1 Introduction

Laryngoscopy can be challenging during anesthesia due to anatomical features or clinical scenarios. Examples include limited atlantooccipital joint extension or mouth opening, Mallampati grade III or IV views of the glossopharyngeal space, or an immobilized cervical spine after trauma [1, 2]. Failure to expose the glottis can have severe, potentially life-threatening consequences [3, 4]. In addition to a comprehensive airway assessment before anesthesia induction, the choice of appropriate intubation instrument is crucial [5].

The Disposcope endoscope (Disposcope Taiwan, Hsinchuang, Taiwan) is a newly introduced medical device for endotracheal intubation [6] (Fig. 1). It consists of a wired transmitter [7] for handling and wireless connection to a portable 14.2 cm display screen, and a wire body [8] with a microcamera at the tip. Park et al.’s simulation study on a manikin showed that the Disposcope provides easier glottis visualization compared to the Macintosh laryngoscope [9]. The wire body is made of flexible memory metal, allowing operators to adjust it to optimal angles based on the patient’s condition [6].

Fig. 1
figure 1

Composition of Disposcope endoscope. A The Disposcope (Disposcope Taiwan, Hsinchuang, Taiwan) consists of a flexible wire transfer (wire body), wire transmitter and portable screen. B The Disposcope endoscope with an endotracheal tube

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This study aims to evaluate whether the Disposcope could improve glottis visualization, minimize hemodynamic disturbance, and reduce movement of the upper cervical spine (C-spine) during laryngoscopy, particularly in patients predicted to have difficult intubation.

2 Methods

2.1 Case description

The study protocol was reviewed and approved by the Anesthesia Ethics Committee of the Changzhou NO.2 People’s Hospital. From March 1, 2019, to October 31, 2020, 60 patients were initially enrolled in the study. Informed written consent was obtained from all patients before study enrollment. All patients who underwent lumbar spine surgery. Prior to their participation. The patients included in the study were classified as the American Society of Anesthesiologists physical status grade I-II and were scheduled to undergo elective surgery requiring general anesthesia with tracheal intubation. They had no history of hypertension, heart disease, or other cardiovascular diseases. However, all patients enrolled patients exhibited one or more the following features that predicted difficult intubation, such as a body mass index of more than 25, interincisor distance less than 3.5 cm, thyromental distance less than 6.5 cm, or Mallampati grade III and IV view of the glossopharyngeal space. Patients with known difficulty in mask ventilation, or extensive maxillofacial tumors, or serious head and facial trauma, or respiratory tract obstruction were excluded from the study.

2.2 Conduct of anesthesia

Patients were premedicated with 0.1 g intramuscular phenobarbital and 0.01 mg·kg−1 penehyclidine hydrochloride 30 min before anesthesia induction. Standard monitoring techniques were applied, such as electrocardiography, non-invasive blood pressure measurement, and pulse oxymetry. Patients received 3 L of 100% for several minutes before anesthesia. Anesthesia induction was performed with 0.05 mg·kg−1 intravenous midazolam, 2 mg·kg−1 propofol, 0.15 mg·kg−1 besylate cisatracurium, and 0.5 μg·kg−1 sufentanil. Lidocaine 2%, 2 mL, was administered intravenously to prevent pain associated with the propofol administration. Before laryngoscopy, patients were placed in the supine position without a pillow on the operating table (i.e., neutral position). An accelerated muscle relaxation monitor (Organon, Ireland) was connected to monitor contraction of the muscle flexor digiti brevis contraction.

Laryngoscopy was initiated when the first twitch (T1) of the train-of-four reached its maximum depression. When using the Macintosh laryngoscope, the operator employed the cross-finger method to open the patient’s mouth and used the blade to displace the tongue, facilitating its insertion and providing visibility of the exposure of epiglottis and glottis. On the other hand, when using the Disposcope, the operator used the left thumb and fingers to open the patient’s mouth, holding the lower incisor and chin. The distal third of the wire body was grasped and kept it parallel to the oral fissure. With the screen display in view, the operator inserted the tube downward along the oropharynx, guiding it to the vocal cords and confirming visualization of the glottis.

2.3 Assessment methods

With standard precautions taken to protect against radiation exposure, lateral radiographs of the C-spine were obtained before and after exposure of the glottis exposure using the Macintosh or Disposcope devices to observe the movement of C-spine movement. The Cormack and Lehane classification and hemodynamic changes during the laryngoscopy were also recorded. All patients underwent sequential exposure to both devices successively, and were intubated 5 min after the second exposure with the second device (Disposcope was used when the Macintosh failed to intubate). A crossover trial design was employed, with half of the patients initially exposed to the Macintosh followed by the Disposcope, and the other half of patients were exposed to the Disposcope first followed by the Macintosh. Following the withdrawal of the first device, the second device was not immediately inserted until the patient’s blood pressure and heart rate (HR) returned to within ± 10% of their original levels. Patients who did not recover to their original blood pressure and HR levels within 5 min were excluded from the study. The Cormack and Lehane classification was as follows according to the previous criteria: grade I indicated a full view, grade II indicated visibility of only the arytenoid cartilages, grade III indicated visibility of only the epiglottis, and grade IV indicated no visibility of the epiglottis [10]. HR and mean arterial pressure (MAP) were recorded before exposure, as well as at 1 min, 3 min, and 5 min after exposure. All procedures were performed randomly by one of two skilled operators who were blinded to the data analysis of this study.

2.4 Definitions of intervertebral angles

Due to obscuration of views of the fifth cervical vertebral body in some patients were obscured by the patients’ shoulders, only the angles between the first cervical vertebra and the occipital bone, and the adjacent segments from the first cervical vertebra to the fourth cervical vertebra were measured on the lateral radiographs. The McGregor line [11], which connects the most dorsal and caudal part of the occiput to the posterior hard palate, served as the reference line for the occipital bone (C0). The reference for C1 was an imaginary line between the lower cortical margin of the anterior and posterior arch. Similarly, the reference for C2–C4 was an imaginary line between the anterior, inferior margin of the respective vertebral bodies (Fig. 2). Evaluation of the upper C-spine movement involved measuring the angles between adjacent vertebral reference lines from the occipital bone to C4. The change in angle between before and after exposure was defined as the maximum change in the angle between adjacent cervical vertebrae. A negative value indicated flexion of the C-spine, while a positive value indicated extension of the C-spine. The cumulative upper C-spine movement was determined by summing the maximal changes in the angles of adjacent vertebrae from the occipital bone to C4.

Fig. 2
figure 2

Reference lines of different cervical vertebra. The reference for the occiput was the McGregor line, which joins the occiput’s most dorsal and caudal part of the occiput to the posterior hard palate, as a reference line for the occipital bone (C0). The reference for the cervical segment C1 was an imaginary line between the lower cortical margin of the anterior and the posterior arch. The references for C2–C4 were lines between the anterior, inferior margin of the respective vertebral bodies, and the lower cortical margin of the respective spinous process. DE Disposcope endoscope; ML Macintosh laryngoscope

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2.5 Analysis and statistics

Of the 68 initially enrolled patients, only 60 patients were included in this study. Six patients were excluded because the lateral radiographs of the neck were not accurately recorded (two with Macintosh and four with Disposcope), and the other two patients were excluded because the blood pressure and HR did not recover to the original level within 5 min. Then we followed the minimal sample formula as follows: n = Z2 × (P × (1−P))/E2, n: sample size; Z: statistics; E: error value; P: probability value. In our study, Z = 1.5, E = 10%, P = 0.5, so the sample size (n) is 56. Because the allowable error is 10%. Based on this calculation, the sample size of 60 patients would meet the requirement. Patient age, body mass index, atlantooccipital joint extension, intubation time, degree of mouth opening, thyromental distance and Oxygen Saturation were presented as mean and standard deviation (SD). The difference in Cormack and Lehane grades was analyzed using the Wilcoxon pair rank-sum test. Maximum changes in the angle between adjacent cervical vertebrae and cumulative upper C-spine movement were reported as median values with ranges (the 25th and 75th percentiles) and analyzed using the Brown-Mood median test. Changes in hemodynamics at different time points were analyzed using one-way ANOVA (Analysis of Variance), while the changes in hemodynamics with different devices were analyzed using paired t-tests. All statistical analyses were performed using SPSS 22.0 software. A P-value less than 0.05 was considered statistically significant.

3 Results

Of the 68 initially enrolled patients, only 60 were included in this study. Six patients were excluded due to inaccurate recording of lateral radiographs of the neck were not accurately recorded (two with Macintosh and four with Disposcope), and the other two patients were excluded because their blood pressure and HR did not recover to the original level within 5 min. The clinical characteristics of all patients are shown in Table 1. The intubation time in the Disposcope endoscope group was significantly lower than that in the Macintosh laryngoscope group, P < 0.01. Cormack and Lehane scores were obtained from all patients. Using Disposcope resulted in a higher proportion of straightforward views (grades I and II) compared to Macintosh (Table 2). Among the seven patients with less than 3 cm of mouth opening; the glottis was visible in only three cases (42.8%) with Macintosh, but in all cases with Disposcope (data not shown). Additionally, compared to the blood pressure and HR before exposure, there was a significant increase at 1 and 3 min after exposure. Furthermore, hemodynamic fluctuations were significantly reduced with Disposcope compared to Macintosh (Figs. 3 and 4).

Table 1 All patient data are displayed with mean (SD) except sex ratio and Mallapati classification
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Table 2 The Cormack-lehane classification of Disposcope and Macintosh
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Fig. 3
figure 3

Mean arterial pressure (MAP) changes associated with glottic exposure in two groups of patients: Disposcope group (Red), Macintosh group (Blue) (n = 60, for each group). Data are displayed with mean ± SD. Asterisks indicate significant differences (*P < 0.05). T1:1 minate after exposure; T2: 3 minate after exposure; T3: 5 minate after exposure

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Fig. 4
figure 4

Heart rate (HR) changes associated with glottic exposure in two groups of patients: Disposcope group (Red), Macintosh group (Blue) (n = 60, for each group). Data are displayed with mean ± SD. Asterisks indicate significant differences (*P < 0.05). T1: 1 minate after exposure; T2: 3 minate after exposure; T3: 5 minate after exposure; SD Standard deviation

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The median cumulative upper C-spine movement was 9.4° (range: 2.6° to 15.3°) in the Disposcope group. In contrast, the median cumulative movement was greater in the Macintosh group (26.9°, range: 21.6° to 29.7°) (Fig. 5). The median maximal changes in the angle with Macintosh between adjacent cervical vertebrae with Macintosh were 13.9°, 8.3°, 5.0°, and −1.4°, respectively. On the other hand, the changes in angle with Disposcope were 9.3°, 0.25°, −0.25°, and −0.3°, respectively. These results indicate that using Disposcope reduces the angulation of the cervical spine (Table 3).

Fig. 5
figure 5

Cumulative movement of the upper cervical spine with Disposcope and Macintosh. Disposcope means to expose the glottis by disposcope endoscope, Macintosh means to expose the glottis by Macintosh. **P < 0.01, vs. Macintosh group

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Table 3 The median maximal changes in the angle between adjacent cervical vertebrate with Macintosh or Disposcope
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4 Discussion

This study showed that the intubation time in the Disposcope endoscope group was significantly less than Macintosh laryngoscope (P < 0.01), and the body of the Disposcope endoscope, a wire, is made of flexible memory metal, so it bends easily, which allows the surgeon to adjust it to the optimal angle and advance the tracheal tube faster, depending on the patient and the situation speed. Furthermore, the Disposcope endoscope wire is encased in an endotracheal tube with an outer diameter of < 10 mm. For patients with difficult mouth opening, the Disposcope endoscope can overcome this obstacle.

This study demonstrates that Disposcope improved views of the glottis, particularly in patients with mouth opening less than 3 cm, as indicated by the Cormack and Lehane classification. Disposcope is a novel fiber optic intubation device that offers additional support in situations where glottis exposure is challenging [12]. It features a rapid image output and an antifogging lens with a wide-angle view of 70°, ensuring clear visualization of the glottis. The device is constructed with bypass memory metal, allowing for significant bending and adjustment to accommodate various angles [6]. In a simulation study on a manikin wearing a semirigid neck collar, Park et al. reported that Disposcope provided a superior glottis view and a higher success rate for endotracheal intubation [9]. However, the model airway used in the manikin cannot precisely replicate the diverse features that contribute to difficult intubation. Therefore, our study is designed to evaluate the device’s clinical performance using patients who exhibit one or more characteristics associated with difficult laryngoscopy.

It is well established that traditional Macintosh laryngoscopy increases arterial blood pressure and HR [13], which can be particularly detrimental to patients with cardiovascular and cerebrovascular diseases [14]. Mitigating this reflex response during laryngoscopy remains a significant concern for anesthesiologists. The exposure of the glottis during laryngoscopy requires the elevation of the epiglottis through forward and upward lifting of the laryngoscope blade [15]. Studies have revealed that the force exerted during laryngoscopy is a key factor in stimulating stretch receptors in the respiratory tract [16]. Therefore, any technique requiring less lifting force during laryngoscopy would result in a proportional reduction in sympathetic discharge [17], leading to fewer changes in HR and blood pressure. Previous research has demonstrated that the use of improved equipment, such as the TruView EVO2 fiber optic intubation device, can attenuate the reflex response [18, 19]. As a new fiber optic intubation device, Disposcope also requires less lifting force [17]. Our data suggests that using this new instrument significantly reduces hemodynamic disturbances compared to the traditional Macintosh technique.

With the increasing incidence of cervical diseases and cervical trauma, anesthesiologists frequently encounter patients with limited neck mobility or potential or actual cervical cord compression [20] Therefore, maintaining cervical spine stability of cervical spine during the intubation is crucial. Some studies have demonstrated that the traditional Macintosh laryngoscope can cause the movement of the upper cervical spine and exacerbate cervical spinal cord injury [21, 22]. In our study, on maximal glottal exposure using the Macintosh blade resulted in changes in the angles between the atlas and the occipital bone (C0/C1), atlas and the second cervical vertebra (C1/C2), atlas and the third cervical vertebra (C2/C3), and the third and fourth cervical vertebrae (C3/C4) by 13.9°, 8.3°, 5.0°, and −1.4°, respectively, compared to the angles of patients before exposure. Such movement has the potential detriment to harm the spinal cord in an unstable cervical spine. In contrast, using Disposcope significantly reduced these angles to 9.3°, 0.25°, −0.25°, and −0.3°, respectively. Although there were no significant effects on C1/C2, C2/C3, and C3/C4, there were still exist some effects on occiput/C1. We speculate that this may be associated with the maneuver of gripping the lower incisors and chin with the left thumb and fingers to open the mouth. Therefore, it is crucial to utilize a semi-rigid neck collar for those patients with an unstable cervical spine. Fortunately, according to Park’s study, the use of Disposcope demonstrated a higher success rate in patients wearing a semi-rigid neck collar compared to Macintosh [9].

However, there are several limitations to consider. First, the view may be significantly compromised by airway secretions. Second, when the glottis is positioned far back, and the epiglottis is too short, it can be challenging to expose the glottis solely with the semi-rigid body of Disposcope. In our study, three patients encountered difficulty in glottis exposure with Disposcope alone, and successful intubation required the assistance of the McCoy laryngoscope. Third, the potential biases from the operator biases exist, as it is challenging to blind the operator. This poses a challenge for the widespread adoption of Disposcope among anesthesiologists, particularly for those with less experience.

5 Conclusion

Compared to the Macintosh laryngoscope, the Disposcope offers several advantages. It allows for laryngoscopy with reduced movement of the upper C-spine, facilitating clearer visualization of the glottis. This is particularly beneficial for patients with difficult airways or cervical spine injuries. Additionally, the Disposcope minimizes stimulation of the cardiovascular responses during airway manipulation.