An electrical stimulus is generated by the sinus node (also called the sinoatrial node, or SA node), which is a small area of specialized tissue located in the right atrium (right upper chamber) of the heart. Under normal conditions, the sinus node generates an electrical stimulus every time the heart beats (60 to 190 times per minute, depending on your child’s age of activity level). This electrical stimulus travels down through the conduction pathways (similar to the way electricity flows through power lines from the power plant to your house) and causes the heart's chambers to contract and pump out blood. The right and left atria (the two upper chambers of the heart) are stimulated first and contract a short period of time before the right and left ventricles (the two lower chambers of the heart). The electrical impulse then travels from the sinus node to the atrioventricular (AV) node, where it stops for a very short period, and continues down the conduction pathways via the bundle of His into the ventricles. The bundle of His divides into right and left pathways to provide electrical stimulation to both ventricles.
What is an electrocardiogram • An EKG is one of the simplest and fastest procedures used to evaluate the heart. Before the test, electrodes (small, plastic patches) are placed at certain locations on your child's chest, arms and legs. After the electrodes are connected to the EKG/ECG machine by lead wires, the electrical activity of your child's heart is measured, interpreted and printed out for the physician to examine.
Why is an electrocardiogram (EKG) performed? The electrical activity of the heart is measured by an electrocardiogram. By placing electrodes at specific locations on the body (chest, arms and legs), we can make a graphic representation, or tracing, of the electrical activity. Changes in an EKG can indicate that your child has a heart-related condition.
Some medical conditions which may cause changes in the EKG pattern include: conditions in which the heart is enlarged- can be caused by various factors, such as congenital (present at birth) heart defects, valve disorders, high blood pressure, congestive heart failure or conduction disturbances left ventricular hypertrophy- (enlargement of the left ventricle) High blood pressure makes the heart work harder causing the muscle of the left ventricle to enlarge. The echo can detect problems with the heart such as enlargement, abnormalities in motion of the heart wall, blood clots, and heart valve abnormalities. It also gives a good measurement of the strength of the heart muscle (ejection fraction) conduction disorders- a dysfunction in the heart's electrical conduction system, which can make the heart beat too fast, too slow, or at an uneven rate, a condition called arrhythmia or disrhythmia electrolyte disturbances- an imbalance in the level of electrolytes, or chemicals, in the blood, such as potassium, magnesium or calcium pericarditis- an inflammation or infection of the sac which surrounds the heart valve disease- malfunction of one or more of the heart valves may cause an obstruction of the blood flow within the heart chest trauma- blunt trauma to the chest, such as a hitting the steering wheel in an automobile accident.
When your child's doctor studies your child's EKG, she looks at the size and length of each part of the EKG. Variations in size and length of the different parts of the tracing may be significant. The tracing for each lead of a 12 -lead EKG will look different, but will have the same basic components as described above. Each lead of the 12 -lead EKG is "looking" at a specific part of the heart from different angles. Variations in a lead may indicate a problem with the part of the heart associated with that particular lead.
Maturational changes that occur over the early years of childhood account for the major differences in ECG interpretation. These differences center on heart rate and normal duration of intervals, as well as right ventricular dominance. Infants initially have a rapid heart rate, which is necessary to produce adequate cardiac output. Cardiac output equals stroke volume times heart rate. The higher rate is needed to meet the high metabolic needs in the infant whose small ventricular size cannot compensate by increasing stroke volume. The heart rate of the newborn is commonly 120160 beats per minute. This rate gradually declines with age. As the heart size grows, stroke volume increases so a higher rate is no longer necessary to produce an adequate cardiac output. Because normal rates for an infant are higher than those for an adult, definitions of tachycardia and bradycardia are different for the pediatric population. Pediatric tachycardia is defined as a heart rate higher than expected for age. Pediatric bradycardia is defined as a heart rate lower than expected for age. These definitions are less definitive than those used for adults, for whom one consistent rate is used to define an abnormality.
HR PR interval QRS interval 1 - 3 weeks 100 - 180. 07 -. 14. 03 -. 07 1 - 6 months 100 - 185. 07 -. 16. 03 -. 07 6 - 12 months 100 - 170 08 -. 16. 03 -. 08 1 - 3 years 90 - 150 09 -. 16 03 -. 08 3 - 5 years 70 - 140. 09 -. 16 03 -. 08 5 - 8 years 65 - 130. 09 -. 16. 03 -. 08 8 - 12 years 60 - 110. 09 -. 16. 03 -. 09 12 - 16 years 60 - 100 09 -. 18. 03 -. 09
Because of the faster rate and shorter distances to travel in the smaller sized heart, resulting intervals are shorter. The PR and QRS intervals gradually lengthen with age. This difference is key in identifying specific arrhythmias that use the interval as the criteria for abnormality (e. g. , first degree heart block). For pediatric patients, the normal duration of the PR interval used to determine first degree atrioventricular (AV) block increases with age.
The QT interval is based on the distance between the initial component of the QRS and the end of the T wave. But the QT interval is highly influenced by heart rate. Clearly, a faster rate will shorten the interval. To adjust for the rate effect on the QT interval, health care personnel must use a "corrected" QT or [Qt. sub. c]. The [Qt. sub. c] is calculated by using Bazett's equation of [Qt. sub. c] = QT interval / square root of RR interval in seconds.
This is important because of the lethal arrhythmias that can result from congenital prolonged QT syndrome as well as antiarrhythmic and other medications that affect QT duration. For example, hyperkalemia, hypocalcemia, cisapride , digitalis, quinidine, procainaminde, lithium, tricyclics, and phenothiazides all prolong the QT interval and increase the risk of R on T phenomenon.
Common Pediatric Arrhythmias Another major difference in pediatric ECG interpretation is the type of abnormal rhythms most frequently seen. The most common dysrhythmias in children are supraventricular tachycardia, bradycardia, and sinus arrhythmia. Atrial fibrillation, atrial flutter, ventricular tachycardia, or fibrillation often seen in adults are rarely found in children. The only caveat is that children with congenital heart disease may present with any arrhythmia. Abnormal rhythms specifically related to their heart disease or damage caused by surgical repair are the arrhythmias most usually seen. Supraventricular tachycardia (SVT) is the most common arrhythmia seen in children. It includes a variety of rhythms that emanate from the sinus, atrial, or junctional areas of the heart. By definition, all but ventricular rhythms are considered supraventricular. Supraventricular tachycardia is differentiated from sinus tachycardia by the unusually fast rate and the patient's presentation. It can occur in normal healthy infants, children, and adolescents with an accessory pathway such as Wolf-Parkinson-White syndrome (WPW). it can also occur as a result of an ectopic stimulus that initiates a reentry phenomenon
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