Electrocardiogram (ECG) Interpretation

• Electrocardiogram interpretation and related concepts are discussed

The Electrocardiogram

• ECGs are inexpensive, noninvasive, and easy to obtain, making them a popular tool
• They are often used to assess patients suspected of having an acute myocardial infarction
• ECGs serve as a health screening tool for patients over 40 years old
• ECGs cannot predict future heart attacks or detect structural defects like valve stenosis

Cellular Energy

• Myocardial cells' filaments slide together and contract when electrically stimulated, but they depend on pacemaker cells for electrical impulses
• Pacemaker cells are responsible for the spontaneous generation and conduction of electrical impulses

Electrophysiology

• Cardiac cells are normally polarized with a positive charge on the outside
• Stimulation causes cardiac cells to depolarize due to sodium rushing inside
• Depolarization causes muscle cells to contract momentarily
• Repolarization reestablishes the electrical imbalance across the cell membrane

Depolarization

• Anions and K+ exist inside the cell
• During depolarization, Na+ and Ca+ enter through Na+ channels, making the cell more positive
• Cardiac contraction begins when the cell depolarizes
• Depolarization isn't the same as contraction; it's the electrical event that leads to it

Repolarization

• Cells recover during repolarization
• Na+ stops flowing in, K+ moves out, and Cl- moves in
• Positive charges in the cell are reduced, which helps the cell return to its resting state

Impulse-Conducting System

• This impulse-conducting system initiates the heartbeat and controls the heart rate
• The SA node normally has the greatest degree of automaticity and paces the heart
• The AV node acts as the back-up pacemaker when the SA node fails
• Electrical impulse is temporarily delayed at the AV node to allow better ventricular filling
• After leaving the AV node, the impulse travels through the bundle of His, bundle branches, and Purkinje fibers
• The impulse-conducting system coordinates contraction of the chambers.
• Defects in the impulse-conducting system can decrease cardiac output and tissue perfusion
• Any heartbeat generated from outside the SA node is considered "ectopic"
• Sympathetic system increase heart rate while parasympathetic slows it

ECG Waves

• Atrial depolarization is seen as the P wave, with a normal length of 3mm and a normal interval of 0.12-0.20 seconds
• Ventricular depolarization is seen as the QRS complex, representing the spread of electrical impulses through the ventricles; its waveform is bigger due to the strength needed to contract ventricles
• The normal QRS complex is not wider than 3 mm (0.12 second)
• The wave of ventricular repolarization is seen as the T wave
• The PR interval is the time from the start of atrial contraction to the start of ventricular contraction (normally not >0.20 second)
• Long PR intervals indicate heart block
• The ST segment is the time from the end of ventricular depolarization to the start of ventricular repolarization and is normally isoelectric
• Elevated or depressed ST segments indicate ischemia

Axis Evaluation

• The mean axis is normally between 0 and +90 degrees
• Right-axis deviation (+90 to +180 degrees) indicates right ventricular hypertrophy
• Left-axis deviation (0 to -90 degrees) indicates left ventricular hypertrophy

Steps to Follow during Evaluation

• Identify the atrial and ventricular rates by multiplying the number of QRS complexes in 6 seconds by 10; 60 divided by how much time between R & R; 300 divided by number of big boxes between R & R waves
• Measure the PR interval which normally should be is <0.20 second