Cardiac Output (Tillisch)

Question 1

Equation for blood pressure

Answer 1

BP = MAP = CO x Resistance

= 2/3 (diastole) + 1/3 (systole)

= (systolic - diastolic)/3 + diastolic

= PP/3 + diastolic

MAP = (CO x SVR) + CVP

Question 2

Equation for cardiac output

Answer 2

CO = HR x SV

Question 3

What determines Stroke Volume?

Answer 3

1) Contractility (EF)
2) Afterload (MAP)
3) Preload (EDV)

Question 4

Stroke Volume

Answer 4

Amount of blood ejected on one beat

EDV - ESV

Question 5

End diastolic volume

Answer 5

Volume in ventricle before ejection

Question 6

End systolic volume

Answer 6

Volume in ventricle after ejection

Question 7

Ejection fraction

Answer 7

Ejection fraction = SV/EDV = (EDV - ESV)/EDV

EF is normally about 55%

EF is a measure of contractility

Question 8

Preload

Answer 8

Many definitions:

Left ventricular end diastolic volume (LVEDV)

End diastolic fiber length

Volume of ventricle at beginning of contraction

Resting length of myofibrils

Radius of ventricle

“Stretch on myofilament”

Question 9

Afterload

Answer 9

Many definitions:

Tension or wall stress generated during ejection

Force required to eject blood from ventricle

BP = MAP

Systolic wall stress (from LaPlace equation)

Aortic pressure

Impedence to ejection

Question 10

Contractility

Answer 10

Force of contraction at any given preload and afterload

Dependent on Ca2+ units activated, so dependent on:

1) Amount of trigger Ca2+ current
2) Amount of Ca2+ in SR
3) Competition between Ca2+ entry/release and Ca2+ efflux from the cell

Note: Ejection fraction is indicator of contractility (increased EF reflects increase in contractility)

Question 11

Frank-Starling relationship

Answer 11

The volume of blood ejected by the ventricle depends on the volume present in the ventricle at the end of diastole.

As cardiac muscle is stretched passively, the force of contraction when stimulated increases to a point and then plateaus

X axis: Ventricular EDV or Right Atrial Pressure

Y axis: Cardiac Output or Stroke Volume

Frank-Starling relationship is based upon length-tension relationship for ventricles

Question 13

Law of LaPlace

Answer 13

T = (Pr)/(2H)

T = Tension

P = Pressure

r = radius

H = Thickness

Tension required of the shortening myofilament is proportional to the pressure generated in the chamber times the radius

Question 13

Fick principle

Answer 13

There is conservation of mass, which can be applied to the utilization of O₂

CO = (O₂ consumption)/([O₂]_{pulm vein} - [O₂]_{pulm art})

Can calculate CO because you can measure all [O₂]

Remember: pulmonary vein is oxygenated and pulmonary artery is not!

Question 14

When you’re sitting, resting, what controls your heart rate? What about your blood vessel tone?

Answer 14

Vagus nerve using Ach inhibits heart rate when you’re at rest. When sitting, you have no sympathetic stimulation speeding your heart rate. If given a beta blocker (stim of beta1 by NE increases HR) when sitting, won’t have decrease in HR.

Sympathetic system maintains normal blood vessel tone/BP/vascular resistance.

Question 15

Compliance (Capacitance)

Answer 15

Passive distensibility/elastance

Passive tension of myofibrils depends on intrinsic elasticity and degree of stretch of that elasticity

Higher compliance means can hold higher volume at a given pressure

Veins have high compliance and arteries have low compliance

C = V/P

Question 16

Normal pressures

Answer 16

Left ventricle: 130/4 mmHg

Aortic root: 130/70 mmHg

Pulmonary artery: 30/15 mmHg

Right ventricle: 30/0 mmHg

Right atrium: 3 mmHg

Question 17

Systemic vascular resistance (SVR)

Answer 17

SVR = (MAP - CVP)/CO

(times 80 to get dynes-sec-cm^-5)