How to Use Post-Tetanic Count (PTC) in the Clinical Setting

What is PTC?

PTC (post-tetanic count) is the number of responses to single twitch stimuli spaced 1 second apart following high-frequency tetanic stimulation (5-second 50 Hz). The sequence was developed by Viby-Mogensen et al after discovering the phenomenon of post-tetanic potentiation; the ability to briefly detect a response to single twitch stimuli following a tetanus when the patient's TOFC is 0. Viby-Mogensen et al observed that as patients recover from deep neuromuscular block, their PTC increases [1]. 

Thus, post-tetanic count quantifies deep non-depolarizing block—when a patient has a TOF Count of 0 [1, 2, 3]. A typical intubation dose of neuromuscular blocking agent will cause a patient to lose all 4 twitches, but how long does it take a patient to recover their first twitch, and what if you don't want the first twitch to reemerge?

Perfusion, potentiating drugs and several other factors make patient response to neuromuscular blocking agents highly variable [3]. Debaene determined the clinical duration of a typical intubation dose of rocuronium (0.6 mg/kg) can range from 15 to 85 minutes [4]. Post-tetanic count provides data on the individual patient's rate of spontaneous recovery well before they recover their first TOF twitch. This new visibility enables the anesthesia provider to effectively predict recovery timing and proactively intervene to maintain optimal surgical conditions. In a 2007 BJA article, Dhonneur et al. provide evidence that a PTC measurement of the adductor pollicis is a better indicator of early diaphragmatic recovery than a train of four count at corrugator supercilii [5].

Why is PTC Useful?

Beyond proactive intervention, PTC can also help the provider determine the appropriate intervention. For example, you are mid-case and the surgeon indicates that the patient is breathing and requests more roc. Will a redose of neuromuscular blocking agent effectively suppress respiration? Unfortunately, there is no set quantitative measurement indicative of diaphragmatic suppression, and some patients regain or even maintain diaphragmatic function at deep levels of blockade. See Figure 1 for an example of a patient overbreathing the ventilator with a PTC 2 measured at the adductor pollicis. This ability to breathe spontaneously, maintain normal tidal volumes and end-tidal CO2 while profoundly relaxed is well documented [6, 7, 8]. In Chapter 22 - Neuromuscular Monitoring in Monitoring in Anesthesia and Perioperative Care, Dr. Aaron Kopman comments on the potential clinical responses to diaphragmatic activity.

Using PTC, you can identify a recovery threshold and target measurement for each patient and case. If your patient begins breathing at a PTC 14 or TOFC 1, a proactive redose of neuromuscular blocking agent prior to could be considered. If your patient’s recovery threshold resembles the patient in Figure 1, an alternate intervention, as noted by Dr. Kopman may be more effective at suppressing respiration. 

Figure 1: Image displaying patient spontaneous breathing, 43 mmHg EtCO2 and 550 ml tidal volume with a simultaneous post-tetanic count of 2. Source: Author

How does PTC work?

In the post-tetanic sequence, 50 Hz tetanic stimulation (50 stimulations per second) is applied, which causes post-tetanic potentiation making the neuromuscular junction more likely to respond to an incoming stimulus. After a three-second pause, single twitches are repeated once a second. The number of times muscle depolarization occurs equals the post-tetanic count or PTC (Figure 2). A patient with twelve responses to the single stimuli (PTC 12) is very close to recovering the first twitch on a train of four, while a patient with only one response to the single stimuli (PTC 1) is deeply relaxed [5].