Cardiac Electrophysiology and the ECG
Key ionic currents, action potential phases, conduction timing, and ECG waveforms of the heart.
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Questions Covered in This Set
12 cards to master
What causes Phase 0 of the ventricular myocyte action potential?
Rapid opening of fast voltage-gated Na⁺ channels, driving Vm from −90 mV to about +20 mV in ~1 ms.
What maintains the Phase 2 plateau?
Inward Ca²⁺ through L-type channels almost exactly balancing outward K⁺ current, holding Vm near 0 mV for 200–300 ms.
Why can't cardiac muscle tetanise?
The long plateau makes the refractory period last nearly as long as the contraction, guaranteeing relaxation and refilling between beats.
What sets the stable −90 mV resting potential in working myocytes?
High resting K⁺ permeability via the inward-rectifier current I_K1 (plus the Na⁺/K⁺-ATPase).
What is the funny current (I_f)?
A mixed Na⁺/K⁺ inward current through HCN channels activated by hyperpolarisation; it drives slow phase 4 diastolic depolarisation in pacemaker cells.
How do pacemaker cells differ from working myocytes?
No I_K1 (no stable rest), few fast Na⁺ channels (slow Ca²⁺-driven upstroke), and presence of I_f producing spontaneous phase 4 depolarisation.
How do sympathetic and vagal input change heart rate?
Noradrenaline/cAMP steepens the phase 4 slope via HCN channels (tachycardia); acetylcholine flattens the slope and opens I_KACh to hyperpolarise (bradycardia).
Why does the AV node delay conduction (~100 ms)?
Its slow ~0.05 m/s conduction lets the atria finish emptying before ventricular contraction, and it filters fast atrial rates (e.g. in AF).
List intrinsic pacemaker rates and define overdrive suppression.
SA node 70–80/min, AV node 40–60/min, Purkinje 20–40/min; the fastest pacemaker (SA) suppresses slower latent pacemakers.
What is the basic rule linking depolarisation waves to ECG deflections?
Depolarisation moving toward a positive electrode gives an upward deflection, moving away gives a downward one; repolarisation reverses the sign, and uniform potential gives an isoelectric line.
In what direction is the ventricle activated?
Endocardium to epicardium and apex to base, wringing blood upward toward the outflow tracts.
What does the P wave represent?
Atrial depolarisation, lasting about 80–100 ms.
