Original Study
Journal of Veterinary Emergency and Critical Care 24(2) 2014, pp 182–187 doi: 10.1111/vec.12129
The effect of cuff presence and cuff inflation on airway pressure in a canine tracheostomy tube model Jamie R. Wignall, BSc, BVetMed, MS and Stephen J. Baines, MA, VetMB, PhD, DECVS
Abstract
Objective – To evaluate the effect of cuff presence and cuff inflation on airway pressure in an inspiratory model of canine tracheostomy. Design – Ex vivo experimental study. Cadaver tracheas from Beagle dogs were attached aborally to a vacuum. Airway pressure and flow rate was measured before and after placement of tracheostomy tubes. Animals – None. Interventions – Adult uncuffed tubes and cuffed tracheostomy tubes (sizes 4, 6, 8, and 10) were placed within tracheas. Cuffs were investigated without inflation and at maximum cuff inflation. Airway pressure was measured at constant airflow rates at 30 and 60 L/min. Measurements and Main Results – At set flow rates, airway pressures of tracheostomy tubes were compared to the intact trachea. A size 4 uncuffed tracheostomy tube showed the lowest airway pressure and a size 4 cuffed trachestomy tube with inflation showed the highest airway pressures. For sizes 6, 8, and 10 tubes, the presence of a cuff with and without inflation significantly increased airway pressure. Inflation of a cuff always significantly increased airway pressure. Similar pressure is seen between sizes 4 and 6 uncuffed tubes. Conclusions – Cuffed tracheostomy tubes should not be used unless specifically indicated due to increased airway pressure. (J Vet Emerg Crit Care 2014; 24(2): 182–187) doi: 10.1111/vec.12129 Keywords: temporary patent airway, morbidity, complications, trachea
Introduction Tracheostomy tubes are commonly used to provide a temporary patent airway. These tubes have been used in patients with respiratory tract obstruction cranial to the trachea, to allow ventilation in animals with impaired neurological or respiratory function, improve surgical access to the oral cavity and pharynx during anesthesia, and provide access to the lower respiratory tract for airway clearance.1, 2 Current recommendations are that the outer diameter of the tube must not exceed 75%3 of
From the Department of Clinical Science and Services, The Royal Veterinary College, University of London, North Mymms, United Kingdom. Mr. Wignall’s current address: California Veterinary Specialists, 39809 Avenida Acacias, Murrieta, CA 92563. Dr. Baines’ current address: Willows Referral Service, Highlands Road, Shirley, Solihull, West Midlands, B90 4NH, United Kingdom. Presented in abstract form at The British Small Animal Veterinary Association Congress, April 2010, Birmingham, UK, and in poster format at the European College of Veterinary Surgeons Congress, July 2010, Helsinki, Finland. Address correspondence and reprint requests to J. R. Wignall, California Veterinary Specialists, 39809 Avenida Acacias, Murrieta, CA 92563. Email: [email protected] Submitted May 30, 2012; Accepted November 07, 2013.
182
Abbreviation
RG
rima glottidis
the tracheal diameter or to use a tube that does not completely fill the lumen;4 however, there is little scientific evidence to support these recommendations. Potential advantages of cuffed tracheostomy tubes include protecting the lower airways from debris and aspirate, preventing tube dislodgement in conscious patients, allowing effective positive pressure ventilation, and preventing anesthetic gas leaks in anesthetized patients.5–7 Some older texts recommended that cuffs should not be inflated unless positive pressure ventilation is required.6, 8 In a recent study the most common complications of tracheostomy tube use were obstruction, dislodgement, aspiration pneumonia, and stoma swelling but no clinical information was available comparing the use of cuffed and uncuffed tubes.7 Use of cuffed tubes may increase the risk of tracheal necrosis, infection, obstruction, and stenosis.2, 9 Currently, no C Veterinary Emergency and Critical Care Society 2013
Airway pressure in cuffed and uncuffed tracheostomy tubes
information is published regarding comparative airway pressure and flow rates between cuffed and uncuffed tubes of varied sizes. As an airway’s diameter is reduced, greater inspiratory pressure is required to permit the same airflow.10 At a constant airway flow rate, changes in airway pressure may be used to quantify obstruction to flow and hence tracheostomy tubes, which provide lower pressures are expected to minimize obstruction to an animal’s ventilatory effort, although this has not been examined in clinical patients. The aim of this study was to evaluate the airway pressure in canine cadaver tracheas with tracheostomy tubes of different sizes and types, at varied airflow rates. Additionally, at constant airflow, to evaluate if larger diameter cuffed tubes could achieve similar pressures to smaller uncuffed tracheostomy tubes. The null hypothesis was that the presence of a cuff and cuff inflation would have no effect on airway pressure.
Materials and Methods Tracheal specimens were obtained from 9 adult Beagles euthanatized by intravenous administration of a pentobarbital-phenytoin solution, for reasons other than upper respiratory disease. Age and weight information was unavailable. Each specimen comprised the larynx, caudal oropharynx, proximal esophagus, and trachea to the 10th tracheal ring. All larynges and tracheas were grossly normal on examination. Specimens were stored at −20◦ C and warmed to room temperature before use. Cadaver material was periodically moistened throughout the experiments and used within 3 days of initial thawing. Excess cervical tissue was excised and the esophagus was incised dorsally to permit visualization of the rima glottidis (RG). The tracheal diameter was measured at the level of the 5th tracheal ring. Each larynx was secured to a wooden board using 2 stainless steel nails at the epihyoid-stylohyoid articulation to stabilize the larynx without impeding arytenoid motion and allow inversion of the larynx to allow tracheostomy tube placement. A 20-Ga orthopedic wire was inserted through the epiglottis and pulled cranially to keep the glottis open at all times. The trachea was attached by a zip tie to form an airtight seal at the 9th tracheal ring to a 5 cm length of 28 mm diameter anesthetic tubing. The laryngeal end of the construct was left open to the atmosphere and airflow was created by applying a vacuuma in series with a custom-made flow regulator. This simulated negative pressure aboral to the larynx. Flow rate and airway pressure were measured using a ventilator calibration analyzer,b connected in series (Figure 1). The transducer was sensitive to ±0.005 cm H2 O C Veterinary Emergency and Critical Care Society 2013, doi: 10.1111/vec.12129
pressure and ±0.005 L/min flow. The pressure transducer was zeroed at atmospheric pressure with no flow and negative pressure measured as differential pressure (positive). One study found upper airway flow rates of approximately 23.4 and 33 L/min for resting and hyperpneic large breed dogs, respectively, and 30–55.8 L/min for resting and hyperpneic dogs with laryngeal paralysis.11 Flow was set at a steady state at 30 and 60 L/min and pressure was measured in triplicate for each intervention and taken as baseline values for each specimen. An incision was made through the annular ligament and mucosa between the 4th and 5th tracheal ring using a No. 10 scalpel blade. A suture of 2 metric polypropylenec was placed around the 4th tracheal ring and another was placed around the 5th tracheal ring. Metzenbaum scissors were used to extend the incision to 50% of the tracheal circumference. Sutures were manipulated to allow the placement of tracheostomy tubes of varied size and cuff type. Tubes were placed to allow central alignment in the trachea without excessive twisting or pressure on the dorsal wall of the trachea. Flow and pressure measurements were taken in triplicate. Shiley curved adult uncuffed tubesd of sizes 4 (internal diameter [ID]) = 5 mm, external diameter [ED] = 9.4 mm), 6 (ID = 6.4 mm, ED = 10.8 mm), 8 (ID = 7.6 mm, ED = 12.2 mm), and 10 (ID = 8.9 mm, ED = 13.8 mm) were placed. Subsequently, low pressure cuffed Shiley adult tubes of sizes 4, 6, 8, and 10 (same ID and ED as corresponding uncuffed tubes) were placed, initially with no cuff inflation, followed by 100% cuff inflation based on the manufacturer’s specifications. The order of testing was employed in increasing tube size and with all uncuffed tubes followed by all cuffed tubes with no inflation. Lastly, cuffed tubes with inflation were placed. This aimed to minimize tracheal deformation. Statistical methods Differential pressure data are presented as mean and standard deviation and analyzed using one-way repeated measures ANOVA. Pressure data at each intervention were compared with values before intervention in all tracheas. Commercial statistical softwaree was used for all analysis. Significance was set at P < 0.05. Airway pressure between 2 tubes at the same flow was deemed equivalent if no significant difference was found between them.
Results Placement of adult uncuffed tubes had no significant effect on airway pressure compared to the unaltered trachea (Figures 2 and 3). When a cuffed tube was placed 183
J. R. Wignall & S. J. Baines
Figure 1: The laryngeal specimens were mounted and inverted to allow tracheal tube placement. Airflow was created by applying a vacuuma in series with 3 modified 30 mL syringes acting as flow regulators. Flow rate and airway pressure were measured using a ventilator calibration analyzer, connected in series.
without inflation, size 6, 8 and 10 uninflated cuffed tubes resulted in a significant increase in airway pressure compared to the unaltered trachea at 30 and 60 L/min. Noninflated cuffed tubes resulted in an increased airway pressure compared to uncuffed tubes for all sizes and both flow rates. This was significant for tube sizes 6, 8, and 10 at both flow rates. Fully inflated (100%) cuffed tubes resulted in a significant increase in airway pressure compared to the unaltered trachea and uncuffed tubes, for all tube sizes and flow rates. As adult uncuffed tube size increased, airway pressure tended to increase from size 4 to size 8 and pressure decreased from size 8 to size 10. Greatest airway pressure was seen using uncuffed size 8 tubes and least airway pressure using uncuffed size 4 tubes. If use of an inflated cuff is indicated, equivalent airflow (nonsignificant increases in pressure) can be achieved by increasing tube size in the following situation; use of a size 8 or 10 cuffed inflated tube provides equivalent flow to the uncuffed size 4 tube. No inflated cuffed tubes are able to provide equivalent airflow to uncuffed sizes 6, 8, or 10 tubes (Table 1).
Discussion Use of a cuffed tracheostomy tube and cuff inflation significantly increased airway pressures in this experimental model and the null hypothesis can be rejected. Pouseille’s law states that for laminar flow in tube, V = Pr4 /8 L (where V is flow, P is pressure difference, n is gas viscosity, l is length of the passage, and r is radius of the passage).12 For this experiment, where the 184
apparatus and gas viscosity remain constant, pressure is proportional to flow and can be used to assess obstruction to airflow. We postulate that this increase in airway pressure may cause obstruction to airflow or increased respiratory effort in clinical patients. When comparing the unaltered trachea to cuffed tubes, all cuffed tubes resulted in increased airway pressure. In this experimental model, after placement of a tracheostomy tube, air flows through both the RG and tracheostoma. An increase in airway pressure after tube placement suggests that the presence of the tube caused more obstruction to flow through the RG than the tracheostoma can provide. Airflow through both the RG and tracheostoma can be exhibited during this experiment by comparing the effect of tube size between uncuffed tubes and cuffed tubes. In cuffed tubes where the cuff minimized RG flow, as tube size increases, pressure reduced, suggesting only tracheostoma flow occurs. For uncuffed tubes, the size 4 tube allows the lowest pressure airflow and therefore, by Pousielle’s law, must provide the greatest combined area between RG and tracheostoma.12 The presence of flow through both the RG and tracheostoma is not a requirement in clinical tracheostomy; however, this study suggests that it may occur in cases of partial upper airway obstruction, unless inflated cuffed tubes are used. The presence and inflation of a cuff increase airway pressure. If use of an inflated cuff is indicated, for example, to protect against aspirated oral content, it would be ideal to maintain the lowest airway pressure achievable. Since the uncuffed size 4 tube maintains the least airway pressure, it would be of clinical C Veterinary Emergency and Critical Care Society 2013, doi: 10.1111/vec.12129
Airway pressure in cuffed and uncuffed tracheostomy tubes
Figure 2: Differential airway pressure (cm H2 O) at a flow rate of 30 L/min to illustrate the effect of tube size, cuff presence, and cuff inflation. Mean airway pressure is illustrated by the horizontal line, standard deviation by the vertical line. Significant differences between cuffed tubes at 0% inflation and uncuffed tubes are shown by ∗ (P < 0.001). Significant differences between cuffed tubes at 100% inflation and uncuffed tubes are shown by ∧ (P < 0.001). Note: Figures 2 and 3 have different y axes.
Figure 3: Differential airway pressure (cm H2 O) at a flow rate of 60 L/min to illustrate the effect of tube size, cuff presence, and cuff inflation. Mean airway pressure is illustrated by the horizontal line, standard deviation by the vertical line. Significant differences between cuffed tubes at 0% inflation and uncuffed tubes are shown by ∗ (P < 0.001). Significant differences between cuffed tubes at 100% inflation and uncuffed tubes are shown by ∧ (P < 0.001). Note: Figures 2 and 3 have different y axes.
interest to know if larger cuffed tubes could maintain similar pressures, in essence substituting flow through the RG for extra flow through the tracheostoma. No significant differences were seen between airway pressures achieved with a size 4 uncuffed tube, a cuffed size 8 or a cuffed size 10 tube. Placing size 8 and 10 cuffed tubes in a Beagle trachea may be impractical without oversizing the tube and risking pressure necrosis, and we conclude that increasing tube size is unable to practically offset the obstruction to flow caused by the cuff’s presence. C Veterinary Emergency and Critical Care Society 2013, doi: 10.1111/vec.12129
Manufacturer’s specifications for maximum cuff inflation were used in this study to ensure that the minimal allowable airflow would exist through the RG, although this was not clinically tested. This level of inflation would not be advised in the live animal. In addition to an increase in airway pressure, all tubes have risk of displacement and clogging.13 Cuffed tubes are more likely to cause tracheal necrosis, infection, and stenosis.2, 14 No data are available on the complication rate of cuffed tubes in clinical practice. Clinicians using cuffed tracheostomy tubes, particularly in conscious 185
J. R. Wignall & S. J. Baines
Table 1: Mean pressure (cm H2 O) ± SD for the unaltered trachea, uncuffed, and cuffed tubes at different levels of inflation, for tubes of sizes 4, 6, 8, and 10
Flow
Unaltered Uncuffed Cuffed 4
(L/min)
trachea
30 60
Mean SD Mean SD
0.31 0.07 1.15 0.30
Uncuffed Cuffed 6
4
0%
100%
6
0.19 0.01 0.63 0.09
0.53 0.14 1.63 0.56
5.96∗ 1.81 20.73∗ 6.83
0.25 0.06 0.85 0.22
0%
100%
1.02∗ 0.48 3.74∗ 1.85
2.21∗ 0.54 7.89∗ 2.22
Uncuffed Cuffed 8 8 0.36 0.13 1.11 0.44
0%
100%
0.93∗ 0.20 3.12∗ 0.80
1.07∗ 0.20 3.68∗ 0.85
Uncuffed Cuffed 10 10 0.32 0.08 1.10 0.29
0%
100%
0.56∗ 0.08 1.88∗ 0.33
0.60∗ 0.09 2.04∗ 0.37
Significant increase in airway pressure (P < 0.001) compared to the unaltered trachea is annotated by ∗ .
patients, should acknowledge and evaluate these risks against the potential benefits of these tubes. This study utilized a small number of tracheas, although each trachea was subject to all procedures and served as its own control. Cadaver studies are subject to limitations such as loss of supporting peritracheal tissues, loss of muscular tone, lack of response to tissue handling, and possible change in mechanical properties due to loss of osmotic balance and specimen treatment.15 It is granted that the trachea in a live dog is subject to flexion and extension, which will affect airway flow and pressure. This study is designed with standardized extension of the trachea to allow direct comparison between tubes. This is a constant rate inspiratory model of respiration aimed at replicating peak inspiratory flow only. The airflows listed here are not intended to directly translate to the live dog. However, since the characterization of airway pressure is currently limited in the live animal, cadaver experiments provide an alternative and have been used in previous studies.15 Some authors claim that the arytenoid cartilages of the excised larynx maintain a similar intermediate position to the live patient with bilateral laryngeal paralysis.16, 17 This observation is based on the observation of the arytenoids in an intermediate position, and its use has not been ratified as a model of dynamic obstruction. Studies on the cadaver and live animal have been used in documenting the effects of surgical intervention on the larynx but not the trachea.16–18 Tidal breathing flow-volume loops19 and head out plethysmography11 are unfortunately limited to resting animals only. RG flow rates have not been classified in the live animal due to the difficulty of isolating flow from the nares and upper respiratory tract in a conscious animal. One study found upper airway flow rates of approximately 23.4 and 33 L/min for resting and hyperpneic large breed dogs, respectively, and 30–55.8 L/min for resting and hyperpneic dogs with laryngeal paralysis.11 These were generally larger dogs than the Beagles used in this study but 30 and 60 L/min were deemed the most appropriate flow rates to test. With regard to inspiratory pressures employed in this experiment, negative pressures are presented in Table 1. The range of negative 186
pressures elicited across the experiment was −0.19 to −2.20 cm H2 O for 30 L/min and −0.63 to −20.73 L/min for 60 L/min. Peak inspiratory pleural pressures range from −7 to −14.3 cm H2 O (mean, −9.34 cm H2 O), drawing air into the airways and to the lungs;20 however, limited information is available for tracheal pressures in resting medium-sized dogs. These recommendations must be interpreted carefully as statistical significance may not equal clinical significance. Since air flows through both the RG and tracheostomy tube, this model of airway obstruction mimics a single point on the spectrum between no significant obstruction and total airway occlusion, and results should be analyzed as such. Rather than a fixed obstruction as may be seen in a space-occupying lesion, the level of obstruction in this model may also depend on the inspiratory pressure—the increased inspiratory pressure is able to pull the paralyzed arytenoids into the airway, thus reducing the radius of the tube and increasing pressure exponentially.12 A previous study found that high inspiratory pressure in this model can cause laryngeal collapse.15 The relative effect of the cuff may be dependent on the degree of RG obstruction already present. In a model of more significant upper airway obstruction, where less RG flow is present, the relative effect of a cuff, which affects this RG airflow, should also reduce as the flow through the tracheostoma becomes more important. Mean tracheal diameter in the Beagle specimens used was 25.11 mm (±0.484 SD). According to previous recommendations of tube size occupying a maximum diameter of 75% of tracheal diameter,3 all uncuffed tubes used would be suitable; however, the use of cuffs would not be accounted for. Although the comparison of a range of tubes in one trachea is useful, some of the tube usage (specifically the size 8 and 10 cuffed inflated tubes) in this study is in the author’s opinion unrealistic for the size of the trachea and would not be clinically recommended. The order of tube placement was designed to minimize trauma to the tracheal specimens. Subjectively, due to the elastic properties of the trachea, when uncuffed and cuffed uninflated tubes were placed and removed, the C Veterinary Emergency and Critical Care Society 2013, doi: 10.1111/vec.12129
Airway pressure in cuffed and uncuffed tracheostomy tubes
trachea returned to a normal position; however, a significant limitation was that specimen pressures were not measured between tube placements, as in this phase of the experiment, airflow properties may have changed without subjective visual assessment. When fully inflated cuffs were placed, subjective deformation did occur; however, since the aim of the full cuff was to eliminate all airflow around the tube and measure tube flow only, deformation in the trachea would not be expected to affect pressure data for the use of cuffed tubes of increasing size. As an alternative to inflating the cuff to the manufacturer’s specification, a more clinically applicable method of cuff inflation, such as leak testing or utilizing a defined pressure or volume within the cuff, could have been used. This experimental method was chosen to ensure that laryngeal airflow would be reduced to the most achievable minimum, and this level of distension would not be recommended in clinical practice due to the risk of tracheal necrosis and rupture. Further studies would be required to assess the effect of tracheostomy tubes on different tracheal sizes. In conclusion, the benefits of using cuffed tracheostomy tubes and cuff inflation should be considered in light of detrimental increases in airway pressure. Depending on the patient’s tracheal size it may not be possible to negate the effects of a cuff; however, the use of cuffed tubes should be carefully considered. Uncuffed tubes are recommended in patients who need tracheostomies as evidenced by decreased airway pressures when compared to cuffed tubes.
Footnotes a b c d e
Tesco VC406 Bagged Compact Cylinder Vacuum; Tesco, Cheshunt, England. Timeter RT-200; Allied Health Products, St. Louis, MO. Prolene; Ethicon, Edinburgh, Scotland. Tyco Healthcare UK Ltd., Gosport, United Kingdom. GraphPad Software, Inc., San Diego, CA.
References 1. Nicholson I, Baines S. Indications, placement and management of tracheostomy tubes. In Pract 2010; 104–113.
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2. Hess DR. Tracheostomy tubes and related appliances. Respir Care 2005; 50:497–510. 3. Prymak C, Weisse C. The extrathoracic trachea, In: Brockman DJ, Holt D. eds. BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery. Gloucester, UK: BSAVA; 2005, pp. 81–82. 4. Hedlund CS. Surgery of the upper respiratory system, In: Fossum TW. ed. Small Animal Surgery, 3rd edn. St. Louis, MO: Elsevier/Mosby; 2007, pp. 825–826. 5. Ford RB, Mazzaferro EM. Kirk and Bistner’s Handbook of Veterinary Procedures and Emergency Treatment, 8th edn. Philadelphia, PA: W.B. Saunders Company; 2006, pp. 55–56. 6. Colley P, Henderson R, Huber M. Tracheostomy techniques and management. Compend Contin Educ Pract Vet 1999; 21:44– 53. 7. Nicholson I, Baines S. Complications associated with temporary tracheostomy tubes in 42 dogs [1998 to 2007]. J Sm An Pract 2012; 53:108–114. 8. Hedlund CS, Tangner CH. Tracheal surgery in the dog—part II. Compend Contin Educ Pract Vet 1983; 5: 738–750. 9. Goldberg M, Pearson FG. Pathogenesis of tracheal stenosis following tracheostomy with a cuffed tube. Thorax 1972; 27:678– 691. 10. Dye JA, Costa DL. Pulmonary function testing. In: King LG. ed. Textbook of Respiratory Disease in Dogs and Cats. St. Louis, MO Elsevier; 2004, pp. 157–175. 11. Bedenice D, Rozanski EA, Bach J, et al. Canine awake head-out plethysmography (HOP): characterization of external resistive loading and spontaneous laryngeal paralysis. Resp Phys Neurobiol 2006; 151:61–73. 12. West JB. Mechanics of breathing. In: Respiratory Physiology: The Essentials. Baltimore, MD: Lippincott Williams and Wilkins; 1974, p. 185. 13. Brown D, Gregory S. Brachycephalic airway disease. In: Brockman DJ, Holt D. eds. BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery. Gloucester, UK: BSAVA; 2005, pp. 87–88. 14. Andrews MP, Pearson FG. Incidence and pathogenesis of tracheal injury following cuffed tube tracheostomy with assisted ventilation: analysis of a two-year prospective study. Ann Surg 1971; 173:249– 263. 15. Wignall J, Baines S. Effects of unilateral arytenoid lateralization technique and suture tension on airway pressure in the larynx of canine cadavers. Am J Vet Res 2012; 73:917–924. 16. Harvey CE, O’Brien JA. Treatment of laryngeal paralysis in dogs by partial laryngectomy. J Am Anim Hosp Assoc 1982; 18:551– 556. 17. Lussier B, Flanders JA, Erb HN. The effect of unilateral arytenoid lateralization on rima glottidis area in canine cadaver larynges. Vet Surg 1996; 25:121–126. 18. Lozier S, Pope E. Effects of arytenoid abduction and modified castellated laryngofissure on the rima glottidis in canine cadavers. Vet Surg 1992; 21:195–200. 19. Amis TC, Kurpershoek C. Tidal breathing flow-volume loop analysis for clinical assessment of airway obstruction in conscious dogs. Am J Vet Res 1986; 47:1002–1006. 20. Ednick MD, Pagala M, Barakat JP, et al. Telemetric recording of intrapleural pressure. J Surg Res 2007; 138(1):10–14.
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Journal of Veterinary Emergency and Critical Care 24(2) 2014, pp 182–187 doi: 10.1111/vec.12129
The effect of cuff presence and cuff inflation on airway pressure in a canine tracheostomy tube model Jamie R. Wignall, BSc, BVetMed, MS and Stephen J. Baines, MA, VetMB, PhD, DECVS
Abstract
Objective – To evaluate the effect of cuff presence and cuff inflation on airway pressure in an inspiratory model of canine tracheostomy. Design – Ex vivo experimental study. Cadaver tracheas from Beagle dogs were attached aborally to a vacuum. Airway pressure and flow rate was measured before and after placement of tracheostomy tubes. Animals – None. Interventions – Adult uncuffed tubes and cuffed tracheostomy tubes (sizes 4, 6, 8, and 10) were placed within tracheas. Cuffs were investigated without inflation and at maximum cuff inflation. Airway pressure was measured at constant airflow rates at 30 and 60 L/min. Measurements and Main Results – At set flow rates, airway pressures of tracheostomy tubes were compared to the intact trachea. A size 4 uncuffed tracheostomy tube showed the lowest airway pressure and a size 4 cuffed trachestomy tube with inflation showed the highest airway pressures. For sizes 6, 8, and 10 tubes, the presence of a cuff with and without inflation significantly increased airway pressure. Inflation of a cuff always significantly increased airway pressure. Similar pressure is seen between sizes 4 and 6 uncuffed tubes. Conclusions – Cuffed tracheostomy tubes should not be used unless specifically indicated due to increased airway pressure. (J Vet Emerg Crit Care 2014; 24(2): 182–187) doi: 10.1111/vec.12129 Keywords: temporary patent airway, morbidity, complications, trachea
Introduction Tracheostomy tubes are commonly used to provide a temporary patent airway. These tubes have been used in patients with respiratory tract obstruction cranial to the trachea, to allow ventilation in animals with impaired neurological or respiratory function, improve surgical access to the oral cavity and pharynx during anesthesia, and provide access to the lower respiratory tract for airway clearance.1, 2 Current recommendations are that the outer diameter of the tube must not exceed 75%3 of
From the Department of Clinical Science and Services, The Royal Veterinary College, University of London, North Mymms, United Kingdom. Mr. Wignall’s current address: California Veterinary Specialists, 39809 Avenida Acacias, Murrieta, CA 92563. Dr. Baines’ current address: Willows Referral Service, Highlands Road, Shirley, Solihull, West Midlands, B90 4NH, United Kingdom. Presented in abstract form at The British Small Animal Veterinary Association Congress, April 2010, Birmingham, UK, and in poster format at the European College of Veterinary Surgeons Congress, July 2010, Helsinki, Finland. Address correspondence and reprint requests to J. R. Wignall, California Veterinary Specialists, 39809 Avenida Acacias, Murrieta, CA 92563. Email: [email protected] Submitted May 30, 2012; Accepted November 07, 2013.
182
Abbreviation
RG
rima glottidis
the tracheal diameter or to use a tube that does not completely fill the lumen;4 however, there is little scientific evidence to support these recommendations. Potential advantages of cuffed tracheostomy tubes include protecting the lower airways from debris and aspirate, preventing tube dislodgement in conscious patients, allowing effective positive pressure ventilation, and preventing anesthetic gas leaks in anesthetized patients.5–7 Some older texts recommended that cuffs should not be inflated unless positive pressure ventilation is required.6, 8 In a recent study the most common complications of tracheostomy tube use were obstruction, dislodgement, aspiration pneumonia, and stoma swelling but no clinical information was available comparing the use of cuffed and uncuffed tubes.7 Use of cuffed tubes may increase the risk of tracheal necrosis, infection, obstruction, and stenosis.2, 9 Currently, no C Veterinary Emergency and Critical Care Society 2013
Airway pressure in cuffed and uncuffed tracheostomy tubes
information is published regarding comparative airway pressure and flow rates between cuffed and uncuffed tubes of varied sizes. As an airway’s diameter is reduced, greater inspiratory pressure is required to permit the same airflow.10 At a constant airway flow rate, changes in airway pressure may be used to quantify obstruction to flow and hence tracheostomy tubes, which provide lower pressures are expected to minimize obstruction to an animal’s ventilatory effort, although this has not been examined in clinical patients. The aim of this study was to evaluate the airway pressure in canine cadaver tracheas with tracheostomy tubes of different sizes and types, at varied airflow rates. Additionally, at constant airflow, to evaluate if larger diameter cuffed tubes could achieve similar pressures to smaller uncuffed tracheostomy tubes. The null hypothesis was that the presence of a cuff and cuff inflation would have no effect on airway pressure.
Materials and Methods Tracheal specimens were obtained from 9 adult Beagles euthanatized by intravenous administration of a pentobarbital-phenytoin solution, for reasons other than upper respiratory disease. Age and weight information was unavailable. Each specimen comprised the larynx, caudal oropharynx, proximal esophagus, and trachea to the 10th tracheal ring. All larynges and tracheas were grossly normal on examination. Specimens were stored at −20◦ C and warmed to room temperature before use. Cadaver material was periodically moistened throughout the experiments and used within 3 days of initial thawing. Excess cervical tissue was excised and the esophagus was incised dorsally to permit visualization of the rima glottidis (RG). The tracheal diameter was measured at the level of the 5th tracheal ring. Each larynx was secured to a wooden board using 2 stainless steel nails at the epihyoid-stylohyoid articulation to stabilize the larynx without impeding arytenoid motion and allow inversion of the larynx to allow tracheostomy tube placement. A 20-Ga orthopedic wire was inserted through the epiglottis and pulled cranially to keep the glottis open at all times. The trachea was attached by a zip tie to form an airtight seal at the 9th tracheal ring to a 5 cm length of 28 mm diameter anesthetic tubing. The laryngeal end of the construct was left open to the atmosphere and airflow was created by applying a vacuuma in series with a custom-made flow regulator. This simulated negative pressure aboral to the larynx. Flow rate and airway pressure were measured using a ventilator calibration analyzer,b connected in series (Figure 1). The transducer was sensitive to ±0.005 cm H2 O C Veterinary Emergency and Critical Care Society 2013, doi: 10.1111/vec.12129
pressure and ±0.005 L/min flow. The pressure transducer was zeroed at atmospheric pressure with no flow and negative pressure measured as differential pressure (positive). One study found upper airway flow rates of approximately 23.4 and 33 L/min for resting and hyperpneic large breed dogs, respectively, and 30–55.8 L/min for resting and hyperpneic dogs with laryngeal paralysis.11 Flow was set at a steady state at 30 and 60 L/min and pressure was measured in triplicate for each intervention and taken as baseline values for each specimen. An incision was made through the annular ligament and mucosa between the 4th and 5th tracheal ring using a No. 10 scalpel blade. A suture of 2 metric polypropylenec was placed around the 4th tracheal ring and another was placed around the 5th tracheal ring. Metzenbaum scissors were used to extend the incision to 50% of the tracheal circumference. Sutures were manipulated to allow the placement of tracheostomy tubes of varied size and cuff type. Tubes were placed to allow central alignment in the trachea without excessive twisting or pressure on the dorsal wall of the trachea. Flow and pressure measurements were taken in triplicate. Shiley curved adult uncuffed tubesd of sizes 4 (internal diameter [ID]) = 5 mm, external diameter [ED] = 9.4 mm), 6 (ID = 6.4 mm, ED = 10.8 mm), 8 (ID = 7.6 mm, ED = 12.2 mm), and 10 (ID = 8.9 mm, ED = 13.8 mm) were placed. Subsequently, low pressure cuffed Shiley adult tubes of sizes 4, 6, 8, and 10 (same ID and ED as corresponding uncuffed tubes) were placed, initially with no cuff inflation, followed by 100% cuff inflation based on the manufacturer’s specifications. The order of testing was employed in increasing tube size and with all uncuffed tubes followed by all cuffed tubes with no inflation. Lastly, cuffed tubes with inflation were placed. This aimed to minimize tracheal deformation. Statistical methods Differential pressure data are presented as mean and standard deviation and analyzed using one-way repeated measures ANOVA. Pressure data at each intervention were compared with values before intervention in all tracheas. Commercial statistical softwaree was used for all analysis. Significance was set at P < 0.05. Airway pressure between 2 tubes at the same flow was deemed equivalent if no significant difference was found between them.
Results Placement of adult uncuffed tubes had no significant effect on airway pressure compared to the unaltered trachea (Figures 2 and 3). When a cuffed tube was placed 183
J. R. Wignall & S. J. Baines
Figure 1: The laryngeal specimens were mounted and inverted to allow tracheal tube placement. Airflow was created by applying a vacuuma in series with 3 modified 30 mL syringes acting as flow regulators. Flow rate and airway pressure were measured using a ventilator calibration analyzer, connected in series.
without inflation, size 6, 8 and 10 uninflated cuffed tubes resulted in a significant increase in airway pressure compared to the unaltered trachea at 30 and 60 L/min. Noninflated cuffed tubes resulted in an increased airway pressure compared to uncuffed tubes for all sizes and both flow rates. This was significant for tube sizes 6, 8, and 10 at both flow rates. Fully inflated (100%) cuffed tubes resulted in a significant increase in airway pressure compared to the unaltered trachea and uncuffed tubes, for all tube sizes and flow rates. As adult uncuffed tube size increased, airway pressure tended to increase from size 4 to size 8 and pressure decreased from size 8 to size 10. Greatest airway pressure was seen using uncuffed size 8 tubes and least airway pressure using uncuffed size 4 tubes. If use of an inflated cuff is indicated, equivalent airflow (nonsignificant increases in pressure) can be achieved by increasing tube size in the following situation; use of a size 8 or 10 cuffed inflated tube provides equivalent flow to the uncuffed size 4 tube. No inflated cuffed tubes are able to provide equivalent airflow to uncuffed sizes 6, 8, or 10 tubes (Table 1).
Discussion Use of a cuffed tracheostomy tube and cuff inflation significantly increased airway pressures in this experimental model and the null hypothesis can be rejected. Pouseille’s law states that for laminar flow in tube, V = Pr4 /8 L (where V is flow, P is pressure difference, n is gas viscosity, l is length of the passage, and r is radius of the passage).12 For this experiment, where the 184
apparatus and gas viscosity remain constant, pressure is proportional to flow and can be used to assess obstruction to airflow. We postulate that this increase in airway pressure may cause obstruction to airflow or increased respiratory effort in clinical patients. When comparing the unaltered trachea to cuffed tubes, all cuffed tubes resulted in increased airway pressure. In this experimental model, after placement of a tracheostomy tube, air flows through both the RG and tracheostoma. An increase in airway pressure after tube placement suggests that the presence of the tube caused more obstruction to flow through the RG than the tracheostoma can provide. Airflow through both the RG and tracheostoma can be exhibited during this experiment by comparing the effect of tube size between uncuffed tubes and cuffed tubes. In cuffed tubes where the cuff minimized RG flow, as tube size increases, pressure reduced, suggesting only tracheostoma flow occurs. For uncuffed tubes, the size 4 tube allows the lowest pressure airflow and therefore, by Pousielle’s law, must provide the greatest combined area between RG and tracheostoma.12 The presence of flow through both the RG and tracheostoma is not a requirement in clinical tracheostomy; however, this study suggests that it may occur in cases of partial upper airway obstruction, unless inflated cuffed tubes are used. The presence and inflation of a cuff increase airway pressure. If use of an inflated cuff is indicated, for example, to protect against aspirated oral content, it would be ideal to maintain the lowest airway pressure achievable. Since the uncuffed size 4 tube maintains the least airway pressure, it would be of clinical C Veterinary Emergency and Critical Care Society 2013, doi: 10.1111/vec.12129
Airway pressure in cuffed and uncuffed tracheostomy tubes
Figure 2: Differential airway pressure (cm H2 O) at a flow rate of 30 L/min to illustrate the effect of tube size, cuff presence, and cuff inflation. Mean airway pressure is illustrated by the horizontal line, standard deviation by the vertical line. Significant differences between cuffed tubes at 0% inflation and uncuffed tubes are shown by ∗ (P < 0.001). Significant differences between cuffed tubes at 100% inflation and uncuffed tubes are shown by ∧ (P < 0.001). Note: Figures 2 and 3 have different y axes.
Figure 3: Differential airway pressure (cm H2 O) at a flow rate of 60 L/min to illustrate the effect of tube size, cuff presence, and cuff inflation. Mean airway pressure is illustrated by the horizontal line, standard deviation by the vertical line. Significant differences between cuffed tubes at 0% inflation and uncuffed tubes are shown by ∗ (P < 0.001). Significant differences between cuffed tubes at 100% inflation and uncuffed tubes are shown by ∧ (P < 0.001). Note: Figures 2 and 3 have different y axes.
interest to know if larger cuffed tubes could maintain similar pressures, in essence substituting flow through the RG for extra flow through the tracheostoma. No significant differences were seen between airway pressures achieved with a size 4 uncuffed tube, a cuffed size 8 or a cuffed size 10 tube. Placing size 8 and 10 cuffed tubes in a Beagle trachea may be impractical without oversizing the tube and risking pressure necrosis, and we conclude that increasing tube size is unable to practically offset the obstruction to flow caused by the cuff’s presence. C Veterinary Emergency and Critical Care Society 2013, doi: 10.1111/vec.12129
Manufacturer’s specifications for maximum cuff inflation were used in this study to ensure that the minimal allowable airflow would exist through the RG, although this was not clinically tested. This level of inflation would not be advised in the live animal. In addition to an increase in airway pressure, all tubes have risk of displacement and clogging.13 Cuffed tubes are more likely to cause tracheal necrosis, infection, and stenosis.2, 14 No data are available on the complication rate of cuffed tubes in clinical practice. Clinicians using cuffed tracheostomy tubes, particularly in conscious 185
J. R. Wignall & S. J. Baines
Table 1: Mean pressure (cm H2 O) ± SD for the unaltered trachea, uncuffed, and cuffed tubes at different levels of inflation, for tubes of sizes 4, 6, 8, and 10
Flow
Unaltered Uncuffed Cuffed 4
(L/min)
trachea
30 60
Mean SD Mean SD
0.31 0.07 1.15 0.30
Uncuffed Cuffed 6
4
0%
100%
6
0.19 0.01 0.63 0.09
0.53 0.14 1.63 0.56
5.96∗ 1.81 20.73∗ 6.83
0.25 0.06 0.85 0.22
0%
100%
1.02∗ 0.48 3.74∗ 1.85
2.21∗ 0.54 7.89∗ 2.22
Uncuffed Cuffed 8 8 0.36 0.13 1.11 0.44
0%
100%
0.93∗ 0.20 3.12∗ 0.80
1.07∗ 0.20 3.68∗ 0.85
Uncuffed Cuffed 10 10 0.32 0.08 1.10 0.29
0%
100%
0.56∗ 0.08 1.88∗ 0.33
0.60∗ 0.09 2.04∗ 0.37
Significant increase in airway pressure (P < 0.001) compared to the unaltered trachea is annotated by ∗ .
patients, should acknowledge and evaluate these risks against the potential benefits of these tubes. This study utilized a small number of tracheas, although each trachea was subject to all procedures and served as its own control. Cadaver studies are subject to limitations such as loss of supporting peritracheal tissues, loss of muscular tone, lack of response to tissue handling, and possible change in mechanical properties due to loss of osmotic balance and specimen treatment.15 It is granted that the trachea in a live dog is subject to flexion and extension, which will affect airway flow and pressure. This study is designed with standardized extension of the trachea to allow direct comparison between tubes. This is a constant rate inspiratory model of respiration aimed at replicating peak inspiratory flow only. The airflows listed here are not intended to directly translate to the live dog. However, since the characterization of airway pressure is currently limited in the live animal, cadaver experiments provide an alternative and have been used in previous studies.15 Some authors claim that the arytenoid cartilages of the excised larynx maintain a similar intermediate position to the live patient with bilateral laryngeal paralysis.16, 17 This observation is based on the observation of the arytenoids in an intermediate position, and its use has not been ratified as a model of dynamic obstruction. Studies on the cadaver and live animal have been used in documenting the effects of surgical intervention on the larynx but not the trachea.16–18 Tidal breathing flow-volume loops19 and head out plethysmography11 are unfortunately limited to resting animals only. RG flow rates have not been classified in the live animal due to the difficulty of isolating flow from the nares and upper respiratory tract in a conscious animal. One study found upper airway flow rates of approximately 23.4 and 33 L/min for resting and hyperpneic large breed dogs, respectively, and 30–55.8 L/min for resting and hyperpneic dogs with laryngeal paralysis.11 These were generally larger dogs than the Beagles used in this study but 30 and 60 L/min were deemed the most appropriate flow rates to test. With regard to inspiratory pressures employed in this experiment, negative pressures are presented in Table 1. The range of negative 186
pressures elicited across the experiment was −0.19 to −2.20 cm H2 O for 30 L/min and −0.63 to −20.73 L/min for 60 L/min. Peak inspiratory pleural pressures range from −7 to −14.3 cm H2 O (mean, −9.34 cm H2 O), drawing air into the airways and to the lungs;20 however, limited information is available for tracheal pressures in resting medium-sized dogs. These recommendations must be interpreted carefully as statistical significance may not equal clinical significance. Since air flows through both the RG and tracheostomy tube, this model of airway obstruction mimics a single point on the spectrum between no significant obstruction and total airway occlusion, and results should be analyzed as such. Rather than a fixed obstruction as may be seen in a space-occupying lesion, the level of obstruction in this model may also depend on the inspiratory pressure—the increased inspiratory pressure is able to pull the paralyzed arytenoids into the airway, thus reducing the radius of the tube and increasing pressure exponentially.12 A previous study found that high inspiratory pressure in this model can cause laryngeal collapse.15 The relative effect of the cuff may be dependent on the degree of RG obstruction already present. In a model of more significant upper airway obstruction, where less RG flow is present, the relative effect of a cuff, which affects this RG airflow, should also reduce as the flow through the tracheostoma becomes more important. Mean tracheal diameter in the Beagle specimens used was 25.11 mm (±0.484 SD). According to previous recommendations of tube size occupying a maximum diameter of 75% of tracheal diameter,3 all uncuffed tubes used would be suitable; however, the use of cuffs would not be accounted for. Although the comparison of a range of tubes in one trachea is useful, some of the tube usage (specifically the size 8 and 10 cuffed inflated tubes) in this study is in the author’s opinion unrealistic for the size of the trachea and would not be clinically recommended. The order of tube placement was designed to minimize trauma to the tracheal specimens. Subjectively, due to the elastic properties of the trachea, when uncuffed and cuffed uninflated tubes were placed and removed, the C Veterinary Emergency and Critical Care Society 2013, doi: 10.1111/vec.12129
Airway pressure in cuffed and uncuffed tracheostomy tubes
trachea returned to a normal position; however, a significant limitation was that specimen pressures were not measured between tube placements, as in this phase of the experiment, airflow properties may have changed without subjective visual assessment. When fully inflated cuffs were placed, subjective deformation did occur; however, since the aim of the full cuff was to eliminate all airflow around the tube and measure tube flow only, deformation in the trachea would not be expected to affect pressure data for the use of cuffed tubes of increasing size. As an alternative to inflating the cuff to the manufacturer’s specification, a more clinically applicable method of cuff inflation, such as leak testing or utilizing a defined pressure or volume within the cuff, could have been used. This experimental method was chosen to ensure that laryngeal airflow would be reduced to the most achievable minimum, and this level of distension would not be recommended in clinical practice due to the risk of tracheal necrosis and rupture. Further studies would be required to assess the effect of tracheostomy tubes on different tracheal sizes. In conclusion, the benefits of using cuffed tracheostomy tubes and cuff inflation should be considered in light of detrimental increases in airway pressure. Depending on the patient’s tracheal size it may not be possible to negate the effects of a cuff; however, the use of cuffed tubes should be carefully considered. Uncuffed tubes are recommended in patients who need tracheostomies as evidenced by decreased airway pressures when compared to cuffed tubes.
Footnotes a b c d e
Tesco VC406 Bagged Compact Cylinder Vacuum; Tesco, Cheshunt, England. Timeter RT-200; Allied Health Products, St. Louis, MO. Prolene; Ethicon, Edinburgh, Scotland. Tyco Healthcare UK Ltd., Gosport, United Kingdom. GraphPad Software, Inc., San Diego, CA.
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