Other Physiologic Studies
A study by Fabi et al. examined short- and long-term changes in
respiratory mechanics after palliative or corrective surgery in 168
infants and toddlers with congenital heart disease [28]. They found
that these patients demonstrated normal basal respiratory function but
decreased exercise capacity. This decrease in exercise capacity was more
pronounced in patients who underwent total cavopulmonary connection
(TCPC) procedures. This suggests the need to conduct respiratory
monitoring on patients with TCPC more closely and for longer than those
with non-TCPC procedures. They also found that a higher age at surgery
but prior to 24 months was associated with better respiratory outcomes
which suggests that increased development of the lung parenchyma prior
to intervention will lead to improved respiratory outcomes
post-operatively.
Sivieri, et al used a preterm infant lung model to evaluate the effects
on end tidal CO2 (ETCO2) when high frequency oscillation
is superimposed on bubble CPAP (BCPAP) [29]. They found that there
is improved CO2 clearance when high frequency
oscillation is applied to BCPAP, and that this effect was more
pronounced when the model mimicked lung disease in a preterm infant
compared to normal infant lung mechanics. This improvement was not
dependent on BCPAP pressure, CO2 clearance was significantly improved
for both lung models as a function of increased frequency and lower
respiratory rate. These results support conducting a clinical trial to
assess the effect of superimposed high frequency oscillations in infants
receiving BCPAP.
Extrathoracic high frequency chest compression (HFCC) using an
inflatable vest is frequently used for airway clearance in patients with
mucociliary abnormalities, such as CF. Weiner, et al sought to determine
if HFCC augments ventilation by measuring end-tidal CO2 (ETCO2) after
HFCC was performed at varying frequencies (Hz)in a cohort of 15 people
with CF who were experiencing a pulmonary exacerbation [30]. ETCO2
values were significantly lower at all Hz compared to no treatment. This
remained the same when adjusting for patient specific factors such as
age, sex, body mass index, baseline FEV1 and respiratory rate.
Increasing Hz produced a small, but significant improvement in ETC02
clearance. These results demonstrate that HFCC increases clearance of
CO2 in a frequency dependent manner. Further research is needed to
determine the clinical importance of this finding.
Formulas used to calculate the appropriate pediatric tracheostomy or
endotracheal tube (ETT) size have incorporated age, weight, and height,
but there are no formulas that incorporate airflow resistance or
cross-sectional area of the inner cannula. Since resistance varies by
the 4th power of the radius, small changes in ETT
lumen diameter can have marked effects on tube resistance (R) [31].
Khan and Baker applied the Hagen-Poiseuille equation for R under laminar
flow conditions to calculate the R in different ETT and tracheostomy
tubes based on tube inner diameter and cross-sectional area [32]. A
large decrease in R was seen when tube diameter was increased by as
little as 0.5mm in ETTs that would likely be used for patients
<1 year of age. This effect was not as significant in larger
tube sizes. When invasive mechanical ventilation is required, and a
patient can safely tolerate upsizing of ETT or tracheostomy tube, this
should be considered to lower airway resistance and improve airflow
dynamics.