HOM M7 Electrocardiogram PDF

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HOM M7 Electrocardiogram Report: The Human Electrocardiogram The resting ECG (lead I recording)

1.

Above is your resting ECG trace, as recorded on page 5 of the practical. Make sure that you can identify the P wave, QRS complex and T wave from this trace! Which was larger in amplitude in your trace, the QRS complex or the T wave? Which had the longer duration? Were your results consistent with the results of the groups around you? Answer The waves varied in such a way that sometimes the T wave amplitude was higher, whereas with other times the QRS complex was higher. The T wave had the longer duration compared to the QRS complex. These results were not quite consistent with the groups around us because the other groups tended to have more pronounced QRS complexes, and much smaller T waves. However, we are aware that there is always variation in individuals, so there are unlikely to be any pathological implications.

2.

If your ECG trace looks different from the example shown, can you suggest any (non-clinical!) reasons why this might be? Answer

There could be an error in our placing of electrodes, as we may not have placed them in optimal positions on the wrists. Also, different levels of exercise, diet and a difference in genetics (slight variances in heart structure) could influence the slight difference in ECG as, between individuals, every heart is different. 3.

A ventricular action potential recorded intracellularly features a sharp depolarization spike, then a plateau, then a repolarization. To which ECG events do the spike, plateau and repolarization roughly correspond? Answer

The spike event roughly corresponds to the QRS complex of the ECG. The plateau event roughly corresponds to the latent period between the QRS complex and the T wave. The repolarization event roughly corresponds to the T wave. 4.

What was your PR interval (beginning of P to beginning of QRS)? What events contribute to this? Answer 0.14 seconds. The delay of the electrical impulse in the heart as it travels from the SAN to the AVN, where the signal passes slower and can’t escape due to the insulating layer between the atria and ventricles, allowing the ventricles to fill with blood before they are depolarized in the QRS complex.

5.

What was your QRS duration (beginning to end of QRS complex)? Answer

0.11 seconds.

6.

What was your QT interval (start of QRS complex to end of T wave)? Answer 0.38 seconds.

The effects of exercise At rest

Immediately after exercise

2 mins after exercise

4 mins after exercise

7.

At very high heart rates, a further increase in rate may not translate into an increased cardiac output. Why not? Answer

At very increased outputs, venous return is likely to be limiting, and since the heart is unable to pump out nonexistent blood, the cardiac output would remain the same, while stroke volume drops or reaches a steady rate. Make sure that you know how "manually" to calculate the heart rate, in either beats per minute or Hertz, from a trace like the one above.

Compression of the cardiac cycle As heart rate increases, the cardiac cycle must shorten. But do systole and diastole both shorten in proportion, or does one shorten more than the other?

8.

As heart rate increases, what happens to the durations of systole and diastole? If they shorten, which shortens more? Answer

In the case of this heart, durations of diastole definitely shorten and in systole you could argue that they shorten if you ignore the anomalous point at the beginning. As a result, the graphs also show that diastole has shortened more.

Comparing leads I and III

9.

Both leads are measuring the electrical activity of your heart at the same time, so why do the recordings look different? Answer in general terms rather than looking in detail at each part of the ECG trace. Answer

They look different as the two leads are situated in different areas of the body, so they are recording the same trace from different angles. Lead I records the horizontal component as its between the two wrists. Whereas, lead III records the vertical component as it connects the left ankle and wrist.

Ventilation and the ECG

10. What happens to the heart rate when you breathe in, as opposed to when you breathe out? Answer

There is some evidence that the heart rate increases a little upon breathing in compared to breathing out. 11. These effects on heart rate, known as "respiratory sinus arrhythmia", occur due to a signal passing from the respiratory centre in the medulla oblongata to the cardiovascular centre during deep breathing. How do you think the cardiovascular centre, in turn, communicates with the heart? Answer

The cardiovascular centre would stimulate/ reduce the activity of the vagus nerve, initiating the change in heart rate. 12. What changes in AMPLITUDE of the ECG traces do you observe, coming from each lead? Answer

Upon breathing in, the amplitude of Lead I decreases and increases upon exhalation. Whereas, Lead III’s amplitude increases upon breathing in and decreases upon exhalation. 13. What do you think might be causing these amplitude changes? Answer

This could be due to the position of the heart in the different phases of inhalation and exhalation. Upon inhalation, the heart becomes more vertical and less horizontal. So, this means that Lead III, which measures the vertical component, would increase in amplitude, as the electrical impulses would occur down a more favorable position. Whereas, Lead I would decrease in amplitude as the horizontal component is less favoured by the position of the heart. When it comes to exhalation, the idea the same but in reverse.

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