Manjith Narayanan

Alveolarization Continues during Childhood and Adolescence: New Evidence from Helium-3 Magnetic Resonance

RATIONALE The current hypothesis that human pulmonary alveolarization is complete by 3 years is contradicted by new evidence of alveolarization throughout adolescence in mammals. OBJECTIVES We reexamined the current hypothesis using helium-3 ((3)He) magnetic resonance (MR) to assess alveolar size noninvasively between 7 and 21 years, during which lung volume nearly quadruples. If new alveolarization does not occur, alveolar size should increase to the same extent. METHODS Lung volumes were measured by spirometry and plethysmography in 109 healthy subjects aged 7-21 years. Using (3)HeMR we determined two independent measures of peripheral airspace dimensions: apparent diffusion coefficient (ADC) of (3)He at FRC (n = 109), and average diffusion distance of helium (X(rms)) by q-space analysis (n = 46). We compared the change in these parameters with lung growth against a model of lung expansion with no new alveolarization. MEASUREMENTS AND MAIN RESULTS ADC increased by 0.19% for every 1% increment in FRC (95% confidence interval [CI], 0.13-0.25), whereas the expected change in the absence of neoalveolarization is 0.41% (95% CI, 0.31-0.52). Similarly, increase of (X(rms)) with FRC was significantly less than the predicted increase in the absence of neoalveolarization. The number of alveoli is estimated to increase 1.94-fold (95% CI, 1.64-2.30) across the age range studied. CONCLUSIONS Our observations are best explained by postulating that the lungs grow partly by neoalveolarization throughout childhood and adolescence. This has important implications: developing lungs have the potential to recover from early life insults and respond to emerging alveolar therapies. Conversely, drugs, diseases, or environmental exposures could adversely affect alveolarization throughout childhood.

Alveolarization Continues during Childhood and Adolescence

RATIONALE The current hypothesis that human pulmonary alveolarization is complete by 3 years is contradicted by new evidence of alveolarization throughout adolescence in mammals. OBJECTIVES We reexamined the current hypothesis using helium-3 ((3)He) magnetic resonance (MR) to assess alveolar size noninvasively between 7 and 21 years, during which lung volume nearly quadruples. If new alveolarization does not occur, alveolar size should increase to the same extent. METHODS Lung volumes were measured by spirometry and plethysmography in 109 healthy subjects aged 7-21 years. Using (3)HeMR we determined two independent measures of peripheral airspace dimensions: apparent diffusion coefficient (ADC) of (3)He at FRC (n = 109), and average diffusion distance of helium (X(rms)) by q-space analysis (n = 46). We compared the change in these parameters with lung growth against a model of lung expansion with no new alveolarization. MEASUREMENTS AND MAIN RESULTS ADC increased by 0.19% for every 1% increment in FRC (95% confidence interval [CI], 0.13-0.25), whereas the expected change in the absence of neoalveolarization is 0.41% (95% CI, 0.31-0.52). Similarly, increase of (X(rms)) with FRC was significantly less than the predicted increase in the absence of neoalveolarization. The number of alveoli is estimated to increase 1.94-fold (95% CI, 1.64-2.30) across the age range studied. CONCLUSIONS Our observations are best explained by postulating that the lungs grow partly by neoalveolarization throughout childhood and adolescence. This has important implications: developing lungs have the potential to recover from early life insults and respond to emerging alveolar therapies. Conversely, drugs, diseases, or environmental exposures could adversely affect alveolarization throughout childhood.

Breastfeeding, lung volumes and alveolar size at school-age

Background Previous studies found larger lung volumes at school-age in formerly breastfed children, with some studies suggesting an effect modification by maternal asthma. We wanted to explore this further in children who had undergone extensive lung function testing. The current study aimed to assess whether breastfeeding was associated with larger lung volumes and, if so, whether all compartments were affected. We also assessed association of breastfeeding with apparent diffusion coefficient (ADC), which measures freedom of gas diffusion in alveolar-acinar compartments and is a surrogate of alveolar dimensions. Additionally, we assessed whether these effects were modified by maternal asthma. Methods We analysed data from 111 children and young adults aged 11–21 years, who had participated in detailed lung function testing, including spirometry, plethysmography and measurement of ADC of 3Helium (3He) by MR. Information on breastfeeding came from questionnaires applied in early childhood (age 1–4 years). We determined the association between breastfeeding and these measurements using linear regression, controlling for potential confounders. Results We did not find significant evidence for an association between duration of breastfeeding and lung volumes or alveolar dimensions in the entire sample. In breastfed children of mothers with asthma, we observed larger lung volumes and larger average alveolar size than in non-breastfed children, but the differences did not reach significance levels. Conclusions Confirmation of effects of breastfeeding on lung volumes would have important implications for public health. Further investigations with larger sample sizes are warranted.

Reply: On the Use of 3He Diffusion Magnetic Resonance as Evidence of Neo-Alveolarization during Childhood and Adolescence

From the Authors: We thank the editor of the Journal for giving us the opportunity to respond to the letter by Parra-Robles and Wild. Using helium-3 (3He) magnetic resonance (MR), we investigated whether lung growth during childhood was achieved by expansion of pre-existing alveoli or by new alveolarization (1). We measured how the parameters obtained from 3HeMR change during enlargement of the lung by controlled inhalation, and compared them with those occurring during lung growth in a cross-section of over 100 children. The parameters changed very little with growth compared with the predicted changes, suggesting that the peripheral lung structures altered little in size during a period when lung volume increased up to 3.6-fold. The implication is that alveolarization occurred throughout childhood. Our study is just one of several in recent literature supporting alveolarization beyond early childhood in mammals (2–6). Following the publication of our paper, there has been more evidence supporting post-mature alveolarization in humans (7). Three major issues were identified in the letter from Parra-Robles and Wild and are dealt with here: (1) length scale used in our measurements, (2) use of the “expansion model” (based on inhalation) to test the hypothesis of no alveolarization, and (3) the mathematical model on which they base their criticisms. These issues are discussed in more detail and their other concerns are explored in the online supplement to this letter. 1. In free space, helium atoms undergo diffusion. The root-mean-square displacement s due to diffusion is related to the measurement interval (“diffusion time”) t as s=2Dt, (Eq.1) where D is unrestricted diffusion coefficient. In a restricted space, diffusion is limited by the barriers. Therefore, the measured value is an “apparent diffusion coefficient” (ADC). Using ADC as a measure of dimension of the barriers is practical only if the dimensions are of the same order as s (8) (and therefore, the diffusion time t should be appropriately chosen). The diameter of alveoli based on previous histological studies is of the order of 0.25 to 0.3 mm (9–11). For a diffusion time of 2 ms, suggested by Parra-Robles and Wild (12, 13), the value for s in Equation 1 is 0.57 mm, assuming D = 0.8 cm2/s for helium in air. For longer diffusion times, this changes by a factor of (t) (s is only 1.58 times larger for t = 5 ms). Therefore, we hold that our measurements are still sensitive to alveolar dimensions. The alveolar duct and the sacs do not have an independent wall (open geometry). The “boundaries” of the ducts and sacs are in fact the openings of alveoli (14), and so diffusing atoms are in fact restricted by alveolar walls. Our measurements are sensitive to alveolar dimensions as long as the alveolar ducts and sacs do not change in dimension without a corresponding change in alveolar dimension (see online supplement to this letter). There is evidence that the relative size of alveoli and alveolar ducts remains constant between 7 and 14 years (15). We believe that our technique has the sensitivity to distinguish between two alternative hypotheses: enlargement of acinar structures during lung growth happens as a result of either an increase in volume of pre-existing structures (the pre-existing hypothesis), or by formation of new alveoli during lung development as we propose. Our expansion model (below) provides evidence that our technique is sensitive to an appropriate range of changes of dimensions of peripheral lung structures. 2. We used the change in ADC during controlled inhalation to predict its change when lung enlarges without alveolarization (the green line in Figure 2 of our article [1]). This is valid if the inhalation enlarges peripheral lung structures in the same way as growth without alveolarization. We believe that this is the case for gentle inhalation from functional residual capacity, taking care not to overdistend the lung (i.e., not to inflate near the limits of total lung capacity). Other possible models for lung expansion including recruitment of new alveoli (16) will actually cause our predicted line of lung growth without alveolarization (1) to be steeper than depicted, strengthening rather than weakening our argument of neo-alveolarization (see online supplement to this letter). Further, the expansion model was tested on children with a range of functional residual capacity from 0.98 to 2.63 L. 3. Parra-Robles and Wild base their critique heavily on their simulations based on their model alveolar duct (12, 13). We have some concerns about the assumptions underlying the simulation. While the length scales in the model are realistic, the model is mathematically simplified. We enlarge upon the following additional concerns in the online supplement: a. Assumptions about the branching structure are not realistic. b. The branching is assumed to be infinite, but the blind ending alveolar sacs are greater in number than the alveolar ducts. c. The expansion of alveoli is modeled as expansion of one dimension alone (13) (increasing axial dimension of the alveoli without increase in depth or cross-sectional diameter). This is implausible, and the paper from which they draw this assumption (7) never attempted to model alveolar expansion due to inhalation. In fact, it concludes that their findings were due to neo-alveolarization. d. In contrast to the model proposed by Parra-Robles and Wild (12, 13), ADC correlates to histological measures of alveolar dimensions in animal and human lungs (17–20). We feel that the results based on this model are open to question. We suggest that their model could be run with lung expansion modeled in a realistic manner. In conclusion, our results were not based on any geometric lung model. Our only assumption is that the expansion of the lung during moderate degrees of inhalation is an acceptable model of lung expansion during development in the absence of new alveolarization. Given that assumption, all subsequent analysis was based on statistical approaches. In contrast, other techniques proposed by Parra-Robles and Wild in their letter, such as computed tomography and DlCO, are either unethical in healthy children or do not give information about alveolar dimensions. We would welcome further research into lung development in children using shorter diffusion times, and we would also welcome further refinement of computer models to assist the interpretation of physiological measurements.

Alveolarization continues during childhood and adolescence: new evidence from helium-3 magnetic resonance.

RATIONALE The current hypothesis that human pulmonary alveolarization is complete by 3 years is contradicted by new evidence of alveolarization throughout adolescence in mammals. OBJECTIVES We reexamined the current hypothesis using helium-3 ((3)He) magnetic resonance (MR) to assess alveolar size noninvasively between 7 and 21 years, during which lung volume nearly quadruples. If new alveolarization does not occur, alveolar size should increase to the same extent. METHODS Lung volumes were measured by spirometry and plethysmography in 109 healthy subjects aged 7-21 years. Using (3)HeMR we determined two independent measures of peripheral airspace dimensions: apparent diffusion coefficient (ADC) of (3)He at FRC (n = 109), and average diffusion distance of helium (X(rms)) by q-space analysis (n = 46). We compared the change in these parameters with lung growth against a model of lung expansion with no new alveolarization. MEASUREMENTS AND MAIN RESULTS ADC increased by 0.19% for every 1% increment in FRC (95% confidence interval [CI], 0.13-0.25), whereas the expected change in the absence of neoalveolarization is 0.41% (95% CI, 0.31-0.52). Similarly, increase of (X(rms)) with FRC was significantly less than the predicted increase in the absence of neoalveolarization. The number of alveoli is estimated to increase 1.94-fold (95% CI, 1.64-2.30) across the age range studied. CONCLUSIONS Our observations are best explained by postulating that the lungs grow partly by neoalveolarization throughout childhood and adolescence. This has important implications: developing lungs have the potential to recover from early life insults and respond to emerging alveolar therapies. Conversely, drugs, diseases, or environmental exposures could adversely affect alveolarization throughout childhood.