7.4 Human Circulatory System · Part 6

of QRS complex. This is from the start of depolarisation of the atria to the beginning of ventricular depolarisation. It is the time taken for the impulse to travel from the atria to the ventricles ( . - .21sec). It is the measure of AV conduction time. QRS Complex (Ventricular Depolarisation) No separate wave for atrial depolarisation in the ECG is visible. Atrial depolarisation occurs simultaneously with the ventricular depolarisation. The normal QRS complex lasts for . - . sec. QRS complex is shorter than the P wave, because depolarisation spreads through the Purkinjie fibres. Prolonged QRS wave indicates delayed conduction through the ventricle, often caused due to ventricular hypertrophy or due to a block in the branches of the bundle of His. ST Segment It lies between the QRS complex and T wave. It is the time during which all regions of the ventricles are completely depolarised and reflects the long plateau phase before repolarisation. In the heart muscle, the prolonged depolarisation is due to retardation of K+ efflux and is responsible for the plateau. The ST segment lasts for . sec. T wave (Ventricular Repolarisation) It represents ventricular repolarisation. The duration of the T wave is longer than QRS complex because repolarisation takes place $WULDOGHSRODUL]DWLRQLQLWLDWHGE\WKH $QRGH FDXVHVWKH ZDYH  9HQWULFXODUGHSRODUL]DWLRQLVFRPSOHWH  9HQWULFXODUUHSRODUL]DWLRQEHJLQVDWDSH[ FDXVLQJWKH ZDYH  9HQWULFXODUUHSRODUL]DWLRQLVFRPSOHWH  :LWKDWULDOGHSRODUL]DWLRQFRPSOHWHWKH LPSXOVHLVGHOD\HGDWWKH$ QRGH  9HQWULFXODUGHSRODUL]DWLRQEHJLQVDWDSH[ FDXVLQJWKH FRPSOH[$WULDO UHSRODUL]DWLRQRFFXUV  Figure . Stages of ECG graph simultaneously throughout the ventricular depolarisation. XI Std Biology-Zoology Chapter-