ECGs
ECGs detect the sequence of electrical events that occur during the
contraction (depolarization) and relaxation (repolarization) cycle of the
heart.
Depolarization is initiated by the sinoatrial (SA) node, the heart’s natural
pacemaker, which transmits the electrical stimulus to the
atrioventricular (AV) node. From here the impulse is conducted through
the bundle of His and along the bundle branches to the Purkinje fibres,
causing the heart to contract.
The
atrioventricular (AV) node can also function as a pacemaker when there
is a dysfunction of the SA node, when the impulse generated is too slow
(sinus bradycardia) or when the impulse is not conducted to the AV node
(SA block, AV block).
, ECGs are recorded on graphed paper that travels at25 mm/s. It is divided
into large squares of 5 mm width, which represents 0.2 s horizontally.
Each square is then divided into five squares of 1 mm width. Electrical
activity is measured in millivolts (mV). A 1 mV signal moves the recording
stylus vertically 1 cm (i.e. two large squares).
An ECG complex consists of five waveforms labelled with the letters P, Q,
R, S and T, which represent the electrical events that occur in one
cardiac cycle.
The P wave represents the activation of the atria (atrial depolarization).
● P amplitude: 2.5 mm
● P duration: 0.06–0.12 s
The PR interval represents the time between the onset of atrial
depolarization and the onset of ventricular depolarization
● PR duration: 0.12–0.20 s
The QRS complex represents the activation of the ventricles (ventricular
depolarization).
● QRS amplitude: 5–30 mm
● QRS duration: 0.06–0.10 s
The ST segment represents the end of ventricular depolarization and the
beginning of ventricular repolarization.
The T wave represents ventricular repolarization.
● T amplitude: 10 mm
The QT interval represents the total time for ventricular depolarization
and repolarization.
● QT duration: 0.35–0.45 s
The U wave represents repolarization of the HisPurkinje system and is
not always present on an ECG.
ECGs detect the sequence of electrical events that occur during the
contraction (depolarization) and relaxation (repolarization) cycle of the
heart.
Depolarization is initiated by the sinoatrial (SA) node, the heart’s natural
pacemaker, which transmits the electrical stimulus to the
atrioventricular (AV) node. From here the impulse is conducted through
the bundle of His and along the bundle branches to the Purkinje fibres,
causing the heart to contract.
The
atrioventricular (AV) node can also function as a pacemaker when there
is a dysfunction of the SA node, when the impulse generated is too slow
(sinus bradycardia) or when the impulse is not conducted to the AV node
(SA block, AV block).
, ECGs are recorded on graphed paper that travels at25 mm/s. It is divided
into large squares of 5 mm width, which represents 0.2 s horizontally.
Each square is then divided into five squares of 1 mm width. Electrical
activity is measured in millivolts (mV). A 1 mV signal moves the recording
stylus vertically 1 cm (i.e. two large squares).
An ECG complex consists of five waveforms labelled with the letters P, Q,
R, S and T, which represent the electrical events that occur in one
cardiac cycle.
The P wave represents the activation of the atria (atrial depolarization).
● P amplitude: 2.5 mm
● P duration: 0.06–0.12 s
The PR interval represents the time between the onset of atrial
depolarization and the onset of ventricular depolarization
● PR duration: 0.12–0.20 s
The QRS complex represents the activation of the ventricles (ventricular
depolarization).
● QRS amplitude: 5–30 mm
● QRS duration: 0.06–0.10 s
The ST segment represents the end of ventricular depolarization and the
beginning of ventricular repolarization.
The T wave represents ventricular repolarization.
● T amplitude: 10 mm
The QT interval represents the total time for ventricular depolarization
and repolarization.
● QT duration: 0.35–0.45 s
The U wave represents repolarization of the HisPurkinje system and is
not always present on an ECG.
