Cardiac Pressure-Volume Relationships

Question 1

ESPVR

Answer 1

The end systolic pressure volume relationship.

Describes the maximum pressure that can be generated for any given LV volume in a fully contraccted ventricle. This slope is really a measure of contractility. The steeper the slope the greater the inotropic state and the more pressure that can be generated.

Question 2

EDPVR

Answer 2

The end diastolic pressure volume relationship.

Describes the pressure for any given amount of volume in a fully relaxed ventricle. This slope is really the inverse of compliance. The steeper the slope the less compliant the ventricle (ie. it’s stiffer)

Question 3

Ejection fraction

Answer 3

Is often used as a surrogate measure of contracttility. It can be measured using echocardiography or by a nuclear medicine method.

EF (%) = Stroke volume / EDV x 100

Normal EF is 50-70%. An EF less than 30% represents severe systolic dysfunction (heart fialure).

Question 4

The Frank-Starling Curve effect of afterload

Answer 4

Question 5

The Frank-Starling Curve effect of contractility

Answer 5

Question 6

Myocardial oxygen cost

Answer 6

Also called ‘external work’ or ‘stroke work’

Stroke work is a measure of external work done by the heart for each beat and is defined as the area enclosed by a pressure-volume loop.

Stroke work (N cm) = change in pressure x change in volume

The O₂ uptake (or consumption) of the heart (VO₂) is proportional to the work that the heart does.

Myocardial oxygen uptake (VO₂) also depends on the heart’s energetic efficiency. This is affected by a number of factors including substrate selection (e.g. fatty acid oxidation is more ‘oxygen expensive’ than glucose oxidation) and can be reduced with ventricular remodeling in disease states (e.g. heart failure).

Question 7

Effect on the P-V loop of an increase in venous return

Answer 7

An increase in venous return increases the preload (EDV) to the ventricle. Increased preload leads to increased SV (and thus CO) via the Frank-Starling mechanism. Ventricular pressure, on the other hand are not much affected.

Question 8

Effect on the P-V loop of a decrease in venous return

Answer 8

A decrease in venous return decreases the preload (EDV) in the ventricle. Decreased preload leads to decreased SV (and thus CO) via the Frank-Starling mechanism.

Question 9

Effect on the P-V loop of an increase in aortic pressure

Answer 9

An increase in aortic pressure increases the afterload on the ventricle. Stroke volume will fall.

Question 10

Effect on the P-V loop of a decrease in aortic pressure

Answer 10

A decrease in aortic pressure decreases the afterload on the ventricle. Stroke volume will increase.

Question 11

Effect on the P-V loop of changing inotropy

Answer 11

An increase in contractility (positive inotropy) increases the slope of teh ESPVR. What this means is that at any given ventricular volume, the ventricle can generate more pressure.

Increased contractility leads to increased SV (another way of visualising the Frank-Starling mechanism).

Decreased contractility reduces the slope of the ESPVR and reduces stroke volume.

Question 12

Draw the left ventricular pressure volume loop, labelling relevant aspects.

Answer 12

A: mitral valve opens and ventricular filling begins.

B: mitral valve closes and isovolumic contraction begins.

C: aortic valve opens and isovolumic contraction ends.

D: aortic valve closes and isovolumic relaxation begins.