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Characteristic acoustics of transmyocardial laser revascularization

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Characteristic acoustics of transmyocardial laser revascularization

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CHARACTERISTIC ACOUSTICS OF
TRANSMYOCARDIAL LASER REVASCULARIZATION
by
Valina Ghookassian
A Thesis Presented to the
FACULTY OF THE SCHOOL OF ENGINEERING
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE IN BIOMEDICAL ENGINEERING
December 1999
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UMI Number: 1409630
Copyright 2002 by
Ghookassian, Valina
All rights reserved.
_ _ ®
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I
This thesis, written by
Valina Ghookassian
under the guidance of his/her Faculty Committee and
approved by all its members, has been presented to and
accepted by the School of Engineering in partial
fulfillm ent of the requirements fo r the degree of
Master's of Science in Biomedical Engineering
April 19, 1999
D ate: ----------------------------------------------- ---------------------------------------------------------------------
Faculty Committe
Chairman
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noLuied^mentd
2 dedicate tliii research to my beloved grandmothers 3noush a n d
^ydnna ^or th e ir love, sacrifices, a J prayers th a t g u id ed my life . 3 L r
memory has been in stru m en tal in bringing this worh to completion.
2 u/ish to express my sincere g ratitu d e a n d appreciation to «2)r.
IdJarren Z . (jru n d fe s t, « 2 )r. 2 . )a v id Z . 2 ) drginio a n d « 2 )r. P am ez
liehada., fo r the ir advice, Support, a n d u s e fu l discussions throughout the
course o f this research.
naSiS J w o u ld l ih e to thanh 2 > r . jC a u rie Z e lb y , V flr. V h a
P a p a io a n n o u , W)rS. d a r o l P u p p e -d o w a n , W r . - J A e r t J . Q a lL u ,
a n d H ]r . ^ 4 d r ia n Q l e n n o f the d e d a r s -Z in a i W je d ic a l d e n te r fo r their
assistance, advice, a n d contribution to this study.
ii
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TABLE OF CONTENTS
ACKNOWLEDGMENTS.................................................................................................II
LIST OF FIGURES.....................................................................................................nv
LIST OF TABLES......................................................................................................... v
OBJECTIVE...................................................................................................... 1
Chapter 1............................................................................................................4
Introduction.............................................................................................................4
l . I Transmyocardial Laser Revascularization (TMLR)...............................4
1.1.1 Source o f the idea.................................................................................... 4
1.1.2 Historical development o f TMLR............................................................. 5
/. 1.3 Challenges to overcome........................................................................... 7
1.1.4 Why an excimer laser...............................................................................9
1.2 Shock waves and Detection Methods......................................................9
Chapter 2..........................................................................................................11
Effect o f Tissue Attenuation On the Laser-induced Acoustic Waves..............11
2.1 Materials and Methods.............................................................................1 1
2.2.1 Experimental Set-up............................................................................... 1 1
2.1.2 Experimental Protocol.......................................................................... 14
2.1.3 Data Analysis.........................................................................................15
2.2 Results.......................................................................................................16
2.3 Discussion................................................................................................23
Chapter 3......................................................................................................... 25
In-vivo study in a Porcine Model........................................................................25
3.1 Methods and Materials............................................................................ 25
3.1.1 Animal Model........................................................................................ 25
3.1.2 The TMLR System.................................................................................. 27
3.1.3 Data Acquisition System......................................................................... 30
3.1.4 Experimental Protocol...........................................................................30
3.1.5 Data Analysis........................................................................................ 31
3.2 Results...................................................................................................... 33
3.3 Discussion................................................................................................38
REFERENCES............................................................................................... 40
H i
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LIST OF FIGURES
FIGURE PAGE
1.1 Various chromophores and their absorption spectra........................ 8
1.2 Frequency range for a condenser microphone................................. 10
2.1 Ratio of peaks vs. hematocrit............................................................... 20
2.2 Ratio of peaks vs. fluence..................................................................... 21
2.3 Audio signals for three hematocrit levels............................................ 22
3.1 Animal model set-up for T M L R ............................................................ 26
3.2 Schematic TMLR set-up....................................................................... 28
3.3 Diagram of laser probe..........................................................................
29
3.4 Porcine heart, left view with typical locations for TMLR channels ... 32
3.5 Absorption of laser light for whole blood and porcine myocardium . ...35
3.6 Typical audio signal for porcine myocardium.....................................
36
3.7 Histology of a typical channel in porcine myocardium......................
37
iv
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LIST OF TABLES
TABLE PAGE
2.1 Hematocrit levels........................................................................................ 13
2.2 Averaged peak heights of the PSD..........................................................18
v
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OBJECTIVE
Coronary artery disease is the leading cause of death in industrialized
nations (US Department of Health and Human Services 1998). In addition to
drug therapies (nitrates, R-blockers, and calcium antagonists), invasive
procedures are also effective for treating ischemic heart disease.
Percutaneous transluminal coronary angioplasty (PTCA), percutaneous
transluminal coronary rotational atherectomy (PTCRA), and coronary artery
bypass grafting (CABG) are the current surgical revascularization methods.
(Schoebel et al., 1997) However, diffuse atherosclerotic disease, severe small
vessel coronary artery disease, or high periinterventional risk because of
severe left ventricular dysfunction, are unsuitable conditions for such invasive
surgical therapies (Anderson and Gonzales, 1998). Therefore, alternative
treatments are necessary for coronary anatomy unsuitable for mechanical
intervention. (Vincent etal., 1997)
To increase blood flow to the heart, several techniques including
endocardial incisions, cardiac acupuncture, fistula creation, pedicle grafting,
omental implantation, vessel implantation, and sympathectomy, have been
explored with limited success (Fisher et al., 1997). Transmyocardial laser
revascularization (TMLR) is an option for individuals with inoperable
conditions (Horvath and Cohn et al., 1997; Malik et al., 1997).
Currently in clinical trials, TMLR is a surgical procedure whereby laser
energy is used to create transmural channels in the left ventricle (Frazier and
1
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Kadipasaoglu, 1996). The channels allow direct ventricular blood flow to the
myocardium and facilitate revascularization of the ischemic tissue (Horvath
and Mannting, 1996). Advances in laser technologies now allow TMLR to be
performed on a beating heart; however, technical challenges still persist.
(Kadipasaoglu et al., 1997)
One of the difficulties in implementing TMLR is determining the exact
moment of ventricular penetration by the laser (Donovan et al., 1997). When
the laser beam penetrates through the myocardium and contacts the
interventricular blood, intraventricular microcavities, otherwise known as
steam bubbles or gaseous emboli are produced. (Grocott and Amory et al.,
1997) To minimize the creation of the gaseous emboli and to eliminate
unnecessary advancement of the laser fiber within the ventricle, it is essential
to design a mechanism to accurately detect the laser penetration of the
ventricle (Mehan and Hayden, 1998). Presently, no systematic method
exists (Vincent et al., 1997). The triggering of the laser during TMLR
procedure is entirely dependent upon the experience of the surgical team.
(Milano and Pratali et al., 1997)
Laser shock wave is a potential source for detection of ventricular
penetration (Tomaru et al., 1992). Based upon the acoustic phenomenon of
the laser beam at interfaces, it may be possible to precisely know the moment
of ventricular penetration (Djaiani and Hardy, 1997). By interfacing the
detection mechanism with the laser trigger, it is possible to unmistakably stop
the laser from firing (March and Guynn, 1995).
2
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In this study, the laser shock waves are examined as a possible
indication source for ventricular penetration of the laser beam during TMLR.
The study is separated into two distinct sections. In the first section, an in-
vitro study of laser attenuation with respect to the hematocrit level is
examined to gain an understanding of laser effects in blood. In the second
section, an in-vivo study of TMLR in porcine models and an in-vitro adaptation
of TMLR under controlled conditions are examined for differences in the
absorbance levels of the laser energy in blood and in myocardium. The
analysis of the two sections will reveal the feasibility of using laser shock
waves as sources of ventricular penetration during TMLR.
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3
Chapter 1
Introduction
1.1 Transmyocardial Laser Revascularization (TMLR)
1.1.1 Source of the idea
The idea of transmyocardial revascularization (TMR) was to provide
blood flow to ischemic myocardium by creating a series of channels in which
blood from the left ventricle (LV) perfuses the myocardium (Krabatsch et al.,
1996). Transmyocardial revascularization (TMR) is theoretically based on the
reptilian model of circulation, which is devoid of epicardial coronary arteries
(Grocott and Newman et al., 1997). In reptiles, oxygenated blood is delivered
to the myocardium through an extensive vascular network comprised of
intramyocardial sinusoids that directly connect the ventricle, arteries, and
veins (Kim et al., 1997). This method of myocardial blood flow accounts for
approximately 90% of the reptile’s myocardial blood supply. The other 10%
of the flow occur through coronary arteries that supply a thin peripheral layer
of the heart (Anderson and Gonzales, 1998). In TMR, transmural channels
are created either mechanically or by laser to connect the intramyocardial
vascular network to the left ventricle to provide oxygenated blood flow to
ischemic myocardium (Hardy et al., 1987).
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1.1.2 Historical development of TMLR
To improve regional myocardial blood flow, several different
techniques emerged before TMLR (Grocott and Newman et al., 1997). Based
on early histopathological works, Weam et al, in 1933, showed that the
human heart is capable of self-perfusion. Their studies hypothesized the
importance of epicardial conductance vessels and of extracardiac and
ventriculocoronary anastosomes for myocardial perfusion. (deGuzman et al.,
1997 ) Beck, in 1935, attempted to stimulate collateral circulation from the
epicardium by scarification of the epicardial surface (Sundt III and Rogers,
1997). Established in 1954, the Vineberg procedure was an attempt at
achieving direct myocardial revascularization other than the normal coronary
vasculature (Kohmoto and Uzun, et al., 1996). This method involved the
implantation of an internal mammary artery directly into the myocardium to
deliver oxygenated blood to previously ischemic tissue. (Shrager, 1994) In
1957, Mossimo and Boffi performed myocardial revascularization by
implanting T-shaped tubes into the myocardial wall (Cooley et al., 1996).
Further investigation to improve ventricular perfusion and performance
lead to techniques more closely related to TMR. In 1965, Sen et al. created
transmyocardial mechanical openings in canine models using acupuncture
(Sundt III and Rogers, 1997). Based on the works of Sen and his
colleagues, White and Hershey performed needle acupuncture in a 61-year-
old man using a 2.5 mm knobbed cannula (Cooley et al., 1996). They
reported the successful restoration of normal ventricular rhythm. According to
5
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their reported findings, the flow of blood from the left ventricle alleviated the
ischemia and recurrent ventricular fibrillation (Frazier and Kadipasaoglu,
1996). Acupuncture techniques were abandoned after the successful
introduction of aortocoronary bypass grafting and experimental findings that
showed fibrous occlusions of the channels and scarring caused by the
associated trauma (Schoebel et al., 1997).
In 1981, Mirhoseini and Cayton proposed using a laser to create
transmyocardial channels to provide direct perfusion of the myocardium with
ventricular blood (Donovan et al., 1997). The initial TMLR investigation using
a low powered (80 W ) CO2 laser was performed in conjunction to CABG on a
cardioplegic arrested heart. (Kadipasaoglu et al., 1997) In order to achieve
transmyocardial penetration at that low energy level required a cool and
arrested heart (deGuzman et al., 1997). A decade later, Mirhoseini et al.
used a high-powered (850 W) CO2 laser, which can cut through the entire
myocardium without interfering with the electrical activity of the heart (Trehan
et al., 1997).
Since then, TMLR technique has expanded and other lasers have
been used (Fisher et al., 1997). According to studies, patients who undergo
TMLR report a reduction in their Canadian Cardiovascular Society class
rating of III or IV angina to class I or II (Schoebel et al., 1997). A class IV
patient cannot perform any physical activity without discomfort and may
experience angina even at rest. Whereas a class III patient can perform
physical activity but with significant limitations. The individual may be able to
6
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walk one or two blocks or climb a flight of stairs without angina. (Anderson
and Gonzales, 1998)
1.1.3 Challenges to overcome
Although results of clinical studies continue to suggest that TMLR is
effective in relieving angina pectoris, data from animal studies have been
mixed (Kohmoto and Fisher et al., 1996; Kadipasaoglu et al., 1997). Various
groups have reported that channels created with CO2 and holmiumiyttrium-
aluminum-garnet (Ho:YAG) lasers have a short-term patency and significant
tissue damage. (Krabatsch et al., 1996 and Jansen et al., 1997) The tissue
damage is generally due to high absorption of laser energy. Although
absorption is necessary for tissue ablation, excessive absorption causes
severe scaring and subsequently, channel blockage.
Chromophores are responsible for absorption of light at different
wavelengths (Nava et al., 1995). Soft biologic tissue has several
endogenous chromophores, such as hemoglobin, proteins, and water, the
most abundant chromophore (-80% ) (Jansen et al., 1997). CO2 and Ho:YAG
lasers generate near and far-IR laser radiation, which are predominantly
absorbed by water, thus causing severe soft tissue damage (Kohmoto and
Fisher et al., 1996, 1997). The natural biologic healing response by
macrophages and fibroblasts is contingent upon the degree of tissue damage,
the greater the tissue damage, the greater the response, which also
contribute to channel blockage (Fleischer et al., 1996).
7
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M F ' '
Kir UF'M iaifi- N tY M
1E«4
>1U
— 1E- 4 f
-W f
— O j O I i
V I S
IE-1
IE-2
-1
-1 0
1E-4
0 . 1
1 J 0
tm vriM gSt (um)
Figure 1.1 Various chromophores and their absorption spectra.
Insufficient time for thermal relaxation of the ablated tissue is another
problem when using C 0 2 and Ho:YAG lasers. Thermal relaxation prevents
an explosion caused by buildup of water vapor pressure, thereby minimizing
collateral structural damage at high energy levels (Esenaliev et al., 1993).
The long pulse duration of C 0 2 lasers constitutes a problem of long exposure
and tends to increase the thermal damage (Kohmoto and Fisher et al., 1997).
A narrow zone of carbonization and a broad zone of thermal fixation are
central problems in channel patency and contribute to channel collapse
(Gassier etal., 1996; Jansen et al., 1997).
Cerebral embolization is an associated complication of the absorption
and vaporization of the myocardium during TMLR (Grocott and Newman et
8
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al., 1997). Transesophageal echocardiography (TEE) imaging within the left
ventricle during TMLR reveals bubbles or gaseous emboli. To prevent
cerebral damage by the gaseous emboli during TMLR necessitates a
cardiopulmonary bypass, further complicating the surgery (Knobelsdorff et al.,
1997).
1.1.4 Why an excimer laser
Eximer laser causes less thermoacoustic damage to the tissue in
comparison to both the CO2 and the Ho:YAG lasers. (Hillegersberg, 1997)
This reduced heat generation allows sufficient time for thermal relaxation and
minimizes gaseous emboli; hence, eliminating the need for a cardiopulmonary
bypass and the potential for cerebral damage (Esenaliev et al., 1993;
Hillegersberg, 1997). Furthermore, the pulse time of an excimer laser is
shorter than the time required for the heat to spread in the myocardium,
consequently confining the heat damage to a narrow target range (Mack et
al., 1997). Minimal tissue damage elicits negligible macrophage and
fibroblast intervention thus increasing the potential for channel patency.
1.2 Shock waves and Detection Methods
Rapid deposition and absorption of energy in a small space produces
acoustic waves (Eisenaliev et al., 1993). Shock waves, a specialized form of
acoustic waves, have a fast rise time, internal energy, and particle velocity (a
9
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virtual discontinuity of the pressure density). The fast rise time of shock
waves results in a high-pressure gradient, which may propagate deep into the
tissue (Flotte et al., 1991).
Shock waves are commonly detected using either piezoelectric
transducers or condenser microphones depending on the frequency range of
interest (Grad and Mozina, 1993). Piezoelectric transducers are frequency
specific and their thickness determines the sensitivity to the received pressure
waves. Because of their limited bandwidth, piezoelectric transducers are a
poor choice for recording sounds of unknown resonant frequencies (Zweig
and Deutsch, 1992). On the other hand, condenser microphones have a
broad band of sensitivity within the audible band and can record sounds at
different frequencies. (Brown et al., 1988) Hence, condenser microphones
are a better choice for recording sounds of unknown frequencies within the
audible band.
20 Hz 50 190 200 Hz 500 1000 2000 Hz 50001000020000
Figure 1.2 Typical frequency response of a condenser microphone.
80 dB
70 dB
60 dB
50 dB
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3
Chapter 2
Effect of Tissue Attenuation
On the Laser-induced Acoustic Waves
2.1 Materials and Methods
2.2.1 Experimental Set-up
Two liters of fresh porcine blood (less than 3 hours postmortem),
treated with EDTA, an anticoagulant, was obtained from a local
slaughterhouse. The blood was refrigerated for 24 hours in order to allow
sufficient time for the RBCs to settle. The next day, the RBCs had settled,
and the plasma was separated from the RBCs using a pipeting technique.
Thirty ml of the RBCs were pipetted into a 50 ml plastic centrifuge tube. The
tube was capped then shaken; from this tube, five separate samples of the
RBCs were taken and microcentrifuged. Using a ruler, the total height of the
fluid in the microcentrifuge tube and the height of the RBCs were measured
and divided to determine the hematocrit level. The five separate values were
averaged. The hematocrit level of the 30 ml sample of the RBCs was
calculated to be 84.7 + 0.5 %; this sample was labeled as stock.
Six additional samples of varying hematocrit levels were prepared.
Using a micropipet, small amounts of the stock were added to six separate
plastic 50 ml centrifuge tubes, each containing 30 ml of 0.9% NaCI. To
1 1
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ensure homogeneity of the RBCs, the stock solution was mixed before
removing each portion. The volume of stock needed to prepare the desired
hematocrit concentration of each solution was calculated using the following
equations where C is concentration of RBCs and V is the volume of fluid in
ml. Since saline does not have any RBCs, the term C saiine in equation (1)
goes to zero, leaving equation (2). Substituting equation (3) into equation (2)
and solving for Vsto*. gives equation (4), which was used to calculate the
exact volume of stock RBCs needed.
Cstock V stock Csaiine V saiine = C ne w V new (1)
Cnew Vnew
V stock — (2)
Cstock
Vnew = Vstock + V S aline (3)
Cnew Vsaiine
V stock = (4)
Cstock " Cnew
12
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The hematocrit range was based upon observation and preliminary
analysis of the sound recordings of various hematocrit levels as they were
lased. The six samples and their respective hematocrit levels are listed in
Table 2.1. Portions of each sample were microcentrifuged to implicitly
calculate the % hematocrit; however, the samples were very dilute and a
reasonable measurement could not be taken.
Table 2.1 The six samples and their respective hematocrit levels.
Sample Added Stock [ml] Saline Added [ml] Hematocrit
1 0.01 30 0.028
2 0.03 30 0.085
3 0.06 30 0.169
4 0.1 30 0.281
5 0.2 30 0.561
6 0.3 30 0.839
An omnidirectional condenser microphone (Radio Shack Cat. No 270-
092B, Fort Worth, TX) with a broad frequency response between 20-
15,000 Hz and sensitivity of -65dB + 4dB was connected to a personal
computer and used to record the sound generated during the ablation period.
The microphone was firmly sealed with a plastic film, attached to the
13
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fiberoptic delivery bundle, and submerged in the various samples. The
samples were lased with an excimer 308 nm laser (Spectranetics, Colorado
Springs, CO) with the repetition rate set at 25 Hz and at various fiuences. To
measure the energy delivered during lasing, the tip of the fiberoptic bundle
was held 3 inches away from the center of a power meter and the laser was
triggered. The energy was measured each time the fluence was changed.
The sound recordings were made at 44.1 kHz sampling frequency and 16 bits
mono settings on Microsoft Sound Recorder (Microsoft, Windows '95) with an
ESS sound card (model number 1888). Based on the Nyquist criterion, which
states that to sample an analog signal without aliasing, the sampling rate or
frequency must exceed twice the bandwidth, 22.05 kHz is the maximum
detectable non-aliased frequency for the recorded signals.
2.1.2 Experimental Protocol
The microphone was firmly wrapped in a thin plastic film and in
conjunction with the excimer laser fiber was immersed in each sample
carefully so as to avoid touching the container. Each sample was lased for
approximately 6 seconds three separate times at each of the four fluence
settings: 30, 35, 40, and 45 mJ/mm2 .
The samples were lased in increasing hematocrit levels to avoid
contamination of the more dilute sample. Additionally, the fiberoptic bundle
and microphone arrangement was rinsed with a 0.9 % NaCI solution before
changing samples to further avoid contamination of the samples.
14
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2.1.3 Data Analysis
The recorded signals were analyzed using the Spectral Analysis Toolbox
of MATLAB version 5.0. All Microsoft recorded sound files were read into
MATLAB and converted to a vector Y using the WAVREAD function. Each
vector was divided into overlapping sections, detrended, windowed by a
Hanning window, and zero-padded to length NFFT=256. A Hanning window
was chosen because its smoother-tapered ends with a roll-off of 18 dB/octave
gave better leakage control. NFFT of a power of 2 facilitated the use of a fast
radix-2 fast-Fourier-transform (FFT) algorithm developed by Tukey and
Cooley. (Lathi, 1992 and Jackson, 1996) Sufficient zero padding of the
samples before taking the discrete-Fourier-transform (DFT) revealed the
frequency of the isolated spectral peaks.
The power spectral density (PSD) was calculated for each vector Y using
Welch’s averaged periodogram method. To determine characteristic spectral
peaks or specific spectral trends for different hematocrit solutions, the PSD
plots were obtained for the different hematocrit levels and fluence settings.
Additionally, the PSD for the low frequencies, between 0 and 10 kHz, and
high frequencies, between 10 and 25 kHz, were plotted separately to allow
further detailed analysis of the peaks. The original sound waves were plotted
on a time scale to show amplitude changes in the sound recordings with
respect to changes in the hematocrit.
15
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2.2 Results
Each sound file was read into Matlab as a wave file to determine the
Power Spectral Density (PSD). Two distinctive sets of peaks appeared.
Individual sound files were then divided such that the PSD was obtained for
low (0 to 10 kHz) and high (10 to 25 kHz) frequency responses separately.
Distinctive peaks emerged at approximately 0.2 kHz, 1 kHz, and 3 kHz for the
low frequency response and at 19.2 kHz and 21.2 kHz for the high frequency
response. Peak heights were measured and recorded for each hematocrit
level at the specific fluences.
Using Microsoft Excel, all of the peak heights at a given fluence
were compared as follows: 0.2 k H z /1 kHz, 0.2 kHz/ 3 kHz, 1 kHz / 3 kHz, and
19.2 kHz/ 21.2 kHz. The results of the averaged ratios are listed in Table 2.2.
Comparison of the peak heights at low frequencies and at high frequencies
revealed different patterns. As seen in Fig 2.1(a), the ratio of peak heights at
1 kHz/ 3 kHz decreases exponentially as the hematocrit level increases for all
fluences tested. At low hematocrit, 2.82E-04, the ratio of the peaks 1 kHz/ 3
kHz is differentiated with respect to the fluence: however, this distinction
disappears at higher hematocrit of 1.69E-03. Notice that at higher hematocrit
levels all lines coincide irrespective of the fluence. Comparison of peak
heights at 1 kHz and at 3 kHz with respect to the peak at 0.2 kHz revealed no
detectable trend.
The ratio of the peak heights at high frequencies also followed a
pattern. The ratio of the peak heights 19.2 kHz/ 21.2 kHz increased with
16
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increasing hematocrit levels. Figure 2.1(b) shows a relatively linear
relationship for all four fluences tested.
The low frequency peaks were not compared to the high frequency
peaks because of significant magnitude differences.
The ratio of peaks at 1 kHz / 3 kHz versus fluence is graphed in Figure
2.2(a). Notice that hematocrit levels above 1.69E-03 do not appear on this
plot; their ratios were too small to fit the scale. From the data in Table 2.1, it
is apparent that the ratio of 1 kHz/ 3 kHz equilibrates for high hematocrit for
all fluences tested. Figure 2.2(b) reveals the opposite trend for the ratio of
peaks at 19.2 kHz/ 21.2 kHz. The ratio increases as the hematocrit increases
for all fluences tested. The graph begins with a relatively smooth flat line,
which dramatically rises with the increasing hematocrit levels.
The absorption of the laser light in three different hematocrit samples
0.028 %, 0.281%, and 0.839 %, are plotted in figure 2.3. Increases in
hematocrit level correlate with increased absorption, the greater the
hematocrit level, the larger the spikes. In the one-second interval, the 25
pulses of the laser indicative of the 25 Hz frequency are evident particularly in
the medium and high hematocrit levels.
17
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Fluence 30 mJ/mm2
Averaged Ratios of Peak Heights
Hematocrit
0.2kHz/1.0kHz 0.2kHz/3.0kHz 1.0kHz/3.0kHz 19.2kHz/21.2kHz
2.82E-04 0.9 147 176 0.12
8.46E-04 1.04 32 35 0.40
1.69E-03 1.35 11 7.10 1.85
2.81 E-03 0.39 1.31 3.40 3.87
5.61 E-03 0.34 0.77 2.24 5.31
8.39E-03 0.26 0.31 1.22 8.06
(a)
Fluence 35 mJ/mm2
Averaged Ratios of Peak Heights
Hematocrit
0.2kHz/1.0kHz 0.2kHz/3.0kHz 1.0kHz/3.0kHz 19.2kHz/21.2kHz
2.82E-04 1.12 134 134 0.14
8.46E-04 0.25 2.38 9.64 1.31
1.69E-03 0.35 2.20 5.00 2.53
2.81 E-03 0.84 2.25 2.69 4.44
5.61 E-03 0.40 0.92 2.33 5.30
8.39E-03 0.38 0.71 1.85 6.86
(b)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Fluence 40 mJ/mm2
Averaged Ratios of Peak Heights
Hematocrit
0.2kHz/1.0kHz 0.2kHz/3.0kHz 1.0kHz/3.0kHz 19.2kHz/21.2kHz
2.82E-04 0.80 110 129 0.12
8.46E-04 0.21 1.97 8.72 1.57
1.69E-03 0.12 0.42 3.52 3.22
2.81 E-03 0.63 1.89 3.05 4.07
5.61 E-03 0.56 1.25 2.19 5.63
8.39E-03 1.06 2.03 1.92 6.88
(c)
Fluence 45 mJ/mm2
Averaged Ratios of Peak Heights
Hematocrit
0.2kHz/1.0kHz 0.2kHz/3.0kHz 1.0kHz/3.0kHz 19.2kHz/21.2kHz
2.82E-04 0.79 70 91 0.15
8.46E-04 3.02 12.59 5.48 1.88
1.69E-03 0.05 0.18 3.46 2.94
2.81 E-03 0.19 0.50 2.67 4.63
5.61 E-03 1.19 2.75 2.37 5.36
8.39E-03 0.81 1.77 2.01 7.53
(d)
Table 2.2 Averaged ratios of the peak heights from the PSD of the
recordings taken while lasing the different hematocrit levels with the XeCI
308 nm excimer laser operating at the following fluences (a) 30 mJ/mm2
(b) 35 mJ/mm2 (c) 40 mJ/mm2 (d) 45 mJ/mm2.
19
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
200
180
n 160
> X 140
j S 120
* 100
^ 80
| 60
K 40
20
0
O .O O E + O O 2.00E-03 4.00E-03 6.00E-03 8.00E-03 1.00E-02
Hematocrit
(a)
30 mJ/mm2
35 mJ/mm2
40 mJ/mm2
45 mJ/mm2
N
z
x
c m
C M
■ »
x
X
C M
0 >
< 0
- - S ' ‘2 ___
: ii: 3 ■
■: ; -
. . . r . . .. . • ' ------
• \ r . • • ;. •
■ « ' ‘ : i • ‘:-
- 30 mJ/mm2
- 35 mJ/mm2
-40 mJ/mm2
- 45 mJ/mm2
0.00E+00 2.00E-03 4.00E-03 6.00E-03
Hematocrit
8.00E-03 1.00E-02
(b)
Figure 2.1 Ratio of peaks at low frequencies (a) and high frequencies (b)
versus increasing hematocrit at various fluences (mJ/mm2 ).
20
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Ratio 1 9 .2 KHz/ 21.2 K H z Ratio 1 KHz1 3 KHz
200
180
160
140
120
100
80
60
40
20
0
. - a w i . * : ' l ' r - \ •
-
•i V iu.'.
•w.
----------------^
;■ |
a — ---------------------------------------------
................... -1
i
= s f l — :■ ■ = !
25 30 35 40
Fluence mJ/mm2
45
■ 0.000282 Hct
0.000846 Hct
0.00169 Hct
50
(a)
35 40
Fluence mJ/mm2
0.000282 Hct
0.000846 Hct
0.00169 Hct
( b )
Figure 2.2 Ratio of peaks at low frequencies (a) and at high frequencies (b)
versus fluences (mJ/mm2 ) for varying hematocrit levels.
21
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
O S
U
■ 0 4
• O S
O S
AaSa Opal ail (ialaMl
a 0 2
V* A t * V p m
(a)
(b)
1
a *
as
0 L 4
1 “
0
0 3
04
0 5
O S
-I
An4»Sfnlb1 a Wand
I
o oi 02 03 04 as as 07 as os t
TkaaM
(C)
Figure 2.3 Audio signal for a one-second interval of various hematocrit
levels (a) 0.028 % (b) 0.281 % (c) 0.839 % lased with a XeCI 308 nm excimer
laser at a fluence of 35 mJ/mm2 .
22
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.3 Discussion
A better understanding of the laser effects in blood is necessary to
utilize the shock waves generated during the LV penetration of the laser
during TMLR. An in-vitro study of laser attenuation with respect to hematocrit
level was conducted.
Increased hematocrit translates into increased concentration of the
dominant chromophore, hemoglobin. Since chromophores are responsible
for the absorption of the laser light, it is logical that at different hematocrit
levels, different degrees of light absorption occurred and gave rise to varying
peak heights. All of the graphs were based on the ratios of the peaks from
the PSD because by taking the ratios of the peaks, the data was normalized
and changing trends were clearly seen.
In the preliminary phase of this study, the samples were not very dilute
and upon analysis, no distinctive peak trends appeared in the PSD. It was
hypothesized that because beyond a certain concentration, all of the light was
absorbed and no distinctions could be made with respect to the amount of
hematocrit in the samples above that critical concentration. This hypothesis
is supported by figure 2.1(a). At low hematocrit levels, the ratio of the peaks
at 1 kHz/ 3 kHz are differentiated with respect to the fluence. However, this
distinction disappears as the hematocrit level increases. All of the lines in this
reverse exponential graph coincide and are indistinguishable well before
3.00E-03 % hematocrit. Also, notice that in figure 2.2(a), again focusing on
the ratio of peaks at 1 kHz/ 3 kHz, only the three most dilute hematocrit levels
23
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
appear in this chart and that the lines flatten out with each increase in the
hematocrit concentration irrespective of the fluence. This further supports the
argument that there is a saturation level beyond which increasing the
hematocrit level or the fluence has limited affects. Also, notice that the slopes
of the lines in figure 2.1(b) become less steep at high hematocrit levels for all
fluences. The graph seems to be reaching a plateau or a critical saturation
level for the absorbed laser light.
The signals in Figure 2.3 again support the principle that increased
chromophores means increased light absorption. In Figure 2.3(a) the
hematocrit level is very low, 0.028%, and thus very little of the laser energy is
absorbed. The one-second interval of the signal reveals small and almost
indistinguishable spikes. However, the distinctions become apparent for
medium, 0.281%, and high, 0.839 %, hematocrit. The larger spikes appear
for higher hematocrit levels, which supports the principle of more
chromophores causes greater absorption and leads to larger spikes.
Additionally, the 25 spikes correlate with the laser operational setting of 25 Hz
frequency.
The findings in Figure 2.3 also support the sound changes heard
during the ablation period. As the samples were lased in the order of
increasing hematocrit, there was a change in pitch of the laser sound heard
by the investigators.
24
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 3
In-vivo study in a Porcine Model
3.1 Methods and Materials
3.1.1 Animal Model
Eleven farmer pigs weighing between 40-50 kg were used to model
myocardium ablation in TMLR. The animals were pretreated with bretylium
7-10 mg/kg, lidocaine 2-3 mg/kg, and propanolol 0.025-0.03 mg/kg. The
animals were intubated and mechanically ventilated at a constant respiratory
rate of 10 bpm with a tidal volume of 450 mL. An intramuscular injection of a
ketamine (20 mg/kg), acepromazine (0.5 mg/kg) and atropine (0.05 mg/kg)
cocktail was used for sedation. Anesthesia was induced by injecting
thiopental into an ocular vein and maintained by using isoflurane (1-2%).
The pigs were placed in the supine position. The heart was exposed by
either of two surgical approaches: a midline sternotomy or an intercostal
incision between the third and fourth ribs. The pericardial sac was carefully
opened to expose the anterior lateral wall of the heart. In both surgical
approaches, the heart was kept in its anatomical position to avoid any
disturbance to the normal blood flow and was moistened periodically with a
saline soaked gauze to prevent dryness. The schematic set-up of the animal
model is shown in Figure 3.1.
25
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Pentium 133 MHz
notebook computer
Condenser
microphone
6 inches
away from
the heart
Fiberoptic
1.4 mm O.D.
probe
Ventilator
advancement
Figure 3.1 Data acquisition set-up of the animal model during TMLR.
2 6
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All animals received humane care in compliance with the “Principles of
Laboratory Animal Care" formulated by the National Society for Medical
Research and the “ Guide for Care and Use of Laboratory Animals” prepared
by the Institute of Laboratory Animal Resources and published by National
Institute of Health (NIH publication, no. 83-23, revised 1985).
3.1.2 The TMLR System
The selection of the laser parameters for this study were based on
preliminary studies and histologic evaluations that determined the optimum
laser settings to provide well-circumscribed channels with the least thermal
damage. Transmyocardial channels were created on the LV approximately
1cm apart using a 308 nm XeCI excimer laser (Spectranetics, Colorado
Springs, CO). The laser beam was delivered to the tissue by UV fiberoptic
bundle with an outer diameter (O.D.) of 1.4 mm. The laser operated at a
fluence of 35 mJ/mm2, a repetition rate of 25 Hz and an advance rate of 0.037
in/s. Laser energy was measured at the tip of the fiber by means of a power
meter built inside each machine. The schematic set-up of the TMLR system
is shown in Figure 3.2. A scanned picture of the laser probe with the
fiberoptic catheter threaded through the plastic hand grip cover is shown in
Figure 3.3
27
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Catheter
Catheter
1.4 mm O.D.
Fiberoptic
probe
Controller
386-based Computer
Driver
XeCI 308 nm
Excimer
Laser
Figure 3.2 Schematic of the fiber advancement mechanism of the TMLR
system with the XeCI 308 nm excimer laser operating at a fluence of
35 mJ/mm2, an advance rate of 0.037 in/s and a repetition rate of 25 Hz.
2 8
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3.3 Diagram of the laser probe used to drill the TMLR channels.
The fiberoptic bundle is threaded inside the plastic hand held cover.
2 9
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.1.3 Data Acquisition System
To record the sound during the ablation period, an omnidirectional
condenser microphone (Radio Shack Cat. No 270-092B, Fort Worth, TX)
with a frequency response of 20-15,000 Hz and sensitivity of -65d B + 4dB
was connected to a personal computer and positioned in air approximately 6
inches away from the tissue. The recordings were made at a sampling
frequency of 44.1kHz and 16 bits mono settings on Microsoft Sound Recorder
(Microsoft, Windows ’95) with an ESS sound card (model number 1888).
Based on the Nyquist criterion, which states that to sample an analog signal
without aliasing, the sampling rate or frequency must exceed twice the
bandwidth, 22.05 kHz is the maximum detectable non-aliasing frequency for
the recorded signals
3.1.4 Experimental Protocol
The channels were created on the LV of the beating heart from the
epicardial to the endocardial surface. Transmyocardial penetration of the
ventricle was detected phonetically by a sound change during the ablation
and confirmed by the presence of blood seeping from the channel and into
the fiberoptic delivery mechanism. Care was taken to avoid lasing through
epicardial blood vessels and to avoid the apex and papillary muscles.
Possible locations of the lased channels are depicted in Figure 3.4.
Persistent bleeding of the channels was controlled by direct digital pressure
and if necessary a silk suture. The sound recordings were made
30
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
simultaneously as the transmyocardial channels were drilled. To insure
recording of the entire drilling procedure, the recorder was activated 1-2
seconds prior to lasing of the myocardium and was deactivated 1-2 seconds
after the lasing stopped.
3.1.5 Data Analysis
The recorded signals were analyzed using the Spectral Analysis
Toolbox of MATLAB version 5.0. All Microsoft recorded sound files were read
into MATLAB and converted to a vector Y using the WAVREAD function. A
time scale was generated and each vector Y was plotted in time to show the
acoustical waveforms generated during the ablation period.
31
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
/>/? M'na uzggos
.\ art it' arch
I ,• ft xiihrh irittn tttiv ru V
Rulnttniary trun k
/» ’ i-rtch iitcr/thalic
trunk
Uplift rent rid e
[*uhtttnntnj vein#
V a n d a l rtn u
f ' u r n
L e ft a u ricle
L e ft ventricle
'm in a l vena
n e t t
Right auricle
Figure 3.4 Porcine heart, left view. Typical location of lased
transmyocardial channels
32
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.2 Results
Recordings made during the in-vivo segment of the TMLR study were
contaminated with extraneous noise in the operating room. Analyses of the
results were inconclusive and therefore omitted.
An in-vitro adaptation of the TMLR segment of the study was
conducted under controlled conditions. Porcine myocardium submerged in
porcine whole blood was lased with the 308 nm excimer laser. The laser
penetrated through blood-myocardium-blood. A typical audio spectrum of this
arrangement is shown in Figure 3.6(a). As the laser penetrated through
blood, there was greater absorption. The amplitude drops once the laser
reaches the myocardium and again rises when the laser has penetrated
through the tissue and has emerged in the blood.
The opposite effect is seen in the Figure 3.6(b). In this arrangement,
the porcine myocardium is submerged in saline rather than whole blood.
Thus, the laser penetrates through saline-myocardium-saline. Here there is
less absorption when the laser is in the saline rather than when it is
penetrating through the myocardium as can be seen from the drop in the
height of the absorption peaks.
Figure 3.5 shows the absorption of a one-second interval of the audio
signal during TMLR. Frame (a) shows the absorption for whole blood
whereas frame (b) shows the absorption of the myocardium. Notice that the
scale in frame (b) is reduced 20 times to better show the amplitude of the
spikes. In each frame 25 distinctive spikes appear. The spikes have an
33
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
amplitude of approximately 1.0 a.u. for the whole blood and an approximate
amplitude of 0.02 a.u. for the myocardium.
Histological slides of the channels drilled during TMLR were also
prepared to study the tissue surrounding the drilled channels. One such
channel is imaged under polarized light and shown in Figure 3.7. The region
of thermal damage and carbonization is outlined. Notice that the dimensions
of the channel are 1.5 mm lengthwise and 0.5 mm in width. The zone of
thermal damage lies roughly 0.3 mm in periphery.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
34
1
08
06
04
"? 02
f 0
^■02
•0.4
-0. 6
•0. 8
Audn Sgml in 1 • Haml
- 1
0 0.1 0.2 0.3 0.4 0.5 06 0.7 0.8 0 9 1
Tmn(«)
(a)
0.05
A u d io S ig n a l in 1« Mnml
a 0. 01
Ti m ( a )
(b)
Figure 3.5 Absorption of laser light for one-second duration as it penetrates
through (a) whole blood and (b) porcine myocardium at a fluence of 35 mJ/mm .
35
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3.6 Typical audio signal for porcine myocardium lased with a 308
nm excimer laser at a fluence of 35 mJ/mm2 in (a) blood-myocardium-blood
and (b) water-myocardium-water arrangements.
36
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3.7 Histology of a typical channel drilled in porcine myocardium
using XeCI 308 nm excimer laser, as imaged under polarized light. The
channel and its surrounding region of apparent thermal damage are outlined.
37
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.3 Discussion
The hemoglobin content in blood is much higher than in the
myocardium and hence blood absorbs more of the incident laser energy than
myocardium. The higher optical absorption of the laser energy in blood
produces stronger shock waves. Notice that in Figure 3.6(a), the spikes are
much higher in amplitude when the laser is penetrating through whole blood
than through myocardium. This sharp increase in the magnitude of the shock
waves makes them more audible and hence distinguishable from the average
background noise. Specifically, a low frequency 25 Hz (lasing frequency)
tone becomes distinctly audible.
The difference in absorbance levels of the laser energy translated as a
change in the amplitude of the audio signal clearly indicate when the laser is
penetrating through blood and when it is penetrating through the myocardium.
The signal in Figure 3.6(a) begins with a flat line, which is the data collected
before the laser was fired. The sudden increase in amplitude occurs when
the laser is submerged in the blood. Once the laser penetrates the
myocardium, the absorbance level drops and is reflected by the drop in
amplitude. Finally, when the laser fully penetrates the myocardium and
emerges on the other side of the tissue, it is once again in contact with whole
blood and the amplitude increases. The amplitude in the beginning is not as
great as in the final stage because there was less blood covering the surface
of the myocardium.
38
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
In Figure 3.6(b) the amplitude of the signal is small then rises and
drops again. Here, the myocardium is submerged in saline, which has no
proteins and thus absorbs less laser light than myocardium. The increase in
amplitude occurs when the laser is penetrating through the myocardium
rather than the saline. The different layers are easily distinguishable based
on the amplitudes.
Isolated portions of the signals are shown in Figure 3.5. In the top
frame, one-second duration of the absorption of whole blood is shown.
Notice that there are 25 individual spikes indicative of the frequency of the
laser setting. Each pulse of the laser creates a shock wave in the medium.
The condenser microphone is not sensitive enough to record the ultra-high
frequency contents of the shock wave; however, it does record changes in the
magnitude of the overall shock wave. In the Figure 3.5(b), the signal obtained
from lasing the myocardium is magnified. The 25 spikes are again visible but
are not as easily distinguishable as in Figure 3.5(a). The drop in amplitude is
due to less light absorption in myocardium than in whole blood. The sudden
change in amplitude is a possible source of detection for ventricular
penetration during TMLR.
39
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Asset Metadata

Creator Ghookassian, Valina (author)

Core Title Characteristic acoustics of transmyocardial laser revascularization

School School of Engineering

Degree Master of Science

Degree Program Biomedical Engineering

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag engineering, biomedical,engineering, electronics and electrical,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Grundfest, Warren S. (committee chair), D'Argenio, David (committee member), Shehada, Ramez (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-288066

Unique identifier UC11342289

Identifier 1409630.pdf (filename),usctheses-c16-288066 (legacy record id)

Legacy Identifier 1409630-0.pdf

Dmrecord 288066

Document Type Thesis

Rights Ghookassian, Valina

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA