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Anatomy of the Heart

The heart, located in the mediastinum, is the central structure of the cardiovascular system. It is protected by the bony structures of the sternum anteriorly, the spinal column posteriorly, and the rib cage.

Mini Sternotomy
y Clinical Tip:The cone-shaped heart has its tip (apex) just above the diaphragm to the left of the midline. This is why we may think of the heart as being on the left side, since the strongest beat can be heard or felt here.

Copyright2005 F A Davis

Copyright2005 F A Davis

Layers of the Heart

Fibrous Pericardium

The pericardial cavity contains a small amount of lubricating fluid to prevent friction during heart contraction.

Endocardium/

Myocardium (heart muscle) Epicardium (visceral pericardium)

Pericardial cavity

Parietal pericardium

Fibrous pericardium (pericardial sac)

The pericardial cavity contains a small amount of lubricating fluid to prevent friction during heart contraction.

Heart Valves

Properties of Heart Valves

■ Fibrous connective tissue prevents enlargement of valve openings and anchors valve flaps.

■ Valve closure prevents backflow of blood during and after contraction.

Pulmonary semilunar valve

Aortic semilunar valve

Coronary artery

Tricuspid valve

Posterior

The atria have been removed in this superior view.

Fibrous skeleton

Mitral valve

Posterior

The atria have been removed in this superior view.

Heart Chambers and Great Vessels

Brachiocephalic artery pulmonary artery

Superior vena cava Right \

Right pulmonary veins

Pulmonary semilunar valve

Right atrium

Tricuspid, valve

Inferior vena cava

Right pulmonary veins

Pulmonary semilunar valve

'Apex tCehnodrindeaaee Right Papillary tendineae ventricle Papillary

Left common carotid artery Left subclavian artery • Aortic arch

Left pulmonary artery Left atrium Left pulmonary veins Mitral valve Left ventricle

Aortic semilunar valve

Interventricular septum

tCehnodrindeaaee Right Papillary tendineae ventricle Papillary

'Apex

Coronary Arterial Circulation

Coronary sinus

Coronary sinus

Coronary Arteries

Right coronary artery Right coronary vein

Small cardiac vein

Right coronary artery Right coronary vein

(A) Anterior view

Small cardiac vein

(B) Posterior view

Anatomy of the Cardiovascular System

The cardiovascular system is a closed system consisting of blood vessels and the heart. Arteries and veins are connected by smaller structures in which electrolytes are exchanged across cell membranes.

Blood Vessel Structures

Cell Membrane Full Size Images

Arterial Circulation

Occipital Internal carotid Vertebral -Brachiocephalic Aortic arch

Celiac Left gastric Hepatic Splenic Superior.— mesenteric Abdominal aorta Right common iliac

Internal iliac External iliac

Deep femoral Femoral

Popliteal Anterior tibial

Posterior tibial

Maxillary Facial

External carotid Common carotid Subclavian Axillary Pulmonary Intercostal Brachial

Anterior Tibial Vein

Popliteal Anterior tibial

Posterior tibial

Renal Gonadal

Inferior mesenteric Radial Ulnar

Deep palmar arch

Superficial palmar arch

Venous Circulation

Anterior facial Superior vena cava

Venous Dorsal Palmar Arch

Popliteal

Anterior tibial

Anterior facial Superior vena cava

Axillary Cephalic Hemiazygos Intercostal Inferior vena cava Brachial Basilic Gonadal Superior mesenteric Common iliac

Dorsal arch Volar digital

Superior sagittal sinus Inferior sagittal sinus Straight sinus Transverse sinus Vertebral External jugular Internal jugular Subclavian Brachiocephalic Pulmonary Hepatic Hepatic portal Left gastric Renal Splenic Inferior mesenteric Internal iliac External iliac

Femoral Great saphenous

Popliteal

Small saphenous

Anterior tibial

Veins (excluding the pulmonary vein) carry blood low in oxygen and high in carbon dioxide.

Physiology of the Heart

Mechanics of Heart Function

Process

Action

Cardiac cycle

Sequence of events in 1 heartbeat. Blood is pumped through the entire cardiovascular system.

Systole

Contraction phase — usually refers to ventricular contraction.

Diastole

Relaxation phase—the atria and ventricles are filling. Lasts longer than systole.

Stroke volume (SV)

Amount of blood ejected from either ventricle in a single contraction. Starling's Law of the Heart states that degree of cardiac muscle stretch can increase force of ejected blood. More blood filling the ventricles T SV.

Cardiac output (CO)

Amount of blood pumped through the cardiovascular system per min. CO = SV x Heart rate (HR)

Properties of Cardiac Cells

Property

Ability

Automaticity

Generates electrical impulse independently, without involving the nervous system.

Excitability

Responds to electrical stimulation.

Conductivity

Passes or propagates electrical impulses from cell to cell.

Contractility

Shortens in response to electrical stimulation.

Electrical Conduction System of the Heart

Conduction System Structures and Functions

Structure

Function and Location

Sinoatrial (SA) node

Dominant pacemaker of the heart, located in upper portion of right atrium. Intrinsic rate 60-100 bpm.

Internodal pathways

Direct electrical impulses between SA and AV nodes.

Atrioventricular (AV) node

Part of AV junctional tissue. Slows conduction, creating a slight delay before impulses reach ventricles. Intrinsic rate 40-60 bpm.

Bundle of His

Transmits impulses to bundle branches. Located below AV node.

Left bundle branch

Conducts impulses that lead to left ventricle.

Right bundle branch

Conducts impulses that lead to right ventricle.

Purkinje system

Network of fibers that spreads impulses rapidly throughout ventricular walls. Located at terminals of bundle branches. Intrinsic rate 20-40 bpm.

Heart Electrical System

Electrical Conduction System of the Heart

Electrophysiology

Action

Effect

Depolarization

The electrical charge of a cell is altered by a shift of electrolytes on either side of the cell membrane. This change stimulates muscle fiber to contract.

Repolarization

Chemical pumps re-establish an internal negative charge as the cells return to their resting state.

Atrial depolarization

Ventricular Ventricular depolarization repolarization

Depolarization and repolarization of the heart.

y Clinical Tip: Mechanical and electrical functions of the heart are influenced by proper electrolyte balance. Important components of this balance are sodium, calcium, potassium, and magnesium.

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The Electrocardiogram (ECG)

■ An ECG is a series of waves and deflections recording the heart's electrical activity from a certain "view."

■ Many views, each called a lead, monitor voltage changes between electrodes placed in different positions on the body.

■ Leads I, II, and III are bipolar leads, which consist of two electrodes of opposite polarity (positive and negative). The third (ground) electrode minimizes electrical activity from other sources.

■ Leads aVR, aVL, and aVF are unipolar leads and consist of a single positive electrode and a reference point (with zero electrical potential) that lies in the center of the heart's electrical field.

■ Leads V-|-V6 are unipolar leads and consist of a single positive electrode with a negative reference point found at the electrical center of the heart.

■ Voltage changes are amplified and visually displayed on an oscilloscope and graph paper.

■ An ECG tracing looks different in each lead because the recorded angle of electrical activity changes with each lead.

■ Several different angles allow a more accurate perspective than a single one would.

■ The ECG machine can be adjusted to make any skin electrode positive or negative. The polarity depends on which lead the machine is recording.

■ A cable attached to the patient is divided into several different-colored wires: three, four, or five for monitoring purposes, or ten for a 12-lead ECG.

■ Incorrect placement of electrodes may turn a normal ECG tracing into an abnormal one.

y Clinical Tip: Patients should be treated according to their symptoms, not merely their ECG.

y Clinical Tip:To obtain a 12-lead ECG, four wires are attached to each limb and six wires are attached at different locations on the chest. The total of ten wires provides twelve views (12 leads).

Lead Ecg Placement
Standard Limb Leads
Standard Limb Leads

Elements of Standard Limb Leads

Lead

Positive Electrode

Negative Electrode

View of Heart

I

LA

RA

Lateral

II

LL

RA

Inferior

III

LL

LA

Inferior

Augmented Limb Leads

Mcl6 Ekg Lead Placement

Elements of Augmented Limb Leads

Lead

Positive Electrode

View of Heart

aVR

RA

None

aVL

LA

Lateral

aVF

LL

Inferior

Chest Leads

Standard Chest Lead Electrode Placement

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Midclavicular ^^ line

Anterior axillary line

Midaxillary yine

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Elements of Chest Leads

Lead

Positive Electrode Placement

View of Heart

V,

4th Intercostal space to right of sternum

Septum

V2

4th Intercostal space to left of sternum

Septum

V3

Directly between V2 and V4

Anterior

V4

5th Intercostal space at left midclavicular line

Anterior

V5

Level with V4 at left anterior axillary line

Lateral

Ve

Level with V5 at left midaxillary line

Lateral

Copyright'£'2005 F A Davis

17

| Electrode Placement Using a 3-Wire Cable

Three Electrode Placement

Electrode Placement Using a 5-Wire Cable

Mcl6 Ekg Lead Placement
V Clinical Tip: Five-wire telemetry units are commonly used to monitor leads I, II, III, aVR, aVL, aVF, and V1 in critical care settings.

Modified Chest Leads

■ Modified chest leads (MCL) are useful in detecting bundle branch blocks and premature beats.

■ Lead MCL1 simulates chest lead V1 and views the ventricular septum.

■ Lead MCL6 simulates chest lead V6 and views the lateral wall of the left ventricle.

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Lead MCL1 electrode placement.

Lead MCL6 electrode placement.

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y Clinical Tip: Write on the rhythm strip which simulated lead was used.

The Right-Sided 12-Lead ECG

■ The limb leads are placed as usual but the chest leads are a mirror image of the standard 12-lead chest placement.

■ The ECG machine cannot recognize that the leads have been reversed. It will still print "Vi-V6" next to the tracing. Be sure to cross this out, and write the new lead positions on the ECG paper.

Midclavicular

Midclavicular

Ecg Lead Position V7v8v9 Images Free
The Right-Sided 12-Lead ECG

Chest Leads

Position

4th Intercostal space to left of sternum

4th Intercostal space to right of sternum

Directly between V2R and V4R

5th Intercostal space at right midclavicular line

Level with V4R at right anterior axillary line

Level with V5r at right midaxillary line y Clinical Tip: Patients with an acute inferior MI should have right-sided ECGs to assess for possible right ventricular infarction.

y Clinical Tip: Patients with an acute inferior MI should have right-sided ECGs to assess for possible right ventricular infarction.

The 15-Lead ECG

Areas of the heart that are not well visualized by the six chest leads include the wall of the right ventricle and the posterior wall of the left ventricle. A 15-lead ECG, which includes the standard 12 leads plus leads V4R, Vs, and V9, increases the chance of detecting an MI in these areas.

Lead Ecg Indication

The 15-Lead ECG

Chest Leads

Electrode Placement

View of Heart

V4R

5th Intercostal space in right anterior midclavicular line

Right ventricle

Vs

Posterior 5th intercostal space in left midscapular line

Posterior wall of left ventricle

V9

Directly between Vs and spinal column at posterior 5th intercostal space

Posterior wall of left ventricle

y Clinical Tip: Use a 15-lead ECG when the 12-lead is normal but the history is still suggestive of an acute infarction.

Recording of the ECG

Constant speed of 25 mm'sei

Constant speed of 25 mm'sei

Mobitz Type
0.20 sec

Components of an ECG Tracing

Ecg Components

Electrical Components

Deflection

Description

Small rounded, upright (positive) wave indicating atrial depolarization (and contraction)

PR Interval

Distance between beginning of P wave and beginning of QRS complex Measures time during which a depolarization wave travels from the atria to the ventricles

QRS Interval

Three deflections following P wave Indicates ventricular depolarization (and contraction) Q Wave: First negative deflection R Wave: First positive deflection S Wave: First negative deflection after R wave

ST Segment

Distance between S wave and beginning of T wave

Measures time between ventricular depolarization and beginning of repolarization

T Wave

Rounded upright (positive) wave following QRS

Represents ventricular repolarization

QT Interval

Measured from beginning of QRS to end of T wave.

Represents total ventricular activity.

U Wave

Small rounded, upright wave following T wave

Most easily seen with a slow HR. Represents repolarization of Purkinje fibers.

Methods for Calculating Heart Rate

Heart rate is calculated as the number of times the heart beats per minute. It usually measures ventricular rate (the number of QRS complexes) but can refer to atrial rate (the number of P waves). The method chosen to calculate HR varies according to rate and regularity on the ECG tracing.

Method 1: Count Large Boxes

Regular rhythms can be quickly determined by counting the number of large graph boxes between two R waves. That number is divided into 300 to calculate bpm. The rates for the first one to six large boxes can be easily memorized. Remember: 60 sec/min divided by 0.20 sec/large box = 300 large boxes/min.

300 150 100 75 60 50

Counting large boxes for heart rate. The rate is 60 bpm.

Counting large boxes for heart rate. The rate is 60 bpm.

Method 2: Count Small Boxes

Method 2: Count Small Boxes

Sometimes it is necessary to count the number of small boxes between two R waves for fast heart rates. That number is divided into 1500 to calculate bpm. Remember: 60 sec/min divided by 0.04 sec/small box = 1500 small boxes/min.

Examples: If there are six small boxes between two R waves: 1500/6 = 250 bpm.

If there are ten small boxes between two R waves: 1500/10 = 150 bpm.

Methods 1 and 2 for Calculating Heart Rate

Number of Large Boxes

Rate/Min

Number of Small Boxes

Rate/Min

1

300

2

750

2

150

3

500

3

100

4

375

4

75

5

300

5

60

6

250

6

50

7

214

7

43

8

186

8

38

9

167

9

33

10

150

10

30

11

136

11

27

12

125

12

25

13

115

13

23

14

107

14

21

15

100

15

20

16

94

y Clinical Tip: Approximate rate/min is rounded to the next-highest number.

Method 3: Six-Second ECG Rhythm Strip

The best method for measuring irregular rates with varying R-R intervals is to count the number of R waves in a 6-sec strip and multiply by 10. This gives the average number of bpm.

Calculating Heart Rate Ekg Strip
Using 6-sec ECG rhythm strip to calculate heart rate. Formula: 7 x 10 = 70 bpm

y Clinical Tip: If a rhythm is extremely irregular, it is best to count the number of R-R intervals per 60 sec (1 min).

ECG Interpretation

Analyzing a Rhythm

Component

Characteristic

Rate

The bpm is commonly the ventricular rate. If atrial and ventricular rates differ, as in a

3rd-degree block, measure both rates. Normal: 60-100 bpm Slow (bradycardia): <60 bpm Fast (tachycardia): >100 bpm

Regularity

Measure R-R intervals and P-P intervals. Regular: Intervals consistent Regularly irregular: Repeating pattern Irregular: No pattern

P Waves

If present: Same in size, shape, position? Does each QRS have a P wave? Normal: Upright (positive) and uniform Inverted: Negative Notched: P'

None: Rhythm is junctional or ventricular.

PR Interval

Constant: Intervals are the same. Variable: Intervals differ. Normal: 0.12-0.20 sec and constant

QRS Interval

Normal: 0.06-0.10 sec Wide: >0.10 sec None: Absent

QT Interval

Beginning of R wave to end of T wave Varies with HR.

Normal: Less than half the R-R interval

Dropped beats

Occur in AV blocks. Occur in sinus arrest.

Component Characteristic

Pause Compensatory: Complete pause following a premature atrial contraction (PAC), premature junctional contraction (PJC), or premature ventricular contraction (PVC) Noncompensatory: Incomplete pause following a PAC, PJC, or PVC QRS Complex Bigeminy: Repeating pattern of normal grouping complex followed by a premature complex

Trigeminy: Repeating pattern of 2 normal complexes followed by a premature complex

Quadrigeminy: Repeating pattern of 3 normal complexes followed by a premature complex Couplets: 2 Consecutive premature complexes Triplets: 3 Consecutive premature complexes

Notes:

Sinoatrial (SA) Node Arrhythmias

■ Upright P waves all look similar. Note: All ECG strips in this tab were recorded in lead II.

■ PR intervals and QRS complexes are of normal duration.

Normal Sinus Rhythm (NSR)

s Rate: Normal (60-100 bpm)

_< Rhythm: Regular t P Waves: Normal (upright and uniform)

B PR Interval: Normal (0.12-0.20 sec)

Sinus Bradycardia

y Clinical Tip: A normal ECG does not exclude heart disease.

Sinus Bradycardia

■ Results from slowing of the SA node.

■ Results from slowing of the SA node.

Normal Sinus Rhythm Ekg

Rate: Slow (<60 bpm) Rhythm: Regular

P Waves: Normal (upright and uniform) PR Interval: Normal (0.12-0.20 sec) QRS: Normal (0.06-0.10 sec)

V Clinical Tip: Sinus bradycardia is normal in athletes and during sleep. In acute MI, it may be protective and beneficial or the slow rate may compromise cardiac output. Certain medications, such as beta blockers, may also cause sinus bradycardia.

Sinus Tachycardia

■ Results from increased SA node discharge.

Ekstrasistole

Rate: Fast (>100 bpm) Rhythm: Regular

P Waves: Normal (upright and uniform) PR Interval: Normal (0.12-0.20 sec) QRS: Normal (0.06-0.10 sec)

y Clinical Tip: Sinus tachycardia may be caused by exercise, anxiety, fever, hypoxemia, hypovolemia, or cardiac failure.

Sinus Arrhythmia

■ The SA node discharges irregularly.

■ The R-R interval is irregular.

Rate: Usually normal (60-100 bpm); frequently increases with inspiration and decreases with expiration

Rhythm: Irregular; varies with respiration | P Waves: Normal (upright and uniform) J PR Interval: Normal (0.12-0.20 sec) QRS: Normal (0.06-0.10 sec)

© y Clinical Tip:The pacing rate of the SA node varies with respiration, especially in children and elderly people.

Sinus Pause (Sinus Arrest)

■ The SA node fails to discharge and then resumes.

■ Electrical activity resumes either when the SA node resets itself or when a lower latent pacemaker begins to discharge.

■ The pause (arrest) time interval is not a multiple of the normal P-P interval.

Sinus Pause Duration

Rate: Normal to slow; determined by duration and frequency of sinus pause (arrest) | Rhythm: Irregular whenever a pause (arrest) occurs B P Waves: Normal (upright and uniform) except in areas of pause (arrest) t PR Interval: Normal (0.12-0.20 sec) | QRS: Normal (0.06-0.10 sec)

Sinoatrial (SA) Block

■ The block occurs in some multiple of the P-P interval.

■ After the dropped beat, cycles continue on time.

Normal Ekg With Drop Beat

Rate: Normal to slow; determined by duration and frequency of SA block Rhythm: Irregular whenever an SA block occurs

P Waves: Normal (upright and uniform) except in areas of dropped beats PR Interval: Normal (0.12-0.20 sec) QRS: Normal (0.06-0.10 sec)

Atrial Arrhythmias

■ P Waves differ in appearance from sinus P waves.

■ QRS Complexes are of normal duration.

Wandering Atrial Pacemaker (WAP)

■ Pacemaker site transfers from the SA node to other latent pacemaker sites in the atria and the AV junction and then moves back to the SA node.

■ Pacemaker site transfers from the SA node to other latent pacemaker sites in the atria and the AV junction and then moves back to the SA node.

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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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Responses

  • toni
    What are the structures of the sternum?
    8 years ago
  • BALBO
    How to calculate sinus pause pic?
    3 years ago

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