Mean Arterial Pressure (MAP) refers to the average arterial pressure during a single cardiac cycle encompassing both systole and diastole .
MAP is the perfusion pressure seen by organs in the body. It reflects the driving force for blood flow and is vital for maintaining tissue and organ viability.
MAP Calculation
MAP is a time-weighted average , not a simple arithmetic mean, because diastole lasts longer than systole.
Standard Clinical Formula:
Where:
- DBP = Diastolic Blood Pressure
- SBP = Systolic Blood Pressure
- PP = Pulse Pressure (SBP - DBP)
Since diastole occupies about two-thirds of the cardiac cycle, MAP is closer to diastolic pressure .
High-Yield Note
- Normal MAP Range : 70–100 mmHg
- Minimum for organ perfusion : ~60 mmHg
- MAP < 60 mmHg → Risk of organ ischemia
- MAP > 100 mmHg → Risk of vascular injury and end-organ damage
Determinants of MAP
MAP is a function of both cardiac output (CO) and systemic vascular resistance (SVR) .
Stroke Volume (SV) is affected by:
- Preload : End-diastolic volume; ↑ blood volume = ↑ preload
- Contractility : Inotropy of myocardium
- Afterload : Resistance the heart must pump against (↑ afterload = ↓ SV)
Heart Rate is regulated by:
- Chronotropy (rate)
- Dromotropy (conduction speed)
- Lusitropy (relaxation ability)
Clinical Relevance of MAP
| MAP Value | Clinical Interpretation | Potential Consequences |
|---|---|---|
| > 100 mmHg | Hypertensive emergency risk | ↑ Cardiac workload, ↑ clot formation, arterial injury |
| 60–100 mmHg | Optimal perfusion range | Adequate tissue oxygenation |
| < 60 mmHg | Inadequate perfusion | Cerebral ischemia, renal failure, organ damage |
Physiologic and Systemic Regulation
Cardiovascular System
- Controls CO and SVR via:
- Baroreceptor reflexes (carotid sinus, aortic arch)
- Sympathetic and parasympathetic output
Renal System
- Renin-Angiotensin-Aldosterone System (RAAS) increases:
- Sodium & water retention → ↑ blood volume → ↑ preload and MAP
- Vasoconstriction → ↑ SVR
Autonomic Nervous System (ANS)
- Adjusts:
- Heart rate (via beta-1 receptors)
- Vascular tone (via alpha-1 receptors)
Why MAP ≠ Simple Average of SBP & DBP
- Systolic and diastolic phases are not equal in duration.
- At rest :
- Systole = ~1/3 of cardiac cycle
- Diastole = ~2/3 of cardiac cycle
- Simple averaging would overestimate perfusion .
Measurement Methods
| Method | Description |
|---|---|
| Sphygmomanometer | Manual method using BP cuff; MAP calculated |
| Oscillometric BP Monitor | Automated; often provides MAP directly |
| Invasive Arterial Line | Gold standard in ICU settings; real-time measurement |
Clinical Conditions Associated with Abnormal MAP
High MAP (> 100 mmHg)
Causes :
- Hypertension
- Pheochromocytoma
- Vasopressor overdose
- Hypervolemia
Risks :
- End-organ damage (e.g., kidney, brain, heart)
- Left ventricular hypertrophy
- Stroke or myocardial infarction
Low MAP (< 60 mmHg)
Causes :
- Sepsis (vasodilation + capillary leakage)
- Hemorrhage or internal bleeding
- Heart failure
- Stroke or neurogenic shock
Consequences :
- Hypoperfusion of vital organs
- Acute kidney injury
- Loss of consciousness
- Neuronal death
High-Yield Summary
| Concept | Key Point |
|---|---|
| MAP Formula | MAP = DBP + 1/3(SBP - DBP) |
| Organ perfusion threshold | MAP ≥ 60 mmHg |
| Determinants of MAP | MAP = CO × SVR |
| Stroke volume influenced by | Preload, Afterload, Contractility |
| RAAS effect | ↑ Plasma volume & SVR → ↑ MAP |
| Baroreceptors | Regulate MAP via autonomic feedback |
| High MAP risk | Hypertension, stroke, cardiac damage |
| Low MAP danger | Organ ischemia, shock, death |
