause obvious symptoms. Organs that are commonly affected due to uncontrolled BP are the heart, kidneys, brain, and eyes.

Normal sBP is less than 120, and dBP is less than 80. For organs to get optimal blood, sBP of more than 90 is typically sufficient (there are few exceptions). People with sBP between 90 and 120 typically feel fine, which is rarely of concern. sBP lower than 90 may create signs/symptoms of organ hypo-perfusion. BP higher than 120/80 should be addressed to prevent long-term complications. sBP more than 180 can cause acute illness and needs to be managed urgently.

Hypertension occurs when BP is more than 120/80. The American Heart Association proposes a staging system (table below).

Why do people develop Hypertension?
Here are a few of the hypotheses that are thought to cause hypertension:

1. Homeostatic dysregulation
Our body’s parameters like BP, pulse rate, temperature, breathing rate, etc., are tightly regulated by neural and hormonal mechanisms. They work similarly to an air-conditioner thermostat. Once you set a temperature on the thermostat, the air-conditioner tries to bring the ambient temperature to that level. Similarly, in the human body, regulatory mechanisms work towards achieving what it considers normal. Homeostatic dysregulation refers to a state where the human body is unable to revert to normalcy, for some reason or another. While ageing plays an essential role, ageing can not explain all the cases of Hypertension, particularly early onset.

2. Lifestyle choices
Unhealthy lifestyle choices, ranging from consuming food items with high salt content or high caloric value, lack of exercise, high personal or professional stress, smoking, and alcohol intake, all can give rise to high blood pressure, partly contributing to homeostatic dysregulation.

3. Secondary causes
Certain diseases or organ dysfunction can also increase blood pressure. Secondary causes are common in young individuals and in those with late-onset hypertension (diagnosed for the first time at age >60 years). Problems related to kidneys, thyroid gland, adrenal gland etc., medications like steroids and painkillers can contribute to high blood pressure. Night-time snoring is associated with a clinical condition called “Obstructive Sleep Apnea” (OSA). People with OSA typically have difficulty controlling their BP as well. Entities that involve blood flow, like blockages of the kidney arteries, Takayasu arteritis, and Coarctation of the Aorta, are other causes that are sometimes identified as a cause of high BP.

4. Pregnancy
BP is known to increase during pregnancy with its unique features and treatment strategies.

There are multiple ways to reduce blood pressure without medications (table below). Stress management plays an important role as well. Yoga, meditation, personal or community recreational activities, etc., induce positive biochemical changes in the human body, which help reduce blood pressure.

Medications are brought in to control hypertension when:
1. Hypertension is confirmed by repeated BP measurements, or if a single reading itself is very high, and
2. Lifestyle modifications have been tried but optimal BP control could not be achieved, or
3. If BP is so high lifestyle modification alone is unlikely to be effective.

The choice of medications is highly individualized, and patients should refer their treating physicians.

Occasionally, one should look for other causes of high blood pressure. Certain organ dysfunctions can lead to hypertension (a condition called ‘secondary hypertension’), for example,

Hypertension, without any identifiable cause (called Essential Hypertension), typically stays lifelong which means BP reduction measures in some form need to be taken for the rest of the life. This can be in the form of lifestyle modifications alone or a combination of lifestyle with some medications. With current pharmaceutical advancements, medicines these days are very well tolerated with acceptable side effects. Also, many hypertension medications are shared with that of heart or diabetes management. In essence, few hypertension medications have positive effects on the heart as well as kidneys.

I am sure I didn’t answer all your questions on this topic. Feel free to message me with your queries, and I will be happy to answer them.

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