G2iii: CVS Electrical Syllabus: Cardiac Cycle

  • Cardiac cycle = one complete sequence of ventricular systole & diastole (At HR 72bpm, CC takes 0.8sec → diagram)
  • The CARDIAC CYCLE DIAGRAM (aka Wigger’s diagram) depics the L) side of the heart over time
  • It is a graph of these events over time

Pressure

LV P

Aortic P

Atrial P

Volume

LV vol

Electrical

ECG

  • Events of systole: IVC, rapid ejection, reduced ejection, IVR
  • Events of diastole: rapid filling, reduced filling, atrial systole

Isovolumetric Contraction

  • AV & SL valves shut → no volume ∆
    • ECG: peak of R wave → ventricular depol. → myocyte contraction
    • Atrial P: bulging of MV into LA → ↑atrial P = c wave
    • LV P: rapid rise in LVP > atrial P = MV closes
    • Aortic P: decreasing as blood going out to peripheries
    • LV vol: no change
    • Aortic BF: minimal

Rapid Ejection

  • LV P > AoP = opening SL valves
  • Ejection of blood occurs because Total E of blood in LV exceeds Total E of blood in Aorta
  • Total E of blood = Pressure Energy + Kinetic E
    • ECG: isoelectric ST segment
    • Atrial P: no descent, ↓LAP because of atrial stretching, despite MV shut & atria filling
    • LV P: ↑LVP
    • AoP: ↑AoP
  • LV P > AoP → AoV opens → blood ejected
  • LVP at slightly higher P to depict P grad required for forward flow of blood
    • LV vol: rapid ↓ as blood ejected into Aorta
    • Aortic BP: rapid ↑ Aortic BF, highest when LVP & AoP intersect

Reduced Ejection

  • ECG: T wave = ventricular repol. = ↓ventricle tension
  • LAP: progressive ↑LAP as blood filling atria
  • LVP: ↓as large vol. of blood has been ejected
  • AoP: AoP cont. to rise slightly, exceeding LVP due to stored potential E. in stretched arterial wall & then AoP falls as blood runs off to peripheries
  • LV Vol: ↓but less quickly as LVP ↓
  • Aortic BF: outward flow still occurring due to Kinetic E. of blood propelling blood forward, but ↓as AoP > LVP
    • End of this phase concludes systole
    • Aortic BF reverses → AoV shuts
    • End Systolic Vol = 50mL
    • End Diastolic Vol = 120mL
    • ∴EjF ~ > 0.55 = normal

Isovolumetric Relaxation

  • Closure of SL valve until AV valve opens
  • ECG: end of Tw, ventricular repolarisation
  • LAP: v-wave, peak in atrial P as AV valves still closed
  • LVP: rapid ↓ to baseline as all valves closed
  • AoP: flow reversal curves AoV shut & dicrotic notch
    • There, kinetic E of retrograde flow is converted into potential (pressure) E when closed AoV is met
    • Aortic does not drop drastically like LVP because there is potential stored E in the elastic walls & SVR tone impedes all blood flowing out to distributing arterioles immediately
  • LV Vol: no change. ESV = 50mL
  • Aortic flow: remains low, no LV ejection & blood moving out to peripheries

Rapid Filling

  • LAP > LVP → AV valve opens
  • ECG: isoelectric
  • LAP: y descent, atria empty, LAP ↓
  • LVP: ventricular P ↓ despite filling because of ongoing ventricular relaxation
  • AoP: falling through diastole
  • LV vol: rapid ↑, 60% of ventricular filling

(Diastasis) Reduced Filling

  • ECG: isoelectric
  • LAP: follows LVP, gradual rise
  • LVP: gradual rise (v. small) as blood filling a fully relaxed ventricle
    • Small P difference between LAP & LVP indicate AV valves, representing a very low R pathway for blood flow
  • LV vol: ↑ but at slower rate

Atrial Systole

  • ECG: p wave complete → atrial depolarisation & ∴atrial myocyte contraction
    • Beginning of QR
  • LAP: a wave, ↑ P from atrial systole, followed by y descent
  • LVP: ↑with ↑blood flow
  • AoP: still declining in ventricular diastole
  • LV vol: ↑LV vol. (atrial kick = 30% filling)
    • EDV = 120mL
    • EDP = 8 mmHg
Atrial Systole

Wigger’s Cardiac Cycle