P and QRS waves Flashcards
Electrocardiogram (ECG)
Graphic record of the electrical potentials produced by cardiac myocytes (voltage over time)
Dx arrythmias
Positioning of the ECG
Right lateral recumbency
Black: LA, white: RA, red: LL green: RL
Limb leads: frontal plane
Lead 1: right arm to left arm
Lead 2: Right arm to left leg
Lead 3: left arm to left leg
Bipolar leads
Contain a negative pole (-) and positive pole
Limb leads 1, 2 and 3
Unipolar leads (augmented)
Measures the electric potential @ 1 point with respect to null point
Augmented leads aVR, aVL, and aVF
Cardiac conduction system
- Sinus node (initiation of electrical impulse 60-180 bpm)
- AV node (only link between atrium and ventricles 40-60 bpm)
- Purkinje fibers (inherent rate of 20-40 bpm)
Rule of ECG
Rapid depol/ repol= narrow tracing
Slow depol/ repol = wide tracing
P wave
Organized atrial depolarization
absent or sometimes= no atrial depolarizations
What happens when you never see a P wave?
Slow regular ventricular complexes
Atrial flutter or fibrillation
Atrial standstill
What happens when you sometimes see a P wave?
P with ventricular complexes
Sinus node disease
QRS complex
Ventricular depolarization
Q is first - delfection, R is first + deflection
S always second - or first - after + QRS
T wave
Ventricular repolarization
T +, -, or biphasic
Each beat MUST have a T wave
Intervals
The amount of time it takes for a wave of conduction to propagate through the heart
PR interval
Beginning of P to beginning of Q
Atrial depol and conduction through the AV node
QT interval
Beginning of Q to the end of T
Ventricular activity
Calculating HR with 25 mm/sec
Average: 30 large boxes= 6 sec
Number of complexes x 10
Instantaneous: 1500/ # small boxes between consecutive R waves
Calculating HR with 50 mm/sec
Average: 30 large boxes= 3 sec
Number of complexes x 10
Instantaneous: 3000/ # small boxes between consecutive R waves
When is each method for calculating HR used?
Average used to irregular arrhythmias
Instantaneous used to regular rhythms or non-sustained arrhythmias
What happens when there ISN’T a P wave for every QRS?
Early: premature, late: escape
Ectopic v. sinus beat
Looks similar: supraventricular/ atrial
Looks different: Ventricular
Ventricular premature contractions (VPCs)
No associated P waves
Earlier than expected sinus beat
QRS of ectopics look different from sinus QRS
premature: wide and bizarre
Atrial premature contractions (APCs)
No associated P waves
Earlier than the next expected sinus beat
QRS of ectopics look similar to sinus QRS
above the ventricle
Ventricular escapes
Some sinus, bradycardia
Ectopics have no associated P wave, late
QRSs of ectopics are wide and bizarre
Escape beats: safety mechanism
Sinus mode:P wave, narrow QRS complex
AV node: No P wave, narrow QRS
Ventricle (purkinje): No P, wide QRS
beats generated by pacemakers when sinus rate becomes too slow or arrests
Abnormal P waves
P wave + when initiated from sinus node
P wave - from abnorm origin in atrium
___________ cells only work as pacemaker if diseased
Atrial myocardial cells
abnormal P waves
1st degree AV block
Prolongation of PR interval (dogs: 0.13, cat: 0.09)
Slow conduction in the AV node
What caused 1st degree AV block?
Fibrosis in the AV node
↑ vagal tone (physiologic, systemic disease, drugs like betablocks- digoxin)
exception to rule (QRS for every P)
2nd degree Type 1 AV block (Wenckebach)
Gradual prolongation of the PR interval than dropped P
sometimes no QRS for every P
2nd degree Type 2 AV block
Some conduction through the AV node
No pattern of PR prolongation
Dropped Ps are random
sometimes no QRS for every P
3rd degree AV block
Impulse always blocked in AV node (sinus faster than escape rate)
Secondary pacemaker takes over
always no QRS for every P
Bundle branches
Rapidly propagate the wave of depol through the L and R ventricles
Left bundle branch block
Right ventricle depol rapidly and normally via the right bundle branch
Left ventricle activated late and slowly making QRS wide and bizarre
Left ventricular hypertrophy
Sinus rhythm with skinny, tall R waves
Right bundle branch block
Left ventricle depol rapidly and normally via left bundle branch
Right ventricle activated late and slowly making QRS wide and bizarre
Right ventricular hypertrophy
Sinus rhythm with skinny, deep S waves
