Often general practitioners refer  ECGs  with abnormal resting  ST/T wave patterns  to cardiologists .

Following are few of them

  • ST elevation
  • ST depression
  • T wave inversion
  • Tall T waves
  • A relatively uncommon  finding is  a flat ST segment  , which  is discussed here.

The commonest( benign) abnormality   is  T wave inversion  in women and tall ST /T waves   reflecting  early repolarisation  pattern in men. A flat ST segment is an occasional finding in general population.

ST segment is inscribed  during the most important  time  of  cardiac cycle.This is the period the ventricle is doing its prime function , namely ejecting the blood in systole .Hence it is subjected to maximum stress . During times of ischemia  ST segment  gets elevated or depressed depending upon the severity of ischemia. For the same reason , even  subtle changes in this segment is  frowned upon by cardiologists. Most of them would receive a EST.

It is ironical to note  , few normal people  show almost silent electrical activity during this  crucial  phase of   their  ECG .ST segment is often  a flat line  in them . This is a ECG of a women referred as CAD. She was asymptomatic . Echocardiogram  was normal . She was asked to do  a EST.

This asymptomatic women was refered for ECG opinion

The peculiar thing about T waves  are ,   a 10 mm upright  as well as  5 mm inverted T wave ,  both can be normal. So .  there is no element of surprise  to note absent  T waves  or a flat  T wave  to be called as normal .

The curious case of lost ST segment !

* T waves are recorded when K+ efflux occur rapidly out of cells . Hypokalemia  can be an important cause of flat T waves.

It is still a  mystery to me  why some people inscribe a tall T when  potassium comes out  of cell and  an equal number (Esp women)  record a down ward T wave  for the same event !  I wish  I get an answer  to this  lingering  question from  any of the readers !

Is a flat T wave represent  a T wave in  transition  to become inverted T wave  later ?

Possible .But we  are not sure ! A static T wave is safer than a dynamic T wave .

Final message

Flat ST segment and absent T waves  represent a same spectrum of ECG  findings  which  are  referred to as  non specific ST segment changes in  clinical practice .Generally , they have  little clinical significance.* In our experience we have found , female patients, Anemia  hypothyroidism  are  often associated with flat ST segments  . If CAD is suspected exercise stress test  should be done. Some believe a flat ST segment  is more likely to  result in EST positivity (Not necessarily true positive !)

* Non specific ST/T changes by itself is a  huge topic.  Ideally the term non specific ST /T changes should be avoided , as it  primarily came into vogue  to denote non ischemic ST segment (Still , other pathologies are very much  possible) It is estimated there are about  50 causes for non specific ST/T changes , right from a  benign situation  like deep   respirations , to significant  myocardial disorders. However , it still makes   good clinical sense for a  general practitioner  , to refer to a cardiologist , whenever ST  segment deviates  without any reason .

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The most common ECG dilemmas one encounters is to differentiate between the ST segment depression and T wave inversion due to LVH from that of primary ischemia.

Very often  , the entity is misdiagnosed . The implication can be serious , and adding further complexity is exercise stress testing is alos prone for errors in these group of patients as false negative or positive results are very common due to basline  ST/T changes.So it needs a CAG to confirm or rule out CAD in many .

Still the clinical acumen with the help of ECG can help us to a great extent !

A rough and approximate way to identify primary ischemia is given below.

Though these  humble ECG features may not be specific to diagnose CAD . One  need to remember even a normal coronary angiogram is  not synonymous with normal coronary arteries !

Read this blog on limitations of CAG .

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Non ST elevation Myocardial infarction  (NSTEMI) is a major mode of presentation of acute coronary syndrome.

Patients present with clinical unstable angina and  elevated cardiac enzymes or troponin.

ECG features can be any of the following.

1.ST depression (70-80%)

2.T wave inversion(10-20%)

3.Both ST depression and T wave inversion

4Post MI NSTEMI  -ECG changes variable ( Ironically ,even a residual  ST elevation may be present)

5.Normal ECG.

                                     Bulk of the NSTEMI belong to ST depression group.NSTEMI with purely T wave inversion is less common but occurs mainly in perioperative settings, pre existing CAD.NSTEMI with normal ECG is very rare  but can occur.

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