Mean Arterial Pressure
What is blood pressure?
As the heart contracts, blood is forced out of the left ventricle and into the aorta and distributing arteries. The force created by the heart's contraction generates pressure as blood pushes against the walls of the vessels. This is known as blood pressure and consists of two readings: the maximum (systolic) and minimum (diastolic) pressures. The maximum pressure occurs as the heart contracts, and the minimum pressure occurs as the heart fills back up with more blood. The normal blood pressure for an adult is 120/80 mmHg.
What is Mean Arterial Pressure (MAP)?
MAP is the measurement that explains the average blood pressure in a person's blood vessels during a single cardiac cycle. Mean arterial pressure is significant because it measures the pressure necessary for adequate perfusion of the organs of the body. It is considered by many to be a better indication of perfusion than systolic blood pressure. It is vital to have a MAP of at least 60 mmHg to provide enough blood to the coronary arteries, kidneys, and brain. The normal MAP range is between 70 and 100 mmHg. Mean arterial pressures that deviate from this range for prolonged periods of time can have drastic negative effects on the body.
High MAP can cause stress on the heart because it has to work harder than normal to push against the elevated pressure in the vessels. It can lead to advanced heart disease, blood clots, heart attack, and stroke. Prolonged elevated MAP results in heart muscle enlargement, which jeopardizes the life expectancy of the heart.
Low MAP can be life threatening as well. When the MAP gets below 60, vital organs in the body do not get the nourishment they need for survival. When it gets low, it can lead to shock and eventually death of cells and organ systems. Low mean arterial pressure can be caused by sepsis, stroke, hemorrhaging, or trauma.
MAP is directly affected by factors such as:
• Amount of blood pumped out of the heart per minute (cardiac output)
• Heart rate (beats per minute)
• Blood pressure
• Resistance to blood flow in the vessels
A change in any of these factors will alter the mean arterial pressure and cause negative effects on the body
How to Calculate MAP
MAP can be determined by the equation: MAP = CO x SVR where,
CO = Cardiac Output
SVR = Systemic Vascular Resistance (resistance to blood flow throughout the vasculature)
The most accurate MAP can only be determined through an invasive central line. However, it can be measured by another equation using the systolic blood pressure (SBP) and diastolic blood pressure (DBP): MAP = DBP + ⅓ (SBP - DBP)
Fortunately, many monitors and blood pressure machines calculate mean arterial pressure for you when a blood pressure is taken so calculations do not need to be made.