A "typical" ECG tracing is shown to the right. The different waves that comprise the ECG represent the sequence of depolarization and repolarization of the atria and ventricles. Therefore, each small 1-mm square represents 0. Because the recording speed is standardized, one can calculate the heart rate from the intervals between different waves. The P wave represents the wave of depolarization that spreads from the SA node throughout the atria, and is usually 0.
The brief isoelectric zero voltage period after the P wave represents the time in which the impulse is traveling within the AV node where the conduction velocity is greatly retarded and the bundle of His.
Atrial rate can be calculated by determining the time interval between P waves. Click here to see how atrial rate is calculated. The period of time from the onset of the P wave to the beginning of the QRS complex is termed the PR interval , which normally ranges from 0. This interval represents the time between the onset of atrial depolarization and the onset of ventricular depolarization. The QRS complex represents ventricular depolarization. Ventricular rate can be calculated by determining the time interval between QRS complexes.
Click here to see how ventricular rate is calculated. Let us now calculate the direction of depolarization of the ventricular muscle.
We have to arrive at a vector such that the projections of this vector onto the three lead axes is consistent with the height of the QRS complexes in the three leads. It is rather defined to be the time from the beginning of the P-wave to the beginning of the QRS complex.
Thus the PR interval is measured from the beginning of the P-wave to the beginning of the R-wave only if the first deflection in the QRS complex happens to be an R-wave i. It is rather defined as the time from the beginning of the QRS complex to the end of the T-wave. It is generated by electrical activity more specifically depolarization or activation of the muscle. Purkinje fibre cells are NOT nerve cells. Rather, they are specialized cardiac muscle cells.
The sinoatrial node, atrioventricular node, bundle of His, and bundle branches are also made up of specialized cardiac muscle cells. The following statements are true:. The following statements are true: One does NOT see any deflection on the ECG during the time that the sinoatrial node is being depolarized. The excitation and subsequent development of tension and shortening of atrial cells cause atrial pressures to rise. Active atrial contraction forces additional volumes of blood into the ventricles often referred to as "atrial kick".
At normal heart rates, the atrial contractions are considered essential for adequate ventricular filling. As heart rates increase, atrial filling becomes increasingly important for ventricular filling because the time interval between contractions for passive filling becomes progressively shorter. Throughout diastole, atrial and ventricular pressures are nearly identical due to the open atrioventricular values which offer little or no resistance to blood flow.
It should also be noted that contraction and movement of blood out of the atrial appendage auricle can be an additional source for increased blood volume. TPR is a measure of resistance in the blood vessels, which acts as the force by which blood must overcome to flow through the arteries determined by the diameter of the blood vessels. The exact relationship is such that a twofold increase in blood vessel diameter doubling the diameter would decrease resistance by fold, and the opposite is true as well.
CO can also predict blood pressure based on blood volume. Essentially, this means that higher venous blood return to the heart also called the preload will increase SV, which will in turn increase CO. This is because sarcomeres are stretched further when EDV increases, allowing the heart to eject more blood and keep the same ESV if no other factors change.
The main implication of this law is that increases in blood volume or blood return to the heart will increase cardiac output, which will lead to an increase in MAP. The opposite scenario is true as well. For example, a dehydrated person will have a low blood volume and lower venous return to the heart, which will decrease cardiac output and blood pressure. Those that stand up quickly after lying down may feel light-headed because their venous return to the heart is momentarily impaired by gravity, temporarily decreasing blood pressure and supply to the brain.
The adjustment for blood pressure is a quick process, while blood volume is slowly altered. Blood volume itself is another regulated variable, regulated slowly through complex processes in the renal system that alter blood pressure based on the Starling mechanism. Privacy Policy.
Skip to main content. Cardiovascular System: The Heart. Search for:. Physiology of the Heart. Electrical Events Cardiac contraction is initiated in the excitable cells of the sinoatrial SA node by both spontaneous depolarization and sympathetic activity. Learning Objectives Describe the electrical events of the heart.
Key Takeaways Key Points The sinoatrial SA and atrioventricular AV nodes make up the intrinsic conduction system of the heart by setting the rate at which the heart beats. The SA node generates action potentials spontaneously. The SA node fires at a normal rate of 60— beats per minute bpm , and causes depolarization in atrial muscle tissue and subsequent atrial contraction. The AV node slows the impulses from the SA node, firing at a normal rate of bpm, and causes depolarization of the ventricular muscle tissue and ventricular contraction.
Sympathetic nervous stimulation increases the heart rate, while parasympathetic nervous stimulation decreases the heart rate. Key Terms pacemaker : A structure that sets the rate at which the heart beats. Under normal conditions, the SA node serves this function for the heart. Learning Objectives Describe electrocardiograms and their correlation with systole. Key Takeaways Key Points An ECG is used to measure the rate and regularity of heartbeats as well as the size and position of the chambers, the presence of damage to the heart, and the effects of drugs or devices used to regulate the heart, such as a pacemaker.
The ECG device detects and amplifies the tiny electrical changes on the skin that are caused when the heart muscle depolarizes during each heartbeat, and then translates the electrical pulses of the heart into a graphic representation. A typical ECG tracing of the cardiac cycle heartbeat consists of a P wave atrial depolarization , a QRS complex ventricular depolarization , and a T wave ventricular repolarization.
An additional wave, the U wave Purkinje repolarization , is often visible, but not always. The ST complex is usually elevated during a myocardial infarction. Atrial fibrillation occurs when the P wave is missing and represents irregular, rapid, and inefficient atrial contraction, but is generally not fatal on its own. Ventricular fibrillation occurs when all normal waves of an ECG are missing, represents rapid and irregular heartbeats, and will quickly cause sudden cardiac death.
Key Terms fibrillation : A condition in which parts of the ECG do not appear normally, representing irregular, rapid, disorganized, and inefficient contractions of the atria or ventricles. ST segment : The line between the QRS complex and the T wave, representing the time when the ventricles are depolarized before repolarization begins.
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