Background: The flow in the human trachea is turbulent.
Thus, the tracheal resistance (R) and the pressure gradient
(ΔP) required to maintain a given flow across the trachea is
inversely related to its radius raised to the fifth power. If the
caliber reduction ratio (X) after endotracheal intubation is calculated
as X = radius of the endotracheal tube (rETT)/radius of the
trachea (rT), then ΔP and/or R will be increased by
Study objectives: To measure the
actual ratio between rETT and rT following endotracheal intubation of
pediatric patients with respiratory failure and to calculate the
resulting increase in the tracheal R and ΔP for a given inspiratory
Design: Retrospective chart review.
Setting: Pediatric ICU in a tertiary-care teaching
children’s medical center.
Patient enrollment: Twenty
consecutive pediatric patients (mean [± SD] age, 6.4 ± 7.2
years) whose tracheas had been intubated for various causes of
respiratory failure, and who had received a CT scan, were included in
our study. All patients received an endotracheal tube the size of which
was derived from the following formula: (age in years/4) + 4.
Measurements and main results: rT and rETT were measured
from CT scan sections at and around the level of the thoracic inlet,
and the average values were used to calculate X. These values ranged
from 0.33 to 0.65 (mean, 0.55 ± 0.8). The factor (1/X)5
was calculated for each patient and then was multiplied by the known
normal value for tracheal R for adolescents and adults (0.07 cm
H2O/L/s) to obtain the value of R resulting from the
artificial airway, (1/X)5 × 0.07. Our results showed
that tracheal R increased due to caliber reduction of the trachea after
endotracheal intubation by 33.9 ± 52.5-fold (range, 8.6- to
255.5-fold). In order to maintain an inspiratory flow of 1 L/s, the
value of P for the intubated trachea would increase from 0.07 cm
H2O to a mean of 2.4 ± 3.7 cm H2O (range,
0.6 to 18 cm H2O). In two of our patients, the rT/rETT
ratios were < 0.5 (0.33 and 0.44, respectively); this translated into
a more significant increase in the calculated ΔPs, 18 and 4.2 cm
common value of X due to endotracheal intubation is between 0.5 and
0.6, which in and of itself results in an increase in R across the
intubated trachea up to 32-fold. The calculated increase in P as a
result of this is between 2 and 3 cm H2O for adolescents or
young adults. The addition of pressure support of at least 3 cm
H2O during spontaneous ventilation via an endotracheal
tube, which is common practice in pediatric critical care, should
alleviate any respiratory distress emanating from the increased R.
However, a value for X < 0.5, which was found in 10% of our patients
(2 of 20 patients), results in a much higher calculated increase in the
pressure gradient and, therefore, a higher level of pressure support is
required to overcome this increase.