30.03.15 Cardiac Function Assessment via Electrocardiography, Phonocardiography and 2D Echocardiography Conducted by: MinChul Park, Thomas Michael Allen Nairn and Hadassah Patchigalla (Class 14661: Group A5) Aims 1. To record an electrocardiogram and phonocardiogram and use these recordings to: a) Determine the link between electrical activity in the heart and the sequence of events in cardiac cycle b) Investigate a cause of variation in the ECG recording c) Investigate the relationship between the ECG recording, the heart sounds and the pulse as measured at the carotid sinus 2. To analyse data obtained by echocardiogram to determine changes in ventricular function following myocardial infarction Introduction The cardiac cycle is the sequence of events that occurs in the heart during one heartbeat, mainly involving diastole (filling) and systole (contraction) of the atria and ventricles on the left and right sides of the heart (Tortora & Derrickson, 2006). The specific sequence of events in the cardiac cycle can be represented in a graphical form by recording the electrical activity that occurs simultaneously in the heart to produce each contraction and relaxation (and so each subsequent movement of blood from the heart to the systemic or pulmonary circulation). The electrical activity that occurs in the heart is the constant firing of action potentials, originating from the sinoatrial node, which spread throughout cardiac muscle cells to initiate depolarisation and the corresponding contraction of cardiac myocytes, muscle fibres and tissue (Boron & Boulpaep, 2009). This electric current generated in the heart with each cardiac cycle can be detected at the surface of the body (i.e. the skin) because the extracellular fluid of the body acts as a volume conductor, and it is this property that is used in the creation of an electrocardiogram (ECG), a graphical representation of the depolarisation and repolarisation of the heart muscle (Tortora & Derrickson, 2006). The ECG is recorded using four electrodes placed on both arms and both legs of the subject (six more electrodes are placed at strategic points on the subject's chest to record a standard 12-lead ECG, but this experiment obtains only a 6-lead ECG). Leads are created between sets of two electrodes and are used to measure the potential difference (voltage) between them, thus measuring the potential difference between two points of the body and so the conduction of electrical activity within the heart. This is achieved as a potential difference between two
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30.03.15
Cardiac Function Assessment
via Electrocardiography, Phonocardiography and 2D Echocardiography
Conducted by: MinChul Park, Thomas Michael Allen Nairn and Hadassah Patchigalla (Class 14661:
Group A5)
Aims
1. To record an electrocardiogram and phonocardiogram and use these recordings to:
a) Determine the link between electrical activity in the heart and the sequence of events in cardiac
cycle
b) Investigate a cause of variation in the ECG recording
c) Investigate the relationship between the ECG recording, the heart sounds and the pulse as
measured at the carotid sinus
2. To analyse data obtained by echocardiogram to determine changes in ventricular function
following myocardial infarction
Introduction
The cardiac cycle is the sequence of events that occurs in the heart during one heartbeat, mainly
involving diastole (filling) and systole (contraction) of the atria and ventricles on the left and right
sides of the heart (Tortora & Derrickson, 2006). The specific sequence of events in the cardiac
cycle can be represented in a graphical form by recording the electrical activity that occurs
simultaneously in the heart to produce each contraction and relaxation (and so each subsequent
movement of blood from the heart to the systemic or pulmonary circulation).
The electrical activity that occurs in the heart is the constant firing of action potentials, originating
from the sinoatrial node, which spread throughout cardiac muscle cells to initiate depolarisation
and the corresponding contraction of cardiac myocytes, muscle fibres and tissue (Boron &
Boulpaep, 2009). This electric current generated in the heart with each cardiac cycle can be
detected at the surface of the body (i.e. the skin) because the extracellular fluid of the body acts
as a volume conductor, and it is this property that is used in the creation of an electrocardiogram
(ECG), a graphical representation of the depolarisation and repolarisation of the heart muscle
(Tortora & Derrickson, 2006).
The ECG is recorded using four electrodes placed on both arms and both legs of the subject (six
more electrodes are placed at strategic points on the subject's chest to record a standard 12-lead
ECG, but this experiment obtains only a 6-lead ECG). Leads are created between sets of two
electrodes and are used to measure the potential difference (voltage) between them, thus
measuring the potential difference between two points of the body and so the conduction of
electrical activity within the heart. This is achieved as a potential difference between two
electrodes is created only when an action potential spreads through the cells of the myocardium
causing depolarisation of part of the muscle while the rest remains polarised (Klabunde, 2004). A
wave of depolarisation spreading towards a positive electrode will produce a positive ECG
deflection, and a wave of depolarisation moving away from a positive electrode will produce a
negative ECG deflection (conversely, a wave of repolarisation moving towards a positive electrode
will produce a negative ECG deflection, and a wave of repolarisation moving away from a positive
electrode will produce a positive ECG deflection) (Klabunde, 2004).
Therefore, as waves of depolarisation spread throughout the heart from the SA node to the atria,
AV node, bundle of His, Purkinje fibres and the ventricles, voltages will be recorded between pairs
of electrodes as positive or negative deflections from the isoelectric voltage (or baseline voltage)
on the ECG and the direction in which it spreads will be shown by the leads that produce a
positive deflection on the ECG.
The standard leads used to record the ECG in the frontal plane are the bipolar limb leads and the
augmented unipolar leads, which represent the potential difference between two limbs and
between one limb and an average of the other two limbs respectively. The limb leads are: lead I -
positive electrode at left arm, negative at right arm; lead II - positive at left leg, negative at right
arm; lead III - positive at left leg, negative at left arm. The augmented leads measure the potential
difference between one limb electrode (defined as positive) and the heart (defined as negative).
The augmented leads are: lead aVR - positive at right arm; aVL - positive at left arm; aVF - positive
at left leg (Boron & Boulpaep, 2009). These six leads can also be represented in Einthoven's
triangle used for vector analysis, as used in this experiment.
The position of the electrodes and the combination of leads created between them allows
multiple perspectives and therefore recordings to be created of the same electrical activity in the
heart - that is, each lead represents a different segment of the heart over which the voltage
difference is measured, and so the deflections recorded will differ depending on which lead is
aligned with the direction of the electrical activity of the heart (Boron & Boulpaep, 2009). This in
turn depends on the location and orientation of the heart within the thoracic cavity, as discussed
below.
In a standard ECG, the P wave represents right and left atrial depolarisation; the QRS complex
represents right and left ventricular depolarisation; the T wave represents repolarisation of both
ventricles. The P-T segment reflects spread of electrical activity from the atria to the ventricles,
however the depolarisation of the structures between the two chambers is not shown on the ECG
as the deflections are extremely small (Boron & Boulpaep, 2009).
A phonocardiogram is the recording of the heart sounds associated with the closing of the
atrioventricular and semilunar valves using a digital stethoscope or a microphone (as used in this
experiment). The closure of the valves does not itself produce the heart sound; instead the
vibrations in the ventricular wall produced by the closure are recorded by the microphone. The
two major heart sounds that can be easily attained by phonocardiography are the first heart
sound produced by the closing of the AV (mitral and tricuspid) valves, and the second heart
sound produced by the closure of the semilunar (aortic and pulmonary) valves (Boron &
Boulpaep, 2009).
An echocardiogram is the recording of the interior image of the heart using ultrasound waves.
Echocardiography is used to assess ventricular function as the imaging technique shows various
aspects of the heart in motion such as heart size, shape and the volume and velocity of blood
ejected from the ventricles (Tortora & Derrickson, 2012). Images of the heart shape gives
knowledge of how the ventricles act in a healthy heart and how it fails to pump properly in a
damaged heart. In this experiment a patient who had an anteroseptal myocardial infarction (MI) is
assessed (blockage of the coronary arteries ultimately resulting in cardiac myocyte deaths in the
anterior region of the interventricular wall) (Tortora & Derrickson, 2012). Using echocardiography,
evidence for akinesis (no heart wall movement during systole) and dyskinesis (heart wall
bulging/curving during systole) (MEDSCI 205 Laboratory Manual, 2015).
Method
For all ECG recordings the subject was lying supine, palms upturned and breathing normally.
Electrodes were placed on the subject's lower left and right forearms, lower left leg, and a
grounding electrode was placed on the lower right leg (note that this electrode was not used in
any lead configuration for recording the ECG). The labelled lead wires were attached to each
respective electrode to enable recordings to be made for leads I, II, III, aVR, aVL and aVF.
The voltage output (in millivolts) was recorded using the Chart software as an output graph
showing voltage deflections over the time period (in seconds).
Part A:
Experiment A: The ECG was recorded using lead II with subject:
a) Breathing normally for 30s; then
b) Breathing deeply, taking approximately 10s per inhalation and exhalation cycle, for 30s
Heart rate of the subject was then calculated by LabChart during normal breathing, inhalation and
exhalation and recorded.
Experiment B: A short segment of ECG was recorded for each of leads I, II, III, aVR, aVL, aVF. The
recording for each lead was made for the duration of 4 cardiac cycles, and the recording from the
output of the electrodes was paused in between each recording to allow the lead input to be
adjusted.
For each of the six leads, the magnitude of the peak positive and peak negative deflections within
the QRS complex were measured (in millivolts). This was achieved by setting the baseline
recording of the ECG as 0mV, then using the Chart software to measure the change in voltage
between the baseline and each of: the peak positive deflection (the R deflection in all but the aVR
recording) and the peak negative deflection (the Q deflection in all but the aVR recording).
The net deflection of the QRS complex (in mV) was calculated as follows: