Lab 5 - Heart Physiology & Electrocardiography (EKG)

Electrocardiogram (ECG or EKG)

Pattern of electrical activity recorded at the body surface.

Sinoatrial (SA) node

Acts as a pacemaker for the heart, auto-depolarizing at the greatest rate among pacemaker cells, generating sinus rhythm.

Atrioventricular (AV) node

The only electrical connection between the atria and the ventricles, allowing atrial contraction to contribute to ventricular filling.

Cardiac Action Potential

Consists of rapid depolarization, plateau depolarization, and repolarization phases.

P wave

Produced by atrial depolarization, occurring before atrial systole/contraction.

QRS complex

Produced by ventricular depolarization, with atrial depolarization also happening concurrently.

T wave

Produced by ventricular repolarization, occurring before ventricular diastole/relaxation.

Systolic pressure

Peak pressure during the cardiac cycle.

Diastolic pressure

Arterial blood pressure at its lowest before ventricular contraction pushes blood into the arteries.

Diastole

Period of ventricular relaxation.

Dicrotic notch

Small plateau or dip in the pressure wave.

Heart sounds

Audible vibrations produced by valve closures in the heart.

Lub-dup sound

"Lub" sound occurs during early ventricular contraction, "dup" sound occurs when semilunar valves close.

Sinoatrial Node

Located in the right atrial wall, sets the rhythm of the heart at roughly 75 times per minute, known as the heart's pacemaker.

Atrioventricular Node

Located in the inferior segment of the interatrial septum, it briefly delays the signal from the SA node while the atria finish contracting.

delayed because fewer gap junctions and smaller diameter of conduction fibers

Atrioventricular Bundle

Links atrial conduction to ventricular conduction by receiving signals from the AV node.

located in the superior portion of the interventricular septum

Bundle Branches

Located along the left and right sides of the interventricular septum, these branches receive signals from the AV bundle and send them to the apex of the heart.

Purkinje Fibers

Penetrate into the apex and run into the ventricular walls, these fibers facilitate the bulk of ventricular excitation.

receives signal from the bundle branches and completes the conduction cycle

purkinje network is more elaborate in the left chamber

Electrocardiogram (ECG)

The sum of all electrical potentials generated by the cells of the heart at any time, providing key information about heart rhythm.

normal electrical activity of the heart gives 3 distinct waves

P wave

Represents atrial depolarization and results from the signal from the SA node spreading through the atria.

QRS Complex

Represents ventricular depolarization and includes atrial repolarization, resulting from Purkinje fibers exciting the ventricular walls.

T wave

Represents ventricular repolarization caused by Purkinje fibers repolarizing the ventricular walls.

Cardiac Output (CO)

The amount of blood pumped out of each ventricle in a minute, calculated as heart rate multiplied by stroke volume. ( CO = HR x SV)

Stroke Volume (SV)

The volume of blood pumped out by one ventricle with each beat.

correlates with strength of ventricle (SV = EDV - ESV)

End diastolic volume (EDV)

Amount of blood that collects in a ventricle during diastole.

End systolic volume (ESV)

Amount of blood remaining in a ventricle after it has contracted.

Dicrotic notch

A small increase in blood vessel diameter due to a pressure wave generated by blood rebounding off the semilunar valves.

Pacemaker potential

The continual depolarization of autorhythmic cells due to the gating kinetics of the ion channels.

Gap junctions

Structures that allow electrical signals to spread throughout the heart's muscle cells.

Mechanical event

The physical contractile actions of the heart following electrical events of depolarization and repolarization.

ECG lead

A pair of electrodes (one negative, one positive) used to measure ECG signals.

Interatrial septum

The wall dividing the right and left atria of the heart.

Interventricular septum

The wall dividing the right and left ventricles of the heart.

Cardiac Conduction System

1. Sinoatrial Node

2. Atrioventricular Node

3. Atrioventricular bundle

4. Bundle branches

5. Purkinje fibers

Depolarization leads to

contraction

Repolarization leads to

relaxation

Contraction ___ pressure in the respective chamber vice versa for relaxation

increases

Atrioventricular valves __ when ventricular pressure is higher than atrial pressure

close

Semilunar valves ___ when arterial (aorta/pulmonary trunk) pressure exceeds ventricular pressure

close

Blood moves from

an area of high pressure to an area of low pressure

Standard limb leads

I, II, III

Augmented limb leads

aVR, aVL, aVF

Unipolar chest leads

V1 - V6

Direction of electrical vector in the heart determines the ECG trace

1. positive

2. negative

3. isoelectric

Left heart

Blood enters atrial chambers of the heart at

low pressure

Blood leaves the ventricles at

higher pressure

high arterial pressure provides

the energy to force blood through the circulatory system

arteries carry blood __ the heart

AWAY

veins carry blood __ the heart

TOWARDS

Are arteries or veins generally more pressurized ?

arteries

heart is enclosed within

Mediastinum - medial cavity of the thorax

apex is located

near the fifth intercoastal space

heart enclosed in sac called

pericardium

fibrous pericardium

anchors, protects, and prevents over filing of the heart

inner serous pericardium

thin, slippery, dual-layered membranous sac around the heart

2 layers - parietal layer, visceral layer (epicardium)

parietal layer

butts up against the fibrous pericardium

visceral layer (epicardium)

integral part of the cardiac wall

pericardial cavity

between the parietal and visceral layers is filled with serous fluid which allows these two layers to slide past one another as the heart beats

Pericarditis

inflammation of the pericardium

can lead to cardiac tamponade

cardiac tamponade

buildup of inflammatory fluids in the pericardial cavity which compresses and impedes the heart

myocardium

thick cardiac muscle layer of the heart

cardiac “skeleton”

branching cardiac muscles cells are arranged in interlacing spiral bundles tethered by crisscrossing collagen and elastic connective tissue fibers

! strengthens and anchors the heart to itself

endothelial endocardium

innermost layer of the heart; lines the heart valves and heart chambers

auricles of atria

allow for additional filling

Muscular ridges of the atria and ventricles

pectinate muscles (ATRIA); trabeculae carnae (VENTRICLES)

fetus exchanges gases where

placenta NOT the lungs

Why does all fetal blood not need to pass through the lungs ?

foramen ovale - opening connects the right and left atrium (allows most blood to bypass the pulmonary circuit)

fossa ovalis

impression of foramen ovale in the interatrial septum

patent foramen ovale

still open; common in adults but usually small and inconsequential

ductus arteriosus

connects the pulmonary trunk and the aorta

patent ductus arteriosus

if left untreated can lead to congestive heart failure

Some congenital heart defects

(congenital - present at birth)

Holes in the interventricular septum, valve malformations, and transpositions of the great vessels of the heart

Papillary muscles in the ventricles

link to atrioventricular valves via the chordae tendinae and function to keep the valves from everting into the atria after they shut

Why is the wall of the left ventricle more massive than the right ?

left ventricle pumps to almost the entire body against a heavy friction load while the right ventricle only pumps to the lungs which is a low pressure, low resistance, low friction circuit

pathological stenosis of the pulmonary vessels

increases pressure in the pulmonary circulation fed by the right ventricle which is not built to pump under high pressure like the left side of the heart is

! can lead to congestive heart failure

where does the heart get a majority of its nourishment from and why?

coronary circulation

• heart is too thick for the blood within the chambers to adequately supply the cardiac cells

angina pectoralis

blockage in the coronary arterial circulation fleeting insufficiency of blood supply to the myocardium resulting in chest pain

myocardial infarction (heart attack)

prolonged coronary arterial blockage resulting in an oxygen and nutrient deficit in the supplied myocardium and subsequent death of these cells

cardiac cells

do not regenerate; tissue is replaced with a non-contractile scar tissue

because the cardiac cells are connected by gap junctions

the conduction system is critical to first start the contraction at the cranial portion of the atria and move down the wrong blood into the ventricles, and then rapidly shunt the signal to the apex of the heart do that the contraction will spread up to wring blood out of the ventricles and into the great vessels

pacemaker cells of the cardiac conduction system

have an unstable resting membrane potential that continuously discharges in the direction of depolarization until it reaches threshold which activates calcium channels to open and at that point generates an action potential of depolarization

• more and more as time passes, due to slow sodium channels that open in response to hyperpolarization at the end of an action potential

shifting membrane potentials

termed pacemaker potential

repolarization

the falling phase

due to the opening of potassium channels which remain open until hyperpolarization closes them and opens slow sodium channels to begin the porcess again

cardiac contractions

not dependent upon nerve supply

innervation by parasympathetic vagus nerve slows the heart, and sympathetic nerves speed the basic cardiac rhythm

! CNS can affect rhythm

sinus arrythmia

respiratory activity affects the heart rate to increase as you breathe in and decrease as you breathe out

sinus rhythm

generated by SA node because the cells auto-depolarize at the greatest rate

only electrical connection between the atria and the ventricles is via the

atrioventricular (AV) node

arrhythmia

irregular heart rhythm

can be caused by defects in the conduction system

fibrillate

rapid irregular contractions not driven by SA node but instead by rapid activity in other regions of the heart

junctional rhythm

slower, no P wave

excessive use of caffeine or nicotine can cause

small regions of the heart to become hyperexcitable and outpace the SA node

resulting in premature contraction and extra time needed for the heart to fill before next contraction

heart block

ventricles receive deficient pacing signals due to damage at AV node or purkinje fibers

often treated with an implantable artificial pacemaker

second degree heart block

some SA node impulses do not conduct through the AV node

resulting in more P waves than QRS complexes on an ECG

total heart block

ventricles just beat on their own, way too slow, intrinsic rhythm

cardiac cycle

involves sequential contraction of the atria and ventricles; combined electrical activity of the different myocardial cells produces electrical currents that spread through the body fluids (large enough to be detected by recording electrodes placed on the skin)

cardiac action potential

3 phases

1. rapid depolarization

2. plateau depolarization

3. repolarization

rapid depolarization

as a result of fast voltage gated sodium channels opening in the sarcolemma

plateau depolarization

very obvious in ventricular fibers and caused by slow voltage gated calcium channels opening thus prolonging the depolarization