How is thyroid linked to heart disease?
Heart disease can occur with both increased function of the thyroid gland and decreased function of the thyroid gland. When thyroid function is increased, heart rate increases and the work load of the heart increases. In severe cases heart failure may occur. A peculiarity of heart failure associated with increased thyroid function is that it is a high output failure unlike the usual heart failure in which the pumping function of the heart is reduced and low output failure occurs. While in low output heart failure the extremities are cold, in high output failure due to increased thyroid function, the extremities of the limbs are warm. Another important issue with increased thyroid function is that the heart rhythm may become irregular. This condition is known as atrial fibrillation, in which the upper chambers of the heart generate very fast irregular signals, but fail to contract well. Hence the blood stagnates in some parts of the upper chambers (left atrium) and clots may form. If these clots migrate to the blood vessels of the brain, a stroke may result.
Reduced function of the thyroid gland is also associated with heart disease. Cholesterol levels go up when thyroid function comes down. This in turn can enhance the chance of plaque build up in the blood vessels of the heart (coronary arteries). When thyroid function is very low, fluid collects in various parts of the body. This may occur within the covering of the heart known as pericardium. Collection of fluid within the covering of the heart is called pericardial effusion. If it is severe enough to compress the heart, it prevents proper filling of the heart and blood pressure falls.
How does stress cause heart disease?
Stress and heart disease has various aspects. Stress can lead to increase in risk factors for heart disease as well as precipitate symptoms in a hitherto silent heart disease. Most older persons have build up of plaques in the blood vessels of the heart (coronary arteries). When there is a sudden severe stress, heart rate and blood pressure can shoot up. This can lead to increase of shear stress on the fat plaques in the blood vessels of the heart. A small break may form in the covering of the plaque, exposing the inner cells. Blood components known as platelets adhere to these breaks in the inner surface of the blood vessels and initiate the formation of blood clots. Blood clots in turn block the flow of blood in the vessel, leading to damage of heart muscle known as heart attack.
Sudden stress can sometimes lead to a surge in the blood levels of certain hormones like adrenaline. These contribute to the surges in blood pressure and heart rate. In addition this can upset the heart rhythm leading to life threatening rhythm disorders (cardiac arrhythmia) which can sometimes cause sudden stoppage of the heart (cardiac arrest).
Another manifestation of stress in the heart is the ‘broken heart syndrome‘ or stress cardiomyopathy. This was initially described more in elderly females after sudden demise of their spouse. Part of the left ventricle (lower muscular chamber of the heart) becomes enlarged, giving it the name apical ballooning. This occurs in the absence of any blockage to the blood supply. Most of these cases do recover sooner or later. Stress cardiomyopathy has been described along with multiple other forms of stress other than bereavement.
Stress in general changes our life style and we may eat more of carbohydrate and fat. This has long term consequences like increase in body weight, blood sugar and blood pressure. These can further increase the chance of heart disease. Stress increases the blood pressure and blood sugar through hormonal influences even without a change in the diet pattern.
Electrical system of the heart ensures its rhythmic contractions at normal rate of 60-100 per minute. The signal generator of the heart is known as the sinoatrial node (SA node) and the relay station is known as the atrioventricular node (AV node). Signal conduction between the nodes is through internodal pathways and that from the AV node into the ventricles through the bundle of His and its branches.
SA node is the natural pacemaker of the heart which controls the heart rate. It is in turn under the control of the brain by the autonomic nervous system. SA node increases and decreases its rate in relation to the physiological need of the body. When we take rest, the rate is low and it increases gradually with progressive exertion. Heart rate increases with emotional stress as well. SA node is located in the upper part of the right atrium (upper chamber of the heart), near the entry of the superior vena cava (blood vessel draining deoxygenated blood from the upper part of the body).
There are three signal pathways within the right atrium which connects the SA node to the AV node. One of them gives a branch to the left atrium (left upper chamber of the heart) for giving electrical signals to that chamber.
AV node is situated at the junction of the upper and lower chambers of the heart (AV junction), almost in the middle, at the lower part of right atrium. It delays the signals coming from the SA node so that contraction of the upper chambers are fully over before the lower chambers start contracting.
Bundle of His
Bundle of His is the electrical conductions system of the heart which crosses the AV junction and passess into the wall between the right and left lower chambers (ventricles) of the heart. It divides into two bundle branches – right and left for each ventricle. Left bundle further divides into two divisions, one of which goes to the front region and the other to the back region of the left ventricle (left anterior and left posterior fascicles).
The bundle branches divide further into numerous small branches which take the electrical signals to all the muscle cells of the ventricles. These tiny branches are known as Purkinje fibres.
When the electrical system of the heart fails, the heart beats slow down and the person can get a black out. If the slowing is due to a permanent cause, an artificial electronic pacemaker is implanted under the skin and connected to the heart by a lead wire to restore the heart rhythm. If the cause is temporary (reversible), a temporary external pacemaker is used instead, which can be removed after the spontaneous rhythm of the heart is restored.
Blood vessels supplying oxygenated blood to the heart are known as coronary arteries. The two important coronary arteries are the right coronary artery (RCA) and the left main coronary artery (LMCA). Left main coronary artery has two branches – left circumflex coronary artery (LCX) and left anterior descending coronary artery (LAD). Each of these give several branches to supply the heart muscle. Both the left and right coronary arteries arise from the root of the aorta, the biggest blood vessel arising from the heart, carrying blood to the whole body.
Blood returning from the heart, containing deoxygenated blood are known as coronary veins. The biggest coronary vein is the coronary sinus (CS), which drains into the right atrium, the upper right chamber of the heart.
Left anterior descending artery (LAD)
LAD is so called because it descends on the front (anterior) surface of the heart. It is by far the most important blood vessel supplying the heart muscle so that blockage can be catastrophic. Hence it has earned the name ‘widow maker‘ artery. Blockage of LAD causes anterior wall myocardial infarction – damage to the heart muscle in the front portion of the left ventricle, which is the lower muscular chamber of the heart. Significant damage to the musculature of the left ventricle can impair the pump function of the heart very much so that the blood pressure can fall very low.
Left circumflex coronary artery (LCX)
LCX, as the name implies, curves around the left border of heart, to run in a groove between the upper and lower chambers of the heart (atrioventricular groove or AV groove). It supplies blood to the left side and back of the left ventricle and left atrium (upper left chamber of the heart).
Right coronary artery (RCA)
RCA gives branches to the right atrium right ventricle (lower right chamber of the heart) and part of the back side of the left ventricle. Blood supply to two important electrical structures of the heart can be from the RCA. The upper one is known as sinus node (SA node), which is the usual pacemaker of the heart, causing the heart to beat regularly. The lower one is atrioventricular node (AV node) which is situated at the junction between the upper and lower chambers of the heart (AV Junction). AV node passess on the signals conducted down from the SA node, into the lower chambers of the heart after a short delay. The delay is useful to delay the contraction of the lower chambers for a short while after the contraction of the upper chambers. This allows completion of emptying of the upper chambers into the lower chambers, before they start contracting.