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Transcript
Electrical Activity in the Heart Heart muscle has several unusual properties, one of which is that it conducts electrical impulses in much the same way as nervous tissue does. This is important for the coordinated and sequential contraction of the cardiac muscle cells (myocytes) during the cardiac cycle. The details of exactly what a nerve impulse consists of isn’t covered until A2 (it’s all about the flow of ions across the cell membrane, changing the potential difference between the inside of the cell and the extracellular fluid), but you can think of a neurone (nerve cell) or a myocytes as being able to alternate between a resting stage (when there is no electrical activity in the cell), a short period when it has been stimulated and a nerve impulse passes along it (the cell is depolarized – when the potential difference across the membrane is changing), and then a brief period when it recovers back to the resting stage (the cell is repolarizing and the potential difference across the membrane is returning to normal). The recovery stage is also known as the refractory period – during that stage it is unable to conduct another nerve impulse until it is back in the resting stage. Electrical activity can be monitored using an electrocardiogram or ECG. A standard ECG trace is shown below and consists of a series of waves. Note that the graph does not show an individual nerve impulse – they are far too small in magnitude to be detected that way – but a summation of the nerve impulses across the heart at a given time. The ECG does not measure the mechanical changes that occur during the cardiac cycle, but since the wave of excitation that spreads across the heart causes the heart to contract there is obviously a correlation between the two. L8th | ECG Interpretation HW (version 2) 1 Midway along the P wave the atria contract. What initiates this? .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. Why is there a gap between the end of the P wave and the start of the QRS complex? Why is this important? .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. What is happening during the QRS complex? .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. Why is the QRS complex of greater magnitude than the P wave? .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. There are in fact two separate electrical events taking place during the QRS complex. What do you think the other is and why is it masked by the greater magnitude of the ventricle depolarization? .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. What does the T wave represent? .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. L8th | ECG Interpretation HW (version 2) 2 Ventricular fibrillation refers to when the muscle fibres of the ventricles fail to contract in a coordinated way. How would you expect the ECG trace to look in a patient with ventricular fibrillation? .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. This is a life-threatening condition and a defibrillator can be used to provide a strong electrical shock that resets the normal heart beat. Why is ventricular fibrillation a lifethreatening condition? Why is atrial fibrillation not immediately life-threatening? .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. .................................................................................................................................................. L8th | ECG Interpretation HW (version 2) 3
