7.1: Background Information

Blood Flow

Blood flow refers to the movement of blood through a vessel, tissue, or organ, and is usually expressed in terms of volume of blood per unit of time. It is initiated by the contraction of the ventricles of the heart. Ventricular contraction ejects blood into the major arteries, resulting in flow from regions of higher pressure to regions of lower pressure, as blood encounters smaller arteries and arterioles, then capillaries, then the venules and veins of the venous system. A relatively constant flow is required by the body’s tissues, so pressure and resistance are altered to maintain this consistency. A too-high flow can damage blood vessels and tissue, while flow that’s too low means tissues served by the blood vessel may not receive sufficient oxygen to function.

Blood Pressure

Hydrostatic pressure is the force exerted by a fluid due to gravitational pull, usually against the wall of the container in which it is located. One form of hydrostatic pressure is blood pressure, the force exerted by blood upon the walls of the blood vessels or the chambers of the heart. Blood pressure may be measured in capillaries and veins, as well as the vessels of the pulmonary circulation; however, the term blood pressure without any specific descriptors typically refers to systemic arterial blood pressure—that is, the pressure of blood flowing in the arteries of the systemic circulation. In clinical practice, this pressure is measured in mm Hg and is usually obtained using the brachial artery of the arm. Arterial blood pressure in the larger vessels consists of several distinct components: systolic and diastolic pressures, pulse pressure, and mean arterial pressure.

Figure \(\PageIndex{7}\) is a line graph showing how blood pressure changes across the systemic circulation, from the aorta through arteries and microvessels to the venae cavae. The vertical axis is pressure in mm Hg (ranging from about 0 to 120). The horizontal axis lists vessel types in order: aorta, elastic arteries, muscular arteries, arterioles, capillaries, venules, medium and large veins, and venae cavae.

A red, pulsating waveform represents arterial pressure pulses. The peaks of the waveform are labeled systolic pressure, and the troughs are labeled diastolic pressure. A darker central line is labeled mean arterial pressure. The graph shows that:

• In the aorta and large arteries, pressure is high and strongly pulsatile, with systolic values near the top of the scale and diastolic values lower, creating a large pulse pressure.
• As blood moves through elastic and muscular arteries, the overall pressure gradually declines and the pulsations become smaller.
• The largest drop in mean pressure occurs across the arterioles, and the pulsations decrease markedly.
• In the capillaries, pressure is much lower and essentially non-pulsatile.
• Pressure continues to fall through venules and medium/large veins, approaching very low values in the venae cavae near the end of the graph.

Systolic and Diastolic Pressures

When systemic arterial blood pressure is measured, it is recorded as a ratio of two numbers (e.g., 120/80 is a normal adult blood pressure), expressed as systolic pressure over diastolic pressure. The systolic pressure is the higher value (typically around 120 mm Hg) and reflects the arterial pressure resulting from the ejection of blood during ventricular contraction, or systole. The diastolic pressure is the lower value (usually about 80 mm Hg) and represents the arterial pressure
of blood during ventricular relaxation, or diastole.

Pulse Pressure, as shown in Figure \(\PageIndex{7}\), the difference between the systolic pressure and the diastolic pressure is the pulse pressure. It is calculated using the formula: Pulse Pressure = Systolic Blood Pressure (SBP) - Diastolic Blood Pressure (DBP)

For example, if a person's blood pressure is recorded as 120/80 mm Hg, the pulse pressure would be: Pulse Pressure = 120 mm Hg (SBP) - 80 mm Hg (DBP) = 40 mm Hg

A wider pulse pressure (generally considered to be greater than 40 mmHg) can indicate increased stiffness of the arteries or other cardiovascular issues, while a narrow pulse pressure may be a sign of reduced stroke volume or heart problems.

Mean Arterial Pressure

Mean arterial pressure (MAP) represents the “average” pressure of blood in the arteries, that is, the average force driving blood into vessels that serve the tissues. MAP is a key measure of blood flow and organ perfusion, calculated using both systolic and diastolic blood pressure values. The most common formula is:

\(\displaystyle MAP = \frac{\text{Systolic Blood Pressure (SBP)+2} × {\text{Diastolic Blood Pressure (DBP)}}}{3}\) 

This formula weights diastolic pressure more heavily because the heart spends more time in diastole during the cardiac cycle. For example, with a blood pressure of 120/80 mm Hg: MAP = [120 + (2 × 80)] ÷ 3 = [120 + 160] / 3 = 280 ÷ 3 ≈ 93 mm Hg 

A MAP of at least 60 mm Hg is needed for adequate organ perfusion, with normal values typically between 70 and 100 mm Hg.

Pulse

Pulse refers to the rhythmic expansion and contraction of the arteries as blood is pumped through them by the heart. It is a vital sign that reflects the heart's function and overall cardiovascular health. The pulse can be palpated manually at various points in the body, most commonly at the wrist (radial pulse) and neck (carotid pulse) or measured electronically.

Figure \(\PageIndex{8}\) explains that the pulse can be most easily measured at the radial artery, located at the wrist, but it can also be assessed at various pulse points shown in the image. These points include the carotid artery in the neck, the brachial artery in the arm, the femoral artery in the groin, and others, allowing for flexibility in pulse measurement depending on the situation.

Figure \(\PageIndex{8}\) - The pulse is most readily measured at the radial artery, but can be measured at any of the pulse points shown.

• Temporal Pulse: Located at the temple, just in front of the ear.
• Carotid Pulse: Found on either side of the neck, alongside the trachea.
• Apical Pulse: Located at the fifth intercostal space at the left mid-clavicular line on the chest;
• this is the point of maximum impulse over the heart.
• Brachial Pulse: Situated on the inner aspect of the elbow, along the brachial artery.
• Radial Pulse: Found at the wrist, on the thumb side.
• Femoral Pulse: Located in the groin, where the femoral artery passes.
• Popliteal Pulse: Felt at the back of the knee.
• Posterior Tibial Pulse: Found behind the medial malleolus (inner ankle bone).
• Dorsalis Pedis Pulse: Located on the top of the foot, near the center.

A typical resting pulse rate for adults ranges from 60 to 100 beats per minute (bpm). Factors such as age, fitness level, and overall health can influence this rate. To measure your pulse, lightly press on an artery with your fingers (not your thumb) and count the beats for 30 seconds, then multiply by 2 to get the beats per minute.

Resistance

Resistance to flow must be overcome to push blood through the circulatory system. If resistance increases, either pressure must increase to maintain flow, or flow rate must reduce to maintain pressure. Numerous factors can alter resistance, but the three most important are vessel length, vessel radius, and blood viscosity. With increasing length, increasing viscosity, and decreasing radius, resistance is increased. The arterioles and capillary networks are the main regions of the circulatory system that generate resistance, due to the small caliber of their lumen. Arterioles in particular are able to rapidly alter resistance by altering their radius through vasodilation or vasoconstriction. The resistance offered by peripheral circulation is known as systemic vascular resistance (SVR), while the resistance offered by the vasculature of the lungs is known as pulmonary vascular resistance (PVR).

Measurement of Blood Pressure

Blood pressure measurement is a fundamental clinical skill that provides vital information about cardiovascular health. The most common method in laboratory and clinical settings is the noninvasive, intermittent cuff-based technique using a sphygmomanometer and stethoscope, known as the auscultatory method. In this approach, a blood pressure cuff is wrapped snugly around the upper arm at heart level and inflated to temporarily stop blood flow in the brachial artery. As the cuff is gradually deflated, a stethoscope is used to listen for Korotkoff sounds—the first appearance of these sounds marks the systolic pressure (occurs when the ventricles contract and eject blood into arteries), while their disappearance indicates the diastolic pressure (occurs when the ventricles relax and fill with blood from the atria). The average blood pressure of healthy
young adults (about 20 yrs old) is 120 / 80 mm Hg.

Figure \(\PageIndex{9}\) explains how systolic and diastolic blood pressure are determined during a manual blood pressure measurement using a cuff and a stethoscope.

Axes and lines

• The vertical axis is pressure (mm Hg).
• The horizontal axis is time.
• A red wavy line represents the arterial pressure pulse over time (repeating peaks and troughs).
• A gray sloped line labeled cuff pressure shows cuff pressure gradually decreasing as the cuff deflates.

Key labeled events

• A vertical dashed line on the left is labeled “Systolic blood pressure (first sound heard)”. This marks the moment when the decreasing cuff pressure first allows blood to pass intermittently through the artery, producing the first audible Korotkoff sound.
• A vertical dashed line on the right is labeled “Diastolic blood pressure (last sound heard)”. This marks the moment when cuff pressure falls below diastolic pressure and blood flow becomes smoother, so Korotkoff sounds disappear.

Korotkoff sounds

• Along the bottom of the figure is a trace labeled “Korotkoff sounds in stethoscope.”
• The sounds begin at the systolic marker (first sound), continue while cuff pressure is between systolic and diastolic pressures, and stop at the diastolic marker (last sound).

Figure \(\PageIndex{10}\) is a three-panel figure shows how cuff pressure relates to blood flow in the brachial artery and the presence or absence of Korotkoff sounds during a manual blood pressure measurement.

Panel 1 (left): cuff pressure above systolic

• A rubber blood pressure cuff is inflated around the upper arm.
• Text indicates pressure in the cuff above 120 and no sounds audible.
• The brachial artery is closed, meaning blood flow is temporarily stopped by the cuff.

Panel 2 (middle): cuff pressure between systolic and diastolic

• The cuff pressure is shown below 120 but above 70.
• A stethoscope is placed over the artery.
• Text indicates sounds audible in the stethoscope.
• This represents intermittent blood flow through the partially compressed artery, producing Korotkoff sounds.

Panel 3 (right): cuff pressure below diastolic

• The cuff pressure is shown below 70.
• Text indicates no sounds audible.
• This represents blood flowing freely through the artery without the turbulent flow that produces Korotkoff sounds.

Automated devices can also be used, which inflate and deflate the cuff electronically and provide digital readings. Another commonly used method is the oscillometric technique, which measures pressure oscillations in the cuff to estimate systolic, diastolic, and mean arterial pressures; this method is widely used in home and ambulatory monitors. Proper positioning of the arm at heart level, relaxed posture, and correct cuff size are essential for accuracy.

Using an ECG to Calculate Heart Rate

The heart rate can be calculated based on the amount of time elapsed between the “R” of the QRS complexes on the ECG, this is called the R-R interval. To calculate heart rate from the R-R interval, you need to divide 60 by the R-R interval measured in seconds. The R-R interval is the time between two consecutive "R" waves on an electrocardiogram (ECG) and represents the duration of one heart cycle.
Example:

• If your R-R interval is 1 second, your heart rate is 60/1 = 60 bpm.
• If your R-R interval is 0.8 seconds, your heart rate is 60/0.8 = 75 bpm.

Figure \(\PageIndex{13}\) shows an electrocardiogram (ECG) tracing on grid paper. Two repeating heartbeat cycles are visible: each cycle includes a small rounded wave before a tall, narrow spike (the main complex), followed by a flat segment and then another small rounded wave. The pattern repeats across the strip, illustrating a regular rhythm.

We will use Figure \(\PageIndex{13}\) to practice how to measure heart rate on an ECG.

1. Locate two consecutive R waves on the ECG strip. The R wave is the peak of the QRS complex. Measure R-R interval on the ECG. In Figure 13 only one R-R interval is seen for practice purposes.
2. Counting Small Squares: Count the number of small squares between two R waves (R-R interval):
   • Number of small squares you counted (_______) × 0.04 seconds = ______ seconds
     • This gives you the R-R interval using small squares.
3. Counting Big Squares: Count the number of big squares between two R waves (R-R interval):
   • Number of big squares you counted (_______) × 0.2 seconds = ________ seconds.
     • This gives you the R-R interval using big squares.
4. Counting Both Small and Big Squares:
   • If you counted both small and big squares:
   • Number of big squares you counted () × 0.2 seconds + Number of small squares you counted () × 0.04 seconds = __________ seconds.
     • This total gives you the R-R interval using both small and big squares.
5. The total amount of time you found is for one heartbeat. So how many heartbeats will take place in one minute (60 seconds)? We can use the following formula to calculate heart rate (beats per minute/bpm) Heart Rate (beats per minute) = 60 / R-R interval (in seconds).

Another way to calculate heart rate (HR):

1. Convert the R-R interval to seconds: If the R-R interval is measured in millimeters, remember that each small square (1 mm) on an ECG usually represents 0.04 seconds.
   • For example: If the R-R interval is 17.5 mm, then 17.5 mm * 0.04 seconds/mm = 0.7 seconds.
2. Apply the formula: Heart Rate = 60 ÷ 0.7 seconds = 85.7 beats per minute (bpm).

The 6 second method:

1. Denotes a 6 second interval on ECG strip.
2. Count the number of QRS complexes or R waves occurring within the 6 seconds interval, and then multiply that number by 10.
   • For example, if there are 8 R waves in a 6 second strip, the heart rate is (8x10) = 80 bpm.

Alternatively, you can quickly and easily calculate the heart rate from an ECG by using the following formula:

• Heart Rate (bpm) = 1500 Number of small squares in the R-R interval

This method is particularly useful for beginners as it provides a straightforward calculation based on the number of small squares between two R waves. It allows for rapid calculations during clinical assessments.

Example Calculation: If you count 10 small squares between two R waves, Heart Rate = 1500 ÷ 10 = 150 bpm