P wave
The P wave represents right by left atrial depolarization and is low amplitude positive deflection preceding the QRS complex. Atrial repolarization occurs simultaneously with depolarization of the ventricular myocardium and is hidden by the QRS complex.
Assessment of the P wave allows an idea of where the atrial depolarization begins and whether or not the atrial are enlarged
- The best leads to look for P waves in are leads II and V1.
- Right Atrial Enlargement (RAE) tends to present as peaked P wave >2.5mm in lead II.
- Left Atrial Enlargement (LAE) is seen as a biphasic P wave in V1 with a 1 small box deep and 1 small box deep negative deflecton.
- Inverted P waves in leads II, III and aVF imply a negative axis of the P wave or retrograde conduction in the atrial due to a low atrial or junctional pacemaker.
PR interval
The PR interval is measured from the beginning of the P wave to the first part of the QRS complex. It includes time for atrial depolarization, conduction through the AV node, and conduction through the His-Purkinje system. Disruption at any point can prolong the PR interval. The length of the PR interval changes with heart rate, but is normally 0.12-0.20 seconds.
QRS complex
The QRS complex consists of the time needed for ventricular depolarization.
The first positive deflection of the QRS (R wave) is very steep, reflecting the quick distribution of electrical impulses through the His-Purkinje system. This represents depolarization of the left ventricular myocardium. The negative deflection following the R wave is the S wave which represents terminal depolarization of the high lateral wall. The lowercase letters are used for relatively small amplitude waves of less then 0.5mV or 5 mm (one large box).
A lengthened QRS complex can result from 3 possibilities:
- Impairment of the conduction system exists so that the wave of depolarization coming down from the atria does not or cannot travel the normal conduction system (bundle branch block, abberrant ventricular conduction, or Wolff-Parkinson-White syndrome).
- A beat initiated in the ventricle (PVC, Ventricular Tachycardia, or idioventricular).
In a normal ECG the R waves of the QRS complex begin small in V1-V2 and get larger as we progress through the precordial leads laterally. The R wave will be the dominant deflection in the QRS by V3-V4.
Early large R waves in V1-V2, as large as those in following leads, can reflect posterior wall infarction, lateral infarction, right ventricular hypertrophy or septal hypertrophy.
Poor R wave progression is when the R waves do not begin to dominate the QRS until V5 or V6. This may represent infarction or injury of the anterior left ventricle and carries almost as much significance as Q waves.
The size of the QRS can be determined by: the size of the patient (larger patients have smaller complexes), the left ventricular muscle mass, and the age of the patient (older patients tend to have smaller complexes).
ST segment
The ST segment occurs after ventricular depolarization has ended and before repolarization has begun. The ST segment is usually isoelectric and has a slight upward concavity. It is of particular interest in diseased states where it may have other configurations.
ST segments are measured for elevation or depression against the TP segment, not the PR interval. The TP segment is the true isoelectric line. The ST segment is always measure from the J point, where the QRS and ST segments meet. Many J points are an approximation with no clear transition from the QRS to the ST segment.
In young people, J-point elevation is seen as concave up-sloping ST segments that take off a bit above the isoelectric line and this is common. It does not indicate pathology.
T wave
The T wave represents the period of ventricular repolarization. Since repolarization is slower than depolarization, the T wave is broad and has a slow upstroke.
- Flattened T waves may indicate ischemia.
- T waves are normally upright in I, II, V3-V6 and inverted in lead aVR.
- T waves can be observed for indications of ischemia or electrolyte disturbances.
QT interval
The QT interval consists of the QRS complex (representing only a brief part of the interval), along with the ST segment and T wave, which constitutes the majority of the duration. The QT interval is used primary as a measure of membrane repolarization. Since the QT interval varies with heart rate, the QT is "corrected" (QTc) to make comparisons between ECG's. The corrected QT interval equals the QT when the ventricular rate is 60 bpm (the RR=1).
Causes of prolonged QT intervals include:
- Electrolyte abnormalities
- hypothermia
- Idiopathic long QT syndrome
- Tricyclic antidepressants
- Phenothiazines
- Antiarrhytmics (quinine and procainamide)
U wave
The U wave may be seen in some leads, especially the right precordial leads: V2-V4. U waves are associated with metabolic disturbances, typically hypokalemia and hypomagnesemia. Additionally, it may be seen following the T wave and can make interpretation of the QT interval especially difficult.
Determining heart rate
Standard ECG paper is standardized at 25mm/sec. The vertical lines on the ecg graph paper can be used to measure varying time intervals. There is a 0.20 sec between two of the large lines.
To count the number of heart beats (QRS complexes) in a regular rhythm, between 30 large boxes (6 seconds) you just multiply by 10, and this will give you a estimate of how many beats per minute. ECG paper also has additional markings every 3 seconds so you don't have to count 30 large boxes.
Other way that is quicker and easier to estimate the heart rate is to remember the table below:
Rhythm
