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Cardiovascular system anatomy and physiology
Lymphatic system anatomy and physiology
Blood pressure, blood flow, and resistance
Pressures in the cardiovascular system
Laminar flow and Reynolds number
Resistance to blood flow
Compliance of blood vessels
Control of blood flow circulation
Microcirculation and Starling forces
Measuring cardiac output (Fick principle)
Stroke volume, ejection fraction, and cardiac output
Law of Laplace
Cardiac and vascular function curves
Altering cardiac and vascular function curves
Changes in pressure-volume loops
Physiological changes during exercise
Cardiovascular changes during hemorrhage
Cardiovascular changes during postural change
Normal heart sounds
Abnormal heart sounds
Action potentials in myocytes
Action potentials in pacemaker cells
Excitability and refractory periods
Cardiac excitation-contraction coupling
Electrical conduction in the heart
Cardiac conduction velocity
ECG normal sinus rhythm
ECG QRS transition
ECG rate and rhythm
ECG cardiac infarction and ischemia
ECG cardiac hypertrophy and enlargement
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Robyn Hughes, MScBMC
Tanner Marshall, MS
Marisa PedronRishi Desai, MD, MPH
Tanner Marshall, MS
An electrocardiogram is also known as an ECG; the Dutch and German version of the word, elektrokardiogram, is shortened to EKG. It is a tool used to visualize, or “gram,” the electricity, or “electro,” that flows through the heart, or “cardio.” Specifically, an ECG tracing shows how the depolarization wave, which is a wave of positive charge, moves during each heartbeat by providing the perspectives of different sets of electrodes. This particular set of electrodes is called lead II; one electrode is placed on the right arm and the other on the left leg. Essentially, when the wave’s moving toward the left leg electrode, you get a positive deflection. This big, positive deflection corresponds to the wave moving down the septum. When reading an ECG, there are a few key elements to keep in mind; one of them is looking at the intervals.
In a typical waveform, there’s a p-wave, QRS complex, and t-wave. In addition, there are certain intervals, including the PR interval, the QRS complex itself, and the QT interval.
The PR interval spans from the beginning of the p-wave to the beginning of the QRS complex, and it represents the time from the beginning of atrial depolarization to the beginning of ventricular depolarization. It’s normally 0.12-0.20 seconds, which is three to five little boxes, since each little box is 0.04 seconds. Therefore, the PR interval shown is about four boxes or 0.16 seconds.
Any deviation in the normal depolarization pathway from the SA node to the ventricles can change the PR interval. For example, consider if the atria are depolarized by an ectopic atrial focus, such as an irritable atrial cell outside of the SA node. If it was farther away from the AV node, it’d result in a longer PR interval, because the signal has to travel a greater distance. Alternatively, if it was really close to the AV node, the PR interval might be super short. Another example is first degree heart block, which is when the electrical signal travels more slowly through the AV node than it normally does, causing the PR interval to lengthen beyond 0.2 seconds.
The QRS complex represents depolarization of the ventricles; it’s normally less than 100 milliseconds or two and a half little boxes. Just like the PR interval, the QRS duration can differ in its path from the av node to the ventricles. For example, if an ectopic ventricular focus, such as an irritated ventricular cell, fires off, the resulting depolarization wave will move slowly from muscle cell to muscle cell, instead of traveling quickly through the electrical conduction system. Therefore, it takes a longer time to depolarize the ventricles, and the QRS is wider. It’s considered intermediate if it’s 100 to 120 milliseconds, and prolonged if it’s over 120 milliseconds, or three little boxes.
ECG intervals are major elements to look at when reading an ECG strip. These include the PR interval, the QRS complex itself, the QT, and the RR intervals. The PR interval is the time from the beginning of the P wave to the beginning of the QRS complex. It represents the time between the beginning of atrial depolarization and the beginning of ventricular depolarization, which normally lasts about 0.12-0.20 seconds. The QRS complex represents ventricular depolarization and typically lasts less than 100 milliseconds. Next, the QT interval is measured from the beginning of the QRS complex to the end of the T wave. It represents the ventricular systole and typically lasts less than half of the individual's cardiac cycle. Finally, the RR interval is measured from one R wave peak to the next. It helps to calculate the heart rate and tell if it's regular.
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