1. 
   

   The ST segment and T wave in ischemia

   
   
   
   
   1. 
      

      There are over 100 identified causes of ST segment and T wave changes, so the diagnosis of ischemia and infarction frequently requires comparison with previous ECGs and correlation with the clinical presentation and laboratory data.

      
      
   2. 
      

      Myocardial ischemia produces a range of changes in the ST segment and T wave, depending on the severity of ischemia and the timing of the ECG.

      
      
   3. 
      

      The specificity of the ST segment for ischemia is dependent on its morphology. (Day 4-01)

      
      

       

      
      

      

      
      

       

      
      
   4. 
      

      In exercise stress testing, 1 mm or more of horizontal or downsloping ST segment depression 80 msec from the J point is considered an ischemic response. (Day 4-02)

      
      
   5. 
      

      The specificity of ST segment and T wave changes is decreased in patients with resting abnormalities, particularly LBBB and LVH. (Day 4-03)

   
   

    

   
   

   DAY 4-01

   
   

   

   
   

    

   
   

   DAY 4-02

   
   

   

   
   

    

   
   

   DAY 4-03

   
   

   

   
   

    

   
   
2. 
   

   Myocardial infarction(MI)

   
   
   
   
   1. 
      

      ECG patterns in infarction

      
      
      
      
      1. 
         

         A zone of ischemia typically produces ST segment depression.

         
         
      2. 
         

         A zone of injury produces ST segment elevation.

         
         
      3. 
         

         A zone of infarction produces a large Q wave in the QRS complex.

      
      

       

      
      

      

      
      

       

      
      
   2. 
      

      Genesis of the Q wave in infarction

      
      
      
      
      1. 
         

         The normal situation

         
         
         
         
         1. 
            

            For example, in Lead I, the QRS complex begins with a small Q wave because left ventricular depolarization begins in the septum and the electrical forces are directed away from Lead I.

            
            
         2. 
            

            The small Q wave is rapidly succeeded by forces directed inferiorly and laterally, resulting in a large R wave in Lead I.

         
         
      2. 
         

         The infarct situation

         
         
         
         
         1. 
            

            If there is a lateral myocardial infarction, however, the electrical vectors in the lateral direction are lost, the forces directed medially are unbalanced.

            
            
         2. 
            

            A large Q wave results in Lead I.

   
   

    

   
   

   

   
   

    

   
   
3. 
   

   The time course of myocardial and ECG changes during infarction

   
   

    

   
   

   

   
   

    

   
   
4. 
   

   Anatomical and ECG locations of MI (Day 4-04) (Day 4-05)

   
   

    

   
   

   

   
   

    

   
   

   DAY 4-04

   
   

   

   
   

    

   
   

   DAY 4-05

   
   

   

   
   

    

   
   

   DAY 4-06

   
   

   

   
   

    

   
   

   DAY 4-07

   
   

   

   
   

    

   
   
5. 
   

   Non-ST-segment elevation MI (NSTEMI) (Day 4-08)

   
   
   
   
   1. 
      

      About half of the 750,000 MIs that occur annually in the U.S. do not develop new Q waves.

      
      
   2. 
      

      Usually there are ST segment and T wave changes, but about 20% of NSTEMIs have no obvious ECG abnormalities and the diagnosis of MI is based on the clinical presentation and elevated cardiac markers.

      
      
   3. 
      

      Anatomically, NSTEMIs are frequently associated with patchy subendocardial necrosis.

      
      

       

      
      

      

DAY 4-08

Sample Tracings

ECG 1

ECG 2

ECG 3

ECG 4

ECG 5

ECG 6

ECG 7

ECG 8

ECG 9

ECG 10

ECG 11

ECG 12

ECG 13

ECG 14

ECG 15

ECG 16

ECG 17

ECG 18

ECG 19

ECG 20

Ischemia and Infarction

ECG 1

Atrial rate:

Ventricular rate: 150

Rhythm: Atrial fibrillation with rapid ventricular response

P wave:

PR interval:

QRS complex:

 Axis: -75°

 Duration: 110 msec

 Voltage: Normal

 Morphology: Q waves in V₁ and V₂

ST segment: Diffuse ST segment depression, and elevation in V₁ and V₂

T wave: Normal

QT interval: 310 msec

U wave:

Diagnosis: Atrial fibrillation with rapid ventricular response, left axis deviation, and diffuse ST changes consistent with ischemia and possible acute septal MI

ECG 2

Atrial rate: 54

Ventricular rate: 54

Rhythm: Sinus bradycardia

P wave: Normal

PR interval: 160 msec

QRS complex:

 Axis: 110°

 Duration: 180 msec, RBBB

 Voltage: Normal

 Morphology: Q waves in V₁ to V₃

ST segment: Normal

T wave: Normal

QT interval: 480 msec

U wave:

Diagnosis: Sinus bradycardia with right axis deviation, RBBB, and old anteroseptal MI. It is essential that one recognize the presence of concomitant RBBB and the infarct. The expected R wave of the RSRí in V₁ has been replaced by a Q wave.

ECG 3

Atrial rate: 68

Ventricular rate: 68

Rhythm: Sinus rhythm

P wave: Normal

PR interval: 180 msec

QRS complex:

 Axis: -75°

 Duration: 110 msec, with an incomplete RBBB

 Voltage: Normal

 Morphology: Q wave in V₁ to V₄, III and aVF

ST segment: Elevated in V₁ to V₅

T wave: Normal

QT interval: 380 msec

U wave:

Diagnosis: Sinus rhythm with left axis deviation, incomplete RBBB (the morphology of RBBB is present but the QRS deviation is <120 msec), old inferior MI, and anterior MI, probably recent. Once again, the R wave of the RSR’ in V₁ of the incomplete RBBB has been replaced by a Q wave.

ECG 4

Atrial rate: 86

Ventricular rate: 86

Rhythm: Sinus rhythm

P wave: Normal

PR interval: 140 msec

QRS complex:

 Axis: 80°

 Duration: 115 msec, with an incomplete RBBB

 Voltage: Normal

 Morphology: Normal