Cardio Lecture 2 Part 2 - Intro to ECG

Horizontal Electrical Position (of the heart)

(Horizontal QRS Axis) Ventricular depolarization is directed mainly horizontally and to the left in the frontal plane In Frontal Plane Diagram, Positive poles of leads I and aVL are oriented horizontally and to the left, therefore, when the heart is electrically horizontal, the *QRS voltages are directed toward leads I and aVL* --> *Tall R wave in Lead I and Lead aVL* (usually as part of a qR complex)

QRS Calculation: Example 4

(Pic) - QRS complex is positive in leads II, III, and aVF. Therefore the axis is relatively vertical. - Because the R waves are of equal magnitude in leads I and III, the mean QRS axis must be oriented between these two leads (+60°) Alternatively: Lead aVL, shows a biphasic RS-type complex = axis must be at right angles to lead aVL (-30°); (either -120° or +60°) - Electrical axis must be oriented toward lead II, which shows a relatively tall R wave so we know Axis is at +60°

QRS Calculation: Example 5

(Pic) Electrical axis is oriented away from leads II, III, and aVF and *toward leads aVR and aVL, which show positive complexes* - R waves are of equal magnitude in leads aVR and aVL --> axis must be oriented precisely between these leads (-90°) Alternatively: Lead I shows a biphasic RS complex = axis must be directed at right angles to lead I (0°); (must be either -90° or +90°) - Because the axis is oriented away from the positive pole of lead aVF and toward the negative pole of that lead, it must be -90°

Vertical Electrical Position (of the heart)

(Vertical QRS Axis) Ventricular depolarization mainly directed downward - In Frontal Plane Diagram, Positive poles of leads II, III, and aVF are oriented downward, therefore, when the heart is electrically vertical, the *QRS voltages are directed toward leads II, III, and aVF* --> *Relatively Tall R wave in Leads II, III, and aVF* (usually as part of a qR complex)

Mean QRS (Electrical) Axis

*= general direction in the frontal plane toward which the QRS complex is predominantly pointed* (Depolarization stimulus spreads through the ventricles in different directions from instant to instant. A vector arrow to represent the overall (mean) direction in which the QRS complex is pointed in the frontal plane of the body = electrical axis of the QRS complex)

Polarity of T Wave in Extremity Leads

*Horizontal heart*: main QRS deflection is positive in leads I and aVL and the T wave is also positive in these leads *Vertical heart*: QRS is positive in leads II, III, and aVF and the T wave is also positive in these leads - on some normal ECGs with a vertical axis T wave may be negative in Lead III

Normal vs Shifted P Wave

*Normal*: sinus rhythm present, P wave Negative in lead aVR and Positive in lead II = P wave directed toward the positive pole of lead II, axis ~+60° *Shifted*: If AV junction is pacing the heart, atria are stimulated in a retrograde way --> atrial depolarization spreads upward, toward lead aVR and away from lead II: Lead aVR: Positive P wave Lead II: Negative P wave --> Mean P wave axis ~-150°

Left Atrial Abnormality (LAA)

*P mitrale* Normally L atrium depolarizes after R atrium L atrial enlargement (LAE) prolongs total duration of atrial depolarization --> *abnormally wide P wave w/ duration of 0.12 sec or more (at least three small boxes)* - P Wave *Amplitude may be normal or increased* - P wave frequently has *humped/notched appearance*

Important Causes of RAA

*Usually associated w/right ventricular enlargement* Two of the most common clinical causes of RAA are: 1.) *Pulmonary Disease*: - acute (bronchial asthma, pulmonary embolism) - chronic (emphysema, bronchitis) 2.) *Congenital Heart Disease*: - pulmonary valve stenosis - atrial septal defects - Ebstein's anomaly (malformation of tricuspid valve) - Tetralogy of Fallot

Lead V6 in QRS

*qR pattern* 1.) q wave is a septal q wave, reflecting the left-to-right spread of the stimulus through the septum away from lead V6 2.) Positive (R) wave reflects the leftward spread of ventricular stimulation voltages through the left ventricle

Lead V1 in QRS

*rS type of complex* 1.) Small initial r wave represents the *left-to-right spread of septal stimulation* in Phase 1 = Septal R Wave 2.) The Negative (S) wave reflects the spread of ventricular stimulation forces during phase 2, away from the right and toward the dominant left ventricle

What determines Mean QRS Axis?

- Anatomic Position of the Heart - Direction in which the stimulus spreads through the ventricles (direction of ventricular depolariation)

Horizontal Electrical Position of the Heart - Comparison to Leads V5 and V6

- Leads I and aVL show qR complexes similar to the qR complexes seen normally in the left chest leads (V5 and V6) - Leads II, III, and aVF show rS or RS complexes similar to those seen in the right chest leads normally

Vertical Electrical Position of the Heart - Comparison to Leads V5 and V6

- Leads II, III, and aVF show qR complexes similar to those seen in the left chest leads - Leads I and aVL show rS-type complexes resembling those in the right chest leads

Important Causes of LAD

- Pts with Left Ventricular Hypertrophy (LVH) - Left Anterior Fasicular Block (Hemiblock) (fairly common cause of LAD) - Left Bundle Branch Block - May occur in Absence of apparent cardiac disease

Normal T Wave on ECG

1.) As a rule, the *T wave follows the direction of the main QRS deflection* (main QRS deflection is positive (upright), the T wave is normally positive and vice versa) 2.) Normal T wave is always *Negative in Lead aVR* but *Positive in lead II* 3.) *Left-sided chest leads (V4-V6) normally show a *Positive T wave* 4.) T wave in the other leads may be variable - *Right chest leads (V1 and V2)* T wave may be normally negative, isoelectric, or positive but it is almost *always positive by lead V3 in adults* 5.) *If the T wave is positive in any chest lead, it must remain positive in all chest leads to the left of that lead. Otherwise, it is abnormal.*

Normal Sinus Rhythm (Lecture Version)

1.) P waves positive in lead II (and negative in lead aVR) 2.) HR between 60-100 bpm 3.) 1:1 P to QRS ratio (P for every QRS)

Normals (Lecture Version)

1.) Sinus P wave (generated in high right atrium), should be upright (in sinus rhythm) in lead II, because vector and direction of depolarization are toward 60 degrees 2.) QRS complexes should be mostly upright in leads pointed in direction of left ventricle (leads I, II, V5-V6) 3.) There are exceptions with normal variants

In summary, the mean QRS axis can be determined on the basis of one or both of the following rules:

1.) The mean QRS axis points midway between the axes of two extremity leads that show tall R waves of equal amplitude 2.) The mean QRS axis points at 90° (right angles) to any extremity lead that shows a biphasic (QR or RS) complex and in the direction of leads that show relatively tall R waves

Sinus Rhythm "Criteria" (bla bla convuluted bla)

1.) When asked to define criteria for sinus rhythm, most mention the requirement for a P wave before each QRS complex and a QRS after every P, along with a regular rate and rhythm. However, these criteria are NOT necessary or sufficient. Term "Sinus Rhythm" answers the question of *what pacemaker is controlling the atria*. - Can see sinus rhythm with any degree of heart block, including complete heart block, and even with ventricular asystole (no QRS complexes during cardiac arrest!). 2.) Can also have a P wave before each QRS and not have sinus rhythm, but an ectopic atrial mechanism

Along with tall right chest R waves, RVH often produces two additional ECG signs:

1.) right axis deviation (RAD) 2.) T wave inversions in right to mid-precordial leads

Is This Normal Sinus Rhythm? HR = 65 bpm P wave upright in II, inverted in aVR P for every QRS

?

Three Basic "Laws of Electrocardiography: Law 2

A *Negative (downward) deflection* appears in any lead if the wave of *depolarization spreads toward the negative pole* of that lead (or away from the positive pole) - Thus, if the *atrial stimulation* path spreads downward and to the left, a negative P wave is seen in lead aVR - If the *ventricular stimulation* path is directed entirely away from the positive pole of any lead, a negative QRS complex (QS deflection) is seen

Three Basic "Laws of Electrocardiography: Law 1

A *positive (upward) deflection* appears in any lead if the wave of *depolarization spreads toward the positive pole* of that lead - Thus, if the path of atrial stimulation is directed downward and to the patient's left, toward the positive pole of Lead II, a positive (upward) P wave is seen in lead II - Similarly, if the ventricular stimulation path is directed to the left, a positive deflection (R wave) is seen in lead I

Pulmonary Embolsim on ECG

A sudden shift in the mean QRS axis to the right (not necessarily causing actual RAD) may occur with acute pulmonary embolism (Dr. Leinicke is always RIGHT about the diagnosis of PE!)

P pulmonale

Abnormal P wave in RAA is sometimes referred to as P pulmonale because the atrial enlargement that it signifies often occurs with severe pulmonary disease

Right Ventricular Hypertrophy (RVH)

Affects both depolarization (QRS complex) and repolarization (ST-T complex) - hypertrophy of the heart muscle alters normal repolarization sequence RVH: characteristic repolarization change = appearance of *inverted T waves in the right (V1) and middle chest leads*

Left Axis Deviation (LAD)

An axis of *-30° or more negative* is described as left axis deviation - an abnormal extension of the mean QRS axis found in persons with an electrically *horizontal heart*

Right Axis Deviation (RAD)

An axis that is *+100° or more positive* - an abnormal extension of the mean QRS axis in persons with an electrically *vertical heart*

Anterior wall MI Effect on normal R wave progression

Anterior wall infarction results in the death of myocardial cells and the *loss of normal positive (R wave) voltages*. One major ECG sign of an anterior wall infarction is the loss of normal R wave progression in the chest leads

QRS Calculation "Rule 1"

As a general rule, the mean QRS axis points midway between any two leads that show tall R waves of equal height

*Normal R Wave Progression* (What happens between leads V1 and V6?)

As you move across the chest (in direction of the electrically predominant left ventricle), the *R wave tends to become relatively larger* and the *S wave becomes relatively smaller* - increase in height of the R wave usually reaches a max around lead V4 or V5

Transition Zone

At some point (around V3 or V4 position) the *ratio of the R wave to the S wave becomes 1* When amplitude of the R wave = S wave = Transition Zone *Early Transition*: in ECGs of some normal people the transition may be seen as early as lead V2 *Delayed Transition*: in other cases transition zone may not appear until leads V5 and V6

AV Junctional Rhythm P Wave

Atrial *depolarization must spread up the atria in a retrograde direction* = opposite normal sinus rhythm Vector points *upward and to the right* 1.) *aVR Lead* records *Positive* P (depolarization moving towards positive pole) 2.) *Lead II* records *Negative* P wave (depolarization moving away from positive pole)

Atrial Hypertrophy vs Ventricular Hypertophy

Atrial Hypertrophy affects P Wave Ventricular Hypertophy affects QRS Complex

Normal Sinus Rhythm P Wave

Atrial depolarization First waveform seen in any cycle Path spreads (from the SA Node) from *right to left and downward toward the AV junction* (away from positive pole of led aVR, towards positive pole of Lead II) - Represented by an arrow (vector) pointing downward and to pt's left 1.) *Lead aVR* always shows a *negative P wave* 2.) *Lead II* always records a *positive* (upward) P wave

Causes of Cardiac Enlargement

Both dilation and hypertrophy usually result from some type of *chronic pressure or volume load* on the heart muscle. In rarer cases, cardiac enlargement can result from genetic abnormalities. (e.g. arrhythmogenic right ventricular cardiomyopathy ("dysplasia") and hypertrophic cardiomyopathy syndromes)

Direction of Ventricular Depolarization (QRS Complex): Phase 1 *Septal Stimulation*

Brief duration (<0.04sec) and Small amplitude Results from *spread of stimulus through the interventricular septum* - septum is the first part of the ventricles to be stimulated Left side of the septum is stimulated first (by a branch of the left bundle of His) --> *depolarization spreads from the left ventricle to the right across the septum* Phase one of ventricular depolarization (septal stimulation) can therefore be represented by a *small arrow pointing from the left septal wall to the right*

QRS Calculation: Example 3

By inspection the mean QRS axis is obviously horizontal, because *Leads I and aVL are positive* and *Leads II, III, and aVF are predominantly Negative* Precise electrical axis can be calculated by looking at lead II, which shows a biphasic RS complex (therefore the axis must be at right angles to lead II) - Because lead II is at +60° on the hexaxial scale, the axis must be either -30° or +150° --> If it were +150°, leads II, III, and aVF would be positive --> Clearly the axis is -30°

P mitrale

Characteristic P wave changes seen in LAA, P wave sometimes has distinctive humped/notched appearance - *Second hump corresponds to the delayed depolarization of the left atrium* - humped P waves usually best seen in one or more of the *extremity leads* [Term P mitrale sometimes used to describe wide P waves seen with LAA bc they were first observed in pts w/ rheumatic mitral valve disease]

Important Causes of LAA

Clinically, LAA may occur in a variety of important settings, including: 1.) Valvular heart disease, particularly aortic stenosis, aortic regurgitation, mitral regurgitation, and mitral stenosis 2.) Hypertensive heart disease, which causes left ventricular enlargement and eventually LAA 3.) Cardiomyopathies (dilated, hypertrophic, and restrictive) 4.) Coronary artery disease (CAD)

QRS in Extremity Leads I, II, III, aVL, and aVF

Complicated - QRS patterns in the extremity leads show considerable normal variation e.g. some normal people may show qR-type complexes in leads I and aVL and rS-type complexes in leads III and aVF (Fig. 4-10) - Other people may be opposite, w/ qR complexes in leads II, III, and aVF and RS complexes in lead aVL and sometimes lead I (Fig. 4-11)

Cardiac Enlargement

Dilation of a heart chamber or Hypertrophy of the heart muscle (often occur together) 1.) *Dilation of a chamber*: heart muscle is stretched and chamber becomes enlarged (e.g. in heart failure caused by acute aortic or mitral valve regurgitation, the left ventricle dilates) 2.) *Cardiac Hypertrophy*: heart muscle fibers increase in size, w/ resultant chamber enlargement (e.g. aortic stenosis, which chronically obstructs outflow of blood from left ventricle, leads to hypertrophy of the left ventricular muscle)

Figure 4.8

Examine the set of normal chest leads in Pic. Notice the rS complex in lead V1 and the qR complex in lead V6. - The R wave tends to become gradually larger as you move toward the left chest leads. *Transition zone* (R wave = S wave) is in *Lead V4* - In normal chest leads R wave voltage does not have to become literally larger as you go from leads V1 and V6 BUT overall trend should show a *relative increase*. (e.g. Pic: notice that the complexes in leads V2 and V3 are about the same and that the R wave in lead V5 is taller than the R wave in lead V6)

QRS Calculation: Method 2 (Using Biphasic Complexes)

If a wave of depolarization is oriented at right angles to any lead axis, a biphasic complex (RS or QR) is recorded in that lead --> if you find a biphasic complex in any of the extremity leads, the mean QRS axis must be directed at 90° to that lead (Note in pic: aVL and aVR are Not biphasic bc r and s are not equal in amplitude)

Three Basic "Laws of Electrocardiography: Law 3

If the *mean depolarization path is directed at right angles (perpendicular) to any lead, a small biphasic deflection (consisting of positive and negative deflections of equal size) is usually seen* - If the *atrial stimulation* path spreads at right angles to any lead, a biphasic P wave is seen in that lead - If the *ventricular stimulation* path spreads at right angles to any lead, the QRS complex is biphasic - A biphasic QRS complex may consist of either an RS pattern or a QR pattern

Sinus rhythm with 1:1 AV conduction"

If you state that the rhythm is "normal sinus" and do not mention any AV node conduction abnormalities, listeners will assume that each P wave is followed by a QRS and vice versa. The more technical and physiologically pure way of stating this finding would be to say, "Sinus rhythm with 1:1 AV conduction."

Influence of *Cardiac Anatomic Position* on Mean QRS/Deviation

Illustrated by effects of respiration 1.) Pt breathes in, diaphragm descends and heart becomes more vertical in the chest cavity --> shifts QRS electrical axis vertically (to the Right) - Pts w/ emphysema and chronically hyperinflated lungs usually have anatomically vertical hearts and electrically vertical QRS axes 2.) Pt breathes out, diaphragm ascends and the heart assumes a more horizontal position in the chest --> shifts QRS electrical axis horizontally (to the Left)

Influence of *Direction of Ventricular Depolarization* on Mean QRS/Deviation

Illustrated by: 1.) Left anterior fascicular block --> spread of stimuli through the more superior and leftward portions of the left ventricle is delayed --> mean QRS axis shifts to the Left 2.) w/right ventricular hypertrophy (RVH) the QRS axis shifts to the Right

LAA Biphasic P Wave

In LAA, *Lead V1* sometimes shows distinctive *biphasic P wave* w/ a small, initial positive deflection and a prominent, wide negative deflection (*"That's One (V1) Loopy (LAA) P-Wave"*) Negative component is >0.04sec duration or 1mm or more in depth - corresponds to the delayed stimulation of the enlarged left atrium Initial Positive deflection indicates R atrial depolarization; deep Negative deflection result of left atrial depolarization voltages directed posteriorly (away from Lead V1 positive pole)

Calculating the Mean QRS Example: Method 1 (Using R Waves)

In Pic for example: - notice the tall R waves in leads II, III, and aVF (indicate that the heart is electrically vertical). - R waves are equally tall in leads II and III Therefore, by simple inspection the mean electrical QRS axis can be seen to be directed between the positive poles of leads II and III and toward the positive pole of lead aVF (+90°)

LAA

In some cases of LAA, you may see both *broad, often humped P waves in leads I and II* AND *biphasic P wave in lead V1* - In other cases only one or the other is seen LAA by ECG, esp when marked, indicates *increased risk of A-Fib*; conversely, patients with a history of paroxysmal A-Fib often have ECG signs of LAA when in sinus rhythm

Normal Mean QRS Axis

In the ECGs of most normal people the axis lies *between -30° and +100°*

MI of the lateral wall of the left ventricle on ECG

In this setting, loss of the normal leftward depolarization forces may lead "by default" to a rightward axis

Axis Deviation: Instant Recognition

Inspecting the QRS complex from Leads I and II 1.) QRS complexes in both leads Positive --> axis must be normal 2.) QRS complex predominantly Positive in Lead I and Negative in Lead II --> LAD is present 3.) QRS complex predominantly Negative in Lead I and Positive in Lead II --> RAD (or at least borderline RAD) is present 4.) Rarely QRS will be predominantly negative in both Leads I and II. In such unusual cases, Extreme right or left axis deviation will be present

Direction of Ventricular Depolarization (QRS Complex): Phase 2

Involves simultaneous stimulation of the main mass of both the left and right ventricles from the *inside (endocardium) to the outside (epicardium)* of the heart muscle. - In the normal heart the *left ventricle is electrically predominant* (it electrically overbalances the R ventricle) --> *Arrow representing phase two of ventricular stimulation points toward the left ventricle*

Ventricular Depolarization QRS Complex: Phase 2 *Lead V1 vs Lead V6*

Large arrow pointing in the direction of the left ventricle - points away from the positive pole of lead V1 and toward the negative pole of lead V6 1.) *Negative deflection* in the right precordial leads --> *Lead V1 shows a deep negative (S) wave = rS complex.* 2.) *Positive deflection* in the left precordial leads --> *Lead V6 displays a tall positive (R) wave = qR pattern*

Recognizing LAD on ECG

Lead I shows a tall R wave Lead III shows a deep S wave Lead II shows either a biphasic RS complex (with the amplitude of the S wave exceeding height of the r wave) or a QS complex Leads I and aVL both show R waves.

QRS Calculation: Example 6

Lead aVR shows a biphasic RS-type complex = axis must be at right angles to the axis of that lead. The axis of aVR is at -150°; therefore, the electrical axis in this case must be either -60° or +120°. Clearly it is -60° because lead aVL is positive and lead III shows a negative complex.

Right vs Left Atrium Anatomical Positions

Left atrium is situated posteriorly, up against the esophagus Right atrium lies anteriorly, against the sternum

Effect of Left or Right Ventricular Hypertrophy on Chest Lead Patterns

Left ventricle is normally electrically predominant and L ventricular depolarization produces deep (negative) S waves in the right chest leads with tall (positive) R waves in the left chest leads *L Ventricular hypertrophy*: L ventricular voltages are further increased, resulting in *very tall R waves in the left* chest leads and *very deep S waves in the right* chest leads *Right ventricular hypertrophy*: shifts the balance of electrical forces to the right, producing *tall positive waves (R waves) in the right* chest leads

RVH: Chest leads to the Left of Leads showing tall R waves

May display a variable pattern: - Sometimes the middle and left chest leads show slow R wave progression, with rS or RS complexes all the way to lead V6 - In other cases, normal R wave progression is preserved and the left chest leads also show R waves

Right Atrial Abnormality (RAA)

May produce *abnormally Tall P Wave (2.5mm or more)* = *P pulmonale* Occasionally associated w/a deep (negative) but narrow P wave in lead V1, due to the relative inferior location of the right atrium relative to this lead --> may cause confusion with L atrial abnormality (LAA). However: pure RAA does not increase total duration of atrial depolarization = *width of the P wave is normal (<0.12sec)*

Electrical Position

Mean QRS Axis - In simplest terms the electrical position of the heart may be described as either *horizontal or vertical*

QRS Calculation "Rule 2"

Mean QRS axis is oriented at *right angles to any lead showing a biphasic complex* - In this situation the mean QRS axis points *in the direction of leads showing tall R waves*

Normal vs Shifted T Wave

Mean T wave axis and mean QRS axis normally point in the same general (but not identical) direction: 1.) Heart is horizontal: T waves Positive in Leads I and aVL, w/ tall R waves in those leads 2.) Heart Vertical: T waves Positive in Leads II, III, and aVF, w/ tall R waves in those leads - T wave is often negative in Lead III normally, regardless of electrical position of heart

Cardiac Hypertrophy vs Hyperplasia

NO HYPERPLASIA IN THE HEART!!!! When cardiac hypertrophy occurs, the total number of heart muscle fibers does NOT increase; rather, each individual fiber becomes larger. One predictable ECG effect of cardiac hypertrophy is an *increase in the voltage or duration of the P wave or QRS complex*

Normal R Wave in Lead V1

Normal R wave in lead V1 is generally smaller than the S wave in that lead - R wave > S wave in lead V1 is suggestive but not diagnostic of RVH, sometimes a small q wave precedes the tall R wave in lead V1

Normal ST Segment on ECG

Normal ST segment, represents early phase of ventricular repolarization - usually isoelectric, slight deviations <1mm) may be seen *Early Repolarization*: ECGs of certain normal people show more marked ST segment elevations as a normal variant Normal Variant of Early Repolarization: right chest leads (V1-V3) show short ST Segments, T waves appear to take off almost from the J point - not an uncommon finding in normal individuals

Right Ventricle Hypertrophy R Wave

Normal electrical predominance of L ventricle overcome Instead of normal rS complex, *Right chest leads (V1)* show *Tall (positive) R waves* (indicating spread of positive voltages from hypertrophied right ventricle toward the right) - Tall positive (R) wave indicates marked hypertrophy of R ventricle *Lead V1: R wave > S wave is suggestive but not diagnostic of RVH* - Sometimes a *small q wave precedes the tall R wave* in lead V1

Normal Ventricle QRS

Normally L and R ventricles depolarize simultaneously, and the left ventricle is electrically predominant because it has greater mass 1.) Right side chest leads (e.g., V1) record *rS-type complexes* - deep negative S-wave indicates spread of depolarization away *from right towards left* side 2.) Left side chest leads (e.g. V5, V6) record a qR-type complex - tall positive R wave indicates depolarization that points to the left and is generated by the L ventricle

Normal P Wave Amplitude and Width

Normally, at rest, the P wave in every lead is <2.5mm (0.25mV) in amplitude and <0.12sec (3small boxes) in width

Cardiac Enlargement Side Effects

Pathologic hypertrophy and dilation often accompanied by *fibrosis (scarring) and changes in myocardial geometry (remodeling)* that may worsen myocardial function and lead to arrhythmias and CHF - ANS, inadequate myocardial perfusion, the NO system, and the renin- angiotensin axis (RAAS) may all play roles in the linking hypertrophy and fibrosis of heart muscle cells with dysfunction of other organs

RAA plus LAA

Patients with enlargement of both atria (*biatrial enlargement or abnormality*) may show a combination of patterns (e.g., tall and wide P waves). This finding is most likely with *severe cardiomyopathy or valve disease*

RAA and LAA Summary Pic

Pic

QRS Calculation: Method 2 Example (Using Biphasic Complexes)

Pic: *Lead I is biphasic and shows an RS pattern* = mean electrical axis must be directed at *right angles to lead I* - Because lead I on the hexaxial lead scale is at 0°, the mean electrical axis must be at right angles to 0° or at either -90° or +90° -> if the axis were -90°, the depolarization forces would be oriented away from the positive pole of lead aVF and that lead would show a negative complex. In Figure 5-3, however, lead aVF shows a *positive complex (tall R wave)*; therefore the axis must be +90°.

Measuring Mean QRS Axis: Scale

Positive pole of lead I is said to be at 0° All points below Lead I Axis are positive All points above axis are negative - Thus, toward the positive pole of lead aVL (-30°), the scale becomes negative, Downward toward the positive poles of leads II, III, and aVF, the scale becomes more positive (lead II at +60°, lead aVF at +90°, and lead III at +120°)

Lead aVR QRS Complex

Positive pole of lead aVR is oriented upward and toward the right shoulder - Ventricular stimulation forces are oriented primarily toward the left ventricle --> a *predominantly negative QRS Complex* - Lead aVR may display any of the QRS-T complexes shown in Pic. In all cases the QRS is predominantly negative - *T wave in lead aVR is also normally negative*

Recognizing RAD on ECG

QRS axis +100° or more positive - When leads II and III show tall R waves of equal height, the QRS axis must be +90° - As an approximate rule, *if leads II and III show tall R waves and the R wave in Lead III > R wave Lead II --> RAD* - In addition, *Lead I shows an RS pattern* with *S wave deeper than the R wave is tall*

Measuring Mean QRS Axis

QRS axis is defined in the frontal plane = described only in reference to the 6 extremity leads (the frontal plane leads)--> Scale of reference used to measure the mean QRS axis = diagram of the frontal plane leads *hexaxial lead diagram*

LAD and RAD vs Horizontal and Vertical

RAD = extreme form of a vertical mean QRS axis LAD = extreme form of a horizontal mean QRS axis

Important Causes of RAD

RAD, with a mean QRS axis +100° or more, sometimes seen in normal hearts - RVH - MI of the lateral wall of the left ventricle - Left posterior Fasicular Block (Hemiblock) - Chronic Lung Disease (emphysema of chronic bronchitis) - Acute Pulmonary Embolism

Indeterminate Axis

Rare All 6 extremity leads show biphasic (QR or RS) complexes --> impossible to calculate the mean frontal plane QRS axis May occur as a normal variant, or may be seen in a variety of pathologic settings

Right vs Left T Wave Inversion Terminology

Right chest T wave inversions formerly called: right ventricular "strain" pattern Left precordial T wave inversions due to LVH were called: left ventricular "strain" Preferable and more contemporary names are: *T wave inversions associated w/ Right ventricular or Left ventricular Overload*, respectively

RAA P pulmonale (tall, narrow P Waves)

Seen best in *Leads II, III, aVF, and sometimes V1. P wave >2.5 mm in any of these leads = P pulmonale* - Echocardiographic evidence indicates that the finding of a tall, peaked P wave does not consistently correlate with RAA and pts may have actual right atrial overload and no tall P waves = tall peaked P waves are of *limited sensitivity and specificity in the diagnosis of right atrial enlargement*

Broad P Wave with No Detectable Left Atrium Enlargement

Seen esp in coronary artery disease (CAD) Abnormal P waves probably represent an *atrial conduction delay in a normal sized chamber* --> Therefore, rather than L Atrial enlargement, the more general term *left atrial abnormality is recommended to describe these abnormally broad P waves*

Ventricular Depolarization QRS Complex: Phase 1 *Lead V1 vs Lead V6*

Septal stimulation represented by small arrow pointing to the right, reflecting the left-to-right spread of the depolarization stimulus through the septum 1.) *points toward the positive pole of lead V1* --> produces a *small positive deflection (r wave) in lead V1* 2.) points away from positive pole of lead V6 --> produces a *small negative deflection (q wave) in lead V6*

Respiratory Sinus Arrhythmia

Sinus rhythm does not have to be strictly regular. If you feel your own pulse, during slower breathing you will note *increases in HR with inspiration and decreases with expiration*. - These phasic changes are called respiratory sinus arrhythmia and are a normal variant, especially pronounced in young, healthy people with high vagal tone.

Septal R Wave

Small initial r wave (or rS complex) represents the *left-to-right spread of septal stimulation* = Septal R Wave

Direction of Ventricular Depolarization (QRS Complex): Summary

Ventricular depolarization process divided into 2 main phases: 1.) stimulation of the interventricular septum (short arrow pointing through septum into R ventricle) 2.) simultaneous left and right ventricular stimulation (larger arrow pointing through the left ventricle and toward the left side of the chest)

Calculating the Mean QRS Axis tells you...

What general direction or toward which lead axis the QRS complex is predominantly oriented

AV Junctional Rhythm

When heart is being paced by the AV Junction (AV Node) rather than the SA Node

Lead V6

a *left* chest lead, shows voltages detected in the left midaxillary line

Of the six limb (extremity) leads (I, II, III, aVR, aVL, and aVF), which is the easiest to visualize?

aVR

Leads V5 and V6

generally show a qR-type complex.

Lead V1

shows voltages detected by an electrode placed on the *right* side of the sternum (fourth intercostal space).

Ectopic Rhythm

sinus node is NOT controlling atria

Sinus Rhythm

sinus node is controlling atria Caution: sinus rhythm is not always "normal" sinus rhythm