Unless specified, blood pressure in medical parlance almost always refers to systemic arterial blood pressure; central venous pressure and pulmonary artery pressure being regarded separately. Systemic arterial blood pressure drives tissue perfusion. A low blood pressure that impairs this results in the clinical state of shock. A high blood pressure is known as hypertension and is so common and commonly sought that "Blood Pressure" is a common lay synonym for "Hypertension".
Systole and Diastole
Blood pressure is usually quoted as two numbers, the systolic pressure and the diastolic pressure. A normal value is about 120/70mmHg, which is usually spoken aloud as "one hundred and twenty over seventy". Normal values vary widely in a healthy population (see below).
The systolic pressure is produced by contraction of the left ventricle of the heart. As the left ventricle pumps blood out into the aorta (the stroke volume), the aorta and large arteries (such as the renal and iliac arteries) expand elastically to accommodate the stroke volume.
At the end of systole, the pressure in the ventricle falls rapidly to almost zero to admit blood from the left atrium. There is a rapid reversal of pressures, with the pressure in the aorta exceeding the pressure in the ventricle. This would tend to cause blood to reflux back down into the ventricle if not for the presence of the aortic valve, which closes.
As the aorta and large arteries recoil elastically (and the aortic valve is closed), this has two important functions. First, it squeezes the bolus of blood distally round the body. Secondly, it perfuses the coronary arteries. (Coronary blood flow to the left ventricle only happens during diastole.)
If the patient is very vasodilated (e.g. because of sepsis syndrome) the diastolic pressure will be low, even if the systolic pressure remains high. In this situation, the systemic vascular resistance (SVR; also termed the afterload) is low.
If the patient is very vasoconstricted, the diastolic pressure will be much higher. In this situation, the SVR (afterload) is high.
Pulsatile pressure seems to be important for capillary networks. If some capillary networks are perfused at constant pressure, their perfusion goes down.
The pulse pressure is the difference between systolic and diastolic pressures.
Why we measure blood pressure
Measurement of blood pressure is of use in several key areas. Blood pressure is easy to measure, repeatedly and non-invasively.
Firstly, an adequate blood pressure is necessary to perfuse organs such as the kidneys and brain. These organs are particularly sensitive to hypoperfusion.
Secondly, blood pressure is a surrogate measure of cardiac output. Cardiac output (CO; a flow), is the product of mean arterial pressure (MAP; a pressure), and systemic vascular resistance (SVR; a resistance). Assuming SVR remains constant, the higher the MAP, the higher the CO.
Thirdly, blood pressure can be a useful marker in several diseases. For example, pre-eclampsia, phaeochromocytoma, and raised intracranial pressure all cause hypertension. Even essential hypertension is not innocuous. A prolonged elevated BP requires that the heart does more work of pumping; a long-term increase in work causes left ventricular hypertrophy, which in turn puts the patient at risk of ischaemic heart disease.
Mean arterial pressure
It is convenient, during calculations involving blood pressure, to assume that the blood pressure is constant and not pulsatile. This constant pressure, the mean arterial pressure (MAP) is usually calculated and displayed automatically.
A more convenient (but less accurate) method is to estimate the mean arterial pressure by adding one third of the pulse pressure to the diastolic pressure.
Measurement of Blood pressure (BP)
Although blood pressure is conveniently measured in the arm, the desired pressure to measure is in the aorta. It is very invasive to measure aortic pressure directly, and therefore more peripheral pressures are usually acceptable. It is assumed that the aortic pressure and the more peripheral pressures are interchangeable; for several reasons, this may not be the case.
- By estimation from palpation of the Pulses, which, although extremely unreliable, may be all that is possible in difficult situations (a palpable radial pulse usually indicates systolic BP of over 80mmHg);
- With a cuff, Sphygmomanometer and Stethoscope using the sounds described by Korotkoff.
- With a cuff and oscillotonometer (now obsolete) (see also Anaesthetics)
- Using automatic devices based on oscillation in a cuff
- 24 hour measurements : Use may increase now it is known that nocturnal BP in treated patients has prognostic implications 
- Directly by intra-arterial cannulation (see also Anaesthetics).
BP in both arms
The pressure is most commonly nearly identical in each arm, although the arterial anatomy is slightly different. It is desirable that people should have the pressure measured in each arm on some occasion, to demonstrate a lack of great variation, and the higher one would usually be used thereafter. 23% of people in one study were found to have a difference of more than 10mmHg and the suggestion made that this indicates peripheral arterial disease. The variant group were reported to have an increased or acelerated rate of cardiovascular morbidity. This idea remains incompletely evaluated.
Sources of error in measurement
The measurement of blood pressure in the arm (by any method) assumes good equilibrium between the pressures in the aorta and the brachial artery. If there is arterial disease, such as a coarctation of the aorta, or a dissecting aneurysm of the aorta, blood pressure in the arm may not accurately reflect the pressure in the aorta.
Measuring the blood pressure with a cuff (including automated devices) requires a cuff of the appropriate dimensions. A cuff which is too small for the arm will overestimate blood pressure (because extra pressure must be put into the cuff to adequately compress the artery).
Measuring blood pressure by arterial cannulation requires that there be a clear column of fluid in the line between the cannula and the transducer. If there are air bubbles or clots in the line, the contour of the waveform will be affected. Technically this is known as damping. A damped waveform will cause both the systolic and diastolic pressures to apparently read closer to the mean arterial pressure, but the MAP should still be accurately measured.
Automated blood pressure monitors which mechanically inflate and deflate the cuff are sensitive to vibrations of any kind, such as those produced by movements of the patient, or those produced in motor vehicle transport. Such interference may cause the device to read either high or low.
Automated devices are also very unreliable at low blood pressures.
Blood Pressure and the Population
Blood pressure as a statistical value within an untreated population is distributed as a normal or Gaussian distribution. High blood pressure as a defined value is arbitrary. One could define high blood pressure as the value at which lowering it has clinical benefits. Generally speaking, in a normal patient the lower the blood pressure the better. Given thresholds for treatment, and targets to achieve, a population including treated patients (say, the population of a complete General Practice) will show a notch and a shelf in the Gaussian curve, due to treatment. Actual measurements of BP in a whole population, made by actual people, will show the well-known effects of digit preference, and of reduced precision outside interesting ranges.
Stroke, Myocardial infarction and other ischaemic heart disease, and Renal failure are reduced by reductions in the population average BP. Current thought is that the risk falls according to the reduction in BP, mmHg by mmHg, and that apparent differences in end-events between different drugs and combinations are more likely to be accounted for by small differences in end-point BP in trials than subtle differences in effects of the drugs at the same BP reduction. See individual classes of drugs for more information.
Blood pressure as a variable in physiological laws
Blood pressure is a vital variable in calculating many physiological functions, many of which are only of direct interest to Intensivists in the ICU. One of the most important the Frank-Starling law of the heart.
- ↑ BHS guidance 2007
- ↑ Boggia J, Li Y, Thijs L et al. Prognostic accuracy of day versus night ambulatory blood pressure: a cohort study The Lancet 2007; 370:1219-1229
- ↑ Online: Journal of Human Hypertension 2007. (via Pulse)