CHAPTER 2 : Basic waves

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CHAPTER 2 : Basic waves

Post by Admin on Sat Mar 06 2010, 15:40

Basic ECG Complexes: P, QRS, ST, T, and U Waves
The spread of stimuli through the atria and ventricles followed by the return of stimulated atrial and ventricular muscle to the resting state produces the electrical currents recorded on the ECG. Furthermore, each phase of cardiac electrical activity produces a specific wave or complex . The basic ECG waves are labeled alphabetically and begin with the P wave:

P wave--atrial depolarization (stimulation)

QRS complex--ventricular depolarization (stimulation)

ST segment, T wave, and U wave--ventricular repolarization (recovery)

You are probably wondering why no wave or complex represents the return of stimulated atria to their resting state. The answer is that the atrial ST segment (STa) and atrial T wave (Ta) are generally not observed on the normal ECG because of their low amplitudes

ECG Paper
Each of the small boxes is 1 millimeter square (1 mm2 ). The paper usually moves at a speed of 25 mm/sec. Therefore horizontally, each unit represents 0.04 second (25 mm/sec × 0.04 sec = 1 mm). Notice that the lines between every five boxes are heavier, so that each 5-mm unit horizontally corresponds to 0.2 second (5 × 0.04 = 0.2). The ECG can therefore be regarded as a moving graph that horizontally corresponds to time, with 0.04- and 0.2-second divisions.

Vertically the ECG graph measures the voltages,of the ECG waves or deflections. The exact voltages can be measured because the electrocardiograph is standardized. In most electrocardiographs the standardization can also be set at one-half or two-times normal sensitivity.

SO WHAT IS STANDARDIZATION ?? wil be the next question

The electrocardiograph must be properly calibrated so that a 1-mV signal produces a 10-mm deflection. ( ie if R wave is of amplitude 10mm in the ecg paper then its voltage is 1mV)
The standardization deflection is also important because standardization can be varied in most electrocardiographs

When very large deflections are present (as occurs, for example, in some patients who have an electronic pacemaker that produces very large spikes or who have high QRS voltage caused by hypertrophy), it may be advisable to take the ECG at one-half standardization to get the entire tracing on the paper. If the ECG complexes are very small, it may be advisable to double the standardization (e.g., to study a small Q wave more thoroughly). The standardization need be set only once on an ECG--just before the first lead is recorded.. this is very iimportant to see whether the standardization is normal or half normal or doubled inorder to avoid mistakes in interpretation. ( eg . if the standardization mark is DOUBLED , then normal R wave will produce a 20 mm amplitude deviation in ECG paper . which will be equal to jus 1mV voltage that is a normal R wave !!!! )

here the one which is rounded is the standardization mark ..

so first before seeing an ecg LOOK AT THE STANDARDIZATION MARK



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