Electrophysiology of the Heart ECG Monitoring The ECG
- Slides: 86
Electrophysiology of the Heart
ECG Monitoring • The ECG is a graphic representation of the heart's electrical activity generated by depolarization and repolarization of the atria and ventricles
ECG Monitoring • Valuable diagnostic tool for identifying cardiac abnormalities including: – Abnormal heart rates and rhythms – Abnormal conduction pathways – Hypertrophy or atrophy of portions of the heart – Approximate location of ischemic or infarcted cardiac muscle
ECG Monitoring • The ECG tracing is only a reflection of the heart's electrical activity • It does not provide information regarding mechanical events such as force of contraction or blood pressure
Voltage • Voltage may be: – Positive--seen as an upward deflection on the ECG tracing – Negative--seen as a downward deflection on the ECG tracing – Isoelectric--no electrical current detected • Seen as a straight baseline on the ECG
Application of Monitoring Electrodes • Electrodes are pre-gelled, stick-on disks that can easily be applied to the chest wall • When applying electrodes: – Cleanse the area with alcohol to remove dirt and body oil – Use the inner surfaces of the arms and legs when attaching electrodes to extremities
Application of Monitoring Electrodes • When applying electrodes: – Trim excess body hair (if necessary) before placing the electrodes – Attach the electrodes to the prepared site – Attach the ECG cables to electrodes – Turn on the ECG monitor and obtain a baseline tracing
Monitoring Electrodes • If the signal is poor, recheck the cable connections and electrode contact with the patient's skin • Other causes of a poor signal include: – Excessive body hair – Dried conductive gel – Poor electrode placement – Diaphoresis
ECG Monitoring
ECG Monitoring
ECG Monitoring
Calibration • The sensitivity of the 12 -lead ECG machine is standardized • When properly calibrated, a 1 -m. V electrical signal produces a 10 -mm deflection (two large squares) on the ECG tracing Figure 28 -21
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram – Refractory Periods • Absolute – no other electrical impulse can be generated • Relative – very dangerous time another electrical impulse can interfere with the rhythm – V Tach can occur or V Fib
Conduction System of the Heart • Sinoatrial (SA) node • Atrioventricular (AV) node / bundle of His • Purkinje fibers Figure 6 -41
Normal Conduction • Sequence of normal impulse conduction – SA node – Both atria • Atrial contraction – – AV node Bundle of His Purkinje fibers Both ventricles • Ventricular contraction Figure 6 -41
Inherent Rates • SA node – 60 -100 • AV node – 40 -60 • Ventricular – 20 -40
P Wave • First positive (upward) deflection on ECG • Represents atrial depolarization • Usually rounded and precedes the QRS complex – Begins with first positive deflection from baseline – Ends at point where wave returns to baseline Figure 28 -22
P Wave • Duration normally 0. 10 second or less • Amplitude normally 0. 5 to 2. 5 mm • Usually followed by a QRS complex unless conduction disturbances are present Figure 28 -22
PR Interval • Represents the time it takes for an electrical impulse to be conducted through the atria and the AV node up to the instant of ventricular depolarization – Measured from the beginning of the P wave to the beginning of the next deflection on the baseline (the onset of the QRS complex) • Normal is 0. 12 -0. 20 second Figure 28 -25
PR Interval • A normal PR interval indicates that the electrical impulse has been conducted through the atria, AV node, and bundle of His normally and without delay • PRI < 0. 2 secs Figure 28 -25
QRS Complex • Generally composed of three individual waves: the Q, R, and S waves • Begins at the point where the first wave of the complex deviates from the baseline • Ends where the last wave of the complex begins to flatten at, above, or below the baseline Figure 28 -25
QRS Complex • Direction of the QRS complex may be: – Predominantly positive (upright) – Predominantly negative (inverted) – Biphasic (partly positive, partly negative)
QRS Complex • The normal QRS complex is narrow and sharply pointed • Duration is generally 0. 08 to 0. 12 second • Amplitude normally varies from less than 5 mm to more than 15 mm Figure 28 -25
Q Wave • The first negative (downward) deflection of the QRS complex on the ECG – May not be present in all leads • Represents depolarization of the interventricular septum Figure 28 -25
R Wave • First positive deflection after the P wave – Subsequent positive deflections in the QRS complex that extend above the baseline and that are taller than the first R wave are called R prime (R’), R double prime (R’’), and so on Figure 28 -23
S Wave • Negative deflection that follows the R wave – Subsequent negative deflections are called S prime (S’), S double prime (S”), and so on • R and S waves represent the sum of electrical forces resulting from depolarization of the right and left ventricles Figure 28 -25
QRS Complex • Follows the P wave • Marks the approximate beginning of mechanical systole of the ventricles, which continues through the onset of the T wave • Represents ventricular depolarization – Conduction of an electrical impulse from the AV node through the bundle of His, Purkinje fibers, and the right and left bundle branches
QRS Complex Figure 28 -22
ST Segment • Represents the early phase of repolarization of the right and left ventricles • Immediately follows the QRS complex and ends with the onset of the T wave Figure 28 -24 A
ST Segment • The point at which the ST segment “takes off” from the QRS complex is called the J point Figure 28 -24 D
ST Segment • The position of the ST segment is commonly judged as normal or abnormal using the baseline of the PR or TP interval as a reference – ST segment elevation – ST segment depression Figure 28 -24 A, B, & C
ST Segment • Abnormal ST segments may be seen in: – Infarction – Ischemia – Pericarditis – After digitalis administration – Other disease states
T Wave • Represents repolarization of ventricular myocardial cells • Occurs during the last part of ventricular systole • May be above or below the isoelectric line and is usually slightly rounded and slightly asymmetrical Figure 28 -25
T Wave • Deep and symmetrically inverted T waves may suggest cardiac ischemia • A T wave elevated more than half the height of the QRS complex (peaked T wave) may indicate a new onset of myocardial ischemia or hyperkalemia
Artifact • A series of deflections on the ECG display or tracing produced by factors other than the heart's electrical activity • Common causes of artifact: – – – Improper grounding of the ECG machine Patient movement Loss of electrode contact with the patient's skin Patient shivering or tremors External chest compressions
Artifact - Muscle Tremors Figure 28 -26 A
Artifact - AC (60 cycle) Interference Figure 28 -26 B
Artifact – Loose Electrode Figure 28 -26 C
Reading ECG’s
ECG Measurements 1 sec 0. 20 sec 0. 04 sec 5 large squares= 25 mm = 1 second 1 large square = 5 mm = 5 small squares =0. 20 seconds 1 small square = 1 mm = 0. 04 seconds 0. 1 m. V Voltage Standard calibration for an ecg is 10 mm=1 m. V, or 1 mm = 0. 1 m. V Time
Rate Determination The Six Second Method 1. ECG paper is marked at three second intervals (15 large boxes) 2. Count the R Waves in 6 seconds (30 large boxes/two 3 -second intervals) 3. Multiply the number of complexes by 10 to determine the rate (in this example 7 X 10 = 70) 1 2 3 4 5 6 7
Interpreting ECG’s • Questions to ask – Are there P waves? – Do they all look alike? – PRI – short or long? – QRS – tight or wide? – Any extra P waves? – Any extra QRS complexes? – What is the rate? – WHAT IS THE UNDERLYING RHYTHM?
Atrial Rhythms • Sinus rhythm – – P waves PRI – normal No extra beats Rate 60 - 100
• Sinus Bradycardia – – P waves PRI – normal No extra beats Rate is < 60
• Sinus Tachycardia – – P waves PRI – normal No extra beats Rate is 100 -150
• Wandering Atrial Pacemaker – P waves are present but they do not look alike – QRS intervals are regular
• Wolfe-Parkinson-White – P waves are present – Delta wave – Reentry rhythm
• Sinus Rhythm with unifocal Premature Ventricular Contractions (PVC) – – – P waves are present PRI is normal Some QRS complexes come early and are wide Premature QRS complexes look alike Must differentiate whether the PVC’s look alike or not
• Sinus Rhythm with multifocal PVC’s – – – P waves are present PRI is normal Some QRS complexes come early and are wide Early QRS complexes are different Must differentiate whether the PVC’s look alike or not
• Sinus Rhythm with couplets – P waves are present – PRI is normal – 2 QRS complexes come early together and are wide
• Sinus Rhythm with a run of V Tach – P waves are present – PRI is normal – 3 or more PVC’s together
• Sinus Rhythm with R on T phenomenon – P waves are present – PRI is normal – Can go into V Fib
• Sinus Rhythm with bigeminy – Underlying rhythm is a SR – Every other QRS complex is a PVC
• Sinus Rhythm with an aberrant conduction or Bundle Branch Block – P waves are present – PRI is normal – QRS complex is wide
• Atrial fibrillation – No discernable P waves – QRS – irregularly irregular
• Atrial Flutter – P waves are flutter waves or “sawtooth pattern” – QRS can be regular or irregular
Heart Blocks • Sinus Rhythm with 1 st degree HB – – P waves are present PRI – longer than normal QRS complex is usually narrow MUST GIVE AN UNDERLYING RHYTHM
• 2 nd degree HB Type I or Wenckebach – P waves are present – Extra P waves – PRI gets progressively longer then you get a “dropped QRS”
• 2 nd degree HB Type II or Classic 2 nd degree HB – P waves are present – Extra P waves – PRI stays the same but then you get a “dropped QRS”
• 3 rd degree HB or Complete HB – – – – P waves are present P waves map out QRS complexes are usually wide QRS complexes map out P waves do not map out with the QRS complexes PRI has no consistency Sometimes it looks like the P waves are marching towards the QRS
Junctional Rhythms • Junctional Rhythm – No P waves – QRS complex is tight – Rate 40 -60
• Junctional Bradycardia – No P waves – QRS complex is tight – Rate < 40
• Accelerated Junctional Rhythm – No P waves – QRS complex is tight – Rate 60 -100
• Junctional Tachycardia – No P waves – QRS complex is tight – Rate 100 -150
Ventricular Rhythms • Idioventricular Rhythm – – No P waves QRS complex is wide Rhythm is regular Rate 20 -40
• Agonal Idioventricular Rhythm – – No P waves QRS complex is extremely wide Will not produce a pulse Rate < 20
• Accelerated Idioventricular Rhythm – – No P waves QRS complex is wide Rhythm is regular Rate 40 -100
• “Slow” Ventricular Tachycardia – – No P waves QRS complex is wide Rhythm is regular Rate 100 -150
• Ventricular Tachycardia – – No P waves QRS complex is wide Rhythm is regular Rate > 150
• Ventricular Fibrillation – – No P waves QRS complex is wide and bizarre Rhythm is erratic Rate > 150
• Fine Ventricular Fibrillation – – No P waves QRS complex is wide, bizarre and small Rhythm is erratic Rate > 150
• Coarse Ventricular Fibrillation – – No P waves QRS complex is wide, bizarre and large Rhythm is erratic Rate > 150
• Torsades de Pointes (Turning of the Points) – – – No P waves QRS complex is wide, bizarre and large QRS complexes seem to face different directions Rhythm is erratic Rate > 150
• Asystole – – No P waves No QRS complexes “Flatline” Must confirm in at least two leads
• Ventricular Pacemaker – P waves may or may not be present – Pacemaker spikes just prior to QRS complex – QRS complexes are wide
• Atrial Pacemaker – Pacemaker spikes just prior to P waves – QRS complexes are narrow
• Atrial Ventricular Pacemaker or AV Sequential Pacemaker – Pacemaker spikes just prior to P wave – Pacemaker spikes just prior to QRS complex – QRS complexes are wide
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