Electrical activity of the heart

The pumping activity of the heart is dependent on the electrical impulses generated by specialized fibers in the heart itself called the pacemaker or the SA node (sinoatrial node). The SA node is located in the wall of the right upper chamber of the heart called the right atrium. The muscle fibers of the heart are structurally different from muscle fibers in other parts of the body and are connected to each other in a special arrangement such that the impulses generated in that special spot in the right atrial wall are propagated and programmed to arrive at each of the other 3 heart chambers (left atrium, right ventricle and left ventricle) in perfect timing.




Electrical conduction pathway of the heart.
Original heart diagram illustrated by Patrick J. Lynch; illustrator; C. Carl Jaffe; MD; cardiologist Yale University Center for Advanced Instructional Media

The result is that the 2 atria receive electrical impulses at the same time and contract in unison, pushing blood out into the ventricles while the ventricles receive electrical impulses a bit later and are thus relaxed when the atria are contracting. By the time the ventricles finally begin to contract, the atria have emptied out, the building pressure in the ventricles cause the atrioventricular valves to close and blood is pushed out of both ventricles; from the right to the lungs and from the left to the aorta, the biggest artery in the body.

lub dub

HEART ATTACK (2)

How do I know if I'm having a heart attack?

A person having a heart attack(myocardial infarction) may experience severe central chest pain often described as squeezing or pressure which may radiate to the shoulders, neck or jaw, with or without headache, dizziness, breathlessness, sweating, vomiting or fainting. The pain lasts longer than a few minutes and is not relieved by nitroglycerin (a drug that widens the coronary arteries, prescribed for patients with angina).Unfortunately however, some people,  will not experience any symptoms at all. This is called silent infarction and is more common in diabetics. If not immediately treated, the heart muscle fibers affected will undergo irreversible damage/infarction/death within 4-5 minutes, lose their functionality and be replaced by scar tissue in a few weeks. Depending on the size of the area affected, this damage may lead to heart failure, arrhythmias, heart rupture, cardiac arrest or death.    A heart attack can occur suddenly but many patients have warning signs days or weeks before the actual event. These warning signs may be in form of recurrent chest pain which is precipitated by exertion and relieved by rest, or increasing frequency and severity of chest pain in people who have already been diagnosed with ischemic heart disease. If medical intervention is sought early, the process can be halted or reversed with appropriate therapy.

In what ways may you be increasing your risk for ischemic disease?

Although there are some predisposing factors which are not modifiable or controllable such as male gender, age(˃40) and heredity (family history of IHD), most of the risk factors for IHD are modifiable. Interestingly, modifying these risk factors simply implies adopting a healthy lifestyle i.e. engaging in regular physical exercises, eating a healthy balanced diet low in saturated fat, avoiding smoking and maintaining optimal body weight. Also, screening for hypertension, diabetes and hypercholesterolemia and controlling them if present will significantly reduce the risk of IHD. Thus, you may be increasing your own risk for heart disease by living a physically inactive life, living on junk foods high in saturated fats and carbohydrate, smoking and not having regular medical checkups.

lub dub

Back to Heart attack 1

HEART ATTACK (1)

Myocardial infarction also known as heart attack is an irreversible damage (death) to the heart muscles due to loss of blood supply following narrowing and occlusion of a coronary artery. The coronary arteries are the arteries which supply the heart muscles (myocardium). The right and left main coronary arteries are the first branches of the aorta, carrying oxygenated blood from the left ventricle. These 2 arteries divide into many branches which supply every part of the heart. Unlike many other parts of the body, the heart does not have an alternate source of oxygenated blood. This is why the coronary arteries are called end arteries. Narrowing of these arteries therefore puts the heart tissue at risk of ischemic injury (Ischemic heart disease -IHD). Also, unlike other parts of the body which get continuous blood supply, the muscle layer of the heart only gets blood supply in pulses. This is because each time the heart muscles contract (systole), they squeeze the arteries running through them and temporarily stop blood flow, when they relax (diastole), the arteries become patent again and blood flows to supply the muscles and inner lining of the heart. [An average heart contracts and relaxes about 70 times each minute]. This intermittent oxygen supply contributes to the susceptibility of the hard working heart muscles to hypoxic damage. Fortunately, the heart muscles are capable of extracting more oxygen from blood than any other tissue in the body. Also, the blood flow to the heart increases almost proportionately to increase in workload and corresponding increase in oxygen consumption, by dilatation of the coronary arteries. These measures normally prevent hypoxia and tissue damage.

Patrick J. Lynch, medical illustrator

Coronary arteries may become narrowed or blocked by plaques (in coronary artery disease), blood clot building over a ruptured plaque or rarely, loose clot (embolus) formed within a heart with damaged valve or wall and carried in blood into the aorta.  Narrowing can also occur due to spasm of the artery. A plaque is mainly composed of fat (in form of cholesterol), calcium and fibrous tissue. Plaques take years to develop. They are commonly seen with ageing but may occur in younger people with high cholesterol levels, diabetes mellitus or hypertension.   The presence of plaques in any artery is called atherosclerosis- meaning hardened artery. Apart from narrowing the lumen, the plaque also hardens the walls of the artery preventing it from dilating to supply more blood in response to increased oxygen demand.

However, even in the presence of a plaque, symptoms of ischemia do not occur until about 70% of the lumen of the artery is occluded. The individual experiences squeezing chest pain (angina) brought on by any increase in workload on the heart such as with exertion (e.g. climbing stairs, physical exercise), in cold weather, emotional situations etc., and relieved by rest. If the plaque ruptures, a raw surface is exposed on the arterial wall and this stimulates the formation of a blood clot much like a bleeding injury anywhere in the body. This further narrows the lumen; worsening the angina which may now occur even at rest. Sometimes, the clot or thrombus can grow large enough to completely occlude the artery, totally depriving the part of the heart muscle (supplied by this artery) of oxygenated blood. This is what is referred to as a heart attack.

lub dub

See Heart attack (2)

click here to know your risk for heart disease

FROM SORE THROAT TO HEART DISEASE

(RHEUMATIC HEART DISEASE)

Rheumatic heart disease is a permanent heart valve damage resulting from one or repeated episodes of rheumatic fever. Acute rheumatic fever is an abnormal immune response by the body to an infection (usually a throat infection) caused by bacteria called group A streptococci. Normally, one of the ways in which the body fights infections is by producing antibodies which attack the infecting organisms. In acute rheumatic fever, the antibodies produced are thought to also attack the host’s tissues (brain, skin, subcutaneous tissues, joints and heart) causing an inflammatory damage several weeks after the throat infection. This autoimmune response is seen in about 0.3-30% of patients with untreated or poorly treated group A streptococcal throat infection. Rheumatic heart disease is the most serious complication of rheumatic fever. Almost half of patients with acute rheumatic fever will have the heart involvement.

Heart valve affected by RHD

The most commonly affected valve is the mitral valve followed by the aortic valve. Other valves (tricuspid and pulmonary are less frequently affected).The damage causes the flaps of the valve to thicken causing narrowing of the valve opening (stenosis). Repeated episodes of rheumatic fever further damage the valves and eventually lead to heart failure as less blood is pumped out of the heart but rather backs up in the left atrium, pulmonary veins and lungs. At this stage, except the valves are replaced with prosthesis, the patient will die of consequences of the heart failure. During an acute episode of rheumatic fever, the valvular damage may be minor and still be reversible. Symptoms of rheumatic heart disease include: shortness of breath, fatigue, irregular heartbeats, chest pain and fainting.

 

Rheumatic fever is principally a childhood disease; sore throat in children should therefore be taken seriously especially if there are no symptoms suggesting that it’s viral in origin. Such symptoms include runny nose, itchy eyes and other symptoms of the common cold. On the other hand, symptoms and signs which are considered as hall marks of streptococcal sore throat include a history of close contact with infected person, tender lymph nodes, skin rash (scarlet fever), swollen tonsils with whitish exudates and abdominal pain, with or without fever.

Tonsils are enlarged and have exudates

 

SCARLET FEVER RASH

However, a better and surer way to distinguish between a viral and a streptococcal sore throat is to go to the clinic and have a throat swab done.

An episode of acute rheumatic fever can go unnoticed. Symptoms however include fever, pain and swelling of the joints, nausea, stomach cramps and vomiting. These symptoms may be non specific, making diagnosis difficult.  A set of criteria have been developed to help doctors in diagnosing rheumatic fever.  The major criteria include: carditis, polyarthritis, chorea, subcutaneous nodules, and erythema marginatum. Minor criteria include: previous rheumatic fever or rheumatic heart disease, arthralgia, fever, elevated erythrocyte sedimentation rate/positive C-reactive protein/ leukocytosis, and prolonged PR interval on an electrocardiogram. Two major criteria, or a combination of one major and 2 minor, provide reasonable evidence for a diagnosis of rheumatic fever.

RHD occurs all over the world but is more prevalent in developing countries in association with a)overcrowding: which facilitates the spread of the throat infection,b) poverty: which partially explains the untreated or poorly treated infections, and c)lack of access to health care facilities in some areas.

There are 2 levels of prevention of rheumatic heart disease. The first is Primary prevention. This involves the prompt administration of antibiotics (Penicillin) for suspected streptococcal sore throat. It prevents the development of acute rheumatic fever. The other is Secondary prevention. The aim is to prevent recurrent episodes of rheumatic fever in someone who has already had a first episode. This will reduce progression of heart disease. It involves the use of penicillin orally (daily) or as injection (monthly) continuously for at least 10 years after the episode of acute rheumatic fever. Secondary prevention is more difficult to comply with.

The lessons here are these:

• An “ordinary” sore throat may not be ordinary after all
• A stitch in time saves nine [and in this case, saves the entire garment]

 

lub dub

CARDIAC ARREST (2)

Middle aged man suddenly slumps, lies unconscious, cannot be roused, not breathing, not making any movements, not making any sounds, no pulse is felt......

Is cardiac arrest reversible?

The treatment of cardiac arrest is a race against time because the brain is very sensitive to hypoxia and brain damage occurs usually after about 5 minutes of not receiving oxygen. Thus, treatment must also occur within 4-6mins. Since ventricular fibrillations occur in the majority of cases, the best treatment of cardiac arrest is defibrillation i.e. delivering a therapeutic dose of electrical energy to the heart using a defibrillator. The aim is to depolarize most of the heart muscles at the same time and give the SA node fibres the opportunity to resume normal pacemaker function since they are the fastest and will be the first to recover from the jolt. Unfortunately, most cardiac arrests happen outside the hospital setting where a defibrillator may not be available. In these circumstances, prompt CPR in form of Chest compressions and assisted ventilation have been shown to save lives by maintaining circulation and ventilation until help arrives or until heart beat is spontaneously restored. Automated external defibrillators are now often found in public places like shopping malls and can be used by bystanders to reverse a cardiac arrest. The cardiac thump, though less effective is another technique which works by the same principle as a defibrillator and can be used by a trained personnel to achieve the same objective. Many attempts at resuscitation are however unsuccessful even in a hospital setting especially if there are ongoing underlying anomalies.

Is cardiac arrest preventable?

As mentioned earlier, most cases of cardiac arrest are due to ischemic heart disease (IHD) which can be prevented by minimising the modifiable risk factors. These include smoking, high cholesterol levels, diabetes, and hypertension. Also early detection and management of ischemic heart disease will arrest the progression to myocardial damage and possible cardiac arrest. Symptoms of IHD include chest pain on exertion, in cold weather or emotional situations (angina pectoris) which may progress to acute severe chest pain not relieved by rest due to heart muscle damage (myocardial infarction or heart attack). It is therefore important to report any severe or recurring chest paint to your physician to rule out the possibility of IHD. Difficulty in breathing or swelling of the extremities may also occur due to weakness of the heart muscle (Heart failure). Unfortunately, not all cases of IHD are symptomatic (silent ischemia). Thus the best prevention is the primary prevention of ischemic heart disease by engaging in regular physical exercises, eating a healthy balanced diet low in saturated fat, avoiding smoking, maintaining optimal body weight/body mass index (BMI),screening for hypertension and maintaining good blood pressure control, screening for diabetes and maintaining good blood sugar control, screening for and treating hypercholesterolemia. This is important for everyone but even more important in individuals with the unmodifiable risk factors for IHD such as male gender, age above 40 and family history of ischemic heart disease.

The outcome of resuscitation from cardiac arrest varies from one individual to another. It ranges from full recovery, to recovery with only partial function, to being in coma for weeks, to death. Screen yourself and learn CPR today; you may just be saving your life or your neighbour’s.

lub dub

To calculate your risk for heart disease, click here

CARDIAC ARREST (1)

From the moment it starts beating in the 21 days embryo, to the day the individual dies, the heart never stops pumping. Well, this statement is not absolutely true; not with the invention of by-pass surgeries and definitely not in people who were fortunate to have survived an episode of cardiac arrest.

Ever heard “He just slumped and died!”? – This is most likely sudden cardiac arrest.

Cardiac arrest refers to a state in which the heart suddenly stops beating due to a lack of electrical activity in the heart muscles. When the heart is not pumping, blood is not being carried around the body to supply oxygen to tissues. Lack of oxygen to the brain almost immediately causes loss of consciousness and cessation of breathing. Within 4 to 6 minutes, permanent brain damage occurs and death quickly follows.

What causes the sudden black out?

The pumping activity of the heart is dependent on the electrical impulses generated by specialized fibers in the heart itself called the pacemaker or the SA node (sinoatrial node). The SA node is located in the wall of the right upper chamber of the heart called the right atrium. The muscle fibers of the heart are structurally different from muscle fibers in other parts of the body and are connected to each other in a special arrangement such that the impulses generated in that special spot in the right atrial wall are propagated and programmed to arrive at each of the other 3 heart chambers (left atrium, right ventricle and left ventricle) in perfect timing.

Electrical conduction pathway of the heart.
Original heart diagram illustrated by Patrick J. Lynch; illustrator; C. Carl Jaffe; MD; cardiologist Yale University Center for Advanced Instructional Media

The result is that the 2 atria receive electrical impulses at the same time and contract in unison, pushing blood out into the ventricles while the ventricles receive electrical impulses a bit later and are thus relaxed when the atria are contracting. By the time the ventricles finally begin to contract, the atria have emptied out, the building pressure in the ventricles cause the atrioventricular valves to close and blood is pushed out of both ventricles; from the right to the lungs and from the left to the aorta, the biggest artery in the body.

Many conditions including the following: low blood volume, hypoxia , acidosis, hyperkalaemia or hypokalaemia (High or low potassium), hypothermia (low body temperature), Low or high blood glucose, toxins, drugs, cardiac tamponade(compression of the heart by blood or other fluid building up around it e.g. following chest injury), tension pneumothorax (a collapsed lung), myocardial infarction (Heart attack), pulmonary embolism, electric shock and other forms of trauma, can ultimately disrupt the electrical activity of the heart muscles and lead to ineffective and  uncoordinated contractions (ventricular fibrillations) which cannot pump blood out of the heart. 

 Some people are also born with heart diseases such as Wolff Parkinson-White syndrome and Marfan syndrome which predispose them to cardiac arrest. However, the most common condition  underlying a cardiac arrest is ischemic heart disease,  which causes  heart muscle damage secondary to low oxygen supply.
lub dub

Go to CARDIAC ARREST (2)

THE HEART BEAT

The human heart starts to beat around the 21^st day of life at a rate similar to the mother’s heart rate of about 75beats per minute. This rate increases and then decreases slightly through the fetal period. By the time the baby is born, the heart rate is about 145beats per minute. The heart beat can be heard by listening with the ordinary ear or with the aid of a stethoscope over the chest.  It is loudest over a point marked by the intersection of an imaginary line running horizontally in the space between the 5^th and 6^th left ribs and an imaginary vertical line running down from a point just to the right of the midpoint of the left collar bone.  This point is called the apex; it is the lowermost and outermost part of the heart and it is the point of maximal cardiac impulse.

The normal heart sounds are the sounds produced by turbulence in the heart as a result of the closure of valves as the different heart chambers contract and relax to pump out and receive blood respectively. The sounds are designated with the letter S, with S1 being the first heart sound (also referred to as   lub) and S2 the second heart sound (also referred to as dub).  

lubdub lubdub lubdub lubdub lubdub lubdub lubdub lubdub lubdub…………………

Sometimes, extra sounds may be heard by a doctor with the aid of a stethoscope. There may be 3^rd and 4^th heart sounds (S3, S4) or Murmurs. A murmur may or may not be a sign of a heart disease; for instance, hyper dynamic states in which there’s more blood flowing through the heart can result in turbulence which is heard as a murmur. Such states may be caused by fever, anemia, hyperthyroidism and pregnancy. With the reversal of the condition, the murmur also disappears. However, a murmur may be caused by heart disease, most often a congenital defect such as holes in the septum or valvular abnormalities. Heart valves may also be damaged by certain infections or age related changes.

In an adult, the normal rate of the heart is between 60-100 beats per minute. A rate faster than 100bpm is generally referred to as tachycardia while one slower than 60bpm is called bradycardia. There are many conditions which may lead to tachycardia or bradycardia. Some of these are physiological i.e due to normal body adjustments e.g the heart rate of an athlete is expected to be slower than average. Sudden fright or anxiety can, on the other hand, cause tachycardia which is only temporary and resolves when the cause is taken away.  Substances such as caffein, nicotine (in cigarette), drugs of abuse, prescription drugs may also cause changes in the heart rate. However, low heart rate or high heart rate may sometimes be manifestations of heart disease or other diseases. 

The most common symptom of a change in the rate or rythm of the heartbeat is palpitation. This refers to a sense of awareness of the heart beat. It could be disturbing and quite distressful.

heartfailurematters.org

 

The heartbeat, normal or abnormal, can provide a large amount of information about what is going on in the heart as well as other parts of the body, therefore auscultation for the heart sounds is a vital part of the medical examination.

lub dub

edited on 28/11/11