ECG changes occur in severe or long-standing hypothyroidism. The major pathological changes causing it are:
1) Interstitial myocardial oedema and
2) Pericardial effusion.
The commonly associated ECG changes are:
1) Sinus bradycardia
2) Low voltage complexes (P, QRS and T waves)
3) ST segment depression (mild)
4) Prolonged P-R interval
5) A-V block
6) Prominent U waves
7) Corrected QT interval may be prolonged
Sinus tachycardia is the most important feature. In addition, arrhythmias may be seen (atrial fibrillation, atrial flutter or atrial tachycardia)
Generalised ST segment elevation is the most typical ECG change in acute pericarditis. This occurs due to a subepicardial inflammation of the myocardium adjacent to the inflamed pericardium.
The differentiating points to distinguish ECG changes due to acute pericarditis from acute MI causing ST elevation are:
1) ST segment appears to be concave upwards in acute pericarditis but is convex upwards in acute MI.
2) The ST segment changes in acute MI are localised but in acute pericarditis, the ST changes are generalised.
Transient, non-specific ECG changes may be present in more than half the patients with subarachnoid or intracranial bleeds. These changes result from autonomic disturbances, but pathological changes in the myocardium are not uncommon. The common ECG changes associated with CNS disease are:
1) Abnormally tall T waves
2) Deep T wave inversion
3) Prominent U waves
4) ST segment elevation/depression
5) Prolonged Q-T interval
6) Arrhythmias (sinus tachy/bradycardia, nodal rhythm, AF or VT)
A decrease in the body temperature is associated with the following ECG changes:
- Sinus bradycardia
- Prolonged P-R interval
- Prolonged Q-T interval.
"J wave": This occurs when the body temperature falls below 25° C. It appears as an extra deflection at the end of the QRS complex just overlapping the beginning of the ST segment.
Hypothermia: J waves or Osborne waves
In hypothermia, a small extra wave is seen immediately after the QRS complex (best seen in Lead I in this example). This extra wave is called a J wave or Osborne wave after the individual who first described it. This wave disappears with warming of body temperature. The mechanism is unknown.
Pulmonary embolism (PE)
Electrocardiographic changes develop in a very small proportion of cases with PE. ECG changes strongly suggestive of a diagnosis of pulmonary embolism are:
1) S1Q3T3 pattern: large S waves in Lead I, large Q waves in V3 and T wave inversion in lead III.
2) Abnormal right axis deviation
3) Transient RBBB
4) T wave inversion in the right precordial leads
But, the commoner findings are non-specific T wave changes and non-specific ST segment elevation or depression.