ECG interpretation

• ECG tracings reflect changes in the magnitude and direction of electrical activity during atrial and ventricular depolarization and repolarization.
• ST segment elevation or depression is measured 0.08 seconds after the J point.
• Unipolar leads measure the electrical potential between a positive electrode and a zero reference point, not between positive and negative electrodes.
• The S wave is always a negative deflection.
• When electrical current flows toward a positive electrode, the ECG will show an upright, or positive, deflection.
• Ventricular depolarization normally begins at the endocardial surface and progresses to the epicardial surface.
• The R wave is always a positive deflection.

ECG Test Points

• V2 is NOT a limb lead.
• In Lead II, the positive electrode is on the left leg.
• The V4 electrode is located on the mid-clavicular line at the level of the 5th intercostal space.
• Leads I, aVL, V5, and V6 view the left lateral surface of the heart.
• In Lead I, the negative electrode is on the right arm.
• In Lead aVL, the positive electrode is on the left arm.
• Lead aVF detects the electrical potential between a positive electrode on the left leg and the central terminal.
• Leads V3 and V4 are placed over the interventricular septum and part of the left ventricle.

ECG Intervals

• The normal duration of a P wave is ≤ 0.10 seconds.
• The normal amplitude of a P wave in Lead II is 0.5 to 2.5 mm high.
• The normal duration of the QRS complex is ≤ 0.10 seconds.
• The TP segment occurs between the end of the T wave and the beginning of the next P wave; it represents the period when the ventricles are relaxed and filling, not late ventricular repolarization.
• The PR interval (PRI) represents the time from the beginning of atrial depolarization to the beginning of ventricular depolarization, from the P wave until the QRS complex.
• It does NOT include the R wave of the QRS complex.

AV Blocks

• Cardiac pacemakers are often needed for 2nd degree AV block, Type II.
• 1st degree and 2nd degree Type I AV blocks typically produce no symptoms and require monitoring.
• 2nd degree, Type I (Wenckebach) AV block: Progressive lengthening of the PR interval until a QRS complex is dropped.
• 3rd degree AV block: Independent pacing of the atria and ventricles with no correlation between P waves and QRS complexes.
• 2nd degree Type II AV block: Constant PR intervals and regularly dropped QRS complexes.
• A P:QRS conduction ratio of 4:1 is labeled as a "high grade" or advanced AV block.
• A sinus rhythm with a constant PR interval of 0.24 seconds and one QRS complex following each P wave describes a 1st degree AV block.
• A 3:1 AV block means there are 3 P waves for each QRS complex.

Coronary Artery Disease (CAD)

• Coronary atherosclerosis involves plaque formation and is the primary disease process in the development of coronary artery disease.
• Small, rounded, upright T waves are NOT a sign of myocardial ischemia.
• ST depression >1 mm in V5 and V6 is considered clinically significant.
• The two major branches off the left coronary artery are the left circumflex and left anterior descending arteries.
• The SA node is supplied by the left circumflex or right coronary artery in 50-60% of people.
• ST depression in leads II, III, and aVF suggests ischemia or injury in the inferior wall of the left ventricle.
• Pathologic or significant Q waves in leads V1, V2, and V3 suggest myocardial infarction affecting the anterior wall of the left ventricle.
• A pathologic or significant Q wave has a depth of at least 25% of the height of the QRS complex and/or is at least 1 mm wide.
• The left circumflex artery typically supplies the lateral wall of the left ventricle.
• Prolonged occlusion of the left anterior descending artery is most likely to produce an infarct in the anteroseptal area of the left ventricle.
• Inverted T waves (where T wave inversions at rest are not a normal variation) and ST depression are signs of ischemia.
• Clinically significant Q waves in leads I, aVL, V5, and V6 suggest a previous high lateral wall infarction.

ECG Interpretation

• Evidence of a previous inferior infarction is seen on the inferior surface of the heart.
• No evidence of current ischemia or a previous anterior infarction is seen on the anterior surface of the heart.
• Evidence of current lateral ischemia is seen on the lateral surface of the heart.
• Anteroseptal ischemia is evident on the ECG.
• There is no ECG evidence of a previous infarction,

Cardiac Cells

• Cells in the SA and AV nodes do not contain actin and myosin, so they cannot contract.
• The function of the electrical conduction system is to transmit impulses to the atria and ventricles, initiating contraction of myocardial cells.
• Myocardial cells have fewer gap junctions than cells in the SA nodes.
• During ventricular diastole, the ventricles are relaxed and filling with blood, not contracting.
• Ectopic beats and rhythms result from spontaneous depolarization of nonpacemaker cells.
• Cardiac cells divide into branches which connect with adjacent cells, forming a network that permits rapid conduction of electrical impulses
• The electrical activity recorded by an ECG results from the movement of ions across cell membranes.
• Myocardial cells in the left ventricle pump against the greatest resistance.
• As pressure in the ventricles increases, the aortic and pulmonic valves open.
• Repolarization is complete at the end of phase 3 of the cardiac action potential and interior of the cell is negatively charged, Na+ is inside, K+ is outside.
• During phase 0 of the cardiac action potential, the cell membrane reaches threshold potential, triggering fast Na+ channels to open. The interior of the cell becomes positively charged relative to the exterior.
• Increased sympathetic activity results in increased heart rate and contractility.
• The AV node delays the electrical impulse approximately 0.1 seconds.
• Bachmann's Bundle is directly responsible for transmitting electrical impulses to the left atrium.
• Blood is returned from the coronary circulation to the right atrium via the coronary sinus.
• The right ventricle pumps blood to the lungs against low resistance.
• The innermost layer of the myocardial wall is the endocardium.
• Myocardial cells do have the ability to depolarize in response to an electrical stimulus.
• Myocardial cells in the right atrium, left ventricle and the AV node can act are all potential escape pacemakers.

Ventricular Hypertrophy

• ST depression and/or inverted T waves are often seen on an ECG that shows characteristics of right or left ventricular hypertrophy.
• Wide, notched P waves in lead II greater than 0.10 seconds are most likely associated with left atrial enlargement.
• The R wave in V1 is taller than the depth of the S wave is most likely associated with right ventricular hypertrophy.
• Symmetric, peaked P waves in lead II measuring at least 3 mm tall is most likely associated with right atrial enlargement.
• The sum of the R wave in V5 or V6 and the S wave in V1 is greater than or equal to 35 mm is most likely associated with left ventricular hypertrophy.
• Chronic obstructive pulmonary disease (COPD) is NOT a likely cause of left ventricular hypertrophy.
• Pulmonary disease, such as COPD or a pulmonary embolism is a likely cause of right ventricular hypertrophy.

Coronary Artery Anatomy

• The left anterior descending artery comes down from the left side of the heart.
• The left circumflex artery wraps around the side and goes to the back of the heart.
• The right coronary artery supplies the SA node in 50-60% of people.
• The right coronary artery supplies the AV node in 85-90% of people.
• The right coronary artery provides blood flow to the inferior wall of the right ventricle.
• The left anterior descending artery supplies blood to the front and bottom of the left ventricle and the front of the septum.
• The left circumflex artery supplies the SA node in 40-50% of people.
• The left circumflex artery supplies blood to the lateral walls of the left ventricle, the left atrium, and the left posterior fasciculus of the left bundle branch.

Ischemia and Infarction

• The left ventricle is more likely to have problems than the right.
• Ischemia is reversible.
• Ischemia on an ECG is defined by ST depression and inverted T waves.
• Subendocardial ST depression affects the innermost part of the myocardial wall.
• Ischemia is a lack of blood flow and oxygen to the heart due to inadequate blood flow.
• Evidence of ischemia requires observation in two leads that view the same area of the heart.
• Transmural ischemia shows T wave inversion.
• Transmural ischemia goes through the myocardial wall.
• An inverted T wave in V5 and V6 represents the left circumflex artery.
• An inverted T wave and ST depression in V1, V2, and V3 represents the left anterior descending artery.
• ECG changes showing previous infarction/heart attack include significant Q waves that are at least 25% of the distance or height of the QRS complex or at least 1 mm wide.
• Infarction is not reversible.
• Transmural infarction shows significant Q waves.
• The subendocardial of infarction show ST depression and T wave inversion in leads facing the infarction.
• The process from ischemia to injury to infarction results from the failure of coronary arteries to supply sufficient oxygen to the myocardial tissue demand.
• V1, V2, and V3: Anteroseptal
• V4, V5, and V6: Anterolateral
• I and aVL: High lateral LV
• II, III, and aVF: Inferior leads
• High lateral leads I and aVL have the positive electrode on the left arm.
• Inferior leads I, II, and aVF have the positive electrode on the left leg.
• You need at least 2 leads to rule out ischemia or random activity on ECG.

Causes of Heart Conditions

• Direct causes of heart conditions include coronary thrombosis and artery spasm.
• Indirect causes include decreased coronary arterial blood flow and increased myocardial workload.
• Atherosclerosis is a disease in which lipids and fibrous materials are deposited in artery walls.
• ST depression can be downslopping, upslopping, or horizontal.
• Upslopping ST depression: Better outcome, heart has energy to depolarize slowly
• 1mm wide or more is significant for Q Waves.

ECG Accuracy and Considerations

• Diagnosing infarction with ECG is approximately 85% accurate.
• ECG diagnosis is useful in LV infarction.
• Diagnosing with ECG is not valid when a LBBB or hypertrophy exists.
• An ECG is more sensitive when trying to detect transmural than subendocardial infarction.

Cardiac Cell Properties

• All resting, polarized cardiac cells have the property of excitability.
• Cardiac cells divide at their ends into branches that connect with adjacent cells, forming a network of cells (syncytium) that permits rapid conduction of electrical impulses.
• Cardiac cells in the right ventricle cannot function as an escape pacemaker.

Electrophysiology: Ions

• Na+ outside cell
• K+ inside cell
• Phosphate, sulfate, and protein ions inside cell

Interior/Exterior Cell

• Interior of cell is - compared to exterior.
• Negative electrical potential exists across the cell membrane.
• Resting membrane potential -90 mV

ECG Components

• P wave (atrial depolarization)
• QRS Complex (Ventricular Depolarization)
• T Wave (Ventricular Repolarization)

ECG Preparation

• Locate sites, clean with alcohol.
• For exercise ECG, abrade sites until somewhat red.
• Use gauze to "dust off" dry skin cells.
• Attach electrodes to each site. For exercise, after attaching leads, use mesh shirt.