Understanding ECGs: Cell Types and Electrophysiology

'- An ECG is a popular, inexpensive, and noninvasive tool for assessing patients, particularly those suspected of having an acute myocardial infarction.

• ECGs are also used as a health screening tool for patients over 40.
• ECGs cannot predict future heart attacks or detect structural defects like valve stenosis.

Cell Types

• Myocardial cells contract when electrically stimulated, relying on pacemaker cells to generate electrical impulses.
• Pacemaker cells spontaneously generate and conduct electrical impulses.

Electrophysiology

• Cardiac cells are normally polarized with a positive charge on the outside.
• Stimulation causes cardiac cells to depolarize as sodium rushes inside.
• Depolarization leads to muscle cell contraction.
• Repolarization reestablishes the electrical imbalance across the cell membrane.

Depolarization

• During depolarization, Na+ and Ca+ enter the cell, making the inside more positive.
• Cardiac contraction begins when the cell depolarizes.
• Depolarization is the electrical event that should result in contraction.
• Depolarization is the movement of charged particles across a cell membrane, causing the inside of the cell to become positive.

Repolarization

• During repolarization Na+ stops flowing into the cell, and K+ moves out, reducing positive charges and returning the cell to its resting state.
• Repolarization stops the flow of Na+ into the cells, allowing K+ to leave.
• The cell becomes more negative than the outside, resulting in polarization.

The Impulse-Conducting System

• This system initiates the heartbeat and controls heart rate.
• The SA node normally paces the heart due to its high automaticity.
• The AV node serves as a backup pacemaker if the SA node fails, with a temporary delay to allow better ventricular filling.
• Impulses travel from the AV node through the bundle of HIS, bundle branches, and Purkinje fibers.
• System initiates heartbeat and controls HR, coordinating contraction of the chambers.
• Defects in this system can decrease CO and tissue perfusion.
• Any heartbeat generated outside the SA node is considered "ectopic."
• The sympathetic system increases HR, while the parasympathetic system slows it.

Basic ECG Waves

• P wave represents atrial depolarization (normal length = 3 mm).
• Normal P-R interval is 0.12-0.20 seconds.
• QRS complex represents ventricular depolarization (spread of electrical impulses through the ventricles).
• Normal QRS complex is not wider than 3 mm (0.12 second). Wave form is bigger because more strength is needed to contract ventricles
• T wave represents ventricular repolarization.
• 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 may indicate heart block.
• ST segment represents the time from the end of ventricular depolarization to the start of ventricular repolarization (normally isoelectric).
• Elevated or depressed ST segments may indicate ischemia. Axis Evaluation •Normally, the mean axis is between 0 and +90 degrees.

•Right-axis deviation (+90 to +180 degrees) is consistent with right ventricular hypertrophy.

•Left-axis deviation (0 to 90 degrees) is consistent with left ventricular hypertrophy

Steps to Follow 1.Identify the atrial and ventricular rates; # of QRS in 6 seconds x 10 = ventricular HR. 60 divided by how much time between R & R. 300 divided by number of big boxes between R & R waves

2.Measure the PR interval; normally this is <0.20 second.

3.Evaluate the QRS complex; it should be no longer than 0.12 second.

4.Evaluate the T wave; normally it should be upright and rounded; inversion = ischemia Steps to Follow (cont.) 5. Evaluate the ST segment; normally it is flat; elevation or depression = ischemia

6. Assess the RR interval to evaluate the regularity of the rhythm.

7. Identify the mean QRS axis by finding the limb lead with the most voltage; if this lead has a positive QRS complex the axis is very close to where this lead is labeled on the hexaxial reference circle. Rhythm •“Sinus” rhythm reflects normal electrical activity (starts in SA Node and goes through proper conduction)

•Has an upright P wave that is identical throughout the strip

•The PR interval is <0.20 second.

•The QRS complexes are identical and no longer than 0.12 second.

•The ST segment is flat.

•The RR interval is regular, and the heart rate is 60 to 100/min. Sinus Tachycardia •Recognized when the heart rate exceed 100/min at rest

•Each QRS complex is preceded by a P wave.

•A common finding in patients with acute illness and may be caused by pain, anxiety, fever, hypovolemia, and hypoxemia.

•It may also be caused by certain medications such as bronchodilators. Sinus Bradycardia •Heart rate is less than 60/min.

•Each QRS is preceded by a P wave.

•PR interval and QRS complex are normal.

•Of concern only when it causes clinical problems such hypotension and syncope

•Atropine is an effective treatment. •Normal for athletes, quadriplegic patients 0 First-Degree Heart Block •The PR interval exceeds 0.20 second.

•Each QRS complex is preceded by a P wave.

•Indicates impulses from SA node is getting through the ventricles, but delayed in passing through AV node or bundle of HIS.

•Typically, the RR intervals are regular.

•May occur after an MI or with the use of beta-blockers (digoxin or beta blockers)

•Treatment may not be needed. Second-Degree Heart Block •Comes in two types –Type I (Wenckebach or Mobitz type I) block; recognized when PR interval gets progressively longer until one does not pass on to the ventricles.

–Type II (Mobitz type II) is less common but occurs with more serious problems such as an MI. Longer, Longer, Longer, Drop – then you have a Wenckeback Second Degree Type I Third-Degree Heart Block •Conduction system between the atria and ventricles is completely blocked. Impulses generated in SA node are not conducted to ventricles.

•The atria and ventricles are paced independently and there is no relationship between the P waves and the QRS complexes.

•The PP intervals and RR intervals are regular.

•Treatment includes medications to speed up the heart and a placement of a pacemaker. Ectopic Beat •Source for the electrical impulse that triggers contraction that lie outside the sinoatrial node are referred to as ectopic foci. Atrial Flutter •Represents rapid depolarization of the atria from an ectopic focus (250 to 350/min)

•Causes a characteristic sawtooth pattern; numerous P waves are present for each QRS

•Caused by a wide variety of disorders such as rheumatic heart disease, coronary heart disease, renal failure, stress, and hypoxemia

•Treated with medications and cardioversion