Summary Part 1 - THE BASICS

1. The resting membrane potential and the action potential
   • Selective ion channels, ion exchangers and energy demanding pumps generate a resting membrane potential of -70 to -90 mV. The cells contain much more potassium, and little sodium and calcium compared to the extracellular compartment. The difference between the intra- and extracellular concentration of potassium is the major determinant of the cell’s membrane potential.
   • The action potential has a phase 0 (sodium influx, depolarization), phase 1 (outward potassium, modest repolarization), phase 2 (calcium inwards – muscle contraction), phase 3 (potassium influx, repolarization), phase 4 diastole (reestablishing the ionic concentration gradients).
   • The cardiac conduction system has pacemaker cells that spontaneous slowly depolarize in diastole until they reach a threshold when calcium channels open and initiate an action potential.
2. Spread of impulses
   • The fastest pacemaker cells are located in the sinus node. The AV node has a complex structure and delays the impulses, allowing optimal filling of the ventricles. From the node, specialized conduction fibers pass through the bundle of His, the fascicles and bundle branches to the purkinje fibers that end in the subendocardium.
   • o Muscle cells have fast longitudinal conduction, but impulse transmission is delayed by the intercalated disks, and hence, transversal conduction outside the specialized conduction system, is rather slow.
3. The ECG recorder
   • The resultant of the electric activity of the myocytes is described by the ECG, provided other muscles in the body are relaxed (recumbent position is optimal), that the electrodes (antennae) are correctly positioned and that filtering of signals is according to guidelines.
   • The printout should be arranged so that adjacent electrodes are shown in neighbour channels (in the frontal plane that order means aVL, I, -aVR, II, aVF, III, and V1-V6 in the horizontal plane). In this way estimation of electrical axes is facilitated, and misplaced electrodes or erroneous coupling is more easily detected.
   • The axes are estimated from the position of the lead that has the largest net amplitude, or perpendicular to an isoelectric recording.
   • For the recommended paper speed of 50 mm/s, 1 mm represents 20 ms (0.02 s), a 5 by 5 mm square covers 100 ms (0.10 s), and an A4 sheet in landscape position shows 5 seconds.