Electrical Activity of the Heart Flashcards
What does the QRS complex represent?
Ventricular depolarization
In one minute how many big boxes will pass on an EKG?
300 large boxes
Which part of the EKG gives you an idea of how long it takes for the AP to go from the SA node to the ventricles?
The PR interval
Which leads are especially useful at providing information about how the septum and left heart are working?
V1 and V2
What is the term for QRS complexes that are not evenly spaced?
Sinus arrythmia
What part of the EKG represents AV nodal delay?
The PR segments
What causes repolarization of fast-response cardiomyocytes?
Closure of calcium channels and Potassium efflux
What is the pathway of cardiac actiion potentials?
SA node–internodal pathway–>AV node–> Bundle of His–> Bundle branches–> Purkinje fibers
What caues rapid depolarization in fast response cardiomyocytes?
Sodium influx
What is the effect of a myocardial infarction on the ST segment?
Elevation
True or False: mean electrical axis is calculated based on ventricular depolarization only
True
What is the nodal delay?
The atrioventricular node slows conduction to 1/20 of the conduction speed of normal myocardium
Where are the electrodes places for lead II?
Negative on R. arm, Positive on L. leg
What is the rate of purkinje fiber action potential conduction relative to myocardium conduction?
4-6x faster than myocardium
If the vector goes the opposite direction as the lead but parallel what will the strength and direction of EKG deflection be?
Strong downward deflection
If there is an R wave every 3 big boxes, what would the heart rate be?
100 bpm
What transfers the action potential from the SA node to the AV node?
Atrial internodal pathways
What is tachycardia?
A heart rate above 100 bpm
What leads to the autorhymicity of pacemaker cells?
Rather than having a resting membrane potential these cells have a decay towards threshold due to leaky ion channels
What is the physiologic advantage of the actional potential spreading from the apex of the heart up?
The pulmonary and aortic valves are located at the top of each ventricle, so squeezing from the apex towards the base maximizes the ejection of blood
How do sympathetics speed up the heart rate?
BY releasing norepinephrine which binds to beta1-receptors on autorhymthic cells allowing the leakiness that drives the threshold graded depolarization to occur faster
What cells of the heart are fast-response?
Cardiomyocytes
What events are represented by the T wave in leads I and aVF?
Repolarization of the ventricles
What does the QT interval represent?
Ventricular depolarization and repolarization
What does a widened QRS complex indicate?
Abnormal conduction
What is the noral range of SA nodal pacemaker rate? AV node?
70-100 bpm; 40-60 bpm
What is the normal range of the mean electrical axis?
-30-120
How do parasympathetics slow down the heart rate?
Releasing ACh which binds to muscarinic receptors on autorhythmic cells allowing potassium efflux and calcium influx, resulting in a hyperpolarized cell that takes longer to reach threshold
What is the travel time for the action potential to go from the start of the bundle branch to the end of the Purkinje network? What is this represented as on the ECG?
0.06 seconds; the QRS complex
Where are the electrodes places for lead I?
Negative on R. arm, Positive on L. arm
What areas of the CNS provide ANS innervation to the heart?
The parasympthetic cardioinhibitory center and the sympathetic cardioaccelatory center in the medulla
What are the X- and Y- axes of an EKG reading?
X= time; Y= voltage
What cells of the heart are slow-response?
SA and AV nodal cells
What causes the Q wave on Lead I?
The movement of signal from left to right in the bundle of His to the bundle branches
What causes the plateau phase of fast-response cardiomyocytes?
L-type voltage gated calcium channels opening in repsonse to depolarization
What is responsible for the upstoke in the action potential of autorhymic cardiac cells?
Opening of voltage-gated L-type calcium channels allowing influx of calcium
What allows the spread of electrical signal though the cardiomyocytes?
Gap junctions and intercalated discs
Where do parasympathetic nerve fibers terminate in the heart?
At the SA and AV nodes
What is the effect of parasympathetic innervation on the PR interval?
The PR interval is increased
What causes the S wave in Lead aVF?
The movement of signal upwards through the ventricles from bottom to top
What does the ST segment represent?
The refractory period
What physiologic event is represented by the P wave on an ECG?
Propagation of the AP through the atria due to the SA node firing
What is the significance of the plateau phase in cardiac muscle?
It prevents the cell from repolarizing quickly, creating a longer refractory period during which the cells cannot depolarize again. This gives the heart enough time to contract, relax and fill with blood before it beats again
What are the physical limits of the PR interval on an EKG?
From the start of the P wave to the upstroke of the R wave
Where do sympathetic nerve fibers terminate in the heart?
SA and AV nodes, all over the ventricular myocardium and a little of the atrial myocardium
Where are the electrodes of the precordial leads?
Positive on the skin, negative from inside the body
Which are the precordial leads?
V1-V6
True or false: The heart exhibites summation and a tetanus response
False
Where are the electrodes placed for lead III?
Negative on L. arm, Positive on L. Leg
What is the mean electrical axis of the heart?
The sum of all the electrical vectors ocuring during depolarization
Why is the SA node the pacemaker of the heart and not the AV node?
The AV does does not have time to reach its own autorhythmic potential- the AV node just receives the faster signal from the SA node which triggers the AV node to depolarize and pass on the signal
If a wave of depolarization is moving parallel to a lead towards the positive electrode, what will be the direction of deflection on the EKG? Will it be a strong or weak signal
Strong Upward Deflection
What events contribute to the R wave on Lead aVF?
Movement of electrical vector down the bundle of His and bundle branches toward the positive pole; additionally the spread of the AP through the ventricular system and reaching the Purkinje fibers
What is the physiologic function of the nodal delay?
Gives the atria time to depolarize, contract, and force blood into the ventricles
How is heart rate modulated by the ANS?
1) impacting the autorhymic firing rate of the SA node and 2) slowing or increasing the transmission of action potentials through the AV node
How would an electrical vector perpidicular to the lead, be represented on an ECG?
No signal (isoelectric point)
What times are represented by each big box? Each little box?
0.2 sec; 0.04 seconds
What kind of cells make up the SA node?
Autorhythmic pacemaker cells
What type of channel allows sodium to leak into autorhythmic pacemaker cells? Calcium?
F-type sodium channels; T-type calcium channels
What is brachycardia?
A heart rate below 60 bpm
What is the site of cardiac action potential initiation?
The sinoatrial (SA) node
