Neuromuscular blockade (NMB), as the name implies, refers to a battery of agents that can specifically bind to the nicotinic receptor at the neuromuscular junction and thus block impulse transmission from the upper nerve to downstream muscle fibers, leading to transient or persistent skeletal muscle relaxation and aiding in easing endotracheal intubation and providing optimal conditions for operating and mechanical ventilation [1]. Many factors have been seen to affect the pharmacokinetics of NMB, such as age, organ function, or the apparent volume of distribution, requiring individualized NMB administration [2,3]. Hence, it is tricky for anesthesiologists to accurately maintain a shallow, moderate, or deep neuromuscular block at a proper time. However, an airway injury during endotracheal intubation, a sudden body-movement in surgical procedure, or a postoperative residual neuromuscular block (PRNB) during the reversal phase caused by the misjudgment of neuromuscular block may end in calamity [4,5,6]. Therefore, to reduce NMB-related complications, perioperative neuromuscular monitoring is highly recommended according to international guidelines to empower NMB with more controllability and to make NMB administration more individualized [7,8,9,10].

Typical perioperative neuromuscular monitoring methods (Figure 1) include acceleromyography (ACC), mechanomyography (MMG), electromyography (EMG), and kinemyography (KMG) [11]. ACC-based equipment chooses a sensor that can evaluate acceleration yielded by muscle contraction [12]. ACC can be applied in many muscles, such as the adductor pollicis muscle, the orbicularis oculi muscle, and the corrugator supercilii muscle [13]. It is currently regarded as the gold standard for scientific research and clinical practice. MMG, which measures force generated by muscle contraction, was once considered the gold standard of neuromuscular monitoring; however, it has been phased out due to its bulky setup. EMG-based equipment records compound action potentials from the target muscle. This technique is a desirable choice for neuromuscular monitoring in clinical settings and is also able to detect neuromuscular blocks at various muscles. KMG-based facilities utilize one kind of piezoelectric mechanosensory to obtain and reflect the contraction of the adductor pollicis muscle [13]. This method is easy to use and clinically reliable.

However, frequently used neuromuscular monitoring patterns are always criticized for their diverse shortcomings (Table 1). Despite its universal utilization in clinical settings, ACC is susceptible to artifacts and will probably generate an overrated block degree [12,14]. A conventional ACC-based apparatus records the monaxial movement of the target muscle; therefore, signal quality relies heavily on the posture of the recording sites [15]. Recently, ACC-based equipment capable of estimating the multiplanar movement of the thumb has been developed. However, its feasibility exploration is still in progress [16]. The application of MMG demands a special posture of the hand and an elaborate setup [17]. In addition, this method can only record signals at the adductor pollicis muscle [18]. For EMG, the working principle limits its application intraoperatively owing to disturbances from other electronic equipment in the operating room [19]. In addition, the recording result of the EMG is an electromechanical compound instead of absolute mechanical contraction of skeletal muscle. Similar to MMG, KMG is also subjected to hand position [11]. Moreover, KMG is not interchangeable with MMG according to existing investigations [20].

Phonomyography (PMG), also named acoustic myography, is a little-known neuromuscular monitoring technique. However, previous studies suggested that PMG may be a feasible alternative neuromuscular monitoring method. Our paper provides an overview of the evolutionary trajectory of PMG on perioperative neuromuscular monitoring chronologically and summarizes the advantages and drawbacks of PMG as a means to present an avenue for further research. We performed a PubMed search using the keywords “phonomyography”, “acoustic myography”, and “muscle sounds” updated to December 2021. Articles describing PMG on perioperative neuromuscular monitoring are displayed in Table 2.