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What Does An ECG Test For?

May. 14, 2024



The heart, as one of the most important organs of the human body, beats continuously every day to provide us with life force. However, the heart is also one of the parts susceptible to various diseases. In modern medicine, the electrocardiograph has become an indispensable instrument to help doctors detect and evaluate heart function. By recording changes in the heart's electrical activity, it provides valuable information that allows doctors to better understand a patient's heart condition. In 1887, Dutch physiologist Willem Einthoven successfully recorded the electrical signals of the heart, which is considered the beginning of electrocardiography. Since then, EKG technology has continued to improve and develop, becoming an important tool in modern medicine for diagnosing heart disease. The electrocardiograph can reflect the functional and structural status of the heart by measuring the intensity and time of these electrical signals, as well as their generation and propagation in different parts of the heart.



The working principle of an electrocardiograph is relatively simple, but very important. By attaching electrodes to the patient's chest, arms, legs, etc., the electrocardiograph can record changes in the heart's electrical signals. These electrical signals can reflect the heart's rhythm, conduction, and potentially abnormal activity. Once the recording is complete, doctors can analyze the EKG patterns to determine whether heart disease or other abnormalities are present.



Electrocardiographs are widely used in clinical practice. It is often used to diagnose and monitor cardiac conditions such as arrhythmias, myocardial ischemia, myocardial infarction, and heart valve disease. For patients with heart disease, regular electrocardiogram examinations can help doctors evaluate the effectiveness of treatment and adjust drug dosage to ensure the best treatment results for patients. The electrocardiograph can also be used in physical examination screening, preoperative evaluation, emergency rescue and other scenarios, providing important reference for doctors.


What is an electrocardiogram?


Electrocardiogram (ECG or EKG) is a technology that uses an electrocardiograph to record the electrical activity changes produced by the heart in each cardiac cycle from the body surface.

In each cardiac cycle, the pacemaker, atria, and ventricles are excited one after another. Along with the changes in bioelectricity, various forms of potential change patterns are elicited from the body surface through the electrocardiograph.

The principle of electrocardiogram generation


The myocardial cell membrane in the heart is a semipermeable membrane. In the resting state, a certain number of positively charged cations are arranged outside the membrane, and the same number of negatively charged anions are arranged inside the membrane. The potential outside the membrane is higher than that inside the membrane, which is called a polarization state. . In the resting state, since myocardial cells in all parts of the heart are in a polarized state and there is no potential difference, the potential curve recorded by the current recorder is flat, which is the isoelectric line of the body surface electrocardiogram. When myocardial cells are stimulated to a certain intensity, the permeability of the cell membrane changes, and a large number of cations flow into the membrane in a short period of time, causing the potential in the membrane to change from negative to positive. This process is called depolarization.


For the whole heart, the potential changes of myocardial cells during the sequential depolarization process from the endocardium to the epicardium. The potential curve traced by the current recorder is called the depolarization wave, which is the P wave of the atrium and the ventricle on the surface electrocardiogram. of QRS waves. After the cell depolarization is completed, the cell membrane discharges a large number of cations, causing the intramembrane potential to change from positive to negative and return to the original polarization state. This process proceeds from the epicardium to the endocardium, which is called repolarization. Similarly, the potential changes during the repolarization process of myocardial cells are recorded by the current recorder and are called repolarization waves. Since the repolarization process is relatively slow, the repolarization wave is lower than the depolarization wave. The repolarization wave of the atrium is low and buried in the depolarization wave of the ventricle, making it difficult to identify on the surface electrocardiogram.


Ventricular repolarization waves appear as T waves on the surface electrocardiogram. After all the myocardial cells are repolarized, the polarization state is restored again. There is no potential difference between the myocardial cells in various parts. The body surface electrocardiogram records the equipotential lines. So how do you see the electrocardiogram? Below we will understand how to see the electrocardiogram through the explanation of electrocardiogram.


ECG recording method


When the heart contracts and relaxes, tiny bioelectricity is generated. Using electrocardiogram, this potential change can be detected and recorded from different parts of the body surface. When tracing an electrocardiogram, electrodes are placed on different parts of the human body. Through this electrocardiogram recording method, the electrical activity of the heart is recorded and reflected. The placement locations of the cardiac electrodes are as shown in the table below. When performing a routine electrocardiogram examination, usually only 4 limb lead electrodes and V1 to V66 chest lead electrodes are placed to record a conventional 12-lead electrocardiogram.

Different leads are formed between two electrodes or between the electrodes and the central potential end. The lead wires are connected to the positive and negative poles of the electrocardiograph ammeter to record the electrical activity of the heart. A bipolar lead is formed between the two electrodes, one lead is positive and the other is negative.


Bipolar limb leads include lead I, lead II and lead III; a unipolar lead is formed between the electrode and the central potential end. At this time, the detection electrode is the positive electrode and the central potential end is the negative electrode. Leads avR, avL, avF, V1, V2, V3, V4, V5, and V6 are all unipolar leads. Since avR, avL, and avF are far away from the heart, the potential difference recorded when the central electric terminal is the negative electrode is too small, so the negative electrode is the average of the sum of the potentials of the other two limb leads except the exploration electrode. Because this recording increases the potential in leads avR, avL, and avF, these leads are also called pressurized unipolar limb leads.

The limb lead system reflects the projection of cardiac potential in the sagittal plane. Including leads I, II, III, avR, avL and avF. The chest lead system reflects the horizontal plane of cardiac potential projection including: V1, V2, V3, V4, V5, and V6 leads. These leads are further grouped to reflect electrical activity in different parts of the heart.


How to read electrocardiogram

(1) Look at the numerical value


1. Heart rate: normal is 60-100.


If the bpm is less than 60 bpm, it is reported as "sinus bradycardia", and if it is more than 100 bpm, it is reported as "sinus tachycardia".


2. P wave: normal time <120ms.


>120ms and double peaks, reporting "mitral valve type P wave" (sometimes visible, but not often)


3. PR interval: normal time 120-200ms.


If it is >200ms, it will be reported as “first degree atrioventricular block”; if it is <120ms, it will be reported as “short PR interval”.


4. The QRS interval should be <120ms. A value greater than this has diagnostic significance (discussed later); amplitude (the sum of the absolute values of positive and negative waves): limb conduction <0.5mv and/or chest conduction <0.8mv will report " "Left ventricular low voltage" (rare), young babies don't need to remember it.


5. If Rv5>2.5mv and/or Rv5+Sv1>4.0mv (men)//>3.5mv (women), "left ventricular high voltage" is reported, which is very, very common! It is a manifestation of left ventricular hypertrophy. Ask for medical history. Many patients have high blood pressure, and the QRS complex is significantly increased by visual inspection of the electrocardiogram.


6. P/QRS/T axis: XX/XX/XX, the number in the middle is -30. —-90. Reported as "left deviation of the cardiac axis" (very common), 90. —180. The report of "right deviation of the cardiac axis" (it's rare, but I have it) has to be graded in detail, from light to moderate to severe, usually moderate, and only moderate and above are reported. I don't know the specifics, and it's not stated in the book. No, please give me some advice, thanks!


(2) See if there are premature beats


If the QRS shape is wide and deformed, it will be reported as "premature ventricular contraction", otherwise it will be reported as "premature atrial contraction". Premature atrial contractions must be distinguished from sinus arrhythmias. The premature wave is added to the RR interval of the two normal waves before and after it and divided by 2 is equal to a normal RR interval (it is difficult to understand, please overcome it). You need to use a compass to measure it. Sinus arrhythmia does not have this rule. When writing a report, it is also necessary to distinguish between frequent premature beats and occasional premature beats. Frequent premature beats are mostly classified as "frequent premature bigeminy/triponym", and less than 3 premature beats per minute are considered occasional.


(3) Look for irregular heart rhythms

(4) Check for atrial fibrillation and atrial flutter

(5) Right bundle branch block

(6) Left anterior fascicular block

(7) Changes in ST segment and T wave

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