Download Cardiac Mechanics The Heart

Cardiac Mechanics
Bioengineering/Physiology 6000
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
The Heart
• Structure
– Macro and micro
• Function
• Cells
– Pacemaker
– Conduction system
– Contractile myocytes
• Electrophysiology
– Action potentials
– Cell to cell coupling
• Mechanics
– EC coupling
– Cardiac cycle
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Average Hemodynamic Values
Variable
Cow
Man
Dog
Rabbit
Rat
Weight
(kg)
414
70
20
4
0.6
Cardiac
Output (ml/
sec)
680
110
42
5.2
1.2
Heart rate
(min-1)
71
76
99
288
349
Stroke
Volume
(ml)
570
87
25
1.1
0.21
16
18
32
22
Velocity in
ascending
aorta
690
560
.2 (5)
2700
.7 (1.4)
Cardiac Mechanics
Cardiovascular Physiology
By William R. Milnor
Bioengineering 6000 -- SystemsPhysiology I
Hummingbird Physiology
(What we need to explain)
• Heart is 20% of body volume
(largest of any animal), 2-3% of
body mass
• Heart rate varies from 30 (deep
rest), 500 (while perching), to 1200
(during high speed chases)
• Highest known mass-specific
metabolic rates among vertebrate
homeotherms (100 times greater
than elephant)
• Eats .5-8 times body weight per day
• Respiratory rate of 250 at rest
• Dives produce 7-10 G at speeds of
up to 100 kph.
R.K. Suarez, Hummingbird flight: Sustaining the highest massspecific metabolic rates among vertebrates. Cellular and
Molecular Life Sciences (CMLS) 48(6), 1992.
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Electrophysiology Review
• Pacemaker cells
– Neurogenic vs.
myogenic
– SA Node
– AV Node
– Purkinje Fibers
• Conduction system
• Ventricular myocytes
• The Electrocardiogram
(ECG)
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Excitation-Contraction Coupling
(ECC)
Cardiac Mechanics
Bioengineering 6000 CV Physiology
Structure-Function Relationship
Cardiac Mechanics
Bioengineering 6000 CV Physiology
EC Coupling
• Action potential causes influx of
Ca2+
• In mammals and birds, this
causes release of more Ca2+
• Ca2+ interacts with actin/myosin
to cause contraction
• Pumps gather up Ca2+ or
remove it from cell
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Calcium-induce Calcium Release
D.M.Bers Nature VOL 415 10 January 2002
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Calcium Measurement
• Calcium-sensitive dye
• Optical recording system
• Stimulate cell and synchronize with
optics
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
EC Coupling and Ca2+
+40 mV
-80 mV
diastole
50 ms
systole
1) Control
2) SR disabled (caffeine)
3) No Ca2+ entry (Ba2+)
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Ca+2 Measurements
http://www.youtube.com/watch?v=x3s2L2rvNLM
Cardiovascular Review
Bioengineering 6900 B-TAC
CICR Summary
• Calcium causes contraction
• Calcium is stored in the sarcoplasmic reticulum (SR)
• A small increase of Ca in the vicinity of the SR causes a much
larger release of Ca from the SR.
• Contraction can be graded
Action Potential
AP
Contraction
100
[Ca++]
gram
Cai from ICa
t
e
ng
Ra
Cai transient
mV
Tension [% max]
ov
er
Twitch (tension)
Rest
0
10-7
Cardiac Mechanics
Cai [M]
10-5
Bioengineering 6000 -- SystemsPhysiology I
Contractile Proteins
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Pretension, Frank-Starling, and Control of
Contraction
Cardiac Mechanics
Bioengineering 6000 CV Physiology
Pretension of Muscle (Explain?)
active tension
tension
Isometric
Contraction
with
Pretension
resting tension
time
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Isotonic Contraction and Preload
2g
2
2g
1
tension [g]
1g
1
1g
3
1g
2
isotonic tension
4
3
3
2
1
2g
3
2
1
2
3
pretension
1
muscle length
Cardiovascular Review
Bioengineering 6900 B-TAC
Intrinsic Regulation of Cellular Contraction
Action Potential
Twitch (tension)
Cai transient
mV
[Ca++]
Cai from ICa
t
Total tension
Twitch tension
120
Tension [% of maximum]
gram
100
80
60
40
20
0
1
1.5
2
2.5
3
3.5
Striation Spacing [µm]
4
• Spacing in contractile elements
– Determines contractility
– Adjusted passively
Passive (rest) tension
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Frank Starling Mechanism
Cell Level
Muscle Level
120
Whole Heart Level
Total tension
80
60
Twitch tension
40
20
0
1
1.5
2
2.5
3
3.5
4
Passive (rest) tension
Striation Spacing [µm]
•
•
•
•
•
Increased pretension can increase contraction
Cell: striation spacing
Muscle: pretension
Heart: Increased filling produces increased output
Does not require neural input
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Extrinsic Regulation of Contractility
AP
Contraction
Muscle
Effect of NE/E
er
100
ov
Total tension
Ra
ng
e
Tension [% max]
Cell
Rest
Muscle tension
Tension [% of maximum]
100
0
10-7
Cai [M]
10-5
Twitch tension
Passive (rest)
tension
Whole Heart
Muscle length
• Free [Ca]i is key component
• Positive inotropic agents:
– epinephrine: stimulate β receptors and increase
Ca influx and uptake (load SR)
• Negative inotropic agents:
– ACh: acts mostly on atria to shorten AP and
reduce [Ca]i
– Acidosis
Cardiac Mechanics
Bioengineering 6000 CV Physiology
The Cardiac Cycle and Afterload
Cardiac Mechanics
Bioengineering 6000 CV Physiology
Cardiac Cycle
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Cardiac Cycle
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Work of the Heart
• Work = Pressure * Volume
(force * distance)
• Cardiac cycle as PV diagram
• Efficiency
– Convert work to O2 consumption
– Compare to total O2
consumption
– 10--15% efficient
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Effect of E/NE on Contraction
tension [g]
maximum
isotonic tension
Pressure
Left Ventricle
4
3
2
1
muscle length
stroke
volume
• constant pretension
• reduced end-systolic
volume
• increased stroke volume
Volume
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Isotonic Contraction and Afterload
3g
2
tension [g]
1g
1
2g
3g
3
1
Afterload reduces
shortening
2g
1g
1g
5
4
isotonic tension
4
3
1g
2g
35
4
2
2
pretension
1
1
Cardiovascular Review
muscle length
Bioengineering 6900 B-TAC
Contraction at the Whole Heart
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Whole Heart Simulation
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Animal Hearts
Cardiac Mechanics
Bioengineering 6000 CV Physiology
Animal Hearts
• Air-breathing vs. water breathing vs. fetal
• Open Systems
• Separate left and right hearts
– Higher pressure good for rapid transport but require
lymphatic system
– Lower pressure (e.g., pulmonary) does not require
lymphatics and stays drier
– Both sides must have equal flow
• Shared ventricles
– Shunts from pulmonary to systemic (P->S)
– Allows adjustment of flow through lungs/gills
– Flows to both parts of circulation are not equal but
pressures are
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Open Systems
• Blood empties into body space
• Bathes tissues directly, blood in
small chambers
B
• Low pressure system (4-10 mm
Hg)
• Typically limited regulation and
low oxygen transport (with
exceptions)
• Insects bypass lungs and
transport oxygen directly so open
circulation does not carry oxygen
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Rigid Pericardium
• Compliant
– Minimal constraint
– Lubrication
• Non-compliant
– contraction of one chamber
assists filling the other
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Water Breathing Fishes
• 4-chambered, sequential heart with valves
• Gills perform gas exchange and also ion balance (like kidneys in
mammals)
• Gill circulation under higher pressure than systemic circulation
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Air Breathing Fishes
• Gills and vascularized air sac both provide O2; gills used for CO2
and ion balance
• Blood directed to different parts of system by the heart, also
between gills and air-breathing organs
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Amphibians (e.g., Frog)
• Shared ventricle but blood
separated
– Spiral fold
– Initial flow through pulmonary
because of reduced pressure
– Resistance to flow varies with
breathing (inhale reduces
resistance)
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Frog II
• Partial mixing of systemic blood flow
– Cerebral flow oxygenated, systemic mixed
• http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AnimalHearts.html
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Reptiles (non crocodilian)
• Partial separation of single ventricle
by 2 septa
• In ventricular systole, flow
determined by relative resistances
of pulmonary and system, e.g.,
breathing, diving
• Pressure differences in arteries also
directs flow: lower pressure value
opens first
•
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I
Fetal Heart
• Lungs collapsed so minimal
pulmonary flow
• Oxygenated blood comes from
placenta and some passes
through foramen ovale to left
atrium
• Rest of returning blood goes from
RV and most through Ductus
ateriosus to aorta
• At birth, lungs inflate, flow to them
increases, placenta flow gone so
systemic resistance increases
• Left side pressure increases, D.A.
and F.O close
Numbers are percentage of ventricular output
Cardiac Mechanics
Bioengineering 6000 -- SystemsPhysiology I