Cardiac cycle

pacemaker-capable tissues in order of rate (fast to slow)

-SA node

-AV node

-bundle of His

-Purkinje fibres

what is the pathway of the impulse that travels through the heart

-SA node → atrial muscle → AV node → Bundle of His → left and right bundle branches → purkinje fibres → ventricular muscle

what is the sinoatrial (SA) node

-small cluster of specialised cells

-in top corner of atrium

-provides heart with spontaneous electrical activity to drive beat

what are the key features of SA node

-diastolic depolarisation

-ap fires once membrane potential reaches threshold, doesnt need external input

-depolarises spontaneously faster than any other pace maker region

what is diastolic depolarisation

-unstable resting membrane potential

-slowly depolarises between beats

what makes SA nodes intrinsic rate high

-vagus nerve is constantly applying a slight brake

-req sympathetic stimulation

what is special about SA to AV conduction

-slow

-travels through ordinary atrial muscle

-not specialist tissue

what is special about AV node

-introduces a deliberate delay (AV pause)

-gives ventricle time to fill with blood before contracting

-acts as filter; prevents dangerous high atrial rates from being passed to ventricles

what is special AV node to ventricular muscle

-fast

-Bundle of His and Purkinje fibres are specialist conducting tissue

-speeds the impulse to the apex first

-so the ventricle contracts apex-to-base; pushing blood upward and out through the aorta and pulmonary artery

what does ECG measure

-measures potential difference between 2 electrodes places on body surface

-one is a reference and other is a recording electrode

how can ECG measure electrical activity from surface of body

-wave depolarisation spreading through heart creates a moving electrical dipole

-wave of positivity followed by wave of negativity

-can be detected from surface body using electrodes

-body’s fluid conducts electricity

who pioneered ECG

-Augustus Waller created it

-Willem Einthoven improved it

what are the different electrode positions

-Lead I: right arm to left arm

-Lead II: right arm to left foot

-Lead III: left arm to left foot

what changes when you change electrode positions

-timing of waves stay same

-shape of wave changes

what are the different ECG waves

-P wave

-Q wave

-R wave

-S wave

-T wave

what is the P wave

-atrial depolarisation

-impulse spreads from SA node across both atria

-SA node too small to appear on ECG

-large atrial muscle activating is shown

what is Q wave

-first part of ventricular depolarisation

-septum depolarises first

-wave spreads away form recording electrode

-gives small downward deflection

what is R wave

-main ventricular depolarisation

-spreads towards apex (also towards recording electrode)

-gives large upward deflection

what is S wave

-depolarisation spreading back up walls of ventricle towards base

-moves away from electrode

-small downwards deflection

what is T wave

-ventricular repolarisation

-atria also repolarises

-but hidden underneath QRS complex

what is the PQ interval

-time from atrial depolarisation to ventricular depolarization

-shows atrial conduction and AV nodal delay

what does it mean if PQ interval is taking too long

-AV is blocked

-impulse is taking too long or failing to get through to AV node

what is QRS duration

-how quickly depolarisation spreads through ventricles via conduction

what does it mean if QRS duration is long

-bundle branch is blocked

-damage to His bundle or bundle branches

what is ST segment

-period when entire ventricle is depolarised

-no dipole is moving so lien is flat on baseline

what does it mean if ST segment is elevated

-part of ventricle has no AP

-sign of myocardial infraction

what is QT interval

-from ventricular depolarization to repolarisation

-equivalent to AP duration

what does it mean if QT interval is long

-long QT syndrome

what are the AV valves

-between atria and ventricles

-mitral valve (bicuspid): left atrium to left ventricle

-tricuspid valve: right atrium to right ventricle

what are semilunar valves

-at outflow tracts

-aortic valve: left ventricle to aorta

-pulmonary valve: right ventricle to pulmonary artery

how do valves open and close

-pressure driven

-if pressure is higher upstream side: valve opens, blood flows through

-if pressure is higher on downstream side: valve snaps shut

pressure volume changes in cardiac cycle

-A pressure is bit higher than V pressure

-mitral valve opens → blood flows into v → vol rises

-P wave fires → a contract → final top up of blood into v → vol inc (EDV)

-QRS fires → v starts contracting → v pressure rises

-a pressure rises → mitral valve shuts

-v pressure less then a pressure ; a valve stays shut

-both valves shut; pressure rises (isovolumic contraction)

-v pressure more than a pressure: a valve opens

-blood ejected into aorta; vol falls

-v relaxes; v pressure less than a pressure

-a valve shuts

-v pressure more than a pressure; mitral valve stays shut

-v pressure falls below a pressure → mitral valve opens -? v refills

what is EDV

-end diastolic volume

what is dicrotic notch

-small bump in a pressure trace

-v pressure less than a pressure

-a valve shuts

how to find stroke vol from this graph

-EDS - ESV

how to find ejection fraction from this graph

-stroke vol / EDV x 100

how to find stroke work

-area enclosed by loop