1.4 Cardiac Cycle and Heart Sounds

The cardiac cycle is the repeating sequence of contraction and relaxation that allows the heart to pump blood. Each heartbeat represents one complete cycle, and understanding its phases, pressure changes, and associated sounds is essential for interpreting how the heart works in both health and disease.

Systole vs Diastole

Defining Systole and Diastole

Systole is the contraction phase of the cardiac cycle. During systole, the heart muscle contracts and pumps blood out of the chambers.

Diastole is the relaxation phase. The heart muscle relaxes, and the chambers fill with blood in preparation for the next contraction.

One complete cardiac cycle consists of a systole followed by a diastole, and this repeats with every heartbeat (normally 60–100 beats per minute in a resting adult).

Timing and Duration of Systole and Diastole

• Systole is shorter than diastole, taking up roughly one-third of the total cycle.
• At a normal resting heart rate, systole lasts about 0.3 seconds, while diastole lasts about 0.5–0.7 seconds.
• As heart rate increases, diastole shortens more than systole. This matters clinically because diastole is when the ventricles fill with blood and when the coronary arteries receive most of their blood flow. A very fast heart rate can compromise both filling and coronary perfusion.

Pressure and Volume Changes in the Heart

Ventricular Pressure and Volume Changes

These changes drive the opening and closing of valves, so they're worth understanding step by step.

• During systole, ventricular pressure rises sharply as the ventricles contract. Once ventricular pressure exceeds the pressure in the aorta (left side) or pulmonary trunk (right side), the semilunar valves open and blood is ejected.
• During diastole, ventricular pressure drops as the ventricles relax. Once it falls below atrial pressure, the atrioventricular (AV) valves open and blood flows passively from the atria into the ventricles.
• Ventricular volume decreases during systole as blood is ejected. The amount ejected per beat is the stroke volume (typically ~70 mL). Volume increases during diastole as the ventricles refill, reaching the end-diastolic volume (typically ~120 mL) just before the next contraction.

Atrial Pressure and Volume Changes

Atrial pressure changes are much smaller than ventricular changes, but they produce characteristic waveforms you'll see on atrial pressure tracings.

• During atrial systole, the atria contract and push a final bolus of blood into the ventricles. This produces a small pressure rise called the a-wave.
• During atrial diastole, the atria relax and passively fill with blood returning from the venae cavae (right atrium) and pulmonary veins (left atrium). Atrial volume gradually increases while pressure stays relatively low.
• Atrial contraction contributes roughly 20–30% of ventricular filling. The remaining 70–80% occurs passively during the rapid filling phase of diastole.

Phases of the Cardiac Cycle

Ventricular Phases

The cardiac cycle can be broken into four distinct ventricular phases. Follow these in order:

1. Isovolumetric contraction — The ventricles begin contracting, but all four valves are closed. Ventricular pressure rises rapidly with no change in volume (the blood has nowhere to go yet). This phase begins right after the AV valves close (S1).

2. Ventricular ejection — Ventricular pressure exceeds aortic/pulmonary pressure, forcing the semilunar valves open. Blood is ejected into the aorta and pulmonary trunk. Ejection starts rapidly, then tapers off.

3. Isovolumetric relaxation — The ventricles begin to relax and ventricular pressure drops. The semilunar valves snap shut (S2), but the AV valves haven't opened yet. Again, all four valves are closed and volume doesn't change.

4. Ventricular filling — Ventricular pressure falls below atrial pressure, so the AV valves (mitral and tricuspid) open. Blood flows from the atria into the ventricles. This occurs in two sub-phases:

   • Rapid filling: Most blood flows in quickly right after the valves open.
   • Diastasis: Filling slows as pressures equalize between atria and ventricles.

Atrial Phases

• Atrial systole occurs at the very end of ventricular diastole. The atria contract and deliver that final 20–30% of blood into the ventricles, topping off ventricular volume just before the next ventricular contraction.
• Atrial diastole spans the rest of the cycle. The atria are relaxed and passively collecting blood from the great veins during ventricular systole and most of ventricular diastole.

Heart Sounds: Origin and Significance

First and Second Heart Sounds

Heart sounds are produced by the closure of valves, not by the opening of valves or the flow of blood itself.

• S1 ("lub") — Caused by closure of the AV valves (mitral and tricuspid) at the onset of ventricular systole. It marks the beginning of systole and is best heard at the apex of the heart.
• S2 ("dub") — Caused by closure of the semilunar valves (aortic and pulmonary) at the end of ventricular systole. It marks the beginning of diastole and is best heard at the base of the heart (second intercostal space).

Together, the familiar "lub-dub" represents one complete cardiac cycle. The pause between S2 and the next S1 (diastole) is longer than the pause between S1 and S2 (systole).

Clinical Significance of Heart Sounds

Heart sounds give you real-time information about valve function and the timing of cardiac events.

• Murmurs are abnormal whooshing or swishing sounds caused by turbulent blood flow. They can indicate valve stenosis (narrowing that restricts flow) or valve regurgitation (incomplete closure that allows backflow).
• Splitting of S2 can be normal during inspiration (physiological splitting) because the pulmonic valve closes slightly after the aortic valve. Abnormal or fixed splitting may indicate conditions like a bundle branch block or atrial septal defect.
• S3 (ventricular gallop) is a low-pitched sound heard just after S2, during rapid ventricular filling. It can be normal in young adults but in older patients often signals volume overload or heart failure.
• S4 (atrial gallop) is heard just before S1, during atrial contraction. It occurs when the atria contract against a stiff or noncompliant ventricle, as seen in ventricular hypertrophy or hypertrophic cardiomyopathy. S4 is always considered abnormal.