Cardiovascular system
and
Heart ischemia (infarction)
incl. Detection of heart ischemia using
bioimpedance measurement
Andres Kink
2012
CONTENTS
CARDIOVASCULAR SYSTEM
MYOCARDIAL ISCHEMIA
ANATOMY OF THE HEART
CARDIOVASCULAR SUSTEM
AND CORONARY CIRCULATION
CARDIAC RHYTM
AND ARTIFICIAL PACING
PRINCIPLES OF RATE CONTROL
CARDIOVASCULAR SYSTEM
Energy as product of low temperature burning
of food products inside the body
To maintain life, every living animal organism must have
additional energy inflow as food and oxygen.
To save excess of food for future is possible due to
intracellular systems. But the same is not possible
for oxygen. Oxygen is gaseous, and to accumulate it
inside the body in reasonable quantity will take
too much energy.
In this text we will focus on energy as energy units (joule)
or as units of used oxygen to get energy.
CARDIOVASCULAR SYSTEM
Oxygen and food substrate delivery system for cells
Most of animals (not included fishes) have specialized
oxygen carrying system to maintain body tissues oxygenation:

Blood as solute to carry oxygen

Lungs as barrier between atmosphere and blood.

Circulation system as tubing system to carry oxygen rich blood
to every cell in body, and to collect waste from it.

Cellular system to produce ATP from energetic substances
and oxygen.
MYOCARDIAL ISCHEMIA
Definition:
Myocardial ischemia is an imbalance between oxygen supply
of the myocard and oxygen demand of the myocard.
In general ischemia is a decrease in the blood supply to
a bodily organ, tissue, or part caused by constriction or obstruction
of the blood vessels.
In the case of the heart the ischemia means a narrowing of
the coronary artery(s) sufficient to prevent adequate blood supply
to the myocardium.
This narrowing may progress to a point where the heart muscle
is damaged (infarction).
_____________________________________
MYΣ+KAPΔΊA = myocard (muscle + heart, in the contemp. Greek: ο μυς της καρδιάς)
(in Latin: MUS(CULUS) = mouse, muscle)
IΣX…+AΊMIA = isch(a)emia (Greek: stop+blood)
IN+FARCTUS = infarct
(Latin: in+filled)
MYOCARDIAL ISCHEMIA
Possible types of ishemia
GLOBAL MYOCARDIAL ISCHEMIA
TRANSIENT
REVERSIBLE
DYSFUNCTION OF SUBCELLULAR MECHANISMS
NO PERMANENT STRUCTURAL DAMAGE
PERMANENT STRUCTURAL DAMAGE
MYOCARDIAL ISCHEMIA
Ishemia as energy imbalance



Energy imbalance is result of non-equal oxygen supply
related to oxygen consumption.
Ischemia with myocardial cell damage is often
described in heart as myocardial infarction.
Myocardial infarction is not reversible process, cell
necrosis is healed by scar formation.
Short time myocardial ischemia is not dangerous,
because myocardial has limited protection against lack
of oxygen.
MYOCARDIAL ISCHEMIA
Epidemiology


Heart ischemic conditions are most leading reason
for mortality in world.
Silent myocardial ischemia is dangerous condition
witch leads very offen to myocardial infarction
(muscle tissue necrosis)
MYOCARDIAL ISCHEMIA
Ischaemic heart disease world map
DALY - WHO2004
ANATOMY OF THE HEART
CARDIOVASCULAR SYSTEM,
CORONARY CIRCULATION
and
Xxx
CARDIOVASCULAR SYSTEM, and CORONARY CIRCULATION
Physiology of the coronary arteries
CARDIOVASCULAR SYSTEM, and CORONARY CIRCULATION
Coronary artery disease
CARDIOVASCULAR SYSTEM, and CORONARY CIRCULATION
Coronary reserve
CARDIOVASCULAR SYSTEM, and CORONARY CIRCULATION
Special Features of Coronary Circulation


At rest, coronary blood flow BF = 5%
of cardiac output CO = 250ml/min = 60-80ml/100gm/min
During exercise rises by 2 … 5 times
(coronary vasculature has a high vasodilator reserve capacity)

Coronary Blood Flow is phasic

Total Coronary Flow is greater during diastole
Therefore, the most crucial factors for perfusing coronary arteries
are
- aortic pressure
- duration of diastole
CARDIOVASCULAR SYSTEM, and CORONARY CIRCULATION
Myocardial O2 demand




The cardiac muscle always depends on aerobic oxidation
of substrates, even during heavy exercising
The cardiac muscle has the highest O2 uptake (VO2)
compared to other tissues of the body
(12…15 volume%; 7…9 ml O2/100gm/min)
This is achieved by a dense network of capillaries,
all is perfused at rest (no capillary reserve)
Maximal extraction of O2 from RBCs (almost no reserve of
O2 extraction)
CARDIOVASCULAR SYSTEM, and CORONARY CIRCULATION
Pressure volume area inside of ventricles
(left ventricle)
CARDIOVASCULAR SYSTEM, and CORONARY CIRCULATION
Cardiac cycle
and
pressure-volume area
Cardiac Output (CO)
determined thru
Heart Rate (HR) and
Stroke Volume (SV)
CARDIOVASCULAR SYSTEM, and CORONARY CIRCULATION
Frank Starling Curves



Ability of the heart to change
force of contraction in response
to changes in venous return.
If EDV increases, there is a
corresponding increase in
stroke volume, suggesting heart
failure and inotropy.
Reduced stroke volume
suggests increased preload
and decreased ejection fraction.
CARDIOVASCULAR SYSTEM, and CORONARY CIRCULATION
Cardiac Output



Cardiac Output is the volume of blood (in liters)
ejected by the heart in one minute
Stroke Volume is the volume of blood (in liters)
ejected by the heart in one beat
When the body is under stress (physical, emotional),
the heart tries to increase cardiac output …
by increasing the rate according to this formula
Cardiac Output = Heart Rate x Stroke Volume
CO = HR x SV
CARDIOVASCULAR SYSTEM, and CORONARY CIRCULATION
Bradycardia or “low heart rate”
Artifical heart assisting devices
The first artificial pacemaker to maintain heart rhythm was
induced by Steiner in Germany to avoid cardiac arrest
as a side effect of chloroform anaesthesia.
Steiner's study (1871) was performed in chloroform arrested
hearts of horses, donkey, dogs, cats and rabbits.
In the next year the same method was used in humans
by Green in the United Kingdom.
The first pacemakers had interrupted galvanic (direct-current)
stimuli and were connected by 13 cm long needles directly
to the myocardium.
Modern era of implantable pacemakers


The first implantable pacemaker was made by Swedish inventor
Dr. Rune Elmqvist, and implanted in 1958 by Dr. Ake Senning.
The first demand pacemaker was introduced by Berkovits
in June 1964.
Demand pacemaker have additional sensing unit to avoid
competition with heart own pacemaker (sinus node),
and to save battery energy.
Pacemakers
Sensory systems

ECG based interval measurements

Movement analysis (acceleration, ..)

Temperature measurement

Impedance based



Lung impedance
Intraventricular impedance, mostly right
ventricle
Myocardial impedance
Rate adaptive pacing



Heart rate is regulated to maintain body
energetic needs
First generation target was night time heart
rate reduction
New generation is multisensor (accelometer,
ECG, temperature, bioimpedance based, …)
based optimal heart rate calculation
Why Rate Response?



Rate response is the pacemaker’s ability to increase
the pacing rate in response to physical activity or
metabolic demand
Rate response mimics the healthy heart
Special sensor(s) required
 Accelerometer
 Piezoelectric crystal
 Minute ventilation (transthoracic impedance)
 Blood temperature
 Single or combination
Intracardiac bioimpedance measurement
Normal Chronotropic Response
Chronotropic Incompetence
If the patient’s heart cannot increase its rate appropriately
in response to increased activity, the patient is
chronotropically incompetent
Chronotropic incompetence (definitions):


Maximum heart rate < 90% x (220 - Age)
Maximum heart rate < 120 bpm
Causes
 aging
 drugs
 heart disease
Sensors


Rate-responsive pacemakers rely on sensor(s) to detect
patient activity
The ideal sensor should be
 Physiologic
 Quick to respond
 Able to increase the rate proportionally to the patient’s
need
 Able to work compatibly with the rest of the
pacemaker
 Able to work well with minimum energy demands or
current drain
 Easy to program and adjust
Types of Sensors


Activity sensors
 Vibration sensors (piezoelectric sensors)
 Accelerometers
Physiologic sensors
 Minute ventilation
 Temperature
 Evoked response
 QT interval
 Closed loop system (CLS)

Virtual sensors
Activity Sensor/Vibration




Responds rapidly
No special pacing leads required
Easy to manufacture and program
Can be “fooled” by pressure on the can or footfalls (like walking
downstairs)
Activity/ Accelerometer




Responds rapidly
No special pacing leads required
Easy to manufacture and program
Cannot be “fooled” by pressure on the can
Minute Ventilation
Minute Ventilation




Uses low-level electrical signals to
measure resistance across the chest
(“transthoracic impedance”)
Requires no special sensor
Requires bipolar pacing leads
Metabolic
Temperature




A thermistor is mounted in the lead (not the can)
Requires a special pacing lead
Metabolic
Response time can be slow
Evoked Response





Measures the QRS
depolarization area
Theory: the QRS
depolarization decreases
in area with exercise
Requires no special leads
May be affected by
changes in posture
Only works when the
device is pacing
QT Interval




Measures the interval
between the pacing
spike and the evoked Twave
Theory: This interval
shortens with exercise
Requires no special
pacing lead
Works only when the
device is pacing
Rate-Responsive Parameters to
Program







Base rate
Maximum tracking rate (in DDDR devices)
Maximum sensor rate
Threshold
Slope
Reaction time
Recovery time
Rate-Responsive Pacing
Threshold


Threshold is the amount of activity needed to
cause sensor activity
Can also be set to AUTO
 Measures variations in the last 18 hours of
activity
 Adjusts threshold automatically
 Displays Measured Average Sensor value
when pacemaker is interrogated
 Offset values can be programmed for more
fine-tuning
Threshold in Action
Threshold Programming
Considerations



AUTO allows the pacemaker to automatically adjust to the
patient’s changing activity levels
 Updates every 18 hours
AUTO with Offset can further fine-tune the settings
 A negative value makes it more sensitive (less activity is
needed to start rate response)
 A positive value makes it less sensitive (more activity is
needed to start rate response)
Considerations
 Patient age, lifestyle, everyday activities
 Patient’s fitness level (how likely is he to go jogging?)
 How well patient tolerates higher-rate pacing
Slope


Slope describes the sensor-drive pacing rate for
a given level of activity
AutoSlope
 Based on recent activity levels
Slope in Action
Slope Programming Considerations


Slope determines “how much” rate response is
given for a specific activity
Slope factors:
 The patient’s age, activities, lifestyle
 How well he can tolerate rapidly paced
activity
 How much rate response he needs
Reaction Time



When the sensor determines the patient needs
rate response, the Reaction Time parameter
regulates how quickly rate response is delivered
Programmable to: Fast, Medium, Slow
Consider the patient’s age, lifestyle, activities,
and how quickly he would need rate response
 Athletic patients probably need a faster
reaction time than couch potatoes
 Younger, fitter patients probably need a faster
reaction time than older, sedentary patients
Reaction Time in Action
Recovery Time




Recovery time determines the minimum
time it will take the sensor-driven rate at
the maximum sensor rate to go back down
to the programmed based rate
Similar to Reaction Time
Programmable as Fast, Medium, Slow,
and Very Slow
Programming considerations are the
usual:
 Patient age, lifestyle, activity levels
 Tolerance of rate transitions (can he
tolerate a rapid change in rate?)
Recovery Time in Action
Maximum Sensor Rate



Maximum Sensor Rate is the fastest possible rate
the pacemaker will pace in response to sensor input
It does not have to be the same setting as Maximum
Tracking Rate (fastest rate the pacemaker will pace
the ventricle in response to sensed atrial activity)
The Maximum Sensor Rate must be a rate that the
patient can tolerate


Maximum heart rate formula (220-age) x .90 is highest
possible setting
But if patient cannot tolerate the maximum heart rate,
set the Maximum Sensor Rate to a rate he can
tolerate
Threshold



Threshold defines how much activity must occur
before the sensor “sees” activity
Most patients do well with AUTO
If AUTO needs some further adjustment, use the
offset feature
 If sensor seems to react too often or too quickly,
program a positive offset
 If sensor does not seem to react soon enough or
at all, program a negative offset
Reaction and Recovery Times


Reaction time determines how fast rate
response goes to work
 If the patient does not tolerate abrupt changes
in rate, program this to SLOW
Recovery time determines how quickly a sensordriven rate goes back to the base rate
 MEDIUM is a good setting for most patients
 SLOW can expose the patient to prolonged
periods of pacing at a higher-than-necessary
rate
Slope


Slope is “how much” rate response a
patient receives once the sensor
determines rate response is needed
AUTO is a good middle-of-the-road
choice
Problem
How to control the pacing rate avoiding
imbalance between energy consumption
and energy supply of the myocardium,
estimating:
minute volume (MV) of
ventilation
relative stroke volume (SV)
diastolic time (tdiast)
AVOID ISCEMIA, NOT TO MEASURE IT !
Cardiovascular System with a Rate
Adaptive Pacemaker
Measurement of Cardiac (ŻC) and
Respiratory (ŻR) Impedances
Possible gates for heart rate control

No gates, fixed heart rate

Heart rate (slope control)


Ventricular volume, minimal stroke volume to
maintain body needs
Energy based control : ratio of PVA to
myocardial perfusion index during cardiac
cycle
Control system
Optimal v. min-max rate control

Optimal heart rate


Mostly
technical, not
from real heart
physiology
Underestimates
heart rate
variability
importance
Min-max rate gates
Allows to act as
supervisory
system for other
cont. algorithms
Possibility to
increase patient
cardiovascular
system
adaptation
Energy Balance
Simplified Calculations
EV
AVD
k
mc
myocardial
blood volume
oxygen uptake
(arterio-venous
difference)
energetical
coefficient
P
(balance)
Vmc tdiast EW

P
SV
R
hydraulic coronary resistance
tdiast
(energy balance)
AVD

R

k
SV
Simplified Calculations (cont.)
Rrest tdiast , rest  AVDrest SV  k
CRR 


R
tdiast  AVD
SVrest  k
coronary resistance ratio
R
rest
CR


max
CRR

2
to
6
R
min
coronary reserve
arteriosclerosis
healthy heart
the condition for
myocardium’s energy
balance
t
SV
diast
,rest


CR
t
SV
diast
rest
Volume Measurement - Theory



Gmeas = Gblood + Gp
Gp is parallel conductance of
muscle and must be removed
to estimate volume
Hypertonic saline bolus
injection

Conductance signal increases

Gb-ED & Gb-ES both increase

Conductivity of blood changes
but not the conductivity of the
muscle
Experimental setup with an isolated
pig’s heart
“ISHEMIA” data processing
Ischemic damage of
myocardial cells
ECG
Easy to measure,
Lots of data
SV, coronary perfusion
Difficult to measure
Small pieces of data
INFORMATION:
LIVE/DEAD
Rhythm type
INFORMATION:
Pump function
Ischemic status of cells
Diff. to get prognosis
Easy to get prognosis
Conclusion




Rate response is almost a “standard feature”
today
Pacemaker patients often suffer from at least a
degree of chronotropic incompetence
 Many who are not chronotropically
incompetent now will become chronotropically
incompetent with disease progression
There is no “perfect” sensor
Gate based control is important to avoid
“overpacing”
Conclusions

Our experimental studies and theoretical
speculations confirm that:


Increased concern over maintenance of
energy balance within the heart may be
addressed by novel pacing control algorithms
that require only relative stroke volume
information, derivable from bioimpedance
measurements.
New impedance measurement methods can
permit more reliable results to make such
feedback systems feasible for rate control.