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Flashcards in Physiology-Surface ECG Deck (31)
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What is the end goal of an action potential in the heart?

To allow Ca2+ to enter the cell, causing muscle to contract.


Where does the cardiac action potential originate?

Sinoatrial node. It has the fastest nodal pacemaker.


What autonomic nerves stimulate the heart to either increase or decrease heart rate?



How is the SA node related to the graph illustrated below?

The SA node action potential initiates atrial contraction. The illustration depicts the Frank-Starling Mechanism: the strength of ventricular contraction is dependent on the end diastolic volume…which is dependent on atrial contraction.


What would happen if the AV node did not have as slow of conductance as it does?

The atria would contract before all of its contents have been emptied.


Where does the action potential propagate after the AV node?

Down the His-Purkinje system -> working ventricular myocardium from apex to base and endocardium to epicardium. 


Why is AV node conductance slow and the His-Purkinje system fast?

The AV node has slow Ca2+ channels that results in a slow rise in phase 0. The His-Purkinje system utilizes fast Na+ channels.


Why do epicardial cells tend to repolarize before the endocardial cells?

The epicardial cells have a shorter action potential duration


What factors contribute to positive, negative and zero deflections in ECG readings?



What are the standard leads used in an ECG?



What are the different bipolar leads in an ECG?



What are the different unipolar leads in an ECG?

You use "Wilson's central terminus" by using the heart as the central negative electrode. By removing one electrode, you measure the voltage by assessing the voltage of the other two electrodes in its absence.


What happens when you put the bipolar and unipolar leads together in an electrocardiogram?

The leads give you a hexaxial reference system with axis oriented in 30 degree increments around the heart.


What is the purpose of having unipolar precordial leads?

They provide a hexaxial reference system in the horizontal plane.


What wave occurs at this point in an ECG?

The P-wave. The SA node leads to atrial depolarization which causes a positive vector production in lead I and the avF lead.


What wave occurs at this point in an ECG?

The Q-wave and the start of the R-wave. The AV node leads to His-Purkinje depolarization which causes a positive vector in the avF lead (R-wave), a negative vector in lead 1 (Q-wave).


What wave occurs at this point in an ECG?

The R-wave. This is a result of early ventricular depolarization. Note positive vector production in lead 1 and lead avF.


What wave occurs at this point in an ECG?

Continuation of the R-wave and production of the S-wave. This is a result of late ventricular depolarization. Note positive deflection in lead 1 and negative deflection in  lead avF.


What is happening at the plateau of an ECG after the S-wave?

All myocardial cells are depolarized and there is no vector.


What wave occurs at this final point in an ECG?

T-wave. This happens as a result of the epicardium repolarizing before the endocardium. Notice the positive vector produced in lead 1 and in lead avF.


What intervals do we measure when taking an ECG?

PR interval (length of atrial AP), QRS duration (Ventricular depolarization), ST segment (ventricular action potential plateau) and QT interval (ventricular action potential duration).


You see a patient with a history of cardiac issues and decide to do an ECG on him. What can you conclude if you observe a prolonged PR interval? What if the PR interval was shortened?

Prolonged = increased risk of AV conduction failure, AP will die out before it reaches ventricular myocardium. Shortened = Wolfe -Parkinson -White, there is an accessory pathway where the AP bypasses the AV delay and goes directly from atrium to ventricle.


You see a patient with a history of cardiac problems and you are reading her ECG. You notice a widened QRS complex. What do you conclude from this reading?

If the QRS complex is widened, it indicates slow AP conduction in Purkinje fibers or ventricular muscles.


A patient comes to your clinic and before you see him you look over his ECG. You not a lengthened QT interval. What are you worried about because of this reading? What if the QT interval was shortened?

A lengthened QT means that the ventricular AP duration is longer and puts the patient at risk for cardiac arrhythmia. Shortened QT intervals also put you at risk for arrhythmia.


When would your QT interval be shorter or longer in a healthy individual without any arrhythmias? How is this change in QT interval accounted for?

When your heart rate increases or decreases. Fast heart rate = faster AP. Slower heart rate = slower AP. This is the basis for the cQT interval that takes into account heart rate.


In what plane does a normal QRS interval lie?

-30 to 90 degrees.


How do you determine the mean QRS axis?

Add and subtract the voltage seen in each lead on the actual ECG reading. Then, apply the calculated voltage to the line in the hex axial plane for each lead. Find the point where all perpendicular lines intersect and that is your mean QRS axis.


What do you know about the mean QRS axis when looking at this lead reading?

Lead III has a biphasic deflection, which means the QRS axis is perpendicular to it, making it = to the aVR lead. Then looking at the aVR lead, you see the deflection is negative. Draw the line and note the mean QRS axis is at 30 degrees.


If you see this deflection in leads 1 and 2, in what part of the axis do you know the QRS wave has to be?



If you see this deflection in leads 1 and 2, in what part of the axis do you know the QRS wave must be?

Left axis deviation means that the QRS wave is more oriented towards the left side of the heart.

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