What is PTC (Post-Tetanic Count)?

Post-Tetanic Count (PTC): Definition and Clinical Application in Anesthesia

Post-Tetanic Count (PTC) is a neuromuscular monitoring technique used to assess deep neuromuscular blockade when no responses to train-of-four (TOF) stimulation are detectable, allowing clinicians to quantify the depth of blockade and predict recovery time. 1

Mechanism and Technique

PTC involves applying the following stimulation pattern:

• A 50 Hz tetanic stimulation for 5 seconds
• A brief pause (3 seconds)
• Single twitch stimulations at 1 Hz
• The number of detectable responses to these post-tetanic single twitches is counted as the PTC value

Clinical Significance and Interpretation

Depth of Blockade Assessment

• Deep blockade: Defined as 1-2 PTC responses at the adductor pollicis muscle with no TOF responses 1
• Very deep blockade: No PTC responses detectable
• Moderate blockade: TOF responses are present (1-4 responses)

Correlation with Recovery

• PTC values correlate with the time to reappearance of TOF responses:
  • PTC of 8-9 typically indicates imminent return of the first TOF response (T1) 2, 3
  • Higher PTC values indicate shorter time to recovery of TOF responses
  • The relationship can be expressed mathematically for specific muscle relaxants 3

Clinical Applications

1. Guiding Reversal Agent Selection and Dosing

• For sugammadex dosing:
  • Deep blockade (1-2 PTC responses): 4.0 mg/kg sugammadex 1, 4
  • Very deep blockade (no PTC responses): Wait and reassess later 1
  • Immediate reversal after high-dose rocuronium: 8.0 mg/kg sugammadex 1, 4

2. Predicting Recovery Time

• Allows estimation of when TOF responses will return
• Helps with planning surgical timing and extubation
• Different muscle relaxants have specific PTC-to-TOF recovery relationships 2, 3

3. Monitoring During Procedures Requiring Deep Relaxation

• Laparoscopic surgery
• Neurosurgery
• Procedures where patient movement must be prevented

Practical Considerations

Monitoring Sites

• Adductor pollicis is the standard site for PTC monitoring 1
• Different muscle groups have varying sensitivity to neuromuscular blockade:
  • Orbicularis oculi recovers earlier than adductor pollicis 5
  • PTC at adductor pollicis is more reliable for predicting overall recovery 5

Stimulation Frequency

• Repeated PTC stimulation can cause post-tetanic facilitation
• Traditional recommendation: Limit PTC assessment to every 6 minutes
• However, evidence suggests PTC can be repeated every 3 minutes without significantly affecting overall recovery time 6

Common Pitfalls

1. Misinterpreting deep blockade: When no TOF responses are present, PTC is essential to differentiate between deep and very deep blockade

2. Inadequate monitoring: Failure to use PTC when TOF shows no responses can lead to inappropriate timing of reversal agent administration

3. Incorrect site selection: Different muscle groups recover at different rates; adductor pollicis is the standard monitoring site

4. Overestimation of recovery: PTC may recover faster than clinically relevant recovery, especially with certain anesthetic agents 7

Decision Algorithm for Neuromuscular Blockade Monitoring and Reversal

1. Initial assessment: Check for TOF responses at adductor pollicis

2. If no TOF responses detected:

   • Perform PTC stimulation
   • Count number of responses

3. Based on PTC results:

   • No PTC responses: Wait and maintain anesthesia, reassess later
   • 1-2 PTC responses: Deep blockade, use sugammadex 4 mg/kg if reversal needed
   • ≥3 PTC responses: Approaching moderate blockade, continue monitoring

4. When TOF responses appear:

   • 1-2 TOF responses: Moderate blockade, use sugammadex 2 mg/kg if reversal needed
   • 3-4 TOF responses: Shallow blockade, use sugammadex 1 mg/kg if reversal needed

5. Continue monitoring until TOF ratio ≥0.9 to ensure complete recovery

PTC is an essential monitoring technique that provides critical information during deep neuromuscular blockade, guiding appropriate reversal agent selection and dosing while helping predict recovery timing.