Physiology-Surface ECG Flashcards
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?
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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?
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What are the standard leads used in an ECG?
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What are the different bipolar leads in an ECG?
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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 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).
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.
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?
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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.
What happens in ECG readings when cardiac tissue is ischemic?
The beginning and ends of the ECG reading are more negative than the central plateau because ischemic cells depolarize, creating a greater potential difference before the cell depolarizes and when it repolarizes. However, the ischemic cell depolarizes the same as the other cells and will have the same zero potential plateau as the others. Due to how the ECG is read, you will observe an elevated ST segment.
