Cardiac Cycle

  • Opening of the aortic valve takes place during ventricular systole, especially during the phase of ventricular ejection which occur at the end of isovolumetric contraction..
  • Each cardiac cycle consist of an atrial and ventricular cycle. Atrial cycle consist of atrial systole and diastole. Ventricular cycle consist of ventricular systole (0.3s) and diastole (0.5s). Ventricular systole consist of isovolumetric contraction and phase of ventricular ejection. Ventricular diastole consist of protodiastole, isovolumetric relaxation, rapid filling phase, diastasis and last rapid filling phase.
  • The P wave represents the wave of depolarization that spreads from the SA node throughout the atria.
  1. The QRS complex represents ventricular depolarization. Ventricular rate can be calculated by determining the time interval between QRS complexes.
  2. The isoelectric period (ST segment) following the QRS is the time at which the entire ventricle is depolarized and roughly corresponds to the plateau phase of the ventricular action potential.
  3. The T wave represents ventricular repolarization and is longer in duration than depolarization.
  4. The Q-T interval represents the time for both ventricular depolarization and repolarization to occur and therefore roughly estimates the duration of an average ventricular action potential. 
  • Duration of cardiac cycle (at heart rate of 75/min) is 0.80sec. Out of which duration of systole is 0.27sec and diastole is 0.53sec.
  • Coronary blood flow is maximum during the isovolumetric relaxation phase of cardiac cycle. 
  • Isovolumic relaxation phase of the cardiac cycle ends with opening of AV Valve.
  • Action potential in cardiac muscles
  1. Normal RMP in myocardial fibers is about -90 mV The AP in myocardial fibers has 5 phases : 0, 1, 2, 3, 4.
  2. Phase 0 :- It is the phase of rapid depolarization and is due to opening of fast sodium channels.
  3. Phase 1 :- It is the initial phase of rapid repolarization. It is due to closure of fast Na+ channels.
  4. Phase 2 :- This is platen phase. It is due to opening of 'voltage gated slow Ca+ channels, also called calcium-sodium channels which causes calcium influx.
  5. Phase 3 :- This phase of final repolarization is due to opening of IC- channels. The membrane potential comes back to resting membrane potential.
  6. Phase 4 :- It is the resting phase, i.e., the phase of resting membrane potential.
  • Maximum pressure in left ventricle is seen in ventricular ejection phase of cardiac cycle.
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