5.2. MEASUREMENT OF BLOOD PRESSURE

Transcription

1 5.2. MEASUREMENT OF BLOOD PRESSURE Purpose of experiment To record the pressure changes in the vascular system. To set the acoustic signal voltage and time parameters. To calculate the systolic and diastolic blood pressure. Theoretical topics Liquids. Their physical properties. Fluid flow in narrow tubes. Bernoulli equation. Puazeil and Hagen law. Fluid viscosity. The physical model of the blood circulatory system. Blood pressure measurement. Equipment and materials Cuff, phonendoscope, manometer, hand-held pump, booster, pressure-voltage transducer, a digital oscilloscope, a computer. Methodology Blood flow in the blood vessels and its distribution depends on cardiac performance and the overall diameter of the vascular system (the walls of the blood vessels have elastic muscular tissues on whose retraction the inside diameter of the vessel depends). Blood flow is also influenced by the volume and viscosity of the circulating blood. All of these factors are determined by the central nervous system. You can see in Figure the changes in pressure and blood flow velocity in the main types of blood vessels. v, m/s p, mmhg The aorta v Arterioles p Capilla- Venules res Veins pav. The change of pressure p and the blood flow velocity v in main blood vessels Under normal conditions, the vascular system is closed and has no contact with the atmosphere. Blood vessels are oriented in different directions, but most of the arterial and venous blood vessels that carry blood in opposite directions, are located in parallel with each other. Under normal conditions, the blood flow is laminar. In the presence of abnormal conditions, such as a sharp decrease in vascular lumen, partially open, or, conversely, incompletely closed, heart valves or aorta (the resulting sounds are called heart murmurs), the blood flow becomes turbulent. Arterial blood pressure (BP) is blood pressure which presses against the inner wall of the artery. Blood from the heart has to be at a certain pressure to reach the internal organs. The force of the blood flow at the beginning of FBML MEASUREMENT OF BLOOD PRESSURE 1

2 each heart rate increases and then decreases. Therefore, blood pressure is expressed as two values: First, the higher value indicates the pressure created by the contraction of the heart and is called the systolic pressure; Second, the smaller value indicates the pressure in the blood vessels between heartbeats and is called the diastolic pressure. The blood pressure in the vessels can be measured in several ways: directly (invasive) or indirectly (noninvasive). Direct (invasive) measurement is done by catheter or cannula, which is a rubber tube connected to a pressure gauge that is inserted into the vessel lumen. Indirect (noninvasive) measurement is done with sphygmomanometers (sphygmos + manometer], (a device for measuring arterial blood pressure), which may be mechanical, electronic or use mercury for measurement. Two methods are normally used for measuring arterial blood pressure: traditional and oscillometric. Traditional method: In this approach, air is pumped into a cuff and completely stops the blood flow in the artery. The air is then slowly released from the cuff. When the cuff pressure coincides with the pressure in the artery, blood flows through the compression zone causing sounds audible with a phonendoscope. These sounds mean that the pressure in the cuff (the results can be read on a gauge) is equal to the pressure in the artery and the systolic pressure is measured at that time. The diastolic pressure is measured when the sound disappears. This method has the advantage that it is recognized as an official standard non-invasive means of determining arterial blood pressure values. The disadvantages of this approach, however, are that it requires special skills for using the phonendoscope unfamiliarity with the instrument can yield incorrect results, and in addition measurement results can be influenced by poor hearing or vision, distraction, extraneous sounds, and the air outlet velocity. Oscillometric method The oscillometric method is based on recording with a digital device (oscilloscope) and does not depend on personal interpretation. This method has several advantages: measurements are independent of the individual characteristics (good hearing or vision, etc.) of the person measuring, this method is not affected by environmental noise, it is a simple technology you can easily measure the patient s blood pressure yourself. The disadvantage is that during measurement the patient cannot move, or speak. The following rules must be complied with to obtain accurate blood pressure readings: Not less than 10 minutes before measuring, the patient has to sit or lie down, and must not eat, smoke or become nervous. BP is measured with the patient lying down or sitting. The cuff is placed on the arm in advance (1-2 min. without any inflation, - vascular and neural adaptation). Measurements are carried out with the patient sitting, his arm at an angle of 45 o held against the chest. The cuff should be at the level of the heart. Ensure that the cuff is placed onto the upper arm tightly, while completely deflated. The cuff should take up 40% of the upper arm volume. Systolic blood pressure is measured from the first clear, strong tone, diastolic in the slowdown phase, with the complete disappearance of tones. When measuring blood pressure, attention should be paid to the following: Arterial blood pressure in different hands could be different, so blood pressure should be measured for the hand where experience has shown that the results are worse. FBML MEASUREMENT OF BLOOD PRESSURE 2

3 When you need a series of 2-3 measurements, take the measurements at least 1 minute apart, and then calculate the final result as the mean value. When taking a break, do not forget to loosen the cuff. Procedures 1 Measuring blood pressure in the traditional way (manometer) Fasten the cuff (N) on the upper arm and place the appliances so that they are easy to observe; the phonendoscope (F) is added a little above the elbow bend to detect the arterial pulse (Figure 5.2.2) Using the hand pump (RP), the cuff is filled with air (pressure should be 0-20 mmhg higher than that at which the pulse is felt in the artery) The cuff pressure is reduced with the air-vent (O). When the pressure decreases, tones are clearly audible in the phonendoscope headphones. The first appearance of tones corresponds to the maximum or systolic pressure P sist. This is registered on the manometer The sudden disappearance of tones corresponds to the minimum or diastolic pressure P diast. This value is also registered on the manometer. Procedures 2 Blood pressure measured by the oscillometric method. Blood pressure measurement system flowchart is shown in Figure With the hand-pump (RP) inflate the cuff (M) to a pressure P higher than the systolic P sist. Pressure is then released from the cuff with the air-vent. When P P sist,, the blood begins to flow in the patient's blood vessels causing changes of pressure in the cuff P. The pressure variation repeats with the frequency of the heartbeats. At the same time, the blood flow causes acoustic vibrations in the phonendoscope (F) membrane. The change in pressure P in the cuff (M) is transferred into electrical vibrations in the converter P/U. These vibrations are registered by the oscilloscope and can be seen in the F RP M O N pav. Measurement of the blood pressure or a b pav. Scheme of blood pressure measurement system: M - cuff, F - a phonendoscope, RP hand pump, P/U - converter P/U, OS Digital oscilloscope, K computer. a) Gauge as the pressure-voltage converter coupled to an oscilloscope, and b) the latter device is directly connected to OS with the computer FBML MEASUREMENT OF BLOOD PRESSURE 3

4 digital oscilloscope OS (K or computer) monitor. You can read the DC voltage from the oscilloscope and calculate the value of the signal voltage amplitude and then P sist. and P diast Connect the gauge to the oscilloscope with computer (Figure b.) Switch on the computer and run the 2-channel oscilloscope s DSO-2150 USB program. (Figure 5.2.4) After program start-up it is necessary to establish the appropriate operating parameters. Put the oscilloscope measurements 1st-channel baseline at the center, the voltage deflection coefficient k 1 value should be 10 mv / cm. The 2-nd channel baseline should be four divisions below the center (near the bottom of screen), the voltage deflection coefficient k 2 value should be 200 mv/cm. The dissemination indicator s k3 value is 2 s/cm or 4 s / cm (Figure 5.2.5). k 1 k 2 k pav. Computer screen image after startup pav. Computer screen image with operating parameters 2.4. With the manual pump inflate the cuff to a pressure P higher than the systolic P sist. and generate signal amplitude changes on the 2nd channel of the oscilloscope: that is the baseline moves up and is raised up a few divisions from the center. On the 1st channel a visible voltage change should be seen, which is created by the pressure P. At that time heartbeats are heard in phonendoscope (Figure ) The resulting image should be saved in JPG format on the computer desktop From the resulting image calculate the testing signal period T and frequency ν: T Bk 3 ; = 1/T; (5.2.1) where B - horizontal length (cm) of the measured signal, k 3 - dissemination indicator s k 3 value (e.g. 2 S / cm or 4 S / cm). B A sist. A diast pav. Computer screen image showing voltage changes from heartbeats pav. Computer screen image showing systolic and diastolic values FBML MEASUREMENT OF BLOOD PRESSURE 4

5 2.7. Calculate the human heart rate (beats per minute): 60 pulsas (min -1 ) (5.2.2) T pulsas 60 (min -1 ) 2.8. Once again with the manual pump inflate the cuff to a pressure P higher than the systolic P sist. and generate signal amplitude changes on the 2nd channel of the oscilloscope: that is the baseline moves up and is raised up a few divisions from the center Gradually reduce pressure with the air-vent. When pressure P in the cuff is close P sist., blood will flow in the vessels, causing changes of pressure P in the cuff, and together will cause acoustic vibrations in the phonendoscope membrane. At that time the air-vent should be closed and the signal image should be fixed (Fig , A sist. ) The pressure should be further decreased with the air-vent. When the pressure P in the cuff is close to P diast, pressure changes P in the cuff will be lost and the acoustic signal in phonendoscope membrane disappears. At that time the air-vent should be closed and the signal image should be fixed (Fig , A diast. ) The resulting image should be saved in JPG format on the computer desktop Calculate U sist. and U diast.: U sist. A s k 3 ; (5.2.3) U diast. A d k 3 ; (5.2.4) where A s, A d viewable video signal vertical length (cm), k 2 the voltage deflection coefficient value (eg. 200 mv/cm) Knowing the relationship between voltage and pressure: 10 mv = 1 mmhg, systolic pressure P sist. and diastolic pressure P diast. can be calculated Compare blood pressure measured in the traditional way and by the oscillometric method. FBML MEASUREMENT OF BLOOD PRESSURE 5