Tracheal Wall Pressure Research Articles

Case Reports of Acquired and Reversible Tracheomegaly

Dilation of the trachea, or tracheomegaly, is a rare phenomenon that can occur under a variety of disease processes and categorized into either congenital or acquired. In acquired cases, causes range from excessive tracheal wall pressure from prolonged intubation and cuff hyperinflation to inflammatory respiratory pathologies such as chronic obstructive pulmonary disease (COPD), pulmonary fibrosis, and cystic fibrosis. However, due to the low number of reported cases, the nature of these acquired processes is unclear. We present two cases that developed tracheomegaly of unknown etiology, one of which was reversible. Patients with factors predisposing them to tracheomalacia during intubation can experience significant air leaks with difficulty ventilating the patient and that could represent increasing tracheomegaly. Changing the cuff location intermittently during prolonged intubations may prevent tracheomegaly from occurring or may diminish its’ extent once it does occur. To the best of our knowledge, the reversibility of this relatively rare disorder has not been previously reported.

Assessing the accuracy of ultrasound measurements of tracheal diameter: an in vitro experimental study

BackgroundRecent studies indicate that ultrasound can detect changes in tracheal diameter during endotracheal tube (ETT) cuff inflation. We sought to assess the accuracy of ultrasound measurement of tracheal diameter, and to determine the relationship between tracheal wall pressure (TWP), cuff inflation volume (CIV), and the degree of tracheal deformation.MethodsOur study comprised two parts: the first included 45 porcine tracheas, the second 41 porcine tracheas. Each trachea was intubated with a cuffed ETT, which was connected to an injector and the manometer via a three-way tap. The cuff was inflated and the cuff pressure recorded before and after intubation. The tracheal diameter was measured using ultrasound. This included three separate measurements: outer transverse diameter (OTD), internal transverse diameter (ITD), and anterior tracheal wall thicknesses (ATWT). A precision electronic Vernier caliper was also used to measure tracheal diameter. We calculated TWP and the percentage change of tracheal diameter. The Bland–Altman method, linear regression, and locally weighted regression (LOESS) were used to analyze the data.ResultsThere were strong correlation and agreement for OTD (r = 0.97, P < 0.001) and ITD (r = 0.90, P < 0.001) as measured by ultrasound and by precision electronic Vernier caliper, but a poor correlation for ATWT (r = 0.58, P < 0.001). There was a strong correlation between the percentage change of OTD (OTD%, r = 0.75, P < 0.001) and CIV, the percentage change of ITD (ITD%, r = 0.77, P < 0.001) and CIV, TWP (r = 0.75, P < 0.001) and CIV. And a strong correlation was also found between TWP and OTD% (r = 0.84, P < 0.001), TWP and ITD% (r = 0.84, P < 0.001).ConclusionsUse of ultrasound to measure OTD and ITD is accurate, but is less accurate for ATWT. There is a close correlation between OTD%, ITD%, CIV and TWP.

A canine model of tracheal stenosis induced by cuffed endotracheal intubation

Postintubation tracheal stenosis is a complication of endotracheal intubation. The pathological mechanism and risk factors for endotracheal intubation-induced tracheal stenosis remain not fully understood. We aimed to establish an animal model and to investigate risk factors for postintubation tracheal stenosis. Beagles were intubated with 4 sized tubes (internal diameter 6.5 to 8.0 mm) and cuff pressures of 100 to 200 mmHg for 24 hr. The status of tracheal wall was evaluated by bronchoscopic and histological examinations. The model was successfully established by cuffed endotracheal intubation using an 8.0 mm tube and an intra-cuff pressure of 200 mmHg for 24 hr. When the intra-cuff pressures were kept constant, a larger sized tube would induce a larger tracheal wall pressure and more severe injury to the tracheal wall. The degree of tracheal stenosis ranged from 78% to 91% at 2 weeks postextubation. Histological examination demonstrated submucosal infiltration of inflammatory cells, hyperplasia of granulation tissue and collapse of tracheal cartilage. In summary, a novel animal model of tracheal stenosis was established by cuffed endotracheal intubation, whose histopathological feathers are similar to those of clinical cases of postintubation tracheal stenosis. Excessive cuff pressure and over-sized tube are the risk factors for postintubation tracheal stenosis.

An in vitro comparison of tracheostomy tube cuffs.

IntroductionThe Shiley™ Flexible adult tracheostomy tube with TaperGuard™ cuff has been designed through its geometry, materials, diameter, and wall thickness to minimize micro-aspiration of fluids past the cuff and to provide an effective air seal in the trachea while also minimizing the risk of excessive contact pressure on the tracheal mucosa. The cuff also has a deflated profile that may allow for easier insertion through the stoma site. This unique design is known as the TaperGuard™ cuff. The purpose of the observational, in vitro study reported here was to compare the TaperGuard™ taper-shaped cuff to a conventional high-volume low-pressure cylindrical-shaped cuff (Shiley™ Disposable Inner Cannula Tracheostomy Tube [DCT]) with respect to applied tracheal wall pressure, air and fluid sealing efficacy, and insertion force.MethodsThree sizes of tracheostomy tubes with the two cuff types were placed in appropriately sized tracheal models and lateral wall pressure was measured via pressure-sensing elements on the inner surface. Fluid sealing performance was assessed by inflating the cuffs within the tracheal models (25 cmH2O), instilling water above the cuff, and measuring fluid leakage past the cuff. To measure air leak, tubes were attached to a test lung and ventilator, and leak was calculated by subtracting the average exhaled tidal volume from the average delivered tidal volume. A tensile test machine was used to measure insertion force for each tube with the cuff deflated to simulate clinical insertion through a stoma site.ResultsThe average pressure exerted on the lateral wall of the model trachea was lower for the taper-shaped cuff than for the cylindrical cuff under all test conditions (P<0.05). The taper-shaped cuff also demonstrated a more even, lower pressure distribution along the lateral wall of the model trachea. The average air and fluid seal performance with the taper-shaped cuff was significantly improved, when compared to the cylindrical-shaped cuff, for each tube size tested (P<0.05). The insertion force for the taper-shaped cuff was ~40% less than that for the cylindrical-shaped cuff.ConclusionIn a model trachea, the Shiley™ Flexible Adult tracheostomy tube with TaperGuard™ cuff, when compared to the Shiley™ Disposable Inner Cannula Tracheostomy tube with cylindrical cuff, exerted a lower average lateral wall pressure and a more evenly distributed pressure. In addition, it provided more effective fluid and air seals and required less force to insert.

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Introduction: Endotracheal tube (ETT) cuff pressure must be kept less than 30 cm H2O to prevent tracheal wall pressure necrosis, but must maintain an adequate seal for ventilation. Manometry and a minimal leak test (MLT) are both well-described methods for measuring cuff pressure. Methods: During a 2 month period, we used a manometer to obtain initial ETT cuff pressure measurements on all mechanically ventilated patients in a surgery ICU. Two different physicians then separately performed a MLT for each patient and measured the corresponding cuff pressure; these were compared between the two physicians. The VAP rate per ventilator days were calculated for the two months during the study period and compared to the two months immediately prior to the study. Results: Twenty-eight patients were included for a total of 50 events. The post-MLT measured cuff pressures by the two physicians were highly consistent, with an intraclass correlation coefficient of 0.872 (95% CI). The post-MLT measured cuff pressures were greater than 30 cm H2O in only 2 cases (4%; avg 32 cm H2O). During the first half of the study, the incidence of starting cuff pressures elevated above 30 cm H2O was not significantly different than the second month (19% vs 31%, p = 0.38); however, of the patients with elevated cuff pressures, the mean and elevation above the norm were both higher (47.5 ± 7.724 vs 38.89 ± 5.302 cm H20 and 17.5 ± 7.724 vs 8.89 ± 5.302 cm H20, p = 0.037). There was no difference in VAP rate during the study compared to historical controls (2.79 vs 2.99 VAPs per 1000 ventilator days, p = 0.961). Conclusions: The incidence of ventilator-associated pneumonia is not significantly increased due to the performance of a daily MLT. The MLT is a reliable indicator of endotracheal cuff pressure, does not undergo significant inter-observer subjective variation, and produces cuff pressures consistently within the acceptable range of less than 30 cm H2O. Additionally, we suspect that by emphasizing safe cuff pressures and daily cuff measurements during the course of our study we were able to decrease unacceptably high starting endotracheal cuff pressures.

Development of original tracheal model to measure tracheal wall pressure

Development of original tracheal model to measure tracheal wall pressure

An In Vitro Study to Assess Determinant Features Associated With Fluid Sealing in the Design of Endotracheal Tube Cuffs and Exerted Tracheal Pressures*

To assess the structural characteristics involved in the design of high-volume low-pressure endotracheal tube cuffs that are associated with fluid sealing effectiveness and to determine the extent of transmitted tracheal pressures upon cuff inflation. In vitro study. Pneumology laboratories. Eight high-volume low-pressure cuffs of cylindrical or tapered shape, made of polyvinylchloride or polyurethane, were studied. Cuffs were tested within a tracheal model, oriented 30° above horizontal to assess 1 hr leakage of oropharyngeal secretions simulant at cuff internal pressures of 15-30 cm H2O. The four best performing cuffs were evaluated for 24 hrs using an internal pressure of 30 cm H2O. The extent of transmitted tracheal wall pressure throughout the cuff-trachea contact area was determined using an internal pressure sensor within a tracheal model upon cuff inflation up to 30 cm H2O. Outer diameter, length, and compliance of each cuff were assessed. Multivariate regression analysis was performed to identify the main determinants of simulant leakage rate. The cuff-trachea contact area and the percentage of tracheal wall pressure measurements greater than 50 cm H2O were computed. Cuff design characteristics significantly differ among tubes. The cuffs made of polyurethane showed the best short- and long-term sealing efficacy. Nevertheless, in the multivariate analysis, the cuff outer diameter (n: 288, p = 0.003) and length (n: 288, p < 0.001), along with the internal pressure (n: 288, p < 0.001), were the only predictors of simulant leakage rate. The tapered cuffs showed the lowest tracheal wall contact area (n: 96, p < 0.001). The tracheal wall pressure distribution pattern was heterogeneous, and the percentage of high tracheal wall pressure significantly differs among the cuffs (n: 96, p < 0.001). The high-volume low-pressure cuffs' outer diameter, length, material, and internal pressure are the main determinants of sealing efficacy. Despite internal pressure within the safe range, transmitted tracheal pressure is extremely heterogeneous and differs among cuffs, occasionally reaching localized, very high, unsafe levels.

Measurement of tracheal wall pressure: a comparison of three different in vitro techniques

We compared three different tracheal wall pressure measuring techniques in vitro. Using a high-volume, low-pressure, cuffed tracheal tube with an internal diameter of 7.5 mm and a model trachea, the pressure difference technique, the wall pressure membrane technique and the microchip sensor probe technique with and without lubrication were studied. Wall pressures were measured after sequential injections of 0.5 ml of air into the cuff at cuff pressures ranging from 0 to 50 mmHg. The coefficient of variance was largest for the microchip sensor probe technique with lubrication (29%) and without lubrication (214%), and was lower for the wall pressure membrane technique (22%) and the pressure difference technique (19%). The wall pressure membrane and pressure difference techniques provided comparable results. The microchip sensor probe technique considerably underestimated wall pressure. These findings have an impact on the interpretation of published data on tracheal or pharyngeal wall pressure using the microchip sensor probe technique.

Tracheal wall pressures in the clinical setting: comparing Portex Soft Seal and Lotrach cuffs

The tracheal wall pressure (TWP) exerted by a tracheal tube cuff should normally be kept between 20 and 30 cmH2O. This protects against mucosal injury whilst allowing ventilation without an audible air leak [1]. The Portex Soft Seal high-volume low-pressure (HVLP) cuff has a working intracuff pressure of 30 cmH2O, providing a safe TWP of the same value because there is no tension in the cuff wall material. There are, however, folds within the cuff wall that allow passage of subglottic fluid to the tracheobronchial tree below, increasing the risk of ventilator-associated pneumonia [2]. The Lotrach endotracheal tube was designed to prevent this leakage at an equivalent TWP to that of correctly inflated HVLP cuffs [3]. Each Lotrach cuff is individually calibrated to transmit only 30 cmH2O to the tracheal wall, yet because of the lack of folds in the cuff wall it has been shown to prevent of aspiration of subglottic contents [4]. Although extensively tested in benchtop models and pig tracheas, we wished to demonstrate that the Lotrach cuff had an identical sealing pressure, and therefore TWP, as the HVLP cuff in normal clinical practice.

In-vitro comparison of three different tracheal wall pressure measurement techniques

In-vitro comparison of three different tracheal wall pressure measurement techniques

Comparison of the LoTrach and the Portex Soft Seal cuff: tracheal wall pressure and fluid leakage in a benchtop study and a clinical study

The objective of the study is to demonstrate two methods of measuring tracheal wall pressure (in vitro and in patients), with the LoTrach (LVLP) cuff and with conventional high-volume low-pressure (HVLP) cuffs. The LoTrach (VTSL, Singapore) is a new tracheal tube designed for use for mechanical ventilation in the critically ill. The LoTrach has been shown to prevent the ubiquitous problem of micro-aspiration of secretions associated with conventional HVLP cuffs [1]. Aspiration prevention is achieved by the properties of the LoTrach's low-volume low-pressure (LVLP) cuff, and these have been previously described [1,2]. It is important that, alongside achieving aspiration prevention, there is also tracheal wall pressure control. The LoTrach LVLP cuff is calibrated such that the sum of the elastic forces within the cuff remain constant throughout the inflation profile. Thus, at the working intra-cuff pressure of 80 cmH2O, only a consistent and acceptable 30 cmH2O is transmitted to the tracheal wall [2].

An in vitro study of the compliance of paediatric tracheal tube cuffs and tracheal wall pressure

Cuff volume-pressure curves and cuff pressure-tracheal wall pressure relationships were investigated in eight brands of currently available cuffed, paediatric tracheal tubes with internal diameters of 5.0 mm. Cuff volume-pressure curves were measured with the cuff unrestricted and with the cuff placed within a tracheal model with wall pressure measurements. With the tracheal tube cuffs, unrestricted cuff compliance at 20 cmH(2)O cuff pressure varied between 0.06 and 0.3 ml x cmH(2)O(-1). With the cuff restricted within the model trachea, all tracheal tube cuffs became considerably less compliant (0.01-0.09 ml x cmH(2)O(-1)). We found tracheal wall pressure was similar to the cuff pressure as long as the resulting cuff diameter was sufficiently large freely to drape the inner tracheal wall. We found that, regardless of whether a higher or lower compliant tube cuff was used, cuff hyperinflation uniformly resulted in potentially compromised tracheal mucosal blood flow; cuff pressure monitoring using cuff pressure limitation is therefore strongly recommended.

A low-volume, low-pressure tracheal tube cuff reduces pulmonary aspiration*

Leakage of fluid from the subglottic space to the lungs occurs along the longitudinal folds within the wall of an inflated high-volume, low-pressure (HVLP) cuff. The low-volume, low-pressure (LVLP) cuff does not have these folds yet allows for convenient and reliable control of tracheal wall pressure. Pulmonary aspiration during anesthesia has been linked with postoperative pneumonia and during critical illness causes ventilator-associated pneumonia. Prospective, blinded, randomized controlled trial; prospective observational study; and benchtop models. Department of Anaesthesia and Critical Care, Queen Elizabeth Hospital. Anesthetized patients (n=38) and critically ill patients with either an LVLP or HVLP cuffed tracheostomy tube following swallow assessments (n=67). The LVLP cuff was compared with HVLP cuffs for leakage of dye placed in the subglottic space to the tracheobronchial tree in a rigid tracheal model and a benchtop pig trachea model (before and after a standardized cuff movement). In the rigid tracheal model, the incidence of leakage was 0% in the LVLP group and 100% in the HVLP group (p<.01). Dye leakage in the pig tracheal model with HVLP cuffs was 44% before tube movement, increasing to 79% afterward. The LVLP cuff did not leak in the pig tracheal model. Dye leakage in anesthetized patients was 0% before movement and 5% after in the LVLP group and in the HVLP group 22% increasing to 67% after movement (p<.001). Forty-nine percent of swallow assessments were scored as failed in the critical care patients with HVLP tracheostomy tube cuffs, and there were no episodes of aspiration following swallow assessment in the LVLP group (p<.05). The LVLP cuffed tracheal and tracheostomy tubes reduced pulmonary aspiration in the benchtop models and in anesthetized and critically ill patients. The single failure of the LVLP cuff in the anesthesia group was probably associated with accidental endobronchial intubation following tube movement.

Performance of a novel pressure release valve for cuff pressure control in pediatric tracheal tubes

Acute and chronic hyperinflation of tracheal tube cuffs represents a persistent risk factor for airway damage in children when cuffed tracheal tubes are used. In order to overcome this particular risk, a cuff pressure (CP) pop-off valve has been designed to avoid CP exceeding 20 cmH(2)O. The performance of the novel pop-off valve has been evaluated in an in vitro set-up during slow and rapid air insufflation by a syringe or a CP manometer or inadvertent compression of the cuff pilot balloon while measuring cuff and tracheal wall pressure (WP) in ID 3.0 mm cuffed tracheal tubes. Steady-state performance was evaluated during nitrous oxide exposure of tracheal tube cuffs (ID 3.0 mm). The novel CP pop-off valve avoided cuff hyperinflation during rapid air volume changes and showed reliable performance during steady-state nitrous oxide exposure to the tube cuff. These preliminary results show that the CP pop-off valve limits the effect of rapid, potentially dangerous manual cuff inflation maneuvers and reliably prevents CP exceeding the predetermined level of 20 cmH(2)O when exposed to nitrous oxide.

Tracheal Rupture

We previously published a bronchoscopic image of a tracheal tear involving the posterior tracheal wall during an elective surgical procedure.1 We now present a different image in which the entire tracheal was disrupted after emergency endotracheal intubation. A 77-year-old woman presented to an outside institution with atrial fibrillation and respiratory distress requiring urgent intubation. She subsequently developed increasing subcutaneous emphysema prompting x-ray evaluation, which demonstrated pneumomediastinum. Bronchoscopy performed at that time revealed a long membranous tear that extended to the carina. Bilateral chest tubes were placed and the patient was transported to our institution for definitive therapy. On arrival to the intensive-care unit (ICU), a computed tomography (CT) scan was reviewed and revealed extensive disruption of the trachea from above the sternal notch to the carina. The patient was urgently brought to the operating room where a repair was performed after placing the patient on cardiopulmonary bypass for appropriate oxygenation. The bronchoscopic image is presented below in Figure 1.FIGURE 1.: Note the full-thickness tear involving the posterior wall of the midtrachea.Repair was performed through a median sternotomy incision. Postoperatively, the patient was transferred to the thoracic ICU and doing well at the time of this writing. DISCUSSION Although uncommon, tracheal rupture after endotracheal intubation is well described and is said to occur characteristically in elderly females.2–4 Early and emergent repair is imperative to prevent life-threatening complications. Factors predisposing to tracheal laceration are summarized in Table 1.3,4Table 1: Factors Leading to Tracheal Tear or RuptureWomen are more susceptible to iatrogenic tracheobronchial rupture by virtue of their small body size, which puts them at increased risk of placing the tube too far downward in a short and weak membranous trachea.5 The most frequent cause of tracheal injury associated with intubation appears to be excessive cuff to tracheal wall pressure and inadequate tube size.6,7 It is postulated that excessive cuff pressure causes ischemic necrosis with subsequent rupture of the tracheal wall. In most instances, the posterior membranous wall ruptures longitudinally at the junction with the cartilage, and the length of rupture generally corresponds to the length of the cuff. Extensive tearing can be explained by further dissection under positive pressure ventilation.3 Signs of tracheal rupture, commonly subcutaneous emphysema and respiratory distress, appear immediately after intubation. Early radiographic findings include pneumomediastinum, subcutaneous emphysema, and pneumothorax. Pneumothorax occurs only if the tracheobronchial rupture communicates freely with the pleural space. Emergent flexible bronchoscopy is the best means of confirming the diagnosis and determining the exact location and extent of the lesion, which should be performed in the operating room. Aggressive management is essential to avoid life-threatening complications. Immediate complications include tension pneumothorax and anoxia because most of the tidal volume leaks through the tear. Subacute complications include potentially lethal mediastinitis and tracheal stricture.3 Tracheal intubation distal to the lesion or bronchial intubation with a double lumen tube and pleural drainage are of prime importance for control of respiratory distress. A few patients undergoing emergent repair in severe respiratory distress could require cardiopulmonary bypass. In patients with small tears when less than one third of the circumference of the trachea or bronchus is disrupted and in the absence of gross air leak or respiratory embarrassment, conservative management can be opted. However, any patient with a large tear or evidence of respiratory or circulatory compromise, primary surgical repair is the treatment of choice and must be performed at the earliest.3 Authors believe that even if care is applied during intubation, superficial lesions in the transbronchial tree are unavoidable, especially under emergency conditions. Large, iatrogenic disruptions can be avoided by using small endotracheal tubes and double-lumen tubes with carinal spurs should be generally avoided. Continuous monitoring of the cuff pressure is mandatory to overcome the problem of unnoticed cuff distension as a result of diffusion of nitrous oxide.8

Over-inflation of the tracheal tube cuff: a case for routine monitoring

Excessive tracheal tube cuff pressure can cause mucosal ischaemia leading to tracheal stenosis or tracheooesophageal fistulae [1] and under-inflation of the cuff has been associated with an increased risk of ventilator-associated pneumonia [2]. High-volume low-pressure (HVLP) cuffs were introduced in the early 1970s to enable tracheal wall pressure control. The use of a HVLP cuff does not however guarantee an acceptable mucosal pressure (MP) unless the intra-cuff pressure (CP) is maintained lower than 30 cmH2O. CP was measured regularly in only 13% of intensive care units (ICU) in one region of England [3]. This prospective observational study examines the CP recorded after induction of anaesthesia and in two critical care facilities.

The protective effect of cuff lubrication in critically ill patients with tracheostomies

Aspiration of infected secretions past the tracheal tube cuff causes ventilator associated pneumonia (VAP).1 This aspiration occurs despite an adequate intracuff pressure, because the fluid leaks along longitudinal folds within the wall of a high volume low pressure (HVLP) cuff.2 Previousin vitro work has suggested that KY jelly cuff lubrication might prevent this leakage by physically blocking the folds.3 The objective of this study is to quantify the effect of cuff lubrication on subglottic to tracheal dye aspiration in critically ill patients with tracheostomies. Methods This is a prospective open observational study. Seventeen critically ill patients who required a tracheostomy were recruited. All the study patients initially received a size 8 mm ID Portex Profile cuffed tracheostomy tube at percutaneous tracheostomy by the Grigg's technique. Lubrication was determined by the preference of the clinician performing the percutaneous tracheostomy. All the tracheostomy tubes had been modified by the addition of a fine bore catheter that was glued above the cuff to enable dye instillation into the subglottic space. The cuff was inflated to 30 cm H2O (2.9 kPa) and maintained at this pressure by intermittent checks prior to dye instillation and 8 hourly using a cuff inflator (Portex, UK). Blue food dye (E122) 0.5 ml followed by saline 3 ml was instilled daily through the catheter into the subglottic space. The nurse responsible for the patient was instructed to report dye obtained from tracheal aspirates (performed post instillation and at least 4 hourly thereafter). Tracheobronchial aspiration was deemed to have occurred if dye was retrieved from the trachea. The Wilcoxon rank sum test was used for the data analysis. Results Sixteen of the 17 cuffs leaked after a median of 1 day. One cuff did not leak despite 10 days of testing. Of the cuffs that leaked, nine cuffs were lubricated and leaked after a median of 2 days (range 1–5) and seven cuffs were not lubricated and leaked after a median of 1 day (0–1). Lubrication thus appeared to protect against leakage for longer than non-lubrication (P Conclusion This study suggests that cuff lubrication affords protection against aspiration for 1–5 days. Clinicians should be aware that a HVLP cuff cannot reliably prevent the transit of fluid from the upper airway to the lungs after a median of 2 days even with lubrication. Occasionally a HVLP cuff will be an exact fit for the trachea in which it is inflated and folds will not form in the cuff wall. In this circumstance leakage is prevented. It is likely that the one cuff that did not leak despite 10 days of testing was an exact fit for that patient's trachea. Until a cuff is available that prevents aspiration at acceptable tracheal wall pressures we would recommend that all tracheal and tracheostomy cuffs are lubricated with KY jelly prior to insertion.

Evaluation of a new design of tracheal tube cuff to prevent leakage of fluid to the lungs

A new design of tracheal tube cuff was compared with two types of high-volume, low-pressure (HVLP) cuffed tracheal tube for leakage of fluid from the subglottic space into the trachea. Spontaneous and positive-pressure ventilation were simulated using a mechanical lung, an intubated model trachea and a ventilator. Excised human tracheas were intubated and leakage past the cuff assessed. Distention of the tracheal wall was measured. HVLP cuffs leaked rapidly in the model during all modes of ventilation, and also in the excised human tracheas. This leakage occurred preferentially down longitudinal folds that occur in the HVLP cuff wall. The new design completely prevented leakage in the model during all modes of ventilation, during tracheal suctioning, and with tube movement. The new cuff also prevented leakage in the excised human tracheas. Tracheal wall distention and tracheal wall pressures were similar for all cuffs tested.

Change in tracheal blood flow during endotracheal intubation.

Changes in blood flow in the tracheal mucosa of the dog caused by the pressure exerted by high volume, low-pressure cuffs were measured with the hydrogen clearance method. Before inflating the cuffs, the blood flow of the tracheal mucosa was measured as a control for 12 h in order to confirm that the procedures of the hydrogen clearance method itself had little or no influence on the blood flow in the tracheal mucosa. After inflating the cuffs to create a tracheal wall pressure (TWP) of 1.3 kPa (10 mmHg), 2.6 kPa (20 mmHg), 3.9 kPa (30 mmHg) or 6.0 kPa (45 mmHg), local blood flows of tracheal mucosa (TBF) corresponding to each TWP were measured every hour for 12 h. No significant changes in blood flow were observed in the tracheal mucosa with the hydrogen clearance method before inflating the cuffs. In the groups with TWP of 1.3 and 2.6 kPa, the TBF rose 1 h after inflation of the cuffs, and then returned to the baseline values. In the group with TWP of 6.0 kPa, the TBF decreased markedly already 1 h after inflation of the cuffs, and continued to decrease severely thereafter. In the group with TWP of 3.9 kPa, the TBF followed an intermediate course between the groups with TWP of 2.6 kPa and 6.0 kPa. From the results of the present study, it was found that TBF was significantly impaired by a TWP of more than 3.9 kPa. Therefore, in prolonged intubation, TWP should be kept at or below 2.6 kPa.

LABORATORY EVALUATION OF LOW-PRESSURE TRACHEAL TUBE CUFFS: LARGE-VOLUME V. LOW-VOLUME

Large- and low-volume low-pressure tracheal tube cuffs were compared during exposure to nitrous oxide. The compliance of both types of cuff when measured inside and outside an artificial rigid trachea, the volume of intubated cuff, the residual volume, the nitrous oxide diffusion time, and the specific diffusion rate of the cuff membrane to nitrous oxide were measured in the laboratory. From these variables, the diffusion rate of nitrous oxide, the surface area available for nitrous oxide diffusion, the tracheal wall pressure exerted by cuff, and the degree of folding of the intubated cuff were derived. The results showed that the rapid increase in pressure of the large-volume cuff when intubated was caused by low compliance, the large surface area available for nitrous oxide diffusion, and the considerable permeability of the cuff membrane to nitrous oxide.