Lecture 07 Heart Rhythmicity and Normal ECG Flashcards Preview

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Flashcards in Lecture 07 Heart Rhythmicity and Normal ECG Deck (95):

What fibers are connected directly to atrial fibers?

SA node


What is composed of special cardiac muscle 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?



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?



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?



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?



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?



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?



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?



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?

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?



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?



What are the names of the six standard leads?



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