ECG Blog #25 — Regular SVT and ST Depression

Interpret the 12-lead ECG shown below, obtained from a woman with sensation of “rapid heart beat”.  What is your differential diagnosis?  What diagnostic maneuver might help to determine what the rhythm is?

• Additional Questions: 1) Which leads are most helpful to assess for atrial activity?  2) Do you see atrial activity in all leads on this 12-lead tracing?  3) Do you need to be sure of the rhythm diagnosis before initiating treatment?

Figure 1 – 12-lead ECG from a patient with “rapid heart beat”.  What is your differential diagnosis?  What might you do diagnostically? 

INTERPRETATION:  Our preference is to always obtain a lead II Rhythm Strip whenever a question arises as to rhythm diagnosis.  This is shown in Figure 2.

Figure 2 – Lead II rhythm strip from the patient with tachycardia in Figure 1.  What is your differential diagnosis? 

The rhythm is regular at a ventricular rate that is close to 150/minute (the R-R interval is approximately 2 large boxes in duration — and 300 ÷ 2 = 150/minute).  The QRS complex is narrow (ie, not more than half a large box).  Normal atrial activity is absent, since upright P waves are not seen in lead II.  Instead, there is a suggestion of atrial activity having a negative deflection in lead II (as well as in other inferior leads).  There also appears to be a negative notching in the ST segment in each of the inferior leads.  Could this all represent atrial activity?

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Application of a Diagnostic Maneuver: 

The answer to the question of whether atrial activity is embedded within the 12‑lead tracing is forthcoming from application of a vagal maneuver.  Carotid Sinus Pressure (CSP) is applied at the moment marked by the large arrow in Figure 3. 

• Note shortly after application of CSP — there is reduction in the conduction ratio of impulses passing through the AV Node.  This produces a short pause — during which the typical sawtooth pattern of Atrial Flutter can be readily recognized. 

Figure 3 – Lead II rhythm strip showing the result of CSP, which reveals regular sawtooth flutter waves.

DISCUSSION:  A number of important points are brought out by this case.

• We often do not know definitive diagnosis of the rhythm at the time the patient presents.  This need not hinder our approach to treatment.  First priority in the assessment of any tachycardia is to ensure hemodynamic stability.  Although not explicitly stated in this case (we are not told the patient’s blood pressure) — We may presume the patient is stable because the presenting symptom is “rapid heart beat”.  IF the patient was hemodynamically unstable as a result of the rapid rate (ie, with chest pain, shortness of breath, mental confusion, hypotension) — then regardless of what the rhythm turns out to be, immediate synchronized cardioversion would be indicated.
• Given that this patient is stable — We have time to better assess the rhythm.   Use of a 12‑lead ECG during tachycardia is extremely helpful in this regard.  Next to lead II — lead V1 (which lies in proximity anatomically to the right atrium) is usually the next best lead to look at for atrial activity.  After leads II and V1 — we then direct our attention to all 10 of the remaining leads for clues to atrial activity.  Doing so in Figure 1 — we see suggestion of sawtooth activity in leads II, III, aVF, aVR, and V1 — but no sign of atrial activity in most of the other leads.  The 12‑lead tracing also confirms that the QRS complex is truly narrow in all 12 leads.
• The best description of the tachycardia defined by the rhythm in Figures 1 and 2 is that this represents a regular SVT (SupraVentricular Tachycardia) without sign of normal atrial activity.  This description should prompt consideration of the 3 entities shown in Table 1 as the likely cause of the tachycardia:

Table 1 – List of the 3 most common causes of a regular SVT when normal atrial activity is not seen.

• Practically speaking — 90-to-95% of the regular SVTs that primary care clinicians encounter (in and out of the hospital) will be one of the 3 entities on this list.  Clearly — other causes exist (junctional tachycardia, ectopic atrial rhythms, accessory pathway reentry tachycardias, etc.) — but keeping in mind these 3 entities will simplify and facilitate diagnosis in the majority of cases.
• Heart rate may help in distinguishing between the 3 entities in List #1 — IF the rate exceeds 170/minute in an adult.  This is because it is unusual for a nonexercising adult to be in sinus tachycardia at rates above 170/minute.  Atrial flutter with 2:1 AV conduction is also unlikely when the ventricular rate exceeds 170/minute, since the atrial rate of untreated flutter is rarely faster than 340/minute in adults.  However — when the rate of a regular SVT is approximately 150/minute (as it is in Figures 1,2) — any of the 3 entities in Table 1 may be the cause.
• We suspect the rhythm in Figures 1,2 is not sinus tachycardia, because we do not see an upright P wave in lead II.  Application of a vagal maneuver resolves the issue (Figure 3) — as it brings out flutter waves that had been partially hidden within the QRST complex in Figure 1.
• Although not essential for our initial approach to this patient — knowing definitive diagnosis facilitates treatment.  Use of an AV-Nodal-slowing drug (ie, diltiazem, a beta-blocker) now becomes appropriate at this point. Synchronized cardioversion might ultimately be needed if medical treatment does not convert the rhythm.

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  - See also ECG Blogs #23 and #24 -
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