BLOOD PRESSURE

Blood Pressure-Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" refers to the pressure in the large arteries.

  • Arterial blood pressure is measured with a sphygmomanometer.
  • Blood pressure is recorded as a fraction: systolic pressure over the diastolic pressure, and expressed in millimeters of mercury (mm Hg).
  • A typical blood pressure for a healthy adult is 120/80 mmHg.

Definition 

  • Blood pressure is the force or pressure of the blood against the walls of the blood vessels.
  • Blood pressure  is defined as the lateral pressure exerted by flowing blood on the walls of the arteries.

Types of Blood pressure

Blood pressure is mentioned in terms of-

  • Systolic blood pressure
  • Diastolic blood pressure.

Systolic blood pressure is the highest pressure attained in arteries during systole and diastolic blood pressure is the lowest arterial pressure during diastole

Systolic blood pressure

  • The systolic pressure is the pressure as a result of contraction of the ventricles. When the left ventricle contracts and pushes blood into the aorta the pressure produced within the arterial system is called the systolic blood pressure.
  • In adults it is about 120 mmHg.

Diastolic blood pressure.

  • The diastolic pressure is the pressure when the ventricles are at rest..
  • When complete cardiac relaxation occurs and the heart is resting after the ejection of blood, the pressure within the arteries is called diastolic blood pressure.
  • In an adult it is about 80 mmHg.

Characteristics of Blood Pressure

Following are the major characteristics of blood pressure.

1. Cardiac output- Cardiac output depends upon the volume of blood pumped by the heart and if heart pumps more blood that will raise cardiac output and increased cardiac output leads to increased heart rate and blood pressure.


2. Peripheral resistance- Peripheral vascular resistance directly affects the blood pressure. When it is more, BP will be raised and it may occur due to vasoconstriction (due to obstruction) or increased blood volume.

3. Blood volume- Blood volume is directly proportional to the blood pressure. Increase in blood volume leads to increased BP and decrease volume of blood leads to low BP.

4. Elasticity- The elasticity of walls of artery affects the blood pressure. With each heart beat the pressure is created inside the artery. The pressure rises during contraction and falls while relaxation.

5. Viscosity- Viscosity of blood means the percentage of RBCs in blood. Blood viscosity is also directly proportional to the blood pressure. Increase in blood viscosity leads to increased BP and decrease viscosity of blood leads to low BP. 
 

Factors Affecting Blood Pressure

  • Age- The pressure rises with age, reaching a peak at the onset of puberty, and then tends to decline.
  • Exercise- Physical activity increases the cardiac output and hence the blood pressure.
  • Stress- Stimulation of the sympathetic nervous system increases cardiac output and vasoconstriction of the arterioles, thus increasing the blood pressure.
  • Race- African Americans tend to have higher blood pressures than European Americans. The exact reasons are unclear.
  • Sex- Females have lower blood pressures than males due to hormonal variations. After menopause, women have higher blood pressures.
  • Medications- Many medications, including caffeine, may increase or decrease the blood pressure.
  • After meals- The blood pressure increases for a few hours after meals due to an increase in cardiac output . 
  • During sleep- Usually, the pressure is reduced up to 15-20 mm Hg during sleep. It increases slightly during sleep associated with dreams.
  • Obesity- Predispose to hypertension.
  • Positions- Sitting, standing or lying down position cause slight variation in BP.
  • Diurnal variations (time of day)- Pressure is lowest early in the morning, when the metabolic rate is lowest, then rises throughout the day and peaks in the late afternoon or evening.
  • Medical conditions- Any condition affecting the cardiac output, blood volume, blood viscosity, and/or compliance of the arteries has a direct effect on the blood pressure.
  • Temperature- Because of increased metabolic rate, fever can increase blood pressure. However, external heat causes vasodilation and decreased blood pressure. Cold causes vasoconstriction and elevates blood pressure.

  • Hemorrhage- Hemorrhage causes a low blood pressure by decreasing the volume of blood in the blood vessels.

  • Increased intracranial pressure- Increased intracranial pressure usually  produces a rise in the blood pressure. For example, head injury, brain tumors.

Sites for Checking Blood Pressure

  • The upper arm- The blood pressure is usually assessed in the client's upper arm in the brachial artery with a standard stethoscope.
  • Thigh- When the blood pressure cannot be assessed on either arm, check the blood pressure in the thigh. To check the blood pressure, place the cuff around the bottom third of the client's thigh. The cuff's artery line is aligned with the popliteal artery to check the pressure.

Equipment Used for Assessing Blood Pressure

Sphygmomanometer and stethoscope- There are two types of sphygmomanometers.

  1. Aneroid sphygmomanometer -Aneroid sphygmomanometer may require calibration checks, unlike mercury manometers. Aneroid sphygmomanometers are considered safer than mercury sphygmomanometers, although inexpensive ones are less accurate.
  2. Mercury sphygmomanometer- Mercury sphygmomanometers are considered the gold standard. They indicate pressure with a column of mercury, which does not require recalibration. Because of their accuracy, they are often used in clinical trials of drugs and in clinical evaluations of high-risk patients, including pregnant women.It is a calibrated cylinder filled with mercury.

It consists of a cuff, tubing, manometer bulb and valve-

  • Cuff- The cuff contains an airtight flat rubber bladder covered with cloth. Rubber bladder when filled squeezes the arms to obstruct the flow of blood through the artery.
  • Bulb- It is a handheld air pump that is used to pump the air into the cuff.
  • Manometer- It is a part of the sphygmomanometer that measures the blood pressure in mm Hg.
  • Valve- Valve allows for controlled inflation and deflation of the cuff.
  • Tubing- Two tubes are connected to the bladder within the cuff. One is connected to the manometer and the other is attached to bulb used to inflate the bladder.

 Common  Errors in BP measurement

  • BP different in each arm- A difference >10 mmHg suggests peripheral vascular disease and raises the possibility of renal artery stenosis as the cause of hypertension. Record the highest pressure and treat this.
  • Wrong cuff size- A cuff of 12.5 × 23 cm is suitable for only 60% of Europeans.The bladder should encircle between 80% and 100% of the arm. In obese patients with large arms a normal-sized cuff will over- estimate BP and the error is greater when the centre of the cuff is not over the brachial artery.
  • Auscultatory gap- Up to 20% of elderly hypertensive patients have phase 1 Korotkoff sounds which begin at systolic pressure but then disappear for varying lengths of time, reappearing before diastolic pressure. If the first appearance of the sound is missed, the systolic pressure will be recorded at a falsely low level. Avoid this by palpating the systolic pressure first.
  • Excess pressure of stethoscope- Excess pressure can artificially lower the diastolic reading by 10 mmHg.The systolic pressure is not usually affected.
  •  Patient's arm at the wrong level- The patient's elbow should be level with his heart. Hydrostatic pressure effects mean that if the arm is 7 cm higher, both systole and diastole pressures will be 5 mmHg lower. If the arm is 7 cm lower than the heart, they will be about 6 mm higher.
  • Postural change- When a healthy person stands, the pulse increases by about 11 bpm and stabilizes after 1 min.The BP stabilizes after 1-2 min. Check the BP after a patient has been standing for 2 min; a drop of ≥20 mmHg on standing is postural hypotension.
  • Abnormal pulse pressure- The pulse pressure is the difference between the systolic and diastolic pressures. A pulse pressure of ≥80 mmHg suggests aortic regurgitation, while a low pulse pressure may occur in aortic stenosis.
  • Normal Reading Prejudice- Prejudice for normal readings significantly contributes to inaccuracies in blood pressure measurement.

#