Blood Pressure Formulas
Blood pressure is measured as systolic pressure over diastolic pressure (SBP/DBP), while mean arterial pressure (MAP) is most accurately calculated as MAP = DBP + 0.40 × (SBP - DBP), though the traditional formula MAP = DBP + 0.33 × (SBP - DBP) remains widely used despite being less accurate.
Standard Blood Pressure Measurement
Blood pressure consists of two values 1:
Systolic BP (SBP): Maximum arterial pressure during cardiac contraction (systole)
- Measured at the onset of the first Korotkoff sound with auscultatory devices
- Estimated from mean arterial pressure using algorithms in oscillometric devices
Diastolic BP (DBP): Minimum arterial pressure during cardiac relaxation (diastole)
- Measured at complete disappearance of Korotkoff sounds (fifth sound)
- If sounds don't disappear, use the fourth sound (muffling)
Mean Arterial Pressure Formulas
The Traditional Formula (Less Accurate)
The classic formula MAP = DBP + 0.33 × (SBP - DBP) or equivalently MAP = DBP + PP/3 (where PP = pulse pressure) has been standard for decades 2. However, this formula systematically underestimates MAP by approximately 5 mmHg compared to actual measured values 2.
The More Accurate Formula
MAP = DBP + 0.40 × (SBP - DBP) provides significantly better accuracy, with a mean difference of only -1.2 mmHg from actual measured MAP 2. Alternative formulas include:
- MAP = DBP + 0.412 × (SBP - DBP) [difference: -0.6 mmHg]
- MAP = DBP + 0.33 × (SBP - DBP) + 5 mmHg [difference: -0.4 mmHg]
Important Caveats
The percentage of pulse pressure to add to DBP varies dramatically between individuals (ranging from 23% to 58%, mean 42.2%) and is influenced by 2:
- Sex: Higher in women (42.9%) than men (41.2%)
- Age: Lower in elderly (40.9%) versus younger adults (42.8%)
- DBP level: Strongly associated with baseline diastolic pressure
- Physiological state: Increases during mental stress
During or immediately after exercise, all standard formulas fail because tachycardia shortens diastole disproportionately, causing the traditional formula to underestimate MAP by 20-30 mmHg 3. A corrected formula accounting for heart rate changes is needed in these situations.
For noninvasive cuff measurements specifically, a recent corrected formula has been proposed: MAP = DBPcuff + [0.33 + (0.43 - 0.0038 × DBPcuff)] × PPcuff, which showed superior accuracy (R² = 0.818) compared to standard formulas 4.
Practical Measurement Considerations
The standard measurement site is the upper arm at the brachial artery 5, 1. Mean arterial pressure falls only 1-2 mmHg between the aorta and peripheral arteries, but systolic pressure increases and diastolic decreases in more distal locations 5.
All BP devices must be clinically validated before use, yet only 6% of commercially available oscillometric devices have been adequately tested 1. Check validated device lists at www.stridebp.org or www.validatebp.org.