Physiology of The Heart

Electrocardiography (ECG)

M. Djauhari WidjajakusumahDepartemen FisiologiFakultas Kedokteran Universitas Indonesia

isoelectricLead I+--LEAD I

isoelectricLead III+--LEAD III

isoelectricLead AVR+--LEAD AVR

Figure 111Normal electrocardiogram.Guyton & Hall: Textbook of Med Physiol 11th ed, 2006

Effect of Heart Position on the Electrocardiogram

Figure 15-4 Transmembrane potential (Vm) of a cardiac muscle fiber varies inversely with the potassium concentration of the external medium. The straight line (EK) represents the change in transmembrane potential predicted by the Nernst equation for potassium. (Redrawn from Page E: Circulation 26:582, 1962.)

Figure 15-5 Concentration of sodium in the external medium is a critical determinant of the amplitude of the action potential in cardiac muscle (upper line) but it has very little influence on the resting membrane potential (lower line). (Redrawn from Weidmann S: Elektrophysiologie der Herzmuskelfaser, Bern, 1956, Verlag Hans Huber.)

Effects of [K+] on Cardiac Rate and Rhythm Hyperkalemia

Myocardium action potential:Lowers the resting membrane potential (resting Em )Inactivates Na+ channels Na+ fast inward current action potential upstroke and amplitude slow conduction velocity widening of QRS complex the rate and extent of K+ channels re-opening accelerates repolarization shortens the plateau phase and duration of action potentialpeaked T wavesslightly shortened Q-T interval

Pacemaker (SA node) action potential:Lowers the resting membrane potential (resting Em ) phase 4 slopeno change in sinus rate (heart rate)

Effects of [K+] on Cardiac Rate and Rhythm HypokalemiaMyocardium action potential:Nernst equation: resting membrane potential (resting Em ) [K+] < 3mM resting membrane potential slightly Normal action potential upstroke and amplitudeInhibits the re-opening of K+ channels delayed re-opening of K+ channels prolonged plateau, repolarization and duration of the action potentialflat T wavesprolonged Q-T interval

Pacemaker (SA node) action potential:Lowers the resting membrane potential (resting Em ) phase 4 slope sinus rate (heart rate)

Effects of [Ca++] on Cardiac Rate and Rhythm HypercalcemiaMyocardium action potential: [Ca++] o / [Ca++] I ratio Ca++ inward current the height of the plateauNormal action potential upstroke and amplitudeAccelerates the re-opening of K+ channels shortened plateau, repolarization and duration of the action potentialsagging S-T segmentsshortened Q-T interval

Pacemaker (SA node) action potential: threshold potential sinus rate (heart rate)

Effects of [Ca++] on Cardiac Rate and Rhythm HypocalcemiaMyocardium action potential: [Ca++] o / [Ca++] I ratio Ca++ inward current the height of the plateau Action potential upstroke and amplitudeSlowing the re-opening of K+ channels prolonged plateau, repolarization and duration of the action potentialprolonged Q-T interval

Pacemaker (SA node) action potential: threshold potential sinus rate (heart rate)

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Electrocardiography (ECG)

M. Djauhari WidjajakusumahDepartemen FisiologiFakultas Kedokteran Universitas Indonesia

Effect of Heart Position on the Electrocardiogram