Hemodynamics

Hemodynamics is the study of blood flow and how the heart pumps blood throughout the body. Hemodynamics is an intricate field of study in medicine that involves elements of biology, chemistry, and physics. In terms of cardiovascular physiology, hemodynamics is concerned with the forces the heart must develop and maintain to properly circulate blood through the cardiovascular system. These forces include blood pressure and blood flow. Any change or interruption in hemodynamic forces can lead to serious health consequences, including diseases and disorders such as hypertension or congestive heart failure. As a result, testing for hemodynamic instability is often a critical tool in the diagnosis of cardiac ailments and other medical conditions. Various invasive and noninvasive methods of assessment can be used to determine whether a patient is hemodynamically unstable. In the event that a patient is hemodynamically unstable, mechanical or pharmacological support may be required to normalize cardiac output.

Background

Hemodynamics is a key element of the circulatory system. The circulatory system is responsible for transporting blood, oxygen, nutrients, and other materials throughout the body. It is a critical organ system that consists of the heart, lungs, arteries, veins, and other blood vessels. Together, these organs create a loop through which blood is pumped between the heart and lungs and throughout the entire body.

The heart is the circulatory system’s central component. Its pumping action provides the hemodynamic pressure necessary to ensure the continuous flow of blood throughout the body. In addition to pumping deoxygenated blood to and collecting oxygenated blood back from the lungs, the heart is also responsible for pumping that oxygenated blood through the rest of the body. It accomplishes this task with the help of a complex network of blood vessels that includes arteries, veins, and capillaries. The arteries are large blood vessels that transport pressurized blood from the heart to other parts of the body. Veins are low-pressure blood vessels that transport deoxygenated blood back to the heart. Capillaries are the smallest blood vessels in the circulatory system. They are responsible for delivering blood, oxygen, and nutrients to specific parts of the body and collecting carbon dioxide and other waste products that need to be removed. Once this occurs, the process reverses and deoxygenated blood re-enters the circulatory system through the capillaries and ultimately travels back to the heart.

This entire circulatory process is dependent on hemodynamic forces, or the arterial and venous forces that allow for the transportation of blood through the blood vessels. These forces include blood pressure and blood flow. Adequate blood flow is required to ensure that the body’s tissue and organs receive enough oxygen to function normally. As a result, it is imperative that hemodynamic forces are properly maintained at all times. Under normal circumstances, hemodynamic forces keep blood flowing at an even pace. When extra blood flow is required during exercise or some other form of physical exertion, blood flow can be increased as needed to supply tissue with enough blood and oxygen to allow the body to function as well as possible.

Overview

The hemodynamic forces that power the circulatory system make up what is known as the hemodynamic system. The hemodynamic system is composed of heart rate, stroke volume, cardiac output, systemic vascular resistance, and blood pressure. Heart rate, which is also known as one’s pulse, refers to the rate at which a person’s heart beats. It is measured in the number of times one’s heart beats per minute. Stroke volume is the amount of blood a ventricle pumps when it contracts. The left and right ventricles are the two main chambers of the heart. Cardiac output is a measure of how efficiently the heart pumps blood throughout the body. Cardiac output is calculated based on heart rate and stroke volume. To allow for normal function, cardiac output has to be high enough for the body to be able to distribute blood as circumstances require. Cardiac output is also directly tied to systemic vascular resistance and blood pressure. The flow of blood through the body is slowed by resistance generated by the blood vessels. This is called systemic vascular resistance. The force exerted against blood vessel walls by flowing blood is known as blood pressure. All of these factors affect how blood flows throughout the body and help determine how hard the heart has to work to make sure that enough blood reaches the body’s tissues at all times.

Problems with the hemodynamic forces can lead to hemodynamic instability. Hemodynamic instability can also be referred to as circulatory collapse, shock, hypoperfusion, or cardiovascular failure. In any event, hemodynamic instability results when there is an insufficient amount of pressure in the circulatory system to keep blood flowing through the body normally. Hemodynamic instability occurs in connection with another serious medical event, such as a heart attack or hemorrhagic shock. It is not a condition that develops independently. When a patient does experience hemodynamic instability, there are a variety of possible signs and symptoms that can be used to make a positive diagnosis. These include pale and cool skin, sweating, fatigue, abnormally elevated or slowed heart rate, low blood pressure, shortness of breath, chest pain, confusion, and loss of consciousness. Some of the signs of hemodynamic instability are a direct result of the body’s attempts to resolve the situation on its own. When hemodynamic instability begins to occur, the body’s fight-or-flight response is triggered and the sympathetic nervous system starts working to increase blood flow. This results in a stress response that leads to symptoms like elevated heart rate, sweating, and pale and cool skin.

In a patient experiencing a heart attack or some other traumatic cardiac event, hemodynamic instability is a key indicator of the need for immediate medical intervention. A patient exhibiting an abnormally high or low heart rate should be treated more aggressively when he or she is also experiencing hemodynamic instability. Although it can often be identified simply through clinical assessment, there are a number of invasive and noninvasive methods for diagnosing hemodynamic instability. One of these is a hemodynamic test, which is a nuclear imaging procedure that allows doctors to evaluate heart function and circulation. In terms of invasive procedures, hemodynamic instability can be diagnosed by measuring central venous pressure (CVP), pulmonary artery pressure (PAP), or mixed venous oxygen saturation. Measuring CVP means measuring the blood pressure in the vena cava or right atrium. Accomplished through the placement of a cardiac catheter in the vena cava, which is a large vein that carries deoxygenated blood into the heart, CVP measurement yields important information about right ventricular function and venous return to the right side of the heart. A PAP measurement allows for analysis of blood pressure in the pulmonary artery. To take a PAP measurement, a physician simply catheterizes the pulmonary artery. A mixed venous oxygen saturation test, which can be drawn from either a pulmonary artery catheter or a central venous catheter, illustrates the balance between oxygen delivery and oxygen consumption. Noninvasive methods of diagnosing hemodynamic instability include measuring arterial blood pressure or systemic mean arterial pressure (MAP), as well as transthoracic echocardiogram (TTE). Arterial blood pressure can be measured with a simple blood pressure cuff. MAP is the average pressure in a patient’s arteries during a single cardiac cycle. TTE provides physicians with a helpful visualization of the cardiac chambers, valves, and pericardium. This allows for a clear picture of a patient’s overall cardiac function. Hemodynamic evaluation can also be used to diagnose circulatory issues such as hemodynamic instability. Hemodynamic evaluation is a screening method through which doctors can gather information about a patient’s blood volume, systemic vascular resistance, and the presence of inotropes, which are hormone-like substances that circulate in the blood and tell the heart when it is necessary to increase or decrease the heart rate.

A number of conditions can arise as a result of hemodynamic instability. Two of the most common include hypotension and syncope. Hypotension is another term for low blood pressure. While low blood pressure is not always associated with hemodynamic instability, more severe cases, such as those that occur in the event of heart failure, often are. If low blood pressure resulting from hemodynamic instability is not properly treated, permanent tissue damage may occur. Syncope, meanwhile, is a sudden loss of consciousness caused by a temporary decrease in blood flow to the brain. Syncope can be tied to an abrupt fall in blood pressure, a decrease in heart rate, or changes in the volume or distribution of blood. In many cases, syncope resolves quickly with no serious complications.

Treating hemodynamic instability usually means administering some sort of pharmaceutical drugs. The most common types of drugs prescribed to treat hemodynamic instability include catecholamines, adjunctive vasopressors, and nitrovasodilators. Catecholamines are a category of drugs designed to promote blood flow and elevate blood pressure. Some catecholamines typically prescribed in cases of hemodynamic instability include dobutamine, dopamine, and epinephrine. Adjunctive vasopressors are drugs that induce vasoconstriction, which is the narrowing of blood vessels. Common adjunctive vasopressors include vasopressin and terlipressin. Nitrovasodilators are drugs that induce vasodilation through nitric oxide–mediated relaxation of vascular smooth muscle. Nitroglycerin and nitroprusside are two common nitrovasodilators. All of these drugs help to increase hemodynamic forces in the circulatory system and restore normal blood flow.

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