Blood pressure is the force that blood exerts against the walls of the vessels it travels through it. Arterial pressure, that is the pressure within arteries which are the blood vessels that carry oxygen and nutrient-rich blood through the body, and it is higher than venous pressure (pressure in the veins). There are several mechanisms in the body to regulate blood pressure which involves major organs and even individual tissues in the body.
It is a combination of water volume, size of the blood vessel lumen and rate and force of heart contraction. When one or more of these systems are disturbed, the blood pressure is also affected leading to a rise or fall in blood pressure. If these changes are sustained, a person is said to have high blood pressure (hypertension) or low blood pressure (hypotension). Given the prevalence and often disastrous consequences of high blood pressure, most of us consider low blood pressure to be a benign condition. However, low blood pressure can have equally devastating effects on the body and even lead to death in severe cases.
What is hypotension?
Hypotension is an abnormally low blood pressure that hampers the oxygen supply to various parts of the body leading to a host of signs and symptoms. Blood pressure lowers when one is extremely relaxed and inactive like during sleep. Similarly, people who are physically fit, like athletes, will have a lower blood pressure and even pulse rate than the average level. In these cases, the blood pressure is considered normal even though it is slightly low.
It is only when the blood pressure is low enough to affect the oxygen availability to various tissues and organs in the body, in the absence of other diseases that would affect oxygenation, can the blood pressure be labeled as low and a person is said to have hypotension. At this point, there is signs and symptoms indicative of hypotension, the most common of which are fatigue, dizziness, fainting and signs of shock.
Blood pressure is constantly monitored by special pressure receptors (baroreceptors) primarily in the neck and heart. The body then takes the necessary action to increase or decrease the blood pressure and maintain it within a normal range. This regulatory process is controlled by the autonomic nervous system and therefore not under voluntary control. Broadly blood pressure is dependent on three factors :
- Heart rate and the force of contraction of the heart muscle
- Diameter of the blood vessel lumen
- Blood volume
A rise in blood pressure is seen when the heart is beating faster (higher rate), more forcefully, when the blood vessels are narrower, and there is a greater volume of blood. Conversely, a drop in heart rate, weaker heart contractions, dilated blood vessels and lower blood volume will all causes a decrease in blood pressure. Hypotension is therefore attributed to one or more of these processes which may be caused by underlying physiological or pathological factors.
Low Blood Pressure Readings
Hypotension lacks a clear definition in terms of blood pressure levels. Many people may have a low blood pressure that can be considered normal since it allows for adequate oxygen supply to various tissues and does not cause any signs or symptoms. While the average normal blood pressure is said to be 120/80 mmHg (systolic = 120 mmHg; diastolic = 80mmHg), this does not mean that every person with a levels lower than this have hypotension. With hypotension, the signs and symptoms of decreased oxygen supply needs to be present and this is usually seen at levels below 90/60 mmHg.
In fact normal blood pressure cannot be marked definitely at 120/80 mmHg since it can extend upwards towards 130/85 and downwards to 100/60mmHg without causing any signs of symptoms of hypertension or hypotension respectively. This range can therefore be said to be within the norm. Lower blood pressure readings have been noted in people in deep meditation without any negative physiological effect on the body.
Types of Hypotension
There are three main types of hypotension :
- Orthostatic hypotension, which is also known as postural hypotension.
- Neurally-mediated hypotension
- Severe hypotension with shock
There are several other types and sub-types of hypotension which may either be clinically insignificant or uncommon and are therefore not a major consideration in a general approach to hypotension. The causes of these different types of hypotension may vary and so does the pathophysiology to some extent.
Orthostatic hypotension is a drop in blood pressure with a change in position. Hence the term postural hypotension. It usually arises when a person suddenly stands up from a lying or sitting position. Normally the heart compensates for this change in position by beating faster and the blood vessels of the leg constrict to return more blood to the heart. With orthostatic hypotension, the compensatory mechanisms to regulate the blood pressure within a normal range is impaired. Postprandial hypotension is a subtype of orthostatic hypotension.
Neurally Mediated Hypotension
Neurally mediated hypotension (NMH) tends to occur with standing for long periods. Blood pools in the legs but the compensatory mechanisms fail to act to regulate the pressure. In fact, it sends the wrong signals back to the brain thereby leading to further incorrect measures by the autonomic nervous system. The heart rate drops and vasoconstriction of the leg vessels do not occur thereby dropping the blood pressure further.
Severe Hypotension with Shock
Severe hypotension may be seen with shock – a state of drastically reduced blood flow to the organs and tissues. There are several mechanisms which may lead to the different types of shock.
- Hypovolemic shock is a drastic reduction of blood volume, often due to causes such as excessive bleeding.
- Cardiogenic shock arises with failure of the heart to contract with the rate and force to maintain a normal blood pressure. It is seen with various types of heart disease.
- Distributive shock is caused by decreased resistance of the blood vessels which dilate (open widely) more so than is normal. This is seen with severe allergic reactions (anaphylactic shock), infections (septic shock) or damage and injury to the sympathetic nervous system (neurogenic shock).
Causes of Hypotension
The causes of low blood pressure may vary depending on the type of hypotension. This can be either pathological, in that it is associated with disease, or physiological, in that it is related to a normal although disruptive process. At times more than one mechanism and cause may account for hypotension. The list of causes of hypotension include :
- Anemia, which may be associated with chronic mild blood loss, genetic disorders or nutritional deficiencies.
- Dehydration, which may occur in hot weather, with severe diarrhea and vomiting, low fluid intake, strict dieting and fasting, fever and increased urination.
- Blood loss, particularly moderate to severe hemorrhage in acute settings, or even mild loss associated with anemia.
- Heart disease including arrhythmias, cardiomyopathy, myocarditis, pericarditis, congestive heart failure, heart valve disease, and myocardial infarction (heart attack).
- Endocrine diseases including hypothyroidism, adrenal insufficiency, hypoglycemia and diabetes mellitus (sugar diabetes).
- Neurological disorders like Parkinson’s disease and multiple system atrophy (Shy-Drager syndrome).
- Severe inflammation in conditions like pancreatitis.
- Medication used for the treatment and management of angina, anxiety, depression, erectile dysfunction (ED), hypertension (high blood pressure), Parkinson’s disease and pain. Hypotension may also be a result of drug interactions when more than one drug is used simultaneously.
- Substances like alcohol and certain narcotics.
- Vasovagal reaction which may be associated with extreme emotions like fear or pain.
Signs and Symptoms of Hypotension
The clinical features of hypotension depend on the severity of the condition. It may also be accompanied by concomitant signs and symptoms associated with the underlying disease.
- Dizziness or lightheadedness
- Rapid shallow breathing
- Cold clammy skin
- Blurred vision