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Monday, 11 February 2013

Describing The ST Segment

I thought it might be useful to discuss the ST segment in a bit more detail.  

As with any other aspect of an ECG, when you come to comment on the ST segments, just describe what you see

As you know, ST segments can take on the following forms:
  • Isoelectric (Normal baseline)
  • Elevated
  • Depressed
Each of these forms should then be further described in accordance with the pattern/shape they take on. 

1. Isoelectric ST segments


Whilst an isoelectric ST segment is usually considered the norm, it may appear slightly 'squared' off with a sharper than usual cornering effect between the baseline and the T wave.  This can be an early sign of impending ischaemia, during exertion.  It may also appear slightly 'slurred' or 'scooped'.  This scooped pattern may sometimes appear slightly depressed (perhaps only 0.5mm - half a small square). 

Baseline 'slurring' of ST segments is a commonly seen normal variant in well, middle-aged women, and is a non-specific finding. These non-specific changes can make interpretation more tricky, if presenting with chest pain. 

It's important to comment on these variations and they need to be considered in conjunction with the history. Any ST changes occurring during an episode or chest pain, or exertion, need to be considered in terms of 'variation form the baseline'.

A classic example of this was seen on last weeks ECG in leads I and aVL (compare the scooped appearance seen in aVL with the nicely formed ST segment seen in the middle of the rhythm strip):


Given this lady's non-cardiac presentation, and lack of any cardiac risk factors, I would consider these ST segments 'non-specific'. 

2. Elevated ST segments

We are, hopefully, all pretty familiar with the localised convex pattern of ST elevation, which alerts us to an acute STEMI.  More detailed analyses of the acute settling changes of ACS and STEMI's are not really in keeping with this Primary Care blog, and I will leave for others to discuss. All you need to do is recognise it quickly and set in motion acute management (aspirin/GTN/999) in accordance with your local Acute Chest Pain guidelines. 

Other forms of ST elevation can be described as non-localised, saddle-backed, notched, scooped and can be seen in pericarditis, left bundle branch block and high take off. 

Examples of these different patterns can be seen in the July 2012's post Different Patterns of ST elevation.

3. ST Depression

ST depression is a sign of acute ischaemia, and is usually transient. This may be during and episode of chest pain, or may occur silently during exertion. The ST segments usually return to baseline once the angina/exercise episode is over. (Unless the stressor continues, or symptoms are ignored, and the patient progresses to develop a STEMI.)

Patterns of ST depression can be sub-divided into
  • Scooped/slurring (as above - may be significant; or normal variant in some women)
  • Upsloping
  • Horizonal
  • Downsloping
The latter three patterns denote increasing severity of ischaemia.


Scooped ST segments with minimal , if any, depression

These need to be considered in conjunction with history, presentation and risk factors.
In an otherwise well, female patient, with non-cardiac chest pain, and no risk factors, they should probably be ignored. If in any doubt - perhaps in a patient with atypical chest pain, and risk factors, a referral to the RACPC,  for consideration of CT Coronary Angiogram (CTCA) may be reassuring.  

In men, they are less often seen as a normal variant, and their presence should raise your suspicions.

However, if seen during an episode of acute chest pain, even in women, they may be indicative of an Acute Coronary Syndrome and should be managed accordingly. See "Heart and education" 2003 article by Tom Hyde (@thecardyologist) suggesting even 0.5mm ST depression is associated with an increased 4yr mortality). 

Upsloping ST Segments

Whilst this might be the very first and early sign of progressive ischaemia, it can also be a normal feature during faster pulse rates, such as seen during exercise. 
Uplsoping ST depression is often be seen during treadmill ECG's. As long as there is no progression (i.e to horizontal or downsloping ST changes) or no associated symptoms, and there is rapid resolution in the resting/recovery phase, it can usually be disregarded. 

Benign Upsloping ST depression during exercise induced tachycardia:




Horizontal ST depression

This is a more significant form of ST change, indicative of ischaemia. During exercise testing it is described in terms of deviation for the baseline: "1mm of Horizontal ST depression". By definition, for Horizontal ST depression to be considered 'significant' during ETT it should be > or equal to 2mm. less than this can be commented on as "non-diagnositic ST depression". 

Horizontal ST depression of 4-5mm during exercise testing is really quite impressive (see lateral leads):


Whilst the lateral leads above show good going downsloping ST depression, the inferior leads, whilst still a little upsloping, are heading that way soon, I suspect!



Downsloping ST Depression

This is the most advanced form of ST depression, denoting marked ischaemia. 
ST depression will frequently progress through upsloping - horizontal - downsloping pattern during progressive ischaemia/exercise. Most ETT protocols dictate that the test should be stopped if 2mm ST depression is associated with symptoms, or >3mm ST depression occurs, even in the absence of symptoms.


In advanced cases, the downsloping ST depression can eventually progress to cause a 'flipping' of the T-waves and thus T wave inversion is seen, before slowly returning back to baseline during recovery.



Hope this clears up ST segments.
H :)









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