Med-Surg Cardiovascular System, part 2: Heart Conduction System, Cardiac Function, Blood Pressure
In this article, we’ll explain the heart conduction system, cardiac function, and blood pressure.
The Med-Surg Nursing video series follows along with our Medical-Surgical Nursing Flashcards, which are intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI, and NCLEX.
Heart Conduction System
In order for the heart to squeeze and pump blood, it needs an electrical impulse. The heart conduction system is known as such because it's the process by which the heart conducts electricity. So, the steps in the heart conduction system are the steps of how the heart transmits its electrical impulse. The sinus node of the heart, which is also known as the SA node, initiates an electrical impulse, which stimulates the atria to depolarize and contract.
This impulse travels down to the AV node, where there is a delay to allow the blood in the atria to empty into the heart's ventricles. Then, the electrical impulse travels to the Bundle of His, then the left and right bundle branches, then to the Purkinje fibers, then to the myocardial contractile cells (ventricular myocardium). Depolarization of the myocardial contractile cells triggers ventricular contraction.
Check out our EKG Interpretation Flashcards for more information.
Cardiac function is made up of the cardiac cycle, cardiac output, ventricular ejection fraction, preload and afterload.
The cardiac cycle is made up of ⅔ diastole and ⅓ systole. Diastole is the relaxation and filling of the atria and ventricle. Systole is contraction and emptying of the atria and ventricles.
Cardiac output is the volume of blood in liters ejected from the left ventricle each minute. Cardiac output is determined by heart rate, contractility, preload and afterload. A normal cardiac output is 4 - 8 liters per minute.
Cardiac output is the heart rate times stroke volume.
Heart rate is the number of times the ventricles of the heart contract each minute. A normal heart rate is between 60 and 100 beats per minute.
Stroke volume is the volume of blood, in liters, ejected from the left ventricle with each heartbeat.
Decreased vardiac output can be caused by hypovolemia, MI, vasodilation, and heart failure. Increased cardiac output can by caused by sepsis, anemia, and hyperthyroidism.
Check out our Lab Values Flashcards for more information.
Left Ventricular Ejection Fraction (LVEF)
Left ventricular ejection fraction is the percentage of blood that leaves the left ventricle of the heart each time it contracts. A normal level for LVEF is 55 - 70%. Causes of a low LVEF include congestive heart failure, cardiomyopathy, coronary artery disease, myocardial infarction, and valvular heart disease.
These diseases will be covered later in this series, and if you need to study them for your Med-Surg classes or nursing practice, we recommend our Medical-Surgical Nursing Flashcards!
The preload is the volume of blood in the ventricles of the heart at the end of the diastole (just prior to contraction). The preload determines the amount of stretch placed on myocardial fibers.
The afterload is the peripheral resistance the left ventricle of the heart must overcome to push the blood into the systemic circulation.
Blood pressure is the force of blood exerted against the arterial walls, and it is made up of systolic and diastolic blood pressure.
Systolic blood pressure
Systolic blood pressure is the amount of pressure generated against the arterial walls during left ventricular systole (contraction of the heart).
Diastolic blood pressure
Diastolic blood pressure is the amount of pressure generated against the arterial walls during left ventricular diastole (relaxation of the heart).
Regulation of blood pressure
The autonomic nervous system controls blood pressure via input from sensory receptors like the baroreceptors, chemoreceptors, and stretch receptors, located in blood vessels near the heart.
Hi, I'm Cathy with Level Up RN. In this video, we are going to continue our anatomy and physiology review of the cardiovascular system. If you have our medical-surgical nursing second-edition cards, be sure to pull those out and follow along with me. So in this video, we are going to talk about the heart conduction system, cardiac function, and blood pressure. And at the end of the video, I'm going to give you guys a quiz to test your understanding of some of the key concepts that I'll be covering in this video. So definitely stay tuned for that.
Let's first talk about the steps of the heart conduction system using an illustration from our medical-surgical nursing flashcard deck. So we have our SA note here, our sinus node, which will initiate an electrical impulse which stimulates the atria to depolarize and contract. The impulse then travels to the AV node, where there is a delay to allow the blood in the atria to empty into the ventricles. Then, from the AV node, the impulse travels to the bundle of His, then the left and right bundle branches, and then through the Purkinje fibers. And ultimately, this causes depolarization of the myocardium, which triggers contraction of the ventricles. So understanding this path, the conduction system is going to be important for you to know when we start talking about dysrhythmias, which will definitely be covering later on in this video playlist. Now let's talk about some important terms and concepts that relate to cardiac function. Within the cardiac cycle, we have diastole and systole. So during diastole, this is where the ventricles and the atria are relaxed and they are filling with blood. Then, during systole, the ventricles and atria contract and eject that blood. So within the cardiac cycle, about two-thirds of the time is spent in diastole, and about one-third of the time is spent with systole. Cardiac output is the volume of blood in liters that is ejected from the left ventricle every minute. And you would calculate cardiac output by taking the patient's heart rate and multiplying that times the stroke volume. So the heart rate is the number of times that the ventricles contract within a minute. So for an adult, this usually varies between 60 and 100 beats per minute. The stroke volume is the volume of blood in liters that is ejected from the ventricle with each heartbeat. So you take that heart rate times the stroke volume, and that would give you cardiac output. And cardiac output for an adult usually varies between 4 and 8 liters per minute. Another important term to know is something called left ventricular ejection fraction, or LVEF. This is the percentage of blood that leaves the left ventricle each time it contracts. And in a healthy adult, this will vary between 55 and 70 percent. And when we talk about heart failure in another video in this playlist, you will see that most of these patients will have a reduced LVEF that we would determine through an echocardiogram. Then we have preload, which is another important thing to understand. This is the volume of blood in the ventricles at the end of diastole, so right before contraction. And this determines the amount of stretch that we are placing on those myocardial fibers in the heart. This differs from afterload. So afterload is the peripheral resistance that the left ventricle must overcome in order to push that blood into systemic circulation. So when we are caring for a heart-failure patient, we are going to administer medications often that will reduce preload and/or afterload. So we're trying to reduce the amount of stretch on that heart and the amount of workload that heart needs to do by reducing preload. And we're also trying to reduce afterload to make it easier for the heart to push that blood into systemic circulation.
The last topic I want to cover in this video is blood pressure, which is comprised of systolic blood pressure and diastolic blood pressure. So systolic blood pressure is the amount of pressure that is exerted against those arterial walls during left ventricular contraction, as opposed to diastolic blood pressure, which is the amount of pressure exerted against those arterial walls during left ventricular relaxation, or diastole. Blood pressure in the body is regulated by the autonomic nervous system. So the autonomous nervous system will regulate blood pressure based on input it receives from different receptors. So this includes baroreceptors, chemoreceptors, and stretch receptors as well in the blood vessels near the heart.
All right. Time for a quiz. I have three questions for you. First question. How do you calculate cardiac output? You calculate cardiac output by taking the heart rate times the stroke volume. Question number two. What do you call the volume of blood in the ventricles at the end of diastole? If you said preload, you are correct. Third question. The percentage of blood leaving the left ventricle each time it contracts is called what? If you said left ventricular ejection fraction, you're right. I hope this review has been helpful. If so, be sure to like this video. Leave me a comment. And if you haven't already done so, be sure to subscribe. In my next video, we will talk about some A&P concepts as it relates to the hematologic system. So definitely stay tuned for that.
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