Authors: Shanmukha Mukthapuram (1The Perinatal Institute, Cincinnati Children’s Hospital Medical Center; 2Cincinnati Bronchopulmonary Dysplasia Center, Cincinnati Children’s Hospital Medical Center; 3Department of Pediatrics, University of Cincinnati College of Medicine), Addison Donaher (1The Perinatal Institute, Cincinnati Children’s Hospital Medical Center), Nara S Higano (2Cincinnati Bronchopulmonary Dysplasia Center, Cincinnati Children’s Hospital Medical Center; 3Department of Pediatrics, University of Cincinnati College of Medicine; 4Center for Pulmonary Imaging Research, Cincinnati Children’s Hospital Medical Center; 5Division of Pulmonary Medicine, Cincinnati Children’s Hospital Medical Center; 6Department of Radiology, Cincinnati Children’s Hospital Medical Center), James A Rowe (1The Perinatal Institute, Cincinnati Children’s Hospital Medical Center; 2Cincinnati Bronchopulmonary Dysplasia Center, Cincinnati Children’s Hospital Medical Center; 3Department of Pediatrics, University of Cincinnati College of Medicine), Jean A Tkach (4Center for Pulmonary Imaging Research, Cincinnati Children’s Hospital Medical Center), Jason C Woods (2Cincinnati Bronchopulmonary Dysplasia Center, Cincinnati Children’s Hospital Medical Center; 3Department of Pediatrics, University of Cincinnati College of Medicine; 4Center for Pulmonary Imaging Research, Cincinnati Children’s Hospital Medical Center; 5Division of Pulmonary Medicine, Cincinnati Children’s Hospital Medical Center; 6Department of Radiology, Cincinnati Children’s Hospital Medical Center), Paul S Kingma (1The Perinatal Institute, Cincinnati Children’s Hospital Medical Center; 2Cincinnati Bronchopulmonary Dysplasia Center, Cincinnati Children’s Hospital Medical Center; 3Department of Pediatrics, University of Cincinnati College of Medicine; 4Center for Pulmonary Imaging Research, Cincinnati Children’s Hospital Medical Center)
Categories: Article, Neonatal, respiratory physiology, pulmonary hypertension
Source: Neonatology
Doi: 10.1159/000539545
Authors: Shanmukha Mukthapuram, Addison Donaher, Nara S Higano, James A Rowe, Jean A Tkach, Jason C Woods, Paul S Kingma
Pulmonary hypertension often complicates bronchopulmonary dysplasia (BPD) and infants with BPD plus pulmonary hypertension experience higher mortality rates. Current methods to evaluate pulmonary hypertension fail to evaluate the primary cause of this disease. We hypothesize that preterm infants with BPD experience altered pulmonary vascular growth and that MRI can be used to assess vascularity in BPD.
In this observational cohort study, preterm infants with BPD (n=33) and controls (n=6) received a postnatal chest MRI that included a 2-dimensional time-of-flight acquisition. Semi-automatic segmentation was performed to measure vascularity parameters including vascular volume & density (vascular density = vascular volume/lung volume).
Vascular volume on MRI increases with post menstrual age (877.2 mm3/week), however the vascular density does not significantly change. Vascular volume is higher in infants with more severe BPD (p<0.002), but vascular density did not significantly change when comparing mild, moderate, and severe BPD. Vascular density in infants with severe BPD requiring tracheostomy trended lower when compared to infants not requiring tracheostomy (0.18 mm^3^/mm^3^ vs 0.27 mm^3^/mm^3^, p=0.06). Vascular density increases with increasing days on inhaled nitric oxide (iNO) therapy in infants with severe BPD (0.02 mm^3^/mm^3^ /week of iNO, rho=+0.56, p=0.03)
Neonatal MRI can be used to assess pulmonary vascularity in preterm infants with BPD. Infants with BPD experience altered vascular growth and while higher vascular volume is associated with more severe BPD, lower vascular density trends towards worse clinical outcome. Vascular density increases with iNO therapy in severe BPD.
Since Northway et al. first described Bronchopulmonary Dysplasia (BPD) in the late 1960’s we have witnessed several advances in the field of neonatology [1}. Improved ventilation strategies, the advent of surfactant and modernized neonatal care practices resulted in increased survival of very preterm infants [2]. BPD is a common problem in these survivors resulting in high morbidity and mortality rates [3]. Pulmonary hypertension (PH) often complicates BPD and infants with BPD plus PH experience higher mortality rates [4–8]. PH in infants with BPD may also contribute to significant long-term pulmonary morbidity and impaired neurodevelopmental outcomes [6].
Echocardiogram and cardiac catheterization are currently the primary methods to evaluate PH. Cardiac catheterization remains the gold standard for evaluation of PH, however the invasiveness of this procedure explains its frugal use in neonates [9,10]. As a result, echocardiography despite its limitations and inability to assess pulmonary vascularity directly, remains the most common tool to screen and monitor PH in BPD infants. Tricuspid regurgitation velocity, interventricular septum position, direction of shunts and right ventricular pressure, size and function are the parameters on echocardiography commonly used to assess the severity of PH. However, evidence suggests that echocardiographic assessment of pulmonary artery pressure may not be reliable in determining the PH severity [10]. This limitation of echocardiography may in part be explained by the lack of direct assessment of vascularity beyond the pulmonary artery bifurcation and the fact that echocardiography only measures the cardiac sequalae of changes in pulmonary vasculature. Despite the clear shortcomings of cardiac catheterization and echo in evaluating PH, they continue to be the main diagnostic tool used in clinical practice. Clearly a tool that evaluates the primary problem of pathological vascular growth in the lungs rather than the secondary effects of PH on the heart is needed.
Lung imaging modalities such as computer tomography and magnetic resonance imaging (MRI) are now capable of detecting pulmonary blood vessels. Thoracic computer tomography has been used to evaluate pulmonary vasculature and parenchyma, however it is associated with risk of radiation exposure reducing its potential for regular use in neonatal care [11–13]. Cardiac (MRI) has been used to evaluate changes in cardiac function and pulmonary artery blood flow in infants with BPD and in infants with Congenital Diaphragmatic Hernia who are at risk for developing PH [14–16]. Improved pulmonary magnetic resonance imaging (MRI) techniques have allowed us to assess structural abnormalities involving airway and lung parenchyma in BPD infants and can be used in predicting short-term outcomes accurately [17–18]. In addition, we have recently demonstrated in infants with CDH that it is now possible to measure the pulmonary vascular bed to a resolution of 0.7 mm using time of flight MRI [19]. Our evaluation of pulmonary vascular volume and density (vascular density = vascular volume divided by lung volume) in congenital diaphragmatic hernia demonstrated a strong correlation between these variables and markers PH severity in infants with this diagnosis. This correlation in infants with diaphragmatic hernia and the known pulmonary vascular pathology seen post-mortem in infants with BPD, leads to the hypothesis that altered pulmonary vascular growth as measured by changes in vascular volume and density may also correlate with and potentially lead to PH in infants with BPD. Therefore, in the current study we evaluated pulmonary vascularity in preterm infants with BPD using MRI and the correlation between MRI measured pulmonary vascular density and clinical outcomes.
This study was reviewed and approved by the Institutional Review Board of Cincinnati Children’s Hospital Medical Center, Cincinnati Ohio, approval IRB#2013–6101 and 2018–0958. Written informed consent was obtained for all study subjects from the parents or legal guardians. Infants admitted to Cincinnati Children’s Hospital Medical Center with a birth gestational age < 32 weeks and a diagnosis of BPD were consented to participate as part of the BPD group in the study (N=33). Infants that were born greater than 32 weeks gestation with no known respiratory or cardiac disease were included as controls. Infants with weight greater than ~4.5 kg (due to size constraints of the MRI coil), presence of an implanted device incompatible with MRI, uncontrolled atrial or ventricular arrhythmia, congenital heart disease (not including patent ductus arteriosus and hemodynamically insignificant ventricular or atrial septal defects), other congenital malformations affecting life expectancy or cardiopulmonary development, or had insufficient image quality for data analysis (i.e. motion artifacts) were excluded. Infants underwent pulmonary MRI after they were deemed medically stable by the clinical team. A small-footprint neonatal-sized 1.5 T orthopedic MRI scanner adapted for use within our NICU was used to obtain axial, coronal and sagittal 2-dimensional time of flight (2D-TOF) images that averages vessel volume in all patients with ~5-minute scan time. 2D TOF imaging technique used in our study is often utilized to better study blood flow perpendicular to the plane of imaging without the use of contrast by acquiring multiple thin slices with a flow compensated gradient echo sequence. This approach allows 2D TOF imaging to capture pulmonary arterial and venous blood flow perpendicular to the plane of imaging. Neither sedation nor IV contrast was used for the MRI unless sedation was a part of the patient’s ongoing clinical care. Infants on varying degrees of respiratory support including room air, nasal cannula and mechanical ventilator underwent imaging.
Analyze Pro (Analyze Direct Inc, Kansas) software was used to analyze the images as previously described [19]. Semi-automatic segmentation from the 2D-TOF images was performed to identify vessels that produced signal above the detection threshold and create a three-dimensional image of the vessels. The software was then used to measure left lung vessel volume and right vessel volume which were summed to calculate total vessel volume. Vascular density was calculated by dividing total vessel volume with the total lung volume. Clinical data including birth weight, weight at the time of MRI, sex, gestational age at birth, corrected gestational age at the time of MRI, severity of BPD at 36 weeks corrected gestational age, need for inhaled nitric oxide therapy, need for sildenafil therapy and respiratory support at time of discharge (categorized as room air, nasal canula oxygen <2 liters per minute, mechanical ventilation) were obtained from medical records retrospectively.
Total vascular volume and vascular density were compared with weight at the time of MRI, post menstrual age (PMA) at the time of MRI and days on inhaled nitric oxide therapy using Spearman’s correlation. Vascular density amongst infants with different severity of BPD was compared using t-test or ANOVA with post-hoc analysis where appropriate.
A total of 33 infants with BPD and 9 controls who met the inclusion criteria and had good quality images were identified during the study period. The infant characteristics and demographic information are presented in table 1. All infants included in our study were born at less than 32 weeks gestational age and had a diagnosis of BPD based on NICHD/NHLBI definition [20]. Seven infants had mild severity, 11 had moderate and 15 infants had severe BPD. Mean gestational age at birth in infants with BPD was 25.5±1.8 weeks. Mean corrected gestational age at the time of imaging was 41.1 ± 2.6 weeks. Three infants had a PDA that was closed 7–30 days prior to the MRI while 3 and 3 infants had a small PDA and ASD, respectively at the time of MRI. There was no significant difference in vascular density between infants with PDA or ASD (0.202 mm^3^/mm^3^) and those without cardiac anomalies (0.218 mm^3^/mm^3^)(p=0.58). Mean gestational age at birth for the control group was 36.5 weeks (range 34–39.1 weeks) with a corrected gestational age of 41.3 weeks (range 37.7–45.4 weeks) at the time of MRI.
Vascular volume increased with increasing weight (R=0.44, p=0.008) and an increasing trend in vascularity was noted when compared with post menstrual age in days (R=0.32, p=0.06) at the time of MRI (Figure 1). However, vascular densities (i.e., total vascular volume divided by total lung volume) did not significantly change when compared to weight or post menstrual age at the time of MRI, suggesting vascular growth mostly parallels lung volume growth. A representative image of the vasculature detected by 2D-TOF in a control infant is shown (Figure 1E).
The impact of vascular density on clinical outcomes was evaluated by comparing vascular densities to severity of BPD and need for tracheostomy (Figure 2). Total vascular volume increased with BPD severity when infants were categorized as mild, moderate and severe BPD based on NICHD/NHLBI criteria (p<0.002)). However, vascular density did not significantly change (p=0.54). Infants with BPD that required tracheostomy and long-term mechanical ventilation were noted to have a trend towards lower vascular density compared to infants who did not require tracheostomy (0.18 mm^3^/mm^3^ vs 0.22 mm^3^/mm^3^, p=0.06).
A total of 15/33 (45.4%) infants were diagnosed with pulmonary hypertension by echo in our cohort. Vascular densities were similar in infants with and without PH diagnosed by echocardiography (Figure 3). To further this analysis, infants with PH were subdivided into less severe versus more severe PH, based on the need for sildenafil therapy. Higher vascular density was noted in infants with presumably more severe PH that required sildenafil therapy when compared to infants who did not require sildenafil treatment, but this did not reach statistical significance (Figure 3).
To determine if therapy with anti-pulmonary hypertensive agents such as inhaled nitric oxide (iNO) influences vascular density, we compared total number of days on iNO with vascular density specifically in infants in the most severe BPD groups (discharged home on ventilator or deceased) who were treated with iNO prior to the MRI but were not on iNO at the time of MRI. In this severe group, vascular density had a significant direct correlation with total days on iNO prior to the MRI such that infants with longer exposure to iNO had higher vascular densities (R=0.59, p=0.03) (Figure 4) at the time of MRI.
Pulmonary vascular growth and reactivity in preterm infants is complex and influenced by several postnatal factors. When this vascular growth is disrupted in preterm infants with BPD, pulmonary hypertension develops and greatly influences their survival and long-term outcome [21–24]. Our current tools for evaluating pulmonary hypertension are indirect and have limited accuracy. Echocardiography and cardiac catheterization only measure the changes in the right ventricle and pulmonary artery that are secondary to the changes in pulmonary vasculature. Flow across the pulmonary vasculature past pulmonary artery bifurcation or vascular growth is not directly assessed by these existing tools. Our study adds to the existing literature that neonatal pulmonary MRI is a safe and non-invasive tool that can be used to evaluate pulmonary vascularity and directly assess pulmonary vascular abnormalities in infants with BPD [25]. Our study suggests that vessel volume grows in infants with BPD when plotted against weight or corrected gestational age at the time of MRI. However, when vascular growth is normalized by comparing with lung volume growth, we found that vascular density is fairly constant suggesting that vessel volume growth mostly parallels lung volume growth.
While we did not observe a decline in vascular density with worsening BPD severity in our cohort, we did observe a trend towards decreasing vascular density in infants who required tracheostomy and long-term mechanical ventilation. BPD is a complex and multifactorial lung disease. Therefore, it is highly possible that while lower vascular density may worsen the clinical status of a given category of BPD severity and increase the risk of such outcomes as long-term mechanical ventilation, BPD severity itself may not influence and therefore correlate with vascular density. Studies examining postmortem lung samples of infants with severe BPD suggest pulmonary microvascular growth when compared to parenchymal growth is 60% more than in controls, is very complex and needs better understanding [26]. Further studies with larger sample size will be needed to determine if BPD severity influences vascular density measured by MRI.
There was no correlation between vascular density and markers of PH on echocardiography. We initially considered that this lack of correlation may be due in part to the presence of milder disease in infants categorized with PH (i.e., PH+ group), therefore we further stratified this group based on the need for therapy with sildenafil. However, this stratification also failed to produce a correlation between lower vascular density and the diagnosis of PH on echocardiography. In fact, the vascular densities trended higher in the infants that required therapy. Since previous animal studies have suggested vascular growth after exposure to anti pulmonary hypertensive medications, we hypothesized that the slightly higher vascular density in the sildenafil treatment group may reflect vascular growth caused by the anti-pulmonary hypertension therapy.
We couldn’t address this potential impact of sildenafil on vascular density in the infants who required sildenafil therapy since the vascular density in this group was confounded by a mixture of mild to severe BPD and the fact that while all infants in the sildenafil+ category eventually required this therapy, four infants were on sildenafil at the time of MRI while the remaining started sildenafil after the MRI. Therefore, to address the hypothesis of anti-pulmonary hypertensive therapy potentially increasing vascular growth, we specifically examined the 13 infants with severe BPD that were treated with iNO prior to obtaining MRI. Since all these infants had similar levels of BPD severity and had completed iNO therapy by the time of MRI, this comparison should focus on the influence of iNO therapy. Our study demonstrated a significant direct correlation between days on nitric oxide prior to MRI and vascular density suggesting a potential positive therapeutic impact of iNO on vascular growth. It is also possible that findings of our study in infants who received anti pulmonary hypertension therapies are attributed to the disease severity. Though further studies are needed to demonstrate the effect of inhaled nitric oxide and other anti-pulmonary hypertensive therapies on pulmonary vascularity in humans, to our knowledge this is the first study to show this correlation. These results suggest the exciting possibility of using pulmonary vascular density as a direct measure of the therapeutic effect of anti-pulmonary hypertensive therapies.
Although this study includes a novel and objective evaluation of pulmonary vascularity and its relation to pulmonary hypertension in infants with BPD, it still has limitations. Our study is limited by our ability to capture vascularity and blood flow to a resolution of 0.7 mm which means our measurements do not include the distal pulmonary vessels or capillary beds. In addition, in the absence of lung biopsy, it is very difficult for us to compare our MRI-based vascular density with histological findings in lung tissue. The 2D-TOF MR images detect the presence of blood flow and therefore demonstrate only the size of interior lumen of the vessel and not the entire vessel. Although our measurements are not influenced by the amount of blood flow once it reaches a detectable level, the measured size of the vessel lumen may be influenced by temporary changes in vasodilation (although only four of the 33 patients were on vasodilator therapy at the time of MRI). Given the limited number of infants in our cohort, evaluation of nuances associated such as an open patent ductus arteriosus, atrial septal defects were limited but within our limited data, we saw no significant association between vascular density and these cardiac anomalies. Our findings are also limited by lack of comparison to the gold standard cardiac catheterization due to limited number of infants requiring this invasive procedure. Moreover, each MRI only gives a single snapshot of the vascularity at the time of MRI which may change with day-to-day variability or changes in clinical status. Serial MRI to measure these changes as a response to therapy or support offers exciting potential for future investigations.
In conclusion, our study suggests that neonatal MRI can be used to directly evaluate pulmonary vascularity in infants, providing a novel and direct measure of pulmonary vascular disease in infants with BPD. Infants with BPD experience altered vascular growth such that those with severe disease that require long term mechanical ventilation trended towards decreased vascular density. Finally, our results suggest anti-pulmonary hypertensive therapy with medications like iNO may increase vascular density in severe BPD infants and MRI may provide a direct measure of the efficacy of current and future anti pulmonary hypertensive therapies.