Interpret Your Blood Pressure Readings - Systolic, Diastolic and Pulse Pressure

Interpreting the blood pressure readings is simple - just keep them low. However most people ignore or do not understand the pulse pressure. All three numbers, systolic, diastolic and pulse pressure are important as hypertension is dangerous.

Your blood pressure monitor normally returns two numbers - a high value and a low value. The high value is the systolic blood pressure and is the pressure exerted against the arterial walls by blood as it is being pumped from the heart. The low value is the diastolic blood pressure and is the pressure exerted by the elastic recoil of the arteries during the heart relaxes.

Systolic and Diastolic blood pressure

The following table is from the American Heart Association showing the categories of hypertension and applies to adults above 18 years old . In the UK, a reading above 140/90 mmHg is considered hypertension.

CategorySystolic (mmHg)Diastolic (mmHg)
Hypotension<90<60
Normal90 - 11960 - 79
Prehypertenson120 - 13980 - 89
Stage 1 hypertension140 - 15990 - 99
Stage 2 hypertension160 - 179100 - 109
Hypertensive crisis≥180≥110

Pulse pressure

Another important number is the pulse pressure, which is defined as the difference between the systolic and diatolic blood pressure. Normal pulse pressure at rest is 40 mmHg. This can go up to 100 mmHg during exercise.

PP = SBP - DBP

Theoretically, the pulse pressure is proportional to the amount of blood pumped by the heart in each contraction and is inversely proportional to the aorta compliance (elasticity). One important function of the aorta is to dampen the pulsatile output of the left ventricle and reduce the blood pressure. If the aorta becomes rigid, the pulse pressure can be very high.

A pulse pressure is abnormally low if it is less than 25% of the systolic pressure. This normally indicates a drop in the left ventricular stroke volume. If the pulse pressure falls below 25 mmHg, congestive heart failure or shock becomes very likely.

If the pulse pressure is consistently at 60 mmHg or above, causes can be stiffiness in major arteries, leak in aortic value, anemia, and hyperthyroidism. High pulse pressure tends to accelerate normal aging of body organs, including the heart, brain and kidneys.

Mean arterial pressure (perfusion pressure)

The mean arterial pressure (MAP) is the average blood pressure in a cardiac cycle and it represents the average pressure imposed on the patient's arterial walls. A minimum of 60 mmHg is required to circulate blood throughout the organs and body. The normal MAP value is 70 to 110 mmHg.

Map can be estimated using this formula: MAP = diastolic pressure + (1/3)*pulse pressure.

For patients under certain conditions, such as head injury and septic shock, this is a critical vital sign to monitor.

Blood pressure daily cycle

Hypertension increases the risk of having health complications, such as heart attack and kidney failure. The goal is to have normal blood pressure at all time and to decrease the risk of having health issues. The following diagram shows a person's blood pressure throughout the day. Its varies during the day and is depending on the activities being performed by the individual.

24-hour blood pressure chart

In general, blood pressure rises in the morning and after a meal. At night, while sleeping, it should drop by 10-20%. The lack of nighttime blood pressure drop is associated with a significant increase with the risk of having heart and kidney problems.

Blood pressure measured at doctor's office is often misleading due to white coat sydrome. The anxiety being in a doctor's office is sufficient to increase the blood pressure for many people. Blood pressure should be measured through out the day and night (even while sleeping) to get an accurate assessment of the situation. The goal is to minimize excessive blood pressure 24-hours a day.

References

Importance of arterial pulse pressure as a predictor of coronary heart disease risk in PROCAM