Inflation of a tracheal cuff can impair tracheal mucosal capillary blood flow when cuff pressure exceeds capillary perfusion pressure.1–3 Thus, excessive inflation of the tracheal cuff is associated with tracheal morbidity, such as postoperative laryngopharyngeal pain.4 Moreover, mucosal ischaemic damage may lead to serious complications, such as tracheal stenosis or tracheal perforation.5–7 Consequently, it is recommended that cuff inflation pressure does not exceed 30 cmH2O (22 mmHg).1,8
Transoesophageal echocardiography (TOE) is a useful intraoperative monitoring device, as it provides valuable cardiac pathophysiology information.9,10 The TOE probe is generally inserted after induction of anaesthesia and is left in place until the end of surgery. One study, however, reported that insertion of a TOE probe significantly increased the tracheal cuff pressure of a single-lumen endotracheal tube (SLT).11 Because the oesophagus lies posterior to the trachea, the TOE probe may compress the adjacent tracheal wall and increase tracheal cuff pressure. A TOE examination is often performed in patients who are haemodynamically unstable and decreased tracheal mucosal perfusion pressure in hypotensive patients may further increase the risk of tracheal mucosal injury after overinflation of the tracheal cuff.
As lung transplantation or minimally invasive cardiac surgery has increased in popularity, simultaneous use of a TOE probe and a double-lumen endotracheal tube (DLT) has become a common practice. Double-lumen endotracheal tubes have a larger external diameter relative to SLTs and the increase in tracheal cuff pressure following insertion of the TOE probe may be greater with a DLT than that with an SLT. The effect of inserting a TOE probe in patients receiving a DLT, however, has never been reported. We hypothesised that the insertion of a TOE probe would increase the tracheal cuff pressure to a greater degree in patients who receive a DLT than in those who receive an SLT. The aim of this study was to evaluate the effect of TOE probe insertion on tracheal cuff pressure and to compare these effects in patients who received an SLT with those who received a DLT.
Materials and methods
Ethical approval for this study (reference number H-1308-057-512) was provided by the institutional review board of Seoul National University Hospital, Seoul, Korea (chairperson Prof W.H. Kim) on 16 September 2013. This trial was registered at clinicalTrials.gov (NCT02034643). The study complied with the Declaration of Helsinki and each patient gave his or her written consent to participate. From October 2013 to January 2014, patients scheduled for elective cardiothoracic surgery were screened for inclusion. Forty-four patients who required intraoperative TOE monitoring were subsequently recruited to this study. Patients were enrolled into the SLT or the DLT groups according to the clinical requirements for intraoperative lung separation. Patients were excluded if they had a tracheal or oesophageal stricture, tracheo-oesophageal fistula, history of oesophageal surgery, oesophageal trauma, oesophageal varix, oesophageal diverticulum, Barrett's oesophagus, hiatus hernia, large descending aortic aneurysm or vocal cord paralysis.
Upon arrival in the operating room, patients were placed on a surgical table with a 5-cm high pillow. Standard monitoring devices, including electrocardiography, noninvasive blood pressure monitoring and pulse oximetry, were applied. All tracheal tube cuffs were tested for air leaks prior to tracheal intubation. No lubrication jelly was used around the tracheal tube cuff. Anaesthesia was induced with propofol/remifentanil or midazolam/sufentanil. Vecuronium was used as a neuromuscular blocking drug. In the SLT group, a 7.0 mm SLT (Hi-Lo; Mallinckrodt Medical, Athlone, Ireland) was used for women and a 7.5 mm SLT was used for men. In the DLT group, a left-sided DLT (Mallinckrodt endobronchial tube; Covidien, Mansfield, Massachusetts, USA) was used. DLT size was determined by bronchial and tracheal internal diameter as measured preoperatively by computed tomography (CT).12 The trachea was intubated and mechanical ventilation was started with volume-controlled mode with the following settings: tidal volume 8 to 10 ml kg−1 of predicted body weight; respiratory rate adjusted to maintain an end-tidal carbon dioxide tension 4 to 4.7 kPa; inspiratory/expiratory ratio 1 : 2; and inspiratory oxygen fraction 0.5 with an air-oxygen mixture. Positive end-expiratory pressure was not applied. We did not use nitrous oxide in order to avoid its effect on cuff pressure.13,14
Tracheal cuff pressure was measured continuously as described previously.15–17 A pressure transducer system (Auto Transducer pressure monitoring set; Ace Medical, Seoul, Korea) was employed and displayed on a patient monitor (Solar 8000M; GE Healthcare, Milwaukee, Wisconsin, USA) (Fig. 1). The pressure transducer was set to zero and connected to the pilot balloon via a three-way stopcock. The connections of the tubing to the transducer were tightened and double-checked. The remaining port on the pressure transducer was sealed with a male plug. As in the previous study, levelling the stopcock was not needed because an air-filled system was used.16 Air was withdrawn from or injected into the pilot balloon using a 5-ml syringe via a side port on the stopcock,18,19 and tracheal cuff pressure was adjusted to 18 to 19 mmHg (25 to 26 cmH2O). The TOE probe (6Tc-RS; GE Healthcare) was unlocked and lubricated with jelly. The TOE probe was inserted by a single experienced anaesthesiologist (T.K.K.). A brief jaw thrust manoeuvre was performed to assist in passage of the TOE probe, but the patient's head was maintained in a neutral position aligned with the body. The TOE probe was placed and insertion depth was determined when a midoesophageal four-chamber view was displayed. Tracheal cuff pressure was monitored continuously for 5 min after insertion of the TOE probe. The TOE probe was left in situ without any manipulation and cuff pressure was recorded manually every minute. We defined steady-state pressure as no change in pressure over a 30-s period. During the study period, the patient was maintained in a supine position on the bed and no additional procedures were performed to the patient. After all the measurements had been completed, cuff pressure was readjusted to 18 to 19 mmHg (25 to 26 cmH2O) to prevent tracheal morbidity.
Variations in tracheal diameter between individuals might confound the association between the type of tracheal tube and the cuff pressure. Thus, the anteroposterior and transverse tracheal diameter was measured in the preoperative chest CT scan at the level of suprasternal notch, which corresponds to where the tracheal cuff is placed.20
The primary endpoint was tracheal cuff pressure after insertion of the TOE probe. One study reported that tracheal cuff pressure in patients who received an SLT increased by 8.5 ± 6.4 cmH2O after TOE probe insertion.11 We hypothesised that the increase in cuff pressure would be more significant in the DLT group and that an 80% difference in cuff pressure was clinically relevant. A sample size of 20 patients was needed in each group to detect a difference between groups, with a type I error of 0.05 and a power of 0.9. Allowing for possible dropouts, we chose to examine 22 patients in each group. Data are expressed as counts and percentages for discrete variables and as mean ± SD or median (interquartile range, IQR) for continuous variables. All data were tested for normality of distribution with the Kolmogorov–Smirnov test and visual inspection of Q–Q plots. Continuous variables were compared using independent t-test or Wilcoxon rank sum test after normality test. Categorical variables were compared by means of Chi-square test. Wilcoxon signed rank test was used to compare the cuff pressures before and after insertion of the TOE probe in each group and the cuff pressures between the SLT and DLT groups. The percentage of patients with steady-state cuff pressure more than 40 cmH2O was compared between the two groups using the Chi-square test. Cuff pressure was also analysed using generalised estimating equations after the TOE probe had been inserted for 5 min. The influence of the time variable was investigated between the two groups. Univariate linear regression analysis was performed for potential variables, including type of endotracheal tube, sex, height, weight and anteroposterior/transverse tracheal diameter. Multivariate models were used to adjust for variables including sex, height, weight and anteroposterior/transverse tracheal diameter, which might confound the association between the type of tracheal tube and the cuff pressure. A P value less than 0.05 was considered to indicate significance. Statistical analyses were performed using SAS version 9.2 (SAS institute Inc., Cary, North Carolina, USA).
Forty-four patients met the inclusion criteria and were enrolled in the study. The patient demographic data are summarised in Table 1. The endotracheal tube and TOE probe were inserted without difficulty in all patients. Fourteen patients in the SLT group underwent valve replacement surgery and eight patients off-pump coronary artery bypass surgery. Seventeen patients in the DLT group underwent lung surgery, three patients minimally invasive cardiac surgery and two patients combined heart and lung surgery. Three patients in the SLT group received a 7.0 mm tracheal tube and 19 patients a 7.5 mm tracheal tube. Three patients in the DLT group received a 32-Fr tracheal tube, nine patients a 35-Fr tracheal tube, seven patients a 37-Fr trachea tube and three patients a 39-Fr tube.
Baseline tracheal cuff pressures were 25.3 ± 1.9 cmH2O in the SLT group and 25.4 ± 1.4 cmH2O in the DLT group (P = 0.87). Immediately following insertion of the TOE probe, tracheal cuff pressure increased to 34.6 (32.3 to 41.1) cmH2O in the SLT group (P
< 0.001) and to 43.5 (35.4 to 48.9) cmH2O in the DLT group (P
< 0.001). The increase in the cuff pressure in the DLT group was significantly greater than that in the SLT group (P
Tracheal cuff pressure reached a steady state at 3 (2 to 3) min in the SLT and at 2 (1 to 3) min in the DLT group (P
= 0.15) (Fig. 2). The tracheal cuff pressure change over time was significant (P
< 0.001) and the steady-state tracheal cuff pressure was significantly higher in the DLT than in the SLT group [36.7 (31.3 to 44.1) vs. 31.3 (29.6 to 35.7) cmH2O respectively; (P
= 0.03) (Fig. 3)]. Steady-state tracheal cuff pressure more than 40 cmH2O was observed in two patients (18.2%) in the SLT group and nine patients (40.9%) in the DLT group (P
Univariate linear regression revealed that only the type of endotracheal tube affected the steady-state cuff pressure (P
= 0.03). When adjusted for sex, height, weight and anteroposterior/transverse tracheal diameter, the DLT group was associated with an increased steady-state cuff pressure (P
= 0.007) (Table 2).
Insertion of the TOE probe increased tracheal cuff pressure in both groups. Furthermore, the cuff pressure increase was greater in the DLT group than in the SLT group.
TOE is considered relatively noninvasive and well tolerated. Insertion and manipulation of a TOE probe, however, can cause various kinds of mechanical trauma, from minor lacerations to severe complications such as bleeding or oesophageal perforation.21 In addition, manipulation of the TOE probe can induce respiratory complications such as tracheal tube malpositioning and airway compression.21–23
The oesophagus lies directly posterior to the trachea. Due to this anatomic relationship, insertion of a TOE probe may compress the adjacent tracheal wall and increase tracheal cuff pressure. One study reported that a TOE probe increased SLT tracheal cuff pressure (>35 cmH2O) in 45% of patients 1 min after insertion.11 In the current study, however, steady-state pressure was achieved 2 to 3 min after TOE probe insertion and was higher in the DLT group than that in the SLT group. A steady-state cuff pressure of more than 40 cmH2O was also more frequent in the DLT group than in the SLT group. Although the same size of the TOE probe was used for the both groups, due to the larger external diameter of the DLT, the noncompressible part might have already occupied larger space in the same limited tracheal space than SLT group. Thus, the same degree of change in cuff volume might have induced a greater increase of cuff pressure in the DLT group.
We demonstrated that the increase in cuff pressure varied among individuals after insertion of the TOE probe, indicating the presence of individual anatomical variations in tracheal and oesophageal diameter. In a study using MRI, the mean anteroposterior oesophageal diameter below the oesophageal inlet was 8.9 ± 2.0 mm, whereas transverse diameter was 17.0 ± 3.3 mm.24 Another MRI study showed that the cricoid cartilage and the oesophagus are not aligned in almost half of patients.25 The biomechanical properties of the oesophageal wall to prevent overstretch may also have influenced the individual variations observed in the present study.26 Further studies are needed to determine the anatomical factors that could predict the increase in tracheal cuff pressure after insertion of a TOE probe.
Insufficient cuff pressure can result in microaspiration, whereas cuff overinflation can compromise tracheal capillary flow and may lead to tracheal injuries. The routine monitoring of tracheal cuff pressure has, therefore, been recommended in many studies,27–29 although it is not a minimum standard in anaesthetic monitoring guidelines (ASA Standards for basic Anesthetic Monitoring. Available at: http://www.asahq.org/For-Members/Standards-Guidelines-and-Statements.aspx) and little attention has been directed towards its importance.30 Palpation of the tracheal tube pilot balloon is a conventional method to estimate tracheal cuff pressure,20 but this method alone cannot precisely determine cuff pressure.31 As a result, tracheal cuff overinflation frequently occurs in clinical practice.29,32 Furthermore, tracheal cuff pressure is affected by various factors, which can further increase the pressure. Compared with the neutral neck position, cuff pressure is higher in the rotated, extended and flexed positions.33 Body temperature can also affect cuff pressure.34 In the present study, tracheal cuff pressure was higher in patients using a DLT with TOE. This result suggests that when intraoperative TOE is used for long-duration surgical procedures in patients using a DLT, frequent measurement and adjustment of cuff pressure is recommended to prevent tracheal cuff overinflation.
Several limitations of this study should be discussed. First, the patients’ characteristics were not comparable between the SLT and the DLT groups because the study was not a randomised controlled trial. Patients were not randomised due to the ethical issue of using a DLT without the need for single-lung ventilation. However, the study protocol was simple and the data including cuff pressure were numerical and objective. Furthermore, multivariate models were used to adjust for the potential confounders. Second, tracheal cuff pressure indirectly reflects tracheal mucosal pressure. A direct measurement of tracheal mucosal pressure may be needed to precisely evaluate the clinically important mucosal contact pressure.
In summary, insertion of a TOE probe increased tracheal cuff pressure when used with both SLT and DLTs. The increase in cuff pressure was greater in patients who received a DLT. Frequent measurement and adjustment of cuff pressure should be employed, particularly when TOE is used in patients receiving a DLT.
Acknowledgements relating to this article
Assistance with the study: the authors thank the Medical Research Collaborating Centre (MRCC) of Seoul National University Hospital (Seoul National University Hospital, Seoul, Korea) for statistical assistance and supervision.
Financial support and sponsorship: none.
Conflicts of interest: none.
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