In the non-invasive cardiac diagnostic settings worldwide, a transesophageal echocardiography (TEE) can be performed with or without conscious sedation. According to the guidelines for performing a TEE, the procedure is well tolerated by an unsedated patient who is adequately given oral anaesthesia [1]. Comparing with a sedated TEE, the unsedated patients show a lower incidence of cardiopulmonary complications and also receive more in benefit in terms of recovery time and medical care cost [1, 2]. However, performing a TEE without sedation requires a well cooperative patient since the procedure can easily injure organs, including lips, teeth, oropharynx, larynx, esophagus and stomach [3, 4]. In addition, the patients who show gagging during the procedure tend to have more oropharygeal injury than the absent gagging group [5, 6].

As gagging is a significant obstacle to succeed in performing an unsedated TEE, oropharynx assessment should be considered as an important process. However, previous studies mention that only operator’s experience and patient’s cooperation are the two influencing factors [1, 4]. In the field of gastrointestology, Huang, et al. compare the tolerance in esophagogastroduodenoscopy (EGD) among the patients based on Modified Mallampati Classification (MMC) [6]. The result clearly shows that the patients with MMC class III and class IV mostly present gagging during the procedure which leads the patient to be intolerant and be given sedation. Also, in the field of anaesthesiology, the MMC has been accepted as one of the factors affecting a successful endotracheal tube intubation [7–9]. Focusing on the field of cardiology, there is a lack of data supporting the correlation between MMC and the TEE outcomes. Even though TEE probe insertion is technically easier than endotracheal tube intubation, some complications can occur since the long probe has to be passed oropharynx before being inserted into the esophagus. From this point of view, our present study aims to identify additional factors on top of operator experience and patient co-operation which can influence the success of a TEE procedure in unsedated patients with heart disease.

Based on the study results, the high grade MMC (class III and class IV) was statistically significant associated with the gag reflex and the insertion time. These findings are additional clinical information for performing a TEE since previous studies mention only operator’s experience and patient’s cooperation as the key success factors [1, 4]. However, one of the most important problems in performing a TEE is insertion of the probe, especially in the unsedated patients.

During the TEE probe insertion, even though topical anaesthetic agent has been applied throughout the oropharynx, gagging still remains in some cases. This physical reaction is induced by the touch of the transducer on any six sensitive oropharyngeal parts, including soft palate, uvula, fauces, posterior pharyngeal wall, back of the tongue and epiglottis [5, 6]. The effect of gagging can cause a failure of the probe insertion or aspiration during the procedure [5]. As reported by Huang, et al., the patients who have gagging are tended to have lower tolerance for esophagogastroduodenoscopy (EGD) than the patients in the opposite group. They also find out that the patients with high grade MMC (classes III and IV) are found to have more gagging than the low grade MMC patients (classes I and II) [6]. In agreement with our results, the patients who presented with MMC class III and class IV had a 5.2–fold and 3.4-fold more gagging than MMC class I patients. This finding was similar to the insertion time which also associated with MMC.

In reference to our results, the mean time of the fastest probe insertion was 5.32 ± 1.67 s which was found in the group of MMC I while the other three groups of the higher classes showed longer times as in MMC class II was 2.35 ± 2.5 s (P = 0.099), MMC class III was 3.72 ± 3.72 s (P = 0.001) and MMC class IV was 5.16 ± 6.53 s (P = 0.000).

Therefore, according to regression equation, Y = ax + b [11, 12], the successfully inserted time of the patients with MMC class II, class III and class IV are as follows: 7.67 ± 2.5 s, 9.04 ± 3.72 s, and 10.48 ± 6.53 s. Comparing to the another study, there is a lack of data on the TEE probe insertion time, but an approximation is within 1 min [13].

To the best of our knowledge, even though all participants were successfully performed the TEE without sedation, MMC should be considered as one of determining factors affecting the unsedated TEE’s outcome since it is related to gagging and probe insertion time. These correlations may be explained using MMC criteria classified by oropharyngeal cavity [14–19]. By the view of fully opened mouth and protruded tongue without any sounds, MMC class III and class IV allow the examiner to see only soft palate and maybe uvular because the size and position of the tongue which are larger and farther than MMC class I and class II [15]. This specific anatomy is an obstacle to performing the TEE because of the compression of the probe which spontaneously creates a direct pressure on the posterior of the tongue leading to a spasm of the pharynx, a natural mechanism of choking prevention [20–23]. Moreover, the narrow oropharyngeal cavity also affects the procedure in terms of difficulty passing the TEE probe into esophagus. For these two reasons, the patients with the narrow oral cavity (MMC class III and class IV) are tended to experience longer successful insertion time than those who have wider oral cavity (MMC class I and class II).

The other interesting finding was the patients with MMC class III and class IV had a tendency to have oropharyngeal pain at 1 h after the procedure (P = 0.086). This result could be explained based on the successful insertion time and gagging which were related to MMC. As mentioned above, the patients with high grade MMC had narrow oral cavity which might be abraded easily on oropharyngeal mucous membrane by the TEE probe during insertion, especially when having gagging. That is, the patients who present more gagging during the TEE procedure are likely to experience more oropharyngeal pain at 1 h after the procedure than others [13, 24, 25]. This finding supports the TEE is not only a safe procedure but also a non-admitted procedure. According to the TEE guideline, an outpatient can be discharged if there is non-serious complication after the procedure [26].

The reduction of gagging during performing endoscopic procedure has been studied worldwide in order to increase patients’ tolerance and comfort [17, 18],such as using a micro TEE probe and intra cardiac echocardiography probe (ICE) instead of using a conventional probe [27, 28]. Moreover, Tsuboi et al., claim that performing an unsedated EGD by passing the EGD probe through nasal cavity shows better outcomes than passing through oral cavity [15]. Apart from the equipment and the passage, Ulusoy and Kucukarslan state that the sitting position can help the patient to be successfully inserted the TEE probe [6]. Similar to Samsoon and Young, in the field of anesthesiology, neck flexion and head extension are the two important factors facilitating the operator to successfully intubate endotracheal tube.

However, in a busy non-invasive cardiac testing setting or a non-anesthesiology setting, the TEE may be performed without sedation as well as using the conventional probe and technique. In such a limited resource setting, MMC can be used for a quick assessment of gagging which will be helpful in terms of administrating topical anaesthesia. Moreover, the patients with MMC class III and class IV may need to be placed in a particular position of head and neck instead of placing them on the conventional left lateral decubitus position which focuses only on aspiration prevention [28]. In summary, optimizing the unsedated TEE outcomes, the patients with high grade MMC should be given effective oropharyngeal anaesthesia and be placed in a proper position.

Our study demonstrates that MMC is positively associated with the successful TEE probe insertion time. Moreover, the high grade MMC patients (MMC class III and class IV) are found to be correlated with gagging during the TEE probe insertion and found to have a tendency toward oropharyngeal pain at 1 h after the TEE. From these reasons, MMC should be considered as one of determining factors in the success of unsedated TEE procedure in the patients with heart disease. Therefore, in order to optimize unsedated TEE outcomes, the patients should be assessed MMC which will benefit in terms of administrating topical anaesthesia.