Cardiac Pressure Volume Loop

Question 1

State the phases of the ventricular AP.

The ventricular AP is the same as the ______.

Answer 1

The ventricular AP is the same as the atrial muscle.

See figure in pg. 26 in notes 2 or 219 in notes 1

Phase 0 – Depolarization – Rapid Na+ channels are stimulated to open, flooding the cell with
positive sodium ions. This causes a positively directed change in the transmembrane
potential. This shift in voltage is reflected by the initial spike of the action potential = overshoot.

Phase 1 - (dip) is the initial stage of repolarization triggered by closing of the Na+ channels and the brief activation of ItO the transient outward current.

Phase 2 - is the plateau stage where the rate of repolarization by potassium is slowed by the influx of Ca+ ions into the cell. The Ca ions enter the cell slower than the Na ions and help prevent the cell
from repolarizing too quickly, thus extending the refractory period. This mechanism helps
regulate the rate at which cardiac tissue can depolarize. Note that phases 1 & 2 correspond
to the absolute refractory period.

Phase 3 - is the later stages of repolarization where potassium is leaving the cell. Once repolarization is complete, the cell will be able to respond to a new stimulus.

Phase 4 - (no net current) occurs after repolarization is complete.

Question 2

Draw and label the ventricular cardiac cycle on a pressure volume graph

Answer 2

See pgs. 85-87 & note 1) the 4 stages & 2) when the valves open & close 3) systole & diastole

Remember that filling occurs until the pressures on both sides of the valve, equilibrate & that ejection occurs when the pressure before the (aortic valve) in the left ventricle is higher than in the aorta

Question 3

Explain the definitions of preload, afterload, and work (and their relationship to ventricular work)

Answer 3

See pg. 88

Preload = end diastolic volume*. At end diastolic volume the ventricle has been stretched and this increases tension (preload). Note: this stretching also increases sensitivity of the cardiomyocytes as described by the Frank – Starling Law.

Afterload is the ventricular pressure* @ the end of systole. The Aortic Valve opens when blood pressure in the ventricle equals or exceeds the blood pressure in the aorta. This point is considered the ventricular afterload and is related to the amount of work that the heart must perform.

The afterload (highest point) multiplied by the volume change gives the amount of work expended during a contraction (W=P X V).

Question 4

Define stroke volume & describe the ventricular cardiac mechanisms that change cardiac stroke volume

SV =

Answer 4

Stroke volume or SV is the volume of blood ejected by the ventricle in a single contraction.

SV = EDS - ESV

SV is affected by changes in preload, afterload, & inotropy.

See pg. 90

Question 5

It is the left or right ventricle that does the major cardiac output for the heart?

Answer 5

left ventricle produces pumping pressure of the body!

It is more muscular too!

Question 6

1) The pressure in the left ventricle must exceed the pressure in _______ to pump blood through it. One the pressure in the _____ & the ventricle equilibrates, the ______ closes.
2) Discuss the structure & function of cardiac valves

Answer 6

1) Aortic valve
2) Remember that cardiac valves function only in 1 way & that movement goes from area of high pressure before the valve to low pressure on the air side of the valve. If pressure is higher on the air side, the valve closes to prevent regurgitation. If the valve collapses & fails, regurgitation will occur. Heart valves ensure 1 way movement.

Question 7

State the mnemonic for heart valves

Answer 7

LAB RAT - left atrium Bicupsid/Mitral & right atrium tricupsid

Question 8

State the equations for work & power when dealing with the heart

Answer 8

Work = Pressure x Volume change

Power = (Pressure x Volume change)/ time

Question 9

Define isovolumic contraction & relaxation

Answer 9

During the other parts of the cycle the heart contracts without changing volume (isovolumic contraction) or relaxes without changing volume (isovolumic relaxation).

Question 10

Is the heart a muscle?

Answer 10

Yes, the most important muscle; furthermore, it functions as a pump.

Question 11

1) Define compliance & elastance both qualitatively & quantitativley.
2) Describe what would happen graphically if compliance decreased on a P vs V curve

Answer 11

1) Compliance = ΔV/ΔP
High compliance = ventricle is easy to fill. Healthy ventricles are compliant during diastole & not during systole.

Elastance = ΔP/ΔV
A low elastance ventricle = ventricle is easy to fill
Elastance & compliance are inverses

2) Decreased compliance shifts the curve up – see pg. 87

Question 12

The slope of ESPVR (end systolic pressure-volume relatonship) represents the end-systolic _______, which provides an index of myocardial contractility.

Answer 12

elastance

see pg. 87

Elastance = ΔP/ΔV. Remember that a low elastance ventricle = easy to fill

Question 13

Name a condition where compliance is decreased

Answer 13

ventricular compliance is decreased in ventricular hypertrophy, since the ventricle is stiffer & less easy to fill.

Question 14

Aortic stenosis ______ the amount of work the heart must do.

Answer 14

Increases

Question 15

Describe EF or ejection fraction

Answer 15

EF is the amount of end diastolic volume that is ejected out of the ventricle during each contraction. Therefore, not all of the blood in the ventricle is ejected (normal EF = 55%).

EF = Stroke volume/EDV

See pg. 91

MI & systolic disfunction reduce EF

Question 16

Increasing ventricular afterload (arterial pressure)

Answer 16

decreases the stroke volume–see pg. 95

High aortic pressure (high blood pressure) raises the ventricular afterload. Essentially the ventricle has to raise its pressure to meet the aortic pressure. Unfortunately at this higher pressure the stroke volume is lower, meaning less blood is ejected per heart beat.

Question 17

Describe the effects of norepi qualitativley & on a P vs v graph & Length v Tension graph

Answer 17

Norepi increases stroke volume & contractility–both curves shift up

see pg. 96

Question 18

If filling pressure increases, preload ________, & stroke volume ________.

Answer 18

increase (via starling’s law)

Question 19

When looking @ pathologic P v V loops, the non linear isovolumic filling pressure & relaxation =

Answer 19

some sort of leakyness or regurgitation (& increase in volume)

Question 20

Look @ the screenshoted figures in the physiology questions folder

Answer 20

-

Question 21

In a healthy subject, running is usually associated with a decrease in the end-systolic volume of the right and left ventricles and with an increase in their stroke volume. The mechanism for this response is probably:

Answer 21

Starling’s law of the heart–see pg. 96

Running increases the cardiac output in part by increased sympathetic tone to the ventricle. This sympathetic input increases contractility moving the pressure volume curve (Starling’s Curve) to the left. A shift left means that at any aortic pressure, the end diastolic volume (left over blood in ventricle after contraction) is decreased, meaning that more blood was pumped out of the ventricle during the contraction = increased ejection fraction.

Question 22

Positive inotropy means

Answer 22

increase in ventricular contractility

Question 23

Which of the following changes in cardiac function can be related to the effects of norepinephrine?

Answer 23

1) increased contractility
2) increased rate of phase 4 depolarization of the sinoatrial node which increases heart rate
3) increased intraventricular pressure occurs to due increased contractility

The sympathetics increase the rate of depolarization of the pacemaker potential (phase 4 depolarization) and in doing so the cell reaches threshold sooner.

An increase in the force of contraction associated with an increase in end-diastolic volume is the preload effect. Contractility actually decreases preload (end diastolic volume) because of the increased ejection fraction induced.

Question 24

An acute decrease in ventricular compliance during diastole would be indicated by

Answer 24

a decrease in end diastolic volume

Question 25

Increased contractility leads to ______ ESPVR (stiffness) of the fully contracted muscle. This ______ ESPVR corresponds the slope of the ∆P/∆V relationship of the fully contracted ventricle. Therefore, _______ contractility increases ________.

Answer 25

Increased contractility leads to increase ESPVR (stiffness) of the fully contracted muscle. This increase ESPVR corresponds the slope of the ∆P/∆V relationship of the fully contracted ventricle. Therefore, increased contractility increases elastance.

The slope of the ESPVR is a reflection of the “stretchiness or stiffness” = elastance P/V of the fully contracted muscle.