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

Figure Calibration The sensitivity of the 12-lead ECG machine is standardized When properly calibrated, a 1-mV electrical signal produces a 10-mm deflection (two large squares) on the ECG tracing

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

Figure 6-41 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

Inherent Rates SA node – AV node – Ventricular – 20-40

Figure 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 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 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 second

Figure 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 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

QRS Complex Direction of the QRS complex may be: –Predominantly positive (upright) –Predominantly negative (inverted) –Biphasic (partly positive, partly negative)

Figure 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 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 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 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

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

Figure QRS Complex

Figure A 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 D ST Segment The point at which the ST segment “takes off” from the QRS complex is called the J point

Figure A, B, & C 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

ST Segment Abnormal ST segments may be seen in: –Infarction –Ischemia –Pericarditis –After digitalis administration –Other disease states

Figure 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

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

Figure A Artifact - Muscle Tremors

Figure B Artifact - AC (60 cycle) Interference

Figure C Artifact – Loose Electrode

Reading ECG’s

ECG Measurements 1 small square = 1 mm = 0.04seconds 1 large square = 5 mm = 5 small squares =0.20 seconds 0.1 mV Standard calibration for an ecg is 10mm=1 mV, or 1mm = 0.1mV Time Voltage 0.04 sec 0.20 sec 1 sec 5 large squares= 25 mm = 1 second

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)

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

Sinus Bradycardia –P waves –PRI – normal –No extra beats –Rate is < 60

Sinus Tachycardia –P waves –PRI – normal –No extra beats –Rate is

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

Junctional Tachycardia –No P waves –QRS complex is tight –Rate

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

“Slow” Ventricular Tachycardia –No P waves –QRS complex is wide –Rhythm is regular –Rate

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

Any Questions???