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Electrocardiogram The electrocardiogram is a recording of the voltage generated by the heart from the surface of the body during each heartbeat. The P wave is caused by spread of depolarization across the atria, which causes atrial contraction. The QRS waves appear as a result of ventricular...

Electrocardiogram The electrocardiogram is a recording of the voltage generated by the heart from the surface of the body during each heartbeat. The P wave is caused by spread of depolarization across the atria, which causes atrial contraction. The QRS waves appear as a result of ventricular depolarization about 0.16 second after the onset of the P wave, initiating ventricular contraction. The ventricular T wave is caused by repolarization of the ventricle. The Atria Function as Primer Pumps for the Ventricles About 80 percent of ventricular filling occurs during diastole before contraction of the atria. contraction of the atria causes the remaining 20 percent of ventricular filling. contraction of the atria increase the ventricular pumping effectiveness as much as 20 percent When the atria fail to function properly, little difficulty is encountered unless a person exercises, and then shortness of breath and other symptoms of heart failure may occur. The Ventricles Fill With Blood During Diastole The following events occur just before and during diastole: During systole, the A-V valves are closed, and the atria fill with blood. At the beginning of diastole is the period of isovolumic relaxation, caused by ventricular relaxation. When ventricular pressure decreases below that of the atria, the A-V valves open. During diastole the higher pressure in the atria pushes blood into the ventricles. The period of rapid filling of the ventricles occurs during the first third of diastole and provides most of the ventricular filling. At the middle third of diastole blood flow to the ventricles continuouslyl. Atrial contraction occurs during the last third of diastole and contributes about 20 percent of the filling of the ventricle. This contraction is commonly known as the “atrial kick.” Outflow of Blood From the Ventricles Occurs During Systole. The following events occur during systole: At the beginning of systole, ventricular contraction occurs, the A-V valves close, and pressure begins to build up in the ventricle. No outflow of blood occurs during this period of ventricular contraction (the period of isovolumic or Isometric contraction). isovolumic means “the same volume” and refers to the ventricular volume. Isometric means increasing in cardiac muscle tension but no shortening of the muscle fibers. When the left ventricular pressure exceeds the aortic pressure of about 80 mm Hg and the right ventricular pressure exceeds the pulmonary artery pressure of 8 mm Hg, the aortic and pulmonary valves open and Ventricular outflow occurs, called the period of ejection. 70% of ejection occurs during the first third of this period (period of rapid ejection). 30% of ejection occurs during the later two third of this period (the period of slow ejection) At the end of diastole, the volume of each ventricle is 110 to 120 milliliters; this volume is called the end-diastolic volume. The stroke volume, which has a value of about 70 milliliters, is the amount of blood ejected with each beat. The end-systolic volume is the remaining volume in the ventricle at the end of systole and measures about 40 to 50 milliliters. The ejection fraction is calculated by dividing the stroke volume by the end-diastolic volume; it has a value of about 60 percent. The stroke volume of the heart can be doubled by increasing the end- diastolic volume and decreasing the end-systolic volume.