ECGs and the Mean Cardiac Vector

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From the 23/09/13 Human Physiology prac.

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What is the physiological process on which ECGs are based? Excitation-contraction coupling in the myocardium.
Where are action potentials generated in the myocardium? In the sinoatrial node, where the cells have pacemaker activity.
The myocardium contracts as a single unit (a functional syncytium). How is this possible? Electrical impulses can travel rapidly between cells via intercalated discs/gap junctions, which are unique to heart muscle.
What is the significance of the myocardium contracting as a functional syncytium for an ECG? Individual muscle cell electrical activity is so tiny that it would not be possible to measure it. However, that excitation-contraction coupling occurs in a co-ordinated fashion across an entire tissue means that fairly large electrical changes occur across the chest cavity, which can then be recorded as voltages with suitably placed electrodes.
What does an ECG allow us to assess? Examining the electrical activity of the heart can tell us a lot about its mechanical effectiveness. An ECG shows how fast your heart is beating (e.g. may show brachycardia in hypothermia), whether the rhythm of your heartbeat is steady or irregular (this could suggest that a part of the heart is not conducting properly, or that the sinoatrial node is out of rhythm), and the strength and timing of electrical signals as they pass through the heart (this could help locate areas of decreased function, e.g. in a myocardial infarction).
What does the P wave in an ECG correspond to? Depolarising activity in the atria (atrial systole).
What does the QRS peak in an ECG correspond to? Depolarising activity in the ventricles (ventricular systole).
What does the T wave in an ECG correspond to? Repolarising activity of the ventricles (ventricular diastole).
Why is the QRS peak bigger than the P wave in an ECG? Because the mass of ventricular tissue is greater than that of atrial tissue, so it has greater electrical activity.
What is Einthoven's triangle? Einthoven's triangle is produced by the positioning of electrodes on the body in order to record the electrical activity of the myocardium. By having a positive, negative and neutral 'terminal', one can track electrical impulses as they move across the heart (as when an electrical impulse moves towards an electrode, the trace will deflect upwards from the baseline, and vice versa). The heights of these deflections (from the ECG trace produced) can be plotted on Einthoven's triangle, a mathematical triangle which allows one to calculate to the size and angle of the vector. Both of these can tell us about the health or pathology of the heart.
What is the mean cardiac vector? The overall direction and magnitude of the heart’s electrical activity during systole. Anything that has direction and magnitude is a vector.
How can we tell which angle of the heart we are 'looking' at from an ECG? Given that we know where the electrical activity originates in the myocardium (the sinoatrial node), and we know which electrodes and positive and negative and where they are positioned, we can tell from the deflections whether the electrical impulse is travelling towards or away from an electrode, and which one. Thus, we can tell if we are 'looking' at the anterior or posterior by whether the deflections are positive and negative, and their relative sizes.
When plotting data in Einthoven's triangle, what number of mm of a deflection on an ECG trace is equivalent to 1 mV? A 10 mm deflection.
To work out the mean cardiac vector, we plotted perpendicular lines on to Einthoven's triangle at the appropriate mV for each pair of electrodes. How was the angle of the vector calculated? By measuring the angle between the centre of the crossover/triangle produced by the perpendicular measurement lines, and the zero degrees line present in Einthoven's triangle.
How was the size of the mean cardiac vector established using Einthoven's triangle? The size was the distance between the centre of Einthoven's triangle, and the centre of the triangle/crossover of the perpendicular measurement lines we had drawn on (in mm).
What is the normal range of a vector angle in a healthy person? -30° to +90°
Using an unpaired T-test, it was established from the class data that there was no significant difference between the size and angle of the mean cardiac vector in males and females. Suggest why. It would be expected that the mean cardiac vector and angle would be increased in males as their blood pressure (and therefore, likely, their ventricular systole) is higher than in females. This would result in a larger amount of electrical activity and, of course, contraction. The diagram of Einthoven's triangle was not particularly accurate, the experimental conditions for gathering the ECG were quite casual, and the ECG device proved temperamental, so the class data may not be accurate enough to draw this conclusion from.
Using a paired T-test, there was no difference between normal and breath-holding vector sizes and angles. Why would one expect there to be a difference in the angle? During inhalation, the heart deviates to the size as a result of increased intrathoracic space. This should result in a change in mean cardiac vector angle, making the angle different when holding one's breath. The subject's breath may not have been held for long enough to establish a difference before recording the ECG, or the diagram of Einthoven's triangle may not have been accurate enough to work out a subtle difference.
An axis (of the vector angle) of between -30° and -90° is referred to as what? What might cause this? A left axis deviation. This can be caused by ventricular tachycardia, inferior myocardial infarction, left ventricular hypertrophy, or left anterior hemiblock.
An axis (mean vector angle) of between +120° and +180° is referred to as what? What might cause this? A right axis deviation. This can be caused by right ventricular hypertrophy (e.g. in fibrosis of lung tissue as more blood has to be pumped to be oxygenated and it is against more resistance), anterolateral myocardial infarction, or left posterior hemiblock.
What is an axis (mean cardiac vector angle) between -90° and +180° referred to as? What might cause this? An extreme axis deviation. This can be caused by ventricular tachycardia, emphysema, or a pacemaker.
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