Cardiac Cycle, Phases & Pressure-Volume Relationships

The Cardiac Cycle: One Heartbeat in Detail

The cardiac cycle is the repeating sequence of mechanical and electrical events making up a single heartbeat — contraction (systole) and relaxation (diastole) of the atria and ventricles. Sonographers time nearly every 2D, M-mode, and Doppler measurement to a specific point in this cycle, so mastering the sequence is a prerequisite for every later chapter. The Wiggers diagram is the classic reference figure stacking the ECG, heart sounds, aortic pressure, left ventricular (LV) pressure, left atrial (LA) pressure, and LV volume on a shared time axis, showing how these signals relate to one another.

The seven phases

By convention the cycle is divided into seven phases:

1. Atrial systole — triggered by the P wave, atrial contraction delivers the final increment of ventricular filling (the "atrial kick"), pushing LV end-diastolic volume and pressure to their peak just before the QRS complex.
2. Isovolumic contraction (IVCT) — begins the instant the mitral valve closes and ends when the aortic valve opens. All four valves are closed, so LV volume cannot change; LV pressure rises steeply until it exceeds aortic diastolic pressure.
3. Rapid ejection — once LV pressure exceeds aortic pressure, the aortic valve opens and blood is ejected quickly; LV and aortic pressures rise together to their systolic peak during this phase.
4. Reduced ejection — ejection continues but decelerates as the ventricle begins to repolarize (T wave); LV pressure begins to fall, though forward flow continues until LV pressure drops below aortic pressure.
5. Isovolumic relaxation (IVRT) — begins at aortic valve closure and ends at mitral valve opening. All valves are again closed; LV pressure falls rapidly at fixed volume until it drops below LA pressure.
6. Rapid filling — the mitral valve opens once LV pressure falls below LA pressure, and blood stored in the LA during systole empties quickly into the ventricle, generating the E wave on transmitral pulsed-wave Doppler.
7. Diastasis — LA and LV pressures nearly equalize, so filling slows to a trickle; this phase shortens markedly as heart rate rises, which is why tachycardia erodes diastolic filling time first.

Valve events, heart sounds & the "four closed valves" rule

Each phase transition is triggered by a specific valve event:

A frequently tested concept: all four cardiac valves are never open at the same time, and during both isovolumic phases all four valves are simultaneously closed, which is exactly why ventricular volume cannot change during those two phases — the chamber is briefly a sealed box.

The right heart runs the identical four-event sequence in parallel, but not in perfect synchrony: lower pulmonary vascular resistance lets right ventricular ejection persist slightly longer, so the pulmonic valve closes just after the aortic valve. This produces the normal ("physiologic") splitting of S2 into A2 and P2, which widens further with inspiration — an expected finding, not valve dysfunction.

Cycle duration and heart rate

At a typical resting heart rate of 75 bpm, one full cardiac cycle lasts about 0.8 seconds (cycle length in seconds = 60 ÷ heart rate). At rest, systole occupies roughly one-third of that time (~0.3 sec) and diastole the remaining two-thirds (~0.5 sec), giving the ventricle ample time to fill passively before atrial systole tops it off. As heart rate rises, the two halves do not shorten proportionally: diastole contracts disproportionately more. This is why tachycardia — from exercise, stress, or arrhythmia — preferentially compromises filling time and, when coronary disease is present, coronary perfusion time, since the coronary arteries fill predominantly during diastole.

The pressure-volume loop

Plotting instantaneous LV pressure against LV volume across one full cycle produces the pressure-volume (PV) loop, condensing everything the Wiggers diagram shows across time. Tracing the loop counterclockwise from end-diastole:

• Isovolumic contraction appears as a nearly vertical line — pressure rises steeply while volume stays fixed at end-diastolic volume (EDV).
• Ejection appears as the upper curve moving toward lower volumes as blood leaves the ventricle, ending at end-systolic volume (ESV) when the aortic valve closes.
• Isovolumic relaxation appears as a second vertical line — pressure falls steeply at fixed volume (ESV) until the mitral valve opens.
• Diastolic filling appears as the lower curve moving back toward higher volumes as the ventricle fills, returning to the starting EDV point.

Two relationships fall directly out of the loop's geometry: the horizontal width equals stroke volume (EDV − ESV, so EF = stroke volume ÷ EDV), and the area enclosed equals the external stroke work performed by the left ventricle in that beat. Shifting the loop — widening it (increased preload), steepening its upper border (increased contractility), or raising its ejection segment (increased afterload) — is the standard way physiology questions test how loading conditions and inotropic state each change performance independently.

Why this matters for measurement

Every timed echocardiographic interval traces back to these valve events: isovolumic contraction and relaxation times are measured between valve-click events on Doppler tracings, ejection time is measured across the LVOT/aortic Doppler envelope, and the myocardial performance (Tei) index combines both isovolumic times relative to ejection time as a combined systolic-plus-diastolic marker. Recognizing which phase a tracing represents — and which valve is opening or closing at each transition — is the foundation for every quantification chapter that follows.