SA Medical - General Medicine And Imaging of Disease

Remember the waves precede the contraction. The PR interval has a normal value of 0.16 seconds and is the delay between the atrial contraction and the beginning of the ventricular contraction. The QT interval has a normal value of 0.35 seconds - the time of ventricular contraction. The heart rate can be determined by the reciprocal of the time interval between each heart beat. During depolarisation process, the average electrical current flows from the base of the heart toward the apex. ECG leads - there are three bipolar limb leads. Lead I: negative lead is connected to the Right arm and the positive terminal is connected to the left arm. Lead II: the negative terminal of the ECG is connected to the right arm and the positive terminal is connected to the left leg. Lead III: the negative terminal is connected to the left arm and the positive terminal is connected to the left leg. Einthoven's Law states that the electrical potential of any limb lead equals the sum of the

The atria contract 1/6th of a second before the ventricles. Specialised excitatory and conductive system of the Heart. SA node - Sinus node or sinoatrial node, initiates the cardiac impulse Internodal pathway - SA to AV node AV node - delays impulse from the atria to the ventricles AV bundle - conducts from AV node to the ventricles Purkinje Fibers - conducts impulses to all parts of the ventricles. The Sinus Node controls the heart rate: The membrane potential of sinus node is -55 to -60 mV The heart rate is caused by: The fast sodium channels are inactivated at the normal resting potential, but there is slow leakage of sodium in the fiber. Between action potentials the resting potential gradually increases because of slow leakage sodium until the potential reaches -40mV At this time, the calcium-sodium channels become activated, allowing rapid entry of sodium and calcium and thus causing the action potential.  Greatly increased numbers of potassium channels open, allow