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How can cardiac output be controlled physiologically and pharmacologically?

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  1. Introduction
  2. The length tension relationship in the cardiac muscle
  3. Adrenaline and nor-adrenaline
  4. Cardiac dysrhythmias
  5. Causes of a dysrhythmia
  6. The actions of Class I drugs
  7. The actions of Class II drugs
  8. The actions of Class III drugs
  9. Conclusion

Cardiac output is the amount of blood ejected by a ventricle in a given unit of time, for example the amount of blood ejected by the left ventricle into the aortic arch per minute. The cardiac output can be changed by variations in its controlling factors. Changes in cardiac output are vital to the functioning of the body and as such can be caused to increase or decrease to aid said function. Alternatively illness or disease can cause adverse, unwanted effects on cardiac output. Anxiety and excitement, eating, exercise, high environmental temperatures, and pregnancy will all increase cardiac output. Sitting or standing from a lying position, heart disease, and rapid arrhythmias will all cause cardiac output to decrease.

[...] This causes potassium to leak out of the cells and accumulate in the tight extra cellular space of the myocardium. The high potassium levels leads to depolarization and spontaneous firing of the action potential. This spontaneous firing is also seen in hyperkaleamia. A high sympathetic drive will stimulate the pacemaker cells leading to spontaneous depolarization. Abnormal depolarization circuits, cause abnormal propagation i.e. re-entry of the depolarization, where a wave of depolarization chases itself in a circuit throughout the myocardium causing repeated depolarisations. [...]

[...] These actions are important as adrenaline can cause dysrythmias by its effects on the pacemaker potential and on the slow inward calcium current. This drug class includes atenolol, esmolol, propranolol, and metoprolol. Class III act mainly by blocking potassium channels, and thus prolonging depolarization. These drugs do not affect the sodium channel, and therefore conduction velocity is not decreased. This prolongation of the action potential duration and refractory period, combined with the maintenance of normal conduction velocity, prevent re-entrant dysrythmias, i.e. [...]

[...] Cardiac dysrhythmias are classified by site (atrial, nodal, ventricular) and by type (ectopics, tachycardia, flutter, fibrillation) in order of increasing sensitivity. The rhythms tend top be progressive from flutter to fibrillation. There are both atrial and ventricular dysrhythmias, yet atrial dysrhythmias are much less likely to affect an individual, with the main cause for concern being the formation of a blood clot. In the normal rhythm of the heart, there are two distinct action potentials, those of the pacemaker cells, and those of the ventricles. [...]

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