What is the priority between capture and sensing in transcutaneous pacing (temporary pacing using electrodes on the skin)?

Transcutaneous Pacing: Capture vs Sensing Priority

In transcutaneous pacing, capture takes priority over sensing because ensuring mechanical capture is essential for maintaining hemodynamic stability and patient survival. 1, 2

Understanding Transcutaneous Pacing Priorities

Primary Goal: Achieving Capture

• Transcutaneous pacing is used in emergency situations for symptomatic bradycardia unresponsive to medical therapy and ventricular asystole, where establishing effective cardiac output is critical 2
• Capture refers to the successful electrical stimulation of the myocardium resulting in mechanical contraction, which is the ultimate goal of pacing 3
• Without effective capture, the pacing intervention fails to provide hemodynamic support, regardless of sensing capability 1, 3

Challenges with Capture in Transcutaneous Pacing

• Capture is variable in transcutaneous pacing compared to transvenous pacing, making it a priority concern 1
• Higher current (40-80 mA) is typically required to achieve capture, with even higher thresholds in patients with emphysema or pericardial effusion 2, 3
• Anteroposterior pad placement provides better capture with lower energy requirements (mean 93 mA) compared to anterolateral placement (mean 126 mA) 4

Confirming Mechanical Capture

• Electrical capture alone is insufficient - mechanical capture must be confirmed by:
  • Assessing arterial pulse manually 1, 2
  • Using arterial pressure monitoring 1, 2
  • Employing pulse oximetry monitoring 1, 2
• These confirmation methods are essential because the large pacing artifact may obscure or mimic the QRS complex on ECG 1

Role of Sensing in Transcutaneous Pacing

• Sensing is the ability of the pacemaker to detect intrinsic cardiac electrical activity 1
• While important, sensing is secondary to capture in emergency situations where transcutaneous pacing is deployed 2
• Sensing can only be checked if there is spontaneous cardiac activity, which may not be present in many emergency situations requiring transcutaneous pacing 1
• Oversensing issues (from large P or T waves, muscle artifact, or faulty equipment) are less immediately life-threatening than failure to capture 1

Clinical Approach to Transcutaneous Pacing

Initial Setup

• Position electrodes properly - anteroposterior placement is superior to anterolateral for achieving capture 4
• Start with appropriate energy settings, typically 40-80 mA for most patients 3
• Provide adequate sedation and analgesia for conscious patients due to significant discomfort associated with transcutaneous pacing 2, 3

Troubleshooting Capture Issues

• If capture is not achieved, increase output gradually until capture is confirmed 3
• Try different ECG monitoring leads to minimize pacemaker artifact and maximize QRS visualization 1
• If ECG assessment remains difficult, rely on non-ECG methods (arterial pressure, pulse oximetry) to confirm capture 1, 2

Monitoring and Management

• Continuous cardiac monitoring is necessary until transcutaneous pacing is no longer required 2
• Consider transcutaneous pacing as a bridge to more definitive therapy (transvenous or permanent pacing) 2
• For patients likely to require prolonged pacing, plan for transition to transvenous pacing 2

Practical Considerations

• Success rates for transcutaneous pacing are higher in bradycardia (with some intrinsic electrical activity) than in complete asystole 5
• Pediatric patients can be effectively paced transcutaneously using appropriately sized electrodes 6
• Patient discomfort and variable capture reliability are significant limitations of transcutaneous pacing 2