6. Cardiac System 3 Flashcards
What is a electrocardiogram (ECG/EKG)?
Graphic recording/display of the biopotentials generated by the myocardium during cardiac cycle
Shows rhythmic electrical depolarization and repolarization wave (AP) that starts contraction and relax of atria and ventricles
Slide 4-9 oct 3
Which is longer, ventricular systole or ventricular diastole?
For normal cardiac cycle;
Ventricular systole is substantially shorter than that of the ventricular diastole
What are these parts of an ECG; P wave QRS complex T wave U wave
P wave- depolarization of atria
Normally less than 2.5mm tall and 0.11 sec width
QRS complex- depolarization of ventricles
Normally less than 0.12 seconds
T wave- repolarization of ventricles
Should be upright, if not then maybe myocardial ischemia or anxiety attack or intraventricular conduction delay
U wave- unknown, possible repolarization of papillary muscles
Slide 9 oct 3
What is the PR interval of an ECG?
How does it initiate?
Start of atrial depolarization to start of ventricular depolarization
Requires SA node to initiate depolarization wave (P wave) and spread through the right atrium through gap junctions
SA node must also depolarize left atrium though bachmans bundle then gap junctions
Normally between 0.12-0.20 seconds
Slide 10 oct 3
What is the PR segment of ECG?
End of atrial depolarization to the start of ventricular depolarization
Length of segment is related to the time of delay for the action potential passing through AV node
Slide 11 oct 3
What is the QRS duration of an ECG?
Time of this indicates duration of ventricular depolarization
Functional integrity of ventricular conduction system (bundle of his, bundle branches, purkinje fibers) and contractile myocardium play a role in this
Slide 12 oct 3
What is the ST segment of an ECG?
What does changing in this usually indicate?
What does elevation and depression if this segment mean?
End of ventricular depolarization to the start of ventricular repolarization
Also known as isoelectric line
Deviation from this isoelectric line is an indication of acute myocardial ischemia
Elevation- epicardial ischemia
Depression- endocardial ischemia
Slide 13 oct 3
What is the QT interval of an ECG?
What can it be used to estimate?
Beginning of ventricular depolarization to the end of ventricular repolarization
Used to roughly estimate the duration of ventricular action potentials
Directly correlated with heart rate
Slide 14 oct 3
What could short and long PR interval indicate?
Short PR interval= wolff-Parkinson-white syndrome (pre-excitation of ventricle)
Long PR interval= heart block (disruption of conduction)
What is hyperkalemia (hi K+)?
Gives a wide QRS complex
Increase in extracellular potassium levels cause partial depolarization of membrane potentials
- membrane cannot return to its resting state
How are fast Na+ channels involved in hyperkalemia?
Activation gate closed at rest
Inactivation gate opens as rest
At end of depolarization, I activated gates close themselves and repolarization begins
As membrane potential comes closer to resting state during repolarization, inactivation gate reset themselves and start opening their gates
Opening of inactivation gates normally follows closing of activation gates when membrane potential is back to resting state
Hyperkalemia causes partial depolarization so some of activation gates fail to reset themselves and are still in opening position allowing some Na influx which makes wife QRS
Slides 17-19 oct 3
How to calculate the corrected QT interval (QTc)?
QTc=QT/square root(RR)
RR= preceding R-R interval
Normal QTc is between 0.3 and 0.44 seconds
Slide 20 oct 3
What are the limb and chest leads used for an ECG?
Normally;
ECG contains 6 limb leads (I, II, III, aVR, aVL, and aVF)
6 chest leads (V1-V6)
Leads I, II, III are bipolar limb leads
Leads aVR, aVL, aVF, and V1-V6 are unipolar ECG
Slides 24-27 oct 3
Where do you place leads I, II, III?
Lead I:
Negative lead at right arm
Positive lead at left arm
Lead II:
Negative lead at right arm
Positive lead at left leg
Lead III:
Negative lead at left arm
Positive lead at left leg
Slides 23-27 oct 3
Which leads are; Anterior Anterolateral Anteroseptal Anteroapical Posterior Inferior
Anterior- V1-V6 Anterolateral- V5-V6, I, aVL Anteroseptal- V1-V2 Anteroapical- V3-V4 Posterior- V1-V2 Inferior- II, III, aVF
Slide 25 oct 3
What is overdrive suppression?
Phenomenon of pacemaker cells with higher intrinsic rhythm prevent all latent (ectopic) pacemakers in the heart from firing
Higher intrinsic rhythm pacemaker tends to suppress latent pacemakers automaticity
What is the mechanism by which overdrive suppression works?
Based on activity of Na-K ATPase pump (3Na out 2K in)
Higher the rate of Na influx (depolarization) higher activity of Na-K ATPase pump
Hyperpolarization creates a more negative maximum diastolic potential for the pacemaker cells so it requires more time for If to reach threshold
Slides 3-6 oct 8
What is the re entry loop?
Cardiac impulse might re-excite the myocytes through which it had passed previously within the same cardiac cycle
Consequence of conduction disturbance which could result in the onset of tachycardia of fibrillation
What are the 3 conditions required for re-entry to occur?
- It is a closed conduction loop
- Has to be a region of unidirectional block
(Impulse can pass in one direction but not in the other) - Requires sufficiently slow conduction of impulse around the loop
(Effective refractory period of the re-entered region must also be less than the propagation time)
Slides 9-10 oct 8
What are the 5 types of cardiac arrhythmia?
Sinus rhythm- SA node controls the cardiac rhythm (60-100bpm)
Sinus bradycardia- sinus rhythm with a rate less than 60bpm
Sinus tachycardia- sinus rhythm with a rare greater than 100bpm
Arrhythmia (dysrhythmia)- onset of abnormal heart rhythms, symptoms range from benign palpitation (rapid/irregular heart rate) to decrease in cardiac output
Escape rhythm- prolonged impairment of SA node
How does escaped rhythm work?
The prolonged impairment of SA node allows latent pacemaker (AV node) to initiate an escape beat
Escape beats continuing gives rise to escape rhythm
Ventricular conduction system (bundle of his or bundle branches) become latent pacemaker
Heart rate usually low
What is the junctional escape rhythm?
Situation when SA node has a lower automaticity than that of the AV node
Ischemia of the SA node could trigger the onset of junctional escape beat and turn into junction rhythm (AV node becomes principle pacemaker and bradycardia will occur)
It is a form of ectopic beat (any heart beat generated by impulse other than the one from SA node)
What are the 2 directions the impulse from a latent pacemaker of junctional escape rhythm can travel?
- One towards the ventricular contractile myocytes and cause ventricular contraction
- Also travels toward the atrial contractile myocytes and cause atrial contraction
Slide 15 oct 8
What is the ventricular escape rhythm?
Could be a benign physiological response to intense sympathetic outflow
May be related to pathophysiological conditions like ischemic heart disease or myocardial infarct
What is ventricular fluttering?
Highly unstable form of arrhythmia and is the transitional condition between ventricular tachycardia and ventricular fibrillation
Usually deteriorated to ventricular fibrillation which is a form of no functional muscle contraction (quivering)
What is heart block and it’s 3 types?
Impaired conduction through the AV conduction system- including the AC node, bundle of his, left and right bundle branches
Common causes in cardiac arrhythmia
First second and third degree AV block
What is first second and third degree AV block?
First degree AV block- delay between atrial and ventricular depolarization (prolong PR interval)
Second degree AV block- intermittent failure of AV conduction
PR interval progressively lengthens until a single QRS complex is absent
Without lengthening of PR intervals
Third degree AV block- complete heart block with complete failure of conduction between atria and ventricles
Total dissociation between P wave and QRS complex
Slides 20-23 oct 8
What is right bundle branch block?
Occurs when the electrical impulse from the bundle of his does not conduct along the right bundle branch
With normal conduction down the left bundle branch
Interventricular septum and left ventricle depolarize normally
Depolarization of right ventricle occurs later and is slower
Slide 25 oct 8
What is left bundle branch block?
Occurs when transmission of impulse is delayed or fails along the main left bundle branch or in both left anterior and posterior fascicles
Left ventricle slowly depolarizes by means of cell to cell conduction that spreads from the right ventricle to the left ventricle
Slide 27 oct 8
