vert physio - the ECG

gap junctions

connections between myocardial cells

contractile myocardial cells

contain abundant actin and myosin; create force of contractions

autorhythmic myocardial cells

non-contractile (“pacemaker”); contain little actin and myosin; unstable resting membrane potentials that cause them to spontaneously generate APs

intrinsic conduction system

connected to autorhythmic cells; population of highly modified myocardial cells that distribute AP to myocardium in coordinated manner, resulting in 3D contraction

cardiac cycle step 1

atrial contraction/ atrial systole; fills ventricles

cardiac cycle step 2

ventricular systole: isovolumetric contraction; pressure begins to mount in ventricles, but not high enough to open semilunar valves

cardiac cycle step 3

ventricular systole part 2: ventricular ejection

cardiac cycle step 4

ventricular diastole: second isovolumetric period: relaxation; pressure drops, blood volume constant

cardiac cycle step 5

ventricular filling; pressure on atrioventricular valves high enough for passive fill

cardiac cycle step 6/1

atrial systole and active filling

sinoatrial node

½ of start of cardiac cycle with concentrated autorhythmic cells in right atrium; depolarizes faster than AV node; drives heart rate; AP causes atria to contract and push blood to ventricles

atrioventricular node

AP carried here from SA node; serves as a regulator (delay in conduction between AV node and AV bundle)

AV bundle

electrical connection between atria and ventricles

AP travel in heart

SA node → AV node → AV bundle → ventricular muscle → L and R bundle branches → Purkinje fibers

ECG Lead I

records activity from upper left and right chest on horizontal axis

ECG Lead II

records activity between upper L chest to lower chest

ECG Lead III

records activity between upper R chest and lower chest

aVR, aVL, and aVF leads

trigonometrically calculated from voltages on leads I-III

Einothoven’s triangle

arrangements of leads

P wave

atrial systole - contraction of atrial muscle following depolarization; last 100ms

flat line following P wave

PR interval; time when L and R atria both completely depolarized (isoelectric, systole); R and L atrial contraction

QRS complex

ventricular systole; contraction of ventricles following rapid depolarization; finish before T wave begins; lasts 270 ms

T wave

repolarization of ventricles

normal sinus rhythm (NSR)

normal state with SA node as lead pacer; 60-100 BPM

sinus bradycardia

same as NSR but rate <60 BPM; vagal stimulation leading to nodal slowing, or medicine; conditioned athletes

sinus tachycardia

same as NSR, but rate >100 BPM; medications, exercise, etc

asystole

when heart’s electrical system ceases functioning; heart stops

atrial fibrillation

chaotic firing of numerous intrinsic conduction cells in atria in haphazard fashion; no discernible P waves, QRS complexes irregular

atrial flutter

from short circuit in heart, electrical current circulates through R atrium quickly

first degree AV block

from prolonged block in signal conduction to AV node; medication, vagal stimulation, disease, etc.; PR interval > 0.2 seconds

junctional rhythm

SA node is non-functional and cannot initiate normal pace making; HR becomes firing rate of AV node; no interval between P wave and QRS; 40-60 BPM

ventricle tachycardia

very fast ventricular rate with wide QRS complex; similar to atrial flutter, but fast ventricle firing; 100-200 BPM

ventricular fibrillation

“cardiac chaos”; ventricular pacers firing at own pace with no organized contraction

premature atrial contraction

when some pacemaker cell in atria fires before SA node; complex that comes sooner than expected

premature ventricular contraction

premature firing of ventricular cell, before normal SA node

isovolumetric relaxation

ventricular diastole: relaxation of ventricles; 50mL blood into each ventricle; semilunar and AV valves closed

ventricular filling

75% of ventricular filling occurs as blood flows through atria and AV valves; all chambers relaxed

myocardial contractile cells

majority of cells in atria and ventricles; responsible for contraction

myocardial conducting cells

autorhythmic cells that form conduction system; similar to neurons

intercalated discs

at junctions between cells

gap junctions

channel between muscle fibers for passage of cations and spread of APs

desmosome

anchors muscle fibers together

sinoatrial node

myocardial conducting cells; superior and posterior right atrium; highest inherent rate of depolarization; initiates sinus rhythm

internodal pathways

conduct signal from SA node to AV and atrial mocardia

Bachmann’s bundle/ interatrial band

conducts from R to L atrium

atrioventricular node

myocardial conductive cells on inferior right atrium; impulse must pass through AV node before ventricles; slow transmission - small diameter

atrioventricular bundle

travels from AV node to interventricular septum

atrioventricular bundle branches

travel to L and R

Purkinje fibers

extend from the apex and spread impulse to myocardial contractile cells

P wave: atrial depolarization

isoelectric: atrial depolarization complete; atria contract; impulse delayed at AV node

QRS complex: impulse to heart apex; ventricular depolarization; atrial repolarization (obscured by QRS)

isoelectric; ventricular depolarization complete; ventricles contract

T wave; ventricular repolarization

1

Bachmann’s bundle

4

Purkinje fibers

5

left bundle branch

6

right bundle branch

7

bundle of His

8

AV node

9

SA node