3.1.4. Basis of the Surface ECG Flashcards
What is neuronal input within the heart responsible for?
Modulating activity (electrical signal originates within the heart itself)
How many APs per contraction (heart beat)?
1
Why do APs take on different morphologies in different parts of the heart?
It is a reflection of the ion channel population in each cell and its function
Where does cardiac Ap originate?
Specialized cardiac myocytes that exhibit pacemaker activity (SA node)
What nerve is responsible for parasympathetic innervation and what does it do?
Vagus. Slows down HR via ACh
What nerve is responsible for sympathetic innervation and what does it do?
T1-4 Spinal Nerves. Speeds up HR via Norepinephrine
Why does excitation start in the SA node?
Because it is the fastest pacemaker
SA: 70-80 APs/min
AV: 40-60
Purkinje: 15-40
What is the significance of atrial contraction?
Provides a “kick” to fill the ventricle
What is the Frank-Starling mechanism?
Strength of contraction is proportional to the end diastolic volume/ pressure
How is AP conducted to the ventricle?
Slowly (allows complete emptying of atrial contents), through the AV node.
What is the most important difference b/w fast- and slow-response cardiac APs
The ion responsible for the phase 0 upstroke:
Fast-response AP = fast inward Na+ current
Slow-response AP = slow inward Ca current
This is why conduction is slow in the AV nodes, but fast in the His-Purkinje system
Which ventricle is excited first?
Both, simultaneously
Where is AP conducted from in terms of the heart’s anatomy?
Base to Apex, then back Basal after reaching the Ventricles
Endocardium to the Epicardium
How does repolarization occur?
Epicardium to Endocardium
In which layer of the heart is the AP the shortest?
Epicardium
The total time from impulse initiation in the SA node to
repolarization of the ventricles
600 msec
What is an ECG?
An extracellular recording of the mean cardiac vector over time
Does the ECG measure heart contraction?
No. Only electrical activity
Which part of the body serves as the ground electrode?
Left leg
Which leads provide frontal plane information?
Limb leads
Which leads provide horizontal plane information?
Chest leads
Why does the R wave increase in amplitude for chest leads?
Because the left ventricle is larger than the right
What is the zone of transition?
The lead where R- and S-wave amplitudes are roughly the same
Which ECG intervals and durations are important to look out when determining if the heart in functioning properly?
PR interval
QRS duration
ST segment
QT interval
What does a shortened PR interval mean?
Wolfe-Parkinson-White Syndrome
Signals are entering from other areas than the AV node
What does a widened QRS duration mean?
Slow conduction in Purkinje fibers and/pr ventricular muscles
What is someone at greater risk for who has shortened or increased QT intervals?
Cardiac arrhythmias
What does Bazett’s correction account for?
The heart’s affect on the AP duration
What is the normal axis of the apex?
-30 to 90 degrees
What does the ST segment reflect?
AP plateau (changes = ischemia)
What happens when the heart becomes ischemic?
An ATP-sensitive K+ channel opens after sensing the metabolic distress. This causes the AP to shorten dramatically in ischemic cells.
What is atrial flutter?
Back to back P waves; creates a sawtooth appearance on the ECG
What will you see on the ECG of patients with hypokalemia or bradycardia?
a U-wave (small bump after the T wave)
What can irregularly spaces QRS be a sign of?
Atrial fibrillation (no discernible P wave)
Medications that can prolong QT
Sotalol, Risperidone (antipsychotics), Macrolides, Chloroquine, Protease Inhibitors, Quinidine, Thiazides
Congenital long QT syndrome
- Inherited disorder of myocardial repolarization
- Romano-Ward Syndrome
- Jarvell and Lange-Nielsen Syndrome (deafness)
- Predisposes patients to Torsades de pointes (polymorphic ventricular tachycardia)
What is ventricular fibrillation?
Completely erratic rhythm. It is fatal w/o immediate CPR and defibrillation
