Question

The P wave in the ECG indicates
A. Atrial depolarization
B. Rapid ventricular depolarization
C. Absolute refractory period
D. Repolarization of the ventricles

Answer 1

Hint: Body is a volume conductor that means body fluids are good conductors of electricity because they contain large quantities of electrolytes.
The record of these electrical fluctuations during cardiac cycle is called ECG or electrocardiogram.
An ECG gives information about conductivity, electrical activity and site of pacemaker.

Complete answer: ECG or electrocardiogram: It is the record of these electrical fluctuations during cardiac cycle.
Thus, the ECG recorded at the surface of the body represents the resultant activity in the individual myocardial fibre.
The waves associated with the electrical activity of the various parts of the heart tissue during each cardiac cycle are represented by the letters P, Q, R, S, T and U.
P wave: It is the first wave of ECG of duration 0.1 sec, directed upwards, rounded or pointed.
It is due to atrial depolarization and also represents the spread of impulse from SA Node to atrial muscles.
Its height is up to 0.5 mV which represents the functional activity of atrial muscles.
Q wave: It is a small negative deflection of height less than 0.2 mV less than 0.04 second.
Beginning of the Q wave represents the invasion of the midpoint of the interventricular septum by the excitation process.
R wave: It is a prominent, pointed positive wave.
Its upstroke corresponds with the onset of ventricular systole.
It represents the excitation process suddenly invading both ventricles.
S wave: It is negative pointed deflection which follows the R wave.
It represents excitation of more basal parts of ventricles.
T wave: It is rounded, positive deflection of duration 0.27 second and 0.5 mV heights.
U wave: It is rarely seen, positive small round wave of 0.08 second duration and 0.2 mV height.

So, the correct option is A. Atrial depolarization.

Note: ECG is recorded on an mm square graph paper moving at a speed of 25 mm/sec.
Any deflection of the record above the baseline is regarded as positive deflection.
Any deflection below the baseline is regarded as Negative deflection.
No deflection from the baseline means the isoelectric line or isoelectric segment.