Despite the universal use of high-volume low-pressure endotracheal tube (ETT) cuffs, complications related to overinflation of the ETT cuff remain frequent and vary in severity from transient sore throat with hoarseness (15%–80%) and tracheal mucosa ulcers (10%–15%), to more serious and disabling adverse events such as nerve palsy, tracheal rupture or fistula (<1%), and subglottic stenosis (0%–11%).1–9 Subglottic stenosis is most frequently subclinical and not severe (<50% reduction in airway diameter).10 Tracheal ulcers occur more frequently after relatively long periods of intubation in intubated critically ill patients. Most complications occur because high ETT cuff pressures can cause ischemia of the mucosa, particularly in the anterolateral area of the trachea. Fortunately, the risks of devastating complications such as tracheal rupture or fistula are now rare with low-pressure high-volume cuffs. Nevertheless, prevention of these complications may be achieved by maintaining the tracheal cuff pressures in the range of 20–30 cm H2O, which have been suggested to be safe, as they are below values that impair the perfusion of the tracheal mucosa and above those that allow aspiration.11–15
The incidence of ETT cuff overinflation is unacceptably high if a manometer is not used to monitor cuff pressures.16–21 Furthermore, despite a meticulous establishment of the cuff pressures within safe ranges immediately after tracheal intubation, they can later increase significantly because of diverse factors including patient movement, positioning, temperature, and degree of neuromuscular blockade.22–25 In addition, changes in airway pressures during mechanical ventilation may also influence ETT cuff pressures. In vitro studies on mechanical ventilation models have demonstrated that ETT cuff pressures increase in a linear relation to increases in airway pressures.26 However, this relationship has not been extensively evaluated in clinical settings.27 Ignoring this relationship may have significant safety implications if patients with low respiratory compliance are exposed to high airway pressures for prolonged periods of time.
Pelvic laparoscopic surgery under general anesthesia provides a unique opportunity for studying in vivo effects of increasing airway pressures on the ETT cuff pressures.27 Therefore, pelvic laparoscopic surgery may be considered as a model of reversible acute decrease in respiratory compliance, in otherwise healthy lungs, which translates to significant increases in the airway pressures during key time points of the procedure.28 The reduction in respiratory compliance is caused by cephalad displacement of the diaphragm and decreased intrathoracic volume due to several factors, including intraperitoneal carbon dioxide (CO2) insufflation, steep Trendelenburg positioning,29 and chest compression by security straps used to prevent patient movement. The effects on respiratory compliance are further magnified in obese patients.
This prospective observational cohort study was designed to assess the relationship between ETT cuff pressures and airway pressures in obese patients undergoing pelvic laparoscopic surgery. We hypothesized that the ETT cuff pressures will rise in direct relationship to increases in the peak and mean airway pressures. An increase in ETT cuff pressure from baseline to >30 cm H2O was considered to be clinically significant.15
The institutional review board of the University of Texas Southwestern Medical Center approved this study. Because the data collected for the study were deidentified, the requirement for a written informed consent was waived. This manuscript adheres to the applicable Enhancing the Quality and Transparency of Health Research (EQUATOR) guidelines. The study population consisted of adult obese female patients (body mass index [BMI] ≥30 kg/m2) scheduled for elective laparoscopic (manual or robotically-assisted) pelvic gynecologic procedures, including total or partial hysterectomy, myomectomy, oophorectomy, resection of adnexal mass, or treatment of endometriosis. Patients were excluded if (1) they had a history of asthma, chronic pulmonary disease, tracheostomy, tracheal stenosis, tracheomalacia, or other airway condition with altered laryngo-tracheal anatomy; (2) nitrous oxide was administered because its diffusion would increase the ETT cuff pressure; and (3) the ETT cuff or cuff pilot were found to be defective during anesthesia.
All the patients received a standardized general anesthetic technique utilizing ETTs with low-pressure high-volume cuffs (7.0 or 7.5 internal diameter Mallinckrodt, Hi-Lo Oral/Nasal Endotracheal Tube, Cuffed, Intermediate, Murphy Eye; Covidien LLC, Mansfield, MA). After preoxygenation with 100% oxygen (O2), induction of anesthesia was achieved with fentanyl 1–2 µg/kg, intravenous (IV), lidocaine 50 mg, IV, propofol 1–2 mg/kg, IV, and rocuronium 0.6–1 mg/kg, IV. After tracheal intubation, anesthesia was maintained with desflurane or sevoflurane in all cases. Nitrous oxide was not administered. Additional doses of rocuronium were administered to maintain adequate neuromuscular blockade (train of 4 response of ≤1 twitch, particularly during abdominal insufflation). Neostigmine and glycopyrrolate were used for reversal of neuromuscular blockade at the end of the case in all patients. Intraoperative mechanical ventilation was determined by the anesthesiologist in charge of the case but according to our institution protocol (tidal volume 6–8 mL/kg of ideal body weight, I:E ratio of 1:2, positive end-expiratory pressure [PEEP] of 5 cm H2O, and initial respiratory rate of 10–12 breaths/min with a goal of maintaining end-tidal CO2 values around 40 mm Hg, achieved by adjustments in the respiratory rate).
Monitoring included standard American Society of Anesthesiologists monitors. All patients had the procedure performed in low lithotomy position, with the arms tucked to the side. A foam mattress and a safety strap placed around the patient’s chest were used to prevent the patient sliding off the operating table. An upper-body warming blanket (Bair Hugger, 3M, St Paul, MN) was placed on the chest on top of the safety straps and temperature was set to 43°C. The surgical procedures started with the operating table in neutral position with 0° of inclination. After peritoneal insufflation was achieved, a steep Trendelenburg position was attained. The degree of inclination and maximal intraabdominal pressures were determined by the surgeon. Toward the end of surgery, the patient was returned to a neutral position and abdominal exsufflation was achieved.
Tracheal Tube Cuff and Airway Pressure Measurements
After tracheal intubation, the ETT cuff was inflated with air using a 10-mL syringe. Initially, the anesthesia providers determined the volume of air used for cuff inflation by palpation of the pilot balloon. The adequate position of the ETT was verified by auscultation of the thorax and presence of end-tidal CO2 waveform. Once the ETT was secured, a member of the research team proceeded to measure the ETT cuff pressures by connecting the ETT cuff pilot to calibrated pressure transducers (TruWave PX600; Edwards Lifesciences, Irvine, CA). The transducers were primed with normal saline, connected to an anesthesia monitor (GE DASH 4000; GE Health care, Waukesha, WI), and zeroed at the level of the patient’s trachea, at the suprasternal notch. A 3-way stopcock was attached to the transducer to facilitate inflation or deflation of the ETT cuff. The pressure transducer was positioned and secured on the surgical table, in proximity to the patient’s neck, at the level of the trachea. The ETT cuff pressures were adjusted to a baseline value of 25 cm H2O by adding or removing air from the cuff during a brief period of apnea with zero end-expiratory pressure. The ETT cuff pressures were continuously monitored and recorded throughout the procedure until emergence of anesthesia. The volume of air within the ETT cuff was not modified during anesthesia, unless a significant leak was detected. Peak and mean airway pressures were continuously monitored, intraoperatively, in a similar fashion using a pressure transducer connected to the “Y” piece of the anesthesia circuit.
Data on ETT cuff pressures, airway pressures, tidal volumes, respiratory rate, inspiratory-to-expiratory time (I:E) ratio, mode of mechanical ventilation, level of PEEP, intraabdominal pressures, degree of surgical table inclination, and degree of neuromuscular blockade were collected every 10 minutes from the start of mechanical ventilation until tracheal extubation. In addition, patients’ age, weight, height, and BMI, as well as duration of abdominal insufflation and tracheal intubation, were recorded. For descriptive purposes, the study period was divided into 3 phases: (1) preabdominal insufflation, the time from tracheal intubation to initiation of intraperitoneal insufflation; (2) abdominal insufflation, the period from initiation of intraperitoneal insufflation to exsufflation; and (3) postinsufflation, the period from peritoneal exsufflation to tracheal extubation.
Univariate statistical analyses were conducted to describe the baseline characteristics of the study sample. Categorical variables are presented as frequencies and percentages, and continuous variables as mean (standard deviation) or median (interquartile range) as appropriate. Mixed linear regression analyses were conducted to assess for differences in ETT cuff pressures and airway pressures among the 3 phases of the study period. ETT cuff pressure and airway pressure were the dependent variables in the models, respectively. Study phase was the only independent variable. Patient study ID identified the subjects in the random statement of the SAS PROC MIXED procedure. Nonparametric (Kruskal-Wallis) tests were used to assess for differences in nonnormally distributed intraoperative variables among the study phases.
General linear regression models were fit to assess the effect of increases in airway pressures on ETT cuff pressure, after adjusting for covariates. Various diagnostics were performed to ensure that the usual multiple linear regression assumptions were met. We verified that our outcome variables did not depart significantly from normality using histograms, boxplots, normal probability plots, and tests for normality. Residual plots demonstrated that the residuals in the models followed a normal distribution. Diagnostic plots also showed homogeneity of variances. Collinearity diagnostics confirmed that the predictors in the model were not significantly correlated (maximum variance inflation factor was 2.6 and maximum condition index was 24.1).
The structure of the data consisted of individual measures of cuff and airway pressures repeated over time on the same subjects. Pressure measurements were also clustered according to the 3 phases of the study period. Therefore, to account for the correlated nature of the data on the same subjects over time as well as over different study phases, multilevel mixed linear regression models with fixed and random effects were constructed. The ETT cuff pressures were considered the dependent variable and peak airway pressures were considered the independent variable of interest in the models. Airway peak pressures and covariates including phase of the study period, patients’ age, BMI, duration of abdominal insufflation, level of surgical table inclination, intraabdominal pressures, and ventilator settings (level of PEEP, I:E ratio, tidal volume, and ventilation mode) were entered as fixed effects. Clustering of data at the patient level as well as patient and study phase level was accounted for in the analyses by fitting random intercepts for patients and study phases and a random slope coefficient for time. Other variables included in the final mixed model included peak airway pressures, study phase, degree of table tilt, and I:E ratio as fixed effects, after backward elimination of the other nonsignificant variables (intraabdominal pressure, age, BMI, PEEP, tidal volume, and ventilation mode). A similar model for mean airway pressures instead of peak airway pressures was also fitted.
Based on our pilot data on a simulation device, which revealed a strong correlation between increasing airway pressures and ETT cuff pressures, a power and sample size analyses for type III tests of sets of predictors in multiple linear regression, using a mixed linear model, indicated that to detect a partial correlation of at least 0.6 between ETT cuff pressure (dependent variable) and peak airway pressure (predictor of interest) controlling for 4 predictors, 27 patients were required to achieve a power of 0.85, assuming an α level of .05. All the analyses were conducted using SAS 9.4 statistical software (Cary, NC). P values of <.05 were considered statistically significant.
Twenty-eight obese female patients with a mean age of 42 years were enrolled in the study. Patient characteristics are displayed in Table 1. After tracheal intubation, the ETT cuffs were overinflated (pressures >30 cm H2O) in 25 (89.3%) patients, underinflated (cuff pressures <20 cm H2O) in 2 (7.1%) patients, and properly inflated in only 1 patient. Of note, in 13 (46.4%) patients, the ETT cuff pressures were >100 cm H2O. Research personnel adjusted the ETT cuff pressure in all the patients to 25 cm H2O before starting data collection.
Table 2 displays changes in airway and ETT cuff pressures, ventilatory parameters, and other intraoperative characteristics during the 3 study phases. The peak airway pressures significantly increased from a mean (standard error) value of 23.2 (0.84) cm H2O before peritoneal insufflation (phase 1) to 32.1 (0.46) cm H2O after peritoneal insufflation and Trendelenburg positioning (phase 2) and decreased again to 21.9 (0.61) cm H2O after peritoneal deflation (phase 3) (P < .0001). Similarly, the ETT cuff pressure increased significantly from 29.6 (1.30) cm H2O during phase 1 to 35.6 (0.68) cm H2O during phase 2, and decreased to 27.8 (0.79) cm H2O during phase 3 (P < .0001).
The multilevel mixed regression analyses (Table 3) revealed that after controlling for covariates and clustering of the data at the patient and study phase levels, increased peak airway pressures were significantly associated with increased pressures in the ETT cuff. Each 1 cm H2O increase in peak airway pressures produced an increase in the ETT cuff pressures of 0.25 cm H2O (P < .0001). A similar mixed model where peak airway pressures were substituted for mean airway pressures demonstrated that each 1 cm H2O increase in mean airway pressures was significantly associated with an increase in ETT cuff pressures of 0.2 cm H2O (P = .011). Other variables associated with increasing ETT cuff pressure included degree of surgical table inclination and I:E ratio. The ETT cuff pressures increased about 0.1 cm H2O per each 1 degree of Trendelenburg inclination (P = .0003). Similarly, the ETT cuff pressures increased about 4.47 cm H2O when the I:E ventilation ratio was 1:1 compared to a 1:2 ratio (P = .0002).
Our study reveals that in a clinical model of decreased respiratory compliance in mechanically ventilated patients, the ETT cuff pressures change significantly in direct relation to changes in the peak and mean airway pressures. These high ETT cuff pressures may compromise the perfusion of the tracheal mucosa if not recognized and corrected. As a secondary finding, our study revealed a high incidence of ETT cuff overinflation (89.3%) after the tracheal intubation. Importantly, about half of our patients had ETT cuff pressures >100 cm H2O. Cuff overinflation is frequent when a manometer is not routinely used for checking ETT cuff pressures.30,31
Our mixed regression models found other variables independently associated with increased ETT cuff pressures. Although the degree of head down inclination had significant statistical association with increased ETT cuff pressures, the clinical implication was modest (<0.1 cm H2O increase in cuff pressures per 1 degree of inclination). In contrast, an I:E ratio of 1:1 was associated with an increase in the ETT cuff pressures of about 5 cm H2O when compared to a 1:2 ratio. This difference may be explained by the longer duration of the inspiratory phase in a 1:1 ratio that exposes the ETT cuff to higher airway pressures for a longer period.
Our findings are coherent with the principles of fluid mechanics. During positive-pressure ventilation, the trachea and the ETT cuff are part of the same closed pneumatic system. According to Pascal’s principle, any pressure applied externally to the system is transmitted unreduced to every part of the fluid, as well as to all the walls of the container.32 Our results are consistent with an in vitro study by Guyton et al,26 who studied the relationship between airway pressures and the cuff inflation pressures that prevented a 5% or 10% leak in delivered tidal volumes. Using a laboratory model to simulate reduced respiratory compliance with high peak inflation pressures, the investigators found that increases in airway pressure resulted in higher ETT cuff pressures. In a subsequent small clinical study, the same authors investigated the relationship between the minimum occlusive pressure (the lowest cuff pressure that prevented an airway leak) and peak inflation pressures in adults undergoing mechanical ventilation during general anesthesia.33 Despite the limitations such as small sample size (n = 15), measurements at only 3 time points per patient, and peak inflation pressures >30 mm Hg in only 3 patients, the study found that the minimum occlusive ETT cuff pressures increased linearly in relation with the peak inflation pressures. From their regression equation, the investigators concluded that patients requiring ventilation with peak airway pressures >35 mm Hg will require cuff inflation pressures above the safe cuff inflation pressure limit of 25 mm Hg to prevent airway leak.
In contrast to Guyton et al,26 we did not observe any significant airway leak despite increased peak airway pressures, particularly during the intraabdominal insufflation period (ie, phase 2 of the study period). A leak would be expected whenever the airway pressure exceeds the ETT cuff pressure. Furthermore, the cross-sectional area of the trachea increases during inspiration making the chance of a leak more likely.34 The absence of major leak during high positive airway pressure ventilation is explained by the self-sealing action of large volume cuffs present in currently available ETTs. The airway pressures are transmitted to the cuff as they increase during inspiration. The increasing pressure in the distal part of the cuff causes a redistribution of the air contained within the cuff away from the distal higher-pressure area toward the proximal part of the cuff, which is subject to the lower atmospheric pressure. The redistribution of pressures results in a change in the shape of the cuff from cylindrical to conical.35 The change in cuff shape results in an increased proximal cross-sectional area preventing a leak despite no changes in the cuff volume. Accordingly, in patients undergoing mechanical ventilation with peak airway pressures >30 cm H2O a major leak would not be observed, despite a baseline inflation pressure <30 cm H2O, as long as the volume of the cuff is enough to allow redistribution of the air to the proximal region of the cuff.
Our findings have clinical relevance for patients requiring high ventilatory pressures during mechanical ventilation and in those undergoing prolonged surgical procedures. A study found significant association between cuff overinflation and increased incidence of hemorrhagic ulceration of the trachea, particularly when the duration of intubation was longer than 180 minutes.36 Of note, steep head down positioning might lead to venous engorgement of the head and neck, which may reduce tracheal mucosal perfusion; thus, even modest increases in ETT cuff pressures may further compromise perfusion of the tracheal mucosa.
Our study has several limitations. First, we used ETT cuffs of a single design from a single manufacturer. Therefore, our findings may not be applicable to other types of ETT. Second, for logistic reasons, we enrolled only female patients in our study. Although men have on an average larger trachea than women, we believe that the same hydraulic principles would apply regardless of sex. Our study population consisted of relatively young patients. However, the trachea becomes less elastic with aging, and so we speculate that the correlation we observed between airway and cuff pressures would be even stronger in older patients. Although nitrous oxide was not used, we did not measure the concentrations of other gases (eg, CO2 or O2) in the ETT cuff, and thus, were unable to assess their effects on ETT cuff pressure changes. Finally, we did not assess clinical outcomes resulting from the increased cuff pressures. However, others have found increased incidence of sore throat related to increases in ETT cuff pressures during laparoscopy.29,37 Furthermore, a significantly larger sample size would be required to assess the clinical outcomes.
In conclusion, this prospective observational cohort study in patients undergoing laparoscopic pelvic surgery reveals that high airway pressures are associated with increased ETT cuff pressures. Overinflation of ETT cuff pressures is common and despite meticulous establishment of baseline cuff pressures within the recommended safety thresholds, elevations in the airway pressures are transmitted to the tracheal cuff, resulting in increased in ETT cuff pressures above the perfusion pressure of the tracheal mucosa. Our results suggest that baseline ETT cuff pressures should be monitored routinely in all patients and continually in patients with increases in airway pressures. Future larger studies are necessary to assess the clinical importance of routine and continual monitoring of cuff pressures in patients undergoing prolonged surgical procedures and those with high airway pressures.
Name: Eric B. Rosero, MD, MSc.
Contribution: This author helped conceive and design the study, analyze and interpret the data, write the manuscript, critically revise the manuscript for important intellectual content, approve the final version to be published, and is in agreement to be accountable for all aspects of the study.
Name: Esra Ozayar, MD.
Contribution: This author helped conceive and design the study, collect the data, write the manuscript, critically revise the manuscript for important intellectual content, approve the final version to be published, and is in agreement to be accountable for all aspects of the study.
Name: Javier Eslava-Schmalbach, MD, PhD.
Contribution: This author helped analyze and interpret the data, write the manuscript, critically revise the manuscript for important intellectual content, approve the final version to be published, and is in agreement to be accountable for all aspects of the study.
Name: Abu Minhajuddin, PhD.
Contribution: This author helped analyze and interpret the data, write the manuscript, critically revise the manuscript for important intellectual content, approve the final version to be published, and is in agreement to be accountable for all aspects of the study.
Name: Girish P. Joshi, MBBS, MD, FFARCSI.
Contribution: This author helped conceive and design the study, interpret the data, write the manuscript, critically revise the manuscript for important intellectual content, approve the final version to be published, and is in agreement to be accountable for all aspects of the study.
This manuscript was handled by: Richard C. Prielipp, MD.
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Copyright © 2017 International Anesthesia Research Society
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