Mean Arterial Pressure (MAP)

Average arterial pressure throughout one cardiac cycle, systole, and diastole. 

Surrogate indicator of blood flow and believed to be a better indicator of tissue perfusion.

To perfuse vital organs requires the maintenance of a minimum MAP of 60 mmHg. 

MAP = [Cardiac Output (CO) x Systemic Vascular Resistance (SVR)] + Central Venous Pressure (CVP)

MAP = (CO × SVR) + CVP

Because CVP is usually at or near 0 mmHg, this relationship is often simplified to:

MAP ≈ CO × SVR.

Cardiac output (CO) = Heart Rate (HR) X Stroke Volume (SV).

Stroke Volume is by ventricular inotropy and preload. 

Preload is affected by blood volume and the compliance of veins. 

Increasing the blood volume increases the preload, increasing the stroke volume and therefore increasing cardiac output. 

Afterload also affects the stroke volume in that an increase in afterload will decrease stroke volume. 

Heart rate is affected by the chronotropy, dromotropy, and lusitropy of the myocardium. 

Systemic vascular resistance is determined primarily by the radius of the blood vessels. 

Decreasing the radius of the vessels increases vascular resistance. 

Increasing the radius of the vessels would have the opposite effect. 

Blood viscosity can also affect systemic vascular resistance. 

An increase in hematocrit will increase blood viscosity and increase systemic vascular resistance. 

Viscosity, however, is considered only to play a minor role in systemic vascular resistance.

Common formula:

MAP = Diastolic blood pressure + 1/3 (Systolic Blood pressure – Diastolic Blood Pressure)

          = DBP + 1/3(SBP – DBP) or 

MAP = DBP + 1/3(Pulse Pressure)

MAP = [Systolic Blood Pressure + (2 x Diastolic Blood Pressure)]

                                                      3

Example, if blood pressure is 82 mm Hg/50 mm Hg,

MAP = SBP + 2 (DBP) = 82 +2 (50) = 182 = 60.67 mmHg; or

                3                              3              3

MAP = 1/3 (SBP – DBP) + DBP = 1/3 (82-50) + 50 = 10.67 + 50 = 60.67 mmHg

In sepsis, vasopressors are often titrated based on the MAP. 

In the guidelines of the Surviving Sepsis Campaign, it is recommended that MAP be maintained ≥ 65 mm Hg.

Chronotropy = Heart Rate

Dromotropy = Speed of electrical conduction in the Heart

Lusitropy = Rate of myocardial relaxation

Inotropy = Contractility

Mean Arterial Pressure (MAP) = 70-100 mmHg

Cardiac Index (CI) = Cardiac Output (CO)/ Body Surface Area (BSA) 

                               = 2.5-4 L/min/m2.

Stroke volume (SV) = Cardiac output / Heart Rate 

                                 = 60-120 mL/beat.

Systemic vascular resistance (SVR) = (MAP – Mean Right Atrial Pressure) x 80 / CO 

                                                          = 800-1200 dynes x sec/cm3.

Pulmonary Vascular Resistance = (Mean Pulmonary Artery Pressure – Mean Pulmonary Capillary Wedge Pressure) X 80 / Cardiac Output 

                                                 =125-250 dynes X sec/cm3.

Pulse Pressure (PP)

Pulse Pressure (PP) = Systolic Blood Pressure – Diastolic Blood Pressure

Normal pulse pressure, approximately 40 mmHg.

Change in pulse pressure (Delta Pp) = Volume change (Delta-V) = Stroke volume (SV)

                                                                 Arterial compliance (C)     Arterial compliance (C)

                                                                         

                                                            = Approximately 80 mL = Approximately 40 mm Hg

                                                                       2 mL/mm Hg

Arterial compliance (C) = Delta V/Delta P

Because the aorta is the most compliant portion of the human arterial system, the pulse pressure is the lowest. Compliance progressively decreases until it reaches a minimum in the femoral and saphenous arteries, and then it begins to increase again. 

Narrowed PP (Low) < 25% of the SBP.

Widened PP (High) > 100 % of SBP.

Widened (High) Pulse Pressure (PP)

> 100 % of SBP

Indicative of a noncompliant stiff aorta with a reduced ability to distend and recoil.

With age there is a decrease in compliance of the aorta & small arteries.

In majority, SBP increase while DBP remain near normal. 

In aortic regurgitation (AR), backward, or regurgitant flow, increase SBP and decrease DBP, and therefore increased PP.

Heart valve conditions (Aortic regurgitation, Aortic sclerosis)

Reduced blood viscosity (Severe Iron deficiency anemia)

Increased systolic pressure (Hyperthyroidism), 

Less compliant arteries (Arteriosclerosis)

Narrow (Low) Pulse Pressures (PP)

< 25% of the SBP

Decreased pumping (Heart failure), 

Decreased Stroke Volume (Aortic Stenosis)

Decreased Blood Volume (Blood loss), 

Decreased Filling Time (Cardiac Tamponade/Pericarditis). 

Dysautonomia/postural orthostatic tachycardia syndrome (POTS)