Can Dehydration Cause High Blood Pressure?

Dehydration, in its simplest terms is a condition where the fluid intake is far less than the fluid loss.¹ Medically, it is an imbalance between water intake and water loss from the body, which usually results in high morbidity or mortality, especially among elderly persons. A loss in body weight of up to 3% that is associated with water loss is considered to be dehydration.

Water as a universal solvent, plays important roles in the body system of an organism, ranging from the body temperature regulation, protection of organs, tissues and cells, dissolution of minerals and other nutrients for easy absorption and so on. Water makes up 60% of the human body weight and hence is a major component of the body fluids like the blood.² This and many more make dehydration a serious medical condition that can result in several diseases like high blood pressure. 

Signs and symptoms of dehydration

The clinical signs and symptoms of dehydration are dry mouth, inelastic skin, sunken eyes, weakness and fatigue. In extreme cases, such symptoms as lethargy, headache, decreased sweating and urinating, fever and unconsciousness may be experienced. 

Causes

Looking out at the causes of dehydration is the first step in preventing it. Dehydration can result from the following;

Diarrhoea: According to World Health Organization, diarrhoea is defined as the passage of watery stool more than three times a day. During the passage of loose stool, a large amount of water and electrolyte is lost as well resulting in dehydration.

Excessive sweating: Excessive sweating is another cause of dehydration. This could be a result of increased physical activities, disease conditions or hormonal imbalance. Infections, malignancies and neuronal disorders are some of the disease conditions that can result in hyperhidrosis- excessive sweating.

Weather condition: The body loses more water through sweat when the temperature is high.

What is high blood pressure?

Dehydration and high blood pressure

High blood pressure is a chronic disorder that results in a persistent rise in the pressure with which the heart pumps blood to the other parts of the body. Blood is circulated to the other parts of the body through the blood vessels. The pressure asserted on the walls of these vessels results in blood pressure.

Two values are important in blood pressure measurement. Systolic blood pressure is the pressure on the walls of the arteries when it beats while diastolic blood pressure is the pressure when it rests between the beats. The numbers assigned to these parameters are significant in measuring blood pressure.

A blood pressure reading of 120/80mm Hg means that the systolic pressure is 120mm Hg while the diastolic is 80mm Hg.

According to the American Heart Association, Hypertension is diagnosed when blood pressure is consistently≥130 and/or ≥80 mm Hg. This brings us to the different degrees of blood pressure depending on the value. A blood pressure of 120/80mm Hg is taken to be normal. When the blood pressure goes up to 120-139/80-89mm Hg, it is called pre-hypertension. Persistent elevation of blood pressure to this value indicates that the person is at risk of developing high blood pressure. The persistent rise in systolic blood pressure from 140mm Hg and above 90mm Hg and above for diastolic reading means the person is suffering from high blood pressure- hypertension, Dehydration and high blood pressure

How does dehydration cause high blood pressure?

Blood pressure is expressed mathematically as; Cardiac Output (CO) x Total Peripheral Resistance (TPR)

Cardiac output is the total amount of blood pumped by the heart through the left ventricle to the other parts of the body. The force exerted on the walls of the arteries(blood vessels) as the blood is pumped out is the measure of the blood pressure.

Cardiac Output (CO) is determined by two parameters; heart rate (HR) and Stroke Volume (SV).

Heart rate is the number of times the heart beats every minute while at rest. Stroke Volume is the volume of blood pumped by the heart when it beats once. Cardiac output (CO) is expressed as;

CO = HR X SV

When one is dehydrated, the effect of cardiac output on high blood pressure will not be significant since the blood pressure is expressed as; BP = CO X TPR

High blood pressure can occur here as a result of the resistance to the flow of blood through the blood vessels. This is a result of the high viscosity of the blood due to insufficient water, hence leading to an increase in TPR, which is the resistance exerted on the blood vessels as the blood is pumped.

Therefore, during dehydration, the volume of water in the blood is reduced. The blood becomes so thick that it would have to be forced through the walls of the artery. When this happens, the total peripheral resistance increases, increasing blood pressure Dehydration and high blood pressure

Dehydration and low blood pressure

When one is dehydrated, the volume of blood in circulation drops. This leads to a decrease in blood pressure, hence resulting in hypotension - low blood pressure.

Dehydration also causes a drop in blood pressure. This is a result of the reduction in blood volume. Since water is the major component of the blood, anything that affects the water volume of the body will also affect the blood volume. Going by the two parameters used in measuring blood pressure which is Cardiac Output (CO) and Total Peripheral Resistance (TPR);

As the Stroke Volume (SV) reduces as a result of dehydration which results in low blood volume, the blood pressure decreases as well since CO is a direct multiplier of TPR to give the blood pressure.

How much water should I drink each day?

It is clinically recommended that an average adult weighing about 65-70kg should drink 2.5-3 Litres of water per day.

How to stay hydrated?

Staying hydrated would not only prevent you from being at risk of developing blood pressure issues but also enhance your general health. Drinking enough water will reduce the risk of coming down with constipation, kidney stone, and organ and tissue damage. Therefore, you need to stay hydrated at all times by drinking water and other drinks that have high water content as well as eating foods, fruits and vegetables that are rich in water.

When one is dehydrated as a result of acute gastroenteritis, Oral Rehydration Therapy (ORT) is the safest and easiest way to replace lost fluid and electrolyte that results from mild-to-moderate dehydration. In severe cases of dehydration, intravenous fluid replacement with fluid containing 0.9% saline is preferred for rehydration. WHO recommends that the IV fluid be rapidly completed over 3-4 hours.

Other causes of high blood pressure (A brief note)

The risk factors of high blood pressure, according to WHO, are classified into modifiable and non-modifiable. Modifiable risk factors are those that can be changed by the patient and include; unhealthy lifestyles like smoking, drinking alcohol, lack of exercise and sedentary lifestyle, consumption of foods rich in fatty acids, obesity and being overweight. 

The non-modifiable risk factors include hereditary (family history of high blood pressure), age, and other disease conditions like diabetes and kidney failure.

Treatment

The first thing to do to improve the condition is lifestyle modifications: eat healthy foods, quit smoking and alcohol, avoid fatty foods and exercise regularly.

Management of high blood pressure is with such drugs as; diuretics, furosemide, hydrochlorothiazide, spironolactone etc, ACEs like lisinopril, and ARBs like losartan. Other drugs like amlodipine are used singly or in combination with other drugs mentioned to achieve a reduction in blood pressure.

When to seek medical advice

Dehydration can be mild, moderate or severe.

It is good to seek medical advice when you experience any of these persistently.

Summary

High blood pressure may not be an obvious sign when it occurs. Severe dehydration can result in serious medical conditions of which the risk of hypertension is high. Therefore, it is advisable to stay hydrated every time and always seek medical advice when you are at risk of moderate to severe dehydration.