Electrical Activity of the Heart Flashcards
In (non-nodal) cells of the heart what ions/channels set the RMP?
Ungated potassium channels
inward K+ rectifying channels (IK1)
What does phase 0 of non-nodal cells of heart represent?
upstroke of AP
What mediates Phase 0 of AP for non-nodal cells of heart?
Similar to nerve and sk muscle mediated by opening of voltage-gated, fast Na+ channels (note high Na+ conductance)
What mediates Phase 1 of AP for non-nodal cells of heart?
slight repolarization mediated by transient potassium current
Sodium channels transition to inactivated state.
What mediates plateau phase of the AP of non- nodal cells of the heart?
depolarization opens voltage-gated Ca2+ channels (primarily L -type and voltage-gated K+ channels (IKr current)
Inward Ca2+ current is offset by outward K+ current
What occurs during the plateau phase of AP for non-nodal cells of heart?
influx of Ca2+ triggers release of Ca2+ from SR resultant in cross-bridge cycling and muscle contraction
Which segment of the ECG correlates with the plateau phase of non nodal cardiac cells?
ST segment
What is a benefit of the long duration of AP caused by plateau phase?
prevents tetany in cardiac muscle
What is phase 3 of the non nodal cardiac AP?
repolarizaton phase
What ions/channels are implicated in phase 3 repolarization phase of non-nodal cell AP?
L-type Ca2+ channels begin closing, but rectifying K+ currents (Ikr current) still exists resulting in repolarization
IK1 channels open and aid in repolarization?
Which part of the ECG does phase 3 of non-nodal cardiac cells correspond with?
T wave of the EKG
What does Phase 4 of non-nodal cardiac cells correspond with?
resting membrane potential
What are the ions/channels responsible for phase 4 of the cardiac non-nodal cells? What ions/channels are closed?
fast Na+, L-type Ca2+, and rectifying K+ channels (IKr) close, but IK1 channels remain open.
so again ungated K+ channels
Inward rectifying channels IK1 (voltage gated channels that open at rest)
Phase 4 resting membrane potential in nodal cells is maintained by what ions/ channels?
Inward Ca2+ current
Inward Na+ current If (funny sodium channels)
channel involves a HCN hyperpolarization-activated cyclic nucleotide gated channel
Outward K+ current that is reduced to produce pacemaker potential
What is Phase 0 of cardiac AP in nodal cells mediated by? (ions/channels)?
L type primarily Ca2+ channels
What is phase 3 of cardiac AP in nodal cells mediated by? (ions/channels)?
voltage-gated K+ channels
Ivabradine MOA in the heart?
blocks funny current in SA node, thereby reducing HR
Ivabradine indications for heart?
for systolic heart failure when beta blockers fail to reduce HR sufficiently
For idiopathic sinus tachycardia
What is torsade de pointes?
When there is a long QT interval leading to ventricular tachycardia
Chronotropy?
HR related
Dromotropy
conductance related mostly deals with AV node
Inotropy
deals with heart muscle and force of contraction
R wave of ECG?
first upward deflection after P wave
S wave?
first downward deflection after R wave
T wave?
ventricular repolarization
PR interval? (caused by what) (where does it start and end)
Start of P wave to start of QRS complex mostly due to conduction delay in AV node
QT interval?
Start of QRS complex to end of T wave; represent duration of AP
ST segment?
Ventriciles are depolarized during this segment; roughly corresponds to plateau phase of AP
end of S wave start of T wave
J point?
end of S wave; represents isoelectric point
What is the trick to counting bpm on ECG?
numbers relate to boxes:
1 =300
2 = 150
3 = 100
4 = 75
5 = 60
6 = 50
Step 1 to reading an ECG?
Detect rate and rhythm
Step 2 of reading an ECG?
Detect presence of P, QRS, and T wave do they look normal?
Step 3 of reading an ECG?
Find the PR interval and determine if it is normal range that’s 3-5 small boxes look at several cycle to see if PR interval is consistent
What does a + lead 1 and neg aVF indicate?
left axis deviation
Causes of left axis deviation?
- left heart enlargement, either left ventricular hypertrophy or dilation
- conduction defects in the left ventricle, except in posterior bundle branch
- Acute MI on right side tends to shift axis to left unless right ventricle dilates
If lead 1 is negative and aVF is positive what does this indicate?
Right axis deviation
What are causes of rt axis deviation?
- right heart enlargement, hypertrophy, or dilation
- Conduction defects of right ventricle or the posterior left bundle branch
- Acute MI on left side tends to shift the axis right unless the left ventricle dilates
If lead one is (+) and aVF is (+) then what does this indicate?
normal axis deviation
What does this image indicate and why?
FIRST DEGREE HEART BLOCK
long PR interval (slowed conduction through AV node)
Rate and rhythm typically normal
What does this image represent and why?
second degree heart block
QRS complex is preceded by a P wave but not every P wave is followed by QRS complex (some impulses are not transmitted through AV node
Mobitz type I (Wenckebach) progressive prolongation of PR interval until a ventricular beat is missed and then the cycle begins again. This arrhythmia will have an unsteady rhythm
What does this image represent and why?
Second degree heart block
QRS complex is preceded by a P wave but no every P wave is followed by QRS complex (some impulses are not transmitted through AV node
Mobitz type II: PR interval is consistent, i.e it doesn’t lengthen and this separates it from Wenckebach. Rhythm can be steady or unsteady depending on block ratio.
What does this image represent and why?
Third-degree (complete) block
there is complete dissociation of P waves and QRS complexes (impulses not transmitted through AV node
steady rhythm (usually) and very slow ventricular HR (usually)
No consistent PR interval because impulses no transmitted through the AV node
DANG THAT RATE IS SLOW
What does this image represent and why?
Atrial Flutter
very fast atrial rate atrial conduction is intact and coordinated
Characteristic saw tooth appearance
What does this image represent and why?
Atrial Fibrillation
uncoordinated atrial conduction
lack of a coordinated conduction results in no atrial contraction
characteristics: unsteady rhythm (usually) and no discernible P waves
What does this image represent and why?
Wolff-Parkinson-White Syndrome
Accessory pathway (Bundle of Kent) between atria and ventricles
Characteristics: short PR interval; steady rhythm and normal rate (usually); slurred upstroke of R wave; widened QRS
Risk with Wolff-Parkinson-White Syndrome?
cardiac impulse can travel in retrograde fashion to the atria over accessory pathway and initiate a reentrant tachycardia
Risk with Wolff-Parkinson-White Syndrome?
cardiac impulse can travel in retrograde fashion to the atria over accessory pathway and initiate a reentrant tachycardia
Elevated ST segment change may have what clinical correlate?
transmural infarct or Prinzmetal angina (coronary vasospasm)
Depressed ST segment change may indicate what?
subendocardial ischemia or Exertional (stable) angina
Hyperkalemia has what effect on ECG?
increases rate of repolarization resulting in sharp-spiked T waves and shortened QT interval
Hypokalemia has what effect on ECG?
decreases rate of repolarization, resulting in U waves and a prolonged QT interval
Hypercalcemia has what effect on ECG?
decreases the QT interval
Hypocalecmia has what effect on ECG?
increases the QT interval
