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.
During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle. Again, as you consider this flow and relate it to the conduction pathway, the elegance of the system should become apparent. At the beginning of the cardiac cycle, both the atria and ventricles are relaxed diastole. Blood is flowing into the right atrium from the superior and inferior venae cavae and the coronary sinus.
Blood flows into the left atrium from the four pulmonary veins. The two atrioventricular valves, the tricuspid and mitral valves, are both open, so blood flows unimpeded from the atria and into the ventricles.
Approximately 70—80 percent of ventricular filling occurs by this method. The two semilunar valves, the pulmonary and aortic valves, are closed, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left. Contraction of the atria follows depolarization, represented by the P wave of the ECG.
As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular tricuspid, and mitral or bicuspid valves. At the start of atrial systole, the ventricles are normally filled with approximately 70—80 percent of their capacity due to inflow during diastole. Atrial systole lasts approximately ms and ends prior to ventricular systole, as the atrial muscle returns to diastole.
Ventricular systole see image below follows the depolarization of the ventricles and is represented by the QRS complex in the ECG. It may be conveniently divided into two phases, lasting a total of ms. At the end of atrial systole and just prior to atrial contraction, the ventricles contain approximately mL blood in a resting adult in a standing position. This volume is known as the end diastolic volume EDV or preload.
Initially, as the muscles in the ventricle contract, the pressure of the blood within the chamber rises, but it is not yet high enough to open the semilunar pulmonary and aortic valves and be ejected from the heart. However, blood pressure quickly rises above that of the atria that are now relaxed and in diastole. This increase in pressure causes blood to flow back toward the atria, closing the tricuspid and mitral valves.
Since blood is not being ejected from the ventricles at this early stage, the volume of blood within the chamber remains constant. Consequently, this initial phase of ventricular systole is known as isovolumic contraction , also called isovolumetric contraction see image below. In the second phase of ventricular systole, the ventricular ejection phase , the contraction of the ventricular muscle has raised the pressure within the ventricle to the point that it is greater than the pressures in the pulmonary trunk and the aorta.
Blood is pumped from the heart, pushing open the pulmonary and aortic semilunar valves. Pressure generated by the left ventricle will be appreciably greater than the pressure generated by the right ventricle, since the existing pressure in the aorta will be so much higher.
Nevertheless, both ventricles pump the same amount of blood. This quantity is referred to as stroke volume. Atrial systole is the contraction of the myocardium of the left and right atria. Electrical systole of the atria begins with the onset of the P wave on the ECG. Atrial systole, which normally occurs in the late portion of ventricular diastole , causes increased pressure in the atrium and added blood flow into the ventricles.
This added blood flow is known as atrial kick , and is absent if there is loss of normal electrical conduction in the heart, such as during atrial fibrillation , atrial flutter , and complete heart block. Aortic and pulmonary valves are closed. Mitral and tricuspid valves are open due to the increased pressure in the atria. Ventricular systole is the contraction of the myocardium of the left and right ventricles.
At the beginning of ventricular systole, the pressure in the left ventricle increases. This soon eclipses the pressure in the left atrium, closing the mitral valve. We found that onset of mechanical systole occurred on and usually shortly after the peak of a first dominant QRS complex deflection, and onset of diastole occurred at the earliest on and most commonly beyond the peak or midpoint of the T wave.
A DTV should ideally be able to stop vibrating on or before the peak of the first dominant deflection of a QRS complex, and begin vibrating near the peak of the T wave.
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