Blood pressure is the force exerted by circulating blood pushing against the walls of the arteries. Two different pressures are of clinical importance: systolic blood pressure as a measure of blood pressure while the heart is beating, and diastolic blood pressure as a measure of relaxed heart (i.e. between heartbeats). Contrary to hypertension which is a term used for high blood pressure, hypotension represents abnormally low blood pressure.
In a healthy person, low blood pressure without any signs or symptoms is not a problem and no treatment is necessary. On the other hand, if it is associated with some underlying disease process (e.g. severe blood loss), there is a need to find and treat the condition causing hypotension. Blood pressure measurement for the diagnosis of hypotension should follow recommended procedures; otherwise incorrect diagnoses can ensue and result in incorrect treatment and cardiovascular events.
Blood pressure measurement
Blood pressure is measured by century-old conventional sphygmomanometry and by auscultation of the Korotkoff sounds or pulse wave. Most devices for measuring use an inflatable cuff, and the value is expressed in units called “millimeters of mercury” (mmHg). Still, the procedure itself is prone to error which may arise in the observer, the subject, the device or in the overall application of the technique.
Systematic error in the auscultatory blood pressure measurement can be a result of inadequate mental concentration, observer prejudice or bias, poor hearing or confusion of visual and auditory cues. Another major drawback of sphygmomanometric measurement stems from the fact that blood pressure is highly variable and often exhibits large diurnal fluctuations.
The automated devices on the market have been designed for self-measurement of blood pressure and they all employ the oscillometric technique. Three categories of different devices are available – those that measure blood pressure on the finger, the wrist and the upper arm – with the latter being most accurate. Self-measurement is currently performed mostly by patients on their own initiative, with the use of devices bought on the free market.
Newer techniques, such as ambulatory blood pressure monitoring, are slowly being embraced in clinical medicine as a way to surpass some of the limitations of conventional sphygmomanometry. Ambulatory blood pressure monitoring enables recording of the blood pressure throughout the day when patients are engaged in their normal activities; therefore it can provide a sound estimate of their blood pressure within 24 hour-period.
Definition of hypotension
The mechanisms involved in regulation of blood pressure are divided into two broad categories: short-term and long-term regulation. Short-term regulation is dominantly neural and involves baroreceptor reflex, chemoreceptor reflex, stress relaxation and capillary fluid shift. Long-term regulation is hormonal and balances the effects of sympathetic and parasympathetic nervous systems.
It is difficult to define hypotension in absolute terms as there is considerable variability in systemic blood pressure. For someone with a usual blood pressure of 140/85 mmHg, a value of 90/60 mmHg can be regarded as hypotensive. Still, when systolic pressure is less than 80 mmHg and the diastolic pressure less than 50 mmHg, a diagnosis of hypotension can be made.
Many people have blood pressure recordings that decline below the normal range over course the day, most often during sleep. In the absence of cardiovascular disease, people with lower blood pressure values are thought to be protected by this condition.
When hypotension is associated with significant symptoms, reduced perfusion to an organ or some underlying disease process, hypotension can be considered clinically important. The condition may be temporary or chronic, but it can also evolve into circulatory shock. Several cardiac conditions are associated with hypotension.
- Humes HD. Kelley's Essentials of Internal Medicine. Lippincott Williams & Wilkins, 2001; pp. 64-71.
- Pal GK, Pal P. Textbook of Practical Physiology – 2nd Edition. Orient Blackswan, 2006; pp. 193-203.