Electrocardiogram (ECG) waves

• Electrical impulses generated by the SA node can be detected on the skin's surface through the cardiac conduction circuit.
• Electrocardiography measures and records the depolarization and repolarization of the cardiac muscle.

Phases of the Electrocardiogram

• P wave represents atrial depolarization and the conduction of the impulse throughout the atria.
• PR interval is the time from the beginning of atrial depolarization to the beginning of ventricular depolarization (from the start of the P wave to the start of the QRS complex).
• QRS complex represents ventricular depolarization (and atrial repolarization) and the conduction of the impulse throughout the ventricles, triggering ventricular contraction, it is measured from the beginning of the Q wave to the end of the S wave.
• ST segment is the period between ventricular depolarization and the beginning of ventricular repolarization.
• T wave represents ventricular repolarization as the ventricles return to a resting state.
• QT interval is the total time for ventricular depolarization and repolarization (from the beginning of the Q wave to the end of the T wave); its duration varies with heart rate.
• U wave may or may not be present; if present, it follows the T wave and represents the final phase of ventricular repolarization.

Cardiac Cycle

• The cardiac cycle involves the filling and emptying of the heart's chambers.
• Diastole is the relaxation of the ventricles (filling), lasting about two-thirds of the cardiac cycle.
• Systole is the contraction of the ventricles (emptying), lasting the remaining one-third of the cycle.

Diastole

• AV valves are open and ventricles are relaxed during ventricular diastole, atrial pressure is higher than ventricular pressure.
• Early or Protodiastolic Filling involves early, rapid, passive filling as blood rushes from the atria into the ventricles.
• Protodiastolic Filling is followed by slow passive filling.
• Presystole, Atrial Systole, or Atrial Kick occurs near the end of ventricular diastole; the atria contract, completing the emptying of blood into the ventricles and raising left ventricular pressure.

Systole

• Filling during diastole results in a large amount of blood in the ventricles, causing ventricular pressure to be greater than atrial pressure.
• Closure of the AV valves produces the first heart sound (S1), marking the beginning of systole.
• Valve closure prevents blood from flowing backward into the atria during ventricular contraction.
• At this point, all four valves are closed as the ventricles contract (isometric contraction).
• High pressure inside the ventricles causes the aortic valve to open on the left and the pulmonic valve to open on the right.
• With ventricular emptying, ventricular pressure falls, and the semilunar valves close as this produces the second heart sound (S2), signaling the end of systole.
• After semilunar valve closure, the ventricles relax while Atrial pressure is now higher than ventricular pressure, causing the AV valves to open again, and diastolic filling to begin again.

Heart Sounds

• Heart sounds are produced by valve closure, the opening of valves is silent.
• Normal heart sounds are "lub-dub" (S1 and S2); extra heart sounds and murmurs can be auscultated over the precordium using a stethoscope.

Normal Heart Sounds

• The first heart sound (S1; "lub") results from the closure of the AV valves (mitral and tricuspid), correlating with the beginning of systole.

• S1 is usually heard as one sound but may be heard as two.

• If heard as two sounds, the first component represents mitral valve closure (M1) and the second represents tricuspid closure (T1).

• M1 occurs first due to increased left-side heart pressure and myocardial depolarization.

• S1 is heard best at the apex (left MCL, fifth ICS) but is audible across the precordium.

• Intensity of S1 depends on the position of the mitral valve at the start of systole, the structure of the valve leaflets, and how quickly the pressure rises in the ventricles.

• Accentuated S1 is louder than S2 when the mitral valve is wide open and quickly closes and may be present in hyperkinetic states or mitral stenosis.

• Diminished S1 is softer than S2.

• Split S normally occurs only when left and right ventricles contract at different times.

• The second heart sound (S2; "dub") results from closure of the semilunar valves (aortic and pulmonic), correlating with the beginning of diastole.

• If S2 is heard as two sounds, the first component represents aortic valve closure (A2) and the second component represents pulmonic valve closure (P2).

• Accentuated S2 is louder than S1 and the opposite is true for diminished.

• Normal split S2 can be heard over the second or third left ICS (best during inspiration and disappearing during expiration).

• Wide Split S2 is a larger splitting when compared to normal, while still changing between inspiration and expiration.

• Fixed Split S2 is a wide splitting that does not change between inspiration and expiration.

• Reversed Split S2 appears on expiration and disappears on inspiration, and it is also known as paradoxical split.

• A2 accentuated is louder over the right, second ICS, while Diminished A2 would be soft or absent over the right, second ICS.

• An accentuated P2 is louder than or equal than an A2, and diminished P2 is soft or absent.

• S3 and S4 are diastolic filling sounds (extra heart sounds) resulting from ventricular vibration secondary to rapid ventricular filling.

• S3 can be heard early in diastole, after S2.

• S4 results from ventricular vibration secondary to ventricular resistance during atrial contraction.

• S3 is called ventricular gallop, and S4 is called atrial gallop.

Auscultating Heart Sounds

• Heart sounds can be auscultated in the traditional five areas on the precordium, the anterior surface of the body overlying the heart and great vessels.
• The traditional areas include the aortic area, the pulmonic area, Erb point, the tricuspid area, and the mitral or apical area.
• Sounds always travels in the direction of blood flow.

Traditional Areas of Auscultation:

• Aortic area: Second ICS at the right sternal border (the base of the heart).
• Pulmonic area: Second or third ICS at the left sternal border (the base of the heart).
• Erb point: Third ICS at the left sternal border.
• Mitral (apical): Fifth ICS near the left MCL (the apex of the heart).
• Tricuspid area: Fourth or fifth ICS at the left lower sternal border.