Cardiac cycle Study Guide

📖 Core Concepts Cardiac cycle – One complete heartbeat from the start of one beat to the start of the next. Diastole – Heart muscle relaxes; chambers fill with blood. Systole – Heart muscle contracts; blood is ejected into the great vessels. Atrioventricular (AV) valves – Mitral and tricuspid; open during ventricular diastole to allow filling. Semilunar valves – Aortic and pulmonary; open during ventricular systole to permit ejection. Isovolumic phases – Brief periods when all four valves are closed; pressure changes occur without volume change (contraction → relaxation). Electrical conduction – SA node → AV node (delay) → Bundle of His → Purkinje fibers; ensures atrial contraction precedes ventricular contraction. ECG correlates – P wave = atrial depolarization (atrial systole). Cardiac output (CO) – $CO = HR \times SV$ (heart rate × stroke volume). --- 📌 Must Remember Cycle duration is inversely proportional to heart rate: $\text{duration} \propto \frac{1}{\text{HR}}$. At 70–75 bpm, one cycle ≈ 0.8 s. Valve timing AV valves close → isovolumic contraction. Semilunar valves open → ventricular ejection. Semilunar valves close → isovolumic relaxation. AV valves open → ventricular diastole (early filling). Atrial systole adds the final 20 % of ventricular volume (“topping‑off”). Pressure waveforms – Incisura (sharp drop) follows aortic valve closure; dicrotic notch follows the incisura. SA node = primary pacemaker; located in the upper right atrial wall. --- 🔄 Key Processes Mechanical Sequence of One Cardiac Cycle Early ventricular diastole (rapid filling) – Ventricles relax, AV valves open, blood rushes from atria. Atrial systole (late diastole) – Atria contract, push the remaining 20 % of blood into ventricles. Isovolumic contraction – Ventricles contract, pressure rises, AV valves snap shut, volume unchanged. Ventricular ejection – Pressure exceeds aortic/pulmonary pressure, semilunar valves open, blood expelled. Isovolumic relaxation – Ventricular pressure falls below arterial pressure, semilunar valves close, all valves closed. Return to diastole – Ventricular pressure falls further, AV valves reopen, cycle restarts. Electrical Conduction Flow SA node fires → atrial depolarization (P wave). Impulse spreads across atria → atrial contraction. AV node delay → gives ventricles time to fill. Bundle of His → Purkinje network → rapid ventricular depolarization → ventricular contraction. --- 🔍 Key Comparisons AV valves vs. Semilunar valves AV: mitral/tricuspid, open during diastole, close before systole. Semilunar: aortic/pulmonary, open during systole, close before diastole. Isovolumic contraction vs. Isovolumic relaxation Contraction: pressure rises, volume constant, all valves closed. Relaxation: pressure falls, volume constant, all valves closed. Diastole vs. Systole (mechanical) Diastole: muscle relaxes, chambers fill. Systole: muscle contracts, blood ejected. P wave vs. QRS complex (ECG) P wave → atrial depolarization (atrial systole). QRS → ventricular depolarization (ventricular systole). --- ⚠️ Common Misunderstandings “Higher HR = longer cycle” – It’s the opposite; faster HR shortens each cycle, especially diastole. Confusing valve closure with pressure rise – Valves close because pressure exceeds the opposite chamber, not because pressure drops. Assuming the SA node is the only pacemaker – In SA node failure, AV node can take over (lower intrinsic rate). Thinking the dicrotic notch is a new heartbeat – It’s just a pressure rebound after aortic valve closure. --- 🧠 Mental Models / Intuition Pump Analogy – Think of the heart as a two‑stage pump: the atrial “pre‑pump” (topping‑off) followed by the ventricular “main pump.” Pressure‑Volume Loop – Visualize the square‑shaped loop: flat bottom = filling, vertical up = isovolumic contraction, top = ejection, vertical down = isovolumic relaxation. “All‑closed” moments – Treat isovolumic phases like a pressurized cylinder with a sealed lid – pressure builds or falls, but volume stays the same. --- 🚩 Exceptions & Edge Cases Tachycardia (>100 bpm) – Diastolic filling time shrinks dramatically; atrial contribution becomes more critical. AV block – Delay may be prolonged or absent, leading to “cannon A waves” in jugular venous pressure. Aortic stenosis – Increases systolic pressure needed to open aortic valve, lengthening isovolumic contraction. (If a topic lacks source detail, the heading still appears with a placeholder.) --- 📍 When to Use Which Identify valve status → Compare ventricular pressure to atrial or arterial pressure: If ventricular > atrial → AV valves close. If ventricular > arterial → Semilunar valves open. Choose ECG interpretation → Presence of P wave → atrial activity → consider atrial systole timing. Absence of P wave → possible atrial arrhythmia → look for junctional rhythm. Calculate cardiac output → Use $CO = HR \times SV$ when both heart rate and stroke volume are known; not useful if either is pathologically altered (e.g., severe regurgitation). --- 👀 Patterns to Recognize “Incisura → dicrotic notch” on aortic pressure trace → indicates aortic valve closure. “Topping‑off” spike on ventricular pressure curve → marks atrial systole. Flat‑top pressure plateau → ventricular ejection phase (semilunar valves open). Vertical lines on pressure‑volume loop → isovolumic phases (no volume change). --- 🗂️ Exam Traps Trap: “Semilunar valves close during diastole” – Correct, but they close after ejection, before isovolumic relaxation; the phrase “during diastole” can mislead if not tied to timing. Trap: “P wave represents ventricular depolarization” – Actually the QRS complex does; the P wave is atrial. Trap: “Heart rate and cycle duration are directly proportional” – They are inverse; faster HR = shorter cycle. Trap: “Isovolumic contraction occurs while AV valves are open” – Wrong; all valves are closed during isovolumic phases. Trap: “Dicrotic notch signifies a new systolic wave” – It’s just a brief pressure rise after aortic valve closure, not a new beat. ---