Ch 13 Part 2 - The Cardiac Cycle.docx

The Cardiac Cycle The cardiac cycle is defined as all events associated with the flow of blood through the heart during a single, complete heartbeat. This occurs in two phases: systole and diastole,which refer to ventricular contraction and relaxation, respectively. Dr. Nguyen highlighted that the e...

The Cardiac Cycle The cardiac cycle is defined as all events associated with the flow of blood through the heart during a single, complete heartbeat. This occurs in two phases: systole and diastole,which refer to ventricular contraction and relaxation, respectively. Dr. Nguyen highlighted that the events in the cardiac cycle are always the same for both sides of the heart at the same time. Systole = contraction Diastole = relaxation The basic events in the cardiac cycle are as follows: 1. Ventricular filling and atrial contraction - When describing the cardiac cycle, we generally start at mid to late diastole, at which point the ventricles are filling with blood. During this point in the cardiac cycle, the AV valves are open (because blood is flowing into the ventricles) and the semilunar valves are closed. At the end of this stage, the atria contract to "squish out" the lost bit of blood into the ventricles. 2. Isovolumetric contraction - At this point, we are at the isovolumetric contraction in the early systole portion of the cardiac cycle. Here, the contraction of the ventricles has not yet changed the volume of these chambers because the pressure is not high enough to open the semilunar and aortic valves. 3. Ventricular ejection - Next. the ventricles eject their contents in the systole portion of the cardiac cycle. At this point, the AV valves are closed (to prevent backflow into the atria) and the semilunar valves leading to the aorta and the pulmonary circuit are open. 4. Isovolumetric relaxation - At the onset of early diastole, the ventricles relax after they have emptied their contents, and the pressure in the aorta is greater than that of the ventricles. This is referred to as the stage of isovolumetric relaxation, which occurs when the semilunar valves close and the volume of blood in the ventricles is not changing. 5. Ventricular filling - The ventricles re-fill, and the process continues. Note that if the atria were to stop contracting, blood could still fall down into the ventricles with the help of gravity. However, if the ventricles were to stop contracting, blood flow would stop and you would die. Atrial Pressure, Ventricular Pressure, and Aortic Pressure The following diagram shows how atrial pressure, ventricular pressure, and aortic pressure vary throughout the cardiac cycle: Note that the pressures listed on the left side of this graph example are relative to atmospheric pressure (760 mm Hg). So, for example, a measure of 40 mm Hg on the graph would really be an absolute pressure of 800 mm Hg. A brief description of these patterns is as follows: Atrial pressure - The changes in atrial pressure throughout the cardiac cycle are modest compared to the changes in ventricular pressure. We see a small increase in atrial pressure at first, followed by a slight decrease in atrial pressure during systole. Ventricular pressure - There are significant fluctuations in ventricular pressure throughout the cardiac cycle. Beginning in mid- to late-diastole, we see an increase in ventricular pressure as the ventricles begin to fill. The closure of the AV valves marks the beginning of ventricular contraction, which significantly increases ventricular pressure during isovolumetric contraction. Around 70-85mm Hg, the semilunar valves open. The ventricular pressure peaks at the point where the heart has emptied its blood, and the pressure begins to fall off. There is a significant decrease in ventricular pressure during isovolumetric relaxation. At this point, the heart goes back into diastole, the pressure falls to baseline, and the process repeats. Aortic pressure - During mid- to late-diastole, the aortic pressure drops slightly because there is no blood flowing through the aorta. The aortic valve opens when the ventricular pressure is greater than the aortic pressure (around 70-85 mm Hg). We then see a drastic increase in aortic pressure as the blood moves from the left ventricle into the aorta. Once the aortic valve is shut, we see a specialized feature of aortic pressure called a dicrotic notch-a transient increase in aortic pressure before it slowly decreases. The aortic pressure is also defined in different terms to help quantify the pressure changes in the heart: Systolic blood pressure (SP) is the maximum pressure in the aorta during the cardiac cycle. Diastolic blood pressure (DP) is the lowest blood pressure in the aorta during the cardiac cycle. Mean arterial pressure (MAP), also known as organ perfusion pressure, is the weighted overage pressure occurring in the aorta during a cardiac cycle. Because it is a weighted average, we give more weight to the diastolic pressure than to the systolic pressure because the heart spends more time in diastole than in systole. More time is spent relaxing than contracting! (A longer contracting period would vastly increase the heart's energy demands.) Ventricular Volumes The difference in end-diastolic volume (EDV) and end-systolic volume (ESV) represents the volume of blood ejected from the heart during one cardiac cycle. This is referred to as the stroke volume (SV): Stroke Volume (SV) = End-Diastolic Volume (EDV) - End-Systolic Volume (ESV) Normal values for these measurements would be about 130ml for EDV and 60ml for ESV. Therefore, a normal stroke volume is about 70 ml. EXAM TIP: Know these formulas for the exam: Stroke Volume (SV) = End-Diastolic Volume (EDV) - End-Systolic Volume (ESV) Ejection Fraction (EF) = Stroke Volume/End-Diastolic Volume Another way of expressing ventricular volumes is through ejection fraction (EF), which is a measure of the percentage of the EDV ejected from the heart during one cardiac cycle: Ejection Fraction (EF) = stroke Volume/End-Diastolic Volume So, for example, if a normal stroke volume is 70ml and a normal end- diastolic volume is 130 ml, a normal ejection fraction is 70/130 = 0.54, or 54%. This means that only 54% of what you start with in the ventricles at the end of diastole is ejected from the heart. The ejection fraction is commonly used to diagnose heart failure. The ventricular volume increases during ventricular filling and at the start of atrial contraction. All of the valves are closed during isovolumetric contraction, so the ventricular volume remains stable. When the aortic and pulmonary valves open, blood leaves the ventricles, so the ventricular volume drops precipitously. Eventually, the ventricles re-fill and the ventricular volume increases again. Note, also, that the stroke volume is the vertical difference between the highest ventricular volume (at the end of diastole) and the lowest ventricular volume (at the end of systole). Cardiac Output and Factors that Affect it The rate at which a single ventricle pumps blood is referred to as cardiac output (CO). (You might sometimes see cardiac output represented as Q.) It is expressed in liters per minute and is determined by the heart rate and stroke volume. The formula for cardiac output is as follows: Cardiac Output (CO) = Heart Rate (HR) x Stroke Volume (SV) An average, healthy, adult has a heart rate of 72 beats per minute and a stroke volume of about 0.07 liters per beat. Therefore, this person's cardiac output would be calculated as Follows: CO= HR X SV CO = (72 beats per minute) x (0.07 liters per beat) = so liters per minute This means that your left and right ventricles are pumping out an average of five liters of blood every minute. Dr. Nguyen pointed out that stroke volume is most often represented in milliliters, while cardiac output is most often represented in liters. Be sure to make the appropriate conversion when using one to calculate the other. Cardiac output is the same for both the right and left sides of the heart, even though they are involved in different circuits, because blood flows between the circuits in series. If it were not, there would be an unsustainable imbalance in blood flow. Even as little as 1% mismatch could have a profound negative impact on the body by causing a backup in either the pulmonary or systemic circuit. For example, suppose your right ventricle started to pump 1% more blood than your left ventricle. If your cardiac output was 5L per minute, this means that 50 ml of blood would be transferred from your systemic circulation to your pulmonary circulation every minute. If this went on for 20 minutes, a full liter of blood would have been transferred. This would increase the pressures throughout your pulmonary circulation, and you would eventually die because your lungs would "drown" from the inside due to pulmonary edema.