醫學筆記匯整 ED Notes

• VT
• SVT with aberrant conduction due to BBB
• SVT with aberrant conduction due to WPW
   
• Pace-maker tachycardia
• Metabolic : hyperkalaemia
• Poisoning with sodium-channel blocker (TCA)
   

 VT

- Extreme heart rates and lack of coordinated atrial contraction can lead to 
   hypotension, collapse, and acute cardiac failure. 

-  Decreased cardiac output may result in decreased myocardial perfusion
    with degeneration to VF.

  - Commonest mechanism : Reentry
     Develops due to abnormal myocardial scarring (usually prior ischemia or infarction)

- Monomorphic VT : most common, originates from single focus over ventricles.
   * Ischaemic Heart Disease
   * Dilated cardiomyopathy
   * Hypertrophic cardiomyopathy
   * Chaga' s Disease

ECG features increasing the likelihood of VT :

• Absence of typical RBBB or LBBB morphology
• Extreme axis deviation (northwest axis 西北電軸)
      QRS is positive in aVR and negative in I + aVF
   
• Very broad complexes (>160ms) 太寬 (4 小格)
• AV dissociation 
• Capture, Fusion beats
   
• Positive or negative concordance throughout the chest leads
      Ex : leads V1-6 entirely positive (R) or entirely negative (QS) complexes,
      with no RS complexes
   
• Brugada's sign – The distance from the onset of the QRS complex to the nadir of the S-wave > 100ms
• Josephson's sign – Notching near the nadir of the S-wave
   
• RSR' complexes with a taller left rabbit ear. 耳朵左大右小 ! 
      This is the most specific finding in favour of VT.
      This is in contrast to RBBB, where the right rabbit ear is taller.
   

一小格是 0.04 秒
4 小格就是 0.16 秒 ( 160 msec )

1. Absence of an RS complex in all precordial leads

也就是 positive or negative concordance
If all the precordial leads 都是 monophasic R or S waves = VT
If there are any RS complexes in V1-6 –> next step of the algorithm.

2. RS interval > 100ms in one precordial lead

Measured RS interval if RS complexes are present in V1-6
This is the time from the onset of the R wave to the nadir of the S wave.
  - If the RS interval is > 100 ms –> VT
  - If the RS interval is < 100 ms –> move on to step 3.

3. AV dissociation

The ECG is scrutinised for hidden P waves;
these are often superimposed on the QRS complexes and may be difficult to see.

If P waves are present at a different rate to the QRS complexes –> AV dissociation is present and VT is diagnosed.
If no evidence of AV dissociation can be seen –> go to step 4.

4. Morphological Criteria for VT

  - If there is a dominant R wave in V1 –> see criteria for RBBB-like morphology.
  - If there is a dominant S wave in V1 –> see criteria for LBBB-like morphology.

 關於 RBBB like : Dominant R wave in V1 

Appearance in V1-2

• Smooth tall monophasic R wave 高大 R
• Taller left rabbit ear (= Marriott' s sign) 耳朵左大右小
• qR complex (small Q wave, tall R wave) in V1

Appearance in V6

QS complex - a completely negative complex without R wave (strongly suggestted VT).
rS complex in V6 (R/S ratio < 1) - small R wave, deep S wave (indicates VT only if LAD is also present). 

 關於 LBBB like : Dominant S wave in V1 

Appearance in V1-2

• Initial R wave > 30-40 ms duration. 寬 R 
• Notching or slurring of the S wave (Josephson' s sign).
• RS interval (time from R wave onset to S wave nadir) > 60-70 ms.

Appearance in V6

With a LBBB-like pattern, the presence of Q waves in V6 is indicative of VT.

• QS waves in V6 (as with RBBB-like patterns, this finding is very specific for VT).
• qR pattern = small Q wave, large R wave.

有以上特性則偏向 VT !
 

The likelihood of VT is increased with :

• Age > 35 (positive predictive value of 85%)
• Structural heart disease
• Ischaemic heart disease
• Previous MI
• Congestive heart failure
• Cardiomyopathy
• Family history of sudden cardiac death
  (HOCM, congenital long QT syndrome, Brugada syndrome
  or arrhythmogenic right ventricular dysplasia that are associated with episodes of VT)

The likelihood of SVT with aberrancy is increased if :

• Previous ECGs show a BBB pattern with identical morphology
• Previous ECGs show evidence of WPW 
• History of paroxysmal tachycardias
      that have been successfully terminated with adenosine or vagal manoeuvres.
• > 95% of broad complex tachycardias in children are SVT with aberrancy.

https://lifeinthefastlane.com/ecg-library/basics/vt_vs_svt/
https://wikem.org/wiki/V_Tach_vs._SVT