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Aspects of in vivo endotracheal tube intracuff pressure in cats
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Veterinary Anaesthesia and Analgesia 2018, xxx, 1e9
https://doi.org/10.1016/j.vaa.2018.06.017
RESEARCH PAPER
Aspects of in vivo endotracheal tube intracuff Q11
pressure in cats
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Alice R Birda, David J Birdb & Matthew W McMillana
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a
Department of Veterinary Medicine, University of Cambridge, Cambridge, UK
b
Department of Applied Sciences, University of the West of England, Bristol, UK
Correspondence: Alice R Bird, Animal Health Trust, Newmarket, Suffolk, UK. E-mail: [email protected]
Abstract Objectives To determine the endotracheal tube cuff pressure produced with two inflation techniques, in two brands of endotracheal tube in cats. To determine the inspiratory pressure which produces an audible leak when the intracuff pressure is 30 cmH2O. Study design Prospective, clinical, randomized study. Animals A total of 40 client-owned healthy adult cats. Methods Following induction of anaesthesia, endotracheal intubation was performed with a Parker Flex-Tip PFLP (Parker; n ¼ 20) or Flexicare VentiSeal (Flexicare; n ¼ 20) endotracheal tube. For each cat, the endotracheal tube cuff was inflated using two methods, minimum occlusive volume (MOV) and pilot balloon palpation (PBP). Intracuff pressure was recorded. Cuff pressure was then set at 30 cmH2O and the pressure within the breathing system when a manual breath first caused an audible leak was measured.
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Results PBP pressure was lower for Parker (36 ± 13 cmH2O) compared with Flexicare (45 ± 13 cmH2O, p ¼ 0.048). MOV pressure was not different between tube types (56 ± 28 versus 66 ± 25 cmH2O for Parker and Flexicare, respectively, p ¼ 0.247). MOV produced a higher pressure than PBP for Parker (56 ± 28 versus 36 ± 13 cmH2O, p ¼ 0.001) and Flexicare (66 ± 25 versus 45 ± 13 cmH2O, p ¼ 0.007). When intracuff pressure was set at 30 cmH2O, 95% of cats did not develop an audible leak until the inspiratory pressure was greater than 10 or 12 cmH2O for Parker and Flexicare tubes, respectively.
Conclusions PBP produced lower cuff pressures than MOV, although both techniques produced a cuff pressure above that at which mucosal blood flow is believed to be restricted. A cuff pressure of 30 cmH2O may be sufficient to prevent audible leak in most cats if respiratory pressures are kept at 10e12 cmH2O or below. Clinical relevance To ensure a safe endotracheal tube cuff pressure, use of a specifically designed pressure gauge is recommended. Keywords cat, cuff, endotracheal intubation, minimum occlusive volume, pilot balloon palpation, pressure gauge. Introduction Endotracheal tube cuff inflation is recommended during anaesthesia to prevent aspiration of oropharyngeal contents, reduce environmental contamination with volatile anaesthetic agents and allow assisted or controlled ventilation if required (Bednarski 2015). Excessive cuff inflation, however, can reduce mucosal blood flow (Dobrin & Canfield 1977; Seegobin & Hasselt 1984; Bunegin et al. 1993) and result in mucosal damage (Nordin 1977; Castilho et al. 2003). In cats, tracheal damage and rupture following intubation has been reported. Whilst these reports primarily follow dental procedures (Hardie et al. 1999; Mitchell et al. 2000; Bhandal & Kuzma 2008) where movement of the endotracheal tube, rather than the pressure the cuff exerts on the tracheal wall, may be the most important factor, this does not explain all cases. Excessive endotracheal tube cuff pressure alone has been shown to result in extensive tracheal damage in rabbits and it seems
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Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
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Endotracheal tube cuff pressure in cats AR Bird et al.
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reasonable to suspect that cat tracheas could respond similarly (Nordin 1977). Studies in humans (Jain & Triparthi 2011; Totonchi et al. 2015; Khan et al. 2016), dogs (Briganti et al. 2012) and benchtop models (Hoffman et al. 2006; Faris et al. 2007) have shown that inflation of endotracheal tube cuffs by various inflation techniques, without the aid of a pressure gauge, produces wide-ranging intracuff pressures, with some recordings as high as 100 cmH2O (Briganti et al. 2012; Totonchi et al. 2015). In addition, variations in results have been shown with different endotracheal tube brands, cuff material and cuff shape (Asai & Shingu 2001; Zanella et al. 2011). The use of a manometer in human medicine is now widely recommended (Ham et al. 2013), with maximum intracuff pressure recommendation ranging between 20 and 30 cmH2O (Morris et al. 2007; Chan et al. 2009; Totonchi et al. 2015). The aim of this study was twofold. First, to compare the endotracheal tube intracuff pressure reached in cats with two different ‘blind’ (not involving intracuff pressure measurement) inflation techniques, thought to be in common use in general practice, in two different endotracheal tube designs, with different sized pilot balloons and cuff shape. It was hypothesized that the intracuff pressures reached would be variable and excessively high and that the pilot balloon and cuff design would affect the pressures reached. Second, when the endotracheal tube intracuff pressure was to set to 30 cmH2O, we aimed to determine the peak inspiratory pressure needed during manual ventilation to produce an audible leak of air. It was hypothesized that this intracuff pressure would be sufficient to prevent an audible leak in most cats at recommended ventilation peak inspiratory pressures of 8e12 cmH2O (Hopper & Powell 2013). Materials and methods Ethical statement The study design was approved by the Queens Veterinary School Hospital, University of Cambridge, Ethics and Animal Welfare Committee (ref CR238). Owner consent Informed owner consent was obtained for all animals prior to enrolment in the study. 2
Animals All cats presenting to the Queens Veterinary School Hospital for routine neutering or dental procedures from a cat rehoming charity were considered for inclusion in the study. Cats were excluded if <1 year of age, <2.5 kg or American Society of Anaesthesiologists (ASA) score >I. The ASA score was assigned by the anaesthetist collecting the data and cats were excluded where temperament prevented a full clinical examination prior to sedation. Study design The study was designed as a prospective, randomized, partial-crossover clinical trial; each cat’s endotracheal tube cuff was inflated sequentially by two different techniques, but only one endotracheal brand of the two investigated was used in each cat. A pilot study of 10 cats was performed comparing the two inflation techniques used in this study. This produced a mean ± standard deviation difference of 17 ± 34 cmH2O. To achieve a power of 80% and a level of significance of 5%, the sample size needed for this study was calculated as 33. Because four study groups were used (see below) the sample size was increased to 40 cats to ensure equal weighting of the groups. Cats were allocated to one of the four groups (10 cats per group) during recruitment. Groups 1 and 2 were intubated with a Parker Flex-Tip PFLP (Parker Medical, CO, USA) endotracheal tube (Parker), while groups 3 and 4 were intubated with a Flexicare VentiSeal (Flexicare Medical Ltd, UK) endotracheal tube (Flexicare; Fig. 1). Therefore, 20 cats were assigned to each endotracheal tube brand. These 20 cats were then subdivided into two further subgroups, which determined which inflation method was performed first (groups 1 and 3 one method, groups 2 and 4 the second method). Prior to recruitment of any cats, a list of 1, 2, 3, and 4, repeated 10 times, was input into an excel spreadsheet. The numbers were then randomly reordered and cats were assigned their group based on their order of recruitment into the study (first cat recruited assigned to first number in list, tenth cat the tenth number, etc.). Both endotracheal tube brands used were plastic and designed for single use. The cuffs and pilot balloons were, however, different (Fig. 1). The Parker endotracheal tube cuff had a more oval shape, with smaller subsequent contact area suspected to be
© 2018 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e9
Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
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Endotracheal tube cuff pressure in cats AR Bird et al.
Veterinary Association Guidelines (WSAVA Nutritional Assessment Guidelines Task Force Members 2011). Anaesthetic procedure
Figure 1 The two brands of endotracheal tubes used in the study with their cuffs (a) inflated and (b) deflated. 1, Flexicare VentiSeal, 4 mm internal diameter; 2, Parker Flex-Tip PFLP 4 mm internal diameter.
required to produce a seal with the trachea mucosa. The Flexicare cuff in comparison was more cylindrical, designed to contact a larger surface area when inflated. The Parker pilot balloon was larger than the Flexicare. The endotracheal tubes were all new prior the commencement of the study and reused during the study period up to a maximum of five times. After each use, the tubes were washed with warm water and sterilized with Milton sterilizing fluid (Proctor and Gamble, UK) before being left to air dry. Prior to use, all cuffs were inflated to examine for visible defects or deformity of the cuff and then left for 10 minutes to ensure the cuff remained inflated prior to use in any cat. Endotracheal tubes with 4.0 and 4.5 mm internal diameter were available for each cat, with the anaesthetist collecting the data (anaesthetist B) choosing the size they deemed most appropriate following examination of the larynx. All cats were premedicated and later induced in a sound-proofed cat induction bay. All subsequent measurements were made in this room, which was kept silent throughout the data collection. A body condition score was assigned as assessed by anaesthetist B, based on the World Small Animal
Premedication was performed with sedatives and analgesics as deemed appropriate by the anaesthetist overseeing the case (anaesthetist A). Following intravenous cannula placement, anaesthesia was induced with alfaxalone (Alfaxan; Jurox UK Ltd, UK) to effect (0.5e2.0 mg kge1). A second anaesthetist (anaesthetist B), who remained the same person throughout data collection, then examined the larynx of the cat and applied topical lidocaine spray (Intubeaze; Dechra Pharmaceuticals PLC, UK). Fortyfive seconds later, endotracheal intubation was performed. Following intubation, the endotracheal tube was secured in place with a length of gauze bandage. The cats were placed into right lateral recumbency and connected to a modified paediatric T-piece with a reservoir bag and safety adjustable pressure limiting valve (Infant T-piece anaesthetic breathing system; Intersurgical, UK). A capnography adaptor was placed between the endotracheal tube connector and breathing system. Oxygen was supplied at 3 L minutee1. From this point, anaesthetist A monitored the cat whilst anaesthetist B and a nurse collected the data. Comparison of two endotracheal tube cuff inflation techniques A three-way tap (Vyclic 3-way stopcock; Vygon Ltd, UK) was connected to the pilot balloon valve, as well as an injection valve (Safeflow; B. Braun Medical Ltd, UK) for connection of a 5 mL syringe (BD Plastipak syringe 5 mL; Becton Dickinson, UK) and a Portex cuff inflator pressure gauge (Smiths Medical, UK; Fig. 2). The pressure gauge accuracy was checked against a column of water and was found to read within ±0.5 cmH2O over a range of pressures. The endotracheal cuff for each of the 40 cats was inflated using two methods in a randomized order as determined by the aforementioned method: 1. Minimum occlusive volume (MOV): The rebreathing bag was emptied to approximately 50% ‘full volume’ by a registered veterinary nurse before they closed the adjustable pressure limiting valve of the breathing system. The nurse then squeezed the rebreathing bag to observe what they felt was a ‘normal’ breath, by visualizing the expansion of the cat’s chest and
© 2018 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e9
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Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
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Endotracheal tube cuff pressure in cats AR Bird et al.
between each of the three cuff inflations was 5e7 seconds. The same syringe, pressure gauge, connective tubing (V-green extension; Vygon Ltd), threeway tap and valve were used for all cats. Audible leak pressure when endotracheal cuff pressure was set at 30 cmH2O
Figure 2 Set up of pressure gauge for measuring endotracheal intracuff pressure following inflation with ‘minimum occlusive volume’ and ‘pilot balloon palpation’ methods. This configuration was also used to ensure a pressure of 30 cmH2O in the endotracheal cuff before the leak pressure was recorded. A, pressure gauge; B, rigid extension tubing; C, three-way tap; D, injection port valve for 5 mL syringe attachment.
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through the tension felt in the bag. As this was performed, anaesthetist B listened at the cat’s mouth and injected air from the syringe to inflate the endotracheal cuff until a leak was no longer audible. 2. Pilot balloon palpation (PBP): Anaesthetist B inflated the endotracheal cuff by injecting air from the syringe until the pilot balloon was inflated to what s/he deemed was an appropriate pressure (so the cuff was just inflated and had some give on digital palpation). Anaesthetist B had not trained to feel a PBP of any set value.
Following each inflation of the endotracheal tube cuff, the pressure reached on the pressure gauge was recorded by the nurse. The pressure gauge and measurements were kept hidden from view of anaesthetist B throughout collection of the data. The endotracheal cuff was then completely deflated by withdrawing air from the cuff with the syringe until negative pressure was achieved, prior to performing the next pressure measurement, or setting the pressure to 30 cmH2O as described below. Washout time 4
After recording the pressures reached by MOV and PBP, the endotracheal cuff pressure was adjusted to 30 cmH2O. The three-way tap was then removed from the pilot balloon self-sealing valve and the pressure gauge was connected to the internal pressure of the breathing system via the capnography adaptor. Anaesthetist B emptied the rebreathing bag to 50% ‘full volume’ before closing the adjustable pressure limiting valve. Anaesthetist B then squeezed the rebreathing bag and the nurse recorded the pressure reached when a leak was first audible for anaesthetist B at the cat’s mouth. The intracuff pressure was then left at 30 cmH2O and isoflurane in oxygen administered to the cat at a percentage deemed appropriate by anaesthetist A. The anaesthetic continued to allow completion of the neutering or dental procedure. Anaesthetist B had completed a 3 year ‘residency’ in anaesthesia at time of data collection and had no defects in hearing. All cats were anaesthetized in the morning and remained in the clinic for the remainder of the day. Postsurgery/dental rechecks were performed 2e5 days postanaesthetic at the cat rehoming charity by one of the hospital veterinary surgeons. Data analysis IBM SPSS software (IBM, NY, USA) was used for statistical analysis. Data collected were tested for normality of distribution using the ShapiroeWilk test. Where data appeared to be normally distributed, comparisons between two data sets were made using the two samples t-test or the paired t-test as appropriate. When comparing each endotracheal brand and endotracheal size with each other (four groups) a one-way analysis of variance was performed. Where data did not appear to be normally distributed, and for comparison of body condition score between the two endotracheal brands, the ManneWhitney U-test was performed. Spearman’s correlation coefficient was used to compare the order the cats were recruited into the study against their PBP pressure. Normally distributed data are
© 2018 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e9
Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
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Endotracheal tube cuff pressure in cats AR Bird et al.
A total of 40 cats were enrolled in the study. Complete data sets were obtained for all 40 cats. No adverse event occurred during data collection in any cat. Following anaesthesia, no reports of complications, including coughing, up until the point of recheck were reported. All data sets tested were found to be normally distributed with the exception of cat ages for the Parker and Flexicare groups and leak pressures when intracuff pressure was set at 30 cmH2O. No difference was found between the bodyweights of the cats intubated with Parker and Flexicare tubes (3.67 ± 0.88 and 4.06 ± 1.08 kg, respectively, p ¼ 0.216). The distribution of ages and body condition scores were also not significantly different between tube brands (p ¼ 0.799 and 0.341, respectively).
respectively, p ¼ 0.247), although the PBP pressure for the Parker tube (36 ± 13 cmH2O) was lower than that for the Flexicare tube (45 ± 13 cmH2O), p ¼ 0.048. When the data were divided up into four groups, representing endotracheal tube brand and internal tube diameter, the PBP and MOV pressures, respectively, for each group were as follows: Parker 4 mm (33 ± 11, 57 ± 32 cmH2O), Parker 4.5 mm (44 ± 13, 56 ± 20 cmH2O), Flexicare 4 mm (45 ± 15, 63 ± 27 cmH2O) and Flexicare 4.5 mm (44 ± 10, 70 ± 24 cmH2O). No difference was found between the groups for PBP or MOV (p ¼ 0.074 and 0.656, respectively). When the endotracheal tube internal diameters were compared without accounting for endotracheal tube brand, size continued to have no significant effect on PBP pressure (38 ± 14 and 44 ± 11 cmH2O for 4 and 4.5 mm internal diameter, respectively, p ¼ 0.215) or MOV pressure (59 ± 29 and 64 ± 23 cmH2O for 4 and 4.5 mm internal diameter, respectively, p ¼ 0.584).
Differences in intracuff pressure between MOV versus PBP techniques
Audible leak pressure when endotracheal cuff pressure was set at 30 cmH2O
For both the Parker and Flexicare endotracheal tube, and when the measurements obtained with the separate tube types were combined, the MOV technique produced a significantly higher mean pressure than PBP (Table 1).
No difference was found between the distribution of data for the leak pressures obtained for the two brands of endotracheal tube (p ¼ 0.221). Leak pressure percentiles for the Parker and Flexicare endotracheal tubes, and for all cats irrespective of endotracheal brand are shown in Table 2.
presented as mean ± standard deviation. A p value < 0.05 was considered significant. Results
Effect of time on PBP pressure
Discussion
There was no evidence of a correlation between cat number (in order of recruitment) and PBP pressure (rs ¼ 0.059, p ¼ 0.720). Differences in intracuff pressure between endotracheal tube brands The MOV pressure was not different for Parker and Flexicare tubes (56 ± 28 and 66 ± 25 cmH2O,
The study findings support our hypothesis that endotracheal tube cuff inflation techniques without a pressure gauge can produce varying, excessively high pressures. The mean intracuff pressure produced with both inflation techniques in both tube brands was above the human recommended pressure of 20e30 cmH2O (Morris et al. 2007; Chan et al. 2009; Totonchi et al. 2015). The Parker tubes had a
Table 1 Mean ± standard deviation of endotracheal tube intracuff pressure obtained with the pilot balloon palpation (PBP) and minimum occlusive volume (MOV) inflation techniques for both Parker Flex-Tip PFLP and Flexicare VentiSeal endotracheal tubes and for all cats irrespective of endotracheal tube brand Group
PBP pressure (cmH2O)
MOV pressure (cmH2O)
p
Parker Flex-Tip PFLP (n ¼ 20) Flexicare VentiSeal (n ¼ 20) All cats (n ¼ 40)
36 ± 13 45 ± 13 41 ± 12
56 ± 28 66 ± 25 61 ± 27
0.001 0.007 0.001
MOV pressure ¼ intracuff pressure obtained with minimum occlusive volume inflation technique; PBP pressure ¼ intracuff pressure obtained with pilot balloon palpation inflation technique; p < 0.05 indicates significant difference between inflation techniques.
© 2018 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e9
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Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
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Endotracheal tube cuff pressure in cats AR Bird et al. Table 2 The inspiratory pressures (cmH2O) at which an audible leak could be detected when the endotracheal cuffs were inflated to 30 cmH2O and a manual breath administered Percentile (%)
Inspiratory pressure (cmH2O)
Percentage of cats with audible leak developing at or below the inspiratory pressure indicated
Percentage of cats with audible leak developing above the inspiratory pressure indicated
Parker Flex- Flexicare Tip PFLP VentiSeal (n ¼ (n ¼ 20) 20)
All cats (n ¼ 40)
5 10 25 50 75 90 95
95 90 75 50 25 10 5
10 12 15 20 24 24 26
12 12 19 21 24 24 25
significantly lower intracuff pressure than the Flexicare tubes when the PBP inflation technique was used, which may have been due to the larger pilot balloon design and the subsequent ‘feel’ of the balloon on inflation (Fig. 1), although this pressure was still excessively high. Consequently, neither cuffinflation method can be recommended, based on our findings, to produce a safe intracuff pressure in the described tubes. Care should be taken in extrapolating the results of this study to other anaesthetists. Studies in humans have repeatedly demonstrated variability between individuals (Chan et al. 2009; Wujtewicz et al. 2009), with the appropriate ‘feel’ of the pilot balloon being subjective. It is also likely that the sensitivity of the MOV technique is influenced by surrounding noise and the auditory sensitivity of the person inflating the cuff, and so use of a stethoscope over the larynx may improve this technique. White et al. (2017) reported much lower values for MOV and PBP, although these inflations were performed in a cat model, rather than in vivo, and the authors still concluded that both techniques rarely resulted in optimal cuff pressure. Because of the degree of individual variability reported (Briganti et al. 2012; Totonchi et al. 2015; Khan et al. 2016), it is felt unlikely that the excessive intracuff pressures reported here are a rare occurrence. Lower intracuff pressures may have been obtained during the MOV technique had the inspiratory breath been delivered in a controlled manner using a mechanical ventilator. Whilst an inspiratory breath being supplied by a nurse is most reflective of the situation in general practice, it would have likely produced variable peak inspiratory pressure and 6
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speed of breath delivery and may have contributed to the wide range of intracuff pressures recorded. The use of a single nurse throughout the study would have been preferable and the use of multiple nurses is a limitation of this study. Studies investigating MOV, with an inspiratory peak pressure of 20 cmH2O, have been performed in humans (Al-Metwalli et al. 2011) and dogs (Briganti et al. 2012). In the human study, a low mean intracuff pressure was obtained with a small degree of pressure variation (20.5 ± 3.8 cmH2O). The canine study investigated plastic and silicone endotracheal tubes and found mean intracuff pressures of 47.6 ± 43.5 and 104.7 ± 68 cmH2O, respectively. Comparison of pressures between studies should be made with caution, due to different methodology and species, although it is interesting that even with a controlled inspiratory breath, Briganti et al. (2012) still found the intracuff pressures to be variable. Training in the feel of an appropriately inflated pilot balloon has been shown to increase the tendency to inflate endotracheal cuffs within a desired range, although in a study by Siamdoust et al. (2015) those able to accurately inflate the cuff improved from 24.2% to only 39.7% with training. In this study, no training of the ‘feel’ of the pilot balloon at an ideal pressure had been undertaken by anaesthetist B. Lower intracuff pressures may have been obtained if this had occurred prior to data collection. Whilst it was possible that anaesthetist B could have improved their PBP technique throughout the study, as they became aware of previous pressure recordings, this does not seem to be the case as there was no correlation between the order the cats were recruited into the study and PBP. Training of PBP
© 2018 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e9
Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
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Endotracheal tube cuff pressure in cats AR Bird et al.
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may be useful in clinics where availability of a pressure gauge is not always possible. It is important to note that there is strong evidence that anaesthetist experience is not associated with improved accuracy in cuff inflation and training is likely to need refreshing over time (Sengupta et al. 2004; Wujtewicz et al. 2009; White et al. 2017). Because of our findings and the literature discussed above we would advise the use of a pressure gauge when inflating endotracheal tube cuffs in cats. In addition, a specific endotracheal tube cuff pressure gauge is recommended, as discrepancies in pressure readings between these and handcrafted cuff manometers have been shown (Annoni & Evanir de Almeida 2015). Whilst endotracheal tube cuffinflator devices are available as an alternative to a pressure gauge, differences in their accuracy have been reported, with none of the four cuff inflators tested in one study accurately measuring cuff pressure (Blanch 2004). Endotracheal tubes that incorporate a control valve with the pilot balloon arrangement are also available (Mallinckrodt Hi-Lo Oral/Nasal Tracheal Tube Lanz System; Medtronic, UK). These automatically maintain a low intracuff pressure and may be safer to use than the endotracheal tubes used in this study. Whilst intracuff pressure does not necessarily equal the pressure the cuff exerts on the tracheal mucosa, examination of pressures in a tracheal model does suggest that the two are linearly related (Black & Seegobin 1981). It should be noted, however, that the difference between these two values will likely vary with the endotracheal tube cuff design; Dobrin & Canfield (1977) reported mucosal pressures ranging from 58% to 91% intracuff pressure in 18 brands of endotracheal tube, although the mucosal and intracuff pressures were measured with different techniques. Fernandez et al. (1990) reported pressure versus volume curves to be steep for multiple endotracheal tube bands, with a sudden rise in pressure for small increase in volume. Once the cuff is restricted within the trachea we suspect that this volume pressure relationship is likely to be present for most endotracheal tubes. In this study, when the endotracheal tube cuff pressure was set to 30 cmH2O, 95% of cats did not develop an audible leak around their tube until peak inspiratory pressure was greater than 10.1 and 12.3 cmH2O for Parker and Flexicare, respectively. We therefore believe this cuff pressure to be sufficient to prevent large volume environmental contamination with anaesthetic vapours for most cats breathing
spontaneously or ventilated to a peak inspiratory pressure of 10 cmH2O or below with the endotracheal tubes described. It is possible, however, that a small volume of air was still able to leak past the inflated cuffs at pressures below which the audible leak was detected. It is also unclear whether differing characteristics of an inspiratory breath, such as speed of inspiration and pressure distribution throughout the inspiratory breath, would affect the pressure at which an audible leak occurred. Importantly, lack of an audible air leak during an inspiratory breath does not guarantee protection of the lower airways from aspiration. Whilst species differences are likely to be present, 12/12 pigs anaesthetized with an intracuff pressure of 30 cmH2O showed leakage of methylene blue past the cuff (Negro et al. 2014), suggesting that this pressure may not be sufficient to prevent aspiration, at least in pigs. It is also unclear whether 30 cmH2O is safe to use in cats with regard to mucosal blood flow. In hypotensive dogs, an intracuff pressure of 20.4 cmH2O resulted in reduced tracheal blood flow (Bunegin et al. 1993), whilst biopsies of tracheal mucosa from dogs following an intracuff pressure of 25 cmH2O showed mild changes when examined with electron microscopy (Castilho et al. 2003). Whilst the clinical significance of these changes is uncertain, it seems prudent to assume that cat tracheas would be even more sensitive to intracuff pressures due to their smaller and more delicate nature and more complete tracheal rings. Whilst reports of tracheal rupture in cats are relatively scarce (Hardie et al. 1999; Mitchell et al. 2000; Bhandal & Kuzma 2008), it is unknown how many cats have subclinical damage to their mucosa postanaesthetic due to excessive cuff pressure. The presence of any subclinical tracheal mucosal damage in the cats enrolled in this study was undetermined, although no reports of coughing were made. A definitive recommendation for intracuff pressure in veterinary species is lacking. Whilst a maximum of 34 cmH2O in cats and dogs is recommended in one textbook (Hughes 2016) and 20e30 cmH2O was suggested as optimal in a cat model (White et al. 2017) it appears that these recommendations are extrapolated from the human literature. Even in humans the recommendations vary, although suggested pressures fall mostly between 20 and 30 cmH2O (Morris et al. 2007; Chan et al. 2009; Totonchi et al. 2015). This value appears to be the result at least in part by a study by Seegobin & Hasselt (1984) whereby endoscopy was performed
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Endotracheal tube cuff pressure in cats AR Bird et al.
to visualize the tracheal mucosa under the endotracheal cuff in humans. At 25 cmH2O the mucosa was of uniform hue and the blood vessels of normal calibre. At pressures of 30 cmH2O and above, however, the mucosa over the tracheal rings became visibly less pink, although how this reduction in blood flow relates to subsequent pathology is uncertain. In spontaneously breathing cats, a pressure lower than 30 cmH2O may be sufficient to prevent significant environmental contamination, whilst minimizing compromise of mucosal blood flow, although the risk of aspiration is likely to be higher. The endotracheal tubes used in this study were sold as single use but reused. Whilst reuse is more common in veterinary practice, this may have affected the results by altering the properties of the pilot balloon and cuff or creating deformities in the cuff invisible to visual inspection. Conclusions Neither the MOV nor the PBP technique can be recommended for endotracheal tube cuff inflation in cats and use of a cuff-specific pressure gauge is advised. An intracuff pressure of 30 cmH2O appeared to be sufficient in the majority of cats in this study to prevent an audible air leak at a peak inspiratory pressure of 10 cmH2O or below. Whether this pressure sufficiently protected the airways from aspiration or caused any subclinical mucosal damage, however, is uncertain. Uncited reference Q9
Rokamp et al. 2010. Acknowledgements
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The authors wish to thank the theatre nurses at the Queens Veterinary School Hospital, Cambridge University, for their assistance in collecting the data reported. This research did not receive any specific grant from funding agencies in the public commercial, or not for profit sectors. Authors’ contributions ARB: data collection, statistical analysis and preparation of manuscript. DJB: statistical analysis, editorial input of final document. MWM: study design, editorial input of final document. Conflict of interest statement Authors declare no conflict of interest. 8
References Al-Metwalli RR, Al-Ghamdi AA, Mowafi HA et al. (2011) Is sealing cuff pressure, easy, reliable and safe technique for endotracheal tube cuff inflation? A comparative study. Saudi J Anaesth 5, 185e189. Annoni R, Evanir de Almeida A (2015) Handcrafted cuff manometers do not accurately measure endotracheal tube cuff pressure. Rev Bras Ter Intensiva 27, 228e234. Asai T, Shingu K (2001) Leakage of fluid around highvolume, low-pressure cuffs. A comparison of four tracheal tubes. Anaesthesia 35, 38e42. Bednarski RM (2015) Dogs and cats. In: Veterinary Anaesthesia and Analgesia (5th edn). Grimm KA, Lamont LA, Tranquilli WJ et al. (eds). John Wiley & Sons, UK. pp. 819e826. Bhandal J, Kuzma A (2008) Tracheal rupture in a cat: diagnosis by computed tomography. Can Vet J 49, 595e597. Black AMS, Seegobin RD (1981) Pressures on endotracheal tube cuffs. Anaesthesia 36, 498e511. Blanch PB (2004) Laboratory evaluation of 4 brands of endotracheal tube cuff inflator. Respir Care 49, 166e173. Briganti A, Portela DA, Barsotti G et al. (2012) Evaluation of the endotracheal tube cuff pressure resulting from four different methods of inflation in dogs. Vet Anaesth Analg 39, 488e494. Bunegin L, Albin MS, Smith B (1993) Canine tracheal blood flow after endotracheal tube cuff inflation during normotension and hypotension. Anesth Analg 76, 1083e1090. Castilho EC, Braz JR, Cat^ aneo AJ et al. (2003) Effects of tracheal tube cuff limit pressure (25 cmH2O) and “seal” pressure on tracheal mucosa of dogs. Rev Bras Anesthesiol 53, 743e755. Chan SM, Wong CS, Cherng CH (2009) Determining the optimal tracheal tube cuff pressure by the feel of the pilot balloon: a training course for trainees providing airway care. Acta Anaesthesiol Taiwan 47, 79e83. Dobrin P, Canfield T (1977) Cuffed endotracheal tubes: mucosal pressures and tracheal wall blood flow. Am J Surg 133, 562e568. Faris C, Koury E, Philpott J et al. (2007) Estimation of tracheostomy tube cuff pressure by pilot balloon palpation. J Laryngol Otol 121, 869e871. Fernandez R, Blanch L, Mancebo J et al. (1990) Endotracheal tube cuff pressure assessment: pitfalls of finger estimation and need for objective measurement. Crit Care Med 18, 1423e1426. Ham F, Zuercher M, Bassi M et al. (2013) Prospective observational study on tracheal tube cuff pressures in emergency patients e is neglecting the problem the problem? Scand J Trauma Resusc Emerg Med 21, 83. Hardie EM, Spodnick GJ, Gilson SD et al. (1999) Tracheal rupture in cats: 16 cases (1983e1998). J Am Vet Med Assoc 214, 508e512.
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study of effects of four large volume cuffs. Br Med J (Clin Res Ed) 288, 965e968. Sengupta P, Sessler DI, Maglinger P et al. (2004) Endotracheal tube cuff pressure in three hospitals, and the volume required to produce an appropriate cuff pressure. BMC Anaesthesiol 4, 8. Siamdoust SAS, Mohseni M, Memarian A (2015) Endotracheal tube cuff pressure assessment: education may improve but not guarantee the safety of palpation technique. Anesth Pain Med 5e16163. Totonchi Z, Jalili F, Hashemian SM et al. (2015) Tracheal stenosis and cuff pressure: comparison of minimal occlusive volume and palpation techniques. Tanaffos 14, 252e256. White DM, Redondo JI, Mair AR et al. (2017) The effect of user experience and inflation technique on endotracheal tube cuff pressure using a feline airway simulator. Vet Anaesth Analg 44, 1076e1084. WSAVA Nutritional Assessment Guidelines Task Force Members (2011) WSAVA nutritional assessment guidelines. J Feline Med Surg 13, 516e525. Wujtewicz MA, Sawicka W, Owczuk R et al. (2009) Tracheal tube cuff pressure depends on the anaesthesiologist’s experience. A follow-up study. Anestezjol Intens Ter 41, 205e208. Zanella A, Scaravilli V, Isgr o S et al. (2011) Fluid leakage across tracheal tube cuff, effect of different cuff material, shape, and positive expiratory pressure: a benchtop study. Intensive Care Med 37, 343e347. Received 7 April 2018; accepted 7 June 2018. Available online xxx
Supporting Information
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Supplementary data related to this article can be found at https://doi.org/10.1016/j.vaa.2018.06. 017.
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Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
Veterinary Anaesthesia and Analgesia 2018, xxx, 1e9
https://doi.org/10.1016/j.vaa.2018.06.017
RESEARCH PAPER
Aspects of in vivo endotracheal tube intracuff Q11
pressure in cats
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Alice R Birda, David J Birdb & Matthew W McMillana
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a
Department of Veterinary Medicine, University of Cambridge, Cambridge, UK
b
Department of Applied Sciences, University of the West of England, Bristol, UK
Correspondence: Alice R Bird, Animal Health Trust, Newmarket, Suffolk, UK. E-mail: [email protected]
Abstract Objectives To determine the endotracheal tube cuff pressure produced with two inflation techniques, in two brands of endotracheal tube in cats. To determine the inspiratory pressure which produces an audible leak when the intracuff pressure is 30 cmH2O. Study design Prospective, clinical, randomized study. Animals A total of 40 client-owned healthy adult cats. Methods Following induction of anaesthesia, endotracheal intubation was performed with a Parker Flex-Tip PFLP (Parker; n ¼ 20) or Flexicare VentiSeal (Flexicare; n ¼ 20) endotracheal tube. For each cat, the endotracheal tube cuff was inflated using two methods, minimum occlusive volume (MOV) and pilot balloon palpation (PBP). Intracuff pressure was recorded. Cuff pressure was then set at 30 cmH2O and the pressure within the breathing system when a manual breath first caused an audible leak was measured.
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Results PBP pressure was lower for Parker (36 ± 13 cmH2O) compared with Flexicare (45 ± 13 cmH2O, p ¼ 0.048). MOV pressure was not different between tube types (56 ± 28 versus 66 ± 25 cmH2O for Parker and Flexicare, respectively, p ¼ 0.247). MOV produced a higher pressure than PBP for Parker (56 ± 28 versus 36 ± 13 cmH2O, p ¼ 0.001) and Flexicare (66 ± 25 versus 45 ± 13 cmH2O, p ¼ 0.007). When intracuff pressure was set at 30 cmH2O, 95% of cats did not develop an audible leak until the inspiratory pressure was greater than 10 or 12 cmH2O for Parker and Flexicare tubes, respectively.
Conclusions PBP produced lower cuff pressures than MOV, although both techniques produced a cuff pressure above that at which mucosal blood flow is believed to be restricted. A cuff pressure of 30 cmH2O may be sufficient to prevent audible leak in most cats if respiratory pressures are kept at 10e12 cmH2O or below. Clinical relevance To ensure a safe endotracheal tube cuff pressure, use of a specifically designed pressure gauge is recommended. Keywords cat, cuff, endotracheal intubation, minimum occlusive volume, pilot balloon palpation, pressure gauge. Introduction Endotracheal tube cuff inflation is recommended during anaesthesia to prevent aspiration of oropharyngeal contents, reduce environmental contamination with volatile anaesthetic agents and allow assisted or controlled ventilation if required (Bednarski 2015). Excessive cuff inflation, however, can reduce mucosal blood flow (Dobrin & Canfield 1977; Seegobin & Hasselt 1984; Bunegin et al. 1993) and result in mucosal damage (Nordin 1977; Castilho et al. 2003). In cats, tracheal damage and rupture following intubation has been reported. Whilst these reports primarily follow dental procedures (Hardie et al. 1999; Mitchell et al. 2000; Bhandal & Kuzma 2008) where movement of the endotracheal tube, rather than the pressure the cuff exerts on the tracheal wall, may be the most important factor, this does not explain all cases. Excessive endotracheal tube cuff pressure alone has been shown to result in extensive tracheal damage in rabbits and it seems
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Endotracheal tube cuff pressure in cats AR Bird et al.
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reasonable to suspect that cat tracheas could respond similarly (Nordin 1977). Studies in humans (Jain & Triparthi 2011; Totonchi et al. 2015; Khan et al. 2016), dogs (Briganti et al. 2012) and benchtop models (Hoffman et al. 2006; Faris et al. 2007) have shown that inflation of endotracheal tube cuffs by various inflation techniques, without the aid of a pressure gauge, produces wide-ranging intracuff pressures, with some recordings as high as 100 cmH2O (Briganti et al. 2012; Totonchi et al. 2015). In addition, variations in results have been shown with different endotracheal tube brands, cuff material and cuff shape (Asai & Shingu 2001; Zanella et al. 2011). The use of a manometer in human medicine is now widely recommended (Ham et al. 2013), with maximum intracuff pressure recommendation ranging between 20 and 30 cmH2O (Morris et al. 2007; Chan et al. 2009; Totonchi et al. 2015). The aim of this study was twofold. First, to compare the endotracheal tube intracuff pressure reached in cats with two different ‘blind’ (not involving intracuff pressure measurement) inflation techniques, thought to be in common use in general practice, in two different endotracheal tube designs, with different sized pilot balloons and cuff shape. It was hypothesized that the intracuff pressures reached would be variable and excessively high and that the pilot balloon and cuff design would affect the pressures reached. Second, when the endotracheal tube intracuff pressure was to set to 30 cmH2O, we aimed to determine the peak inspiratory pressure needed during manual ventilation to produce an audible leak of air. It was hypothesized that this intracuff pressure would be sufficient to prevent an audible leak in most cats at recommended ventilation peak inspiratory pressures of 8e12 cmH2O (Hopper & Powell 2013). Materials and methods Ethical statement The study design was approved by the Queens Veterinary School Hospital, University of Cambridge, Ethics and Animal Welfare Committee (ref CR238). Owner consent Informed owner consent was obtained for all animals prior to enrolment in the study. 2
Animals All cats presenting to the Queens Veterinary School Hospital for routine neutering or dental procedures from a cat rehoming charity were considered for inclusion in the study. Cats were excluded if <1 year of age, <2.5 kg or American Society of Anaesthesiologists (ASA) score >I. The ASA score was assigned by the anaesthetist collecting the data and cats were excluded where temperament prevented a full clinical examination prior to sedation. Study design The study was designed as a prospective, randomized, partial-crossover clinical trial; each cat’s endotracheal tube cuff was inflated sequentially by two different techniques, but only one endotracheal brand of the two investigated was used in each cat. A pilot study of 10 cats was performed comparing the two inflation techniques used in this study. This produced a mean ± standard deviation difference of 17 ± 34 cmH2O. To achieve a power of 80% and a level of significance of 5%, the sample size needed for this study was calculated as 33. Because four study groups were used (see below) the sample size was increased to 40 cats to ensure equal weighting of the groups. Cats were allocated to one of the four groups (10 cats per group) during recruitment. Groups 1 and 2 were intubated with a Parker Flex-Tip PFLP (Parker Medical, CO, USA) endotracheal tube (Parker), while groups 3 and 4 were intubated with a Flexicare VentiSeal (Flexicare Medical Ltd, UK) endotracheal tube (Flexicare; Fig. 1). Therefore, 20 cats were assigned to each endotracheal tube brand. These 20 cats were then subdivided into two further subgroups, which determined which inflation method was performed first (groups 1 and 3 one method, groups 2 and 4 the second method). Prior to recruitment of any cats, a list of 1, 2, 3, and 4, repeated 10 times, was input into an excel spreadsheet. The numbers were then randomly reordered and cats were assigned their group based on their order of recruitment into the study (first cat recruited assigned to first number in list, tenth cat the tenth number, etc.). Both endotracheal tube brands used were plastic and designed for single use. The cuffs and pilot balloons were, however, different (Fig. 1). The Parker endotracheal tube cuff had a more oval shape, with smaller subsequent contact area suspected to be
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Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
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Endotracheal tube cuff pressure in cats AR Bird et al.
Veterinary Association Guidelines (WSAVA Nutritional Assessment Guidelines Task Force Members 2011). Anaesthetic procedure
Figure 1 The two brands of endotracheal tubes used in the study with their cuffs (a) inflated and (b) deflated. 1, Flexicare VentiSeal, 4 mm internal diameter; 2, Parker Flex-Tip PFLP 4 mm internal diameter.
required to produce a seal with the trachea mucosa. The Flexicare cuff in comparison was more cylindrical, designed to contact a larger surface area when inflated. The Parker pilot balloon was larger than the Flexicare. The endotracheal tubes were all new prior the commencement of the study and reused during the study period up to a maximum of five times. After each use, the tubes were washed with warm water and sterilized with Milton sterilizing fluid (Proctor and Gamble, UK) before being left to air dry. Prior to use, all cuffs were inflated to examine for visible defects or deformity of the cuff and then left for 10 minutes to ensure the cuff remained inflated prior to use in any cat. Endotracheal tubes with 4.0 and 4.5 mm internal diameter were available for each cat, with the anaesthetist collecting the data (anaesthetist B) choosing the size they deemed most appropriate following examination of the larynx. All cats were premedicated and later induced in a sound-proofed cat induction bay. All subsequent measurements were made in this room, which was kept silent throughout the data collection. A body condition score was assigned as assessed by anaesthetist B, based on the World Small Animal
Premedication was performed with sedatives and analgesics as deemed appropriate by the anaesthetist overseeing the case (anaesthetist A). Following intravenous cannula placement, anaesthesia was induced with alfaxalone (Alfaxan; Jurox UK Ltd, UK) to effect (0.5e2.0 mg kge1). A second anaesthetist (anaesthetist B), who remained the same person throughout data collection, then examined the larynx of the cat and applied topical lidocaine spray (Intubeaze; Dechra Pharmaceuticals PLC, UK). Fortyfive seconds later, endotracheal intubation was performed. Following intubation, the endotracheal tube was secured in place with a length of gauze bandage. The cats were placed into right lateral recumbency and connected to a modified paediatric T-piece with a reservoir bag and safety adjustable pressure limiting valve (Infant T-piece anaesthetic breathing system; Intersurgical, UK). A capnography adaptor was placed between the endotracheal tube connector and breathing system. Oxygen was supplied at 3 L minutee1. From this point, anaesthetist A monitored the cat whilst anaesthetist B and a nurse collected the data. Comparison of two endotracheal tube cuff inflation techniques A three-way tap (Vyclic 3-way stopcock; Vygon Ltd, UK) was connected to the pilot balloon valve, as well as an injection valve (Safeflow; B. Braun Medical Ltd, UK) for connection of a 5 mL syringe (BD Plastipak syringe 5 mL; Becton Dickinson, UK) and a Portex cuff inflator pressure gauge (Smiths Medical, UK; Fig. 2). The pressure gauge accuracy was checked against a column of water and was found to read within ±0.5 cmH2O over a range of pressures. The endotracheal cuff for each of the 40 cats was inflated using two methods in a randomized order as determined by the aforementioned method: 1. Minimum occlusive volume (MOV): The rebreathing bag was emptied to approximately 50% ‘full volume’ by a registered veterinary nurse before they closed the adjustable pressure limiting valve of the breathing system. The nurse then squeezed the rebreathing bag to observe what they felt was a ‘normal’ breath, by visualizing the expansion of the cat’s chest and
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Endotracheal tube cuff pressure in cats AR Bird et al.
between each of the three cuff inflations was 5e7 seconds. The same syringe, pressure gauge, connective tubing (V-green extension; Vygon Ltd), threeway tap and valve were used for all cats. Audible leak pressure when endotracheal cuff pressure was set at 30 cmH2O
Figure 2 Set up of pressure gauge for measuring endotracheal intracuff pressure following inflation with ‘minimum occlusive volume’ and ‘pilot balloon palpation’ methods. This configuration was also used to ensure a pressure of 30 cmH2O in the endotracheal cuff before the leak pressure was recorded. A, pressure gauge; B, rigid extension tubing; C, three-way tap; D, injection port valve for 5 mL syringe attachment.
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through the tension felt in the bag. As this was performed, anaesthetist B listened at the cat’s mouth and injected air from the syringe to inflate the endotracheal cuff until a leak was no longer audible. 2. Pilot balloon palpation (PBP): Anaesthetist B inflated the endotracheal cuff by injecting air from the syringe until the pilot balloon was inflated to what s/he deemed was an appropriate pressure (so the cuff was just inflated and had some give on digital palpation). Anaesthetist B had not trained to feel a PBP of any set value.
Following each inflation of the endotracheal tube cuff, the pressure reached on the pressure gauge was recorded by the nurse. The pressure gauge and measurements were kept hidden from view of anaesthetist B throughout collection of the data. The endotracheal cuff was then completely deflated by withdrawing air from the cuff with the syringe until negative pressure was achieved, prior to performing the next pressure measurement, or setting the pressure to 30 cmH2O as described below. Washout time 4
After recording the pressures reached by MOV and PBP, the endotracheal cuff pressure was adjusted to 30 cmH2O. The three-way tap was then removed from the pilot balloon self-sealing valve and the pressure gauge was connected to the internal pressure of the breathing system via the capnography adaptor. Anaesthetist B emptied the rebreathing bag to 50% ‘full volume’ before closing the adjustable pressure limiting valve. Anaesthetist B then squeezed the rebreathing bag and the nurse recorded the pressure reached when a leak was first audible for anaesthetist B at the cat’s mouth. The intracuff pressure was then left at 30 cmH2O and isoflurane in oxygen administered to the cat at a percentage deemed appropriate by anaesthetist A. The anaesthetic continued to allow completion of the neutering or dental procedure. Anaesthetist B had completed a 3 year ‘residency’ in anaesthesia at time of data collection and had no defects in hearing. All cats were anaesthetized in the morning and remained in the clinic for the remainder of the day. Postsurgery/dental rechecks were performed 2e5 days postanaesthetic at the cat rehoming charity by one of the hospital veterinary surgeons. Data analysis IBM SPSS software (IBM, NY, USA) was used for statistical analysis. Data collected were tested for normality of distribution using the ShapiroeWilk test. Where data appeared to be normally distributed, comparisons between two data sets were made using the two samples t-test or the paired t-test as appropriate. When comparing each endotracheal brand and endotracheal size with each other (four groups) a one-way analysis of variance was performed. Where data did not appear to be normally distributed, and for comparison of body condition score between the two endotracheal brands, the ManneWhitney U-test was performed. Spearman’s correlation coefficient was used to compare the order the cats were recruited into the study against their PBP pressure. Normally distributed data are
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Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
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Endotracheal tube cuff pressure in cats AR Bird et al.
A total of 40 cats were enrolled in the study. Complete data sets were obtained for all 40 cats. No adverse event occurred during data collection in any cat. Following anaesthesia, no reports of complications, including coughing, up until the point of recheck were reported. All data sets tested were found to be normally distributed with the exception of cat ages for the Parker and Flexicare groups and leak pressures when intracuff pressure was set at 30 cmH2O. No difference was found between the bodyweights of the cats intubated with Parker and Flexicare tubes (3.67 ± 0.88 and 4.06 ± 1.08 kg, respectively, p ¼ 0.216). The distribution of ages and body condition scores were also not significantly different between tube brands (p ¼ 0.799 and 0.341, respectively).
respectively, p ¼ 0.247), although the PBP pressure for the Parker tube (36 ± 13 cmH2O) was lower than that for the Flexicare tube (45 ± 13 cmH2O), p ¼ 0.048. When the data were divided up into four groups, representing endotracheal tube brand and internal tube diameter, the PBP and MOV pressures, respectively, for each group were as follows: Parker 4 mm (33 ± 11, 57 ± 32 cmH2O), Parker 4.5 mm (44 ± 13, 56 ± 20 cmH2O), Flexicare 4 mm (45 ± 15, 63 ± 27 cmH2O) and Flexicare 4.5 mm (44 ± 10, 70 ± 24 cmH2O). No difference was found between the groups for PBP or MOV (p ¼ 0.074 and 0.656, respectively). When the endotracheal tube internal diameters were compared without accounting for endotracheal tube brand, size continued to have no significant effect on PBP pressure (38 ± 14 and 44 ± 11 cmH2O for 4 and 4.5 mm internal diameter, respectively, p ¼ 0.215) or MOV pressure (59 ± 29 and 64 ± 23 cmH2O for 4 and 4.5 mm internal diameter, respectively, p ¼ 0.584).
Differences in intracuff pressure between MOV versus PBP techniques
Audible leak pressure when endotracheal cuff pressure was set at 30 cmH2O
For both the Parker and Flexicare endotracheal tube, and when the measurements obtained with the separate tube types were combined, the MOV technique produced a significantly higher mean pressure than PBP (Table 1).
No difference was found between the distribution of data for the leak pressures obtained for the two brands of endotracheal tube (p ¼ 0.221). Leak pressure percentiles for the Parker and Flexicare endotracheal tubes, and for all cats irrespective of endotracheal brand are shown in Table 2.
presented as mean ± standard deviation. A p value < 0.05 was considered significant. Results
Effect of time on PBP pressure
Discussion
There was no evidence of a correlation between cat number (in order of recruitment) and PBP pressure (rs ¼ 0.059, p ¼ 0.720). Differences in intracuff pressure between endotracheal tube brands The MOV pressure was not different for Parker and Flexicare tubes (56 ± 28 and 66 ± 25 cmH2O,
The study findings support our hypothesis that endotracheal tube cuff inflation techniques without a pressure gauge can produce varying, excessively high pressures. The mean intracuff pressure produced with both inflation techniques in both tube brands was above the human recommended pressure of 20e30 cmH2O (Morris et al. 2007; Chan et al. 2009; Totonchi et al. 2015). The Parker tubes had a
Table 1 Mean ± standard deviation of endotracheal tube intracuff pressure obtained with the pilot balloon palpation (PBP) and minimum occlusive volume (MOV) inflation techniques for both Parker Flex-Tip PFLP and Flexicare VentiSeal endotracheal tubes and for all cats irrespective of endotracheal tube brand Group
PBP pressure (cmH2O)
MOV pressure (cmH2O)
p
Parker Flex-Tip PFLP (n ¼ 20) Flexicare VentiSeal (n ¼ 20) All cats (n ¼ 40)
36 ± 13 45 ± 13 41 ± 12
56 ± 28 66 ± 25 61 ± 27
0.001 0.007 0.001
MOV pressure ¼ intracuff pressure obtained with minimum occlusive volume inflation technique; PBP pressure ¼ intracuff pressure obtained with pilot balloon palpation inflation technique; p < 0.05 indicates significant difference between inflation techniques.
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Endotracheal tube cuff pressure in cats AR Bird et al. Table 2 The inspiratory pressures (cmH2O) at which an audible leak could be detected when the endotracheal cuffs were inflated to 30 cmH2O and a manual breath administered Percentile (%)
Inspiratory pressure (cmH2O)
Percentage of cats with audible leak developing at or below the inspiratory pressure indicated
Percentage of cats with audible leak developing above the inspiratory pressure indicated
Parker Flex- Flexicare Tip PFLP VentiSeal (n ¼ (n ¼ 20) 20)
All cats (n ¼ 40)
5 10 25 50 75 90 95
95 90 75 50 25 10 5
10 12 15 20 24 24 26
12 12 19 21 24 24 25
significantly lower intracuff pressure than the Flexicare tubes when the PBP inflation technique was used, which may have been due to the larger pilot balloon design and the subsequent ‘feel’ of the balloon on inflation (Fig. 1), although this pressure was still excessively high. Consequently, neither cuffinflation method can be recommended, based on our findings, to produce a safe intracuff pressure in the described tubes. Care should be taken in extrapolating the results of this study to other anaesthetists. Studies in humans have repeatedly demonstrated variability between individuals (Chan et al. 2009; Wujtewicz et al. 2009), with the appropriate ‘feel’ of the pilot balloon being subjective. It is also likely that the sensitivity of the MOV technique is influenced by surrounding noise and the auditory sensitivity of the person inflating the cuff, and so use of a stethoscope over the larynx may improve this technique. White et al. (2017) reported much lower values for MOV and PBP, although these inflations were performed in a cat model, rather than in vivo, and the authors still concluded that both techniques rarely resulted in optimal cuff pressure. Because of the degree of individual variability reported (Briganti et al. 2012; Totonchi et al. 2015; Khan et al. 2016), it is felt unlikely that the excessive intracuff pressures reported here are a rare occurrence. Lower intracuff pressures may have been obtained during the MOV technique had the inspiratory breath been delivered in a controlled manner using a mechanical ventilator. Whilst an inspiratory breath being supplied by a nurse is most reflective of the situation in general practice, it would have likely produced variable peak inspiratory pressure and 6
12 20 20 22 24 24 25
speed of breath delivery and may have contributed to the wide range of intracuff pressures recorded. The use of a single nurse throughout the study would have been preferable and the use of multiple nurses is a limitation of this study. Studies investigating MOV, with an inspiratory peak pressure of 20 cmH2O, have been performed in humans (Al-Metwalli et al. 2011) and dogs (Briganti et al. 2012). In the human study, a low mean intracuff pressure was obtained with a small degree of pressure variation (20.5 ± 3.8 cmH2O). The canine study investigated plastic and silicone endotracheal tubes and found mean intracuff pressures of 47.6 ± 43.5 and 104.7 ± 68 cmH2O, respectively. Comparison of pressures between studies should be made with caution, due to different methodology and species, although it is interesting that even with a controlled inspiratory breath, Briganti et al. (2012) still found the intracuff pressures to be variable. Training in the feel of an appropriately inflated pilot balloon has been shown to increase the tendency to inflate endotracheal cuffs within a desired range, although in a study by Siamdoust et al. (2015) those able to accurately inflate the cuff improved from 24.2% to only 39.7% with training. In this study, no training of the ‘feel’ of the pilot balloon at an ideal pressure had been undertaken by anaesthetist B. Lower intracuff pressures may have been obtained if this had occurred prior to data collection. Whilst it was possible that anaesthetist B could have improved their PBP technique throughout the study, as they became aware of previous pressure recordings, this does not seem to be the case as there was no correlation between the order the cats were recruited into the study and PBP. Training of PBP
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Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
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Endotracheal tube cuff pressure in cats AR Bird et al.
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may be useful in clinics where availability of a pressure gauge is not always possible. It is important to note that there is strong evidence that anaesthetist experience is not associated with improved accuracy in cuff inflation and training is likely to need refreshing over time (Sengupta et al. 2004; Wujtewicz et al. 2009; White et al. 2017). Because of our findings and the literature discussed above we would advise the use of a pressure gauge when inflating endotracheal tube cuffs in cats. In addition, a specific endotracheal tube cuff pressure gauge is recommended, as discrepancies in pressure readings between these and handcrafted cuff manometers have been shown (Annoni & Evanir de Almeida 2015). Whilst endotracheal tube cuffinflator devices are available as an alternative to a pressure gauge, differences in their accuracy have been reported, with none of the four cuff inflators tested in one study accurately measuring cuff pressure (Blanch 2004). Endotracheal tubes that incorporate a control valve with the pilot balloon arrangement are also available (Mallinckrodt Hi-Lo Oral/Nasal Tracheal Tube Lanz System; Medtronic, UK). These automatically maintain a low intracuff pressure and may be safer to use than the endotracheal tubes used in this study. Whilst intracuff pressure does not necessarily equal the pressure the cuff exerts on the tracheal mucosa, examination of pressures in a tracheal model does suggest that the two are linearly related (Black & Seegobin 1981). It should be noted, however, that the difference between these two values will likely vary with the endotracheal tube cuff design; Dobrin & Canfield (1977) reported mucosal pressures ranging from 58% to 91% intracuff pressure in 18 brands of endotracheal tube, although the mucosal and intracuff pressures were measured with different techniques. Fernandez et al. (1990) reported pressure versus volume curves to be steep for multiple endotracheal tube bands, with a sudden rise in pressure for small increase in volume. Once the cuff is restricted within the trachea we suspect that this volume pressure relationship is likely to be present for most endotracheal tubes. In this study, when the endotracheal tube cuff pressure was set to 30 cmH2O, 95% of cats did not develop an audible leak around their tube until peak inspiratory pressure was greater than 10.1 and 12.3 cmH2O for Parker and Flexicare, respectively. We therefore believe this cuff pressure to be sufficient to prevent large volume environmental contamination with anaesthetic vapours for most cats breathing
spontaneously or ventilated to a peak inspiratory pressure of 10 cmH2O or below with the endotracheal tubes described. It is possible, however, that a small volume of air was still able to leak past the inflated cuffs at pressures below which the audible leak was detected. It is also unclear whether differing characteristics of an inspiratory breath, such as speed of inspiration and pressure distribution throughout the inspiratory breath, would affect the pressure at which an audible leak occurred. Importantly, lack of an audible air leak during an inspiratory breath does not guarantee protection of the lower airways from aspiration. Whilst species differences are likely to be present, 12/12 pigs anaesthetized with an intracuff pressure of 30 cmH2O showed leakage of methylene blue past the cuff (Negro et al. 2014), suggesting that this pressure may not be sufficient to prevent aspiration, at least in pigs. It is also unclear whether 30 cmH2O is safe to use in cats with regard to mucosal blood flow. In hypotensive dogs, an intracuff pressure of 20.4 cmH2O resulted in reduced tracheal blood flow (Bunegin et al. 1993), whilst biopsies of tracheal mucosa from dogs following an intracuff pressure of 25 cmH2O showed mild changes when examined with electron microscopy (Castilho et al. 2003). Whilst the clinical significance of these changes is uncertain, it seems prudent to assume that cat tracheas would be even more sensitive to intracuff pressures due to their smaller and more delicate nature and more complete tracheal rings. Whilst reports of tracheal rupture in cats are relatively scarce (Hardie et al. 1999; Mitchell et al. 2000; Bhandal & Kuzma 2008), it is unknown how many cats have subclinical damage to their mucosa postanaesthetic due to excessive cuff pressure. The presence of any subclinical tracheal mucosal damage in the cats enrolled in this study was undetermined, although no reports of coughing were made. A definitive recommendation for intracuff pressure in veterinary species is lacking. Whilst a maximum of 34 cmH2O in cats and dogs is recommended in one textbook (Hughes 2016) and 20e30 cmH2O was suggested as optimal in a cat model (White et al. 2017) it appears that these recommendations are extrapolated from the human literature. Even in humans the recommendations vary, although suggested pressures fall mostly between 20 and 30 cmH2O (Morris et al. 2007; Chan et al. 2009; Totonchi et al. 2015). This value appears to be the result at least in part by a study by Seegobin & Hasselt (1984) whereby endoscopy was performed
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Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017
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Endotracheal tube cuff pressure in cats AR Bird et al.
to visualize the tracheal mucosa under the endotracheal cuff in humans. At 25 cmH2O the mucosa was of uniform hue and the blood vessels of normal calibre. At pressures of 30 cmH2O and above, however, the mucosa over the tracheal rings became visibly less pink, although how this reduction in blood flow relates to subsequent pathology is uncertain. In spontaneously breathing cats, a pressure lower than 30 cmH2O may be sufficient to prevent significant environmental contamination, whilst minimizing compromise of mucosal blood flow, although the risk of aspiration is likely to be higher. The endotracheal tubes used in this study were sold as single use but reused. Whilst reuse is more common in veterinary practice, this may have affected the results by altering the properties of the pilot balloon and cuff or creating deformities in the cuff invisible to visual inspection. Conclusions Neither the MOV nor the PBP technique can be recommended for endotracheal tube cuff inflation in cats and use of a cuff-specific pressure gauge is advised. An intracuff pressure of 30 cmH2O appeared to be sufficient in the majority of cats in this study to prevent an audible air leak at a peak inspiratory pressure of 10 cmH2O or below. Whether this pressure sufficiently protected the airways from aspiration or caused any subclinical mucosal damage, however, is uncertain. Uncited reference Q9
Rokamp et al. 2010. Acknowledgements
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The authors wish to thank the theatre nurses at the Queens Veterinary School Hospital, Cambridge University, for their assistance in collecting the data reported. This research did not receive any specific grant from funding agencies in the public commercial, or not for profit sectors. Authors’ contributions ARB: data collection, statistical analysis and preparation of manuscript. DJB: statistical analysis, editorial input of final document. MWM: study design, editorial input of final document. Conflict of interest statement Authors declare no conflict of interest. 8
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Supporting Information
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Supplementary data related to this article can be found at https://doi.org/10.1016/j.vaa.2018.06. 017.
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Please cite this article in press as: Bird AR, Bird DJ, McMillan MW Aspects of in vivo endotracheal tube intracuff pressure in cats, Veterinary Anaesthesia and Analgesia (2018), https://doi.org/10.1016/j.vaa.2018.06.017