Medical instrumentation- cardiac arrhythmia, pacemaker, defibrillator

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UEEG002
Medical
Instrumentation
Cardiac Arrhythmia, Pacemaker, Defibrillator
D.Poornima, AP(Sr.Gr)/EEE,
Sri Ramakrishna Institute of Technology, Coimbatore

01
Cardiac Arrhythmias

• Arrhythmias occur when the electrical signals that coordinate heartbeats are not
working correctly.
• An irregular heartbeat may feel like a racing heart or fluttering.
• Many heart arrhythmias are harmless.
• If highly irregular or result from a weak or damaged heart can cause severe and
potentially fatal symptoms and complications.
• Cardiac arrhythmia refers to a group of conditions that cause the heart to beat
irregular, too slowly, or too quickly.
• There are several categories of arrhythmia, including:
 bradycardia, or a slow heartbeat
 tachycardia, or a fast heartbeat
 irregular heartbeat, also known as a flutter or fibrillation
 early heartbeat, or a premature contraction

• Arrhythmia monitor is a sophisticated alarm system
• Constantly scans ECG rhythm patterns and issues alarms to events that may be
premonitory or life threatening.
• Available in various degrees of sophistication
• Complex computerised systems have multi-patient set-ups and detect arrhythmias
of a wide variety at graded alarm levels
• Simpler instruments mostly look for widened QRS waves and heart timing for
premature beats.
• The arrhythmias, which the instruments are designed to detect are
• premature QRS complexes,
• Widened QRS complexes
• runs of widened complexes
• Each patient’s ECG may differ, the instruments generally base their determination
of abnormal or ectopic beats upon a reference obtained from the patient himself.
Arrhythmia Monitor

• Arrhythmia monitoring instrument will operate in the following
sequence:
– Stores a normal QRS for reference, particularly QRS width and R–R interval.
• An external ECG recorder activated during the store normal mode so that the reference heart
beats may be visually examined and determined as to whether they are truly representative.
– Initiates an alarm automatically, when ectopic beats are detected—either the
ventricular prematured or widened varieties.
– Gives alarm light signals whenever the prematured or widened ectopic beat
exist up to the rate of 6/min or 12/min.
– Detects and triggers alarm when artifacts are present at the source,
e.g. muscle tremor due to patient movement, base line shift and improperly
connected electrodes.

Basic Arrhythmia Monitoring System
Signal Conditioning:
• Single or multiple ECG leads may be used for
arrhythmia monitoring.
• ECG signal is amplified, filtered for diagnostic
purposes and digitized
Noise Detection:
• Unwanted noise and artifact still remain in ECG sig
nal even after filtering.
• Baseline wander, motion artifact and muscle noise
have some energy that overlaps the ECG signal
spectrum.
• Using specialized signal processing techniques,
unwanted noise and artifact are minimized.
• The ECG waveform is processed by two digitalfilters
: a detection filter and a classification filter.
• Resulting ECG output can be used for feature
measurements and beat classification

QRS Detection:
• Arrhythmia monitors require reliable R wave detect
ors for subsequent analysis.
• Most analog devices use various filtering methods
to extract the QRS complex by attenuating P and T
waves and artifacts.
• QRS detection is performed digitally in a two-step
process.
• The ECG is first preprocessed to enhance the QRS
complex while suppressing noise, artifact & non-QRS
portions of the ECG.
• The output of the preprocessor stage is subjected to
a decision rule that confirms detection of QRS if the
processor output exceeds a threshold.
Morphology Characterization:
• Based on shape of the QRS complexes beats are
separated into groups of similar morphology.

Timing Classification:
• Categorizes QRS complexes as on time, prematu
re or late.
• The observed R–R interval is compared to an
estimate of the expected R-R interval.
• Declared premature if R-R interval is less than 85
% of the predicted interval.
• Long if it is greater than 110% of the predicted
value.
Beat Labelling:
• A physiologic label is assigned to each QRS com
plex.
• The possible beat labels that can be attached by
a beat classification module are
 normal,
 supraventricular premature beat,
 Premature Ventricular Contractions, etc.
• Special detectors are employed to identify atrial
fibrillation or ventricular fibrillation

Atrial Fibrillation Detection:
• Based on detecting abnormal rhythms from the
timing sequence of QRS complexes.
Ventricular Fibrillation:
• Usually detected by frequency domain analysis.
The system is characterized as a narrow-band, lo
w frequency signal with energy concentrated in
a band around 5–6 Hz. It can be distinguished fr
om noise (16–18 Hz) by appropriately designing
bandpass filters.
Summary Statistics:
Characterize the cardiac rhythm over long time
periods.
• These statistics may be presented in the form of
a table or graphically.
• Trend plots of heart rate and abnormal beats are
particularly useful to the clinician.
Alarms: These are necessary to bring to the attentio
n of the nursing staff

02
Cardiac Pacemaker

• The rhythmic beating of the heart is due to the triggering
pulses that originate in an area of specialized tissue in
the right atrium of the heart known as sino-atrial node.
• The natural and normal synchronization of the heart
action gets disturbed when
 if this natural pacemaker ceases to function
 becomes unreliable
 if the triggering pulse does not reach the heart muscle
because of blocking by the damaged tissues.
 This shows as a decrease in the heart rate and changes
in the electrocardiogram (ECG) waveform.
 By giving external electrical stimulation impulses to the
heart muscle, it is possible to regulate the heart rate.
 These impulses are given by an electronic instrument
called a pacemaker’
Need for Cardiac Pacemaker

• A pacemaker basically consists of two parts:
 an electronic unit
 which generates stimulating impulses of controlled rate and
amplitude, known as pulse generator
 the lead
 which carries the electrical pulses from the pulse generator
to heart.
• The lead includes
• the termination which connects to the pulse generator
• the insulated conductors, which interface with
electrodes and terminate within the heart
• A variety of pacemakers with various
possibilities of operation are available
• Waveforms used for pacing are round-topped
rectangular pulses of 1–3 ms duration with
rates adjustable from 50-150 pulses per minute.

Types of Pacemakers
01.External Pacemakers - used when the heart block presents
as an emergency and when it is expected to be present for a short
time.
02. Internal pacemakers - used in cases
requiring long-term pacing
• Pacemaker itself may be implanted in the
body.
• The patient is able to move about freely
and is not tied to any external apparatus

• Employed to restart the normal rhythm of the heart in cases of cardiac standstill
• Used in situations where short-term pacing is considered adequate, while the patient is in the
intensive care unit or is awaiting implantation of a permanent pacemaker.
• The pacing impulse is applied through metal electrodes placed on the surface of the body
• Electrode jelly is used for better contact and to avoid burning of the skin underneath.
• Procedure is painful and therefore is used only in an emergency or a temporary situation
External Pacemakers

• The pulses may be delivered:
 Continuously
 When it is felt that the heart rate is below the pre-set value.
 The impulse frequency is independent of the electrical activity of the heart.
 On demand R-wave synchronous pacing
 Normally the pacemaker is inoperative but it is activated when the heart rate falls below the normal
or pre-set value.
 Beat to beat examination of the time interval between two R-waves is done.
 When this interval exceeds the pre-set value, the pacemaker comes into operation.
 Eliminates any competition between the heart’s own pacemaker and externally applied pacemaker
pulses.
• Pacing with external pacemakers through the chest requires a maximum of 150
V pulses across an impedance of the order 1 kW.
• Disadvantages
 The electrodes eventually tend to burn the skin and the electrical pulses become painful.
 Each impulse causes an uncomfortable contraction of the thoracic muscles around the
area of the electrodes

• The stimulating pulses can be preferably applied to a heart through a pacing catheter passing
through a vein and connected to the heart - called internal pacing
• The pacing current required is much less than when it is applied through the chest
• The voltage output of internal pacemakers is about 0–15 V and the available output current
ranges from 1–20 mA.
• They can be worn in the pocket or strapped to a limb or to the torso.
• The electronic circuit of a pacemaker consists of two parts
 Impulse-generating circuit - determines the frequency and duration of the impulses
 The output circuit.-determines the shape and amplitude of the impulse.
• There are three types of pacemakers based on the type of output waveform
 Voltage Pacemakers
 The current in the circuit is determined by the available voltage during the entire duration of the impulse.
 The voltage output from the pacemaker remains constant and changes of resistance in the circuit will
influence only the current.
 Current Pacemaker
 The current in the circuit is determined by the internal resistance of the pacemaker
 Current Limited Voltage Pacemakers
 This is primarily a voltage circuit, but the maximum current in the circuit is limited, preventing too large a
current impulse to circulate when there is a low resistance in the electrode circuit.

• The implantable pacemaker, along with its electrodes, is designed
to be entirely implanted beneath the skin.
• Its output leads are connected directly to the heart muscle
• Pacemaker is located just beneath the skin, the replacement of the
pacemaker unit involving relatively minor surgery is easy
• For any implantable circuit, the basic requirements are:
• The components used in the circuit should be highly reliable
• The power source should supply sufficient power to the circuit for
long periods of time
• The circuit should be covered with a biological inert material so that
the implant is not rejected by the body
• The unit should be covered in such a way that body fluids do not
find a way inside the circuit and thus short-circuit the batteries or
other malfunctioning of the circuit
Implantable Pacemakers

03
CARDIAC DEFIBRILLATORS

• Ventricular fibrillation is a serious cardiac emergency resulting from asynchronous contracti
on of the heart muscles.
• The uncoordinated movement of the ventricle walls of the heart may be due to coronary
occlusion, from electric shock or from abnormalities of body chemistry.
• Due to irregular contraction of the muscle fibres, the ventricles simply quiver rather than
pumping the blood effectively.
• Results in a steep fall of cardiac output and can prove to be fatal
Need for Defibrillator

• Ventricular fibrillation can be converted into a more efficient rhythm by applying
a high energy shock to the heart.
• This sudden surge across the heart causes all muscle fibres to contract simultane
ously.
• Possibly, the fibres may then respond to normal physiological pacemaking pulses.
• The instrument for giving the shock is called a defibrillator
• The shock can be delivered to the heart by means of
• electrodes placed on the chest of the patient (external
defibrillation)
• electrodes may be held directly against the heart when the
chest is open (internal defibrillation)
• Higher voltages are required for external defibrillation
than for internal defibrillation

Types of defibrillators
• In present-day defibrillators, an energy
storage capacitor is charged at a relati
vely slow rate (in the order of second
s)
• from the AC line by means of a step
-up transformer and rectifier
arrangement –AC Defibrillators
• from a battery and a DC to DC
converter arrangement.-
DC Defibrillators

• Oldest and simplest type.
• Construction is such that appropriate values are available for internal and
external defibrillation.
• A shock of 50 Hz a.c frequency is applied to the chest for a time of 0.25 to 1
second through electrodes.
• Applying electric shock to resynchronize heart is
known as Countershock.
• Defibrillation continues until patient responds
to the treatment
AC Defibrillator

• Consists of a step-up transformer with primary and secondary winding, and two
switches.
• A.C supply is given through switches and fuse to primary winding of the
transformer.
• The timing circuit is connected with switch - used to preset the time for the
defibrillator to deliver shock to the patient.
Working of AC Defibrillator

• A resistive and a simple capacitor network or monostable multivibrator forms the
timing circuit.
• Triggered with a foot switch or a push button switch.
• Various tapping are available along the secondary winding.
• Tappings are connected to the electrodes that delivers electric shock to the heart
of the patient.
• Voltage value ranging between 250 V to 750 V is applied for AC external
defibrillation.
• For safety reasons, secondary coil should be isolated from earth to avoid shock.
• For internal fibrillation voltage values between 60 V to 250 V is applied.

• To produce uniform and simultaneous contraction of heart muscles large currents
are used for external defibrillation.
• Results in skin burn under electrodes and violent contraction of heart muscles.
• It also results in atrium fibrillation and stops ventricular fibrillation

• In DC defibrillation, the energy stored in the capacitor is delivered at a relatively
rapid rate (in the order of milliseconds) to the chest of the subject through his/her
own resistance (R).
• DC defibrillator consists of auto transformer T1 that acts as primary of the high
voltage transformer T2.
• A diode rectifier rectifies the output
voltage from T2.
• It is connected to vacuum type-high
voltage over switch.
DC Defibrillator

• At position A, switch is connected to one end of the capacitor and capacitor
charges to a voltage.
• A foot switch present on the handle of the electrode is used to deliver shock to
the patient.
• Now the high voltage switch changes it position to B that makes the capacitor to
discharge to the heart through electrodes.
• To slow down the discharge from the
capacitor an inductor L is placed in
one of the electrode lead.
• This L induces a counter voltage that
reduces the capacitor discharge value

Thank you

Medical instrumentation- cardiac arrhythmia, pacemaker, defibrillator

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