The blood circulation is a closed system in which the pressure varies constantly. It rises to a peak, at the height of the contraction of each heartbeat as the heart pumps blood out. It then falls to a lower level, which it reaches just before each heartbeat (NHS, 2002) This variance of pressure is necessary for a healthy circulatory system however, “if the pressure is consistently higher than normal at rest, this is high blood pressure, also known as hypertension” (BUPA, 2002).
BUPA (2002) describes blood pressure (BP) as “a measure of the force that the blood applies to the walls of the arteries as it flows through them”. It is measured in millimetres of mercury (mmHg) and is expressed using two numbers, written as 120/80mmHg (“one hundred and twenty over eighty”). These two numbers represent the systolic and diastolic blood pressures respectively.
Systolic blood pressure:
The systolic blood pressure (SBP) is the pressure exerted on the artery walls, when the cardiac muscle is contracting (ventricular systole) and pumping blood. This is the higher of the two numbers, and is usually noted first. It is caused by the increased volume of blood flowing through the arteries with each beat, therefore increasing the pressure within the arteries. There is a linear increase in SBP with increasing levels of exertion (Franklin, 1998), approximately 8-12mmHg per MET (metabolic equivalent). It is also increased in the short and long term by a number of factors.
Diastolic blood pressure:
The diastolic blood pressure (DBP) is the pressure exerted on the artery walls, when the heart is in a relaxed state. The heart goes through this period of relaxation, or diastole, to allow the chambers of the heart to fill with blood prior to contraction. “The DBP is the running or ‘remaining’ pressure between beats” (NHS, 2002) and is always smaller than the SBP. During exertion DBP may decrease slightly, due to vasodilation, or will remain unchanged (Franklin, 1998), except in hypertensives where it may rise as a result of an impaired vasodilatory response (Gordon, 1997). The DBP is more stable and less influenced by acute factors than SBP, although over a period of time, it will rise in accordance with chronic variables. Therefore changes in this measure are usually clinically significant in diagnosing contra-indications to exercise, such as hypertension.
Optimal blood pressure:
The ACSM define optimal blood pressure, with respect to cardiovascular risk, as being below 120 mmHg for systolic and 80 mmHg for diastolic pressure (Franklin (ed.), 2000). It should be noted that unusually low readings could be of some clinical significance.
Blood pressure is an expression of the arterial blood flow and the peripheral resistance the blood encounters as it flows round the body. It can therefore be expressed in the equation (McArdle, Katch & Katch, 1996):
Blood Pressure = Cardiac Output x Total Peripheral Resistance
The volume of blood pumped out by the heart in one minute (ml/min). The greater the cardiac output, the higher the blood pressure.
Total peripheral resistance:
The resistance the blood vessels offer to blood flow; the greater the resistance, the higher the blood pressure. Peripheral resistance is increased or decreased by constriction or dilation, respectively, of the blood vessels (arterioles).
Adapted from NIH 2003
Prescription drugs* – decongestants, NSAIDs, oral contraceptives, adrenal steroids
Ilicit drugs** – cocaine, amphetamines, and others.
Adapted from the Seventh Report of the JNC, 2003
* – SBP = Systolic blood pressure, DBP = Diastolic blood pressure.
– not taking hypertensive medication and not acutely ill, based on the average of 2 or more readings, taken at each of 2 or more separate visits.
– should the SBP and DBP fall into different categories, the higher BP category should be selected to classify the individuals BP status. e.g. a recorded BP of 164/94mmHg would be classified as Hypertension Stage 2 (medical referral) and not as Stage 1 (special attention).
Method of Assessment
BP can be measured most accurately through direct measurement of intra-arterial pressure (Heyward, 2002). However, this is very invasive and requires catheterisation, so indirect measurement by auscultation (listening to the internal sound of the body) is preferred. This uses a stethoscope and sphygmomanometer, consisting of either an aneroid or a mercurial column manometer.
The contraction of the heart causes the distinctive sounds heard when listening to the heart with a stethoscope. The “lub-dub” sound is the sound of the valves in the heart closing. When the atria end their contraction and the ventricles begin to contract, the blood is forced back against the valves between the atria and the ventricles, causing the valves to close. This is the “lub” sound, and signals the beginning of ventricular contraction, known as systole. The “dub” is the sound of the valves closing between the ventricles and their arteries, and signals the beginning of ventricular relaxation, known as diastole. A physician listening carefully to the heart can detect if the valves are closing completely or not. Instead of a distinctive valve sound, the physician may hear a swishing sound if they are letting blood flow backward. Where the swishing is heard, tells the physician where the leaky valve is located.
As the pressure is released further the sound will lessen in magnitude until it disappears. The upper and lower pressures give the blood pressure, systolic over diastolic.
Position of blood pressure cuff and stethoscope head.
Guidelines for taking BP
- Ensure that your client is relaxed and has been seated in a quiet room for at least five minutes. The client’s arm should be bare and resting at a 45 degree angle, supported on a flat surface so that the middle of the arm is level with the heart, and the palm is facing up.
- Check that the bladder of the cuff encircles 80% of an adult’s arm and 100% of a child’s arm.
- Palpate the brachial artery (see RHR for anatomical position) and then wrap the deflated cuff firmly around the upper arm so that the midline of the cuff is directly over the brachial artery pulse. The edge of the cuff should be approximately 2.5cm (1inch) above the antecubital fossa (inner elbow crease).
- Ensure the cuff is snug around the arm, if it is too loose then BP will be underestimated
- Do not put the cuff over clothing and ensure that the circulation is not occluded if the sleeve is rolled up.
- Place the manometer at eye level and ensure that the cuff’s tubing is not overlapping or obstructed
- Position the earpieces of the stethoscope so that they are aligned with the auditory canals (i.e. angled anteriorly).
- Place the head (bell) of the stethoscope over the brachial pulse (about 1cm superior and medial to the antecubital fossa). Make certain that the entire head of the stethoscope is in contact with the skin.
- To avoid extraneous noise, do not place any part of the head of the stethoscope underneath the cuff, and be aware that heavy pressure may distort the reading and result in sounds being heard below the diastolic pressure.
- Close the valve, by rotating the switch clockwise and quickly and steadily inflate the cuff pressure to 20-30 mmHg above the estimated systolic value. This will collapse the brachial artery and there will be no blood flow.
- Partially open the valve, by turning the switch anti-clockwise slightly, and slowly deflate the cuff at a constant rate of 2-3 mmHg/sec.
- Note when you hear the first sharp thud caused by the sudden rush of blood as the artery opens (the arterial pressure and cuff pressure are equal). This is known as the first Korotkoff sound and corresponds to the SBP.
Continue to reduce the pressure at the same rate and note when the tapping sound (intermittent arterial blood flow) becomes muffled and when the sound disappears. The sound disappears because the cuff pressure is now equal to the diastolic pressure and normal blood flow is restored.
Potential Sources of Error in BP Assessment
- inaccurate/damaged sphygmomanometer or stethoscope
- inappropriate cuff width or length
- cuff not centred, too loose or over clothing
- arm unsupported or elbow lower than heart level
- improper rate of inflation or deflation of the cuff pressure
- improper stethoscope placement or pressure
- poor auditory acuity of technician
- expectation bias and inexperience of the technician
- slow reaction time of the technician
- error in reading the manometer
- background noise
- certain physiological abnormalities (damaged brachial artery)
- client holding treadmill handrail or cycle ergometer during exercise BP measurement