Electrical Impulse - Depolarization

- Sodium influx and potassium efflux causes depolarization.

- Calcium influx holds depolarization above threshold.

- Repolarization prevents backflow of action potential.

Figure 1: Spread of Action Potential (1, p. 507)

ECG Readings

- Voltage A – Voltage B = ECG readings

- Electrical activity towards electrode = Positive deflection

- Electrical activity perpendicular to electrode = No deflection

- Electrical activity away from electrode = Negative deflection

Figure 2: ECG Interpretation (1, p. 518)

Limb Leads

- Lead I = R arm L arm

- Lead II = R arm L leg

- Lead III = L arm L leg

- aVR = Center R arm

- aVL = Center L arm

- aVF = Center L footFigure 3 – Limb Lead Interpretations (1, p. 516)

Normal axis is -30° to +90°

Finding Ventricular Axis:1. Look for isoelectric line – Impulse is traveling 90° from this lead.2. Look for the most positive lead – Provides rough value of vector.

Figure 4 - Einthoven's Triangle (1, p. 517)

Example

1. Lead III is the closest to isoelectric.

2. Lead III = 120°, therefore the vector is pointed either +30° or +210°(-150°).

3. This is equivalent to either aVR or lead II.

4. Lead II is more positive, therefore vector is around 30°.Figure 5 – Example (1, p. 520)

Re-entrant Conduction

- During bifurcation, there is no retrograde transmission because of repolarization.- When there is a block, i.e. dead cells, the action potential can travel in reverse.- If reentry frequency is greater than the SA node, tachyarrhythmias develop (V-tach, V-fib).Figure 6 – Re-entrant Conduction (1, p. 525)

Cellular conduction during an MI

Calcium and other electrolytes spill out of cell B and decreases the resting potential.

Figure 6 – Conduction during an MI (1, p. 523)

"J" - Point Identification

Figure 7 – J point Identification(2, p. 14)

Figure 8 – J Point: A. Normal, B. with ST-elevation, C. with ST-depression

ECG Progression of STEMI's

- Peaking of T-waves (ischemia - reversible)

- T-wave inversion (ischemia - reversible)

- ST-elevation (earliest sign of infarction)

- Deep Q-waves and T-wave inversion (dead cells – irreversible)

MI Progression

Figure 9 – Anterior Wall MI Progression (2, pg. 90)

Unusual MI's

- Right Bundle Branch Blocks• QRS complex is greater than 0.12 seconds

• V1 has a positive deflection

• ST-elevation in anterior chest leads

• ST-depression in II, III, and aVf

RBBB with MI

Figure 10 – RBBB with Anterior Wall MI (2, p. 105)

Unusual MI's

- Left Bundle Branch Block• More Difficult to Assess• 12-lead may show without MI

- Poor R wave progression- QS waves and ST-elevation in right chest leads (V1 – V2)

• Indicative of ischemia- QR complex in left chest leads (V5 – V6)- T-wave inversion in right chest leads with prominent S waves- ST-elevation in left chest leads (or in other leads with prominent R waves)- ST-depression in (V1-V2)

LBBB with Ischemia

Figure 11 – LBBB A. Normal, B. Ischemia with T-wave inversion (2, p. 106)

LBBB with Ischemia

Figure 12 – LBBB A. Normal, B. Ischemia with QR Complex in Left Chest Leads (2, p. 106)

Unusual MI's- Subendocardial Ischemia/Infarction

• Inner layer of heart is MORE prone to ischemia due to coronary blood supply on epicardium and high pressure of ventricles

• ST-depression with possible ST-elevation in VR• Infarction might also show with T-wave inversion (Figure 14)

Figure 13 – Subendocardial and Transmural MIs (2, p. 110)

Subendocardial MI

- Deep T-wave Inversion in Leads I, aVL, and V2 - V6

Figure 14 – Non-Q Wave MI (2, p. 114)

MI "Look-A-Likes"

- Prinzmetal's Angina: ST-elevation seen with chest pain at rest or at night. Caused by coronary spasm.- Pericarditis

• Generalized ST-elevation• PR elevation and ST-depression in aVR• PR depression with ST-elevation in other leads.• No abnormal Q-waves – occurs in cardiac death because of loss positive

depolarization.

Pericarditis

Figure 15 – Pericarditis with Generalized ST-Elevation (2, p. 140)

MI "Look-A-Likes"

- Acute myocarditis

- Systemic hypothermia

- Low Potassium

- ST-depression from left ventricular hypertrophy

- Digitalis effect: scooping of ST-T complex

Digitalis Effect

Figure 16 – Scooping of ST Segment (V5 and V6) due to Digitalis (2, p.126)

MI "Look-A-Likes"

- Angina from exertion: ST-segment depression with T-wave

inversion• May also present with real MI due to incomplete infarctions• Treatment integral due to risk of complete infarction

Myocardial Infarction ?

Figure 17 – 65 yom complaining of Epigastric Pain

Ascites

Figure 17 – 65 yom complaining of Epigastric Pain

Pacemaker- Temporary (emergency setting) or permanent- Typically present in right ventricular, but may also be right atrium- Pacer spike will appear with a left bundle branch pattern due to delayed left ventricular depolarization.- Modes

• Fixed – preset to specific rate• Demand – only when heart rate falls under certain value. A specific

magnet over batter will change it to "fixed."- Biventricular Pacemakers

• Both ventricles are paced at the same time• Typically present in pt's with CHF• Right bundle branch morphology• QRS duration shorter

- May also contain Implantable Cardioverter Defibrillator

Pacemaker- Malfunction

• Problem with sensing or pacing function• Failure to pace

- Pacemaker spikes without QRS complex (failure to capture)- No spikes with slow heart rate

• Causes- Low Battery: Rate slows as the battery runs out- Dislodgment- Fibrosis around tip of pacing wire

- Pacemaker Syndrome: Light-headedness, fainting, SOB, and cough due to loss of normal physiologic timing.

Atrial and Ventricular Pacemaker

Figure 18 – Ventricular Pacing without Atrial Capture

Sgarbossa Criteria for Pacemakers and LBBB

- Total score ≥3 is indicative of anterior MI1. ST-elevation ≥ 1mm concordant with QRS in any lead 5 Points2. ST-depression ≥ 1mm in V1, V2, or V3 3 Points3. ST-elevation ≥ 5mm discordant with QRS in any lead 2 Points

- Criteria has specificity of >95%, but sensitivity of 20%.

Figure 19 – Sgarbossa Criteria (3, p. 411)

ST/S Ratio- Divided ST-elevation by height of R or S wave (which ever is more pronounced) - Suggested to replace third criteria of Sgarbossa with ST/S Ratio ≤ -0.25- Sensitivity raised to 91% and specificity lowered to 90%.

Figure 20 - ST/S Ratio (3, p. 412)

Figure 21 – Algorhythm Using Sgarbossa and ST/S Ratio to Determine Need of PCI Treatment.

Conclusion

- Treat the patient, not the monitor.

- Follow local protocols.

References

1. Lederer, J. (2012). Cardiac Electrophysiology and the Electrocardiogram. In Medical Physiology (pp. 504-528). Philadelphia, PA: Saunders.

2. Goldberger, A.L. (2006). Clinical Electrocardiography: A Simplified Approach.Philadelphia: Mosby Elsevier.

3. Cai, Q., Mehta, N., Sgarbossa, E.B., Pinski, S. L., Wagner, G. S., Califf, R. M., & Barbagelata, A. (2013). The left bundle-branch block puzzle in the 2013 ST-elevation myocardial infarction guideline: From falsely declaring emergency to denying reperfusion in a high-risk population. Are the Sgarbossa Criteria ready for prime time? American Heart Journal, 166(3), 409-413.