L06 CVS Physiology 2 Cardiac cycle

Overview

The lecture covers one complete cardiac cycle: what happens mechanically, electrically and valvularly during a single heartbeat, how these events are summarised together in the Wiggers diagram, how the picture differs between right and left heart, how atrial pressure waves relate to the jugular venous pulse, how heart rate changes the balance of diastole and systole, and how the ECG and heart sounds map onto the mechanical events (including the basis of murmurs). At rest (70 bpm) the heart beats roughly 2,759,400,000 times over a 75-year lifetime, framing why even small per-beat changes matter.

Heart Anatomy and Valves

• Chambers and great vessels: right/left atrium, right/left ventricle, interatrial and interventricular septa, superior/inferior vena cava, pulmonary arteries and veins, aorta, pulmonary trunk, myocardium/epicardium/pericardium with pericardial fluid.
• AV (atrioventricular) valves: right heart = tricuspid valve; left heart = mitral (bicuspid) valve.
  • Free margins attach via chordae tendineae to papillary muscles (projections of ventricular muscle); papillary muscle contraction helps prevent the valve everting into the atrium.
  • Allow uni-directional flow, atrium into ventricle.
  • Opening and closing are passive, driven purely by the pressure difference across the valve (open when atrial P > ventricular P).
• Semilunar valves: aortic valve (left ventricle to aorta), pulmonary valve (right ventricle to pulmonary artery).
  • Allow forward flow into the arteries during systole, prevent backflow into the ventricles during diastole.
  • Also open and close passively, driven by pressure difference (open when ventricular P > aortic/pulmonary arterial P).

Slide 5 is an image-only intraoperative photograph of a valve sizer/annuloplasty ring being positioned in an opened cardiac chamber, with no caption. Its specific link to the surrounding AV-valve anatomy content is not stated on the slide.

Phases of the Cardiac Cycle

The cardiac cycle is one complete set of contraction (systole) and relaxation (diastole), comprising electrical events (ECG), mechanical events (volume/pressure changes) and valvular events (opening/closing, heart sounds). It has two overall phases, ventricular diastole (ventricles relaxed) and ventricular systole (ventricles contracting), each subdivided into two phases:

• Diastole 1 - isovolumetric (isometric) ventricular relaxation (~0.05 s)
  • All valves shut, ventricles relaxing.
  • Ventricular volume at minimum (end-systolic volume, ESV) and constant; no blood flow.
  • Atrial pressure is initially lower than ventricular pressure; as the atria fill with returning blood, atrial pressure rises above ventricular pressure, so the AV valve opens passively, beginning filling.
• Diastole 2 - ventricular filling (~0.5 s at rest)
  • Roughly 80% of filling is passive, down a pressure gradient (rapid filling).
  • In late diastole, atrial depolarisation occurs (P wave on ECG) and the atria contract, raising atrial pressure to ~5 mmHg and ejecting the final ~20% of filling volume into the ventricle (atrial “top-up”).
  • Ventricular volume reaches its maximum, end-diastolic volume (EDV), ~130 mL at rest.
• Systole 1 - isovolumetric (isometric) ventricular contraction (~0.05 s)
  • Ventricles depolarise (QRS complex) and begin contracting, raising left ventricular pressure (LVP).
  • The AV valves close rapidly; their closure produces the first heart sound (S1).
  • Both AV and semilunar valves are closed, so ventricular volume does not change.
  • Pressure builds in the ventricles to overcome aortic/pulmonary arterial pressure.
  • When LVP exceeds aortic pressure (and right ventricular pressure exceeds pulmonary arterial pressure), the semilunar valves open passively, marking the start of ejection.
• Systole 2 - ventricular ejection (~0.3 s)
  • Blood is ejected into the aorta, increasing arterial volume and pressure; LVP and aortic pressure rise together to a peak at mid-systole.
  • About two-thirds of ejection occurs in the first third of this phase (rapid ejection); in late systole (reduced ejection) both LVP and aortic pressure begin to fall.
  • The same pattern occurs on the right side (right ventricle/pulmonary artery).
  • The ventricles repolarise during this phase (T wave on ECG).
  • The cycle then returns to diastole 1: as ventricular pressure falls below aortic pressure, the semilunar valves close, producing the second heart sound (S2, made up of an aortic (A2) and pulmonary (P2) component); this marks the end of systole and ventricular volume is again at its minimum (ESV, 60 mL).

Pressures and Volumes at Rest (Left and Right Heart)

Left ventricle / aorta:

• Left ventricular pressure: 0 mmHg (diastole) to 120 mmHg (systole).
• Aortic pressure: 80 mmHg (diastole) to 120 mmHg (systole).
• Arterial pulse pressure (systolic - diastolic): 40 mmHg.
• End-diastolic volume (EDV, maximum): 130 mL.
• End-systolic volume (ESV, minimum): 60 mL.
• Stroke volume (SV = EDV - ESV): 70 mL.

Right ventricle / pulmonary artery:

• Events mirror the left heart and right ventricular volumes equal left ventricular volumes, but pulmonary vascular resistance is lower than total peripheral (systemic) resistance, so right-sided pressures are lower.
• Right ventricular pressure: 0 mmHg (diastole) to 25 mmHg (systole).
• Pulmonary artery pressure: 8 mmHg (diastole) to 25 mmHg (systole).

The Wiggers Diagram

A multi-panel chart plotted against a shared time axis (0-1 s), stacking:

• A valve-event/phase timeline (mitral valve closes -> aortic valve opens -> aortic valve closes -> mitral valve opens; atrial systole, ventricular systole, ventricular diastole; isovolumetric contraction/relaxation; filling/ejection).
• Pressure (mmHg) vs time: aortic, left-ventricular and atrial pressure curves, with the incisura (dicrotic notch) marked.
• Ventricle blood volume (mL) vs time: EDV, ESV and SV.
• Aortic-root velocity (cm/s).
• A phonocardiogram (heart sounds).
• The ECG trace (P, QRS, T waves).

How each variable behaves across the cycle:

• Ventricular volume: maximum at end of diastole (EDV), constant during isovolumetric contraction, falls rapidly during ejection, minimum at end of systole (ESV), constant during isovolumetric relaxation, rises rapidly during diastolic filling.
• Ventricular pressure: minimum at the start of diastole, rises a little during filling and after atrial contraction, rises dramatically during isovolumetric contraction, continues to a maximum during ejection then falls in late systole, falls dramatically during isovolumetric relaxation back to minimum.
• Arterial pressure: minimum at the end of diastole (diastolic pressure), rises to a maximum during ejection (systolic pressure, tracking ventricular pressure); when ventricular pressure falls below arterial pressure the aortic valve closes (S2), producing the incisura/dicrotic notch; pressure then continues to fall through diastole back to the diastolic minimum.

Right Atrial Pressure Waves and the Jugular Venous Pulse

• Right atrial pressure shows three classically described waves: A, C and V (plus X and Y descents, and dP/dt(max), labelled on the enlarged pressure trace). The transcript names these waves and shows them on the pressure trace but does not state what mechanistically produces each individual wave [slide does not elaborate].
• There are no valves between the venae cavae and the right atrium, so pressure changes in the right atrium are transmitted back into the large veins (with a slight delay).
• These changes appear as A, C, V waves in the jugular venous column (JVP): they can be seen but not felt (unlike an arterial pulse), especially with the subject lying down.
• Clinical uses of JVP: assessing right atrial pressure, other changes in intrathoracic pressure, or filling volume.

Slide 29 is an image-only photograph of a person singing with a green arrow pointing to their neck, with no caption. Its apparent purpose is to illustrate a visible neck/jugular pulsation linked to the preceding JVP discussion, but this connection is not stated on the slide.

Effect of Heart Rate on the Cardiac Cycle

• At rest (HR 60/min): cardiac cycle duration 1.00 s, of which diastole is 0.66 s (two-thirds) and systole is 0.34 s (one-third).
• As heart rate increases, both diastole and systole shorten, but diastole shortens proportionally more than systole, i.e. systolic time is relatively preserved.
• At maximum heart rate (~180/min): cardiac cycle duration falls to 0.33 s; diastole 0.13 s, systole 0.2 s, i.e. diastole is shortened far more than systole.
• The ventricular volume curve has rapid and slow components of both ejection and filling; it is the slow ejection and slow filling portions that are curtailed as heart rate rises, so rapid filling/ejection are preserved and cardiac output can still be maintained.

ECG, Heart Sounds and Murmurs

ECG waves and their mechanical correlate:

• P wave: atrial depolarisation, precedes atrial contraction.
• QRS complex: ventricular depolarisation, precedes ventricular contraction.
• T wave: ventricular repolarisation, occurs during ventricular relaxation.
• General principle: electrical events precede mechanical events (excitation before contraction).

Heart sounds:

• S1: closure of the mitral and tricuspid valves (mitral component before tricuspid); low, long sound (“lub”).
• S2: closure of the aortic and pulmonary valves (aortic component A2 before pulmonary P2); short, high-pitched sound (“dub”).
• S3: rapid ventricular filling in early diastole; may be heard in the young, or may indicate left ventricular failure (?LVF).
• S4: ventricular filling during atrial “top-up”; may indicate reduced ventricular compliance; produces a “gallop rhythm”.

Murmurs:

• Normal blood flow is laminar (streamlined) and silent.
• Murmurs arise from turbulent blood flow.
• Causes of turbulent flow: narrowing of a valve (stenosis), or reversal of flow through a valve (valve insufficiency/incompetence, i.e. “back-leak”).
• Murmurs are an important sign of valve lesions.

Self-test

1. Distinguish the AV valves from the semilunar valves in terms of location and the mechanism by which they open and close.
2. Describe the role of the chordae tendineae and papillary muscles in AV valve function.
3. List the phases of the cardiac cycle in order, with the approximate duration of each at rest.
4. Describe the sequence of events during isovolumetric ventricular contraction.
5. What causes the semilunar valves to open, and what does this event mark the start of?
6. Describe the two components of ventricular filling during diastole, including the approximate proportion of blood each contributes and the associated change in atrial pressure.
7. Define end-diastolic volume, end-systolic volume and stroke volume, and give their resting values.
8. Distinguish the mechanism that produces the first heart sound (S1) from the mechanism that produces the second heart sound (S2).
9. Compare right heart pressures to left heart pressures at rest, giving values, and explain why they differ.
10. What are the A, C and V waves seen in the atrial/jugular venous pressure trace, and what does the transcript say (or not say) about how each is individually produced?
11. List two clinical uses of the jugular venous pulse (JVP).
12. What is the incisura (dicrotic notch) and what causes it?
13. Describe the effect of increasing heart rate on the duration of diastole versus systole, giving values at rest and near-maximal heart rate.
14. Distinguish the S3 heart sound from the S4 heart sound.
15. Distinguish valve stenosis from valve insufficiency as causes of a murmur.
16. Describe the temporal relationship between the ECG waves (P, QRS, T) and the mechanical/valvular events of the cardiac cycle.
17. During exercise, heart rate rises toward its maximum. Using what you know about the rapid and slow phases of the ventricular volume curve, explain how the heart maintains filling and output despite the much shorter cycle length.

Answers

Reveal answers

1. AV valves (tricuspid on the right, mitral on the left) sit between atria and ventricles and open when atrial pressure exceeds ventricular pressure; semilunar valves (pulmonary, aortic) sit between the ventricles and the great arteries and open when ventricular pressure exceeds arterial pressure. Both types open and close passively, driven only by the pressure difference across them.
2. The chordae tendineae anchor the free margins of the AV valves to the papillary muscles (projections of ventricular muscle). Papillary muscle contraction during ventricular systole helps prevent the AV valve leaflets everting back into the atrium.
3. Diastole 1, isovolumetric relaxation (~0.05 s); Diastole 2, ventricular filling (~0.5 s at rest); Systole 1, isovolumetric contraction (~0.05 s); Systole 2, ventricular ejection (~0.3 s).
4. Ventricles depolarise (QRS) and begin contracting, raising LVP; the AV valves close rapidly, producing S1; with both AV and semilunar valves closed, ventricular volume stays constant while pressure rises to overcome arterial pressure.
5. The semilunar valves open when ventricular pressure exceeds the corresponding arterial pressure (LVP > aortic pressure for the aortic valve; RVP > pulmonary arterial pressure for the pulmonary valve). This marks the beginning of ventricular ejection.
6. About 80% of filling is passive, occurring down a pressure gradient (rapid filling); the remaining ~20% is delivered by atrial contraction in late diastole (atrial “top-up”), which raises atrial pressure to ~5 mmHg.
7. EDV is the maximum ventricular volume, at the end of filling (~130 mL at rest). ESV is the minimum ventricular volume, at the end of ejection (~60 mL at rest). Stroke volume is EDV minus ESV (~70 mL at rest): $SV=EDV-ESV$.
8. S1 is produced by closure of the AV valves (mitral and tricuspid) at the start of isovolumetric contraction. S2 is produced by closure of the semilunar valves (aortic and pulmonary) at the start of isovolumetric relaxation, once ventricular pressure falls below arterial pressure.
9. Right ventricular pressure is 0 mmHg (diastole) to 25 mmHg (systole) and pulmonary artery pressure is 8 mmHg (diastole) to 25 mmHg (systole), both lower than the corresponding left-sided values (LVP 0-120 mmHg, aortic pressure 80-120 mmHg), because pulmonary vascular resistance is lower than total peripheral (systemic) resistance. Right and left ventricular volumes are equal.
10. The transcript labels three waves, A, C and V (with X and Y descents), on the atrial pressure trace and states they are “classically described,” but it does not explain what specifically produces each individual wave [slide does not elaborate].
11. Assessing right atrial pressure, and assessing other changes in intrathoracic pressure or filling volume.
12. The incisura (dicrotic notch) is a small notch in the arterial pressure trace that occurs when ventricular pressure falls below arterial pressure and the semilunar (aortic) valve closes.
13. At rest (HR 60/min), the cycle lasts 1.00 s: diastole 0.66 s, systole 0.34 s. As heart rate rises, both shorten, but diastole shortens proportionally more. At near-maximal HR (~180/min), the cycle is 0.33 s: diastole 0.13 s, systole 0.2 s.
14. S3 occurs from rapid ventricular filling in early diastole; it may be heard in young people or may signal left ventricular failure. S4 occurs during atrial “top-up” filling and may indicate reduced ventricular compliance; it produces a “gallop rhythm.”
15. Stenosis is narrowing of a valve, obstructing forward flow; insufficiency (incompetence) is failure of a valve to close properly, allowing backward “back-leak” flow. Both create turbulent flow, which is heard as a murmur (normal laminar flow is silent).
16. The P wave (atrial depolarisation) precedes atrial contraction; the QRS complex (ventricular depolarisation) precedes ventricular contraction; the T wave (ventricular repolarisation) occurs during ventricular relaxation (which occurs during the ejection phase of systole). Electrical events always precede the mechanical events they trigger.
17. The ventricular volume curve has rapid and slow phases of both filling and ejection. As heart rate rises, it is mainly the slow filling and slow ejection portions of the cycle that are shortened/cut, while the rapid filling and rapid ejection phases are relatively preserved, allowing the ventricle to still fill and eject an adequate volume per beat despite the much shorter overall cycle time.