The Cardiac Cycle Correlated To ECG Trace

A word of advice, and caution. We recommend for a full and better understanding to look into our sources & further reading section, after you have taken the quiz. Of course do not hesitate to reach out to us with questions, we are busy but will do our best to get back to you and explain it in terms that have gotten ourselves through the various academic rigors. Further we recommend to maintain academic integrity and avoid the loss of your agency and subsequently advise you to take proper handwritten notes, and pair your research today with flashcards.

Phases of the Cardiac Cycle Each cardiac cycle begins with both the atria and ventricles in a relaxed state known as diastole. During this resting phase, venous blood returns to the right atrium via the superior and inferior venae cavae and the coronary sinus, while oxygenated blood enters the left atrium through the four pulmonary veins. With the tricuspid and mitral valves open, blood moves passively from the atria down into the ventricles — accounting for roughly 70–80% of total ventricular filling. The pulmonary and aortic semilunar valves remain closed throughout, preventing any backflow from the great vessels into the ventricles.

Atrial Systole and Diastole Atrial depolarisation — visible on the ECG as the P wave — triggers contraction of the atrial myocardium, progressing from the superior atria downward toward the atrioventricular septum. This rise in atrial pressure drives the remaining 20–30% of ventricular filling through the still-open AV valves, a mechanism commonly referred to as the atrial kick. By the end of atrial systole the ventricles have received their full preload. Atrial contraction lasts approximately 100 ms, after which the atrial muscle relaxes and returns to diastole — just before the ventricles begin their own contraction.

Ventricular Systole Ventricular depolarisation is represented by the QRS complex on the ECG and marks the onset of ventricular systole, which spans approximately 270 ms across two distinct phases. At this point the ventricles hold their end-diastolic volume — roughly 130 mL in a resting adult — also referred to as the preload. *Phase 1 — Isovolumic Contraction* As the ventricular walls begin to contract, intraventricular pressure climbs rapidly. It quickly surpasses the pressure in the now-relaxed atria, causing blood to press back against the tricuspid and mitral valves and forcing them shut — producing the first heart sound (S1, "LUB"). The semilunar valves have not yet opened, so no blood leaves the ventricles and the chamber volume remains unchanged. This is isovolumic contraction. *Phase 2 — Ventricular Ejection* Continued contraction raises ventricular pressure above that of the pulmonary trunk and aorta. The semilunar valves are pushed open and blood is ejected into the circulation. The left ventricle generates considerably higher pressure than the right in order to overcome the greater resistance of the systemic circulation, yet both ventricles eject an equal stroke volume — typically 70–80 mL. The blood remaining in each ventricle after ejection is the end-systolic volume, approximately 50–60 mL.

Ventricular Diastole Ventricular repolarisation — the T wave on the ECG — signals the onset of ventricular relaxation, or diastole, which lasts approximately 430 ms and unfolds in two phases. *Phase 1 — Isovolumic Relaxation* As the ventricular muscle relaxes, intraventricular pressure begins to fall. Once it drops below the pressure maintained in the aorta and pulmonary trunk, blood momentarily flows back toward the heart — producing the dicrotic notch visible in arterial pressure tracings — and the semilunar valves close, generating the second heart sound (S2, "DUB"). The AV valves remain closed at this stage, so ventricular volume again stays constant. This is isovolumic ventricular relaxation. *Phase 2 — Late Ventricular Diastole and Passive Filling* Pressure within the ventricles continues to fall until it drops below atrial pressure. At this point the tricuspid and mitral valves swing open and blood flows passively from the atria into the ventricles. Simultaneously, venous return refills the atria from the great veins. Both chambers are now in diastole, the AV valves are open, and the semilunar valves remain closed. The cardiac cycle is complete — and the next P wave will begin it again.