Aim
Blood pressure is the result of cardiac left ventricular pump function on the one hand and peripheral resistance in the arterial system on the other. During blood pressure measurement the systolic blood pressure (maximum arterial blood pressure during ventricular systole) and diastolic blood pressure (minimum arterial blood pressure during ventricular diastole) are determined.
Normal/target values for blood pressure depend amongst other things on the patient’s age and pathology. An elevated systolic and/or diastolic blood pressure is referred to as hypertension. Systolic (and possibly also diastolic) blood pressure which is too low is referred to as hypotension.
Principle
A sphygmomanometer and a stethoscope are needed to measure blood pressure. Many different types of sphygmomanometers and stethoscopes are available. Further information about the various types can be found in textbooks and product information. During the measurement of blood pressure an inflatable cuff is applied to the upper arm. The cuff is inflated enough to ensure that the brachial artery is completely occluded. This is checked by feeling the pulsations in the radial artery whilst inflating the cuff. Then a stethoscope is placed on the brachial artery at the height of the cubital fossa. At this point no sounds should be heard during auscultation. After that the cuff pressure is gradually reduced via a valve until the pressure in the artery is just above the pressure in the cuff. At that moment a small amount of blood can just pass through the forearm again during systole. This gives rise to pulsating sounds. Then allow the cuff to gradually empty further. Sounds and murmurs (the so-called Korotkoff sounds (see below)) are heard until the blood flow once again proceeds unhindered (and normally nothing more is heard hereafter!).
The Korotkoff sounds
Whilst the cuff is deflating, sounds and murmurs are heard that are named after their discoverer Korotkoff. These sounds and murmurs are classified in different phases.
They are used to read systolic and diastolic blood pressure.
Phase 1
Pulsating snapping sounds that gradually become louder
These arise as a result of the vessel walls knocking against each other during diastole. Duration about 10 -15 mmHg.
The start of the pulsating sounds in phase 1 is equivalent to the maximum pressure in the artery (generated during the ventricular systole) and is therefore read as the systolic blood pressure.
Phase 2
Pulsating sounds becoming softer and at the same time the presence of a murmur.
This murmur develops as a consequence of turbulence due to the increased blood flow. Duration about 14 – 20 mmHg.
Phase 3
Thumping pulsating sounds, continuing into Phase 4
The murmur has disappeared because the turbulence has disappeared. Duration 10 – 15 mmHg.
Phase 4
Muted, blowing pulsating sounds.
The pressure in the brachial artery has become so low that the walls of the artery no longer touch each other during diastole.
The pulsating sounds are now caused by collisions of the blood flow (that has increased because the constriction of the artery is still present) with the vessel wall. Duration 10 mmHg.
Phase 5
Silence
The sounds disappear as soon as the artery is compressed less than the minimum (diastolic) pressure in the blood vessel. Under normal conditions the blood flow and vessel wall do not collide so strongly that audible sounds arise. The moment at which the sounds disappear (the start of phase 5) is equivalent to the minimum pressure in the blood vessel (during ventricular diastole) and is read as diastolic blood pressure.
In some situations (in the event of increased cardiac output) blowing sounds remain audible at a pressure below the minimum pressure in the blood vessel and therefore phase 4 persists. For example, this is seen in children, pregnant women, and patients with anaemia or hyperthyroidism. In such cases the start of the 4th Korotkoff phase is used as the diastolic blood pressure.