ECG: Normal & Abnormal

Question 1

Cardiac Contraction: Intro

Answer 1

The period that begins with contraction of the atria and ends with ventricular relaxation is known as thecardiac cycle. The period of contraction that the heart undergoes while it pumps blood into circulation is calledsystole. The period of relaxation that occurs as the chambers fill with blood is calleddiastole. Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated to ensure blood is pumped efficiently to the body.

Contraction of the atria follows depolarization, represented by the P wave of the ECG. As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular (tricuspid, and mitral or bicuspid) valves. At the start of atrial systole, the ventricles are normally filled with approximately 70–80 percent of their capacity due to inflow during diastole. Atrial contraction, also referred to as the “atrial kick,” contributes the remaining 20–30 percent of filling. Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole.

Question 2

Cardiac Contraction: Ventricular Diastole

Answer 2

Ventricular relaxation, or diastole, follows repolarization of the ventricles and is represented by the T wave of the ECG. It too is divided into two distinct phases and lasts approximately 430 ms.

During the early phase of ventricular diastole, as the ventricular muscle relaxes, pressure on the remaining blood within the ventricle begins to fall. When pressure within the ventricles drops below pressure in both the pulmonary trunk and aorta, blood flows back toward the heart, producing the dicrotic notch (small dip) seen in blood pressure tracings. The semilunar valves close to prevent backflow into the heart. Since the atrioventricular valves remain closed at this point, there is no change in the volume of blood in the ventricle, so the early phase of ventricular diastole is called theisovolumic ventricular relaxation phase, also called isovolumetric ventricular relaxation phase.

In the second phase of ventricular diastole, called late ventricular diastole, as the ventricular muscle relaxes, pressure on the blood within the ventricles drops even further. Eventually, it drops below the pressure in the atria. When this occurs, blood flows from the atria into the ventricles, pushing open the tricuspid and mitral valves. As pressure drops within the ventricles, blood flows from the major veins into the relaxed atria and from there into the ventricles. Both chambers are in diastole, the atrioventricular valves are open, and the semilunar valves remain closed. The cardiac cycle is complete.

Question 3

Cardiac Contraction: 1st Degree Heart Block

Answer 3

Prolonged PR interval > 0.2s (normally 0.1s)
‘Block’ at AV node or Bundle of His
Not really a block, but a delay in conduction
Early sign of cardiac disease

Question 4

Cardiac Contraction: 2nd Degree Heart Block

Answer 4

Mobitz I
Progressive lengthening of PR Interval
Greater ratio of P waves to QRS complexes (above: 4:3)
Block in AV Node
Normally temporary and benign

Question 5

Cardiac Contraction: 2nd Degree Heart Block

Answer 5

Mobitz II
PR Interval constant, but a ventricular contraction is missed.
Greater ratio of P waves to QRS complexes, ratio usually varies, (above 3:2)
Block in Bundle of His
Usually indicates serious heart disease, can progress to 3rd degree block.

Question 6

Cardiac Contraction: 3rd Degree Heart Block

Answer 6

No atrial impulses conducted to ventricles Block at AV Node or lower
Ventricles generate a lifesaving bradycardic ESCAPE RHYTHM (VER ~40 bpm) Atria contract at 60 to 100 bpm
Atria and ventricles controlled by separate intrinsic pacemakers (AV DISSOCIATION).