The electrocardiographic determination of AV conduction disturbances is reasonably straight forward because the markers of atrial and ventricular depolarization (the P wave and QRS complex ) can be detected on the body surface and recorded on the standard body surface electrocardiogram. In contrast, the ECG determination of disturbances in conduction from the sinus node to the surrounding atrial myocardium, i.e. sino-atrial (SA) conduction disturbances, is more difficult because the electrical signal caused by sinus node depolarization is not strong enough to be recorded on the body surface. For this reason, sinus node activity must be deduced from the rate and rhythm of P waves that originate in the sinus node.
This ECG is from a 23 year old female with no known heart disease. Try to determine where sinus node depolarization occurs relative to the P wave.
