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.