dynamic muscle pump

What is cardiac output (CO) and its units?

Volume of blood pumped by the heart each minute to meet the body's metabolic demands; measured in mL/min or L/min

Cardiac output formula

CO = HR x SV (Heart Rate x Stroke Volume)

What are the two main determinants of cardiac output?

Heart Rate and Stroke Volume

What system modulates heart rate?

The Autonomic Nervous System (sympathetic and parasympathetic)

How does the ANS alter heart rate?

By altering resting membrane potential (RMP) and the slope of phase 4 spontaneous depolarization in the SA node

Normal RMP and heart rate

RMP ~ -65mV; slope of phase 4 depolarization sets the heart rate (~75 bpm)

Parasympathetic effect on RMP and HR

RMP ~ -70mV (more negative/hyperpolarized); less steep phase 4 slope = slower heart rate (~40 bpm)

Sympathetic effect on RMP and HR

RMP ~ -55mV (less negative); steeper phase 4 slope = faster heart rate (~120 bpm)

Stroke volume formula

SV = EDV - ESV (End Diastolic Volume - End Systolic Volume)

What is preload?

The load on the heart during diastole (before contraction); alters End Diastolic Volume; related to venous return to the ventricles

What is afterload?

The force (load) the contracting myocyte must overcome during systole (after the heart); alters End Systolic Volume; related to arterial blood pressure

What is contractility?

The "strength of the myocardium"; alters End Systolic Volume; example: sympathetic-driven flight or fright response

Three ways to increase stroke volume

Increase contractility, increase preload, reduce afterload

Effect of increased contractility on the pressure-volume loop

Increased stroke volume due to enhanced myocyte contraction; results in a smaller End Systolic Volume (point F)

Effect of increased preload on the pressure-volume loop

Increased stroke volume due to a larger End Diastolic Volume (point C)

Frank-Starling Mechanism

A greater preload (End Diastolic Volume) results in increased stroke volume

Effect of reduced afterload

Increases stroke volume

What can cause decreased cardiac output?

Decreased heart rate and/or decreased stroke volume (via decreased preload, contractility, or increased afterload)

Shock and cardiac output

Massive vasodilation reduces pressure in vessels, so less blood returns to the heart, reducing preload, stroke volume, and cardiac output

Why give IV fluids in shock?

IV fluids increase preload, which increases stroke volume and cardiac output

Why give epinephrine in shock?

Epinephrine increases heart rate and contractility, increasing cardiac output

Sympathetic nervous system neurotransmitter (cardiac)

Norepinephrine (NE) and Epinephrine

Parasympathetic nervous system neurotransmitter (cardiac)

Acetylcholine (ACh)

SNS cardiac receptor type

Beta adrenergic receptors

PNS cardiac receptor type

Muscarinic receptors

SNS effect on heart rate (chronotropy)

Increased heart rate (positive chronotrope)

PNS effect on heart rate (chronotropy)

Decreased heart rate (negative chronotrope)

SNS effect on contractility (inotropy)

Increased contractility (positive inotrope)

SNS effect on myocardial relaxation (lusitropy)

Increased relaxation (positive lusitrope), with an associated increase in preload