Ultrasonographic assessment of abdominal aortic elasticity in hypertensive dogs

Abstract Background Systemic hypertension (SH) is a persistent and pathological increase in arterial blood pressure (BP). Chronic SH leads to an increase in aortic (Ao) stiffness, and measuring Ao elasticity is useful for estimating Ao stiffness in humans. Currently, no literature in veterinary medicine describes noninvasive assessment of abdominal Ao elasticity in dogs with SH. Objective Compare ultrasonographic‐derived abdominal Ao strain (AoSt) between hypertensive (HT) and normotensive (NT) dogs. Animals Fifty privately‐owned dogs with clinical signs, conditions, or both potentially associated with SH. Methods Prospective observational case‐control study. Aortic stiffness was estimated by calculating AoSt as follows: AoSt = ([AoDs − AoDd]/AoDd) × 100, where AoDs and AoDd are the Ao diameter in systole and in diastole, respectively. Aortic stiffness was calculated from 2 different Ao transverse sections, the first caudal to the left renal artery (K_AoSt), and the second cranial to the external iliac arteries (I_AoSt). Results Thirty‐two dogs were included in the HT group and 18 in the NT group. Both K_AoSt and I_AoSt in HT dogs were significantly lower (P < .05) than in NT dogs (7.4 ± SD 3.6) vs 10.3 (±3.8) and 5.7 (interquartile range [IQR], 3.9‐7.5) vs 8.1 (IQR, 7‐10.3), respectively. Only K_AoSt was significantly influenced by age. Conclusions and Clinical Importance Ultrasonographic Ao elasticity assessment was feasible to compare HT and NT dogs. Results indicated that K_AoSt and I_AoSt indices can be used to assess SH‐related Ao stiffness, especially when indirect BP measurements are inconsistent or inaccurate. Additional studies to assess the AoSt in healthy dogs of various ages are needed.

Affected target organs include the kidneys, eyes, central nervous system, heart and vessels. 1 Early and accurate diagnosis of SH is essential to minimize TOD. In clinical practice, Doppler and oscillometric devices are commonly used to noninvasively and indirectly estimate BP. 1 However, excessive movements or tremors can make it difficult to measure BP using such devices. Moreover, anxiety or excitement can induce situational SH in dogs and cats, leading to an erroneous diagnosis of SH. 1 The aorta (Ao) is the main distributing artery of the animal body.
It dampens the pressure pulsations generated by intermittent left ventricular ejection, and transforms the pulsatile flow into continuous blood flow. During systole, the volume of blood ejected into the Ao dilates the vessel and promotes the storage of elastic energy within its walls. This energy then is returned during diastolic recoil, which forces blood into the circulatory system.
The dampening function is a consequence of the elastic properties of the Ao walls that allow it to dilate during systole and recoil during diastole. 2 The elastic properties of the Ao are determined mainly by the greater proportion of elastin fibers versus smooth muscle and collagen contained in its walls. 2,3 In both humans and animals, chronic SH causes structural alterations in the arterial walls, characterized by smooth muscle cell hypertrophy and an increase in collagen. [4][5][6][7] This pathological remodeling results in an increase in Ao wall thickness and stiffness. [8][9][10][11] As Ao stiffness increases, Ao elasticity decreases. Several noninvasive diagnostic methods have been used to measure Ao elasticity in human medicine. [11][12][13][14][15][16][17][18][19][20][21] In small animal practice, noninvasive measurement of Ao elastic properties could help clinicians differentiate between secondary or idiopathic SH and situational SH, as well as to overcome measurement difficulties caused by animal movements and tremors.
Our main objective was to assess abdominal Ao elasticity using ultrasonographic-derived Ao strain (AoSt) in hypertensive (HT) and normotensive (NT) dogs. We also evaluated the effect of age, sex, reproductive status, body weight (BW), and heart rate (HR) on AoSt.

| MATERIALS AND METHODS
The study was an observational case-control study carried out at the Veterinary Teaching Hospital of the University of Sassari. The local Ethical Committee of the University of Sassari (OPBA) approved the study protocol, and all owners signed an informed consent form before enrollment of their animals. Dogs with clinical signs or conditions potentially associated with SH were prospectively included in the study.
The following diseases or conditions were considered potentially associated with SH: chronic kidney disease (CKD), acute kidney injury (AKI), spontaneous hyperadrenocorticism (HAC), diabetes mellitus (DM), hypothyroidism, ultrasonographic evidence of adrenal neoplasia, and glaucoma. 1 The following clinical signs were considered potentially associated with SH: acute onset of blindness, hyphema, 1,22-24 and epistaxis 25 as well as intracranial neurological signs characterized by acute or hyperacute onset, suggestive of cerebrovascular diseases, such as seizures, altered mentation, altered behavior, disorientation, ataxia, head tilt, and nystagmus. 1,24,26,27 Traumatized dogs and dogs receiving antihypertensive agents such as angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, calcium channel blockers, α 1 -blockers, hydralazine, spironolactone, β-blockers, thiazide and loop diuretics were excluded from the study. Dogs under treatment with drugs known to induce secondary SH (eg, glucocorticoids, phenylpropanolamine, toracenib phosphate) and dogs receiving anesthetic, sedative or opioid drugs during the 12 hours before starting the study procedures also were excluded. Diagnosis of CKD, AKI, spontaneous HAC, DM and hypothyroidism was based on a combination of anamnestic, clinical, laboratory, and ultrasonographic variables consistent with these diseases. Azotemia was defined as serum creatinine concentration ≥1.8 mg/dL (upper laboratory reference value). Proteinuria was defined as being persistent when a urinary proteinto-creatinine ratio of >0.5 was found repeatedly in ≥3 specimens obtained ≥2 weeks apart. 29 Ultrasonographic changes consistent with CKD were increased echogenicity, irregular contour, decrease or absence of corticomedullary distinction, decreased kidney size, abnormalities in kidney shape or architecture, or a combination of these. 30 A diagnosis of AKI was made if an acute onset of clinical signs attributable to AKI (anorexia, vomiting, diarrhea) was associated with ≥2 of the following criteria 31 : (1) presence of renal azotemia persisting at least 24 hours after correction of prerenal factors in a previously healthy dog; (2) ultrasonographic findings compatible with AKI, such as perirenal free fluid, and hyperechoic or enlarged kidneys or both; (3) increase in serum creatinine concentration >0.3 mg/dL or >25% from documented baseline during a 48-hour interval in the absence of prerenal factors; (4) persistent pathological oliguria or anuria (<1 mL/kg/min) after volume repletion; and (5) evidence of acute renal tubular injury on urinalysis (renal glucosuria, urinary casts).
A diagnosis of spontaneous HAC was made based on the presence of clinical signs and laboratory findings suggestive of HAC associated with a positive result on a low-dose dexamethasone suppression test. 32 Diabetes mellitus was diagnosed by persistently marked hyperglycemia (plasma glucose concentration >250 mg/dL) and glucosuria in dogs with clinical signs consistent with the disease. 33

| Statistical analysis
An ad hoc electronic form was used to collect all study variables.
Qualitative variables were described using absolute and relative frequencies, whereas quantitative variables were summarized by means and SDs or medians and interquartile range (IQR) for parametric and nonparametric distributions, respectively. Chi-squared or Fisher's exact tests were used to detect any statistical differences in the comparison of qualitative variables between HT and NT dogs. In-between group comparisons of quantitative variables were performed using the Student t tests and the Mann-Whitney test for parametric and nonparametric variables, respectively. Linear regression analyses were carried out to assess the relationship between sex, reproductive state, age, BW, HR, and AoSt.
Within-day and between-day intraobserver I_AoSt and K_AoSt measurement variability was assessed using the interclass coefficient correlation (ICC), which was reported with 95% confidence intervals     Table 2). Intraobserver within-day and between-day I_AoSt and K_AoSt measurement variability was considered moderate to excellent (Table 3).

| DISCUSSION
Our results indicate that Ao elasticity assessment, measured using ultrasonographically-derived AoSt, was feasible in all dogs included in the study, with good intraobserver measurement variability. Aortic St indices measured at 2 different levels (I_AoSt and K_AoSt) were significantly lower in HT than in NT dogs, but only I_AoSt was not influenced by age. These preliminary results suggest that the I_AoSt index is useful when assessing dogs with SH, especially when errors in measurements, obtained using noninvasive indirect methods, are suspected.
Our findings are consistent with those published in human medicine. Several studies have shown an increase in Ao stiffness in humans suffering from chronic arterial hypertension. [9][10][11][12]20,41 In veterinary medicine, the echocardiographic-derived right pulmonary artery distensibility index [42][43][44][45] has been used as an index of PH in dogs. 45,46 To the best of our knowledge, ours is the first study in which Ao stiffness has been assessed using ultrasonography in dogs with SH. Arteries subjected to a chronic increase in BP likely undergo remodeling of their walls, characterized by smooth muscle cell hypertrophy and increased collagen content, which then results in a decrease in vessel elasticity. [4][5][6][7] Although more advanced noninvasive methods are available for measuring Ao elasticity in humans, 10,16 ultrasonography is the most practical and readily available imaging technology for clinical use in dogs. Several ultrasonographic-derived indices of Ao elasticity have been proposed to evaluate arterial stiffness. 16,20,21,47 Most of them include in their formula pulsatile pressure, which is the difference between systolic and diastolic BP.
We assessed Ao elasticity by measuring the percentage change in Ao diameter (AoSt) for 2 main reasons: first, in clinical practice measurement of BP by the Doppler method, which only indicates the systolic pressure, is more common than measurement by the oscillometric method; second, we sought an index that would complement BP measurement and therefore be independent of it. In addition, AoSt is a useful index of Ao elasticity in humans. 11,15,20,21,47 In both groups, K_AoSt was higher than I_AoSt. The greater elasticity of the K-level compared to the I-level could be a result of the different composition of the Ao wall along its course. In fact, the concentration of elastic fibers gradually decreases from the proximal to the distal Ao, as the latter is subject to lower pulse pressure. 2,3,16 The decrease in arterial elasticity is an alteration that also occurs with age 48-50 and in CKD. 51 In our study, these effects were minimized because no significant difference in age or CKD prevalence between the 2 groups was observed (

CONFLICT OF INTEREST DECLARATION
Authors declare no conflict of interest.

OFF-LABEL ANTIMICROBIAL DECLARATION
Authors declare no off-label use of antimicrobials.

INSTITUTIONAL ANIMAL CARE AND USE COMMITTEE (IACUC) OR OTHER APPROVAL DECLARATION
The study protocol was approved by the Ethical Committee of the University of Sassari (OPBA) protocol number 50675/18.

HUMAN ETHICS APPROVAL DECLARATION
Authors declare human ethics approval was not needed for this study.