The formula
MAP ≈ DBP + ⅓(SBP − DBP), equivalently (SBP + 2·DBP) / 3. The heart spends roughly two-thirds of the cardiac cycle in diastole at rest, so diastolic pressure is weighted twice.
Why MAP ≥ 65 mmHg in adults
65 mmHg is the most-cited minimum for adequate organ perfusion in adults (sepsis bundles, intraoperative targets). Below about 60, the kidneys and brain begin to lose autoregulation and ischemia risk rises sharply. The safe floor is patient- and organ-specific, so 65 is a starting point, not a universal answer.
MAP scales with age in children
Pediatric blood pressure rises from birth through adolescence, then converges on adult values. Normal-range bands come from bedside-reference tables near the 50th percentile, and the dangerously-low MAP floor for each age uses published cutoffs — e.g. term neonate under 40, school age under 60, adolescent under 65.
A quick preterm rule is MAP ≥ gestational age in weeks. PALS critical-hypotension thresholds are defined by systolic pressure, not MAP, and are tracked separately.
Why diastolic matters
Coronary perfusion happens largely during diastole, so a low diastolic pressure — especially with tachycardia or LVH — matters as much as systolic for myocardial ischemia risk. MAP should not be read alone: it can look reassuring while a low diastolic starves the coronaries or a low systolic signals failing output.