What is the formula to calculate mean arterial pressure?

Mean Arterial Pressure Calculation

Mean arterial pressure (MAP) is calculated using the standard formula: MAP = diastolic pressure + 1/3(pulse pressure), or equivalently, MAP = diastolic pressure + 1/3(systolic pressure - diastolic pressure). 1

Standard Clinical Formula

The most widely accepted formula for calculating MAP in clinical practice is:

• MAP = Diastolic BP + 1/3(Systolic BP - Diastolic BP) 1
• This can also be expressed as: MAP = (2 × Diastolic BP + Systolic BP) / 3 1
• This formula assumes that diastole comprises approximately 2/3 of the cardiac cycle and systole comprises 1/3 at normal resting heart rates 2, 3

Key Physiological Principle

• Mean arterial pressure falls by only 1-2 mmHg between the aorta and peripheral arteries, making it a relatively stable measure throughout the arterial tree 1, 4
• This contrasts with systolic pressure, which increases in more distal arteries, and diastolic pressure, which decreases peripherally 1

Important Limitation: Heart Rate Dependency

The standard 1/3-2/3 formula becomes increasingly inaccurate at elevated heart rates because the systolic period occupies a greater proportion of the cardiac cycle during tachycardia. 2, 3, 5

Heart Rate-Corrected Formula

For patients with elevated heart rates (particularly during exercise or stress):

• MAP = Diastolic BP + [0.33 + (HR × 0.0012)] × Pulse Pressure 3
• This formula accounts for the increasing time dominance of systole as heart rate increases 3
• The fraction of systole increases from approximately 0.35 at rest to 0.47 at heart rates of 180-190 bpm 5
• The error from using the standard formula increases linearly with heart rate, with the standard equation consistently underestimating MAP during tachycardia 5

Clinical Application Contexts

For General Clinical Use

• Use the standard formula (diastolic + 1/3 pulse pressure) for routine clinical decision-making at normal heart rates 1
• This is the formula referenced in major guidelines including KDIGO blood pressure management guidelines 1

For Exercise or Tachycardia

• Apply the heart rate-corrected formula when HR exceeds 100 bpm to avoid systematic underestimation of MAP 3, 5
• The standard formula should not be applied during exercise without heart rate correction 5

For Organ Perfusion Pressure Calculations

• When calculating organ perfusion pressure, remember that MAP represents the inflow pressure 1
• Organ perfusion pressure = MAP - venous outflow pressure - compartment pressure 1
• If compartment pressure is elevated (e.g., intraabdominal pressure of 15 mmHg) and target organ perfusion is 65 mmHg, the MAP target should be approximately 80 mmHg 1

Common Pitfalls to Avoid

• Do not use the standard formula during exercise or significant tachycardia without recognizing it will underestimate true MAP by failing to account for the altered systolic:diastolic time ratio 5
• Do not assume MAP can be accurately estimated from finger monitors, as these devices are inaccurate and not recommended 1
• Do not confuse MAP with pulse pressure (systolic minus diastolic), which is a different hemodynamic parameter used for cardiovascular risk stratification 6
• When measuring MAP for critical applications, ensure the blood pressure cuff is at heart level, as hydrostatic differences can introduce errors of approximately 2 mmHg per inch of height difference 4