In summary, MAP = (Systemic vascular resistance) × (Cardiac output). We are interested in this variable because it is the pressure which seems to have the greatest influence on bloodflow autoregulation within organs, and on whole-body haemodynamic homeostatic mechanisms (such as the baroreceptors). It is resistant to confounding measurement factors, and is attractively easy to calculate. However, it has a series of drawbacks, not the least of which being the lack of agreement regarding what an appropriate MAP is in any given situation. The golden "MAP of 65" rule is another one of those things which has widespread support and absolutely no evidence.
MAP = (Systemic vascular resistance) × (cardiac output).
MAP = geometrical mean of the invasive arterial blood pressure waveform; or the mathematical adjusted mean of the non-invasive measurement
NIBP MAP = (diastolic pressure) + (pulse pressure divided by 3)
Why are we interested in measuring this variable?
Relevance to hemodynamics
- Organ bloodflow autoregulation is thought to depend on MAP rather than systolic or diastolic pressures.
- The left ventricle is an exception, in that it "autoregulates" according to the diastolic pressure - we call that the Frank-Starling principle
- Baroreceptors also seem to respond to the mean pressure rather than systolic or diastolic
- The MAP-RAP (right atrial pressure) gradient determines the driving pressure for organ perfusion.
- A MAP of 65-70 is empirically thought to be the minimal adequate MAP for organ perfusion.
Resistance to confounding factors
- Not altered by damping of transducers
- Independent of measurement technique and site - unlike systolic pressure
Disadvantages of MAP as a haemodynamic goal
- Diastolic pressure is what fills your coronaries
- Systolic pressure is what bursts your aneurysms and dislodges haemostatic thrombi.
- The exact MAP goal is uncertain for patients with chronic hypertension; some evidence (SEPSISPAM) seems to suggest that a higher MAP is nephro-protective.