Introduction

Pulmonary arterial hypertension (PAH) is a disease characterized by right heart catheter measurement of mean pulmonary arterial pressure (mPAP) >20 mm Hg at rest, according to the recent 6th World Symposium.1 Over time PAH can lead to right ventricular failure and death in any age group. In infants and children, PAH can occur as a result of the chronic lung disease of prematurity, bronchopulmonary dysplasia (BPD). The World Health Organization (WHO) PH classification system places PAH associated with BPD into group 3,2 wherein the pulmonary vasculature is relatively contracted and/or remodeled, decreasing vessel diameter. Monitoring for PAH in high-risk populations such as BPD patients is crucial for early PAH diagnosis and management. Catheter-based hemodynamic measurements are invasive, usually requiring anesthesia, and therefore not ideal in many clinical situations, such as a patient who is small (neonatal/infant), has poor vascular integrity, and/or requires long-term right ventricular pressure (RVP) monitoring. Alternatively, RVP monitoring by pulsed-wave Doppler echocardiography using pulmonary artery acceleration time (PAT): ejection time (PATET) ratio is noninvasive, quantitative, and more easily assessed than tricuspid regurgitation measurement3 with high intra- and interobserver agreement.4 Since early diagnosis and treatment of PAH are vitally important to slow PAH development and prevent PAH-associated morbidity and mortality, PATET has evolved as a useful clinical predictor of PAH in many clinical settings in adults. However, its use is relatively new in small children, particularly infants. Therefore, the aims of this review are to (1) examine the use of PATET as a quantitative clinical tool to assess PAH in the current pediatric literature and (2) discuss the utility of PATET measurement in small animal models of PAH.

PAT and PATET ratio

PAT, also known as PAAT, PAcT, or time to peak velocity, is the time (in ms) it takes for blood from the right ventricle to exit through the pulmonary valve and reach peak velocity after the pulmonary valve opens during systole. PAT is inversely related to heart rate (HR),5,6,7 and adjustment for HR is made by dividing by the right ventricular ET ((RV)ET) to get the PATET.8 When PAP is elevated, as in the case of PAH, tachycardia has less of an effect on the PAT.9 The (RV)ET is the total time (in ms) it takes for blood from the right ventricle to exit through the pulmonary valve during one systolic cycle, from the opening to the closing of the pulmonary valve.10 The PATET then is the ratio of these two time intervals, which gives a unitless (ms/ms) proportion, whereby the smaller the proportion (shortening of the PATET), the less time per cycle is spent getting to peak velocity, indicative of higher mPAP. Stated another way, when mPAP is elevated, as measured by elevated right ventricular systolic pressure (RVSP), it will take a shorter time for blood flow from the pulmonary artery to reach maximum velocity (faster flow acceleration),11,12 and the PATET ratio will be decreased/shortened. In summary, the shorter the PATET, the worse the PAH, reflecting the indirect relationship of PATET and measures of PAH including RVSP, PAP, and pulmonary vascular resistance (PVR), as well as the direct relationship of PATET and measures of PAH including pulmonary arterial compliance and RV mechanical performance.3,5,7

Adult PAT

PAT research was first conducted in adults who normally have baseline HR < 100 b.p.m. The correction for HR with ET was, therefore, less of a concern and had not yet been introduced in the literature. The following studies are in adults and evaluated by transthoracic echocardiography unless otherwise noted. In 1983, Kitabatake et al. were the first to discover that PAT shortened as mPAP increased (mean PAT = 137 ms when mPAP <20 mm Hg, mean PAT = 97 ms when mPAP >20 mm Hg).12 Also, in the 1980s, the linear, inverse relationship between PAT and mPAP was established, within a PAT range of 74–170 ms.5,12,13,14,15 Currently, PAT is used either independently or in conjunction with tricuspid regurgitation velocity (TRV) to predict PAH,6,7,14,16,17 and PAT and TRVmax are inversely correlated.18 Granstam et al. confirmed that PAT is a clinically useful tool, especially in high-risk patients, and reported that a PAT < 100 ms is strongly associated with PAH.19 In 2015, Tousignant and Van Orman studied patients undergoing elective cardiac surgery, where simultaneous hemodynamic measurements by trans-esophageal echocardiography and catheter-based pulmonary capillary wedge pressure showed that PAT correlated with invasive hemodynamic parameters.20 In 2016, Cowie et al. also studied cardiac surgical patients and found that PAT < 107 ms predicted PAH.21 More recently, Zhan et al. utilized PAT to classify PAH severity (mild 70–90 ms, medium 50–70 ms, and severe <50 ms).22 Interestingly, adults exposed to hyperoxia (100% O2) had modestly lengthened PAT, whereas hypoxia (17% O2) had minimal effect.23 PAT can also be used to predict cardiovascular events in young adults.24

Children PAT and PATET

For children, the use of PAT to predict PAH can vary by age, body surface area, and HR. There is a positive correlation of PAT and age, PAT and body surface area, and a negative correlation of PAT and HR.7 Overall in children, detection of elevated PVR, decreased pulmonary arterial compliance, and RV dysfunction occurs at PAT <90 ms and PATET < 0.3,3,10 and PATET < 0.29 predicts PAH with 100% sensitivity and 96% specificity.6,7 In 1984, Kosturakis et al. reported a range of PAT in children of 51–140 ms.25 In a cohort of both children and adults with cystic fibrosis, PAT < 101 ms was proposed as a useful decision-making tool for clinicians regarding the timing of lung transplantation.26

Infant PAT and PATET

PATET and TRV are the major quantitative echocardiographic measurements in older children and adults to predict PAH. As patients become smaller, the TRV is more difficult to assess, thus the importance of measuring PAT and PATET. In both infants and children, PAT has been shown to predict PAP, PVR, and vascular compliance as measured by cardiac catheterization.3,6,10 In 2008, at a time when PATET was not yet routinely measured, Mourani et al. stated that the accurate estimate of catheter-based systolic PAP by echocardiographic quantitative measurements was much needed, especially in children <2 years of age.27 In 2020, PAT was found to correlate with systolic PAP as estimated by TRV in neonates and young infants using the equation: sPAP = 82.6 − 0.58 × PAT + right atrial (RA) mean pressure.28

Preterm infant PAT and PATET

A study by Carlton et al. in 2017 found that quantitative (PATET and TRV) echocardiographic measurements of PAH were more reliable among cardiologists than qualitative measurements in the assessment of pulmonary vascular disease in preterm neonates.29 Indeed, much of the PATET literature in infants has focused on preterm infants, with and without comorbidities such as bronchopulmonary dysplasia.

In healthy preterm infants, PATET increases longitudinally from birth to 1 year of age, indicative of the normal decrease in RV afterload that occurs postnatally.4 In the 1990s, Gill and Weindling studied the natural history of PATET in a cohort of very low birthweight infants and found that the PATET becomes greater over the first 3 weeks of life, corresponding to the normal drop in PVR. In addition, those patients who continued to require supplemental oxygen after 36 weeks postmenstrual age had a lower PATET than room air controls,30,31 and PATET could be used to predict preterm patients that would develop chronic lung disease.32,33 Levy et al. published PATET values for a cohort of preterm patients at 1 year of age and found that PATET was lower in patients with PAH than in those without PAH.3,5 In patients ≤1 year of age, some of which were preterm with BPD and/or patent ductus arteriosus (PDA), infant PAT was inversely related to mPAP, systolic PAP, and PVR.5 Interestingly, when BPD-PH patients received either 100% O2 or 20 p.p.m. nitric oxide, PATET lengthened, indicative of improved PAP.34

Prenatal PAT and PATET

In 1987, Machado et al. reported the first human fetal (16–30 weeks gestation) PAT, as well as aortic AT, and found that PAT was shorter than aortic AT, consistent with the fetal circulatory pattern.35 In 2003, Fuke et al. predicted pulmonary hypoplasia using shorter fetal PATET obtained antenatally (20–39 weeks gestation). Normal values for mean PATET were also obtained in both right (0.17) and left (0.15) fetal pulmonary arteries.36 Additional antenatal human fetal studies have shown that mean PAT and PATET lengthen with increasing gestational age (18 weeks: PAT = 24 ms, PATET = 0.14; 38 weeks: PAT = 36 ms, PATET = 0.20), likely reflecting normal fetal pulmonary vascular development, decreasing PVR, and increasing pulmonary blood flow.37,38,39

PAT and PATET in small animals

PAH has been studied in canines, which, like humans, can develop PAH in response to underlying respiratory disease and left-sided heart disease. In addition, canine PAH is commonly associated with degenerative valve disease and parasites, such as heartworms. Veterinarians routinely use echocardiography clinically to diagnose and assess PAH; however, this can be challenging given the diverse number of species, and factors such as size, breed, and anatomical complexities.40 The American College of Veterinary Internal Medicine recently published a consensus statement providing a guideline for the diagnosis, classifying, treating, and monitoring of canine PAH.40 One of the recommendations includes evaluating anatomical sites, such as the pulmonary artery using echocardiography. RV outflow doppler time (similar to PAT) < 52–58 ms or PATET < 0.30 has been shown to suggest a higher probability of PAH in canines.40,41,42,43 In addition, systolic notching of the Doppler RV outflow profile can assist in determining PAH in veterinary species.40

Early studies in mice had difficulty visualizing the heart by echocardiography because of high heart rate (>400 b.p.m.) and small size.44,45 However, recent advances in technology, as well as comprehensive protocols to assess the right heart in mice, have overcome these barriers with great potential to advance the study of PAH in mouse models.46,47,48 Echocardiographic probes are now miniaturized with frequencies >30 MHz designed for rodents.49,50,51 Echocardiographic landmarks in mice that are relevant to PAH include right ventricle, right ventricular outflow tract (PAT measurement), and tricuspid valve (TRV measurement).52 PAT and TRV can then be used to calculate PVR.53,54,55

In a seminal article by Thibault et al. in 2010, mice underwent echocardiography and PATET was compared to pressures obtained from right ventricle catheterization at 3 months of age, which are currently the youngest mice studied using catheterization and echocardiography. It was determined that PATET < 0.39 detected elevated RVSP (>32 mm Hg) with sensitivity of 100% and specificity of 86%, while keeping both intra- and interobserver variability at <6%.56 This demonstrated that PATET by echocardiography could be used to accurately assess PAH in mice >3 months of age. However, the question still remains as to whether PATET by echocardiography can be utilized to assess PAH in mice <3 months of age.

Recently, PAT and PATET have been routinely measured in several PAH mouse models, adult and neonatal. Although the great strength of mouse models is the ability to interrogate genetic molecular targets causal to the disease phenotype of interest, many of the PAH mouse models are environmental exposures. A study in adult mice exposed to 3 weeks of hypoxia and repeated exposure to SU5416 (VEGF receptor inhibition) showed shortening of the PATET, as well as increases in RV wall thickness measured in diastole, compared to control.44 In a study by Zhu et al. 2019, an 8-week hypoxic mouse model and two rat models (hypoxia/sugen and MCT) showed shortening of the PATET compared to control.57 Adiponectin knockout mice have shortening of the PATET and evidence of PAH at 1 year of age.58 PATET has also been used to longitudinally evaluate adult mice with a congenital diaphragmatic hernia for PAH.59 Reynolds et al. in 2016, evaluated newborn C57BL/6J mice in 70% O2 at 14 days and found evidence of shortening of the PATET, demonstrating PAH in the youngest cohort of mice to be reported at this time.60

Experiment: PAT and PATET in a PAH hypoxic mouse model

We recently evaluated PATET by echocardiography55 in anesthetized 28-day-old C57BL mice before, during an acute hypoxic exposure-induced PAH, and recovery in hyperoxia (50% O2) (Fig. 1). We measured appreciable shortening (by 18%) of the PATET during acute hypoxic exposure (mean PATET = 0.211) compared to pre-hypoxia (mean PATET = 0.257), and recovery of PATET with hyperoxia at 1 min, p = 0.014). These data confirm that PATET can be reliably repeatedly measured in 28-day-old mice and that shortening of the PATET, consistent with elevated PAP, can be observed in acute exposure to hypoxia. These data confirm previous studies56,60 that PAT and PATET measurements are important tools to assess PAH even in very young mice. A mouse model such as this will be very useful in studying the progression of PAH disease, particularly in the context of models of human disease such as persistent pulmonary hypertension of the newborn or BPD.

Fig. 1: PATET shortening in PAH hypoxic mouse model.
figure 1

Echocardiography with PATET measurement in 28-day-old C57BL mice before (pre-21% O2), during hypoxia (10% O2), and recovery with hyperoxia (50% O2). Mice were anesthetized with inhaled 1–3% isoflurane during the experiment. Each mouse (N = 9) had 3–10 data points that were averaged prior to the calculation of mean ± SE presented. Repeated-measures one-way ANOVA with Tukey’s multiple comparisons test. *10% O2 vs. 50% O2—1 min, p = 0.014; 10% O2 vs. 50% O2—3 min, p < 0.001; 10% O2 vs. 50% O2—10 min, p < 0.01.

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PAT and PATET limitations

There are general limitations of small animal models, including the areas of studying breathing mechanics, gas exchange, and pulmonary hemodynamics,61 especially catheterization. Because of the difficulty to catheterize extremely small neonatal mice, we focus on developing PATET by echo as a potential solution. However, there are several limitations and/or considerations that need to be addressed when analyzing PAT and PATET data. The operator technique is important in acquiring data from Doppler echocardiography, including operator variation in determining the optimal angle between the transducer and pulmonary artery.62 This is an issue especially with smaller individuals, and it has been shown that the measurement of blood velocity decreases as the misalignment in the angle increases, and at >20% difference clinically the test is no longer considered accurate.46,63 Another consideration is that PAT measurements when expressed as a range are not continuous because the Doppler waveform is derived from intermittent Fourier analysis.23 Some researchers have proposed magnetic resonance imaging (MRI) as a better modality to estimate PATET,62 and at least three studies in patients with bronchopulmonary dysplasia have determined that another quantitative measurement, eccentricity index, may be a better measure of right ventricular mechanics.64,65,66 Guidelines for measuring cardiopulmonary physiology by both echocardiography and MRI have been published and are an important step forward in the development of preclinical animal models.67,68 Despite superior imaging of MRI and micro-computed tomography,69 the primary advantage of echo assessment is that the echo probe technique and PATET measurement can be (1) studied on a mouse prior to clinical use on a neonatal patient and (2) measured in real-time (functional echocardiography 70) with relatively little anesthesia. Anesthesia can have negative impacts include hypercarbia, acidosis, hypothermia, alteration to ventilation/perfusion matching, tidal volume, and hypoxia resulting in pulmonary vasoconstriction.71,72,73 Any of these changes could alter the PAT and PATET ratio, but have so far not been studied. In addition, exploration of alternative anesthetic protocols to achieve a reduction in minimum alveolar concentration for volatile agents and their negative impacts is needed.

It is important to note that PAP estimation by maximal TRV in the absence of pulmonary stenosis continues to be the gold standard echocardiographic measurement to estimate systolic PAP.5,11,74,75,76 However, the TR envelope is not accessible much of the time,5,77,78 whereas PATET is accessible in >90% of patients.11,68 In mice, TR occurs only in severe PH, and because of problems with flow alignment, measurements by echo are prone to inaccuracy.45,60 In addition, studies in both children and adults have put the TRV under scrutiny, with some published data that TRV does not accurately predict right heart catheterization measurements.10,79

In preterm neonates, PATET may not be useful in the context of PDA with left-to-right shunting. PDA causes turbulent flow in the pulmonary artery, potentially altering PATET measurement.5,6,10,80 Gaulton et al. studied 57 infants (77% with PDA) who had both an echocardiogram and catheterization done within 10 days of each other and found that there was a correlation between PAT and PAP at catheterization, but no correlation between PATET and PAP at catheterization,7 highlighting the ambiguity of data interpretation in the context of PDA.

Discussion on the correction for HR made in the calculation of PATET was presented earlier under the heading “PAT and PATET ratio.” However, further adjustments have been proposed by correcting for the RR interval. Tossavainen et al. in 2013 corrected for HR by calculating a corrected PAT using the formula PAT/√RR interval, finding in adults that a corrected PAT < 90 ms is a good predictor of PVR > 3 Wood units.17 Interestingly, in neonatal patients, studies have not shown a correlation between PAT or PATET and RR interval.10,80,81

Besides HR and large left-to-right shunts, RV dysfunction due to reasons other than PAH can also confound the PATET, such that the PATET should routinely be interpreted in the context of measurements of RV systolic function and shunts.80 There are also considerations specific to the fetus including early RV and LV developmental effects on intrinsic contractile properties, vascular impedance, the capacitance of the vascular bed, and ventricular preload, which should be taken into consideration while interpreting the fetal PATET.82

Conclusions

PAT and PATET are useful tools for the study of PAH in neonates, infants, and children, although further studies are needed to determine optimal cut-off values that have a robust correlation with PAH as diagnosed by catheterization. Furthermore, studies are needed to differentiate normal developmental changes from changes that are attributable to disease progression. In terms of preclinical studies, the mouse model is of particular interest given its widespread use for studying neonatal diseases and developmental changes. Further study to establish PATET as a routine and accurate measure of PAH in small and large animal models is needed to provide a noninvasive, longitudinal measure of PAH. For preclinical studies in neonatal mice to study PAH in bronchopulmonary dysplasia, early time points (day of life 14–28), when catheterization may be limited, validation of PATET against Fulton’s index and/or pulmonary vascular histology may be necessary. Consideration of neonatal functional echo studies70 that include PATET as a real-time tool for a bedside PAH evaluation that is reliable and accurate would greatly advance assessment and treatment plans for these very sick infants.