MAP

Mean Arterial Pressure (MAP)

Overview

• Transition from stroke volume and cardiac output to mean arterial pressure.

• Utilization of Weger's diagram to represent arterial pressure.

Definitions

• Systolic Pressure:

  • Maximum pressure in the aorta during the cardiac cycle.

  • Example: approximately 120 mmHg.

• Diastolic Pressure:

  • Lowest pressure in the aorta during the cardiac cycle.

  • Example: approximately 80 mmHg.

Blood Pressure Measurement

• Standard recording: 120/80 mmHg

  • Systolic pressure over diastolic pressure.

• Importance in medical therapeutics and critical care.

Mean Arterial Pressure (MAP)

• Definition:

  • The mean arterial pressure is the mean pressure throughout the entire cardiac cycle.

• Calculation Method:

  • MAP = Diastolic Blood Pressure + (1/3)(Systolic Pressure - Diastolic Pressure).

  • Example Calculation: Using 120/80 mmHg leads to MAP of approximately 90 or 93 mmHg.

Importance of MAP

• MAP is the driving force of blood delivery to tissues.

• Constant systemic pressure pushing blood out to the body.

Formula for Understanding MAP

• General Relationship: MAP = Stroke Volume × Heart Rate × Systemic Vascular Resistance (SVR).

• Note: This formula is about relationships, not just plugging in numbers.

Systemic Vascular Resistance (SVR)

• Definition:

  • Systemic vascular resistance is the resistance the heart must overcome to pump blood.

  • Synonymous with afterload.

• SVR is affected by:

  • Vascular smooth muscle changes in arterioles.

  • Outflow tract valves (aortic and pulmonary valves).

• Illustration of aortic valve functionality:

  • Typically a three-leafed valve that can become narrowed due to scarring or conditions like a bicuspid aortic valve.

Effects of SVR on Stroke Volume

• Stroke Volume (SV): Volume of blood pumped out of the ventricle in one contraction.

• Three scenarios of afterload and stroke volume:

  • Baseline SVR = certain stroke volume.

  • Reduced SVR → Wider opening → Reduced heart resistance → Increased stroke volume.

  • Increased SVR → Greater resistance → Decreased stroke volume.

• Important Relationship:

  • Direct relationship between SVR and stroke volume.

Changes in MAP with Variable Manipulation

• Understanding influence of each independent variable on MAP.

  • Increase in Stroke Volume → Increases blood pressure.

  • Increase in Heart Rate → Increases blood pressure.

  • Increase in SVR → Increases blood pressure.

• Notable Clarification:

  • Increasing SVR leads to an increase in MAP, despite its effect of decreasing stroke volume in terms of heart strain.

  • Key Point: Changes in SVR result in greater influence on MAP than changes in stroke volume or heart rate.

Regulatory Mechanisms of MAP

• Autonomic Control:

  • Two systems: Parasympathetic and Sympathetic.

  • Overview of their opposing effects on heart rate and vascular resistance:

  • Parasympathetic System:

    • Decreases heart rate (decreasing cardiac output → lower MAP).

  • Sympathetic System:

    • Increases heart rate (increasing cardiac output → higher MAP).

    • Increases contractility → increases stroke volume → higher MAP.

    • Vasoconstriction of arterioles → increases SVR → higher MAP.

    • Vasoconstriction of veins → decreases compliance/capacitance → increases venous return → increases stroke volume → higher MAP.

Homeostasis of Blood Pressure

• Baroreceptors: Sensory receptors that monitor blood pressure changes, primarily located in:

  • Aortic arch

  • Bifurcation of the carotid artery.

• Function:

  • Detect pressure changes and signal the cardiovascular control center located in the brain's medulla.

  • Integration of information to maintain blood pressure homeostasis.

Responses to Blood Pressure Changes

Drop in Blood Pressure

• Detected by baroreceptors.

• Cardiovascular Control Center's Response:

  • Increased sympathetic activity:

  • Increased heart rate via beta-1 receptors on SA node.

  • Increased contractility to enhance stroke volume.

  • Vasoconstriction in arterioles to raise SVR.

  • Increased venous return by constraining veins (decreased compliance).

  • Decreased parasympathetic activity leading to a rise in heart rate.

Increase in Blood Pressure

• Detected by baroreceptors.

• Cardiovascular Control Center's Response:

  • Decreased sympathetic output:

  • Decreased heart rate.

  • Decreased contractility and stroke volume.

  • Vasodilation in arterioles leading to a decrease in SVR.

  • Decreased sympathetic activity in veins, allowing for vasodilation (increased compliance, decreased venous return).

  • Increased parasympathetic activity to lower heart rate.

Conclusion

• MAP is crucial for ensuring adequate blood flow to maintain tissue perfusion.

• The interactions between stroke volume, heart rate, and systemic vascular resistance must be properly understood for effective treatment of cardiovascular-related diseases.