MAP = diastolic pressure + (systolic pressure – diastolic pressure) / 3
Mean arterial pressure (MAP) is a measure of the average blood pressure in a patients arteries over the course of a cardiac cycle, which is one complete heartbeat. It is calculated by taking into account both the systolic and diastolic blood pressures.
During a cardiac cycle, blood pressure fluctuates between two main values: systolic blood pressure, which is the maximum pressure when the heart is contracting and pumping blood out into the arteries, and diastolic blood pressure, which is the minimum pressure when the heart is relaxed and filling with blood.
MAP is calculated as:
MAP = diastolic pressure + (systolic pressure – diastolic pressure) / 3
This formula takes into account that the heart spends more time in diastole than systole during the cardiac cycle. Therefore, the diastolic pressure is weighted more heavily in the calculation of MAP.
MAP is an important measure of blood pressure because it reflects the pressure that is driving blood flow through the organs and tissues of the body. It is used clinically to monitor patients with certain medical conditions, such as hypertension, hypotension, or shock and is often used to guide the dosing of medications that affect blood pressure.
Pathophysiology
Mean arterial pressure (MAP) is a measure of the average blood pressure over the course of a cardiac cycle, and is determined by the interaction between cardiac output, systemic vascular resistance and blood volume.
Cardiac output is the amount of blood that the heart pumps out into the circulation in one minute, and is determined by the heart rate and stroke volume. Stroke volume is the amount of blood that the heart pumps out with each contraction, and is influenced by the force of the heart’s contractions and the filling pressure of the heart during diastole.
Systemic vascular resistance is the resistance to blood flow in the systemic circulation, and is determined by the tone of the blood vessels and the viscosity of the blood. Blood volume is the amount of blood in the circulation, and is determined by the balance between fluid intake and output, as well as the regulation of fluid balance by the kidneys.
In general, an increase in cardiac output or a decrease in systemic vascular resistance will result in an increase in MAP, while a decrease in cardiac output or an increase in systemic vascular resistance will result in a decrease in MAP. Changes in blood volume can also affect MAP, with an increase in blood volume resulting in an increase in MAP and a decrease in blood volume resulting in a decrease in MAP.
Various pathophysiological conditions can affect the factors that determine MAP and lead to changes in blood pressure. For example, hypertension (high blood pressure) is a condition characterised by chronically elevated blood pressure, which can result from increased systemic vascular resistance, increased cardiac output, or a combination of both. Hypotension (low blood pressure) can result from decreased cardiac output, decreased systemic vascular resistance, or decreased blood volume.
Systolic Blood Pressure
Systolic blood pressure is the maximum pressure that occurs in the arteries during a cardiac cycle when the heart is contracting and pumping blood out into the circulation. It is the top number in a blood pressure reading and is measured in millimetres of mercury (mmHg).
When the heart contracts, it generates pressure that pushes blood out of the left ventricle and into the arteries, causing a temporary increase in blood pressure. This pressure wave travels through the arteries and can be felt as a pulse in the arteries. This maximum pressure is known as the systolic blood pressure.
A systolic blood pressure reading is usually recorded as two numbers, for example, “120 over 80” (written as 120/80 mmHg). In this example, 120 mmHg is the systolic blood pressure and 80 mmHg is the diastolic blood pressure, which is the minimum pressure that occurs in the arteries when the heart is relaxed and filling with blood.
Systolic blood pressure can be affected by a variety of factors, such as age, physical activity, stress and certain medical conditions such as hypertension (high blood pressure), atherosclerosis (hardening of the arteries) and heart disease.
Diastolic Blood Pressure
Diastolic blood pressure is the minimum pressure that occurs in the arteries during a cardiac cycle when the heart is relaxed and filling with blood. It is the bottom number in a blood pressure reading and is measured in millimetres of mercury (mmHg).
When the heart is relaxed and filling with blood during diastole, the pressure in the arteries decreases. This minimum pressure is known as the diastolic blood pressure.
A diastolic blood pressure reading is usually recorded as two numbers, for example, “120 over 80” (written as 120/80 mmHg). In this example, 120 mmHg is the systolic blood pressure and 80 mmHg is the diastolic blood pressure.
Similar to above, diastolic blood pressure can be affected by a variety of factors, such as age, physical activity, stress, certain medical conditions such as hypertension (high blood pressure), atherosclerosis (hardening of the arteries) and heart disease.