Hemodynamics

Overview
Hemodynamics, meaning literally "blood movement", is the study of blood flow or the circulation.

Physiology
All animal cells require oxygen (O2) for the conversion of carbohydrates, fats and proteins into carbon dioxide (CO2), water and energy in a process known as aerobic respiration. The circulatory system functions to transport the blood to deliver O2, nutrients and chemicals to the cells of the body to ensure their health and proper function, and to remove the cell wastes.

The circulatory system is a connected series of tubes, which includes the heart, the arteries, the micro-circulation, and the veins.

The heart is the driver of the circulatory system generating cardiac output (CO) by rhythmically contracting and relaxing. This creates changes in regional pressures, and, combined with a complex valvular system in the heart and the veins, ensures that the blood moves around the circulatory system in one direction. The “beating” of the heart generates pulsatile blood flow which is conducted into the arteries, across the micro-circulation and eventually, back via the venous system to the heart. The aorta, the main artery, leaves the left heart and proceeds to divide into smaller and smaller arteries until they become arterioles, and eventually capillaries, where oxygen transfer occurs. The capillaries connect to venules, into which the deoxygenated blood passes from the cells back into the blood, and the blood then travels back through the network of veins to the right heart. The micro-circulation, the arterioles, capillaries and venules, constitutes most of the area of the vascular system and is the site of the transfer of, O2, glucose and substrates into the cells. The venous system returns the de-oxygenated blood to the right heart where it is pumped into the lungs to become oxygenated and CO2 and other gaseous wastes exchanged and expelled during breathing. Blood then returns to the left side of the heart where it begins the process again. Clearly the heart, vessels and lungs are all actively involved in maintaining healthy cells and organs, and all influence hemodynamics.

The factors influencing hemodynamics are complex and extensive but include CO, circulating fluid volume, respiration, vascular diameter and resistance, and blood viscosity. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs or alcohol, obesity and excess weight.

Our understanding of hemodynamics depends on measuring the blood flow at different points in the circulation. A basic approach to understanding hemodynamics is by “feeling the pulse”. This gives simple information regarding the strength of the circulation via the systolic stroke and the heart rate, both important components of the circulation which may be altered in disease. The blood pressure can be simply measured using a plethysmograph or cuff connected to a pressure sensor (mercury or aneroid manometer). This is the most common clinical measure of circulation and provides a peak systolic pressure and a diastolic pressure, often quoted as a normal 115/75. Sometimes the mean arterial pressure is calculated.

MAP ~= BPdia + (BPsys - BPdia)/3 mmHg

Where
 * MAP = Mean Arterial Pressure
 * BPdia = Diastolic blood pressure
 * BPsys = Systolic blood pressure

The arterial pulse pressure can be measured by placing a tonometer or pressure sensor on the skin surface above an artery. This provides a continuous pressure trace or arterial pulse pressure waveform which reflects cardiovascular performance (Fig1). A non-invasive Doppler can also be used to measure blood flow at any point in the circulation, including within the heart, the CO, and can be converted to a pressure difference using the modified Bernoulli equation, P=4V2. An invasive manometer (pressure sensor) can be inserted into an artery on the end of a catheter to measure intra-arterial pulse pressures providing information on cardiovascular performance. Importantly all of these measures should be accompanied by a measure of CO so that the function of the heart and vessels can be distinguished. This allows for more effective understanding and treatment of the cardiovascular system.

The heart and the vascular beds are a dynamic and connected part of the circulatory system and combine to effect efficient transportation of the blood. Circulation is influenced by the resistance of the vascular bed against which the heart is pumping. For the right heart this is the pulmonary vascular bed, creating Pulmonary Vascular Resistance (PVR), while for the systemic circulation this is the systemic vascular bed, creating Systemic Vascular Resistance (SVR). The vessels actively change diameter under the influence of physiology or therapy, vasoconstrictors decrease vessel diameter and increase resistance, while vasodilators increase vessel diameter and decrease resistance. Put simply increasing resistance (narrowing the vessel) decreases CO, and conversely decreased resistance (widening the vessel) increases CO.

This can be explained mathematically:

By simplifying D'arcy's Law, we get the equation that

Flow = Pressure/Resistance

When applied to the circulatory system, we get:

CO = 80 x (MAP – RAP)/TPR

RAP = Mean Right Atrial Pressure in mmHg and TPR = Total Peripheral Resistance in dynes-sec-cm-5.

However, as MAP >> RAP, and RAP is approximately 0, this can be simplified to:

CO ~= 80 x MAP/TPR For right heart CO ~= MAP/PVR For left heart CO ~= MAP/SVR

Physiologists will often re-arrange this equation, making MAP the subject, to study the body's responses. 80 x MAP ~= CO x TPR