Diastolic pressure occurs near the beginning of the cardiac cycle. It is the minimum pressure in the arteries when the pumping chambers of the heart — ventricles — fill with blood. Near the end of the cardiac cycle, systolic pressure, or peak pressure, occurs when the ventricles contract.
As the heart beats, it pumps blood through a system of blood vessels, which carry blood to every part of the body. Blood pressure is the force that blood exerts on the walls of blood vessels. All or any of the events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next is called a cardiac cycle. Problems in the cardiac cycle can cause low or high blood pressure.
|Definition||It is the pressure that is exerted on the walls of the various arteries around the body in between heart beats when the heart is relaxed.||It measures the amount of pressure that blood exerts on arteries and vessels while the heart is beating.|
|Normal range||60 – 80 mmHg (adults); 65 mmHg (infants); 65 mmHg (6 to 9 years)||90 – 120 mmHg (adults); 95 mmHg (infants); 100 mmHg (6 to 9 years)|
|Importance with age||Diastolic readings are particularly important in monitoring blood pressure in younger individuals.||As a person's age increases, so does the importance of their systolic blood pressure measurement.|
|Blood Pressure||Diastolic represents the minimum pressure in the arteries.||Systolic represents the maximum pressure exerted on the arteries.|
|Ventricles of the heart||Fill with blood||Left ventricles contract|
|Blood Pressure reading||The lower number is diastolic pressure.||The higher number is systolic pressure.|
|Etymology||"Diastolic" comes from the Greek diastole meaning "a drawing apart."||"Systolic" comes from the Greek systole meaning "a drawing together or a contraction."|
Blood Pressure Reading
Blood pressure readings are measured in millimeters of mercury (mmHg) and are provided as a pair of numbers. For example, 110 over 70 (written as 110/70) systolic/diastolic.
The lower number is the diastolic blood pressure reading. It represents the minimum pressure in the arteries when the heart is at rest. The higher number is the systolic blood pressure reading. It represents the maximum pressure exerted when the heart contracts.
The following video from Khan Academy explains the two numbers in greater detail.
Measuring Systolic and Diastolic Blood Pressure
The instrument used to measure blood pressure is called a Sphygmomanometer. The blood pressure cuff is snugly wrapped around the upper arm, positioning it so that the lower edge of the cuff is 1 inch above the bend of the elbow. The head of the stethoscope is placed over a large artery then air is pumped into the cuff until circulation is cut off, then air is let out slowly.
Air is pumped into the cuff until circulation is cut off; when a stethoscope is placed over the cuff, there is silence. Then as the air is slowly let out of the cuff, blood begins to flow again and can be heard through the stethoscope. This is the point of greatest pressure (called Systolic), and is usually expressed as how high it forces a column of mercury to rise in a tube. At its highest normal pressure, the heart would send a column of mercury to a height of about 120 millimeters.
At some point, as more and more air is let out of the cuff, the pressure exerted by the cuff is so little that the sound of the blood pulsing against the artery walls subsides and there is silence again. This is the point of lowest pressure (called Diastolic), which normally raises the mercury to about 80 millimeters.
Normal Ranges for Diastolic and Systolic Pressure
In children, the diastolic measurement is about 65 mmHg. In adults it ranges from 60 – 80 mmHg. Systolic measurement in children ranges from 95 to 100 and in adults it ranges from 90 – 120 mmHg.
An adult is considered suffering from
- hypotension if the diastolic reading is < 60 mmHg and systolic reading is < 90 mmHg
- Prehypertension if the diastolic reading is 81 – 89 mmHg and systolic reading is 121 – 139 mmHg
- Stage 1 Hypertension if the diastolic reading is 90 – 99 mmHg and systolic reading is140 – 159 mmHg
- Stage 2 Hypertension if the diastolic reading is 100 mmHg and systolic reading is 160 mmHg
Clinical Significance and Cardiovascular Risk
In the past, more attention was paid to diastolic pressure but it is now recognized that both high systolic pressure and high pulse pressure (the numerical difference between systolic and diastolic pressures) are risk factors. In some cases, it appears that a decrease in excessive diastolic pressure can actually increase risk, probably due to the increased difference between systolic and diastolic pressures.
Cardiovascular risk in those middle-aged and older is often more accurately predicted by using systolic blood pressure measurements than diastolic blood pressure measurements. Diastolic blood pressure can then be used to better understand the risks identified by systolic blood pressure.
In a video titled What is the Clinical Importance of Systolic and Diastolic Blood Pressure, Dr. Len Saputo cites a research study published in the journal The Lancet examining how systolic and diastolic blood pressure in 30-year olds could predict risk of cardiovascular disease later in life. He explains that the difference between the two types of blood pressure is probably more important than either number alone.
Diastolic readings are particularly important in monitoring the blood pressure in younger individuals. Systolic blood pressure is known to rise with age as a result of hardening of the arteries.