Cardio 6 Flashcards
How are cardiac cells at rest? (resting potential)
They are polarized, at a negative potential. (how polarized depends on the type of cell)
Do all cells have a resting potential?
No, cells that have a pacemaking current don’t have a resting potential (AV node, Purkinje)
Name and explain the “steps” in an AP in a ventricular cell
- Upstroke: depolarization
- Plateau: the LONG action potential
- Recovery: repolarization
What does having pacemaker cells intel?
they slowly depolarise until they reach the threshold, which is why they don’t have resting potential
What is the state of an atrial or ventricular cell before an AP?
ionic channels
Open K+ channels
Closed Na+ channels
Closed Ca2+ channels
What happens when an atrial or ventricular cell starts to depolarized and reaches the threshold?
Na+ channels open very quickly, and Na+ enters the cell at a rapid rate before the channels close again
This depolarisation closes the K+ channels
More slowly, the Ca2+ channels open and triggers the Ca induced - Ca release
What is the Ca induced - Ca release?
In tubules, Ca flows into the cell, activating ryanodine receptors, causing the SR to release internal stores of Ca to cause a muscle contraction
What does the delay in the Ca channels opening indicate?
The delay between the AP and the contraction of the muscles
What differentiates the fast and slow AP?
the rate of the upstroke (Na influx)
Which parts of the conducting system have fast/slow AP?
F: ventricular, atrial, bundle of His, bundle branches and Purkinje fibres
S: SA node and AV node (few Na channels)
What will the speed of the AP affect? How?
The conducting velocity
If a cell has a slow AP, the conduction between neighbouring cells (within a same structure) will be slower
How is the conducting speed of the AV node? Why?
It is slow because it has slow AP cells to allow a delay in contraction of the atria and ventricles
What is the normal sinus rhythm, what is bradycardia and what is tachycardia?
Normal sinus: 70 bpm
Brady: abnormally slow - <60 bpm
Tachy: abnormally fast - >100 bpm
T or F
It is possible to have a healthy bradycardia or tachycardia
T
Trained athletes have abnormally slow heartbeats and exercises can bring a HB over 100
This is Physiological brady/tachycardia
What can go wrong at the AV node? What will it cause?
Electrical current can be blocked by damage to the conducting system, meaning that the impulses have a hard time/ can’t pass from the atria to the ventricle
AV block
What will an AV block look like on an ECG?
P waves (impulse in the AV node) have a missing QRS complex (ventricular) and T wave
What is the notable difference between a partial and total AV block?
Complete: No impulses from the SA node reach the ventricle
Partial: every second impulses don’t make it to the ventricle (2:1)
T or F
It is possible to have ventricular contraction even with an AV block
T
the heart has backup pacemaking sites that can contract the ventricle even if the sinus node impulse doesn’t pass through the AV node
What is premature ventricular contractions (PVC)
It is when, under pathological conditions, regions of the ventricle generate an activation wave of their own
What are PVC pacemaker cells called?
Ectopic pacemakers
What is an example of a PVC mechanism?
A localised group of ventricular cells starts spontaneously depolarising, effectively giving them a pacemaking current, resulting in a second pacemaking site in the heart.
(automaticity)
How can you recognise a PVC on an ECG?
Since the current will not have to pass through the Purkinje fibres (fast current), the QRS complex will appear as slower and wider at an unusual place in the cycle
T or F
PVCs are common and can be begnin
T
They become dangerous when they generate a reentry rythm
What is a reentry cycle?
When a pulse circles back on itself to re-excite cardiac tissue. Here, the sinus node is no longer driving the activity
How is a Ventricular reentrant tachycardia (VT) dangerous?
When the heart beast fast with an impulse re-exciting cardiac tissue, the heart is no longer able to pump blood effectively
What can VT generate into?
Ventricular fibrillation (v-fib): many small reentrant waves travelling in the heart muscle and the ventricles no longer contract in a coordinated way (completely ineffective)
How long can someone have V-fib for?
It is fatal if not treated within minutes
Where does our understanding of VT and V-fib come from?
Sock array, a mapping of the heart’s current
What is atrial fibrillation?
A premature arterial contraction (PAC)
How does arterial fibrillation affect the heart when compared to v-fib?
It is not as dangerous as blood can still be pumped though the body, just less effectively
