Abnormalities of Cardiac Impulse

• Factors that influence heart rate (altered frequency of pacemaker cell firing rate)

  • Heart rate determined (interval between pacemaker firing) by the sum of: Action potential duration + Diastolic duration interval

  • More important -- Diastolic duration interval: determined by 3 factors:

    • Maximum diastolic potential (most negative membrane potential reached during diastole

    • Slope of phase 4 depolarization: (increased slope: threshold is reached quicker causing a faster heart rate; decreased slope: longer to reach threshold resulting in a slower heart rate

    • Threshold Potential (membrane potential at which in action potential is initiated)

    
    

  • Decreased Heart Rate:--

    • Vagal Effects: (cholinergic influences on the heart rate)

      • more negative maximum diastolic potential (the membrane potential starts farther away from the threshold potential)

      • reduced slope of phase 4 depolarization (takes longer to reach threshold potential)

  • Increased Heart Rate:-

    • Adrenergic Effects: (sympathetic/sympathomimetic influences on heart rate)

      • Beta adrenergic receptor blockers (reduced phase 4 depolarization slope)

  • Factors that can increase automaticity:

    • hypokalemia

    • cardiac fiber stretch

    • beta-adrenergic receptor activation

    • injury currents

    • acidosis

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  • Latent Pacemakers -- cells not normally serving pacemaker function, but exhibits slow phase 4 depolarization: conditions favoring latent pacemaker activity noted above

    • All cardiac cells (including normally inactive atrial/ventricular cells) may show pacemaker activity, particularly in hypokalemic states

  • Failure of impulse initiation can lead to excessively slow heart rate,bradycardia .

  • If an impulse fails to propagate through the conduction system from the atrium to the ventricle, heart block may occur.

  • An excessively rapid heart rate, tachycardia, is also encountered clinically

Category: Physiology Notes