The Cardiac Cycle

• Cardiac Cycle: A coordinated sequence of electrical and mechanical events occurring from the start of one heartbeat to the start of the next. A single cardiac cycle includes a complete relaxation and contraction of both atria and ventricles. Relaxation is Diastole and Contraction is Systole

• Changes in pressure and volume: A series of pressure changes take place within the heart during the cardiac cycle resulting in the movement of blood. Valves within the heart's direct movement of blood and pressure changes are brought about by conductive electrochemical changes within the myocardium that result in the contraction of cardiac muscle.

• Phases:

  • Atrial Systole: Atrial contraction forces blood into the ventricles. Sets up the P-wave which covers the atria depolarization

  • Ventricular Systole: Ventricular contraction pushes AV valves closed (first phase) and sets up the QRS complex which covers ventricle depolarization. Semilunar valves open and blood is ejected (second phase) and sets up the T-Wave which initiates ventricular repolarization.

  • Ventricular Diastole: Semilunar valves close and blood flows into the atria (early), and then chambers relax and blood fills ventricles passively.

• ECG:

  • P: Atrial Depolarization. Does not represent a contraction of the muscle or the firing of the SA node, we assume the SA node fires at its start and atrial contraction begins at its peak, atrial repolarization however is too minor of an amplitude to be recorded by surface electrodes

  • QRS: Ventricular Depolarization. Q represents initial downward deflection, R represents initial upward deflection and S represents downward deflection and return to baseline - isoelectric point. Contraction commences at peak of the R portion of the complex. Can be confirmed clinically by palpitating pulse.

  • T: Ventricular Repolarization.

  • PR interval: Measured from the beginning of the P wave to the beginning of the R portion of the QRS complex, Starts with atrial muscle depolarization and ends with ventricular depolarization and it is assumed that an impulse passes through the AV node during this interval. Determines if impulse conduction from the atria to the ventricles is normal.

  • PR segment: Flatline between the end of the P wave and onset of QRS complex and reflects the slow impulse contraction through the AV node, serves as a baseline for the isoelectric line of the ECG trace.

Electrophysiology

• Cardiac Pacemakers: The cells of the SAN depolarise over time, with the movement of ions causing the resting membrane potential to gradually decrease (pacemaker potential), once the membrane potential exceeds a threshold, an action potential is triggered  (this happens automatically about every 0.8 seconds at rest). The cells of the AVN do the same, but more slowly; the result of this is that an action potential is triggered in the AVN before they depolarise enough to trigger their own.

• Pacemaker Potentials:

  • At a membrane potential of about -60mv sodium channels open in the SAN cell membrane

  • Sodium enters the cell through the sodium channels taking a positive charge into the cell

  • The inside of the cell becomes less negative in relation to the outside

  • A voltage-gated calcium channel opens and calcium enters the cell slowly

  • The cell continues to depolarise gradually (pacemaker potential)

  • When the threshold is reached, another type of voltage-gated calcium channel opens and calcium enters the cell rapidly.

  • This results in rapid depolarisation which is the cardiac action potential.

  • Potassium outflow when some of its channels open and repolarization occurs when more potassium voltage-gated channels open.

Conduction and ECG

• ECG: As action potentials travel through the heart muscle, they produce electrical currents that can be detected using electrodes on the body surface. The ECG is an electrical trace resulting from action potentials in all the heart muscle fibers.

• The trace varies depending on:

  • The direction of travel

  • Whether the cells are depolarising or repolarizing

  • The size of the change in potential

• 3 vs 12: A 3 lead ECG can give a broad overview of whether the heart is functioning correctly, and can be used for diagnostic purposes such as detecting rhythm changes but a 12-leady system is used for picking up more subtle changes and giving more information regarding defects in specific areas of the heart.