ecg Flashcards
what are the characteristics of the P-Wave
The P-wave is a smooth, round, upright shape in lead II. It is the first of the waveforms in a Normal Sinus Rhythm.
PR Interval
Measure from the start of P-wave to the start of the QRS complex
Normally lasts 0.12 (3 small boxes) to 0.20 seconds (5 boxes). It is usually constant.
QRS
A normal QRS complex is narrow with sharply pointed waves and a duration of less than 0.12 seconds.
Q wave
It is any negative deflection that precedes an R wave.
A normal Q-wave, if present is small and usually seen in the lateral (left-sided) leads of a 12-lead ECG. If present represents electricity spreading Right to Left through the ventricular septum.
There is not always a Q-wave in lead II and a Q-wave that is greater than 1/3 the height of the QRS is abnormal.
ST Segment
The line between the QRS complex and the beginning of the T-wave
Normally isoelectric. An ST segment > 1 mm above or below the isoelectric line is highly suggestive of myocardial ischemia or injury though a 12-lead is required to determine significance.
Represents the interval between ventricular depolarization and repolarization.
T wave- 1st half represents?
Waveform after the QRS and represents ventricular repolarization.
Should be asymmetric.
1st half is the absolute refractory period where the ventricles have not sufficiently repolarized to enable another depolarization (NCECITS compares this to flushing a toilet -> if try to flush too soon, won’t flush). Nothing can stimulate the ventricles again.
Second half of T wave represent?
2nd half is the relative refractory period where some cells have repolarized sufficiently to depolarize again. A large stimulus can initiate depolarization of these cells and result in a dangerous rhythm.
TP Segment
Regarded as the isoelectric line or baseline.
Reference where we compare the J-point and ST Segment
J Point
The junction where QRS complex ends and ST segment begins.
QT Interval
Measured from the beginning of the QRS complex to the end of the T-wave.
Represents all the electrical activity of one complete ventricular cycle (depolarization and repolarization).
Length depends on heart rate and varies by gender and age. It can be affected by many factors.
A prolonged QT puts the heart at risk of lethal dysrhythmias.
As QT varies with rate, a formula is used to “correct” the QT and the result is the QTc. This “corrects” the QT to a rate of 60 and we become concerned when this number reaches greater than 450 ms).
approach to ECG
-Assess Regularity (Atrial and Ventricular)
-Assess Rate (Atrial and Ventricular)
Identify and examine waveforms
-Assess intervals (PR, QRS duration, QT) and examine the ST
Interpret rhythm and determine clinical significance.
Interpret Rhythm- include:
Specify site of origin (pacemaker) of the rhythm (sinus, atrial, junctional, ventricular).
Specify rate
Specify abnormalities (blocks, Wide QRS, Long QT, elevation or depression of T waves etc)
sinus rhythm includes?
A “normal” heartbeat starts with an electrical impulse in the Sinoatrial Node.
A rhythm that starts in the SA node has the following characteristics:
Positive P wave before each QRS (in lead II)
P-waves that are upright and look alike
Constant PR interval
Usually has a regular atrial and ventricular rate
SA block characteristics
-Irregular as a result of the pause(s). –the pause is the same or exact -multiple of distance between two other P-P intervals. Underlying rhythm is regular.
-Rate: Usually normal but varies due to pause
-P-Waves: Positive, 1:1 with QRS and -P-waves all look the same
-PR Interval: 0.12-0.20 seconds
-QRS Duration: < 0.12 seconds unless abnormally conducted.
what is an SA block?
Sinoatrial Block (aka Sinus Exit Block)
Pacemaker in the SA node initiate an impulse but it is blocked before it can exit the SA node resulting in an absent PQRST complex.
This is a disorder of impulse conduction as an impulse was initially generated.
How is sinus arrest different from a sinoatrial block?
Sinus Arrest is a disorder of impulse formation whereas SA block is a disorder of impulse conduction
The difference from a Sinus Exit Block is that the pause will not be an exact multiple of other P-P intervals.
what are the three main mechanisms that result in Atrial Dysrhythmias:
Abnormal Automaticity
Triggered Activity
Re-entry (aka Reactivation)
what is it called when a heart beat comes prematurely and is identified by the site of origin of the beat? (three possible complexes)
(Premature Atrial Complex, Premature Junctional Complex and Premature Ventricular Complex).
patterns seen in PAC
Couplet: Paired beats – 2 premature beats in a row
Run/Burst: 3 or more premature beats in a row
Bigeminy: Every other beat is a premature beat
Trigeminy: Every third beat is a premature beat
Quadrigeminy: Every fourth beat is a premature beat.
what causes a PAC?
PAC occurs when an irritable site within the atria fires before the next sinus impulse is expected.
describe how the p wave may appear in PAC’s
P-wave of the PAC may be:
Very similar to Sinus P if the irritable site is close to the SA node
Biphasic
Flattened
Pointed
Notched
Lost in preceding T-wave
Wandering Atrial Pacemaker (aka Multiform Atrial Rhythm)
There is a gradual shifting of the dominant pacemaker among the SA node, atria, and/or the AV junction. This results in variable-looking P-waves.
There need to be at least 3 different P-waves seen in the same lead for the diagnosis.
Characteristics of WAP
Usually irregular as the pacemaker site shifts
Rate is usually 60-100 but may be slower (if rate is greater than 100, this rhythm is known as Multifocal Atrial Tachycardia)
P-waves: Size, shape, and direction may change beat to beat. May be upright, inverted, biphasic, rounded, flat, pointed, notched, or buried in QRS.
PR interval: Varies as pacemaker site shifts
QRS duration: < 0.12s unless abnormally conducted.
Multifocal Atrial Tachycardia
Diagnosed when the wandering atrial pacemaker rate is greater than 100 beats per minute.
May be very fast and can be difficult to determine that there are variations.
characteristics of MAT
Irregular as pacemaker shifts but can be difficult to ascertain at very fast rates
Rate is > 100 bpm
P-waves: One before each QRS but may also be buried in the QRS. Need at least 3 different P-wave configurations.
PR Interval: Varies as the pacemaker shifts
QRS: Is usually < 0.12 unless abnormally conducted in the ventricles
what are Atrioventricular Nodal reentrant tachycardia (AVNRT) & Atrioventricular Reentrant Tachycardia
Regular, narrow complex tachycardias. In the emergency setting, in most cases won’t be able to differentiate.
AVNRT
Atrioventricular Nodal Reentrant Tachycardia: Is the most common and is caused by reentry circuit within or near the AV node allowing tachycardia to continue
AVRT
Atrioventricular Reentrant Tachycardia: An accessory pathway connects atria to ventricles and impulses can travel in similar circuit between AV node and accessory pathway. Wolff-Parkinson-White Syndrome (WPW) is an example of this.
characteristics of SVT
Very regular
Rate is > 100 beats/minute -> usually greater than 150 beats/minute
P-waves: Often not detectable as hidden in QRS.
PR Interval: Not usually measurable as unable to determine P-wave
QRS duration: < 0.12 seconds unless abnormally conducted.
Atrial Flutter
Reentrant rhythm in which irritable site in the atria fires regularly at a very rapid rate. This produces atrial waveforms that resemble teeth of a saw (commonly referred to as appearing “sawtooth”) which are called Flutter Waves or F waves.
A healthy AV node protects the ventricles from very fast rates and AV node usually can’t conduct more than about 180 impulses/minute. The rhythm may be regular or irregular depending on how the impulses are blocked in the AV node.
rates and characteristics of a flutter
-Atrial rate (F-waves) are regular. —ventricular rate may be regular or irregular depending on AV conduction and blockade
-Atrial rate is 240-300 bpm (F waves). -Ventricular rate varies but usually is not more than 180 bpm.
-P-waves: No P-waves but Flutter waves (F waves)
-PR Interval: Not measurable as no P waves
-QRS: < 0.12 seconds unless abnormally conducted or skewed by F-waves.
Atrial Fibrillation- why does it occur?
Occurs because of abnormal automaticity in rapidly firing sites in the atria or reentry involving one or more circuits in the atria.
characteristics of AFIB
Irregularly, irregular rhythm
Atrial rate is 300-600 bpm and the ventricular rate is variable
P-waves: No P-waves. There may be fibrillatory waves present (sometimes they are too tiny to ascertain)
PR Interval: No P-waves to no PR interval
QRS: < 0.12 s unless abnormally conducted.
Danger of Atrial Fibrillation?
-risk of atrial thrombus formation which can lead to stroke or peripheral thromboembolism if the clot is dislodged and travels through the arterial system to a different site. -If a patient has been in atrial fibrillation for greater than 48 hours, it is recommended that anticoagulation take place prior to trying to cardiovert back into a normal rhythm to minimize stroke risk.
What is the main job of the AV node?
The AV node is a specialized group of cells whose main job is to delay electrical impulse allowing the atrial to contract and fill the ventricles. After leaving the AV node, the electrical impulse enters the bundle of His which has pace-making ability of its own.
what rate does the bundle of His discharge at?
The bundle of His pacemaker cells are capable of discharging at 40-60 bpm.
when does the AV node take over?
The AV junction may assume responsibility for pacing the heart if:
Sinus Arrest
Sinoatrial Exit Block
SA node discharging at a slower rate than AV junction
Impulse from SA node generated and conducted through atria but no conducted to ventricles (ie. AV block)
what are rhythms originating from the AV junction are called?
Junctional Rhythms.
How will p waves be seen in a junctional rhythm?
If the AV junction paces the heart, the electrical impulse must travel in a backward direction (away from positive lead in lead II) to activate the atria. If a P-wave is seen, it will be inverted in lead II because the impulse is traveling away from the positive lead.
P-WAVES in JUNCTIONAL RHYTHMS can be:
Inverted: If atrial depolarize before ventricle this inverted P-wave will be before the QRS and the PR interval will be < 0.12 as the impulse does not need to travel as far.
Not visible: If atria and ventricles depolarize at the same time, the Ps are hidden in the QRS
After QRS (or distort the end of the QRS): The inverted P-wave (in lead II) will appear after if atria depolarize after the ventricle.
when do Premature Junctional Complex (PJC) occur?
Occurs when an irritable site in the AV junction fires before next sinus impulse.
how do you tell the difference b/w a PAC and PJC?
Can usually tell the difference between a PAC and a PJC by looking at the P-wave. Sometimes it can be hard to tell if the P-wave of the PAC is buried in the preceding T-wave.
what is a PJC?
-A PJC is an early beat that occurs before the next expected regular beat. It is important to differentiate this from a late or escape beat
-A PJC is not an entire rhythm but a single beat. When we analyze, we need to specify the underlying rhythm and the origin of the ectopic beat.
Junctional Escape Beat
The beat begins in the AV junction and is later than the next expected regular beat.
When an impulse fails to reach the junction or the ventricles (due to lack of sinus beat or a block), the AV junction or the ventricles should assume responsibility for pacing the heart.
A Junctional Escape Beat is PROTECTIVE -> it prevents cardiac standstill
Junctional Rhythm (aka Junctional Escape Rhythm)
This occurs when there are three or more junctional escape beats in a row.
-When the SA node fails to initiate an impulse, an escape pacemaker in the AV junction assumes responsibility for pacing the heart.
what is the intrinsic firing rate of junctional rhythm?
The intrinsic firing rate of the AV junction is 40-60 beats/minute thus a junctional rhythm would be expected to be 40-60 beats/minute. If the rhythm is junctional but slower than the expected minimum 40 beats/minute, it would be described as junctional bradycardia.
Characteristics of a junctional rhythm
-Tends to be very regular
-Rate: 40-60 bpm
-P-waves: Before, during, or after the QRS. If visible, inverted in lead II
-PR Interval: < 0.12 seconds if P-wave before QRS. If not before QRS, there will be no PR interval
-QRS Duration: < 0.12 seconds unless abnormally conducted
Any rhythm over 100 beats per minute is a tachycardia so what do we call a junctional rhythm that is greater than 60 but less than 100 beats per minute?
Accelerated Junctional Rhythm
define junctional tachycardia
Junctional Tachycardia is a junctional rhythm with a rate greater than 100 beats per minute. If there are three PJCs in a row, we have a run of junctional tachycardia.
ventricle intrinsic rate and risk?
20-40 beats per minute. Any rhythm that relies on the ventricles should be considered dangerous as the ventricles often with stop assuming the pacemaker role without warning.
key feature in ventricular rhythms?
A key feature of ventricular rhythms is a QRS equal to or greater than 0.12 seconds in length of time. This is because the electrical impulse bypasses the normal intraventricular conduction pathway so the impulse travels cell to cell.
Premature Ventricular Complex (PVC)
PVC occurs earlier than the next expected beat.
Due to the abnormal conduction, the QRS will be wide (Greater than or equal to 0.12 seconds)
UNIFOCAL PVC:
The PVCs look alike in the same lead and come from the same anatomic site (same focus).
MULTIFOCAL PVC:
PVCs with more than one shape in the same lead. Usually coming from different sites. These are considered more serious as they suggest a greater degree of irritability
R-ON-T PHENOMENON:
Refers to a ventricular dysrhythmia starting as a result of improperly timed electrical impulse on the T-wave. This is often a result of a PVC falling on a T-wave of the preceding beat. Ventricular repolarization is not complete on the second half of the T-wave so it is possible that a PVC landing on the T-wave can cause Ventricular Tachycardia or Ventricular Fibrillation.
difference between ventricular Escape Beats and PVC?
Although a PVC and a Ventricular Escape beat can look the same morphologically, they are very different:
The early PVC is from irritability in some areas of the ventricle
The ventricular escape beat is PROTECTIVE safeguarding the heart from more extreme slowing or asystole. Because is protective, DO NOT want to give any medications that would wipe out the escape beat even though it is ventricular.
Ventricular Escape Rhythm (aka Idoventricular Rhythm )
Is three or more ventricular escape beats in a row at a rate of 20-40 beats per minute (the intrinsic rate of the ventricles).
When the rate is less than 20 beats per minute, the rhythm is sometimes referred to as an “agonal rhythm”.
when do Ventricular Escape Rhythms occur? (3)
1) when the SA node and AV junction fail to initiate an electrical impulse
2)SA node or AV Junction rate is less than intrinsic rate of purkinje fibers
3)SA node or AV Junction impulses are blocked
Accelerated Idioventricular Rhythm
Three or more ventricular beats in a row at a rate of 41-100 beats per minute.
Exactly the same characteristics as a ventricular escape rhythm but the rate is faster.
Ventricular Tachycardia (VT)
Three or more sequential PVCs occur at a rate greater than 100 beats per minute.
Patient MAY or MAY NOT have a pulse
what can sustained VT cause?
The rapid heart rate associated with sustained VT (> 30 seconds) can cause a significant decrease in ventricular function and cardiac output. This can result in acute heart failure, syncope, hypotension, or circulatory collapse within several seconds to minutes after the onset of VT
Monomorphic Ventricular Tachycardia
The QRS complexes are of the same shape and amplitude.
Polymorphic Ventricular Tachycardia
QRS complexes vary and shape and amplitude from beat to beat and appear to “twist” from upright to negative and back again.
an example of polymorphic VT
TORSADES DE POINTES: Is polymorphic VT that occurred due to long QT interval (need dx of long QT prior to pVT)
AV blocks
First Degree AV blocks: Impulses from SA node are delayed but not blocked
Second Degree AV blocks: Some but not all impulses are blocked between the atria and ventricles
Third Degree AV blocks: All impulses between the atria and ventricles are blocked.
where do AV blocks occur?
AV blocks can occur at the AV node, bundle of His or bundle branches.
difference in levels of AV node block- above or below the AV node?
AV blocks at the level of the AV node have an advantage because there is usually a reliable junctional pacemaker available. When the AV block is below the AV junction, the only available pacemaker may be a slow ventricular pacemaker.
characteristics of a 1st degree heart block
Regular
Rate: Depends on underlying rhythm
P-waves: Normal size and shape; Upright in lead II; 1:1 with QRS
PR interval: Prolonged (> 0.20 seconds) but constant.
QRS: Less than 0.12 seconds unless abnormally conducted.
Second-Degree AV Block Type I- what does the PR do?
AKA: Wenckebach or Mobitz I
The PR interval gets longer with each beat until another P-wave appears before a QRS.
Second-Degree AV Block Type II
AKA: Mobitz II
More serious than Type I and more likely to progress to Third Degree AV Block
PR interval is CONSTANT or FIXED (ie. The same every time there is a PR interval).
2:1 Atrioventricular Block
2:1 Atrioventricular block is a form of second-degree AV block but because there are never two PQRST cycles in a row, not able to classify as Type I or Type II (it is one of these but we are not able to differentiate).
There is one conducted P-wave followed by a non-conducted P-wave so there are 2 P-waves for every QRS.
Third-Degree Atrioventricular Block
There is complete block of impulse from the atria to the ventricles. The site of the block may occur at the level of the AV node, bundle of His or distal to the bundle of His.
A secondary pacemaker (junctional or ventricles) stimulates the ventricles as the sinus signal does not reach them. Therefore, the QRS may be narrow or wide.
Both the QRS and P-waves are regular but not related to each other. May need to look where a P-wave is expected as may be hidden in a QRS complex or T-wave.
three types of pacing
Transvenous Pacemaker: Stimulates the endocardium of the heart by means of an electrode introduced into a central vein.
Epicardial Pacing: Placement of pacing leads directly into or through the epicardium. May be used when a patient is undergoing cardiac surgery and the outer surface of the heart is easy to reach.
Transcutaneous Pacing: Stimulation of the heart through pacing pads placed on the patient’s torso.
What is a normal PR Interval?
0.12 - 0.20 Seconds
Less than 0.12 seconds
Greater than 0.12 seconds
0.16 - 0.24 seconds
0.12 - 0.20 Seconds
The P-wave is an electrical picture of:
Depolarization of the atria
The T-wave is an electrical picture of:
Repolarization of the ventricles
