Cardiac Cycle Flashcards
Describe the sino-atrial action potential with reference to the pacemaker potential.
Phase 4 - hyperpolarisation-activated Na+ channels open, conducting funny current - ‘If’. Influx of Na+ causes slow depolarisation = pacemaker potential.
Phase 0 - threshold is reached and VGCCs open. Influx of Ca2+ causes rapid depolarisation - AP upstroke.
Phase 3 - voltage-gated K+ channels open. K+ efflux causes repolarisation.
Describe the atrioventricular action potential.
Phase 0 - voltage-gated Na+ channels open. Na+ influx causes rapid depolarisation - AP upstroke. VGCCs start to open.
Phase 1 - Na+ channels close - early repolarisation.
Phase 2 - VGCCs open fully. Voltage-gated K+ channels start to open. Ca2+ influx balanced by K+ efflux, causing plateau phase.
Phase 3 - VGCCs close. K+ channels open fully. K+ efflux causes rapid repolarisation.
Phase 4 - Na-K-ATPase establishes resting phase by pumping Na+ out and K+ in. Membrane impermeable to Na+ but permeable to K+.
Describe electrical conduction through the heart.
- Electrical activity in SAN spreads out via gap junctions to atria.
- At AV node, conduction is delayed to allow correct filling of ventricles.
- Conduction occurs rapidly through Bundle of His into ventricles.
- Conduction through Purkinje fibres spreads quickly throughout ventricles.
- Ventricular contraction begins at the apex.
What are the relative durations of systole and diastole?
Systole = 0.34 seconds Diastole = 0.66 seconds
What are the mean systolic and diastolic pressures in the aorta and pulmonary arteries?
Aorta = 120/80 mmHg
Pulmonary arteries = 25/10 mmHg
What are the mean systolic and diastolic pressures in the left and right ventricles?
Left ventricle = 120/80 mmHg
Right ventricle = 25/5 mmHg
Describe the pressure and volume changes during the cardiac cycle.
- Ventricular filling/atrial systole - pressure in atria > ventricles. AV valves (tricuspid + mitral) open and blood moves into the ventricles. Atrial contraction adds final 20% to the end diastolic volume (EDV).
- Isovolumetric contraction - the ventricles begin to contract, with the resulting increase in pressure causing the AV valves to shut.
- Ejection - pressure in left ventricle > aorta. Semilunar valves open and blood is ejected from the ventricles. Atria start to refill.
- Isovolumetric relaxation - pressure in the arteries > ventricles. Semilunar valves close. Ventricles relax and pressure decreases to 0 mmHg.
What is stroke volume?
SV = EDV - ESV
What is the consequence of an ejection fraction below 2/3?
Potential heart failure.
What are average EDV, ESV and SV in a healthy heart?
EDV = 120ml, ESV =40ml, SV = 120 - 40 = 80ml
Ventricular systole encompasses which phases of the cardiac cycle?
Isovolumetric contraction + ejection.
Describe the right atrial cycle and the jugular venous pressure changes that are seen in the neck.
A wave - tricuspid valve open, atrium contracts.
C wave - tricuspid valve closes, but right ventricular contraction causes bulging of tricuspid valve into atrium.
X descent - tricuspid valve closed, atrium relaxing and filling.
V wave - atrium tense and full due to venous filling, tricuspid valve still closed.
Y descent - atrium empties into ventricle, tricuspid valve forced open by pressure gradient.
X and Y are seen in the internal jugular vein.
What does the area inside the ventricular pressure-volume loop denote?
Area = stroke work done, as change in pressure x change in volume = work
Explain the origin of the first two heart sounds during the cardiac cycle.
S1 “Lubb” - closure of mitral and tricuspid valves during isovolumetric contraction.
S2 “Dupp” - closure of aortic and pulmonary semilunar valves during isovolumetric relaxation.
What causes the 3rd heart sound and how frequently is it heard?
S3 - turbulent blood flow into ventricles. Heard occasionally.
