Train of Four Ratio Target for Adequate Recovery
The target Train of Four (TOF) ratio for adequate recovery from neuromuscular blockade is ≥0.9, and this must be documented using quantitative neuromuscular monitoring at the adductor pollicis before patient awakening and extubation. 1
Why TOF Ratio ≥0.9 is the Standard
Residual neuromuscular blockade, defined as TOF ratio <0.9, carries significant risks that directly impact patient morbidity and mortality 1:
- Increased 24-hour postoperative morbidity and mortality 1
- Critical respiratory events in the recovery room (higher incidence than with general anesthesia alone) 1
- Postoperative pulmonary complications including aspiration and pneumonia 1
- Pharyngeal muscle dysfunction with impaired airway protection 1
- Reduced chemoreceptor response to hypoxia 1
- Risk of accidental awareness during general anesthesia 1
Critical Monitoring Requirements
Quantitative monitoring is mandatory—qualitative assessment is inadequate 1:
- Clinical tests (head-lift, hand grip, tongue depressor) have sensitivities of only 10-30% and positive predictive values <50% 1
- Tactile or visual assessment of TOF fade only detects recovery to TOF ratio ≥0.4, leaving a dangerous "monitoring gap" between 0.4 and 0.9 1
- Clinical signs (spontaneous respiration, coughing, extremity movement) do not exclude residual blockade 1
Proper Monitoring Technique
Monitor at the adductor pollicis muscle with ulnar nerve stimulation 1:
- The adductor pollicis has high sensitivity to muscle relaxants and slow recovery kinetics, making it the most reliable monitoring site 1
- Use supramaximal stimulation with quantitative measurement of the T4/T1 ratio 1
- If thumb movement is impeded during surgery, consider electromyography devices or TOF-Cuff, but revert to ulnar nerve monitoring before extubation 1
- Facial nerve monitoring carries five times greater risk of residual paralysis and should not be used for final assessment 1
Real-World Incidence Data
The problem is widespread despite standard practice 2, 3:
- 63.5% of patients had TOF ratio <0.9 at extubation in a Canadian multicenter study, even with qualitative monitoring and neostigmine use 3
- 56.5% still had TOF ratio <0.9 on PACU arrival 3
- In another observational study, 48.8% of patients were extubated without adequate recovery (TOF <0.9) 2
- Incidence of residual blockade ranges from 4-64% across studies 1
Reversal Agent Considerations
When using neostigmine for reversal 1:
- Wait for spontaneous recovery to at least four tactile TOF responses (TOF ratio ≥0.2) before administering neostigmine 1
- Even after neostigmine administration at four twitches, only 55% of patients under sevoflurane anesthesia achieved TOF ratio >0.9 within 10 minutes 1
- Propofol-based anesthesia allows more predictable neostigmine reversal than volatile agents 1
Sugammadex provides more reliable reversal 1, 4, 5:
- At moderate blockade (TOF count 2), sugammadex 2 mg/kg reverses to TOF ratio ≥0.9 faster than neostigmine 4
- At threshold TOF-count-four, sugammadex 1.0 mg/kg reverses to TOF ratio 1.0 in 2.1±0.8 minutes versus 8.5±3.5 minutes with neostigmine 5
- Sugammadex reduces postoperative pulmonary complications compared to neostigmine 1
Documentation and Timing
Document TOF ratio >0.9 before extubation 1:
- Apply the monitor after induction but before neuromuscular blockade 1
- Use throughout all phases of anesthesia 1
- Demonstrate and document adequacy of recovery (TOF ratio >0.9) before patient awakening and extubation 1
Important Caveats
Baseline TOF ratio normalization 6:
- Baseline TOF ratios measured by acceleromyography typically range from 0.95-1.47 and vary widely among patients 6
- A displayed TOF ratio of 0.9 may not represent adequate recovery if baseline was >1.0 6
- Consider normalizing postoperative TOF ratio by baseline value for more accurate assessment 6
Special populations requiring heightened vigilance 1: