RCP 100: Ch. 18 Electrocardiogram (ECG) Interpretation

• Chapter 18 discusses interpreting the electrocardiogram (ECG).
• ECG electrodes are places at the right arm and left arm
• Also places are the right and left leg

The Electrocardiogram

• An ECG is inexpensive, noninvasive, and easy to obtain, so it's a popular tool.
• It is often used to assess patients suspected of having an acute myocardial infarction.
• It is also used as a health screening tool in patients over age 40 years.
• An ECG cannot predict future heart attacks or detect structural defects like valve stenosis.

Cell Types and Energy

• Myocardial cells contract when electrically stimulated.
• Typically, myocardial cells do not generate electrical impulses; they rely on pacemaker cells.
• 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.
• Cardiac cells depolarize when stimulated because sodium rushes inside the cells.
• Depolarization causes muscle cells to contract momentarily.
• Repolarization reestablishes the electrical imbalance across the cell membrane.
• Inside the cell are anions and K+.
• During depolarization, Na+ comes into the cell from Na+ channels along with Ca+, making the inside more positive.
• Depolarization is an electrical event expected to result in a contraction.
• Repolarization stops the flow of Na+ into the cell, allowing K+ to leave, resulting in the cell becoming more negative than the outside.

Impulse-Conducting System

• The impulse-conducting system initiates the heartbeat and controls the heart rate.
• Normally, the SA node has the greatest degree of automaticity and paces the heart.
• The AV node serves 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 and defects can decrease CO and tissue perfusion.
• Any heartbeat generated from outside the SA node is considered "ectopic."
• The sympathetic system increases HR, while the parasympathetic system slows HR.

Basic ECG Waves

• Atrial depolarization is seen as the P wave, which has a normal length of 3 mm and a normal interval of 0.12-0.20 seconds.
• Ventricular depolarization is seen as the QRS complex and represents the spread of electrical impulses through the ventricles. The waveform is bigger because more strength is needed to contract the ventricles.
• The normal QRS complex is not wider than 3 mm (0.12 second).
• 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 and is 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

• 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 When Interpreting an ECG

• Identify the atrial and ventricular rates: # of QRS in 6 seconds x 10 = ventricular HR
• Also 60 divided by how much time between R & R, or 300 divided by number of big boxes between R & R waves
• Measure the PR interval, which is normally <0.20 second.
• Evaluate the QRS complex; it should be no longer than 0.12 second.
• Evaluate the T wave; normally it should be upright and rounded; inversion indicates ischemia.
• Evaluate the ST segment; normally it is flat; elevation or depression indicates ischemia.
• Assess the RR interval to evaluate the regularity of the rhythm.
• 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.

Normal Sinus Rhythm

• Sinus rhythm reflects normal electrical activity (starts in the SA Node and goes through proper conduction).
• It 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

• Diagnosed when the heart rate exceeds 100/min at rest.
• Each QRS complex is preceded by a P wave.
• Common in patients with acute illness and may be caused by pain, anxiety, fever, hypovolemia, and hypoxemia.
• Also caused by certain medications like 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.
• Only a concern when it causes clinical problems like hypotension and syncope.
• Atropine is an effective treatment.
• Can be normal for athletes, quadriplegic patients.

First-Degree Heart Block

• The PR interval exceeds 0.20 second.
• Each QRS complex is preceded by a P wave.
• Suggests that impulses from SA node are 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

• The First type is Wenckebach or Mobitz type I
  • PR interval gets progressively longer until one does not pass on to the ventricles.
• The second type is Mobitz type II
  • Is less common but occurs with more serious problems such as an MI.
  • Characterized by 2 p-waves with no corresponding QRS

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

• The source for the electrical impulse that triggers contraction that lies outside the sinoatrial node is 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.

Atrial Fibrillation

• Results when the atrial muscle quivers in an erratic pattern; no true P waves are seen.
• The ventricular rate may be slow and irregular.
• Atrial fibrillation causes cardiac output to drop and may lead to thrombi in the atria due to blood stagnation.
• Cardioversion is used as treatment in most cases.
• Can lead to atrial thrombi= life-threatening i.e. PE or stroke

Premature Ventricular Contractions

• Occur when ectopic beats originate in the ventricles.
• Occasional PVCs are not a major concern and do not require treatment.
• PVCs are commonly the result of hypoxia, electrolyte imbalances, and acid-base disorders.
• QRS complex is wide and has no preceding P wave.
• Frequent PVCs call for treatment of the underlying cause with lidocaine in some cases.

Ventricular Tachycardia

• Represent a run of three or more PVCs
• Easy to recognize as a series of wide QRS complexes with no preceding P wave
• Ventricular rate is usually 100 to 250/min.
• VT represents a serious arrhythmia that often progresses to V fib if untreated.
• Treatment includes cardioversion and medications.

Ventricular Fibrillation

• Represents the most life-threatening arrhythmia
• Defined as erratic quivering of the ventricular muscle mass
• Causes cardiac output to drop to zero
• The ECG show grossly irregular fluctuations with a zigzag pattern.
• Treatment includes cardioversion, CPR, oxygen, and antiarrhythmic medications.