Abstract

The use of muscle relaxant is an integral part of anesthetic management in present-day practice. Clinically, the neuromuscular blockade is evaluated by determining the thumb twitching to train-of-four (TOF) ulnar nerve stimulation at elbow.1, 2, 3 There are many ways for the assessment of twitch response, such as the use of electromyography4, 5; force displacement transducer techniques2, 6, 7, 8, 9; and most recently, the acceleration transducer–based system of neuromuscular monitoring.10, 11, 12, 13, 14, 15 These equipments are, however, bulky, cumbersome, expensive, or time consuming for operation. Because of these limitations, the routine use of such monitors is impractical. The evaluation of neuromuscular blockade, at the present time, relies on either visual or manual (tactile) thumb twitch responses to the TOF nerve stimulation. Visual or tactile evaluation of neuromuscular blockade is subjective and involves uncertainty, especially in the recovery phase. Furthermore, in certain surgical procedures, such as those on the head and neck, both hands are tucked in the sides of the table and become inaccessible, rendering visual or tactile evaluation of the thumb twitch impossible. 

 In such situation, muscle relaxant is often given in the light of guessing rather than on objective basis. Clearly, there is a need for a simple and practical way of measuring the twitch responses to TOF stimulation during anesthesia. Based on the current method of stimulation of the ulnar nerve over the ulnar groove at the elbow, which elicits a string thumb adduction (because of stimulation of the flexor carpi ulnaris muscle), a simple method that can objectively monitor and record the thumb twitch is developed. The unique features of the method or device are its simplicity and its ability to adapt to any pressure transducer to display the twitch response to TOF simultaneously with electrocardiogram, arterial, or central venous pressure.

Keywords

anesthesia; neuromuscular blockade;

1. Introduction

Stimulation of the ulnar nerve at the elbow (groove) elicits thumb adduction. If a finger-sized balloon is placed between the thumb and a hard plate, it is possible that when the thumb adducts, it will press the balloon against the plate. The pressure change is directly proportional to the force of compression. The balloon pressure change can be transduced and recorded.

2. Apparatus

The assembly that consists of anterior forearm splint affixed with a pressure sensor balloon is shown in Fig. 1. Anterior forearm splint is made of Polycaprolactone (North Coast Medical Inc., Morgan Hill, CA, USA) in functional position. Commercially, Rolyan preformed Resting Pan Mitt Splint (Rehabilitation Division, Patterson Medical Inc., Bowling, IL, USA) appeared to be quite suitable for this purpose. It comes in three different sizes: large, medium, and small. An elongation balloon 6 × 2 cm (8 mL in capacity), with connecting tubing is affixed over the thumb bed of the splint. Tubing is connected, through a three-way stopcock, from the balloon to a universal pressure transducer. The pressure transducer can be any conventional physiological pressure transducer that is used to measure arterial or venous pressure (Fig. 2).

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Fig. 1. The hand-board apparatus affixed with a pressure sensor balloon that is connected to a transducer.

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Fig. 2. The forearm and hand immobilized in the commercially made Pan Mitt Splint equipped with the pressure sensor balloon.

3. Operation

The patient’s hand is placed in the splint such that the thumb of the patient contacts the balloon that is disposed between the thumb and the splint. The forearm, wrist, thumb, and other fingers are fastened by Velcro straps (Patterson Medical Inc.), thereby stabilizing the entire forearm and hand. Once the forearm and hand have been well secured to the splint, the arm can be placed anywhere in any position without restriction. The connecting tube from the balloon is connected through a three-way stopcock with a pressure transducer, and the tracing is presented on monitoring screen.

Air is injected through the stopcock connector to pressurize the balloon at a pressure of 10–15 mmHg. This pressure gives the best evoked tension response on stimulation of the ulnar nerve. The ulnar nerve over the ulnar groove at the elbow is stimulated through the surface electrodes in usual manner to elicit thumb adduction that causes the thumb to tap on the balloon against the splint.

3.1. Clinical assessment

(1)The apparatus was tested for its practicality in a patient during craniotomy in the supine position. The typical recordings (Direct Digital Writer Series 7200; Marquette Electronics Inc., Milwaukee, WI, USA) of the thumb twitch responses to train-of-four (TOF) stimulation before, during, and after reversal of pancuronium neuromuscular blockade are shown in Fig. 3. Electrocardiogram (ECG) and arterial blood pressure are simultaneously shown. Figs. 3A and B: before the blockade. Note that the height of the fourth twitch is equal to the height of the first, that is, T4/T1 is 100%. Figs. 3C–E: during blockade. There is a gradual decrease in the height of fourth twitch in relation to the first. Eventually, the only response remained to TOF stimulation is the first twitch. Figs. 3F and G: during the recovery phase of the blockade, there is a gradual return of the fourth twitch. Fig. 3H: after the reversal of the blockade, the height of the fourth twitch is equal to the height of the first. T4/T1 ratio is again 100%.

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Fig. 3. A typical recording of train-of-four-evoked responses produced by neuromuscular blocking agent, pancuronium, during the course of craniotomy performed in supine position. The ulnar nerve was stimulated at the ulnar nerve groove at the elbow using a regular nerve stimulator. A, B: before the blockade; C–E: during the blockade; and F, G: during the recovery.

(2)The apparatus was also tested in a patient in the prone position for cervical laminectomy and fusion whose arms were tucked in the sides of table. The apparatus was applied while the patient was still in the supine position. After the apparatus having been properly secured, the patient was turned prone and supported by chest rolls. Both arms were brought down to the side of the trunk and tucked in with the bed sheet. Surface electrodes were placed over the ulnar groove at the elbow for TOF stimulation. Fig. 4 shows the tracing of thumb twitch recorded. Fig. 4A: TOF twitches at baseline before administration of pancuronium. Fig. 4B: 2 minutes after administration of pancuronium. Fig. 4C: 20 minutes after administration of pancuronium. Fig. 4D: 50 minutes after administration of pancuronium. Fig. 4E: 70 minutes after administration of pancuronium. Figs. 4F and G: after reversal of the blockade with glycopyrrolate/neostigmine.

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Fig. 4. A typical recording of train-of-four-evoked responses produced by neuromuscular blocking agent, pancuronium, during cervical laminectomy performed in the prone position, in which both arms are tucked along the sides of the table, unaccessible to the anesthesiologist. A, B: before the blockade; C, D: during the blockade; and E–G: during the recovery.

(3)Correlation with thenar muscle electromyography (EMG) (Fig. 5): the muscle EMG (Fig. 5B) was recorded on TOF stimulation of ulnar nerve at the elbow groove during cervical laminectomy and foraminectomy. At the same time, thumb twitch responses were recorded (Fig. 5A) using the present apparatus. The evoked EMG responses correlate well with evoked thumb twitch responses.

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Fig. 5. Comparison of TOF-EMG of thenar muscle evoked by the stimulation of ulnar nerve at elbow in one hand (A) and recording of TOF-evoked responses using present apparatus in the other hand (B). The TOF-evoked response recorded by present apparatus closely correlated with the recorded EMG-TOF stimulation. EMG = electromyography; TOF = train of four.

(4)Fig. 6 shows TOF twitch responses displayed together with ECG and arterial blood pressure tracings on Marquette monitoring screen (Marquette).

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Fig. 6. A Marquette monitoring screen displaying the train-of-four-evoked responses together with the electrocardiogram and an invasive radial arterial pressure tracings.

4. Discussion

In clinical anesthesia, the extent of neuromuscular blockade is most often monitored by TOF stimulation of the ulnar nerve at elbow groove, and the evoked thumb twitch responses are observed. Stimulation of the median and facial nerves are also sometimes used.16, 17 However, in certain surgeries, especially for the cervical spine either with anterior or posterior approach, both the upper extremities are tucked in, rendering the assessment of the neuromuscular function with conventional means impossible. The present apparatus is developed to solve this problem. It is based on the principle that as the thumb adducts, it compresses the balloon against the plate. The tracings as shown in Fig. 3 are the typical fading TOF responses of nondepolarizing muscle relaxant blockade.

The unique feature of using this apparatus to monitor the neuromuscular blockade function is that once the hand is fastened to the apparatus, the hand can be placed in any position, and the thumb need not have free room for movement. Because the stimulation of ulnar nerve causes the thumb to adduct, its adduction against a hard object will produce a force that can be transduced.

The other unique feature of the device is its simplicity. Unlike other apparatus, it only needs a pressure transducer that is readily available in the operating room of modern times. Once the system is connected, the TOF thumb twitching in response to the ulnar nerve stimulation can be displaced together with ECG and other pressures on the monitor screen.

In summary, the device that consists of an anterior forearm-hand splint with an elongated balloon at the thumb is described. It is based on the principle that when the thumb adducts, in response to ulnar nerve stimulation, it compresses the balloon against the splint. The balloon pressure change is transduced and displayed on the monitoring screen.

 † US Patent No. US 6,454,728 B 1 Sept. 24, 2002.