Understand the ionic basis of action potentials in contractile cardiac myocytes.
Comprehend how the Electrocardiogram (ECG) is generated and interpret its components.
Ionic Basis of an Action Potential for Contractile Cardiac Myocytes
Characterized by 5 phases of depolarization and repolarization:
Phase 0: Rapid Depolarization
Triggered by Na+ influx through fast voltage-gated Na+ channels.
Occurs following the pacemaker potential reaching the threshold of approximately -65 mV.
Phase 1: Early Repolarization
Known for rapid inactivation of fast Na+ channels.
Involves activation of transient outward K+ current resulting in a brief efflux of K+.
Phase 2: Plateau Phase
Characterized by a balance between K+ efflux and Ca2+ influx through the L-type Ca2+ channels.
K+ efflux occurs through delayed rectifier K+ channels.
Duration of this phase is approximately 175 ms.
Phase 3: Repolarization
Continuation of K+ efflux leads to a progressively negative inside of the cell.
Duration is about 75 ms, and the cell is refractory during phases 0, 1, 2, and part of 3 due to inactivation of L-type Ca2+ channels.
Phase 4: Restoration of Ionic Concentrations
Returns Na+ and K+ to resting concentrations via Na+-K+ pumps (Na+-K+ ATPase).
Ca2+ restoration through Na+-Ca2+ exchangers and ATP-driven Ca2+ pumps, with a net movement of 2 K+ entering and 3 Na+ leaving the cell.
Heart Structure and Details
Catecholamines play a significant role in cardiac contraction.
Calcium (Ca2+) interactions are crucial in muscle contraction, via binding to troponin I, influencing muscle fibers.
Cardiac glycosides inhibit the Na+-K+ pump, affecting intracellular Na+ levels.
Electrocardiogram (ECG/EKG)
Definition: A graphical representation of biopotentials generated during the cardiac cycle, reflecting electrical events of depolarization and repolarization, subsequently triggering mechanical contraction and relaxation.
Key Components:
P Wave: Atrial depolarization.
QRS Complex: Ventricular depolarization.
T Wave: Ventricular repolarization.
U Wave: Unknown, possibly related to repolarization of papillary muscles or Purkinje fibers.
Lead Connections:
Bipolar Leads: Leads I, II, III (constructed through references of right arm to left arm leg connections).
Unipolar Leads: aVR, aVL, aVF, and leads V1-V6.
ECG Placement of Electrodes
Limb Leads:
Lead I: RA (-) to LA (+)
Lead II: RA (-) to LL (+)
Lead III: LA (-) to LL (+)
Chest Leads: V1 to V6 positioned across the chest to capture electrical activity from the heart's surface.
ECG Intervals or Segments
PR Interval: Time taken for impulse from SA node to start of ventricular depolarization (normal 0.12 to 0.20 seconds).
QRS Duration: Reflects the time required for ventricular depolarization (should be < 0.12 seconds).
QT Interval: Duration from ventricular depolarization to repolarization; can be corrected for heart rate as QTc. Normal range is 0.3 - 0.44 seconds.
ST Segment: Indicates the period between ventricular depolarization and repolarization (isoelectric line). Deviations can indicate ischemia.
T Wave: Normally upright; an inverted T wave may signal ischemia or conduction issues.
Sample Questions
What is the cause of the plateau phase observed in the myocardial action potential?
Answer: c) Eflux of K+ and influx of Ca++.
What does the QRS complex represent in the ECG?
Answer: a) Ventricular depolarization.
Duration of the QT interval can indicate what relative to heart rate?
QTc = QT/extsqrt(RR) and is affected by HR.
PR segment status can suggest conduction delay if prolonged, indicating possible arrhythmias.
Normal ECG Values
P wave: Amplitude < 2.5 mm, width < 0.11 s.
Short PR interval may indicate pre-excitation syndrome.
Long PR interval might suggest heart block.
Ensure adequate monitoring of QT interval especially in drug administration settings to avoid long QT syndrome.