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CVS1 > Control Of Cardiac Output > Flashcards

Control Of Cardiac Output Flashcards

1
Q

Define afterload

A

Afterload – The load the heart must eject blood against (roughly equivalent to aortic pressure)

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2
Q

Define preload

A

• Preload – Amount the ventricles are stretched (filled) in diastole – related to the end diastolic volume or central venous pressure

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3
Q

Define total peripheral resistance

A

• Total peripheral resistance – sometimes referred to as systemic vascular
resistance – resistance to blood flow offered by all the systemic vasculature

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4
Q

What happens to pressure of fluid in a tube as it encounters resistance

A

• The pressure that the blood exerts drops as
it flows through ‘a resistance’
• The arterioles offer the greatest resistance so have the greatest pressure drop
• Constriction of the arterioles increases the resistance. This will cause pressure in the capillaries and on the venous side to fall but
will cause pressure on the arterial side to rise.
See slide for graph

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5
Q

What are the effects of decreasing total peripheral resistance?

A

• If TPR falls and CO is unchanged
– Arterial pressure will fall
– Venous pressure will increase

Dilate vessels at periphery - fall in arterial pressure - easier for bloof to flor through venous end - venous pressure will rise

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6
Q

What are the effects of increasing TPR?

A

• If TPR increases and CO is unchanged
– Arterial pressure will increase
– Venous pressure will fall

If the TPR increases - constriction of vessels - eg aretrial constricted
Pumping out against constriction
Arterial pressure rises
Harder for the blood to get through so venous pressure drops

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7
Q

What are the effects of increasing cardiac output?

A

• If CO increases and TPR is unchanged
– Arterial pressure will increase
– Venous pressure will fall

Heart pumping more
Increase in CO bu TPR the same
Arterial pressure increase but the venous pressure drops

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8
Q

What are the effects of decreasing cardiac output?

A

• If CO decreases and TPR is unchanged
– Arterial pressure will fall
– Venous pressure will rise

Decrease in CO - decrease in arterial pressure
Venous pressure rises 
Low co - increase in venous pressure 
Eg heart failure - otput not high enough
Increase in venous pressure
Peripheral oedema
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9
Q

Why and how does the heart respond to changes in demand for blood?

A
  • The heart must meet changes in demand for blood
  • If the tissues need more blood the arterioles and precapillary sphincters will dilate
  • Therefore peripheral resistance falls
  • The heart needs to pump more so that arterial pressure does not fall and venous pressure doesn’t rise
  • The heart ‘sees’ changes in this demand as changes in arterial blood pressure (aBP) and central venous pressure (CVP)
  • The heart responds to changes in CVP and aBP by INTRINSIC and EXTRINSIC mechanisms
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10
Q

What are intrinsic and extrinsic mechanisms the heart uses to respond to change in demand for blood?

A

Intrinsic - what the heart will do anyway

Extrinsic - sympathetic nervous system and circulating adrenaline

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11
Q

How is cardiac output calculated

A

Cardiac Output = Stroke Volume x Heart Rate

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12
Q

What is stroke volume

A

How much blood the heart pumps out per beat
Heard pumps out roughly 2/3 of edge

  • Stroke Volume = end diastolic volume – end systolic volume
  • SV = EDV - ESV
  • Typical stroke volume in ‘average text-book man at rest’ is about 70ml
  • This is about 67% of normal EDV
  • Can increase SV by increasing EDV or decreasing ESV
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13
Q

What is the relationship between venous pressure and heart filling

A

• In diastole the ventricle communicates with
the atrium and the veins but is isolated from the outflow tract
• The ventricle fills until the walls stretch enough to produces an intraventricular pressure equal to the venous pressure
• The higher the venous pressure the more the heart fills
• This relationship is the Ventricular Compliance Curve (pressure against volume)
• Compliance can be increased or decreased in diseased states

As ventricles fill, pressure rises - ventricular compliance curve
If ventricle very dilated and thin; increased compliance - if filled to same extent - pressure lower
Compliance is the reciprocal of resistance
Compliance reduced eg hypertrophy - more difficult to fill - steeper curve

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14
Q

What is the frank-starling law o the heart?

A
  • Like skeletal muscle – if you stretch the fibres of the heart before contracting, it will contract harder
  • This is the Frank – Starling Law of The Heart
  • The more the heart fills, the harder it contracts (up to a limit)
  • The harder the heart contracts, the bigger the stroke volume
  • An increase in venous pressure will fill the heart more – How much the ventricles fill depends on the compliance

Cardiac muscle = striated
If fibres stretched before contraction then they will contract harder
In the heart this means. The more the heart fills in diastole, the harder it will contract - bigger stoke volume, up to a limit

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15
Q

Describe the starling curve

A

Stroke volume against left ventricular end diastolic pressure

Increasing venous return leads to increased left ventricular end-diastolic pressure (LVEDP) and volume (‘increased preload’). This causes an increase in stroke volume, so that the extra blood is pumped out of the ventricle.
The ‘normal’ operating point at rest is with an LVEDP around 8 mm Hg and stroke volume of ~70 ml

As EDP increase (goes up as Edv does)
As ventricles stretched, pressur eincrease, bigger volume
Increase heart more - increase end diastolic pressure which increased the amount that is ejected as it increased the force of contraction

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16
Q

Describe the length tension curve for cardiac muscle

A

Contractile force as a % of max against resting length as % optimum

  • If sarcomere length is too short filament overlap interferes with contraction
  • In cardiac muscle also get an increase in calcium sensitivity as the muscle fibres are stretched

If fibres stretched up to optimum, increase in contractole force
At the shorter lengths, the fibres crossing over with each other
Steeper in cardiac than skeletal
As you stretch teh fibres they become more sensitive to calcium
For a given intracellular calcium level - increase sensitivity to calcium

17
Q

What ensures both sides of the pump maintain the same output?

A

• The increased stroke volume with increased filling of the heart is and INTRINSIC control mechanism
• It ensures that both sides of the heart pump maintain the same output so blood doesn’t accumulate in circulation
• The pulmonary and systemic circulations operate in series
– The same volume of blood pumped to the body must also be pumped to the lung

18
Q

What is contractility

A

The force of contraction for a given fibre length

19
Q

How does change in contractility affect the starling curve?

A

• Contractility is the force of contraction for a
given fibre length
• A change in contractility is seen as a change in the slope of the Starling Curve
• An increase in contractility will increase the force of contraction for a given left EDP
• EXTRINSIC factors such as sympathetic stimulation and circulating adrenaline/NA can increase contractility
• Reducing sympathetic stimulation will reduce contractility

20
Q

What factors determine cardiac output

A

How much the ventricle empties (end systolic volume) depends on

1) How had it contracts
determined by the end diastolic volume (how much the heart fills) and contractility (increased by sympathetic drive)
2) How hard it is to eject blood
determined by aortic impedance (roughly arterial pressure)

  • Cardiac Output = Stroke Volume x Heart Rate
  • Contractility and heart rate are controlled by the autonomic nervous system
  • A decrease in arterial BP will reduce parasympathetic NS activity and stimulate sympathetic NS increase heart rate and increase contractility
21
Q

What happens to TPR when metabolism of body increases?

A
  • If the metabolism of the body increases, then TPR will fall to supply more blood (tissues need more blood, vasodilation, decrease in peripheral resistance so…)
  • This will result in a fall in arterial pressure and an increase in venous pressure
  • The heart will respond by pumping more
22
Q

How does the CVS respond to eating a meal

A

SEE SLIDE - local vasodilation in gut
Increase venous pressure increases stroke vol as mor ethe heart fills more contacts
SNS activated increase heart rate and stoke volume
Increase in cardiac output
Brings pressure back to normal
Uses both intrinsic and extrinsic control mechanism

23
Q

What happens to pressure when standing up?

A

• Standing up causes ‘pooling’ of blood in legs due to effect of gravity on a column of liquid
Venous Pressure decreases -> CO decreases -> Arterial pressure decreases
• Now both arterial and venous pressure have changed in the same direction
• Cannot adjust be intrinsic mechanisms
• Baroreceptor reflex and autonomic nervous system increase heart rate AND increase TPR
• If reflexes don’t work you get postural hypotension
• Return to this in week 5

24
Q

What happens to CO when exercising?

A
  • Initially muscle pumping and venoconstriction returns more blood to the heart
  • Later decreased TPR also increases venous return
  • Very early response of increased heart rate (decrease parasympathetic drive, increase sympathetic drive)
  • Increased contractility (increased sympathetic drive)
  • Note: increased venous pressure alone would move ventricular function to the top (flat) part of the Starling curve

Venous pressure, heart rate and contractility increase so CO increases

25
Q

How is a jugular venous pulse measured?

A

• Measured in right internal jugular vein
• Biphasic pulse observed
• Direct column of blood connected to right atrium
• Pulse is see behind sternocleidomastoid muscle
– Estimate highest visible pulsations (JVP height) above sternal angle +4cm = JVP in cm H – Normally 5 to 8 cm H2O
• Can also be measured with a central line inserted into internal jugular vein or SVC
– Allows waveform to be seen
See slide for graph

26
Q

What conditions increase JVP?

A

• If the right side of the heart doesn’t pump blood out properly
• Volume overload with IV infusion
• If something impairs filling of the heat
• Case study last week
– Stab wound to the heart
– Distended neck veins
– Low arterial BP – loss of consciousness

CVS1 (20 decks)

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