L3 The Heart Axis and Cardiac Flow PDF

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L3 The Heart Axis and Cardiac Flow 3.1 Einthoven’s Triangle and Cardiac Axis Rhythm strip: one “lead” (a set of two electrodes) that, when placed on your body record a voltage shift as the waves of electrical activity sweep over the heart Problem with 1 lead: not much info  If a wave is travelling in the direction of (parallel to) the lead, then it will give a strong signal. 

If a wave is travelling in the direction of (parallel to) the lead, then it will give a strong signal- flat line on the trace

More leads ¿ more information • Einthoven’s triangle With a triangle, any wave will be moving parallel to (within 60 degrees) at least one of the leads, while moving somewhat perpendicular to at least one other lead. The ground electrode is needed so that the machine can establish where “zero” should be. Everything else will be given in reference to that baseline. Direction of the recording 1. A wave of depolarisation travelling TOWARD a POSITIVE lead results in a positive deflection of the ECG trace. 2. A wave of repolarisation travelling TOWARD a POSITIVE lead results in the opposite. 3. Waves travelling away from the positive electrode result in opposite deflections. Different magnitudes on different leads  This means that one wave of APs can give higher or lower recordings depending on which lead you are considering.

When you see a single ECG trace, it is only one of the leads- usually lead II, because it shows the strongest response.

L3 The Heart Axis and Cardiac Flow

A) The trace goes DOWN because the direction of the wave of depolarisation is slightly away from the positive electrode. Notice that the mean (red arrow) of the green arrows (each representing local portions of the wave) points slightly above the perpendicular mid-line between positive and negative. The magnitude of the mean is small because there is simply not a lot of mass (i.e. cells) in the septum that is being depolarised. On the ECG trace, this shows up as a very small deflection (the Q wave). B) The ventricle is being depolarised. While this happens quickly, it does not ALL happen at once. The muscle of the ventricle that is closest to the chamber starts to squeeze first and the AP wave travels down the tissue from there (see the grey area). This creates a huge wave in the positive direction on the ECG. C) The ventricle is still depolarising, but the wave has travelled up the sides and is pointing less directly at the positive electrode. The R wave is now sloping back toward zero, but that it is NOT negative yet! The mean arrow still points to the positive side of the lead. The magnitude of the mean is still quite large. It leaves a small trace on lead II because the component of the mean that points in the positive direction is small. D) Now the wave is travelling up the final portions of the ventricle toward the atria. The mean points away from the positive electrode, so the trace is negative. The S wave is small because there is not a lot of tissue to depolarise. This would be the first portion of the ventricles, not the last, to depolarise if it weren’t for the cardiac skeleton causing: 1) the signal to stop at the boundary between the atria and ventricles 2) the signal to travel down the bundles and up the Purkinje fibres more quickly than it would if it just travelled through cardiomyocytes.

L3 The Heart Axis and Cardiac Flow Heart axis: the mean electrical axis, otherwise known as the heart axis

Clinics use more:  3 auxiliary leads (aVR, aVL, 

and aVF) Also commonly use 6 chest electrodes that cover the front to back direction, giving you 6 leads in the 3rd dimension (V1-V6) (ventral to

 

dorsal) 10 electrodes in a full 12 lead ECG The mean electrical vector (or axis) is often calculated quickly by using Lead I and aVF

Aberrant ECG traces

“A patient is found to have normal QRS but inverted T waves in leads II, III, and aVf. Which of the following is the most likely explanation for these findings?  The direction of ventricular repolarization is reversed from normal.  … because the T wave is normally positive when the last cells that depolarise are the first to repolarise. When the direction of repolarisation is reversed, the T wave becomes inverted. 3.2 Cardiac flow and Wiggers

L3 The Heart Axis and Cardiac Flow The axial reference system

Anatomy of the heart

Wigger’s diagram is a way of visualising several aspects of the heart all at once, which makes it much easier to understand how one aspect affects another.

3.3 regulation of cardiac output

L3 The Heart Axis and Cardiac Flow Echocardiography: the use of ultrasound waves to investigate the action of the heart - high frequency sound waves are pointed at the heart and the reflections are detected Pressure-Volume loops

A) The ventricle is filling, even though there is very little pressure, because the blood is entering from the pulmonary circuit, through the atria, and its weight fills the relaxed ventricle. Then, the left atrium squeezes and we see a final little surge in pressure, but the pressure is still pretty low in the ventricle (the red line). B) the ventricle starts to squeeze. This closes the mitral valve. No more blood can enter, so this is the end diastolic volume (black line). Note that it stays the same as the ventricle starts to squeeze. C) The pressure gets so high that it forces the aortic valve open against the aortic pressure, which is trying to keep it closed. As they exercise, the amount of blood that enters the ventricle during diastole will increase. This means that the EDV will be higher in these conditions.

NOTE: area under curve is total amount of work, NOT amount of blood (that’s SV)

Some things that affect cardiac output Preload: how much blood is in the chamber at the end of diastole

L3 The Heart Axis and Cardiac Flow

Frank-Starling mechanism: If the heart is stretched (e.g. due to preload), it will respond by contracting more, so that end systolic volume is maintained. - ↑ preload leads to = ↑ stroke volume

Afterload: the pressure that the heart has to pump against to push the blood forward

Compliance: the inverse of stiffness, so the less compliant something is, the stiffer it is. Also affect preload

Inotropy: increase the speed of contraction  The heart squeezes faster so that it has a lower end systolic volume.  This is INDEPENDENT of the current 

sarcomere length. Therefore, this is NOT the same as the Frank-Starling mechanism.



Still, it can lead to increased SV...