Elementos básicos de una red

1. It may be noted that the Korotkoff sounds are not heard equally well in all individuals. Indeed it may be difficult to identify any phases at all except the first appearance of sounds—the criterion of systolic pressure.

2. Sometimes muffling of the sounds (1st diastolic pressure) may not be distinguished though their disappearance is clear. In such cases 5 mm are added to the level at which they disappeared (2nd diastolic pressure). In cases like aortic regurgitation, the Korotkoff sounds may continue right down to the zero level. In others, the sounds may disappear only after 15–20 mm Hg after muffling. In these cases, placing the stethoscope over an artery and pressing its rim on the vessel may produce sharp tapping sounds_} called ‘’pistol shot” sounds.

3. Criterion of diastolic pressure. The criterion for diastolic pressure is the muffling of the sounds or their disappearance?

Simultaneous recordings of BP with auscultatory method and intra-arterial recordings with pressure transducers have shown that the diastolic pressure correlates better with the disappearance of sounds.

However, in adults after exercise, and in children, the diastolic pressure has better correlation with muffling. Therefore, the blood pressure may also be expressed as: 120/80/76, the last figure indicating the disappearance of sounds (1st and 2nd diastolic, Figure 2-14).

Note

The BP readings are seldom identical in the two arms. It has been suggested that both arms be used, preferably the right arm and then the left arm.

Tabulate your results as shown below (Table 2-1):

For report, express your result as:

Right arm: Systolic/Ist diastolic/2nd diastolic;

(e.g. 120/80/76).

Left arm: Systolic/Ist diastolic/2nd diastolic;

(e.g. 118/76/72).

Table 2-1: Record of systemic arterial blood pressure A. PALPATORY METHOD (mm Hg)

Right arm Left arm

C. AUSCULTATORY METHOD (mm Hg) Systolic

1. The subject should be physically and mentally relaxed and free from tension and anxiety. He/

she should be assured and rested for 5 minutes or so to avoid the condition of “white coat hypertension” (i.e. some people have higher BP readings in the clinician’s office than during their normal daytime activity). It is good practice to compare the pressures in the two arms when recording BP for the first time. If the readings are above the upper normal limits, the measurement must be repeated under basal conditions, i.e. early in the morning before the subject gets up from the bed. A diagnosis of hypertension must never be made lightly and in haste.

2. The arm, with the cuff wrapped around it, should be kept at the level of the heart to avoid the influence of gravity. The cuff tubing should lie anterolateral to the cubital fossa so that they do not rub against the chest-piece of the stethoscope.

3. The cuff should not be too tight nor too loose.

4. The cuff should not be left inflated with high pressures for any length of time, because the discomfort and reflex spasm of the artery and its branches will give false high readings.

5. Do not apply pressure on the artery with the chest-piece as this may produce partial obstruction of the artery and a fake low reading.

6. Check the pulse rate at the time of recording BP as the heart rate affects the BP.

7. The palpatory method must always be employed before the auscultatory method.

8. In suspected and known cases of hypertension, the pressure should always be raised well above 200 mm Hg; or above the level estimated by palpatory method.

9. In obese subjects, a cuff that is wider than the standard should be used. Similarly, when measuring the pressures in the thigh, the cuff should be wider, because the thick layer of fat in the obese, or the large amounts of tissues in the thigh dissipate some of the cuff pressure, thus giving false high results. (The BP may be recorded with the cuff on the forearm while palpating and auscultating the radial artery).

QUESTIONS

Q.1 Define blood pressure. Why does blood exert a pressure on the walls of the blood ves-sels? Is this pressure constant throughout the cardiac cycle? What are the units employed for blood pressure?

The vascular system is “overfilled” with blood so that it is slightly stretched by the blood. As a result, the blood exerts an outward lateral force on the inside of the vessels; this force is known as blood pressure.

Although blood exerts a force (pressure) throughout the vascular system, the term blood pressure, used unqualified, refers to systemic arterial pressure.

(Thus, other pressures would be capillary pressure, venous pressure, pulmonary pressure, etc).

Relation between contents and capacity—the cause of blood pressure.

The relation between the contents and capacity of a distensible container determines whether or not the fluid will exert a pressure. So long as the contents

are equal to or less than the capacity, no pressure is exerted, i.e. the pressure is zero, or atmospheric. (All pressures in the body are described with reference to the atmospheric pressure which is taken as zero.

Thus, a pressure of 120 mm Hg means a pressure of 760 + 120 mm Hg; a pressure of –5 mm Hg is equal to 760 – 5 = 755 mm Hg). A pressure is exerted only when the volume of contents exceeds the capacity, i.e. when extra fluid is injected into the container. (An example will clarify the point: we have a rubber ball of 200 ml capacity. When we inject 200 ml water into it, say, with a syringe, the pressure exerted will be zero or atmospheric. Now, if we inject another 20 ml water, the water will exert a pressure. Introduction of another 20 ml water will further increase the pressure. Thus, pressures are created when the contents become more than the capacity. The arterial tree of an adult has a capacity of about 500 ml, while this space contains 750 ml blood; the blood therefore, exerts a mean pressure of 100 mm Hg. If the volume of blood decreases to 500 ml (say, due to hemorrhage), the blood pressure will fall to zero).

If we suddenly stop the heart in an experimental animal, the blood redistributes itself throughout the vascular tree, and now it exerts a pressure of 8–10 mm Hg, which is called the mean circulatory filling pressure.

How much pressure would be exerted by blood on the vessel walls is determined by:

1. The degree of stiffness of the aorta and its large branches.

2. The inflow of blood into the arterial tree (controlled by cardiac output).

3. The outflow of blood from the arterial tree con-trolled by arteriolar tone (i.e. peripheral resis-tance), and

4. The volume of blood (see Figure 2-15).

Arterial blood pressure is pulsatile.

The BP does not remain constant at one level but rises and falls rhythmically with systole and diastole of the heart, i.e. it is pulsatile. It reaches a maximum during systole and falls to a minimum during diastole (See next Q/A also).

Units employed for blood pressure. Pressure is a force acting on a unit area (e.g. dynes/cm²).

The pressure exerted by blood is usually expressed

in terms of the height of a column of fluid that the pressure will support. The SI unit of pressure is the pascal (Pa). This is the pressure exerted by 1 Newton force on an area of a square meter (1 Pa = 1 N/m²); 1 mm Hg = 133.3 Pa

= 0.1333 kPa).

Q.2 What is systolic blood pressure? How is it produced and what is its significance?

With each systole of the left ventricle, 70–80 ml of blood is ejected into the aorta and its branches and the pressure sharply rises. These vessels, which are highly elastic, get stretched (expanded) and accommodate some of this stroke volume, while the rest runs off down the arterial tree. During diastole of the heart, (when the ventricles are relaxing and getting filled with blood from the atria), the large elastic vessels recoil and the blood that was accommodated earlier, now moves down the arterial tree. Thus, these vessels act as “secondary pumps” which produce a pressure and blood flow during diastole of the heart.

The periodic entry of blood into the arterial tree causes the pressure within to alternately rise to a maximum and fall to a minimum. The maximum pressure is reached during the maximum ejection phase of systole and is called the systolic pressure. The minimum pressure is reached during diastole and is called the diastolic pressure.

Note

It may be pointed out that during systole, the pressure rises to a maximum and then begins to fall as blood runs off down the arteries. The pressure would fall to zero but for the next systole when another stroke volume is ejected into the aorta and the pressure rises again. This rise and fall of blood pressure is repeated over and over again.

Significance of systolic pressure. The systolic pressure indicates the force of contraction of the heart and thus it represents the work done by the heart in overcoming the resistance of the vessels.

Q.3 What is diastolic pressure and what is its criterion and significance?

Diastolic pressure is the minimum pressure reached in the arteries during diastole of the heart, i.e. just before the next systole. The relaxation of the heart cannot be a cause of a pressure in the aorta because the left ventricle itself is getting filled with

blood. Two factors, both outside the heart, combine to produce a pressure in the arteries during diastole.

i. Elasticity of aorta and large branches (i.e. recoil of aorta), and

ii. Peripheral resistance.

If the aorta and large arteries were rigid, there would be no diastolic pressure; also the systolic pressure would rise to a much higher level (thus, the elasticity buffers the systolic pressure and does not allow it to rise very high). Similarly, there would be no diastolic pressure if there were no peripheral resistance, as most of the blood would run off into the periphery.

Criterion of diastolic pressure. (page 172).

Significance of diastolic pressure.

Clinically, greater importance is attached to the DP because this much pressure is being exerted all the time during systole and diastole, while systolic pressure is reached only momentarily during systole.

Since a sustained high pressure causes damage to the vessel walls, diastolic hypertension is much more dangerous than systolic hypertension.

Q.4 What is mean arterial pressure and what is its significance?

The mean arterial pressure (MAP, or mean arterial blood pressure, MABP) is the average of all the pressures measured during the cardiac cycle. Since the duration of systole is shorter than that of diastole, the MAP is slightly less than the average of systolic and diastolic pressures. (The true MAP can be determined only by integrating the areas of the pressure curves).

However, a reasonable approximation is: one-third of pulse pressure plus diastolic pressure (e.g. SP = 120;

DP = 80; so MABP is equal to 13 + 80 = 93 mm Hg).

Another approximation is 40% SP + 60% DP (e.g.

40% of 120 = 48, and 60% of 80 = 48; thus 48 + 48 = 96 mm Hg).

Significance of mean arterial pressure.

The MAP of about 95 mm Hg provides the pressure head, or the driving force (vis a tergo) for the flow of blood through the arteries, capillaries, and veins, etc. The MAP in medium-sized arteries (e.g. radial) is about 90 mm Hg. Thus, most viscera, muscles, and other tissues are perfused at a relatively high pressure. The mean pressure of about 85–80 mm

Hg at the start of arterioles falls to about 32 at their capillary ends. (Thus, maximum fall in pressure occurs in the arterioles). The pressure then continues to fall progressively till it reaches zero in the right atrium. The pressure gradient of about 95 mm Hg is responsible for the circulation of blood and tissue perfusion.

Q.5 What is pulse pressure and what is its significance?

Pulse pressure (PP) is the difference between systolic and diastolic pressures, the average PP being about 40 mm Hg. Other factors remaining unchanged, the magnitude of PP indicates the stroke volume.

Thus, it provides information about the condition of cardiovascular system. For example, conditions such as atherosclerosis (hardening of blood vessels) and patent ductus arteriosus generally increase the PP.

The normal ratio of SP to DP and to PP is about 3:2:1.

Q.6 Name the precautions you will observe while recording blood pressure?

See page 172.

Q.7 What are the advantages and disadvan-tages of palpatory method of recording blood pressure?

See page 169.

Q.8 What will be the effect of using a wrong-sized blood pressure cuff in different age groups, or a standard cuff in a very obese person?

If an over- or under-sized cuff is used, the reading will be higher than actual because more pressure would be required in the cuff to overcome tissue resistance and to form a cone of pressure.

When a standard cuff is used in an obese individual, the reading will be higher than actual because of loss of pressure in overcoming tissue resistance.

Q.9 What are Korotkoff sounds and how are they produced?

Normally, the blood flow through the arteries is laminar or streamline, and no sounds are heard when a stethoscope is placed on them. When the cuff pressure is raised above the expected systolic pressure, and then gradually lowered, a time comes, when at the peak of each systole, the intra-arterial pressure just exceeds the cuff (extra-arterial) pressure. But, in between these peaks, the artery is

still constricted. Now, it is known that constriction of an artery increases the velocity of blood flow through the constricted part. Thus, when the small amounts of blood are jetted through the partially constricted artery, their velocity increases and then exceeds the critical velocity. This produces intermittent turbulence that in turn produces Korotkoff sounds (beyond the constriction) which have a staccato quality (tapping intermittent sounds).

Also, the blood column in the distal part of the artery, i.e. below the cuff, is set into vibration by the jets of blood striking against it, which contributes to the sounds. (The velocity of blood has to increase beyond a certain critical level before turbulence and hence sounds are produced. This velocity is sometimes normally exceeded in the ascending aorta at the peak of systolic ejection. Turbulence also occurs commonly in anemia because the viscosity of blood is low. This probably explains the systolic murmurs in these cases).

When the cuff pressure is near the diastolic level, the artery is still partially constricted, but the turbulent flow is now continuous rather than intermittent, and sounds from continuous turbulent flow have a muffled quality rather than a tapping or staccato quality. As the cuff pressure further falls, the blood flow becomes laminar once again and the sounds disappear. The change in the character of sounds during early phases is related to the degree of turbulence.

In aortic regurgitation (leaking of blood back from the aorta into the ventricle through an incompetent valve), the sounds may continue right down to near zero and only muffling of sounds can indicate diastolic pressure. In fact, a slight pressure with a stethoscope alone (without the cuff on the upper arm) may produce sharp, clear, snapping sounds, called “pistol shot” sounds, in this condition.

Q.10 What is auscultatory gap, and what is its significance?

In some patients of hypertension, there may be a gap in the Korotkoff sounds. As the mercury is lowered, a few faint sounds are heard which soon disappear only to reappear once again at a lower pressure. This brief interruption, which may range from 40 to 60 mm Hg, is called the “auscultatory” or “silent” gap. If the

mercury column is raised to this gap, and then the pressure lowered, one may miss the first appearance of sounds, which indicate systolic pressure, and thus record a false low systolic pressure.

To avoid this mistake, the BP should always be recorded by the palpatory method first. Then during auscultatory method, the mercury column must be raised 30–40 mm Hg above the level found by palpatory method.

Q.11 When recording blood pressure, why should the upper arm with the cuff wrapped around it, be kept at the level of the heart?

The force of gravity exerts an important effect on the blood pressure readings. The degree of its effect varies with the vertical distance above and below the level of the heart. Consult next Expt on the effect of gravity on blood pressure.

Q.12 What is the effect of muscular exercise on blood pressure?

Consult next Expt for the effect of exercise on blood pressure.

Q.13 What is oscillatory method of recording blood pressure?

See page 169.

Q.14 How does the blood pressure recorded in the femoral artery differ from that recorded in brachial artery?

A cannula inserted in an artery, with the artery tied off beyond this, records an end pressure. (Flow in the artery is interrupted and all the kinetic energy is converted into pressure energy). If a T tube is inserted in an artery and pressure is measured in the side arm of the tube, it records the side pressure, which is lower than the end pressure.

The subclavian and the brachial arteries represent the side arms from the wall of the aorta. The pressure recorded in the brachial artery, thus, represents the side pressure or the lateral pressure in the aorta. On the other hand, the femoral arteries are the direct extensions of the aorta. When pressure is recorded from a femoral artery, the end pressure is represented in the recording. For this reason, even in the supine position, pressures recorded in the lower limbs are somewhat higher than those in the upper limbs. A low pressure in the femoral artery with hypertension in the

arms is the basic clue to the diagnosis of coarctation of aorta. With the normal person standing, the femoral pressure is higher than brachial pressure.

Q.15 What are the physiological variations in blood pressure?

Normally, variations in BP occur as mentioned below:

1. Age.

The average SP at birth is about 40 mm Hg, reaches 70 at 2 weeks and 80 at one month. The SP/DP averages 90–100/60–70 mm Hg between 4 and 10 years and adult levels are reached by 18–20 years. Both SP and DP rise with age; at 60 years the BP may be 160/90 mm Hg.

2. Sex.

The BP is generally lower in females by about 8–10 mm Hg. It remains so till the age of menopause, after which it remains slightly higher than the male average.

3. Body build and obesity.

Overweight individuals tend to have higher blood pressure. Since resistance to blood flow through a blood vessel depends on its length, increased length of blood vessels is bound to increase the resistance and hence blood pressure. (Each extra kg of adipose tissue is associated with the develop-ment of an additional 400 km of blood vessels).

4. Diurnal variations.

The BP is lowest under basal conditions, the peak being seen in the late afternoon, mainly in the systolic level. The SP shows a significant fall during sleep.

5. Digestion.

The systolic pressure shows a rise of 8–10 mm Hg after meals and lasts for about 1 hour.

Diastolic is little affected, though it may decrease a little due to vasodilation in the viscera.

6. Emotional stress.

Hypertension is a natural response to pain, and stress in nonhypertensive individuals. The systolic

Hypertension is a natural response to pain, and stress in nonhypertensive individuals. The systolic