What is the clinical significance of a monophasic EKG (electrocardiogram) waveform?

Clinical Significance of Monophasic EKG Waveforms in Defibrillation

Monophasic waveforms are less effective for defibrillation and cardioversion compared to biphasic waveforms, requiring higher energy levels and resulting in lower first-shock success rates and potentially more myocardial damage.

Monophasic vs. Biphasic Waveforms: Basic Characteristics

Monophasic waveforms deliver current in only one direction (single polarity) and come in two main types:

• Monophasic damped sinusoidal (MDS): Current returns to zero gradually
• Monophasic truncated exponential (MTE): Current returns abruptly to zero

Biphasic waveforms, by contrast, deliver current in two directions and are the standard in modern defibrillators.

Clinical Significance in Defibrillation

Efficacy Differences

• Biphasic waveforms demonstrate higher first-shock success rates (85-98%) compared to monophasic waveforms 1
• Monophasic waveforms require higher energy levels (200-360J) compared to biphasic waveforms (≤200J) 1
• In shock-resistant atrial fibrillation, biphasic waveforms achieve 61% success rate compared to only 18% with monophasic waveforms 2

Energy Requirements

• For atrial fibrillation cardioversion with monophasic waveforms:
  • Initial energy should be at least 200J (not 100J, which has only 14% success rate)
  • Energy should be increased in 100J increments up to 400J maximum 1
• For ventricular fibrillation, monophasic waveforms typically require 360J for optimal success 1

Myocardial Impact

• Monophasic waveforms cause significantly greater ECG ST-segment elevation after shock compared to biphasic waveforms (2.09 ± 0.37 mm vs. 0.21 ± 0.38 mm), suggesting more myocardial dysfunction 3
• Higher energy requirements with monophasic waveforms may increase risk of post-shock myocardial damage 4

Practical Implications for Clinical Use

When Using Monophasic Defibrillators

1. Start with higher initial energy (200J) for atrial fibrillation 1
2. Use 360J for ventricular fibrillation 1
3. Allow at least 1-minute intervals between shocks to prevent myocardial damage 1
4. Consider anterior-posterior paddle configuration for improved success rates (87% vs. 76% with anterior-lateral) 1

Limitations of Monophasic Defibrillators

• Few monophasic defibrillators are still being manufactured, though many remain in use 1
• The American Heart Association acknowledges that biphasic waveforms are preferred but states monophasic defibrillators are acceptable when biphasic ones are unavailable 1

Common Pitfalls to Avoid

• Using too low initial energy with monophasic waveforms (100J has only 14% success for AF) 1
• Not allowing sufficient time between shocks (minimum 1 minute) 1
• Poor paddle placement or contact (increases impedance)
• Not synchronizing properly with QRS complex during cardioversion 1

Modern Practice Considerations

The clinical significance of monophasic waveforms is primarily historical, as biphasic defibrillators have largely replaced monophasic ones. However, understanding monophasic waveform characteristics remains important for:

1. Settings where older equipment is still in use
2. Comparing efficacy of different defibrillation technologies
3. Understanding the evolution of defibrillation technology and protocols

If a facility still uses monophasic defibrillators, consideration should be given to upgrading to biphasic devices given their superior efficacy and safety profile 1, 4.