Blood Pressure and its measurement 5.4.12 Blood Pressure and its measurement

Blood flow through stenosis Bernoulli’s principle states that when the fluid flow through a tube is constant, the total fluid energy –the sum of kinetic energy and potential energy-remains constant It explains why fluid pressure is low in blood vessels at places where its radius is less

Blood flow through stenosis Proximal to the structure of stenosis, blood flow is laminar. Blood passing through stenotic orifice also remains laminar, but its velocity increases to maintain the volumetric rate of flow. Here the pressure against the walls is least and hence there is a pressure drop across the narrower area of a vessel

Blood Pressure Blood pressure (BP) is the pressure exerted by circulating blood upon the walls of blood vessels The blood pressure in the circulation is principally due to the pumping action of the heart. Differences in mean blood pressure are responsible for blood flow from one location to another in the circulation The rate of mean blood flow depends on the resistance to flow presented by the blood vessels. Mean blood pressure decreases as the circulating blood moves away from the heart through arteries, capillaries and veins due to viscous losses of energy Mean blood pressure drops over the whole circulation, although most of the fall occurs along the small arteries and arterioles

Forces acting on blood during circulation The main forces acting on blood during circulation Inertial force due to acceleration Flowing blood has inertia and energy is expended in setting the blood into motion. Inertial force has two components (i) time acceleration due to pulsatile ejection from heart (ii) spatial acceleration e.g. at entry into a circulation Viscous force (Fv) Pressure force FP (systolic force) Gravitational force FG

Forces acting on blood during circulation According to Newton’s law of motion which also governs the motion of blood F = FV + FP+ FG Energy of fluids moving in horizontal and vertical vessels differ Energy per unit volume in horizontal vessels: Sum of kinetic energy (dynamic pressure) + Potential energy/Hydrostatic pressure = 1/2ρv2 + P Energy per unit volume in vertical vessels: Sum of kinetic energy+ Potential energy/Hydrostatic pressure + gravitational energy = 1/2ρv2 + P + ρgh ρ is the fluid density in kg/m3, v is the linear velocity in m/s, h is the height above or below the heart

Blood Pressure Profile Blood pressure is highest in the arteries. It decreases as we move to arterioles, capillaries and then to veins

Reference points for measuring blood pressure While measuring pressures in cardiovascular system, ambient atmospheric pressure is used as zero reference point. Thus a blood pressure of 90mmHg means that pressure is 90mmHg above atmospheric pressure The second reference point for measuring blood pressureis anatomical and is the position of heart. For example, the usual convention is to measure blood pressure in the brachial artery above elbow i.e. approximately at hearts level when patient is seated If the blood pressure measurements are to be made in the legs, the patient is brought to lying down position. In this position vessel is approximately at cardiac level

Blood Pressure Effect of gravity on pressure Venous pressures -35 mm Hg 1 mm Hg 105 mm Hg Effect of gravity on pressure Distance heart-head~ 0.4 m Heart-feet ~ 1.4 m DP = rgh Arterial pressures 55 mm Hg 100 mm Hg 195 mm Hg 100 mmHg 95 mmHg 95 mm Hg

The pressure in any vessel above heart level is decreased by the effect of gravity The arterial pressure is increased by 0.77mmHg for every centimeter below the right atrium and similarly decreased for each cm above the right atrium

Blood pooling and skeletal muscles of leg The increased hydrostatic pressure in the veins of the legs upon standing pushes outward upon the veins walls causing marked distension with pool-leg of blood Gravitational force also increase capillary pressure. This in turn causes increased filtration of fluid out of capillaries into interstitial spaces The tendency is counteracted by the skeletal muscles of the leg which contract and compress the veins thereby compressing the column of venous blood from feet to heart

The column of venous blood is also interrupted. In the neck The column of venous blood is also interrupted. In the neck. This interruption helps in neutralizing the effect of gravity and the venous pressure in the neck becomes approximately zero

Posture effects on the blood pressure A sudden change in posture from horizontal to vertical can have a very marked effect on the pressure pattern in the body. Consequences can be faintness one can experience on sudden rising. This is known as “postural hypotension syncope (syncope= fainting)” The reason is that veins are most distensible . On rising suddenly the pressure of blood in veins rises markedly. In the absence of correcting mechanisms, the veins expand markedly and the blood ‘pools’ there The venous return to the heart drops drastically and the blood circulation to the brain consequently falls- resulting in dizziness or even loss of consciousness

Posture effects on the blood pressure Normally the body possesses three mechanisms for the maintenance of good blood return to the heart. These are: 1. Pressure reflexes which induce constriction of diameter of arteries and arterioles 2. reflex acceleration of the heart rate when receptors in the aorta or the carotid artery sense a drop in pressure 3. muscular activity in the limbs which help to maintain the diameter of veins and act to reduce pooling of the blood Dysfunction of these reflexes or too rapid motion which produces blood pooling before the postural reflexes come into play , can produce dizziness and/or fainting

Blood pressure measurement Direct method This is an invasive method in which artery or vein is cannulated or catheterised. Pressure measured by direct method is known as “end pressure” Here the kinetic energy of blood flow is measured in terms of pressure. Direct method is used in patients of ‘shock’ where indirect measurements may be inaccurate or indeed impossible indire

Indirect methods of blood pressure measurement Indirect method (non-invasive, measures lateral/side pressure) Auscultatory Oscillometric

Auscultatory Method The auscultatory method uses a stethoscope and a sphygmomanometer An inflatable cuff encircles the arm. Pressure in the cuff is transmitted through the tissue to compress brachial artery and can be viewed on a manometer A stethoscope is used to listen to sounds in the artery distal to the cuff. The sounds heard during measurement of blood pressure are not the same as the heart sounds 'lub' and 'dub' that are due to vibrations inside the ventricles that are associated with the snapping shut of the valves

Auscultatory Method If a stethoscope is placed over the brachial artery in a normal person no sound should be audible. As the heart beats, pulses (pressure waves) are transmitted smoothly via laminar (non-turbulent) blood flow throughout the arteries, and no sound is produced Similarly, if the cuff of a sphygmomanometer is placed around a patient's upper arm and inflated to a pressure above the patient's systolic blood pressure, there will be no sound audible. This is because the pressure in the cuff is high enough such that it completely occludes the blood flow This is similar to a flexible tube or pipe with fluid in it that is being pinched shut

Korotkoff sounds If the pressure is dropped to a level equal to that of the patient's systolic blood pressure, the first Korotkoff sound will be heard. As the pressure in the cuff is the same as the pressure produced by the heart, some blood will be able to pass through the upper arm when the pressure in the artery rises during systole. This blood flows in spurts as the pressure in the artery rises above the pressure in the cuff and then drops back down beyond the cuffed region, resulting in turbulence that produces an audible sound

Korotkoff sounds As the pressure in the cuff is allowed to fall further, thumping sounds continue to be heard as long as the pressure in the cuff is between the systolic and diastolic pressures, as the arterial pressure keeps on rising above and dropping back below the pressure in the cuff. Eventually, as the pressure in the cuff drops further, the sounds change in quality, then become muted, and finally disappear altogether. This occurs because, as the pressure in the cuff drops below the diastolic blood pressure, the cuff no longer provides any restriction to blood flow allowing the blood flow to become smooth again with no turbulence and thus produce no further audible sound. The pressure where sound just disappears is the diastolic pressure

Oscillometric method The oscillometric method was first demonstrated in 1876 and involves the observation of oscillations in the sphygmomanometer cuff pressure[ which are caused by the oscillations of blood flow, i.e. the pulse It uses a sphygmomanometer cuff, like the auscultatory method, but with an electronic pressure sensor (transducer) to observe cuff pressure oscillations, electronics to automatically interpret them, and automatic inflation and deflation of the cuff.

Oscillometric method The cuff is inflated to a pressure initially in excess of the systolic arterial pressure and then reduced to below diastolic pressure over a period of about 30 seconds. When blood flow is nil (cuff pressure exceeding systolic pressure) or unimpeded (cuff pressure below diastolic pressure), cuff pressure will be essentially constant. When blood flow is present, but restricted, the cuff pressure, which is monitored by the pressure sensor, will vary periodically in synchrony with the cyclic expansion and contraction of the brachial artery, i.e., it will oscillate. The values of systolic and diastolic pressure are computed, results are displayed.

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