ancillary factors venous pooling may significantly reduce …. cardiac output part...– venous...

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Ancillary Factors

Ancillary Factors Affect the Venous System and Cardiac Output

•  Gravity – Venous pooling may significantly reduce CO

•  Muscular Activity and Venous Valves

•  Respiratory Activity

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Venous return depends on how much blood returns to the heart, which is… affected by: • blood volume, • venous pressure and • intrathoracic pressure

•  What effect will increased venous return have on EDV?


Gravity o  Gravity acts on vascular volume

n  Mostly venous due to high compliance n  Preload decreases n  CO and arterial pressure fall n  Baroreceptor reflex

o  HR increases o  Vasoconstriction


Effects of Gravity on the Venous System and Cardiac Output

•  Gravity – Venous pooling may significantly reduce CO


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Effect Of Gravity on Venous Pressure

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Skeletal Muscle Pump (Increase Venous Return)


Muscular Activity and Venous Valves


Effect of Venous Valves


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Effects of Respiration •  Spontaneous respiration

–  Decreased intra-thoracic pressure results in a decreased right atrial pressure which enhances venous return

•  Mechanical ventilation –  Increased intra-thoracic pressure during positive-pressure lung

inflation causes increased right atrial pressure which decreases venous return

•  Valsalva Maneuver –  Causes a large increase in intra-thoracic pressure which impedes

venous return to the right atrium


Increase Pleural Negative Pressure (Increase Venous Return)


10

5

0

CA

RD

IAC

OU

TPU

T (L

/min

)

RIGHT ATRIAL PRESSURE (mmHg)

-4 0 4 8 12

15

IPP=

-4m

mH

g

IPP=

-2m

mH

g

IPP = INTRAPLEURAL PRESSURE

IPP=

-5.5

mm

Hg

IPP=

2m

mH

g

CARDIAC TAMPONADE


•  Intrathoracic pressure (which decreases during inspira;on and increases during expira;on) inversely affects venous return.

•  What effect will inhaling more deeply have on venous return?


•  Blood volume and venous pressure (which increases during venoconstric;on [constric;on of the veins]) directly affect venous return.

•  What effect will blood loss have on EDV?


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Factors that Facilitate Venous Return




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Plateau of CO curve determined by heart strength (contractility + ↑HR) ↑ Sympathetics ⇒ ↑ plateau

↓ Parasympathetics (HR↑) ⇒ (? plateau) ↑ Plateau Heart hypertrophy ⇒↑’s plateau Myocardial infarction ⇒ (? plateau) ↓ Plateau

The Cardiac Output Curve

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NORMAL

HYPEREFFECTIVE

-4 0 +4 +8

25

20

15

10

5

0

CA

RD

IAC

OU

TPU

T (L

/min

)


HYPOEFFECTIVE

CARDIAC OUTPUT CURVES

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•  Valvular disease ⇒ ↓ plateau (stenosis or regurgitation)

•  Myocarditis ⇒ ↓ plateau •  Cardiac tamponade ⇒ (? plateau) ↓ Plateau •  Metabolic damage ⇒ ↓ plateau

The Cardiac Output Curve (cont’d)

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Cardiac and Vascular function curves o  Point of equilibrium à predicts cardiac

output and central venous pressure o  Steady state of this particular system

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25

20

15

10

5

0

CA

RD

IAC

OU

TPU

T A

ND

VE

NO

US

RE

TU

RN

(L/m

in/m

)


-4 0 4 8 12 16

NORMAL CARDIAC

VR CURVE NORMAL

SPINAL ANESTHESIA

SPINAL ANESTHESIA

MAXIMAL SYMPATHETIC STIMULATION

SYMPATHETIC STIMULATION

MAX

Copyright © 2006 by Elsevier, Inc. 4/18/12 badri@GMC 109

Changes in contractility o  Digoxin:

n  inhibits Na-K ATPase n  Ca++ builds up

Ejection fraction is an indicator of contractility

EF= SV/EDV


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Changes in volume: mean systemic pressure

o  Decreased blood volume o  Decreased venous compliance

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Changes in Total Peripheral Resistance o  Constrict arterioles

n  Increased afterload n  Decreased venous return


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Pressure-Volume Loop o  ↑Preload o  ↑Afterload o  ↑Contractility

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Sympathetic and parasympathetic control o  Sympathetic

n  Stimulate: increase HR and increase vasoconstriction

n  Inhibit: decrease HR and decrease vasoconstriction

o  Parasympathetic (vagus) n  Stimulate: decreases HR

and causes vasodilation

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⇧ Stroke Volume = EDV-ESV

⇧ End Diastolic Volume

⇧ Preload

⇩ End Systolic Volume

⇩ Afterload

⇧ Contractility

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Summary of Factors That Influence

Cardiac Output and

Mean Arterial Pressure

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Control of Cardiac Output

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Factors that affect the Cardiac Output

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Cardiac Output Concept Map

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Myocardial Oxygen Consumption

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Myocardial Oxygen Consumption

•  Oxygen consumption is defined as the volume of oxygen consumed per minute (usually expressed per 100 grams of tissue weight)

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Myocardial Oxygen Demand is Related to Wall Stress

•  LaPlace’s Law

hPr

∝σ

Wall Stress

P

r

Wall Stress

h

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Factors Increasing Myocardial Oxygen Consumption

•  Increased Heart Rate •  Increased Inotropy (Contractility) •  Increased Afterload •  Increased Preload

–  Changes in preload affect myocardial oxygen consumption less than do changes in the other factors

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Magnitude & Distribution of CO at Rest & During Moderate Exercise

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In Summary…

•  Heart rate and stroke volume are the two factors that determine cardiac output.

•  Each of these is affected by many factors. •  Chronotropic agents affect heart rate while inotropic agents affect contrac;lity, which affects stroke volume.

•  Some factors (e.g., epinephrine and norepinephrine) affect both.

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Shift to the right and downwards of the (Frank-Starling )curve

by heart failure

Normal stroke volume

Decrease in stroke volume

Stroke volume with uncompensated heart failure Normal end-diastolic volume

Normal heart

Failing heart

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Answers to Ques;ons •  If HR increases, what will happen to cardiac output -‐Cardiac output increases. •  If SV decreases, what will happen to cardiac output?

-‐Cardiac output is expected to decrease (note that heart rate can be increased to compensate). •  What effect will sympathe>c nerve impulses have on heart rate?

-‐The norepinephrine released will increase heart rate. •  What effect will parasympathe>c nerve impulses have on heart rate?

-‐The ACh released will decrease heart rate. 4/18/12 badri@GMC 138

Answers to Ques;ons

•  What effect will increased heart rate have on stroke volume (if other factors stay the same)?

Stroke volume will increase to some extend and if further increases will lead to reduced filling ;me and SV will decrease (note that SV may be maintained if the cause of the increased heart rate also increases contrac;lity). •  What effect will increased venous return have on EDV? EDV will increase. •  What effect will blood loss have on EDV? EDV will decrease (note that the body has compensatory mechanisms to ini;ally maintain SV when blood is lost).

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Answers to Ques;ons

•  What effect will inhaling more deeply have on venous return?

Venous return will increase because deeper inhala;on lowers thoracic pressure more than normal. •  What effect will epinephrine have on stroke volume?

Stroke volume will increase due to the increased contrac;lity.

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Answers to Ques;ons

•  What effect will blocking calcium channels have on stroke volume?

Stroke volume will decrease. •  What effect will hypertension have on aGerload? AQerload will increase. •  What effect will hypertension have on stroke volume?

Stroke volume will decrease (and the heart will have to work harder to eject blood).

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The End

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