Lecture Notes- Heart

Cardiac Conduction

(1)The sinoatrial (SA) node and the remainder of the conduction system are at rest. (2)The SA node initiates the action potential, which sweeps across the atria. (3) After reaching the atrioventricular node, there is a delay of approximately 100 ms that allows the atria to complete pumping blood before the impulse is transmitted to the atrioventricular bundle. (4) Following the delay, the impulse travels through the atrioventricular bundle and bundle branches to the Purkinje fibers, and also reaches the right papillary muscle via the moderator band. (5) The impulse spreads to the contractile fibers of the ventricle. (6) Ventricular contraction begin

Defibrillators

(a) An external automatic defibrillator can be used by nonmedical personnel to reestablish a normal sinus rhythm in a person with fibrillation. (b) Defibrillator paddles are more commonly used in hospital settings.

Tachycardia

+ 100 beats/min

Heart contraction is intrinsic but there are two controlling systems

- Autonomic Nervous System - Intrinsic Conduction system: ensures sequential depolarization of the heart making sure that the heart beats as a coordinated unit.

Congestive heat failure is a progressive condition where the CO is so low that blood circulation is inadequate to meet tissue needs. It is caused by:

-Coronary atherosclerosis -Persistent high blood pressure -Multiple myocardial infarcts -Dilated cardiomyopathy (DCM)

isovolumetric relaxation phase

-ventricular diastole begins -AV valves are still closed -atrial diastole continues -SL valves close

The atrioventricular (AV) node delays the impulse approximately ____ second

0.1

Three main factors affect SV

1. Preload 2. Contractility 3. Afterload

3 Layers of the Heart Wal

1. The epicardium on the outside is the visceral layer of the serous pericardium. 2. The myocardium is the cardiac muscle layer which forms most of the heart. 3. The endocardium is the smooth inner layer which reduces friction

To keep oxygenated and deoxygenated blood flowing the heart has two pumping systems

1. The pulmonary circuit pumps blood through the lungs for oxygenation. 2. The systemic circuit sends oxygenated blood through the body

phases of cardiac cycle

1. ve

Junctional Rhythm

40-60 Regular! -impulse from AV node w/ retro/antegrade transmission - P wave often inverted/buried/follow QRS - slow rate - narrow QRS (not wide like ventricular)

Normal adult blood volume about ____ liters

5

The sinoatrial (SA) node generates impulses about ____ times/minute

75

electrocardiogram

A normal tracing shows the P wave, QRS complex, and T wave. Also indicated are the PR, QT, QRS, and ST intervals, plus the P-R and S-T segments

Dual System of the Human Blood Circulation

Blood flows from the right atrium to the right ventricle, where it is pumped into the pulmonary circuit. The blood in the pulmonary artery branches is low in oxygen but relatively high in carbon dioxide. Gas exchange occurs in the pulmonary capillaries (oxygen into the blood, carbon dioxide out), and blood high in oxygen and low in carbon dioxide is returned to the left atrium. From here, blood enters the left ventricle, which pumps it into the systemic circuit. Following exchange in the systemic capillaries (oxygen and nutrients out of the capillaries and carbon dioxide and wastes in), blood returns to the right atrium and the cycle is repeated.

Cardiac Reserve

Cardiac Reserve is resting minus maximal cardiac output 1. Nonathletes - 4 or 5 x normal 2. Athletes - Up to 7 x normal

Major Factors Influencing Cardiac Output

Cardiac output is influenced by heart rate and stroke volume, both of which are also variable.

Regulation of Stroke Volume: CONTRACTILITY

Contractility refers to the contractile strength at a givenmuscle length, independent of muscle stretch and EDV. • Positive inotropic agents increase contractility - Increased Ca2+ influx due to sympathetic stimulation -Hormones (thyroxine, glucagon, and epinephrine) •Negative inotropic agents decrease contractility -Acidosis -Increased extracellular K+ -Calcium channel blockers

systole

Contraction of the heart

Frank-Starling Law

Critical factor controlling stroke volume is stretch of cardiac muscle

myocardial infarction

Heart attack; Death of cardiac muscle

CPR Technique

If the heart should stop, CPR can maintain the flow of blood until the heart resumes beating. By applying pressure to the sternum, the blood within the heart will be squeezed out of the heart and into the circulation. Proper positioning of the hands on the sternum to perform CPR would be between the lines at T4 and T9

Standard Placement of ECG Lead

In a 12-lead ECG, six electrodes are placed on the chest, and four electrodes are placed on the limb

Relationship between the Cardiac Cycle and ECG

Initially, both the atria and ventricles are relaxed (diastole). The P wave represents depolarization of the atria and is followed by atrial contraction (systole). Atrial systole extends until the QRS complex, at which point, the atria relax. The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction. The T wave represents the repolarization of the ventricles and marks the beginning of ventricular relaxation.

Hypoxia

Low oxygen saturation of the body, not enough oxygen in the blood

Heart Sounds

Lub-dub. 1st- a-v valves close. 2nd- aortic and pulmonary valves close

Intervals

PQ - 0.16 sec. - time between atrial excitation and ventricular excitation QT - 0.365 sec. - Time between beginning of ventricular depolarization and repolarization; includes ventricular contraction

Stethoscope Placement for Auscultation

Proper placement of the bell of the stethoscope facilitates auscultation. At each of the four locations on the chest, a different valve can be heard

path of impulse

SA node AV node AV Bundle Bundle branches Purkinje fibers

Regulation of Stroke Volume (SV)

SV = EDV* -ESV* 120 ml - 50 ml = 70 ml/beat EDV = amount of blood collected in a ventricle during diastole ESV = amount of blood remaining in a ventricle after contraction

Conduction System of the Heart

Specialized conducting components of the heart include the sinoatrial node, the internodal pathways, the atrioventricular node, the atrioventricular bundle, the right and left bundle branches, and the Purkinje fibers

Sympathetic Nervous System (SNS)

The component of the autonomic nervous system that responds to stressful situations by initiating the fight-or-flight response.

Chambers of the Heart

The heart has four chambers: two atria and two ventricles

SA node

The pacemaker of the heart, located in the right atrium. Serves as the stimulus for contraction since it has highest discharge rate. It sets rate of depolarization

Development of the Human Heart

This diagram outlines the embryological development of the human heart during the first eight weeks and the subsequent formation of the four heart chambers

Chemical Regulation

a. The hormones epinephrine and thyroxine increase heart rate. b. Ions must be present in the proper amounts. Imbalances lead to serious problems. c. Hypocalcemia (low calcium levels decreases heart rate. d. Hyperkalemia may lead to heart block and cardiac arrest . It disrupts depolarization by lowering the resting potential making depolarization difficult.

arrhythmias

abnormal heart rhythms

ectopic focus

abnormal pacemaker

Other factors that influence heart rate

age, gender, exercise, body temperature

isovolumetric contraction phase

all valves are closed

dicrotic notch

brief rise in aortic pressure caused by backflow of blood rebounding off semilunar valves

Purkinje fibers

carry the impulse to the heart apex and ventricular walls

bundle branches

carry the impulse toward the apex of the heart

Coronary Circulation

circulation of blood through the coronary blood vessels to deliver oxygen and nutrients to the heart muscle tissue

electrocardiography

electrical currents which spread through body as heart beats are measured as an electrocardiogram

Heart Valves

ensure unidirectional blood flow through the heart

Action Potential at the SA Nod

he prepotential is due to a slow influx of sodium ions until the threshold is reached followed by a rapid depolarization and repolarization. The prepotential accounts for the membrane reaching threshold and initiates the spontaneous depolarization and contraction of the cell. Note the lack of a resting potential

Damage to AV node leads to

heart block which interferes with the ability of the ventricles to receive signals from the SA node

The heart is located

in the mediastinum and occupies an area extending from the second rib to fifth intercostal space. The organ is tipped to left with the base pointed toward the right shoulder and the apex pointing toward hip and resting on diaphragm. The heart is located within the thoracic cavity, medially between the lungs in the mediastinum. It is about the size of a fist, is broad at the top, and tapers toward the base

ischemia

lack of blood flow

bradycardia

less than 60 beats/min

Atrioventricular (AV) valves

lie between the atria and the ventricles and prevent backflow into the atria when ventricles contract.

Cardiac Muscle

made up of branched, multinucleate fibers. Intercalated discs mark the boundaries between cells

total heart block

no signal passes through and the ventricle beats at its intrinsic rate (too slow). In partial heart block, some of the impulses get through. In either case, a pacemaker is used to regulate the beating of the heart.

a second-degree or partial block

one- half of the P waves are not followed by the QRS complex and T waves while the other half are

ventricular filling phase

period during which blood drains from atria into ventricles

pectinate muscles

prominent muscular ridges along the inner surface of the auricle and across the adjacent anterior atrial wall

fibrillation

rapid, random, and ineffective contractions of the heart

Regulation of Stroke Volume - PRELOAD

refers to the degree of stretch of cardiac muscle cells before they contract (Frank-Starling law of the heart). Cardiac muscle exhibits a length-tension relationship. At rest, cardiac muscle cells are shorter than optimal length. Slow heartbeat and exercise increase venous return. Increased venous return distends (stretches) the ventricles and increases contraction force

Regulation of Stroke Volume: AFTERLOAD

refers to the pressure that must be overcome for ventricles to eject blood. Hypertension increases afterload, resulting in increased ESV and reduced SV

Diastole

relaxation of heart muscle

A-V (atrioventricular) node

specialized cardiac tissue that provides normal conduction pathway between atrial and ventrizular synctia

AV bundle

splits into two pathways in the interventricular septum (bundle branches)

Parasympathetic nervous system (PNS)

stimulation is mediated by acetylcholine and opposes the SNS

congenital heart defects

structural abnormalities caused by the failure of the heart to develop normally before birth

Cardiac Output (CO)

the amount of blood pumped/ventricle / minute CO = Heart Rate* x Stroke Volume (SV) CO = 75 beats/min x 70 ml/beat= 5.2L/min *number of beats per minute

atrial fibrillation

the electrical pattern is abnormal prior to the QRS complex, and the frequency between the QRS complexes has increase

Atria

the receiving chambers of the heart. Each atrium has a protruding auricle which increases its volume Blood enters right atria from superior and inferior venae cavae and coronary sinus Blood enters left atria from pulmonary veins

ventricular tachycardia

the shape of the QRS complex is abnormal

Ventricles

the two lower chambers of the heart, and they pump blood out to the lungs and body.

third-degree block

there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex)

ventricular fibrillation

there is no normal electrical activity

The pericardium serves to:

• Protect and anchor the heart • Prevent overfilling of the heart with blood • Allow for the heart to work in a relatively friction-free environment

Right and left pulmonary veins Vessels carrying blood away from the heart include:

• Pulmonary trunk, which splits into right and left pulmonary arteries • Ascending aorta (three branches) - brachiocephalic, left common carotid, and subclavian arteries

Vessels returning blood to the heart include:

• Superior and inferior venae cavae • Right and left pulmonary veins

The pericardium is a double-walled sac surrounding the heart. It is composed of a superficial fibrous pericardium and a deep two-layer serous pericardium:

• The parietal layer lines the internal surface of the fibrous pericardium. • The visceral layer or epicardium lines the surface of the heart. • They are separated by the fluid-filled pericardial cavity