Elevated Aortic Stiffness after Pediatric Heart Transplantation

Abstract

In adults, arterial stiffness has been linked to the development of target end-organ damage, thought to be related to abnormal transmission of pulse pressure. Increased arterial stiffness and endothelial dysfunction have been hypothesized to contribute to the development of microvascular dysfunction and coronary allograft vasculopathy (CAV), an important comorbidity after heart transplantation. However, little data exists regarding arterial stiffness in pediatric heart transplantation and its influence on development of coronary allograft vasculopathy is not well understood. We sought to assess aortic stiffness and distensibility in pediatric post-heart transplant patients. A prospective, observational study analyzing the ascending (donor tissue) and descending aorta (recipient tissue) using transthoracic echocardiographic M-mode measurements in patients aged < 21 years was conducted. Descending and ascending aorta M-modes were obtained from the subcostal long axis view, and the parasternal long axis view 3–5mm above the sinotubular junction, respectively. Two independent reviewers averaged measurements over 2–3 cardiac cycles, and Aortic Distensibility (AD) and Aortic Stiffness Index (ASI) were calculated using previously validated methods. We recruited 39 heart transplant (HT) patients and 47 healthy controls. Median end diastolic dimension of the ascending aorta (donor tissue) was significantly larger in the transplant group than the control group (1.92 cm vs. 1.74 cm, p = 0.01). Ascending aortic distensibility in post-transplant patients was significantly lower than in the control group (4.87 vs. 10.53, p < 0.001). Ascending aortic stiffness index was higher in the transplant patients compared to the controls (4.63 vs. 2.21, p < 0.001). There is evidence of altered ascending aortic distensibility and stiffness parameters in post-heart transplant patients. Further studies are required to assess its influence on complications like development of coronary artery vasculopathy.

Introduction

Elevated arterial stiffness may have important adverse cardiovascular implications, and its association with an increased risk of cardiovascular disease and end-organ damage has been recognized in adults [1, 2]. Increased central arterial stiffness happens with aging and secondary to disease processes such as diabetes, atherosclerosis and chronic renal disease. A compliant blood vessel limits excessive transmission of arterial pulsatility and protects the microvascular circulation. Increasing stiffness of larger blood vessels can affect organs with low vascular resistance which are more susceptible to injury secondary to excessive transmission of pulsatile energy, Fig. 1 [1]. These changes can also lead to isolated systolic hypertension, increased left ventricular afterload and reduction in coronary perfusion pressure, further promoting left ventricular remodeling, dysfunction, and failure [3, 4]. These complications are of particular interest in patients after heart transplantation, as alterations in arterial stiffness have been described in adults after heart transplantation with lower small and large artery elasticity compared to healthy controls by contour analysis of radial artery pulse waveforms [5]. However, there is paucity of data in pediatric transplant patients.

Fig. 1.

The role of large arterial vessel stiffness in health and disease and consequences of increased arterial stiffness on arterial waveform and target end-organ perfusion. [1] Reproduced with permission from Chirinos JA, et. al: Large-Artery Stiffness in Health and Disease: JACC State-of-the-Art Review. J Am Coll Cardiol 2019;74:1237–63

Increased arterial stiffness may lead to endothelial dysfunction, hypothesized as a mechanism for the development of coronary microvascular dysfunction [4, 6]. Microvascular dysfunction is as an independent risk factor for the development of CAV after transplantation, an important predictor of graft failure with a reported 50% survival 5 years after detection [7].With the paucity of donor organs and relatively stable number of yearly pediatric heart transplants, there is increasing need to develop better non-invasive diagnostic modalities for longitudinal follow up post-transplant and risk stratification. Various factors such as chronic endstage heart failure, neurohumoral alterations, peri-transplant factors such as preservation, and other donor, recipient and post-transplant variables can impact arterial health. Further insight into arterial health may provide a window of opportunity to understand the pathogenesis of complications post-transplant. Non-invasive technologies such as thenar muscle tissue oxygenation levels have shown promise for assessment of microvascular dysfunction in pediatric heart failure patients [8].

Given the association of elevated arterial stiffness and its effect on overall cardiovascular health, we aimed to examine arterial stiffness in donor tissue (ascending aorta) and native tissue (descending aorta) in pediatric heart transplant patients and compare with arterial stiffness in healthy controls. We hypothesized that donor tissue would have higher arterial stiffness compared to controls.

Materials and Methods

The study was a prospective, observational study performed at the pediatric heart transplant center at the University of Florida in Gainesville, Florida, USA. Heart transplant recipients and control patients aged 0 through 21 years of age were enrolled in the study. Heart transplant patients were enrolled from both the inpatient and outpatient setting, while healthy controls patients were enrolled from outpatient cardiology clinic. Any patients with history of Kawasaki Disease, Multisystem Inflammatory Syndrome in Children, rheumatologic or connective tissue disease, heart re-transplantation, and multi-organ transplantation were excluded. In addition, controls did not have a history of malignancy, chemotherapy, radiation, renal disease or diagnosed hypertension. Demographic data including immunosuppression medications, cardiac catheterization data, and laboratory parameters were collected from the electronic medical record. The University of Florida Institutional Review Board approved the study with a waiver of consent.

Arterial Stiffness Assessment

The ascending aorta (3–5 mm distal to sinotubular junction) and the descending aorta at the level of the diaphragm were used to investigate arterial stiffness. In transplant patients, ascending aortic and the descending aortic stiffness were considered reflective of the donor tissue and native tissue, respectively. The descending aortic M-mode images were obtained from the subcostal long axis view above the celiac artery (Fig. 2a) and the ascending aortic M-mode images were obtained from the parasternal long axis view 3–5 mm above the sinotubular junction (Fig. 2b). We performed M-mode imaging as perpendicular to the vessel as possible. Echocardiographic studies were performed using GE Vivid E9/E95, Horten, Norway and Epiq Philips, Eindhoven, The Netherlands. Two independent reviewers (JAC and HVV) performed the absolute measurements over 2–3 cardiac cycles, which were averaged to minimize errors. Both reviewers reviewed any discrepancy in the measurements simultaneously and a consensus was reached. Aortic Distensibility (AD) and Aortic Stiffness Index (ASI) were calculated utilizing the following equations from previously validated methods [9]:

$$\begin{array}{l}\text{Aortic}\text{Distensibility}(\text{AD})\hfill \\ =2(\text{Aortic}\text{dimension}\text{end}\text{systole}\hfill \\ -\text{Aortic}\text{dimension}\text{end}\text{diastole})\hfill \\ ÷((\text{Aortic}\text{end}\text{diastole})(\text{Systolic}\text{Blood}\text{Pressure}\hfill \\ -\text{Diastolic}\text{Blood}\text{Pressure}))\hfill \end{array}$$

$$\begin{array}{l}\text{Aortic}\text{Distensibility}(\text{AD})\hfill \\ =2(\text{Aortic}\text{dimension}\text{end}\text{systole}\\ -\text{Aortic}\text{dimension}\text{end}\text{diastole})\\ ÷((\text{Aortic}\text{end}\text{diastole})(\text{Systolic}\text{Blood}\text{Pressure}\\ -\text{Dialstolic}\text{Blood}\text{Pressure}))\end{array}$$

Fig. 2.

a Descending aorta M-mode images (recipient) were obtained from the subcostal long axis view. b Ascending aorta M-mode images (donor) were obtained from parasternal long axis view 3–5mm above the sinotubular junction

Statistical Analyses

All analyses were performed using the R statistical software package (Vienna, V. 3.1.3). Data was analyzed using Mann–Whitney and Fisher’s exact tests, with p values ≤ 0.05 considered statistically significant.

Results

A total of 86 patients were enrolled from March 2021 through November 2021 of which 39 patients were post-heart transplantation. Table 1 depicts the demographic information of the two groups, which demonstrates a well-matched control group. Of note, the post-transplant patients demonstrated a higher diastolic blood pressure compared to healthy controls (p 0.03).

Table 1.

Demographic data arranged in quartiles for transplant and control patients

	Transplant (n = 39) Median (IQR 25–75)	Control (n = 47) Median (IQR 25–75)	p-value
Median Age [IQR], years	11 (6.5–17.5)	12 (7–14)	0.56
Males, n (%)	26 (66%)	22 (47%)	0.08
Hispanic/Latino, n (%)	7 (2%)	3 (6%)	0.17
Race, n (%)			0.19
African American	14 (35%)	12 (25.5%)
White	23 (59%)	27 (57.4%)
Weight, kg	41 (20–60)	44 (25.65–59.7)	0.69
Height, cm	1.47 (1.21–1.68)	1.5 (1.31–1.65)	0.95
BMI, kg/m2	18.2 (16.1–22.7)	19.4 (16.9–23.1)	0.65
Median SBP [IQR], mmHg	111 (103–118)	108 (97.5–113)	0.08
Median DBP [IQR], mmHg	67 (60.5–72)	62 (57.5–66.5)	0.03

BMI Body Mass Index, SBP systolic blood pressure, DBP diastolic blood pressure

Clinical features of transplant patients are depicted in Table 2. Overall, 18/39 patients had congenital heart disease and 21/39 had cardiomyopathy as the reason for transplantation. The median time from transplant at the time of evaluation was 3 years [IQR 25–75: 1–9 years], 76% patients had a history of rejection and 38% had a history of pre-transplant mechanical circulatory support. The median ischemic time was 210 min [IQR 25–75: 179–234]. The medications that the patients were on at the time of the evaluation are listed in Table 2, and are not mutually exclusive. One patient had chronic kidney disease without the need for renal replacement therapy, and two patients had controlled diabetes mellitus post transplantation. There was no statistically significant difference between donor height, weight and body surface area and controls.

Table 2.

Clinical details of post-transplant patients

Clinical Parameters	Post-transplant (n = 39)
Congenital Heart Disease	46% (18/39)
Median Time From Transplant (years, [IQR 25–75])	3 [1–9]
History of Rejection, n (%)	76% (30/39)
MCS Prior to Transplant, n (%)	38% (15/39)
Donor Ischemic Time (minutes, [IQR25–75])	210 [179–234]
Diuretics, n	28% (11/39)
Aspirin, n	53% (21/39)
Anticoagulation, n	7% (3/39)
Statin, n	15% (6/39)
ACEI, n	43% (17/39)
ARB, n	5% (2/39)
CCB, n	13% (5/39)
BB, n	10% (4/39)
Immunosuppression
Tacrolimus, n	97% (38/39)
Sirolimus, n	23% (9/39)
MMF, n	66% (26/39)
Steroids, n	41% (16/39)
Azathioprine, n	5% (2/39)

ACEI angiotensin converting enzyme inhibitor, ARB angiotensin receptor blocker, CCB calcium channel blocker, BB Beta Blocker, MMF mycophenolic acid

Table 3 depicts the comparison of aortic measurements and arterial stiffness parameters between the post-transplant and control groups. The median end diastolic dimension of the ascending aorta (EDA) was significantly larger than the control group (1.92 cm vs. 1.74 cm, p = 0.01). The ascending aortic distensibility index (donor tissue) was lower than in the control group (4.87 vs. 10.53, p < 0.001). Additionally, the ascending aortic stiffness index was higher in the transplant patients compared to the controls (4.63 vs. 2.21, p < 0.001).

Table 3.

Aortic measurements and arterial stiffness parameters in the transplant and control patients

	Transplant (n = 39) Median (IQR 25–75)	Control (n = 47) Median (IQR 25–75)	p-value
ESD (cm)	1.32 (1.02–1.56)	1.4 (1.11–1.61)	0.22
EDD (cm)	1.09 (0.87–1.36)	1.18 (0.96–1.38)	0.45
ESA (cm)	2.13 (1.86–2.44)	2.02 (1.77–2.33)	0.35
EDA (cm)	1.92 (1.62–2.21)	1.74 (1.49–1.89)	0.01
ADD (/mmHg × 10−3)	7.36 (4.89–9.58)	8.72 (6.01–12.27)	0.05
ADA (/mmHg × 10−3)	4.87 (3.34–8.22)	10.53 (6.02–13.46)	< 0.001
ASI Descending Aorta	3.33 (2.56–4.71)	2.73 (1.95–4.36)	0.08
ASI Ascending Aorta	4.63 (3.06–6.70)	2.21 (1.91–3.95)	< 0.001

ESD end systolic descending, EDD end diastolic descending, ESA end systolic ascending, EDA end diastolic ascending, ADD aortic distensibility descending, ADA aortic distensibility ascending, ASI aortic stiffness index

In our study, only two patients in the transplant group had angiographic evidence of CAV (angiographic and IVUS positive), and they were also the two that had IVUS Stanford Grade III or higher disease in our cohort. Due to the small sample size, we could not deduce meaningful correlation between arterial stiffness parameters and CAV in our cohort of patients.

Among the post-transplant patients, we analyzed the use of angiotensin converting enzyme inhibitors (ACEi), angiotensin receptor blockers (ARBs), statins, and aspirin and also the use of ACEi or ARB on the absolute measurements of the aorta and the stiffness parameters. Among this analysis, the absolute measurements of the descending aorta (end systole and end diastole) were larger in patients who were on statins (p = 0.04) and the other parameters were not statistically significant. Table 4 demonstrates these findings among statin use in our transplant cohort. There was no statistical difference between donor/transplant weight ratio and statin use. There was also no significant difference between any of the variables and time from transplant.

Table 4.

Aortic measurements in the transplant patients on statins

	No Statins (N = 33) Median (IQR 25–75)	Statin Use (N = 6) Median (IQR 25–75)	p-value
Height (m)	1.34 (1.08–1.64)	1.68 (1.62–1.70)	0.06
Weight (kg)	29 (20–58)	70.5 (64.5–76.8)	0.01
Age (years)	9.9 (6.1–14.6)	18.5 (15.7–20.1)	0.01
ESD (cm)	1.25 (0.96–1.5)	1.57 (1.46–1.66)	0.04
EDD (cm)	1.09 (0.83–1.3)	1.35 (1.28–1.4)	0.04
ESA (cm)	2.12 (1.84–2.34)	2.46 (2.14–2.76)	0.08
EDA (cm)	1.89 (1.6–2.14)	2.26 (1.98–2.43)	0.05
ADD (/mmHg × 10−3)	7.36 (4.77–9.68)	6.69 (5.48–8.19)	0.05
ADA (/mmHg × 10−3)	5.43 (3.43–8.32)	3.52 (2.77–4.10)	0.2
ASI Descending Aorta	3.34 (2.59–4.78)	3.05(2.55–3.69)	0.69
ASI Ascending Aorta	4.16 (2.93–5.69)	6.21 (5.31–7.26)	0.26

ESD end systolic descending, EDD end diastolic descending, ESA end systolic ascending, EDA end diastolic ascending, ADD descending aortic distensibility, ADA ascending aortic distensibility, ASID aortic stiffness index descending, ASIA aortic stiffness index ascending

Discussion

Our study is among the first to report alterations in arterial stiffness in pediatric heart transplant patients compared to healthy controls using a non-invasive imaging modality. With increasing evidence linking arterial stiffness and target end-organ damage in the adult population, it is important to assess how this can provide insight into the outcomes of pediatric patients, and, in particular, heart transplant patients. While many ways to measure arterial stiffness exist, we sought to use echocardiography as it is a non-invasive and widely available tool which can be easily performed in a variety of inpatient and outpatient settings. We used the aortic end systolic and end diastolic dimensions for the calculation of various parameters as these have been previously validated and showed to be reproducible using transthoracic echocardiography [9, 10]. In literature, to accurately describe the properties of the artery, both the arterial stiffness index and distensibility are reported together, as done in our study.

Hauser et. al, attempted to determine baseline normative arterial stiffness data while others have assessed aortic stiffness in patients with Kawasaki Disease and children with bicuspid aortic valves [10, 11]. In patients with Kawasaki disease, there is evidence of elevated arterial stiffness with no association to the acute phase coronary disease [12]. Previous studies in pediatric heart transplant patients using multi-site plethysmography pulse measurement and analysis, and pulse wave velocity measured using high-fidelity applanation tonometry have shown evidence of increased arterial stiffness [13, 14]. Using cardiac MRI, Hussain et. al demonstrated increased central aortic stiffness after transplantation and independent correlation between elevated pulse wave velocity and CAV in pediatric heart transplant patients [15]. Compared to previous studies, we measured arterial stiffness using a clinical assessment routinely used in this population, thus demonstrating the feasibility of our approach and proof of concept. Our study was not adequately powered to deduce a correlation between CAV and arterial stiffness. In the adult heart transplant population, Patel et. al assessed arterial stiffness in post-transplant patients who were supported on continuous flow ventricular assist devices. They showed that the aortic stiffness was markedly increased immediately post-transplant with attenuation over the first-year post-transplant, highlighting the dynamic nature of aortic stiffness [9]. In contrast to their study, arterial stiffness parameters in our patient’s did not differ with history of ventricular assist device use and as a function of time since transplant. However, we did not investigate the temporal changes in arterial stiffness in individual patients over time.

We also investigated if medications that have been shown to affect arterial pressure and endothelial function affected any of these parameters among the post-transplant cohort. Adjusting for ACEi or ARBs in a patient, we found no significant difference in the parameters. We did find significant difference in the absolute measurements of the descending aortic end systolic and end diastolic dimensions among those with statin use. Further studies will be needed to analyze if this changes overtime and with the duration of statin use. However, this could also reflect higher statin use in older patients.. It has been well established that statins do favorably affect arterial stiffness. However, these results need further validation as there were only six patients with statin use (Table 2) in our cohort. Our study demonstrates changes in patients taking these medications but did not show the degree of significance that has been shown in previous studies [16, 17].

Although not a primary aim, our study did demonstrate higher diastolic blood pressure measurements among transplant patients. Similar to our findings, Klinge et. al demonstrated elevated blood pressures in a small number of pediatric heart transplant patients in a cross-sectional study [14]. The exact cause and effect of the elevated diastolic pressures is something that requires further investigation in this population. Is it that they require higher pressures to ensure sufficient coronary perfusion or is it that abnormal arterial stiffness parameters lead to elevated blood pressure? Additionally, Klinge and colleagues showed that the pulse wave velocity in brachial and aortic arteries using high-fidelity applanation tonometry was elevated in heart transplant patients. Additionally, these pulse wave findings also correlated with time from transplantation, leading to the conclusion that arterial rigidity is higher in post-transplant patients [14]. In our study, we did not find an association between time from transplant and the degree of arterial stiffness; this assessment is limited by the lack of longitudinal data in an individual patient and our small sample size. Also, Bansal et. al, have demonstrated that 60% of pediatric heart transplant patients have evidence of masked hypertension, predominantly isolated nocturnal hypertension [18]. Our study further adds to these important findings, utilizing a non-invasive method in heart transplant patients. However, it still stimulates an important question whether this has any long-term impact on other complications like CAV further influencing graft survival.

There is a need for innovative, routine non-invasive methods to monitor transplant patients to help predict outcomes. Data from our study highlight significant alterations in arterial stiffness in pediatric heart transplant patients and may provide an additional tool for clinicians to follow these patients serially. Additional serial follow-up data is required to study changes in arterial stiffness over time and its possible association with development of complications like CAV, allowing better risk stratification for patients.

Our study has several limitations including it being conducted at a single center study. The quality of the M-mode images can be influenced by the consistency of the angle during image acquisition thereby can impact the measurements. Prior to the study, we reviewed with our sonographers the best angle to obtain images to minimize variability and adherence to a standard image acquisition protocol. Our patients were enrolled from inpatient and outpatient settings; however, none of the patients had acute hemodynamic compromise. Another limitation is that we assumed that vessel wall properties have a linear relationship to pressure (as done in previously reported studies in the literature [19, 20]. Limited data was available for coronary flow reserve or other invasive measures, such as index of microcirculatory resistance, fractional flow reserve for assessing the microvascular coronary disease. This may have led to an unintentional underestimation of the disease burden of CAV in our patients limiting our ability to investigate the association of arterial stiffness and abnormal coronary microcirculation. Despite these limitations, we believe that the ability to assess arterial stiffness using transthoracic echocardiography is a valuable resource and would allow for serial assessment of post-transplant patients.

In conclusion, our study demonstrated elevated ascending aortic arterial stiffness and reduced aortic distensibility in pediatric heart transplant patients, using non-invasive tools. This study emphasizes the need for multi-institutional, longitudinal study with a larger patient population which will allow a better understanding of the implications of elevated arterial stiffness post-transplant, temporal changes and potentially provide insight into early diagnosis of significant complications such as CAV in pediatric post-transplant patients.