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ORIGINAL ARTICLE
Year : 2022 | Volume
: 23
| Issue : 2 | Page : 105-110
Prediction of endotracheal tube size for pediatric patients from the length of the middle finger in comparison with standard age-based formula in South Indian population
Rama Rani Krishna Bhat1, Ramya Lakshmi Kamlekar2, Malavika Kulkarni1
1 Department of Anesthesiology, Kasturba Medical College, Manipal, Karnataka, India
2 Department of Anesthesiology, ESIC Medical College, Hyderabad, Telangana, India
Correspondence Address:
Dr. Malavika Kulkarni
Department of Anesthesiology, Kasturba Medical College, Manipal, Karnataka
India
Source of Support: None, Conflict of Interest: None
CheckDOI: 10.4103/TheIAForum.TheIAForum_40_22
Background: Determining the appropriate size of an endotracheal tube (ETT) in infants and children remains a challenge for anesthesiologists. We conducted this study to assess the accuracy of middle finger length (MFL) in predicting the appropriate ETT size for pediatric patients in comparison with the age-based estimation and derive a formula based on MFL for the estimation of ETT size.
Materials and Methods: In the study, South Indian children 1–10 years of age, requiring general anesthesia were intubated by consultant anesthesiologists based on their discretion with an appropriately sized ETT. Subsequently, the MFL of the children was measured and tracheal tube size calculated from the age-based formula was also noted. The actual size of the ETT inserted was compared with the MFL and age-based formula using Pearson's correlation.
Results: In children between 1 and 10 years of age, the age-based formulae (ABF) was found to correlate with ETT estimated with the correlation coefficient (r = 0.885; P < 0.001) and MFL (r = 0.783 and P < 0.001). ABF showed a stronger correlation with the ETT inserted compared to the MFL, nevertheless, we were able to arrive at a formula to predict tracheal tube size based on MFL: ETT ID (mm) =1.1+ (0.7 × MFL [cm]).
Conclusion: Although age-based Cole's and Motoyama's formulae are better predictors of pediatric ETT size, MFL can still be used to predict the tube size in cases when age and weight are unknown.
Keywords: Age-based formula, endotracheal tube, middle finger length, pediatric
In the pediatric population, the selection of the appropriate size of an endotracheal tube (ETT) is extremely crucial as an inappropriately larger size ETT can cause airway edema, postintubation croup, and subglottic stenosis, whereas too small ETT can result in inadequate ventilation and increase the risk of aspiration. This could lead to multiple reintubation attempts resulting in life-threatening hypoxemia. To prevent the complications of multiple trials of intubation, many methods which can precisely predict the appropriate size of an ETT have been suggested like age-based formula, height-based formula, weight-based formula, and little finger diameter-based formula.[1],[2] Ultrasonographic and radiological means of measurement of subglottic diameter are time-consuming and operator-dependent techniques hence may not be suitable in all settings, especially when quick decision-making is necessary.[3],[4] Selection of the appropriately sized ETT is based on the anesthesiologist's experience or using formulae which are based on the children's demographic data such as height,[3] weight,[5] or age.[6] However, Cole's formula is still considered one of the prevalent methods in spite of its poor prediction power.[1] The measurement of bony cartilage growth of the hand is worthy of attention as a surrogate marker of tracheal diameter because the bone and cartilage growth of the body are supposed to be related to each other and bone age for the hand is generally accepted as an indicator of growth. Tracheal rings are composed of cartilages and the measurement of growth of cartilage of the body parts, especially the hand as a surrogate marker of cartilage growth in the trachea can be used to choose the appropriate ETT size.[2] Studies in the past have shown the middle finger as a useful predictor of tracheal tube depth.[7] A single-center study demonstrated that middle finger length (MFL) has the potential to be a better guide for selecting the appropriately sized tracheal tube in children compared to the commonly used age-based formulae (ABF).[1] Therefore, we decided to conduct this observational study with the primary objective of testing the correlation between MFL and the internal diameter of an ETT size in the South Indian population. The secondary objective was to assess the accuracy of Cole's and Motoyama's formulae in predicting the appropriate size of ETT.
Materials and MethodsIt was a prospective observational study started after approval from the institutional ethics committee (IEC: 806/2018) and the Clinical Trial Registry of India (Ref. number CTRI/2019/03/018161). Children between 1 and 10 years of age and belonging to the American Society of Anesthesiologists (ASA) class 1 and 2 posted for elective surgery requiring general anesthesia and endotracheal intubation were included in the study.
Patients requiring nasal intubation, predicted difficult airway, with known airway or limb abnormalities, musculoskeletal disorders, congenital developmental disorders, known allergy to any anesthetic medications, and use of throat packs, upper respiratory tract infection, tracheal and laryngeal pathology, in whom supraglottic airway device was inserted or patient requiring postoperative mechanical ventilation were excluded.
All the patients were evaluated 1 day before the surgery. Patients who met the inclusion criteria were included after taking written informed consent from parents/legal guardians. Adequate fasting was ensured before the surgery. Premedication and induction of anesthesia were done as per the standard protocol by the attending anesthetist. Venous access was either secured on the day before surgery or after inhalational induction with sevoflurane and appropriate intravenous (IV) fluids were started. Anesthesia was induced with IV propofol 2–2.5 mg/kg, IV and Fentanyl 2 μg/kg, followed by IV atracurium 0.5 mg/kg to facilitate endotracheal intubation. The choice of the anesthetic agent and relaxant was as per the discretion of the attending consultant. The children were ventilated with a bag and mask till complete muscle relaxation was achieved. After complete muscle relaxation, the first attempt of intubation was carried out. The correct placement of the ETT was confirmed by waveform capnography and the five-point auscultation method. The consultant anesthesiologist providing clinical care was unaware of middle finger measurement and chose the type and size of ETT they felt most appropriate for the procedure. ETT size was deemed as an appropriate fit if the tube passed the cricoid area without any resistance. Whenever a cuffed ETT was used, the cuff inflation was guided by a cuff pressure manometer. The cuff pressures were maintained between 20 and 25 cm H2O. Immediately after placement of the tube in the trachea, air leakage was monitored by performing auscultation on the front of the neck with a stethoscope, as a part of standard practice. If a leak occurred at an airway pressure of <10 cm of H2O when the lungs were ventilated, the tube size used was considered to be small to fit into the tracheal lumen, in that case, the tube was changed to a tube having a one-step larger size. If there was no audible leak above an airway pressure of 25 cm of H2O, the tube was considered to be large and it was changed to a tube having a step smaller size. This process was continued until the leak occurred only at an airway pressure between 10 and 25 cm of H2O.[8] All children were mechanically ventilated after confirming minimal ETT leak by auscultation method to obtain an expired tidal volume of >8 ml/kg and a square waveform. Anesthesia was maintained in O2 and air (50:50%) in isoflurane using a circle system with a mean flow rate of 1.5 L/min. Following the procedure, observer 2 in the study recorded the type and size of ETT used along with other details such as the need to change over to an alternative tracheal tube size and depth of insertion. The pediatric ETTs used for the study in our institute included microcuffed ETTs by Kimberly Clark, Health care Atlanta, GA, USA, Portex®-Smiths medical uncuffed tracheal tubes. Once the airway was secured in the patient, the MFL was measured on the palmar aspect of the hand from the palmar crease to the tip of the middle finger in centimeters with the help of a measuring scale by observer 2 [Figure 1]. The size of the uncuffed or cuffed ETT for the child was estimated according to the age-based formula suggested by Cole/Motoyama.[9],[10]
Sample size
Based on the previous study,[1] the sample size was calculated based on the correlation coefficient formula with a reliability coefficient of 1.96, an acceptable margin of error of 0.1, and a correlation coefficient of 0.8 which worked out to be 139.
Correlation coefficient formula: n = ([Z1–α/2] [1– R2])/d2
(Z1–α/2) – reliability coefficient, d – acceptable margin of error, and R2 – correlation coefficient.
Statistical analysis
Interpretation and result of the obtained data were analyzed using software Microsoft Office Excel 2010 and Statistical Package for the Social Sciences (SPSS) version 20 (IBM, Bengaluru, Karnataka, India). Data were correlated using Pearson's correlation and quantitative data was represented in numbers and percentages as appropriate. Using linear regression analysis, we predicted the size of ETT using MFL and employed a regression equation to derive a formula.
ResultsA total of 150 children were screened and recruited out of which six participants were excluded as there was a change in anesthetic plan or surgery was canceled, thus, 144 of them were included in the study for analysis [Figure 2]. The demographic characteristics and other basic parameters are shown in [Table 1]. The number of children in the 1–5 years age group were more compared to that in the age group of 6–10 years. Similarly, males (72.2%) outnumbered females and most (93.7%) of the participants belonged to ASA status 1.
Table 1: Demographic characteristics and the American Society of Anesthesiologists status of participantsAmong the 144 children, (118 cuffed and 26 uncuffed) a total of 12 (8.3%) ETT changes were noted with a switch from an uncuffed type to the next larger size cuffed ETT in seven children. Similarly, there was a change among five of them from a cuffed type to one size larger cuffed tracheal tube [Table 1].
Pearson's correlation test was used to evaluate the extent of correlation between ETT inserted with ETT estimated with ABF and MFL. In children between 1 and 10 years of age, the ABF was found to correlate with ETT estimated with the correlation coefficient (r) =0.885.; P < 0.001)
Linear regression analysis was used to determine the relationship between ETT inserted and ETT estimated (ABF and MFL).
Regression analysis between the inserted ETT and the ABF, namely, Cole/Motoyama reveals R2 = 0.783 and infers that there is a 78% variation in tracheal tube size that can be accounted for the ABF with a statistically significant P < 0.001 and a standard error of 0.385 [Figure 3].
Figure 3: Linear regression analysis plot of estimation of ETT size based on the age-based formula. ETT: Endotracheal tubeIn the linear regression analysis, the plot indicates that the ETT estimated by ABF (X-axis) was observed to have 78% correct predictions with ETT inserted (Y-axis) (R2 = 0.783).
Pearson's correlation test between ETT estimated by MFL and ETT inserted was (r) = 0.783 and P < 0.001. Regression analysis between the inserted ETT and the MFL showed that R2 = 0.63 and infers that 63% of the variation in tracheal tube sizes can be accounted for the length of the middle finger with a statistically significant P < 0.001 and a standard error of 0.503 [Figure 4].
Figure 4: Linear regression analysis plot of estimation of ETT size based on the middle finger length. ETT: Endotracheal tubeIn the linear regression analysis, the plot indicates that the ETT estimated by MFL (X-axis) has 63% correct predictions with ETT inserted (R2 = 0.628) [Figure 5].
Figure 5: Comparison of estimation of ETT size using ABF and MFL. ETT: Endotracheal tube, ABF: Age-based formulae, MFL: Middle finger length[Figure 4] depicts the performance of the age-based formula and the MFL with the ETT actually used. The estimated ETT is represented as “smaller than actual”, correctly sized, and larger than actual in both types of estimations. It is observed that ABF had a higher proportion of correct-sized ETT estimation (83.3%) compared to that MFL (67.4%).
Therefore, the ABF showed a stronger correlation with the ETT inserted compared to the MFL. Hence, the results indicate a better agreement of ABF with ETT inserted.
DiscussionThe present study was done to ascertain the scope of using MFL to predict the ETT size and the study reveals that MFL faired lower than the existing ABF. The principal findings of our study were that the ABF still continues to have a better predictive value in the estimation of appropriate size ETT when compared to the MFL in South Indian children.
Various studies have been done to find alternate methods to determine the appropriate ETT size. The rationale for using MFL is based on the evidence that the growth of the bony cartilage corresponds with that of tracheal cartilage. Kim et al. have reported that as there is a correlation in the growth of the cartilage in the body, measurement of cartilage growth in the hand could be used as a surrogate marker for the cartilage growth in the trachea.[2] In a study by Ritchie-McLean et al. conducted among children under 12 years of age in the UK, a linear relationship was demonstrated between uncuffed ETT inner diameter and MFL suggesting a simple formula to predict ETT size: MFL (cm) (round up to nearest 0.5) = uncuffed tracheal tube internal diameter (mm).[1] Our study included both cuffed and uncuffed tubes and probably ethnicity was also the reason due to which a similar association was not observed.
Shibasaki et al. conducted a study on 192 patients aged 1 month–6 years and developed a formula to predict ETT size based on the ultrasonographically measured subglottic diameter that validated with a high rate of agreement between the predicted and clinically opted ETT.[11] Similarly, the measurement of the epiphyseal transverse diameter of distal radius by ultrasonography (USG) to predict the optimal ETT size has been done with success by Kim et al.[2] The disadvantages of using USG for predicting ETT are that it is time-consuming, subjective, requires expertise, resource limited, and noneconomical. On the contrary, MFL used in our study to assess the ETT size is easy to measure, less time-consuming, and cost-effective.
As reported in a study by Ritchie-McLean et al., the possibility of selecting an optimal ETT at the first attempt with lesser calculations unlike demographic-based formulae improved with MFL. Being a simple method, noninvasive, and not requiring expertise, we began this study with the primary objective of predicting ETT length from MFL in the pediatric population of South India as our tertiary care center draws patients from other states also. In contrast, in our study, the linear regression analysis plot indicated the ETT estimated by middle finger to have 63% correct predictions with ETT inserted (R2 = 0.628) whereas there were 78% correct predictions with ETT estimated by age-based formula and ETT inserted (R2 = 0.783).
In a study conducted in the South Indian population by Rajasekhar et al. compared little finger width as an indirect measure of cartilage growth and direct measurement of subglottic diameter through ultrasound. Neither little finger breadth nor ultrasound-guided subglottic diameter measurement proved as a useful tool in determining optimal ETT size. The little finger breadth outer diameter had a better liner regression concordance coefficient (0.46) compared to USG subglottic diameter.[12]
A limitation of our study was the underrepresentation of children in the older age group with the majority of the children being in the 1–5 years category. The investigators did not have the choice of selecting the ETT used as it was the discretion of the anesthesiologist in the theater. The results of our study may not be extrapolated to other brands of manufacturers due to variations in the outer diameter and cuff dimensions.
The new generation of cuffed ETTs with thin distal polyurethane cuffs has brought in a transition in the practice of pediatric anesthesiology with increased use of cuffed ETTs right from neonates onward.[13] Chamber et al. conducted a randomized trial on comparing cuffed versus uncuffed tracheal tubes pertaining to leak, tidal volume, and complications in children between 0 and 16 years of age. They concluded that cuffed tracheal tubes produced better ventilation characteristics compared with uncuffed tracheal tubes.[14] Hence, there is a consistently increasing trend in the use of microcuff tubes in the current practice. However, in view of considerable heterogeneity toward the diameter of cuffed tubes as mentioned in the earlier studies, there is a necessity to adapt ISO standards in designing cuffed ETT to have a uniform assessment.[15] This scenario demands further research in line with emerging practices in anesthesiology.
Although our study was conducted in a single center, the hospital draws patients from neighboring states of South India and gives scope to generalize the study findings. The authors recommend further research in this area separately in cuffed, microcuffed, and uncuffed tubes among children of narrow age groups to assess the utility of MFL in determining optimal ETT size.
ConclusionBased on our findings, the age-based Cole's and Motoyama's formulae are better predictors of the ETT size when compared to MFL. While MFL could be useful in situations when demographic parameters such as age and weight are unknown. However, the utility of MFL needs to be explored further to determine the optimum size of ETT.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References
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