Anesth Analg 2004;99:1445-1449
© 2004 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000134799.36294.E5
TECHNOLOGY, COMPUTING, AND SIMULATION
Brand and Size Matter When Choosing a Syringe to Relieve Pressure in a Tracheal Tube Cuff
Stanley D. Mac Murdo, MD, and
Charles W. Buffington, MD
Department of Anesthesiology, University of Pittsburgh, Pittsburgh, Pennsylvania.
Address correspondence and reprint requests to Charles W. Buffington, MD, MUH N-463, 200 Lothrop Street, Pittsburgh, PA 15213. Address email to buffingtoncw{at}anes.upmc.edu
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Abstract
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We studied the use of an inline syringe as a pressure relief valve for tracheal tube cuffs during exposure to nitrous oxide to see if the technique works. Bench testing was done to determine the stick and slip characteristics of syringes of different brands and sizes. Cuffs were inflated with 20 mL of air, producing a cuff pressure of 100–120 mm Hg. Then the plunger of the syringe was allowed to passively rebound to a steady pressure at which the plunger stopped ("stick pressure"). After several minutes, pressure in the syringe was forcibly increased with a second syringe until the plunger started moving again ("slip pressure"). Stick pressure varied from 18 to 82 mm Hg depending on the brand and size of syringe used. Slip pressures exceeded stick pressures by 20–120 mm Hg. Cuff pressure increased in a linear fashion during nitrous oxide exposure, and no syringe demonstrated automatic pressure reduction. We conclude that a syringe attached to the pilot balloon connector can be used to control tracheal tube cuff pressure during nitrous oxide anesthesia. However, not all syringes are suitable for this purpose: large syringes are better than small syringes, and the Terumo brand is more suitable than BD or Monoject. The system does not work automatically, and intermittent compression of the syringe plunger to overcome static friction is required to avoid overdistension
IMPLICATIONS: An inline syringe can be used as a pressure relief valve for tracheal tube cuffs. The brand of syringe and size are important determinants of cuff pressure. No syringe works automatically, so cuff pressure should be adjusted intermittently.
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Introduction
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Various techniques can prevent overdistension of tracheal tube cuffs and consequent damage to the tracheal mucosa during nitrous oxide anesthesia (1). Somri et al. (2) suggest using a 20-mL syringe for initial cuff inflation and then leaving it attached during anesthesia. They claim that this system results in displacement of gas from the cuff to the syringe and a low, stable cuff pressure of approximately 11 mm Hg during 2–3 h anesthetics. Our clinical impression is that cuff pressure does increase during nitrous oxide anesthesia despite use of an inline syringe. Thus, we designed a bench-top experiment to further study the pressure-relief characteristics of commercially available BD (Franklin Lakes, NJ), Terumo (Europe NV, Leuven, Belgium), and Monoject (Sherwood, St. Louis, MO) syringes.
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Methods
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A bench-top system was assembled from commercially available hospital supplies. The barrel of a 60-mL syringe (BD) was used to model the trachea. The distal end and cuff of a 7.0-mm inner diameter cuffed tracheal tube (Hudson Respiratory Care, Temecula, CA) was positioned in the middle of the syringe barrel. The tracheal tube was attached to the breathing circuit of an anesthesia machine (Ohmeda, Madison, WI) capable of delivering oxygen alone or a mixture of nitrous oxide (70%) and oxygen (30%). Gas flows were adjusted to give a barely discernible gas jet from the Luer connector of the syringe when the cuff was inflated. In this manner, the distal third of the tracheal tube cuff was exposed to the gas mixture and the proximal third to air. When inflated to more than its unstressed volume, the middle third of the cuff came into contact with the syringe barrel. The elastance curve of this system is shown in Figure 1.
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Figure 1. Elastance curve for the tracheal tube cuff and mock trachea (the barrel of a 60-mL BD syringe). The slope of the curve is lower at low cuff pressures. Thus, a similar increase in cuff volume produces a smaller increase in cuff pressure at low cuff pressures than at high cuff pressures.
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The connector at the tracheal cuff’s pilot balloon was attached to a solid-state pressure transducer (TruWave®, Edwards, Irvine, CA) either directly or via 1 or 2 3-way stopcocks, depending on the experiment. The transducer was filled with air and a tight cap was fitted to the flush port to avoid gas loss. Pressure was measured with a clinical monitor (HP Merlin®; Hewlett Packard, Palo Alto, CA) and recorded on a paper strip chart (speed 25 mm/min) as well as in tabular form by the monitor. The transducers were zeroed to room air before each experiment, and a zero check was done afterwards. Syringes were obtained from BD, Monoject, and Terumo. A syringe just out of the package was used in each measurement. All syringes were <6 mo old and had been stored at room temperature.
Several experiments were done. In the control experiment, the tracheal tube cuff was inflated with air to a pressure of 35–45 mm Hg and connected directly to the pressure transducer with no intervening stopcocks. Then the cuff was exposed to 70% nitrous oxide for 1 h. Cuff pressure was recorded continuously (strip chart) and abstracted every 5 min.
In the second experiment, we added one stopcock between the pilot connector and pressure transducer and attached a test syringe to the third port. We "warmed up" each new test syringe by moving the plunger fully back and forth twice to break the seal between plunger tip and barrel that results from prolonged storage. Then the syringe was filled with 20 mL of room air and connected to the stopcock. The full 20 mL was injected into the cuff and the plunger was allowed to passively rebound to an equilibrium pressure at which the plunger stopped moving ("stick pressure"). The 3-way stopcock was set to connect cuff, test syringe, and pressure transducer during this test. After this maneuver, the cuff was exposed to nitrous oxide (70%) for 60 or 90 min and pressure was measured as described previously.
In a third experiment we determined the "stick and slip" characteristics of a variety of syringes. We added a second 3-way stopcock between cuff connector and transducer. We used each test syringe to inflate the cuff with 20 mL of air and then allowed the plunger to rebound to its stick pressure (Fig. 2, A). After several minutes, we adjusted the 2 3-way stopcocks so that the test syringe, another syringe attached to the second stopcock, and the pressure transducer were all connected while the cuff was excluded. Then we gradually increased the pressure in the test syringe (by depressing the other one) until the plunger of the test syringe slipped and began moving (Fig. 2, B). This threshold pressure for movement ("slip pressure") was recorded. Three measurements were made in each syringe and averaged as replicates.
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Figure 2. Strip chart record of cuff pressure during the two experimental maneuvers. At A, the cuff was inflated with 20 mL of air and then the plunger of the syringe was allowed to rebound passively. Cuff pressure decreased over 30–60 s, reaching a plateau at which the plunger stopped ("stick pressure"). After several minutes, pressure within the syringe was increased by the forcible injection of air from a second syringe until the static friction of the plunger tip was overcome and the plunger began moving again (B). This threshold pressure for movement is termed the "slip pressure."
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We tested the reproducibility of stick pressures in syringes of different types. Ten inflation/deflation cycles to determine stick pressure were done over about 1 h. The cuff was emptied of air between each cycle.
We tested the effect of ambient temperature on stick and slip pressures. Measurements were made using 5 20 mL Terumo syringes at room temperature (22°C) and repeated after the syringes and area were warmed for 15 min with a forced air patient warmer (Nellcor) set on 45°C.
Data were abstracted and collated by hand. A hand-held calculator (HP-67) was used to calculate means and standard deviations.
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Results
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The pressure in tracheal tube cuffs exposed to 70% nitrous oxide increased by an average of 28 ± 4 mm Hg in the first hour (Fig. 3). When a 20-mL Terumo syringe was used as a pressure relief device, cuff pressure also increased by 26 ± 5 mm Hg during the 1 hour exposure to nitrous oxide. The pressure increase was linear over time in each cuff, and the continuous record revealed no sudden changes; thus, no automatic pressure adjustment occurred. Linear pressure increases of similar magnitude also occurred when 20- and 60-mL BD syringes were tested as pressure relief valves (data not shown).
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Figure 3. Tracheal tube cuff pressure during a 60-min exposure to nitrous oxide in 4 cuffs with no pressure relief device present. Cuff pressure increased about 1 mm Hg every 2 min.
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The variability in stick and slip pressures among syringes of different brands and sizes is shown in Table 1. Slip pressure exceeded stick pressure by 20–50 mm Hg with all syringes except the 20-mL Terumo. The Terumo syringes had consistently low stick pressures (26 ± 5 mm Hg) but very high slip pressures (140 ± 11 mm Hg). Cumulative frequency histograms (Figs. 4 and 5) demonstrate that brand does matter, both in terms of average values and ranges, and that syringe size is an important determinant of stick pressure.
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Figure 4. Not all syringes are created equal; brand does matter. Cumulative frequency histogram of stick pressure (see the legend of Fig. 2 for definition) for 3 different brands of 20-mL syringes (n = 12 each). The stick pressure of 20-mL Terumo syringes provides an appropriate cuff inflation pressure whereas those of BD and Monoject syringes are higher and more variable.
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Figure 5. Size also matters. Cumulative frequency histogram of stick pressure for 3 different sizes of BD syringes (n = 12 each). The differences probably relate to plunger surface area.
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Terumo syringes (20 mL) displayed very consistent stick pressures over 10 inflation/deflation cycles, but the syringes made by BD got "sticky" with repeated use, especially the 20-mL syringe (Fig. 6).
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Figure 6. Stick pressures of 4 20-mL BD syringes increased dramatically with repeated inflation/deflation cycles.
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A doubling of ambient temperature from 22°C to 45°C did not change stick pressure in five 20-mL Terumo syringes (23.2 ± 1.8 versus 24.2 ± 1.6 mm Hg). Slip pressures were also similar at the two temperatures studied (134 ± 11 versus 148 ± 19 mm Hg).
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Discussion
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The principal findings of this study are that brand and size matter when an inline syringe is used as a pressure relief device to control tracheal tube cuff pressure and that the system is not automatic.
It is clear that overdistension of tracheal tube cuffs in anesthetized patients damages the tracheal mucosa. A cuff pressure of 30 mm Hg for only 15 min produces histologic lesions of the tracheal mucosa in animals (3). Sore throat, as well as more serious complications, can result (1,4–6). There are two related problems: finding the correct initial cuff pressure and relieving increases in cuff pressure caused by diffusion of nitrous oxide into the cuff. Currently, there is no reliable way in the absence of objective measurement to set the initial cuff pressure correctly, and practitioners often forget to adjust cuff pressure during nitrous oxide anesthesia (7).
Use of a syringe to inflate the tracheal tube cuff is standard, but allowing the mechanical properties of the syringe to determine the final cuff pressure is a new idea. Somri et al. (2) reported the use of 20-mL Terumo syringes for this purpose. Subjects in his study had cuff pressures of approximately 11 mm Hg after an inflation/deflation maneuver. In contrast, our 20-mL Terumo syringes stuck at a cuff pressure in the range of 20–30 mm Hg during the initial inflation maneuver. The ideal cuff pressure in patients with normal chest wall and lung compliance should exceed 20 mm Hg to prevent aspiration (8) but should not exceed 30 mm Hg to prevent mucosal ischemia (9).
The brand of syringe is important. Twenty mL syringes made by BD and by Monoject, the most popular brands in the United States, produced stick pressures of 40–60 mm Hg, far above the desired range. The primary factor governing the pressure at which the plunger stops moving is dynamic friction between the surface of the plastic syringe barrel and the rings of the synthetic rubber plunger tip. Syringes are lubricated with a thin layer of silicone that reduces dynamic friction. It is likely that differences in materials, design, and lubrication account for differences in dynamic friction among the brands studied, but it is beyond the scope of this study to explain these differences precisely. Static friction exceeded dynamic friction in all syringes studied. Several factors may contribute to this phenomenon. Plastic deforms when a constant force is applied over time ("creep") and mating between the ins and outs of the surfaces in contact occurs. This connection requires additional force to disrupt. Ultrathin liquid lubricants may become even thinner in the absence of movement, an effect that increases the fluid viscosity by an order of magnitude for each molecular layer lost. Finally, fluid lubricants can undergo a reversible liquid-gel or liquid-solid transformation with an accompanying increase in viscosity that causes static friction to exceed dynamic friction (10). Our slip pressures were determined shortly after the plunger tip had stopped moving. Static friction increases with time, an effect that translates to higher slip pressures if the plunger is undisturbed.
Within a given brand, the size of the syringe is a major factor determining stick pressure. This finding is likely attributable in part to a simple physical relation: pressure equals force per unit area. The larger surface area of the plunger of larger syringes means that less pressure is required to overcome the force resulting from dynamic friction. A commonly appreciated application of this principle is that small syringes can generate higher pressures than large ones when a constant force is applied to the plunger.
The data suggest that the dynamic friction of the 20-mL Terumo syringe makes it most suitable for controlling cuff pressure. The 60-mL BD syringe produced acceptable cuff pressures after the initial inflation/deflation (23 ± 4 mm Hg) and is a possible alternative; however, the stick pressure of these syringes increased with repeated inflation/deflation cycles.
None of the syringes tested automatically adjusted the cuff pressure. Static friction accounts for this finding. Intermittent compression of the syringe plunger that breaks the seal between plunger tip and barrel is an effective way to readjust cuff pressure to a reasonable value. Cuff pressure increases at about 0.5 mm Hg per minute during 70% nitrous oxide anesthesia (11), so this maneuver should be performed every 30 minutes.
In summary, a 20-mL syringe attached to the pilot balloon connector can be used to control tracheal tube cuff pressure during nitrous oxide anesthesia. However, not all syringes are suitable for this purpose: large syringes are better than small syringes, and the Terumo brand is more suitable than BD or Monoject. The system does not work automatically, and intermittent compression of the syringe plunger to overcome static friction is required to avoid overdistension.
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References
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- Combes X, Schauvliege F, Peyrouset O, et al. Intracuff pressure and tracheal morbidity. Anesthesiology 2001; 95: 1120–4.[Web of Science][Medline]
- Somri M, Fradis M, Malatskey S, et al. Simple on-line endotracheal cuff pressure relief valve. Ann Otol Rhinol Laryngol 2002; 111: 190–2.[Medline]
- Nordin U. The trachea and cuff induced tracheal injury: an experimental study on causative factors and prevention. Acta Otolaryngol 1976; 345 (suppl 345): 1–7.
- McHardy FE, Chung F. Postoperative sore throat: cause, prevention and treatment. Anaesthesia 1999; 54: 444–53.[Web of Science][Medline]
- Nguyen Th, Saidi N, Lieutaud T, et al. Nitrous oxide increases endotracheal cuff pressure and the incidence of tracheal lesions in anesthetized patients. Anesth Analg 1999; 89: 187–90.[Abstract/Free Full Text]
- Mandoe H, Nikolajsen L, Lintrup U, et al. Sore throat after endotracheal intubation. Anesth Analg 1992; 74: 897–900.[Abstract/Free Full Text]
- O’Donnell JM. Orotracheal tube intracuff pressure initially and during anesthesia including nitrous oxide. CRNA 1995; 6: 79–85.[Medline]
- Bernhard WN, Cottrell JE, Sivakumaran C, et al. Adjustment of intracuff pressure to prevent aspiration. Anesthesiology 1979; 50: 363–6.[Medline]
- Seeglobin D, Van Hasselt GL. Endotracheal cuff pressure and tracheal mucosal blood flow: Endoscopic study of effects of four large volume cuffs. Br Med J (Clin Res Ed) 1984; 288: 965–8.
- Bhushan B. Principles and application of tribology. New York: Wiley, 1999: 744–9.
- Karasawa F, Matsuoka N, Kodama M, et al. Repeated deflation of a gas-barrier cuff to stabilize cuff pressure during nitrous oxide anesthesia. Anesth Analg 2002; 95: 243–8.[Abstract/Free Full Text]
Accepted for publication May 18, 2004.
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Abstract
Introduction
