Lecture 07 Heart Rhythmicity and Normal ECG Flashcards
What is composed of special cardiac muscle fibers?
SA node
What fibers are connected directly to atrial fibers?
SA node
How fast does the AV node receives signal from the SA node after origin?
.03 seconds
How long is signal delayed in the AV node?
for .09 sec
Why are signals delayed in the AV node?
Due to small size of cells, low amplitude of action potential, and slow rate of depolarization during excitation.
Where does the final delay of .04 seconds occur?
In the penetration bundles
How long is the delay from the initial origin of the signal until onset of ventricular contraction?
.16 sec delay
What is slow conduction caused by?
Mainly by diminished numbers of gap junctions along pathway resulting in an increase in the resistance to conduction
What is the pacemaker of the heart?
SA Node
What is the resting membrane potential of SA node fiber?
-55 to -60mV
What is the threshold of the SA node fiber action potential?
-40mV
Why are fast sodium channels already inactivated at resting potential?
Because activation gates close when membrane potential is less negative than -55mV
What causes the membrane potential to become more positive?
At -60mV only sodium-calcium channels can open. There is a slow leak of sodium ions back into the cells and the membrane potential becomes more positive
Since slow sodium-calcium channels are the only ones that can open at -60mV, what are two results of this?
Atrial nodal action potential is slower to develop
Repolarization is also slower
At what point do ALL sodium-calcium channels become activated?
Threshold has been met at -40mV
What channels become open at the time the sodium calcium channels become inactivated after phase 1?
Large numbers of potassium channels open
When potassium ions move out of the SA node what stage is the action potential at?
Repolarization of the nodal cells. And potassium channels remain open for a few tenths of a second
What is the resting potential of SA node vs Ventricular fiber?
SA node -60
Ventricular fiber -90
When the SA node is the pacemaker of the heart what is the rhythm called?
Sinus
When anything other than the SA node creates the pacemaker of the heart what is the rhythm called?
Ectopic focus or pacemaker
What nerve distributes mainly to the SA and AV nodes?
Vagus Nerve
What neurotransmitter does Vagus nerve have?
Acetylcholine
What type of receptors does the vagus nerve involve?
Muscarinic receptors
Involving the vagus nerve what does it decrease?
Decreases the rate of rhythm of SA node and excitability of the AV junctional fibers, slowing transmission of the cardiac impulse into the ventricles.
The decrease rate of rhythm of the SA node is what type of effect?
Negative chronotropic effect
How does the vagus nerve cause hyperpolarization?
Increases permeability of fiber membranes to potassium ions and cause the potential to go down to -70 to -65
Where is sympathetic innervation distributed to?
To all parts of the heart, mainly the ventricles
What is the neurotransmitter for sympathetic innervation?
Norepinephrine
What receptors do the sympathetic innervation stimulate?
beta-1 adrenergic receptors
Sympathetic innervation increases what?
Depolarization rate (positive chronotropic effect) May increase permeability of fiber membranes to sodium and calcium ions
In a typical ECG, what wave occurs at the beginning of the contraction of the atria?
P
What event is generally not seen on a typical ECG?
Atrial repolarization
What waves represents the depolarization of the heart?
P = atrial depolarization QRS = Ventricular depolarization
Repolarization of the heart on the ECG is represented by what wave?
T Wave = ventricular repolarization
When using a ECG where is the current coming from?
The surface of the heart rather than a transmembrane event.
What is a graphic representation of the electrical activity in cardiac muscle tissue produced by regions of depolarization and repolarization?
ECG
Does an ECG measure Transmembrane potential?
No it measure extracellular potential
When does a deflection from 0 occur in an ECG?
Only when there is current flow between regions of the heart(when there is variation in the membrane potential in different regions of the heart)
Why does current flow not occur when only the atria and the ventricles have different potentials?
Because the atrial musculature and ventricular musculature are electrically isolated.
When is potential recorded on an ECG?
Only when the muscle is partly polarized or partly depolarized does current flow from one part of the ventricle to another
When depolarization and repolarization happen in a single phase but stretch out is what type of action potential is it?
monophasic
What is the time interval of P-Q?
about .16 sec
What is the interval of Q-T?
about .35 sec
What is the direction of bipolar lead III?
120 degrees
What bipolar lead is connected to both left and right arms?
Lead I
Where are the terminals of recording limb lead I? How does it look at the heart?
Negative terminal connected to right arm
Positive terminal connected to left arm
Looks at the heart from left to right
Where are the terminals located with the recording limb lead II? How does it look at the heart?
Negative terminal of ECG is connected to the right arm
Positive terminal of ECG is connected to the left leg
Looks at the heart from upper right to lower left
Where are the terminals involving recording limb lead III and how does it look at the heart?
Negative terminal connected to left arm
Positive terminal connected to the left leg
Looks at heart from upper left to lower left
What is Einthoven’s triangle?
Two arms and left leg form apices of triangle
What is einthoven’s law?
If the electrical potentials of any two of the three bipolar limb ECG leads are known at any given instant, the third can be determined by summing of the first two
How does current flow in the ventricles?
Flows from negative to positive primarily in the direction from the base of the heart toward the apex for most of the heart cycle until the very end
Where will the electrode be negative in reference to the ECG? Where will it be positive?
Negative nearer the base of the heart
Positive nearer the apex of the heart
When does the ECG record positive with recording limb lead I?
When the point where the right arm connects to the chest is negative with respect to the point where the left arm connects, the ECG records positive
For lead II what part of the body is negative and what is positive?
Right arm is negative with respect to the left leg, the ECG records positive
When the left arm is negative with respect to the left leg what will the ECG record? What recording limb does this represent?
Positive
Limb lead III
What is a vector?
An arrow that points in the direction of the electrical potential generated by the current flow, with the arrow head in the positive direction
What does the length of the vector arrow represent?
Drawn proportional to the voltage of the potential
On the axis for lead I, where are the electrodes, what is the direction and what is the charge?
Two electrodes on two arms
Right is negative
Left is positive
The direction of lead is 0 degrees
On the axis for lead II, where are the electrodes, what is the charge, and the direction?
Electrodes on right arm and left leg
Right is negative
Left is positive
Direction 60 degrees
On the axis for lead III, what is the charge and direction of the electrodes and where are the placed?
Electrodes on left arm and left leg
Right is negative
Left is positive
Direction is 120 degrees
When is the voltage recorded in the ECG of this lead is very low means what?
Vector in the heart is in a direction almost perpendicular to the axis of the lead
When the heart vector has almost the same axis as the axis of the lead, how much voltage of the vector will be recorded
Entire voltage
What is instantaneous mean vector
Sum mated vector of the generated potential at a particular instant
What is reference vector?
Vector is horizontal and extends towards a persons left side, the reference direction equals 0 degrees
The average direction of the vector during spread of the depolarization wave through the ventricles results in the apex of the heart remaining as what charge with respect to the base of the heart?
Positive
The QRS vector has what angle in relation to the zero reverence point?
59 degrees (positive)
What is the mean electrical axis of the heart?
+59 degrees
What does the T - wave represent?
Reploarization of the ventricle
What is the greatest portion of the ventricle to repolarize first?
Entire outer surface of the ventricles, especially near the apex
What areas normally repolarize last?
Endocardial areas
What does the ventricular vector point towards during repolarization?
Apex of the heart
Is a normal T wave in all three bipolar leads is negative or positive?
Positive as is most of the QRS complex
Where does atrial depolarization begin?
Sinus node
What is the vector direction in a P-wave?
Generally in the direction of the axes of the three standard bipolar limb leads
During the P wave what does the ECG read in each lead?
Its normally positive in all three leads
Where is the spread of depolarization slower?
Slower through atrial muscle than the ventricles
In an atrial T-wave what is the first part to become repolarized?
Sinus node area
Where is the atrial t-wave on an ECG?
Obscured by QRS wave
How many leads does a typical ECG use?
12
What are the names of the six standard leads?
V1-V6
What are the names of the augmented leads?
aVR, aVL, aVF
Which of the following represents the mean electrical axis of the normal ventricles?
59 degrees
What is the definition of mean electrical axis?
The direction of the electrical potential (negative and positive) from the base of the ventricles towards the apex
What are conditions that can cause mean electrical axis deviation?
Change in position of the heart in the chest Hypertrophy of one ventricle Bundle branch block Fluid in pericardium Pulmonary emphysema
If there is an increased voltage in the standard bipolar leads, this could be a result of what reading from the QRS complex? Give a most common cause
Sum of the voltages of all the ARS complexes of the three standard leads is greater than 4mV. most common cause is hypertrophy of the ventricle
What are causes of decreased voltage in the standard bipolar leads?
Cardiac myopathies
Caused by conditions surrounding the heart
How long does a normal QRS last?
.06 to .08 seconds
In hypertrophy or dilation of the left or right ventricle causes the QRS time to be ________?
Prolonged to .09 to .12 seconds
What are conditions that 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 can cause parts of the heart to remain partially or totally depolarized all the time? And what does this do to current flow?
Different cardiac abnormalities. Since there is always a difference in polarity there will be a current of injury flows between the pathologically depolarized and the normally polarized areas even between heart beats
What are abnormalities causing current of injury?
Mechanical trauma
Infectious processes
Ischemia (most common cause)
What is the effect of current of injury on QRS complex?
Abnormal negative current flows from infarcted area and spreads toward the rest of the ventricles
What is the J point?
Reference point for analyzing current of injury
Where does the J point occur?
At the very end of the QRS wave and is the point at which all parts of the ventricle, including the injured areas, have become depolarized.
How can an ECG show where the injury of the heart is?
Find the vector between Lead I and III and the vector points to the injury
