MR

Wiggers Diagram / Cardiac Cycle Study Notes

Orientation to the Wiggers (Cardiac Cycle) Diagram

  • One cardiac cycle = one diastole + one systole.
  • Diagram integrates electrical, mechanical, acoustic & volumetric events that happen simultaneously in the heart.
  • Initially overwhelming; lecturer recommends:
    1. Identify each panel.
    2. Focus on pressures & volumes first.
    3. Use color-coding or separate sketches to simplify.
  • Left-side values (higher pressures) are shown, but the temporal patterns are identical on the right side (pressures merely scale down to a ~25\,\text{mmHg} peak for the right ventricle).

Panels & What They Represent

  • Panel 1: ECG tracing
    • P wave → atrial depolarization.
    • PQ segment → AV-node conduction.
    • QRS complex → ventricular depolarization.
    • T wave (not labelled here) → ventricular repolarization.
  • Heart Sounds (overlay on Panel 1)
    • S1 “lub” = AV valves close.
    • S2 “dub” = semilunar valves close.
  • Panels 2–4: Pressures
    • Atrial (blue in instructor’s sketch).
    • Ventricular (purple).
    • Aortic (red).
    • Y-axis typically 0!\rightarrow!120\,\text{mmHg}.
  • Panel 5: Ventricular Volume (black line)
    • Tracks filling & emptying of the left ventricle.
    • Peaks at EDV; troughs at ESV.
  • Panels 6–7: Valve status
    • AV valve row (white = open, shaded = closed).
    • Semilunar valve row (white = open, shaded = closed).
  • Panel 8: Phase labels / cartoons
    • Visual arrows indicate blood flow direction.
    • Phase numbers (1–3) vary by textbook; instructor de-emphasizes numbering.

Reference Pressures & Typical Numbers

  • Aorta (systemic): 120\,\text{mmHg}\;\text{(systolic)} \rightarrow 70\,\text{mmHg}\;\text{(diastolic)}.
  • Left Ventricle: 0\,\text{mmHg} in early diastole → \approx120\,\text{mmHg} during peak systole.
  • Left Atrium: very low (just above venous pressures), minor bump during atrial systole.
  • Right-side analogue: peak ventricular pressure \approx25\,\text{mmHg}.

Ventricular Volume Terminology

  • EDV (End-Diastolic Volume) = volume just after filling completes.
  • ESV (End-Systolic Volume) = volume left after ejection.
  • Stroke Volume (SV): \text{SV} = \text{EDV} - \text{ESV}.
  • Filling pattern:
    • ~80 % passive (both chambers in diastole).
    • ~20 % active via atrial systole ("topping off").

Chronological Sequence of Events (Pressures, Valves, Blood Flow)

  1. Ventricular & Atrial Diastole (Early)
    • Atrial P > Ventricular P, so AV valve open.
    • Aortic P\gg Ventricular P, so semilunar valve closed.
    • Passive flow → ventricle fills.
  2. Atrial Systole (Active Filling)
    • Small atrial pressure bump; still P{atria} > P{vent} → more blood enters ventricle.
    • Sets the final EDV.
  3. Isovolumetric Ventricular Contraction (IVC)
    • Ventricular depolarization → rapid pressure rise.
    • AV valve snaps shut → S1 “lub”.
    • Ventricular P < Aortic P, so semilunar still closed.
    • Volume constant (iso-volumetric).
  4. Ventricular Ejection
    • Ventricular P finally exceeds Aortic P → semilunar opens.
    • Blood ejected; ventricular volume plunges toward ESV.
    • Ventricular P > Atrial P so AV valve stays closed.
  5. Isovolumetric Ventricular Relaxation (IVR)
    • Ventricular repolarization → pressure falls.
    • When Ventricular P < Aortic P, semilunar closes → S2 “dub” & dicrotic notch in aortic curve. • AV still closed (ventricular P > atrial P) → no volume change.
  6. Early Ventricular Diastole / Rapid Passive Filling
    • Ventricular P dips below Atrial P → AV valve re-opens.
    • Cycle repeats.

Isovolumetric Periods: Key Points

  • IVC (systole): both valves closed, pressure rising, volume frozen.
  • IVR (diastole): both valves closed, pressure falling, volume frozen.

Heart Sounds & Valve Events (Pressure Crossings)

  • Crossing #1 (Atrium ↔ Ventricle) during IVC → AV close → S1.
  • Crossing #2 (Ventricle ↔ Aorta) upward during IVC → semilunar opens (no sound, but start of ejection).
  • Crossing #3 (Ventricle ↔ Aorta) downward during IVR → semilunar closes → S2.
  • Crossing #4 (Ventricle ↔ Atrium) downward during IVR → AV opens (silent) → rapid filling.

Dicrotic Notch

  • Small rebound in aortic pressure right after semilunar closes.
  • Caused by elastic recoil of aortic wall & brief back-flow against the valve.

Electrical–Mechanical Coupling (ECG vs. Pressure Curves)

  • P wave precedes atrial pressure bump.
  • QRS precedes steep ventricular pressure rise & S1.
  • T wave (ventricular repolarization) starts before ventricular pressure falls & S2.
  • Valve function is purely pressure-driven – no muscular opening/closing.
  • Stroke volume & ejection fraction derive from EDV/ESV; clinical relevance in heart failure.
  • Blood pressure cuff numbers (≈120/70) correspond to aortic curve on Wiggers.
  • Right-side Wiggers is identical in shape; essential for understanding pulmonary hypertension or right-sided failure.
  • Ethical / clinical importance: mis-interpreting heart sounds or pressure traces can lead to mis-diagnosis (e.g., valvular stenosis, regurgitation).

Study Tips

  • Physically sketch the curves; label every crossing & valve state.
  • Color-code pressures like the lecturer (blue = atrial, purple = ventricular, red = aortic, black = volume).
  • Practice overlaying the ECG to see cause-→-effect timing.
  • Use the formula \text{SV} = \text{EDV} - \text{ESV} and relate to ejection fraction \text{EF} = \dfrac{SV}{EDV}.
  • Revisit the diagram multiple times; repetition aids pattern recognition.