00:00 / 00:00
Cardiovascular system anatomy and physiology
Lymphatic system anatomy and physiology
Abnormal heart sounds
Normal heart sounds
Changes in pressure-volume loops
Cardiac and vascular function curves
Altering cardiac and vascular function curves
Law of Laplace
Measuring cardiac output (Fick principle)
Stroke volume, ejection fraction, and cardiac output
Physiological changes during exercise
Cardiovascular changes during hemorrhage
Cardiovascular changes during postural change
Cardiac conduction velocity
Electrical conduction in the heart
ECG normal sinus rhythm
ECG QRS transition
ECG rate and rhythm
ECG cardiac infarction and ischemia
ECG cardiac hypertrophy and enlargement
Control of blood flow circulation
Microcirculation and Starling forces
Blood pressure, blood flow, and resistance
Compliance of blood vessels
Laminar flow and Reynolds number
Pressures in the cardiovascular system
Resistance to blood flow
Action potentials in myocytes
Action potentials in pacemaker cells
Cardiac excitation-contraction coupling
Excitability and refractory periods
0 / 10 complete
0 / 1 complete
Parts of an ECG
An electrocardiogram - an ECG - or the Dutch and German version of the word - EKG, is a tool used to visualize the electricity that flows through the heart And the way it looks depends on the set of electrodes you’re using. This particular set of electrodes is called lead II, for example, with one electrode on the right arm and the other on the left leg, so essentially when the wave’s moving toward the left leg electrode, you get a positive deflection, like this big positive deflection corresponding to the wave moving down into the left and right ventricles. To read an ECG there are a few key elements to keep in mind, one of them includes figuring out the axis.
The axis of an ECG is the average direction of electrical movement through the heart during a depolarization. More specifically, axis usually refers to the mean QRS vector, which is the size and direction of the depolarization wave as it moves through the ventricles. Normally, the QRS axis aims downward and to the left in relation to the body.
So, if we simplify this heart a bit, normally, the sinoatrial (or SA) node sends an electrical signal that propagates out through the walls of the heart and contracts both upper chambers, then that signal moves to the atrioventricular or AV node, where the signal is delayed for a split second, and then goes down into the ventricles or lower chambers where it moves down the bundle of His into the left and right bundle branches and into each ventricle’s Purkinje fibers, causing the ventricles to contract as well. So, in a healthy heart, the upper chambers contract first, then shortly after, the lower chambers contract.
On an ECG, the atrial depolarization and contraction is seen as a p-wave, the ventricular depolarization and contraction is seen as a QRS complex, and the ventricular repolarization, and therefore its relaxation, is seen as a T-wave. A general principle to keep in mind is that a depolarization is caused by the movement of positive charge, so if that movement of positive charge is going toward the positive electrode, then it’s captured as a positive deflection on an ECG.
With that in mind, let’s take a closer look at the mean, or average, QRS vector. After the depolarization wave arrives at the AV node, it travels down the interventricular septum and starts depolarizing the ventricles. The Purkinje fibers sit just below the endocardium, which is the innermost layer of the heart. After the endocardium is the myocardium, the cardiac muscle cells, and then finally there’s the epicardium, which is the outer layer. So the Purkinje fibers initiate depolarization vectors that travel directly outward, starting in the endocardium, going through the myocardium, and ending in the epicardium. And because they transmit a depolarization wave so quickly, they all fire off pretty much in unison. Also, the more muscle tissue in the myocardial layer that a vector travels through, the larger the size of the vector. So like with hypertrophic cardiomyopathy, where the heart muscle gets thicker, you end up with bigger vectors. Although, if the heart muscle has been damaged, from something like a heart attack, then you get smaller vectors because the heart cells can’t depolarize anymore. Another thing that can affect vectors is the position of the diaphragm, which is usually sort of sitting right up against the heart. In obese people, the diaphragm gets pushed upwards, which rotates the heart further to the left, and in thin individuals, the diaphragm lowers, which rotates the heart a bit the other way. When everything is taken into consideration, and all of the individual vectors are added up, there is one overall representative vector arrow, which starts from the AV node, and points in one direction through the ventricles. Now, remember that vectors can be broken down into two perpendicular vector components, so if you look at the component that points at the positive lead electrode, that’s what gets recorded on the ECG, and therefore when you plot these vectors over the course of ventricular depolarization, you end up with the QRS complex.
The ECG axis is the direction of the overall electrical activity of the heart. ECG can be used to help identify problems with the heart's rhythm (cardiac arrhythmia) or with the conduction of electricity through the heart. The axis is measured in degrees, from 0 (horizontal) to +90 (vertical) and -90 (inverted).
A normal ECG axis should be anywhere from +30 to -60. An abnormal ECG axis may indicate problems with the heart's electrical activity, such as right or left ventricular hypertrophy, or arrhythmias.
Latest on COVID-19
Nurse Practitioner (NP)
Physician Assistant (PA)
Create custom content
Raise the Line Podcast
Copyright © 2024 Elsevier, its licensors, and contributors. All rights are reserved, including those for text and data mining, AI training, and similar technologies.
Cookies are used by this site.
Terms and Conditions
USMLE® is a joint program of the Federation of State Medical Boards (FSMB) and the National Board of Medical Examiners (NBME). COMLEX-USA® is a registered trademark of The National Board of Osteopathic Medical Examiners, Inc. NCLEX-RN® is a registered trademark of the National Council of State Boards of Nursing, Inc. Test names and other trademarks are the property of the respective trademark holders. None of the trademark holders are endorsed by nor affiliated with Osmosis or this website.