Electrical Properties of the Heart Flashcards
What is a functional syncytium?
Formed by the cardiac muscle.
Electrically connected by gap junctions.
Physically connected by desmosomes.
Forms the intercalated discs.
What is the importance of cardiac muscle having a long action potential?
A long refractory period - no tetanus.
Ca2+ enters from outside the cell and regulates concentration. It does not saturate troponin, and can vary the strength of concentration.
What is excitation-contraction coupling?
Depolarisation - rapid, releases Ca2+, increases the number of actinomyosin cross-bridges.
SR is modulated - controls contraction strength.
What are non-pacemaker action potentials?
At rest - high K+.
Depolarisation - increase in Na+.
Plateau - increase in Ca2+ (L-type), decrease in K+.
Repolarisation - decrease in Ca2+, increase in K+.
What are pacemaker action potentials?
Action potential - increase in Ca2+ L-type.
Pacemaker potential - gradual decrease in K+, early increase in Na+-IF channels (triggered by repolarisation), late increase in Ca2+ (T-type).
What do pacemakers explain, in terms of the physiology of the heart?
Autorhythmicity - the basis for understanding modulation of the activity of the heart.
What is the special conducting system?
SAN - pacemaker.
Annulus fibrosus - non-conducting.
AVN - delay box (allows the atria to depolarise and contract, squeezing blood into the ventricles).
Bundle of His and Purkinje fibres - quick depolarisation for ventricles to contract at the same time, a large pressure to push blood into circulation.
Ensures coordinated contraction of the heart.
How is an ECG recorded?
Many myocytes depolarising and repolarising at the same time can summate to create large extracellular electrical waves that are recorded.
What are the key features of an ECG?
P wave - atrial depolarisation.
QRS complex - ventricular depolarisation.
T wave - ventricular repolarisation.
What is first-degree heart block?
The distance between the P wave and the R wave increases. Conduction from the atria to the ventricles is still occurring, but is taking longer.
What is second-degree heart block?
A failure of conduction.
The PR interval decreases in size before a QRS complex is missed.
What is third-degree heart block?
No atrioventricular contraction.
The atria contract, but not the ventricles.
The pacemakers in the ventricles take over, causing depolarisation and contraction.
Corrected through installing a pacemaker.
What is atrial flutter?
The atria depolarise and contract a lot faster than they should (a type of supraventricular tachycardia). There are many P waves before a QRS complex, creating a sawtooth pattern.
What is atrial fibrillation?
The failure of the pacemaker to spread that wave of depolarisation through the atria. Cells depolarise and contract and different times. No clear P waves; QRS complexes are random.
What is ventricular fibrillation?
Uncoordinated contraction in the ventricles. Blood is not effectively pumped around the body, and consciousness is lost.
