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A Study to Predict Appropriate Endotracheal Tube Size by Ultrasonography in Pediatric Patients

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Discussion

There is a need to choose an appropriate sized ETT from an assortment of sizes available for the pediatric airway given the fact that there is growth of the airway too as the child grows. Over-sized tubes can result in immediate complications like post-extubation stridor and long term complications like tracheal stenosis. While undersized tubes can result in inadequate ventilation, operating room contamination with anesthetic agents, aspiration risk and inaccurate end-tidal gas measurements. With every millimeter narrowing due to oversized tube, gas flow decreases and resistance increases significantly as flow is proportional to fourth power of radius. Various parameters like height, weight, length, CXR, USG and age are used to estimate optimal ETT size in pediatric patients.

The agreement rate of age-based pediatric ETT size selection using the Cole formula was as low as 47–77% in previous studies14,16 . Age- and height based formulas have been shown to poorly predict optimal ETT size in pediatric patients. Many studies show that ultrasound was highly predictive, but the extent to which the ultrasound predicts optimal ETT size varies among the studies. There are studies which show that pediatric ETT size be selected in 90% of patients by a patient length-based technique (e.g., Broselow tape)4 .However, such methods have limitations because these formulas cannot reflect variation in the growth of internal organs. The present study was conducted to know the extent of agreement between USG measured size and optimal clinical fit. The present prospective clinical study reiterated the findings of previous studies by showing a higher degree of correlation between ETT OD and subglottic diameter to an extent of 79% as compared to CXR which showed a correlation up to 70%.However some previous studies showed a prediction rate up to 95%29 .

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There was a high rate of agreement between predicted and clinically selected ETT size in the phase-2 by using the regression equation constructed from 60 pediatric patients in the phase-1 with an error of 6%. OD was used for regression and comparison in our study to avoid variations which can occur with ID among manufactures14,35,36 . Magnetic resonance (MR) is the gold standard for accurate tracheal measurements which have showed optimal correlation with USG measurements 26 . Pediatric trachea is not uniform. Older studies have shown sub-glottis to be the narrowest. On the contrary, newer studies have shown the narrowest region to be at the vocal cord and sub-vocal cord level 15 . Ultrasonographic transverse air-column measurement at vocal cord level is practically difficult and inconsistent due to poor visualization of the cords. So, we measured the airway diameter at the cricoid cartilage level. This measure represented a reliable and consistent value that could be compared among patients. In our study, 4 patients in the phase-2 had to be upsized with a measurement one above amounting to an error of 6%.

Chest radiography also has been shown to predict optimal ETT size 27 . CXR tracheal measurements have been supported to correlate well with the computed tomography measurements28. So,.we also studied the extent of prediction of optimal ETT size by CXR, taking measurement at the level of 6th cervical vertebra .The extent of prediction by CXR varied between 55% to 70% in our study. Ultrasonography is an operator-dependent technique, with a learning curve. Studies have shown that this skill can be reliably and reproducibly acquired as soon as after performing 26 . Calcium poses an acoustic shadow. So, USG may not be reliably used when the laryngeal cartilages calcify with age. However, this is of little concern in pediatric population 25 . .

Limitations

Narrowest region of pediatric airway is at the level of vocal cords and the region immediately below, which is difficult to measure by ultrasonography.

CONCLUSION

Ultrasonography predicts optimal uncuffed endotracheal tube(ETT) in pediatric patients (n=120) undergoing cardiac surgery under general anaesthesia from day -1 to 5 years to an extent of 79% with 6% error(4- ETT exchanges).The regression equation obtained from the phase-1 was ETT size in mm=0.011x(subglottic USG diameter)+0.98,R2 =0.79. CXR varied in predicting optimal ETT size, 55% to 70%.

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