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.