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coronary artery bypass grafting. In addition, in patients who had only venous grafts,
the risk of late myocardial infarction was 1.41 times higher (P <0.0001), the risk of
hospitalization for cardiac events was 1.25 times higher (P <0.0001), the risk of
redo cardiac surgery was 2.00 times higher (P <0.0001) and the risk of all late
cardiac complications was 1.27 times higher (P <0.0001) compared with patients
who used internal thoracic artery grafts. [79]. Subsequently, these studies were
questioned as new, modern, effective drugs and treatment methods emerged for
conservative therapy.
Later, the authors from the Cleveland Clinic published a twenty-year observation
of groups of patients, with the mammary coronary bypass technique used in one
group of 8,123 patients, and the method of using two internal thoracic arteries in
the second group of 2,001 patients. Comparison of these groups showed that the
survival rate of patients in the bimammary bypass group compared with the group
using one internal mammary artery after 7, 10, 15 and 20 years was 89% versus
87%, 81% versus 78%, 67% versus 58%, and 50% versus 37%, respectively
(p<0.0001). [82]. After analyzing seven large studies and examining data from
15,962 patients: 11,269 of whom used one mammary artery and 4,693 used two
mammary arteries, Dr. David Taggart and colleagues from Oxford concluded on
the better survival rate prognosis in patients from the group using two internal
mammary arteries (ratio risk for death 0.81; 95% CI 0.70-0.94) [118].
In June 2014, an international team of scientists published their meta-analysis of
100 randomized trials with 93,553 patients and 262,090 patient-years of follow-up.
The research was aimed to find out whether revascularization improves the
prognosis of survival rate compared to drug treatment in patients with stable
coronary artery disease. The meta-analysis was made using a Bayesian network,
which allowed to compare the results of treatments from different studies while
maintaining randomization. The analysis involved the selected randomized studies
comparing the effectiveness of drug therapy with myocardial revascularization by
coronary artery bypass grafting or percutaneous coronary intervention using
various generations of stents, including bare metal. The end point was considered
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death from any cause. The analysis resulted in the fact that coronary artery bypass
grafting showed the best survival rates compared with drug treatment (rate ratio
0.80, 95% credibility interval 0.70 to 0.91). Coronary artery bypass grafting is also
superior to drug therapy in reducing the risk of myocardial infarction (0.79,
0.63-0.99) and all known stent models in terms of the number of repeated
revascularizations (0.16, 0.13-0.20). In their conclusions, the authors note the
advantage of coronary artery bypass grafting in patients with stable cardiac angina
in terms of reducing mortality, heart attacks and repeated revascularization over
drug treatment, as well as the advantage of using the latest generation of stents.
[133]
In another meta-analysis, American scientists compared the results of 28
randomized studies involving 13,121 patients, of whom 6,476 were randomized for
revascularization and 6,645 were randomized for drug therapy only. In their
analysis, they used the Medline and Cochrane Central Register of Controlled Trials
databases to compare surgical or percutaneous revascularization with drug therapy
to determine the effect of revascularization on death and nonfatal myocardial
infarction in patients with stable coronary artery disease. The findings suggest that
revascularization with coronary artery bypass grafting or percutaneous intervention
in combination with drug therapy in patients with stable cardiac angina is
associated with significantly better survival rate than drug therapy. [60]
The ten-year results of the STICH international multicenter randomized study,
which included 1,212 patients with low (less than 35) ejection fraction, divided
into 4 age quartiles and randomized to the group of coronary artery bypass grafting
in combination with drug therapy and the group of only drug therapy, suggest that
coronary artery bypass grafting combined with drug treatment has significant
advantages in terms of all-cause mortality and the combination of all-cause
mortality and hospitalization for cardiovascular disease in younger patients
compared to older patients. Coronary artery bypass grafting in combination with
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drug treatment reduces cardiovascular mortality in patients with low ejection
fraction, regardless of age. [104]
Thus, coronary artery bypass grafting improved survival in patients with lesions of
the LMCA or three-vessel lesions, especially those involving the proximal LAD, in
patients with reduced left ventricular ejection fraction.
However, there is a new study that has once again brought controversy to the
understanding of the issue of CAD surgical treatment. The latest study by
American scientists ISCHEMIA [84] indicates that there is no difference in
survival rate between the group of patients with current optimal drug therapy and
the group of patients with optimal drug therapy with augmented myocardial
revascularization.
In their work, scientists included patients with moderate or severe ischemia
(according to non-invasive studies). Patients were evaluated on 341 points, 320 of
which were randomized. Patients underwent CT angiography to exclude >50%
stenosis of the left main coronary artery and to confirm coronary stenosis of more
than 70%. There were 5,179 randomized patients in total. The exclusion criteria
were mild ischemia according to functional tests, stenosis of the left main coronary
artery exceeding 50%, and the absence of significant stenosis of the coronary
arteries. The average age of participants was 64 years, 1,168 women, 2,122
patients with diabetes, 4,643 with an ischemic history. After stress tests, 2,799
patients were regarded as severe, and 1,701 as moderate. Of the 3,912 participants
who underwent CT angiography, 79% had three-vessel lesions, 86.8% had lesions
of the anterior interventricular artery, and in almost half of the cases in the
proximal region. Thus, the researchers randomized 5,179 people with severe or
moderate myocardial ischemia and significant coronary bed lesion into two groups.
The main endpoints were death from cardiovascular causes, myocardial infarction,
hospitalization for unstable cardiac angina, heart failure, or cardiac arrest. The
main secondary endpoints: death/myocardial infarction, as well as patient's quality
of life. The authors of the research conclude that the 10% advantage of surgical
tactics in such patients is not reliable, a high level of myocardial infarction in
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patients in the research was achieved precisely due to invasive tactics. The
difference in reaching control points is low and not reliable.
At the moment, only interim results of the research were published, submitted by
way of presentation, but it is worth noting some features of this research. No more
than 20% of enrolled patients to date reached the five-year endpoint and were
statistically analyzed. On the Kaplan-Meier plots, relative to all control points, the
group of patients with revascularization differs for the better by about 2.2%,
although this difference is not presented as significant. 23% of patients in the
conservative therapy group were revascularized within 4 years, while 16% did not
reach the control points. Concerning the limitations, the authors themselves note
that the study was not blind, and not included patients with a clinical pattern of
severe cardiac angina, with stenosis of the left main coronary artery and ejection
fraction of less than 35%. The completeness of revascularization was not assessed,
and the results of studies from clinics with a high rate of complications could not
be taken into account.
Thus, despite the advances in cardiology involving the new modern CAD therapy,
the mortality rate of patients with cardiovascular diseases remains high with
maximum mortality rates compared to other diseases. Probably, among patients
with optimal modern drug therapy, there are groups where, in addition to this type
of treatment, the best survival rates are achieved only in combination with surgical
treatment of myocardial ischemia. Accordingly, the development of diagnostic
methods for identifying these groups of patients remains relevant and requires
further research.
1.3 Invasive diagnostic methods for the arteriostenosis detection
As a few decades ago, invasive diagnostic methods remain supportive in the
study of the anatomy of the coronary arteries and stenosis detection. Until recently,
coronary angiography was considered a "gold standard" for detecting the coronary
bed stenosis and studying its anatomy. Indeed, coronary angiography is a valuable
diagnostic method and can provide an idea of the coronary bed anatomy, including
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the location, length, diameter of the heart arteries, the presence and degree of
coronary obstruction, the nature of obstruction, the presence and degree of
angiographically visible collateral blood flow. Coronary angiography has two main
objectives: 1) assessing the risk of death of the patient and the risk of developing
acute coronary events by assessing the degree of the coronary bed obstruction in
patients with coronary artery disease; 2) assessing the possibility of surgical
treatment and determining the optimal revascularization method. Predicting the
likelihood of a decrease in the stenocardia class and an improvement in the life
quality after surgical treatment. At the moment, the most common method of
coronary anatomy is to distinguish 3 main arteries: The anterior interventricular
branch of the left coronary artery (LCA), the circumflex branch of the left coronary
artery (LCx) and the right coronary artery (RCA) with right, left or balanced type
of coronary circulation of the myocardium. The degree of coronary artery disease
is defined as one-, two- or three-vascular lesion with 70% or more stenosis of the
vessel lumen, as well as lesion of the left main coronary artery (LMCA) with 50%
or more stenosis of the lumen. However, standard coronary angiography still has
significant limitations. First, the technical quality of angiograms can in many cases
make accurate interpretation difficult or impossible. So in a random sample of 300
angiograms taken in the 90s in New York, 4% had unacceptable quality, and 48%
were with technical flaws that make it difficult to objectively evaluate the study.
[76]. Second, there is a problem of subjective stenosis assessment, when the
degree of the coronary artery narrowing is equally estimated between different
specialists only in 51% of cases. Third, isolated coronary angiography provides
only anatomical data on the coronary bed and does not give any idea of the
functional significance of coronary stenosis. Finally, coronary angiography is not
capable of determining the degree of stability of atherosclerotic plaque, the
presence of a lipid nucleus and a thin fibrous cap. Serial angiographic studies made
before and after the development of myocardial infarction or other acute coronary
events indicate the presence of stenosis in the infarction-dependent artery which
was previously estimated as 50% and therefore did not require surgical
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intervention. [6, 78]. Nevertheless, coronary angiography is of great diagnostic
importance despite the existing limitations, since the degree of stenosis and the
number of affected arteries remain important predictors of long-term outcomes in
patients [20, 108, 109, 39, 110]. Many limitations associated with coronary
angiography have been minimized by the emergence of a number of ancillary
studies, such as optical coherence tomography, intravascular ultrasound, and
fractional flow reserve [124, 100].
However, coronary angiography, despite all the auxiliary and accurate
methods of additional diagnosis, is an invasive method, and therefore associated
with a higher risk of complications, remains a study inaccessible and an expensive
for the population, it cannot be used as a screening diagnostic method for all
patients. It is necessary to search for a non-invasive diagnostic method enabling to
resolve the issue of referring patients to undergo invasive methods, and being
easily accessible, fast, inexpensive, sensitive and specific, as well as providing an
idea of the artery function and its degree of lesion even before coronary
angiography.
Therefore, the search and development of new methods of non-invasive
diagnostics is a priority task of world cardiology.
1.4 Non-invasive methods for detecting coronary artery lesions.
One of these types of diagnostics is multislice computed tomography (MSCT)
of the heart vessels. This method can determine both anatomical and functional
changes in the coronary arteries. [37]. The accuracy of diagnostic data resulting
from this method can reach 90% in relation to coronary angiography [31, 99],
however, a patient is exposed to radiation, which can increase the risk of cancer
[38]. The same can be said about radionuclide tomographic types of CAD
diagnostics. Patients are exposed to radiation, while the accuracy of this method
does not exceed that of ultrasound [46, 95].
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Another modern method for non-invasive study of the anatomy and function
of the coronary arteries is magnetic resonance imaging (MRI). This method lacks a
radiation factor, but the availability is less, and the cost is higher than that of other
methods. Moreover, the accuracy, according to various studies, can be significantly
inferior to ultrasound among others. [31, 34, 51].
The most promising available and safe method for selecting patients for
invasive research methods and further identifying groups of patients in whom
revascularization results in an further improved prognosis are ultrasound diagnostic
methods.
The earliest developed method of non-invasive CAD diagnosis, which has become
widespread, is pharmacological tests or exercise tests, supplemented by
echocardiographic analysis, that is, stress echocardiography. Currently, stress tests
are of the greatest diagnostic value as a method for detecting changes in the
coronary bed and are recommended by international guidelines for the CAD
diagnosis, being the basis of non-invasive methods that can prove myocardial
ischemia. [41, 42, 90]. These methods are widely used to diagnose and obtain data
on the prognosis of survival rate in a population of people with suspected or
proven coronary artery disease [41, 42, 67, 90]. Visual assessment of
echocardiographic parameters during exercise test is a valuable prognostic method
and increases the diagnostic accuracy [13, 25, 70, 86, 103, 113, 114, 136]. The
parameters obtained during this study closely correlate with the results of coronary
angiography [27, 85], provide information about exercise tolerance in everyday life,
which has its own prognostic value proven by a large number of scientific papers
[40, 48, 59, 64, 69, 77, 81, 94], even in cases where no significant changes were
detected according to invasive coronary angiography data [43]. When comparing
the stress echocardiography data with samples using pharmacological agents, the
diagnostic value of both methods was comparable, however, the use of vasoactive
substances entails a greater number of complications at the time of the study [21,
30, 107, 116, 127]. Therefore, stress echocardiography with an exercise is
physiological and correlates with invasive diagnostic methods as well as with
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prognosis. However, conventional stress echocardiography does not provide any
quantitative parameters, leaving the technique dependent on the subjective
researcher's assessment [106]. Moreover, failure to reach the submaximal heart rate,
which occurs in about 27% of studies [83], reduces the method accuracy in
diagnosis and determination of prognosis [49, 83, 116, 134]. Supplementing
conventional stress echocardiographic tests with quantitative study of blood flow
velocities directly in the coronary arteries is a recent development of cardiologists.
1.5 Ultrasound examination of coronary bed function
In the early 90s, experts from different countries already reported on the
capabilities of ultrasound in the coronary artery anatomy diagnosis [28, 45, 63,
111], but these were isolated cases that had no practical value. Imaging percentage
was low, and successful cases were generally reported in children or young adults
with small chest sizes. [52]. Improvement of ultrasound equipment, new software
development made it possible to study coronary arteries without radiation and
chemical load on the patient. [2, 35, 119]. During this period, attention was paid
to transesophageal imaging of the coronary arteries [61, 130, 135] and contrasting
of the heart arteries with subsequent ultrasound assessment [12, 17, 97, 142].
Contrasting of the coronary vessels significantly improved imaging performance
and examining the function of all major heart arteries; however, the contrast agent,
in addition to toxicity, significantly increased the examination cost and, therefore,
reduced availability. The use of a transesophageal transducer imposed huge
limitations. Technical difficulties of implementation, inability of many patients to
easily sustain the examination, modest imaging capabilities identifying only small
proximal segments of the main heart arteries – all this subsequently prevented
these methods from being widely used in practice.
Further development of ultrasound technologies made it possible to study the
three main heart arteries in all segments using a serial multifrequency transthoracic
transducer for routine echocardiography. [15, 19, 72, 73, 128, 129].
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The frequency of successful LAD imaging in all segments reaches 97% -[ 15, 72,
73, 128, 129], and in the population of obese patients it reaches 80-83% [97, 119].
Ultrasound imaging is also capable of examining arterial and venous bypasses,
including the assessment of their function in 90% population of patients studied
[23, 44, 87, 89].
Some researchers published papers where they reported on possible measurements
of vascular wall thickness [101, 102, 120]. Earlier large anatomical examination
confirmed the reliability of ultrasound data [36]. The capability of measuring the
coronary artery diameter also became possible with the development of ultrasound
technologies [102, 120], while the degree of correlation with invasive diagnostic
techniques was quite high (r=0.93-0.94) [32, 53, 65].
These techniques are capable of examining coronary blood flow, direct
measurement of velocities and volumes of blood flow through the coronary arteries
upon an increase in the load, and theoretically provides a large amount of
information about the dysfunction of a certain coronary artery. With the
development of technologies and improvement of the imaging quality and Doppler
measurements, stress echocardiography protocols were developed to assess the
arteries when testing with pharmacological agents [68, 116].
Experimental invasive studies in animals [117, 132], and later in humans [131]
showed that, with the introduction of adenosine, the volumetric blood flow in
unaltered arteries increases three times compared to the initial. At the same time, a
close correlation was found between the volumetric ratio of blood flow and the
ratio of peak diastolic velocities [132]. Therefore, it became clear that a simple
measurement of peak diastolic velocities can indicate a change in volumetric blood
flow with a high degree of reliability. Measuring the ratio of peak diastolic
velocities before and after the administration of vasoactive substances has come to
be called the coronary reserve of blood flow velocities and is now actively used in
experimental and clinical studies [116]. It should be noted that the highest
correlation was achieved with transthoracic [57, 58] measurement of diastolic
peaks, compared with the transesophageal method [143]. Thus, the transthoracic
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method of measuring the coronary reserve in the heart arteries proved to be highly
reproducible [98, 112]. Currently, using modern serial multifrequency transducers
for routine echocardiography in determining and calculating coronary reserve
during pharmacological tests, the availability and success rate reaches 95-97% [ 80,
88, 126]. At the same time, both bypasses and native arteries are available for
research in most patients who have undergone CABG [11, 22, 24, 44].
Recently, data on CR in the heart arteries have also been obtained for exercise tests
in patients with suspected or proven CAD. The relationship between functional
changes of non-invasively measured coronary reserve, anatomical invasive data
and further prognosis of patients was shown [26, 138].
Therefore, the function of coronary blood flow and its assessment during exercise
tests was sufficiently studied. However, only symptomatic patients undergo
functional tests, according to current recommendations [66]. For many patients
with the clinical pattern of coronary artery disease, functional exercise tests remain
unavailable due to a number of factors, e.g. the patients with atherosclerosis of the
arteries of the lower extremities, joint diseases, pathology of the musculoskeletal
system, severe obesity, cannot exercise. It should be noted that up to 50% of
patients do not have specific symptoms of coronary artery disease before
myocardial infarction or death [33.], and therefore they do not undergo exercise
tests, nor further invasive diagnostics, which would reveal coronary circulation
issues.
In turn, the availability of conventional echocardiography is much higher,
and transthoracic examination of coronary arteries at rest does not require
additional equipment and requires standard transducers used by one ultrasound
specialist. The first diagnostic studies with a small number of patients appeared
long ago. They proved the relationship between altered blood flow at rest without
stress tests with stenoses visible during invasive studies, in particular, coronary
angiography. Japanese scientists described cases of recording ultrasound low blood
flow of the left main coronary artery with Doppler sonography at rest. Over time,
with the development of ultrasound technologies and the growing need for reliable
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