abnormal ST depression levels [44].
Echocardiography Echocardiography uses high-pitched sound waves sent through a device called a transducer to create images of the heart, seen in figure 2.5. A standard echocardiogram is also known as a transthoracic echocardiogram or cardiac ultrasound. Views of the heart are obtained by moving a the transducer to different locations on the chest. It is the most widely used diagnostic technique in cardiology as it can provide helpful information about the heart regarding the size, shape, pumping capacity and the location and extent of tissue damage. It can also provide an accurate assessment of myocardial ischemia [39]. However, it requires a skilled technician to operate the transducer and interpret the images and is therefore too cumbersome to use for continuous monitoring [28].
Figure 2.5: Figure showing a heart examination using transthoracic echocardiogram [49]
2.2 Thesis background
Having introduced the most relevant medical theory, the past research leading to this project can be presented.
First, a novel technique for monitoring the motion of the heart is given.
The motion data produced by this technique is used in this project. Then, the various cardiac functions that the technique is intended to monitor is presented. Lastly, a summary of the preceding studies regarding the tech-nique is given.
2.2.1 The novel technique
This thesis bases on a novel technique for automatic detection of various cardiac dysfunctions developed at The Intervention Centre, Oslo Univer-sity Hospital, Rikshospitalet, Oslo, Norway. The technique uses a motion
sensor attached to the heart, seen in figure 2.6. The motion sensor is a 3-axis accelerometer, measuring acceleration in the three directions specified in figure 2.3.
Figure 2.6: View of the heart showing the 3-axis accelerometer (motion sensor). Arrow indicates the circumferential motion [30].
Several pigs underwent interventions to simulate the various cardiac dys-functions the technique was intended to monitor. During the interven-tions, measurements from the motion sensor were collected into a dataset.
It is this dataset this project will take use of to make the machine learn-ing model. The reason for attachlearn-ing a motion sensor is that myocardial (heart muscle) dysfunction associates with motion abnormalities [64,66]. A miniaturized motion sensor attached to the cardiac wall may continuously monitor myocardial motion and automatically detect abnormalities. The proposed use of the technique is during and after cardiac surgery [22], as patients undergoing cardiac surgery may experience myocardial dysfunc-tion or ischemia during surgery or the first few days following the opera-tion [15]. Addiopera-tionally, present alternatives, ECG, and echocardiography, have their mentioned shortcomings.
2.2.2 Cardiac dysfunctions
The following paragraphs introduce the four different cardiac dysfunctions the model will try to classify from each other and baseline (healthy).
Consequently, they will also be denoted as classes. The paragraphs will explain its consequences as well as how they can effectively be inflicted to patients. Note that these functions cannot appear simultaneously.
2.2. THESIS BACKGROUND
Figure 2.7: Figure showing myocardial ischemia. [5]
Ischemia/Occlusion Ischemia is the term for what happens when the my-ocardium (heart muscle) does not receive enough blood, usually caused by a narrowing or blockage of one or more of the blood vessels supplying blood to the myocardium, as seen in figure 2.7. When this happens, the muscle cells which doesn’t receive enough blood starts to hibernate. When hibernating, the muscle cell remains viable, but contraction is chronically depressed because the muscle cells try to reduce energy consumption, lead-ing to a change in motion. But if the blood flow to the hibernatlead-ing muscle cells is too low, the muscle cells will die and cannot be recovered. Common symptoms are chest pain or discomfort which may travel into the shoulder, arm, back, neck or jaw. The pain tends to get worse with activity and go away with rest. Other symptoms are shortness of breath, sweating and a feeling of nausea. The most severe complication is a heart attack, which happens if the blood vessel becomes completely blocked. The part of heart muscle dependent on blood going through the obstructed blood vessel will die. A heart attack can be fatal if immediate emergency services is not re-ceived [14].
Myocardial ischemia is the most common cause of death in most Western countries and a major cause of hospital admissions [9]. It might be treated with medications that improve blood flow to the heart muscle. If the pa-tient does not respond to the medications, surgery is needed. The most common surgical procedures are (i) angioplasty and stenting and (ii) coro-nary artery bypass surgery. The former procedure forces the vessel open by inserting a stent while the latter creates a graft around the narrowed area by using a vessel from another body part.
Ischemia is inflicted onto the animals by using a vascular occluder nearby a blood vessel. The vascular occluder can be thought of as a pinch, thus hin-ders the blood flowing through the vessel. The blood vessel occluded is the left anterior descending artery (LAD) as seen in figure 2.8. The LAD and its branches supply the left ventricle with blood and is the most commonly
occluded for research purposes. It is the thickest of the heart’s chambers and is responsible for pumping oxygen-rich blood to tissues all over the body.
Figure 2.8: Illustration of the left ventricle, showing the basis and apex of the heart (also referred to as basal and apical regions), the LAD and the
location of the occlusion and the motion sensor (accelerometer) [28]
Low contractility/Esmolol Myocardial contractility refers to the ability of the myocardium to contract. An injection of the drug esmolol will cause a decrease in the force and rate of heart contraction [35]. It is commonly used in patients during surgery to prevent the heart rate increasing past normal rate, a condition known as tachycardia. Injection too much esmolol can cause the heart to become ischemic.
High contractility/Adrenaline Epinephrine, also known as adrenaline, is a hormone that can be used as a stimulant in cardiac arrest. Cardiac arrest is a sudden stop in effective blood circulation due to a stop in myocardial contraction. Epinephrine will increase blood flow as well as increase the heart rate, muscle strength and blood pressure [38]. Epinephrine is also used to treat anaphylactic shock, the allergic reaction causing the heart to be unable to pump enough blood. The details of the infliction of both esmolol and adrenaline onto the animals can be found in [22].
Fluid loading The intervention regarding fluid loading will enhance preload [47]. Preload is the measure of the initial stretching of the cardiac muscle cells at the end of diastole. Increasing preload will cause an increase in stroke volume while decreasing preload will cause a decrease in stroke
2.2. THESIS BACKGROUND
volume1. To determine how the accelerometer measurements responded to an elevation in volume loading (preload) of the heart, rapid intravenous infusion of body tempered isotonic saline2was performed.
2.2.3 Past research regarding the novel technique
A brief overview of the research regarding the novel technique will be given here3. Elleet al.[19] describes the first study of how accelerometers can be used to recognize cardiac ischemia. An accelerometer was sutured on the left ventricle in the region of blood supply of the left anterior descending coronary artery (LAD) on three different pigs. The LAD was totally occluded, and it was shown that the positive to negative peak systolic acceleration was reduced by approximately 40% from baseline to occlusion. A method to detect the early changes in the measured acceleration just after occlusion and during incipient ischemia were proposed using a difference value based on the FFT frequency pattern. The next study, by Halvorsenet al. [28] concluded that “...accelerometer detects myocardial ischemia with great accuracy. This novel technique has potential to improve monitoring of myocardial ischemia during cardiac surgery”. Further development was made by Grymyret al.[23]. A method for automatically calculating a 3D velocity vector from accelerations in circumferential, longitudinal and radial directions was made. The method facilitates insertion of the accelerometer as no precise alignment is required. The weakness with the studies is that the data is manually analyzed. When a substantial amount of data is collected, creating a rule-based system for prediction is difficult and poses a risk of missing out on valuable pieces of data that. This weakness alleviates with machine learning, which instead of following rule-based models, automatically identifies discriminative values in the measurements to predict the outcome of new motion data.
There has been a limited amount of research using machine learning and a motion sensor attached to the heart to detect various cardiac dysfunctions.
2.2.4 Accelerometer
The motion sensor used by the described technique is an accelerometer.
An accelerometer is a device that measures proper acceleration, meaning the physical acceleration experienced by an object. A conceptual drawing is shown in figure 2.9.
1The volume of blood pumped from the left ventricle per beat
2Sterile sodium chloride solution at physiological concentration (0.87%); used as a wound cleansing agent or irrigating fluid
3A detailed description of the technique can be found in the following research papers:
(sorted by date of publicity): [19, 22, 23, 28–30, 54]
Figure 2.9: A conceptual drawing of an accelerometer. It consists of a mass m, that can move relative to the accelerometer’s housing, a spring that connects the mass to the housingk, and a damper that dissipates energy
and keeps the spring-mass system from vibrating forever. The other constants are: the damping constantc, the position of the moving block relative to an inertial (stationary) frame of referencex, and the position of
the housing relative to the same inertial framey[1].
One can think of an accelerometer as a device that behaves like a damped mass attached at the end of a spring. When the device experiences an acceleration, the mass is displaced to the point that the spring is able to accelerate the mass at the same rate as the casing. The displacement is then measured to give the acceleration. Most studies using an accelerometer and machine learning have been in the field of activity detection and fall detection. Ravi et al. [53] uses an accelerometer worn near the pelvic region and manages to detect different activities such as standing, walking and running with high accuracy. The measurements from the accelerometer were used to calculate the average, standard deviation, energy and correlation, which in turn were used to classify the activities.
Albertet al. used the embedded smartphone accelerometer and a machine learning model to detect if the subject had fallen, as well as classifying the type of fall, were four types of fall were defined. The model achieved 98% accuracy in detecting fall and 99% accuracy in detecting the type of fall. These results demonstrate the potential use of accelerometer, suggests that it could be used to classify more fine-grained motions, such as cardiac function. No past research using an accelerometer to predict cardiac function, except for the studies mentioned in the previous paragraph were found.