Functional morphometry for the estimation of the alveolar surface area in prematurely-born infants

Functional morphometry: non-invasive estimation of the alveolar surface area in extremely preterm infants

BACKGROUND

The main pathophysiologic characteristic of chronic respiratory disease following extremely premature birth is arrested alveolar growth, which translates to a smaller alveolar surface area (SA). We aimed to use non-invasive measurements to estimate the SA in extremely preterm infants.

METHODS

Paired measurements of the fraction of inspired oxygen and transcutaneous oxygen saturation were used to calculate the ventilation/perfusion ratio, which was translated to SA using Fick's law of diffusion. The SA was then adjusted using volumetric capnography.

RESULTS

Thirty infants with a median (range) gestational age of 26.3 (22.9-27.9) weeks were studied. The median (range) adjusted SA was 647.9 (316.4-902.7) cm2. The adjusted SA was lower in the infants who required home oxygen [637.7 (323.5-837.5) cm2] compared to those who did not [799.1 (444.2-902.7) cm2, p = 0.016]. In predicting the need for supplemental home oxygen, the adjusted SA had an area under the receiver operator characteristic curve of 0.815 (p = 0.017). An adjusted SA ≥688.6 cm2 had 86% sensitivity and 77% specificity in predicting the need for supplemental home oxygen.

CONCLUSIONS

The alveolar surface area can be estimated non-invasively in extremely preterm infants. The adjusted alveolar surface area has the potential to predict the subsequent need for discharge home on supplemental oxygen.

IMPACT

We describe a novel biomarker of respiratory disease following extremely preterm birth. The adjusted alveolar surface area index was derived by non-invasive measurements of the ventilation/perfusion ratio and adjusted by concurrent measurements of volumetric capnography. The adjusted alveolar surface area was markedly reduced in extremely preterm infants studied at 7 days of life and could predict the need for discharge home on supplemental oxygen. This method could be used at the bedside to estimate the alveolar surface area and provide an index of the severity of lung disease, and assist in monitoring, clinical management and prognosis.

Factors affecting the arterial to end-tidal carbon dioxide gradient in ventilated neonates

Objective. To determine factors which influenced the relationship between blood carbon dioxide (pCO₂) and end-tidal carbon dioxide (EtCO₂) values in ventilated, newborn infants. Furthermore, to assess whether pCO₂levels could be predicted from continuous EtCO₂monitoring.Approach. An observational study of routinely monitored newborn infants requiring mechanical ventilation in the first 28 d after birth was undertaken. Infants received standard clinical care. Daily pCO₂and EtCO₂levels were recorded and the difference (gradient: ∆P-EtCO₂) between the pairs were calculated. Ventilatory settings corresponding to the time of each blood gas assessment were noted. End-tidal capnography monitoring was performed using the Microstream sidestream Filterline H set capnograph.Main results. A total of 4697 blood gas results from one hundred and fifty infants were analysed. The infants had a median gestational age of 33.3 (range 22.3-42.0) weeks and birth weight of 1880 (395-5520) grams. Overall, there was moderate correlation between pCO₂and EtCO₂levels (r= 0.65,p< 0.001). The ∆P-EtCO₂for infants born less than 32 weeks of gestation was significantly higher (1.4 kPa) compared to infants born at greater than 32 weeks of gestation (0.8 kPa) (p< 0.001). In infants born at less than 32 completed weeks of gestation, pCO₂levels were independently associated with EtCO₂, day after birth, birthweight and fraction of inspired oxygen (FiO₂) (modelr² = 0.52,p< 0.001).Significance. The results of end-tidal capnography monitoring have the potential to predict blood carbon dioxide values within the neonatal population.

Functional morphometry to estimate the alveolar surface area using a premature baboon model

The main respiratory pathophysiological process following premature birth is the delayed or arrested alveolar development that translates to a smaller alveolar surface area (S_A). Histological morphometry is the gold standard method to measure the S_A but requires invasive tissue sampling or the removal of the whole organ for analysis. Alternatively, the S_A could be measured in living subjects by "functional morphometry" using Fick's first law of diffusion and noninvasive measurements of the ventilation to perfusion ratio (V̇a/Q̇). We herein aim to describe a novel functional morphometric method to measure S_A using a premature baboon model. We used both functional morphometry and postmortem histological morphometry to measure S_A in 11 premature baboons born at 135 days who received intensive care treatment for 14 days. For the calculation of the S_A by functional morphology, we measured the septal wall thickness using microscopy, the alveolar arterial oxygen gradient using concurrent measurements of arterial pressure of O₂ and CO₂, and pulmonary perfusion using echocardiography and integrated Doppler signals. The median [interquartile range (IQR)] S_A using functional morphometry was 3,100 (2,080-3,640) cm² and using histological morphometry was 1,034 (634-1,210) cm² (left lung only). The S_A measured by functional morphometry was not related to the S_A measured by histological morphometry. Following linear regression analysis, the V̇a/Q̇ significantly predicted the histologically measured S_A (R² = 0.659, P = 0.002). In conclusion, functional measurements of ventilation to perfusion ratio could be used to estimate the alveolar surface area in prematurely born baboons and the ventilation perfusion ratio was the main determinant of the alveolar surface area.NEW & NOTEWORTHY The main morphological characteristic of chronic respiratory disease in prematurely born infants is the impaired/arrested alveolar growth that corresponds to a smaller aggregated alveolar surface area (S_A). This decreased S_A might be the limiting factor later in life affecting exercise capacity and quality of life. There is paucity of sensitive, noninvasive biomarkers to monitor the evolution of neonatal respiratory disease. Our noninvasive functional morphometric S_A might help to bridge the gap between pathophysiology and clinical monitoring.