Evaluation of Two Methods of Endotracheal Tube Selection in Dogs

Introduction

Selection of a correctly sized endotracheal tube is important in practicing anesthesia. Too large of an oral-tracheal tube may cause laryngotracheal trauma during the intubation attempt or a complete failure to intubate.¹ An inappropriately small tube may result in gas leakage and subsequent pollution of the operation room. If a smaller cuffed endotracheal tube is used, the cuff may require overinflation to maintain a seal, exerting high pressure on the tracheal wall and thereby increasing the risk of tracheal necrosis. Inappropriately small endotracheal tubes may also increase the work of breathing because of increased airway resistance. For each 1-mm decrease in the internal diameter (ID) of the endotracheal tube, the work of breathing has been demonstrated to increase by 34% to 154%, and the airway resistance increases by 25% to 100%.^1,2 Therefore, when selecting an endotracheal tube for a dog, one must select as large of an ID as possible in order to minimize the work of breathing and reduce airway resistance during inhalant anesthesia.

Size selection of endotracheal tubes for oral-tracheal intubation in dogs has not been standardized due to variations in age, breeds, and body weights of animals. In addition to selecting the estimated optimum-sized endotracheal tube, most practitioners also select two additional endotracheal tubes. One is a size larger and one is a size smaller than the estimated appropriate size. These additional tubes are kept at hand to ensure a close fit.

In practice, several methods have been used to select endotracheal tubes. The first commonly used method is based on the dog’s body weight, with the assumption that the dog’s tracheal diameter is directly proportional to its body mass. The accuracy of this method is highly dependent on the experience of the anesthetist. The second method used is based on direct digital palpation of the dog’s tracheal diameter just above the thoracic inlet, with the assumption that the trachea’s outer diameter (OD) will approximate the appropriate size of endotracheal tube to be used. The third method of selecting endotracheal tubes is to hold an endotracheal tube up to the nasal septum (at the narrowest point between two nostrils) of the dog and match the width of the nasal septum with the endotracheal tube OD. A review of current literature reveals no published information supporting the assumption of a direct anatomical correlation between the nasal septal width and the tracheal ID and OD. Furthermore, no information has been published comparing the accuracies of nasal septal width versus direct palpation methods for endotracheal tube size selection.

In this study, the authors hypothesized that 1) the correlation is low between nasal septal width of the dog and its tracheal ID or OD, and 2) endotracheal tube size selection is more accurate when using digital palpation of the dog’s tracheal OD than when using the dog’s nasal septal width. The objectives of this study, therefore, were to 1) evaluate the correlations between the nasal septal width, tracheal ID and OD, age, and body weight of the dog; and 2) compare two methods of selecting endotracheal tube size in dogs using digital palpation of the tracheal OD and nasal septal width of the dog.

Materials and Methods

Twenty-eight fresh dog cadavers were used in this study. Eight dogs were females, and the rest were males. The ages of the mostly mixed-breed dogs were between 6 months and 5 years, with body weights between 5.4 kg and 29.3 kg [Table 1]. All dogs were dolichocephalic. Two methods were used to select the endotracheal tubes for each dog. The first method used the nasal septal width as a guide, and the second method used digital palpation of the tracheal OD as a guide. For the nasal septal width method, the midportion of the endotracheal tube was held against the nasal septum of the dog, and the tube size was estimated at the septum’s narrowest point. An endotracheal tube with an OD matching this septal width was then selected. The same person also selected an endotracheal tube using digital palpation by selecting the OD that most closely approximated the OD of the dog’s trachea. The digital palpation of the tracheal OD was performed immediately cranial to the thoracic inlet, over hair and other tissues situated around the trachea at that location, just as would be done in a clinical setting. The endotracheal tube size selected for each dog with each method was then recorded.

Following the selection of endotracheal tubes, the nasal septal widths of these dogs were measured in millimeters with calipers between the two nostrils at the narrowest point. For actual measurement of the tracheal ID and OD, the whole airway was harvested, and the tracheal ID and OD were measured in millimeters with calipers at the approximate site immediately above the thoracic inlet where digital palpation occurred. Different sizes of endotracheal tubes were used to actually perform tracheal intubation on the harvested cadaver trachea, and the size of the endotracheal tube that best fit the dog’s actual trachea was selected and recorded.

Endotracheal tubes are clinically indexed with their ID. Because both methods of selection were based on the use of the OD, in order to simulate the clinical situation, each endotracheal tube selected by the OD had to be converted to the ID and recorded for comparison between the two methods. The same person recorded all measurements to ensure consistency. All of the endotracheal tubes used were the polyvinyl chloride type.^a

Results

The results for major variables are presented in Table 1, and the linear correlations and P values for major variables are presented in Table 2. As expected, the endotracheal tube selected and inserted directly into the cadaver’s trachea was highly correlated with the trachea’s ID and OD. The age of the dog was poorly correlated with the trachea’s ID or OD, whereas the body weight of the dog was better correlated with the tracheal ID (r=0.85) and OD (r=0.84). The nasal width of the dog was poorly correlated with the age of the dog but was significantly correlated with body weight. The nasal septal width was significantly correlated with tracheal ID (r=0.73; P<0.0001) and OD (r=0.72; P<0.0001). The digital palpation of the tracheal OD was also significantly (P<0.0001) correlated with the dog’s tracheal OD. The percentages of correct estimates are presented in Tables 3 and 4.

The digital palpation method of selecting endotracheal tubes provided a higher percentage (46%) of correct estimates than the nasal septal width method (21%). For both methods, the percentage of correct estimates was calculated by dividing the number of correct estimates by the total number of dogs [Tables 3, 4]. The absolute mean difference also showed that palpation of the tracheal OD (mean difference = 0.857) is a more precise method to select the best-fit tube than the nasal septal width (mean difference = 1.75) method. McNemar’s test was performed on the data in Tables 3 and 4, analyzing the null hypothesis that both methods are equally accurate. The P value was 0.07. This is not statistically significant at the 0.05 level, but it is “marginally significant” and is indicative of evidence that the digital palpation method is more effective than the nasal septal width method.

Discussion

This study hypothesized that 1) the correlation was low between the nasal septal width of the dog and the tracheal ID or OD; and 2) that endotracheal tube size selection is more accurate using digital palpation of the dog’s tracheal OD than using the dog’s nasal septal width. The results of this study did not support the first hypothesis; nasal septal width correlated well with tracheal ID and OD. However, this study confirmed the hypothesis that digital palpation of the OD of the trachea is a more accurate method for selecting a best-fit endotracheal tube than using the width of the nasal septum.

Based on the data from Tables 3 and 4, McNemar’s test was able (with marginally statistical significance, P=0.07) to identify that the percentage of correct tube selection for the digital palpation method was greater than the percentage of correct tube selection for the nasal septal width method. Clinically, the difference in percentage of correct tube selection (46% for digital palpation versus 21% for nasal septal width) is significant. However, McNemar’s test has limited power when used with small sample sizes, such as those in this study.

Although body weight was not used as part of the endotracheal tube selection method, the results of this study revealed that body weight correlated better with tracheal size than did the age of the dog; nasal septal width of the dog correlated poorly with the age of the dog but did correlate well with the dog’s body weight. A strong correlation has been shown between tracheal diameter and body mass in Dalmatian puppies (<5 months of age)³ and in small, mixed-breed dogs.⁴ The current results are similar to these studies. However, because of breed differences, it is difficult to use body weight as a generalized method for selecting a proper endotracheal tube in a clinical setting.⁵

A high value of the correlation coefficient means that the two variables change in a linear fashion. However, just because the correlation is high does not imply that the prediction is accurate. The current results showed that digital palpation of the tracheal OD was a more precise method for selecting a properly fitting tube than the nasal septal width measurement method. The percentages of obtaining a correct estimate with the digital palpation and nasal septal width methods were 46.4% and 21.4%, respectively.

Body masses of adult dogs vary dramatically between breeds, and, therefore, the tracheal size varies accordingly.⁶ Despite this range of variation, a correlation was found between body size and nasal septal width for the dogs in this study. Body mass variation among breeds poses a challenge for unifying a standard method of selecting endotracheal tubes for dogs. Review of literature revealed minimal information about the correlation among age, body weight, and size of the trachea in mixed-breed dogs. It has been suggested that mammalian linear dimension, such as tracheal diameter or nasal septal width, is not expected to vary with body weight, but with cube root of body weight.^7,8 The positive correlation seen with this study does not disprove this; it is most likely related to the relatively limited range of body weights in the study sample. Clinical studies have suggested that endotracheal tube size can be estimated by palpating the dog’s trachea.⁶ However, others consider this to be a subjective method, especially for brachycephalic dogs.⁹ In this study, only dolichocephalic dogs were used.

In the current study, the tracheas of the cadaver dogs were measured with calipers after they were harvested, and the optimal endotracheal tube was selected by direct insertion of the best-fit tube into the trachea. Some factors one would encounter in a clinical setting were not considered during the selection of the endotracheal tube, such as size limitation of the laryngeal openings or obesity of the dog, where the fat surrounding the trachea might also affect the digital palpation of the trachea.

Besides the anatomical considerations, the endotracheal tube itself should also be considered. For example, wall thickness is not the same in all endotracheal tubes; therefore, ID will vary. Also, as an endotracheal tube ages, it may lose wall rigidity, which may increase its tendency to collapse under ordinary cuff pressure. This would cause turbulent air flow and increase the work of breathing. Furthermore, cleaning and sterilizing an endotracheal tube for reuse can cause loss of pliable polymers, resulting in stiffening or hardening of the tube wall. Thus, a slight increase in cuff pressure to achieve tracheal wall cuff sealing can transfer all the pressure to the tracheal wall membrane, suppressing tissue blood flow.

Some challenges were encountered in this study. In a clinical setting, endotracheal tubes are indexed by their ID, yet clinical selections of endotracheal tubes are based on the tube’s OD. In other words, clinicians use either the OD of the endotracheal tube for measuring against the nasal septal width of the dog, or they use digital palpation of the tracheal OD for estimation against the OD of the endotracheal tube. Because of the clinical nature of these processes, both endotracheal tube selection methods used in this study exhibit some error for estimating the optimal best-fit size of the endotracheal tube, resulting in a tendency for selecting smaller sizes of endotracheal tubes rather than optimal best-fit tubes as the size of the trachea increases. For example, the data in Tables 3 and 4 show that a too-large tube was selected only once, whereas all other incorrectly matched endotracheal tubes were ones that were too small. This inaccuracy rate increased as the optimal endotracheal tube ID sizes (that are approximate to the tracheal ID sizes) increased from 7 mm to 11 mm in both selection methods. Minimal impact was observed when the endotracheal tube size was <7 mm ID. However, when the correct tube size exceeded 7 mm, both methods exhibited a tendency to select tubes that were too small by 1 to 2 mm.

Conclusion

These results show that the digital palpation method is more efficient in selecting a “best-fit” endotracheal tube than the nasal septal width method, and both methods are relatively imprecise as tracheal size increases. Future studies are needed in this area, including evaluation of dogs in different breeds and sizes.

Acknowledgments

The authors thank Dr. Jennifer Hubbard and Meaghan McMonagle for their technical assistance.