Test: Electrocardiograph (ECG) (August 19) - NEET MCQ

In a standard ECG, the P wave is a small upward wave that represents depolarisation of atria which leads to contraction of both atria.

• The 4-chambered heart of birds is not superior to the 4-chambered heart of crocodiles. Both have heart as a muscle that pumps blood. Crocodile's heart has two aortic arches helping it to breath underwater, which makes it's heart more complex than bird's heart.
• Crocodile has two aortic arches, the two aortic arches connect with each other with the help of foramen of penizza as soon as they leave the heart but merge completely in the lower parts of the body away from the heart.

The labelled parts '1' and '4' are aorta and left ventricle respectively, which carry oxygenated blood. Left ventricle receives oxygenated blood from left auricle which received it from pulmonary veins and this oxygenaed blood then moves into the aorta to be supplied to the whole body.

Systemic circulation is the movement of the blood between heart and rest of the body (tissues) and the back to the heart. The oxygenated blood moves from the left auricle into the left ventricle. From here the blood moves into the aorta which ultimately divides into arteries that carry oxygenated blood to various tissues in the body. From the tissues deoxygenated blood is carried by the veins into the right atrium via superior vena cava and inferior vena cava.

'A' is vena cava that carries deoxygenated blood
'B' is aorta that carries oxygenated blood
'C' is pulmonary artery that carries deoxygenated blood
'D' is pulmonary vein that carries oxygenated blood.

In a standard ECG, a patient is connected to the machine with three electrical leads, one to each wrist and one to the left ankle.

Pulmonary circulation is the movement of blood between heart and lungs. During this pathway deoxygenated blood entering the right atrium, moves into the right ventricle From here it moves through the pulmonary arch into the lungs for oxygenation. Then from lungs oxygenated blood moves into the left atrium through pulmonary veins.

In ECG, P-R interval is the time from the beginning of the P wave to the beginning of the ORS complex. It corresponds to the time lag from the onset of atrial depolarisation to the onset of ventricular depolarisation. Most of the delay occurs in the AV node.

A normal electrogram (ECG) is composed of a ‘P’ wave, a QRS wave (complex), and a T wave. The ‘P’ wave is a small upward wave that represents electrical excitation or atrial depolarization which leads to contraction of both the atria. QRS wave (complex) begins after a fraction of a second of the P wave. It begins as a small downward deflection (Q) and continues as a larger upright (R) and triangular wave, ending as a downward wave (S) at its base. It represents ventricular depolarization. The ‘T’ wave is dome-shaped which represents ventricular repolarization. The potential generated by the recovery of the ventricle from the depolarization state is called the repolarization wave. The end of the ‘T’ wave marks the end of the systole.

The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction. By counting the number of QRS complexes that occur in a given time period, One can determine the rate of heart beat of an individual.