The heart is the most efficient pump The heart is a hollow muscular organ that acts as a motor pump that sends poorly oxygenated blood to the lungs and oxygen-rich blood to the various organs and tissues. It is divided into a left and a right side by a muscular wall denominated septum. There is an interatrial and an intraventricular septum. Both the right and the left heart contain two upper chambers, named atria, and two lower chambers, named ventricles.
The right atrium (blue clour) receives de oxygenated blood from coming from the all body tissues via the superior vena cava, that carries blood from the upper half of the body, and the inferior cava vein, that that carries blood from the lower half of the body. When the right atria contracts, the blood passes (through a valve called tricuspid valve) into the right ventricle. When the right ventricle is filled it contracts and pumps the venous deoxygenated blood into pulmonary artery. During this process the tricuspid valve shuts preventing the blood from coming back into the right atria and the closure of the pulmonary valve (located in the trunk of the pulmonary artery) from coming back to the right ventricle.
Once the blood is oxygenated in the lungs by the air that we inhale, it r eturns (marked in red) to the left atrium through four pulmonary veins. As the left atrium contracts, the blood is pushed through the mitral valve located between the left atrium and the left ventricle. When the left ventricle contracts, the blood is pushed through the aortic valve to the aorta, which distributes the oxygenated blood to the various tissues of the body. The mitral valve prevents the blood from flowing back to the left atria during the contraction of the left ventricle, while the aortic valve prevents the flow of the blood back to the left ventricle during the diastole, i.e. when the ventricle.
The atria and the ventricles work together in an orderly and rhythmic manner, contracting and relaxing alternatively to pump the blood through the heart to the lungs and to all the tissues of the body.
The heart beats 24 hours a day at a frequency of 50-70 bpm and pumps around 5 litres of blood per minute. This means that it pumps around 9,000 L of blood every day. Do you know an equivalent machine working for 70 years working tirelessly?
Figure 1. Structure of the heart and the pathway of the blood flow throughout the heart
Systole. The contraction of the cardiac muscle tissue (atria or ventricles) is called systole. When the ventricles contract, they pump the blood from their chambers into the arteries, the pulmonary artery the right ventricle and the aorta the left ventricle. The left ventricle empties into the aorta and the right ventricle into the pulmonary artery.
Diastole. The relaxation of the cardiac muscle tissue is called diastole. When the ventricles relax, they make room to accept the blood from the atria.
Conducción de los impulsos eléctricos cardiacos The ability of the heart to act as a pump which sends blood to the various tissues of the body depends on its ability to generate rhythmic electrical impulses that propagate quickly and orderly through the atria and ventricles to make the heart contract in a synchronous and efficient manner.
Normally, heart beats are generated in a small structure located in the upper right chamber (right atrium) of the heart, the so-called sino-atrial node. This structure is the battery that contains a group of specialized"pacemaker"cells. These cells beat spontaneously fire (60-80 times per minute, bpm) and send an organized electrical signal through the heart that results in a coordinated and rhythmic heart beat (Figura 1). The cardiac impulses generated in the sino-atrial nodal cells propagate through the right and left atria at an speed of 0.5 meters/second (m/sec). After 30-50 miliseconds, the impulses reach the atrio-ventricular (AV) node. This is the only pathway that under normal conditions allows the impulses to travel from the atria to the ventricles. In the AV node the impulses travel very slowly (0,01-0,05 m/sec). This allows that the contraction of the atria squeeze the blood contained in these cavities into the ventricles; thus, the ventricles are completely filled with blood before contracting. From the AV node, the impulses travel through the His bundle, which bifurcates into right and left branches which, in turn, branch into multiple Purkinje fibers, which deliver the excitatory impulse to the ventricular muscle. The cardiac impulse spreads rapidly through the His- Purkinje system (2-4 m/sec) and the ventricular muscle (0.3 to 1 m / s). This rapid intraventricular conduction velocity allows both ventricles to contract quickly and synchronized and to expel a volume adequate blood to the lungs (right ventricle) and to the rest of the tissue (left ventricle).
The cardiac valves The heart has four valves which control the blow flow there through.
The atrioventricular valves prevent backflow from the ventricles into the atrium during systole. They are anchored to the wall of the ventricle by chordae tendineae, which prevent the valve from inverting.
The tricuspid valve controls the blood flow between the right atrium and right ventricle and stops the backflow of blood between the two.
The mitral (bicuspid) valve allows oxygen-rich blood coming from the lungs pass from the left atrium into the left ventricle.
There are another two valves located at the base of both the pulmonary trunk (pulmonary artery) and the aorta, the two arteries taking blood out of the ventricles. These valves (called semilunar valves) permit blood to be forced into the arteries and prevent backflow of blood from the arteries into the ventricles.
The pulmonary valve control blood flow from the right ventricle into the pulmonary arteries. The lungs are the place where the blood is oxygenated.
The aortic valve allows oxygen-rich blood to pass from the left ventricle into the aorta, which is the artery that distributes blood to the rest of the body.
There are two types of heart valve defects:
Valve stenosis. This means that the valves thicken and calcify narrowing the hole through which passes the blood. This forces the heart to generate more pressure on the chamber which is above the stenosed valve to push blood through the obstruction.
Valve insufficiency. This means that the valve does not close completely, allowing a backflow of blood (regurgitation) through the leaky valve.
Frequency at which the heart beats Under normal conditions, the heart contracts rhythmically and regularly at a frequency between 50 and 70 beats per minute (bpm). This means that the heart beats (contracts) between 86,400 and 115,200 times a day. However, the rate at which the heart beats varies throughout the day, so that it decreases at rest or during sleep and is accelerated when performing an exercise or in situations of anxiety, emotion, fever or fear. Furthermore, in healthy people, or the elderly, may have a rate < 50 bpm and in trained athletes and young adults at rest or at night heart rate can be reduced to 30-40 bpm. On the contrary, when a person is performing regular physical activity (such as walking or driving), the heart beats within a normal range, i.e. between 60 and 100 beats per minute.
A cardiac arrhythmia indicates an irregular heart beat. An arrhythmia is a disturbance in the frequency at which the heart beats, either because it is accelerated or decreased, or in the ordered sequence in which the heart beats propagates. Thus, a cardiac arrhythmia is a disorder of the rate at which the heart beats. Cardiac a rrhythmias can be classified in various ways.
According to the heart rate, we talk about heart rate:
Tachycardia or tachyarrhythmia, when the heart rate under resting conditions is abnormally fast, faster than 100 beats per minute (bpm), and
Bradycardia or bradyarrhythmias, when the heart rate is slow and the heart beats at a rate of less than 60 bpm.
Arrhythmias can also be classified according to their location, ie, depending on the area of the heart where the arrhythmia originates. In this case, we talk of:
Supraventricular arrhythmia, which originate in the atria or in any cardiac structure located above the ventricles, and
Ventricular arrhythmia, when they originate in the ventricles. These arrhythmias are the most dangerous and may endanger the patient's life, as almost always affect the heart's ability to pump blood to the rest of the body.
Sometimes, the heart generates a premature or extra beat that we call"extrasystole". The extrasystoles appear in 60% of patients taking coffee, snuff, alcohol or other stimulants and in the elderly even under normal conditions.
We should note that in most patients the tachycardia or the bradycardia are not by themselves a cardiac disease, but are clinical signs of a disease (known or unknown) of the patient. Indeed, cardiac arrhythmias occur very frequently in patients with cardiovascular diseases, such as angina pectoris, myocardial infarction, heart failure or hypertension. Other times, arrhythmias occur in patients with congenital heart defects, i.e., they born with this problem. Occasionally, other non-cardiac diseases can associate with the presence of cardiac arrhythmias. This is the case for hypo/hyperthyroidism, chronic pulmonary diseases, diabetes, chronic kidney disease or sleep apnea. Stress, caffeine, snuff, alcohol and some drugs sold over-the-counter to treat minor diseases processes (cough or cold symptoms) can alter the normal frequency of the heartbeat.
The electrocardiogram (ECG or EKG) is the most easy and practical diagnostic tool to assess the rate and rhythm of the heartbeat and can provide indirect evidence of blood flow to the heart muscle. Up to 12 self-adhesive electrodes are attached to select locations of the skin on the arms, legs and chest. The test is completely painless and takes less than a minute to perform once the leads are in position. After the test, the electrodes are removed.
Holter monitoring. A standard ECG performed at the time of a visit to the doctor's office may not allow the diagnosis of a cardiac arrhythmia. The Holter monitoring is an ambulatory electrocardiography device that allows a continuous recording of the heart's rhythm for 24 or more hours. It is very useful for the diagnosis of occasional cardiac arrhythmias. The Holter consists of the hardware (called monitor or recorder) for recording the signal and the software for review and analysis of the record. Holter monitoring is extremely safe and no different than carrying around a small tape recorder for 24 hours. In some cases, it is possible for the patient to activate the monitor when he feels the symptoms of an arrhythmia. The monitor has a memory system that stores and send the information through the phone to a center that analyzes the ECG.
There is an subcutaneously implantable Holter that allows continuous arrhythmia burden assessment (ECG and 24-hour holter monitoring). The device is an implantable patient activated and automatically activated monitoring system that records subcutaneous ECG and is indicated in patients who experience transient symptoms (i.e., dizziness, palpitation, syncope, and chest pain) that may suggest a cardiac arrhythmia.
Echocardiography. It is a non-invasive diagnostic test that uses sound waves to create a moving picture of the heart. A standard echocardiogram uses a transducer (or probe) that is placed on the chest wall (or thorax) of the subject, and images are taken through the chest wall. This allows a highly accurate and quick assessment of the heart beating, and to see the heart valves and other structures of the heart. Therefore, the echocardiography is routinely used in the diagnosis, management, and follow-up of patients with any suspected or known heart disease. Doppler echocardiography uses ultrasound technology to examine the heart blood flow (direction and velocity) avoiding invasive procedures such as cardiac catheterization.
Electrophysiological study. This is an invasive procedure to assess the electrical activity and conduction pathways of the heart. The type of study is indicated to investigate the cause, location of origin and best treatment for the cardiac arrhythmia. This type of study is performed by a cardiac specialist in cardiac arrhythmias (a cardiac electrophysiologist) and using a single or multiple long and thin tubes called"catheters"placed within the heart through a vein or artery. The entire procedure can take several hours.
Clinical relevance of cardiac arrhythmias
Cardiac arrhythmias are common in the general population and often they occur in patients affected with common cardiovascular diseases, such as angina pectoris, myocardial infarction, heart failure or arterial hypertension. Arrhythmias are one of the leading causes of death in the general population and the main responsible for sudden cardiac deth (see latter).
The symptoms are highly variable, depending on the type, frequency and duration of the arrhythmia and on the presence or absence of an structural heart disease or other comorbidities of the patient. Some patients remain asymptomatic and do not perceive the arrhythmia, especially if they occur sporadically or during sleep. In this case usually need to be treated. On the contrary, arrhythmias sometimes produce symptoms even at rest, and require aggressive treatment because of the risk of which can lead to sudden death of the patient.
The most common symptoms are palpitations (sensation of skipping a heart beat or accelerated heart rate), dizziness, fatigue, sweating, chest pain, shortness of breath and even a temporary loss of consciousness (fainting or syncope) followed by the return to full wakefulness. Generally, there is a good correlation between the severity of symptoms and the severity of the arrhythmia. Therefore, it is recommended that all people who detect an abnormal pulse or heart rate to consult the cardiologist or their doctor. The electrocardiogram allows them to detect quickly and easily the heart rhythm disturbances.
In addition, some arrhythmias (e.g., atrial fibrillation) can produce serious complications such as the formation of blood clots (thrombus) in the atria. These thrombus that can completely or partially detach from the atria and travel via the blood flow from a place in the body to another are called and emboli. The embolus can cause an obstruction of blood vessels (arteries, veins). The obstruction of an artery can cause a complete or partial interruption of blood flow and oxygen delivery to a particular tissue (i.e., ischemia). If the vessel obstruction occurs in a coronary artery supplying blood to the heart and lasts for a sufficient time can result in the death (necrosis) of cardiac muscle cells in a particular area of the heart, that is, a myocardial infarction. When the obstruction occurs in a cerebral artery a cerebrovascular accident (also referred as stroke) may occur.
