Cardiac Mechanics
Cardiac mechanics is an interplay between many different systems: electrical, mechanical, and hemodynamic. Everything should be timed optimally to allow for appropriate blood flow.
There are essentially 2 basic things to note and understand before reading this page:
Pressure Gradients: Blood movement will follow a pressure gradient, moving from high pressure to low pressure. For a heart valve to open, the chamber/structure before the valve must have a pressure greater than the chamber after the valve.
Pressure-Volume Relationship: Pressure is inversely proportional to volume. As a chamber (atria or ventricle) squeezes, the volume or space in that chamber decreases and the pressure increases.
As you are reading, try to correlate the ECG waveform (i.e. p wave, QRS, etc.) that occurs with each cardiac mechanic
Wigger's Diagram
Wigger's diagram is a popular diagram that:
Captures the activity of the left side of the heart, which is essentially mirrored on the right side (although technically there is a minor physiological delay between two sides)
Combines pressure, volume, electrical activation, valvular activity, and auscultatory sound all on one diagram
Step 1: Active ventricular filling
- This step occurs during the PR segment
- The left atrium (LA) contracts to push blood through an open mitral valve, allowing the left ventricle (LV) to fill - known as the "atrial kick".
- LV pressures rise while LA pressures drop.
- The slow transit of the electrical signal through the AV node (a.k.a. AV nodal delay), allows time for the atrial kick to complete LV filling. Without the AV nodal delay, the QRS would happen right after the p wave, meaning there wouldn't be enough time between atrial and ventricular contraction to allow for adequate LV filling.
- The mitral valve closes when the LV pressure exceeds the LA pressure.
Step 2: Isovolumic contraction
- This step occurs during the QRS complex.
- With both the mitral and aortic valves closed, the LV start contracting in response to the QRS, leading to rising LV pressure.
- There is no change in volume as the blood has nowhere to go.
Step 3: Ventricular contraction
- This step occurs during the T wave.
- Once LV pressure exceeds the aortic pressure, the aortic valve opens allowing expulsion of blood from the heart through the aorta.
- Aortic pressure rises and LV pressure dissipates, until eventually the aortic valve closes after aortic pressure exceeds LV pressure.
- As this occurs inside the LV, the LA is beginning to fill up with blood from the pulmonary system.
Step 4: Isovolumic relaxation
- This stage begins after the end of the T wave.
- The LV begins to relax, initially occurring while the aortic and mitral valves are both closed.
- LV volume remains constant but LV pressure drops.
- The LA is concurrently continuing to fill.
Step 5: Isovolumic relaxation
- This stage occurs during the TP segment
- LV pressure drops below the LA pressure
- As this happens, the mitral valve opens, allowing blood from the LA to fill the LV along its pressure gradient.
The Cycle Repeats at Step 1
- The cycle then repeats once the p wave occurs again, causing LA contraction.
ECG Waveforms
Since it takes a brief moment for cardiac cells to contract in response to electrical signals, the ECG waveforms slightly precede the corresponding cardiac muscular activity (depicted in the diagram below).
Ventricular Contraction
The Purkinje fibres allow the depolarization of the heart to follow a predictable pattern in these steps:
- Septum Activation: Septal contraction causes the heart to squish vertically
- Apex, Free Wall, and Base Activation: These structures activate sequentially. This, in a way, "wrings" as much blood as possible with each beat, maximizing cardiac output and improving pumping efficiency. This is depicted in the image to the right.
To see the blood flow in action
See this computational model of blood flow https://research.unsw.edu.au/projects/modelling-cardiac-electrical-and-mechanical-function