[2] Lecture 8:Heart rhythmicity/Norm ECG Flashcards
Conduction system of the heart
Sinus(SA) node-intermodal pathways-Atrioventricular node- AV bundle- bundle branches [l/r]
Describe the delay in heart’s electrical system:
AV node receives impulse from SA after .03 s.
Signal is delayed in AV for .09 s.
Signal is delayed in penetrating bundles for .04 s.
=.16 delay from initial SA signal to ventricular contraction
Why the delay from SA to AV?
Takes .03 s to get signal through atrial fibers that are directly connected to SA.
D/t small size of cells, low amplitude of AP, and slow rate of depolarization during excitation the signal is delayed .09 s.
What’s main cause of slow conduction along the electrical pathway of heart?
Slow conduction is d/t diminished # gap junction in pathway resulting in the resistance to conduction
Resting membrane potential and threshold of SA node
Resting: -55 to -60 mV
Threshold: -40mV
What is open @ -55 to -60 mV in SA node
Slow sodium-calcium channels-making membrane more positive.
Atrial nodal AP is slower
What happens @ -40 mV?
Sodium-calcium channels become activated
Inactivated again in 100-150 msec
What happens to SA node @ approx. 0 mV?
Repolarization by means of large potassium channels open [after Na-Ca channels close]
Differ ventricular and SA node resting potential
SA=-55 to -60 mV
Ventricular=-85 to -90 mV
Difference btw sinus rhythm and ectopic focus
AP originating in SA node=sinus
Generated anywhere else is ectopic
Vagus nerve and parasympathetic effects on HR:
Acetylcholine involves muscarinic receptors…decreases rate of rhythm of SA d.t decreased excitable AV junctional fibers.
This is done by increasing permeability of potassium ions in fiber membranes. Hyper polarization.
Vagus nerve and sympathetic effects on HR:
Noepinephrine stimulates beta-1 adrenergic receptors, increases depolarization rate.
May increase permeability of NA+/Ca++in fiber membranes.
Positive chronotropic effect
Increases depolarization rate
Hyperpolarization: - chronogenic effect
What do the P, QRS, and T waves represent on typical ECG?
P- atrial depolarization [contraction]
QRS-ventricular depolarization [contraction]
T- ventricular repolarization
- can’t see atrial repolarization; it’s behind QRS complex.
Difference between ECG recording and transmembrane potential recording
ECG measures extracellular potential; not the same as transmembrane potential: only concerned w/ what is happening across membrane interface.
When there is current flow between regions of the heart what doe the ECG look like
Deflection from 0. Current flows between different membrane potentials. Monophasic potential of ventricular muscle
What potential is recorded when ventricle is completely polarized or depolarized?
No potential.
It is only when the muscle is partially polarized/ depolarized does current flow from one part of the ventricle to another.
PR interval
0.16 s
QT interval
0.35 s
Beginning of atria contraction
Phase 0 of AP through atrial muscle
P wave
Beginning of the ventricle contraction
Phase 0 of the AP’s through the atrial muscle
QRS complex
Repolarization of ventricles occurs after end of
Represents phase 3 of repolarization of ventricle
T wave
- terminal= R arm
+ terminal= L arm
Looks at heart from R to L
Limb lead I
- terminal=R arm
+ terminal=L leg
Looks at heart from R to L
Limb lead II
- terminal=L arm
+ terminal=R arm
Looks at heart from upper L to lower L
Limb lead III
2 arms and left leg form spices of triangle
Einthoven’s triangle
If the electrical potentials of any 2 of the 3 bipolar limb ECG leads are known at any given instant, the third can be determined by summing the first 2
Einthoven’s law
Normal current flow in the ventricle:
Negative to positive in the direction from the base of the heart to apex.
= base (-) and apex (+)
All limb leads record: (-/+)
Positive
An arrow that points in the direction of the electrical potential generated by the current flow, with the arrowhead in the + direction.
Vector
Axis for Lead I
0 degrees
Axis Lead II
60 degrees
Axis lead III
120 degrees
Summated vector of the generated potential at a particular instant
Instantaneous mean vector
Horizontal and extends towards a persons L side; this direction = 0 degrees
Reference vector; lead I
What is the mean electrical axis of the heart
About +59 degrees in relation to the zero reference point
Limb leads and T, P, atrial T wave are all:
Positive
How is the mean electrical axis measured
- to +
Base to apex
Vector addition leads I and III
abnormal Conditions that cause axis deviation:
- Change of heart position in chest
- One sided ventricular hypertrophy
- BBB
- Fluid in pericardium
- pulmonary emphysema
Most common cause of increased voltage ECG [>4mV]
Hypertrophy of ventricle
Causes for decreased voltage ECG:
Cardiac myopathies
Conditions surrounding heart
Norm QRS duration
0.06-0.08
Conditions that may cause bizarre complexes:
- destruction of cardiac muscle and replacement by scar tissue
- multiple small local blocks in the conduction impulses at many points in the purkinje system.
What causes prolongation of QRS:
Hypertrophy or dilation of either ventricle
Reference point for analyzing current of injury
J point
Different cardiac abnormalities cause part of the heart to remain partially or totally depolarized all the time. This goes through both pathologically and normally polarized areas
Currents of injury
Abnormalities causing current of injury:
1: ischemia-most common
2: infectious process
3: mechanical trauma
What is the effect of current of injury?
Abnormal negative current flows from infarcts area and spreads toward the rest of the ventricles
Where dopes J point occur?
Very end of the QRS wave-this is the point where all parts of the ventricle [injured/healthy] depolarize….ex: ST elevation!!
