Original research•

Impact of heart rate, aortic compliance and stroke volume on the aortic regurgitation fraction studied in an ex vivo pig model

Abstract

Introduction Drug therapy to reduce the regurgitation fraction (RF) of high-grade aortic regurgitation (AR) by increasing heart rate (HR) is generally recommended. However, chronic HR reduction in HFREF patients can significantly improve aortic compliance and thereby potentially decrease RF. To clarify these contrasts, we examined the influence of HR, aortic compliance and stroke volume (SV) on RF in an ex vivo porcine model of severe AR.

Methods Experiments were performed on porcine ascending aorta with aortic valves (n=12). Compliance was varied by inserting a Dacron graft close to the aortic valve. Both tube systems were connected to a left heart simulator varying HR and SV. AR was accomplished by punching a 0.3 cm2 hole in one aortic cusp. Flow, RF, SV and aortic pressure were measured, aortic compliance with transoesophageal ultrasound probes.

Results Compliance of the aorta was significantly reduced after Dacron graft insertion (0.55%±0.21%/mm Hg vs 0.01%±0.007%/mm Hg, p<0.001, respectively). With increasing HR, RF was significantly reduced in each steady state of the native aorta (HR 40 bpm: 88%±7% vs HR 120 bpm: 42%±10%; p<0.001), but Dacron tube did not affect RF (HR 40 bpm: 87%±8%; p=0.79; HR 120 bpm: 42%±3%; p=0.86). Increasing SV also reduced RF independent of the stiff Dacron graft.

Conclusion Aortic compliance did not affect AR in the ex vivo porcine model of AR. RF was significantly reduced with increasing HR and SV. These results affirm that HR lowering and negative inotropic drugs should be avoided to treat severe AR.

What is already known on this topic

  • Clinical experience and small studies suggest that high-grade aortic regurgitation (AR) acutely benefits from increased heart rates by shortening diastole and thereby decreasing regurgitation volume. On the other hand, it could be shown that chronically increased heart rates increase vascular stiffening and thus the afterload in HFREF (heart failure with reduced ejection fraction) patients, so that long-term increased heart rates could also have similar unfavourable effects on AR. Therefore, the influence of an extremely stiff vessel (Dacron prosthesis) on the AR was compared with that of a native vessel in an ex vivo porcine aortic model.

What this study adds

  • The results of the study clearly show that restricted aortic compliance has no influence on the AR and thus does not impair the benefit of increased heart rates and stroke volumes in severe AR.

How this study might affect research, practice or policy

  • Bradycardic and negative inotropic drugs should be avoided in patients with high-grade AR. Stiffening of the aorta at high heart rates is acceptable and does not play a significant role at this stage of AR.

Introduction

Patients with severe aortic regurgitation (AR) with only moderately dilated ventricles (Left ventricular end-systolic diamater (LVESD)<50 mm) and preserved left ventricular (LV) systolic function (EF>50%) without symptoms currently have no indication for surgical repair of the aortic valve.1 Even if advanced echo parameters such as global longitudinal strain were included in future guidelines leading to earlier surgery, many patients with severe AR and mild limitations in this parameter would initially need to be treated conservatively.2 3 Drug therapy is a difficult topic in patients with AR, and the evidence base is low.1 A larger study of long-term vasodilator therapy in patients with severe AR has shown neither improved survival nor delayed disease progression.4 From pathophysiological considerations and clinical experience, AR patients benefit from higher heart rates because diastolic duration is acutely shortened, thereby reducing regurgitation volume (RV), which has been demonstrated in several small studies.5–8 However, in contrast to these beneficial acute effects, a chronically elevated heart rate resulted in aortic stiffening with increased LV afterload, as shown in heart failure with reduced ejection fraction (HFREF) patients9 and may also be relevant for AR patients.

These considerations raise the question of whether chronic heart rate elevation with stiffening of the aorta could have long-term adverse effects in AR patients, such that lower heart rates with improved compliance may be more effective in treating these patients. To answer this question, we used an ex vivo porcine ascending aorta with aortic valve in an artificial heart model to study the mutual influence of aortic compliance and heart rate in severe AR. In the second part of the experiment, the ascending aorta was replaced with a Dacron graft simulating a rigid aorta. The heart rate and the stroke volume (SV) were varied under both conditions and the respective effects on the AR in the normal and stiff aorta were examined.

Methods

The study was performed in agreement with the German Animal Welfare Act (TierSchG).

Aortic models

Freshly harvested porcine ascending aortas with aortic root were collected from a local slaughterhouse and selected for the ex vivo model because of their similar anatomy to the human aorta without chemical fixation (figure 1). The supra-aortic branches of the ascending aorta and the coronary arteries were ligated at their origin. The LV outflow tract (LVOT) was dissected. A short Dacron ring of 28 mm was sutured to the LVOT to connect the aortic root with a custom-made mock circulation loop (figure 1A–E). A 0.3 cm² hole was punched into one cusp of the aortic valve resulting in severe AR. Two investigation runs were carried out. First, the established experimental protocol with hydrodynamic measurements was performed on the native aorta with AR. Thereafter, the ascending aorta was replaced with a Dacron prosthesis adapted to the diameter of the supracoronary aorta. The graft was fixed with running 4/0 Prolene sutures and extends just anterior to the origin of the brachiocephalic artery. The identical experimental protocol was then repeated.

Figure 1
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Figure 1

Aortic preparation and experimental setup. (A) Dissected aorta with root, ascending and bow portion with vessel ligations and proximal suture for connection to the mock outflow tract. (B) Ascending aorta with inserted Dacron tube. (C) The native aorta connected to mock outflow tract and afterload column in water bath. (D) The position of the ultrasound probe in relation to the aortic root in the water bath. (E) The supracoronary insertion oft the Dacron tube with running suture in the water bath.

Physiological mock circulation

Physiological circulatory conditions were mimicked using a pulse wave duplicator to assess the hydrodynamic performance of the aortic valve and ascending aorta. The pulsating flow is generated by a short-stroke piston pump driven by waveform-matched cam plates (mock LV). The afterload system consists of three elements: a variable liquid column for constant diastolic pressure, an adjustable air compliance chamber for characteristics of aortic compliance and a resistance element. The root with ascending aorta was mounted free-standing between two mounts in a water bath just in front of the pump to keep the material moist and to connect the pump to the afterload column. Physiological saline at 37°C from a reservoir served as perfusate. The duplicator can be set to produce defined SV at selectable heart rate, preload and afterload, allowing assessment of hydrodynamic valve and vascular function (for details, see Scharfschwerdt et al).10

Experimental protocol and haemodynamic parameters

First, heart rate experiments were performed with systematically increased steps from 40 bpm to 120 bpm with constant afterload and constant SV. Second, the SV tests with systematically varied increments from 60 to 110 mL were carried out with identical afterload and a constant heart rate (see next paragraph). Twelve experiments per group were studied with native aorta and inserted Dacron prosthesis, respectively. The following hydrodynamic conditions were simulated for the haemodynamic measurements:

  • Six different heart rates (40, 50, 60, 80, 100, 120 bpm) with a fixed SV of 70 mL and a fixed systolic duration of 35%.

  • Five different SV (60, 70, 90, 100, 110 mL) with a fixed heart rate of 60 bpm and a fixed systolic duration of 35%.

An ultrasonic flowmeter in the physiological mock heart system measured the SV as well as the RV.

Regurgitation fraction (RF) was calculated by RF=RV/SV×100 (%).

In addition, a high-fidelity pressure transducer was connected to the mock left ventricle and the ascending aorta to measure LV pressure, aortic pressure and transvalvular gradients (see table 1). The following parameters were calculated:

Table 1
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Basic hydrodynamics in high-grade aortic regurgitation (AR) with and without prosthesis of the ascending aorta at a heart rate of 70 bpm and stroke volume (SV) of 72 mL

Pulse pressure (PP)=systolic pressure−diastolic pressure (mm Hg).

Mean arterial Pressure (MAP)=diastolic Pressure+PP/3 (mm Hg).

All haemodynamic data were collected over 10 consecutive cardiac cycles for each level of variation in heart rate or SV in both groups and the arithmetic mean calculated as the final measurement.

Echocardiography

The transoesophageal (TOE) 3D-GE probe was connected to a GE Vivid S 70 (General Electrics, Solingen, Germany). The TOE probe was then positioned in the water bath surrounding the porcine aorta immediately adjacent to the aortic root. A precise long-axis image of the aortic valve and the ascending aorta was then derived (figure 2). A second plane was then inserted perpendicularly through the sinutubular junction of the aorta to precisely adjust the cross-sectional area of the aorta in systole (Amax) and diastole (Amin) (figure 2).

Figure 2
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Figure 2

Ultrasound longitudinal view (left side) and orthogonal cross-sectional area (right side) of ascending aorta in diastole.

Change in aortic Area = Amax– Amin.

Aortic distensibility (D) was calculated: D=((Amax− Amin)/Amin))/PP (1/mm Hg).11 12

To assess aortic distensibility in %, D was multiplied by 100.

Statistics

Continuous variables are presented as mean±SD. Before starting the statistical analysis, a Kolmogorov-Smirnov test for checking normal distribution of the samples was performed. Changes in haemodynamic parameters in the different groups were analysed by analysis of variance test for repeated measurements and Tukey’s post hoc test to correct for multiple comparisons. If data were not normally distributed, a Kruskall-Wallis test and Dunn’s post hoc test was performed. To compare the corresponding groups (AR without and with Dacron prosthesis) at different heart rate or SV steady states, a Mann-Whitney U test was performed, or a two-tailed Student’s t-test, if the data were normally distributed. P values <0.05 were considered to reflect statistically significant differences. Statistical analyses were performed with GraphPad Prism V.8.1.2.

Results

The basic haemodynamics in the model of AR with and without Dacron prosthesis of the ascending aorta are shown in table 1. This pooled data indicate that high-grade AR is present in our experiments and that the model is therefore appropriate to study the effects of SV and heart rate in this severe valvular defect. Apart from that, it turns out that almost all examined haemodynamic parameters did not differ significantly in the initial conditions of the two test arrangements, apart from the difference in the aortic cross-sectional area (A) in systole and diastole and the corresponding distensibility. Remarkably, the MAP was nearly identical in both groups, so that the distensibility of the aorta can be compared in both test setups under this specification. This was also true for all other experimental conditions (data not shown). The visual representation of the differential compliance of the native aorta and the Dacron graft is shown by the larger systolic vascular area of the native aorta compared with the Dacron graft, shown in a typical cross-sectional echocardiographic alignment (figure 3A). With the same RF and RV (figure 3B), the stiffness of the Dacron prosthesis was reflected in the low distensibility and the minimal tube area change in systole and diastole (figure 3B), while the native aorta showed a mean change in systolic/diastolic area of 1.5±0.4 cm2.

Figure 3
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Figure 3

(A) Echocardiographic cross-sectional area of native aorta (left side) and of inserted Dacron graft (right side) in diastole and systole. (B) Regurgitation fraction and volume as well as aortic distensibility and area change in diastole and systole of ascending aorta; open white circles: aorta; filled black circles: Dacron prosthesis; *p<0.01.

Under the different compliances of the native aorta and the Dacron graft, the impact of heart rate on aortic valve RF is shown in figure 4. With increasing heart rates, diastolic pressure remained stable and showed no differences between both groups (A), although aortic distensibility was drastically decreased after the Dacron prosthesis was inserted (B). Despite serious differences in vascular stiffness, RF decreased systematically with increasing heart rate without showing any significant differences between the two groups (C). With increasing SVs (figure 5), diastolic pressure remained stable with no difference in both groups (A), while aortic distensibility was again significantly reduced in the Dacron group (B). Under these conditions, the RF systematically decreased with increasing SVs and showed no significant differences between native aorta and the inserted rigid Dacron prosthesis (C).

Figure 4
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Figure 4

Diastolic pressure (P), aortic distensibility and regurgitation fraction at varying heart rate at constant stroke volume (70 mL) and fixed systolic duration of 35%; #p<0.01; pn and pd=one-way ANOVA test within native aorta (n) and within Dacron prosthesis (d) for different heart rates; open white circles: aorta; filled black circles: Dacron prosthesis. ANOVA, analysis of variance.

Figure 5
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Figure 5

Diastolic pressure (P), aortic distensibility and regurgitation fraction at varying stroke volume with a fixed heart rate (64 bpm) and a fixed systolic duration of 35%; #p<0.01, pn and pd=one-way ANOVA test within native aorta (n) and within Dacron prosthesis (d) for different stroke volumes; open white circles: aorta; filled black circles: Dacron prosthesis. ANOVA, analysis of variance.

Discussion

The results of the ex vivo investigations on the porcine ascending aorta with severe AR showed that only heart rate and SV influence the RF, but interestingly not the strongly reduced compliance after insertion of Dacron prosthesis. This is noteworthy since arterial compliance is an important factor in the haemodynamics of blood flow in the circulatory system.13 In addition, heart rate can modulate aortic compliance across different rate ranges, making it difficult to predict the impact of both in many acute and chronic cardiovascular diseases.

Heart rate affects myocardial oxygen demand, coronary perfusion, and vascular and myocardial function.9 12 14 15 Experimental and clinical studies indicate an association between elevated resting heart rate and a wide range of adverse effects on cardiovascular function and structure.14 15 These results contradict two pathophysiological features of cardiovascular disease. First, clinical experience with patients with restrictive cardiomyopathies and experimental studies in mice with HFPEF and proven restriction show that higher heart rates have a beneficial effect. Restrictive filling pattern is characterised by early diastolic filling only.12 Therefore, the increase in heart rate can be interpreted as a compensatory mechanism to maintain cardiac output by increasing the number of filling cycles of the heart.12 16 The other cardiac disease that benefits, at least acutely, from high heart rates is AR, because increased heart rate is associated with shortened diastole and thus shortens the duration of regurgitation. This clinical experience has been demonstrated in some studies haemodynamically for the acute case.5–7 In contrast, it was shown in patients with HFREF that chronic heart rate reduction resulted in a decrease in afterload due to improved arterial compliance, which in turn improved ventricular arterial coupling at constant contractility.9 12 17 This effect provides a possible pathophysiological explanation for the beneficial effect of heart rate reduction in HFREF in the SHIFT study18 as well as in experimental HFPEF studies.12 19

Chronic increase in heart rate led to a stiffening of the aorta with increased LV afterload.9 20 Therefore, we were interested in whether a chronic increase in heart rate could also have adverse effects in AR patients, such as increased aortic stiffness, which could overcome the beneficial acute haemodynamic effects by reducing diastolic duration. If this theory were true, it could also explain pathophysiologically the very interesting results of a study by Yang et al, who could retrospectively show in 820 patients with relevant AR that especially patients with higher heart rate >60 bpm showed a worse survival compared with AR patients with lower heart rate <60 bpm.21 The results of this study are surprising and contradict all the previously reported haemodynamic data, However, our haemodynamic analysis clearly demonstrates—independent of the above-mentioned prognostic data21—that strongly reduced aortic compliance under higher heart rates has no influence on AR in the used model. Only increase in heart rate and SV were associated with a decreased aortic RF, unaffected by the rigid Dacron prosthesis.

If the results of the retrospective study by Yang et al are correct, other clinical factors must play an important role influencing the interaction of the heart and vasculature that cannot be represented by pure haemodynamics. We hypothesise that higher heart rates in severe AR are indicative of disease duration and severity.21 22 As long as the heart can cope with the increased SV via compensatory mechanisms such as Starling effect and instrinsic contractility, the rate remains low (good prognosis). However, as soon as these compensatory mechanisms fail in prolonged disease, sympathetic activation occurs with an increase in heart rate and eventually decompensation of the circulatory system. At this advanced stage of AR, the increased heart rate can reduce the load on the LV by reducing RF without increased stiffness of the aortic wall affecting the effect, which we showed in our study.

The decisive factors for RV and fraction seem to be the duration of diastole primarily modified by heart rate23 and the pressure difference between the aorta and the left ventricle in diastole. In our experiments, the stiffer aorta could not increase the diastolic pressure, so that the driving force for regurgitation (diastolic aortic pressure minus diastolic LV pressure) was not changed. Somewhat differently, higher SV reduced the RF by increasing forward LV SV. The accelerated blood flow moved downstream from the heart in diastole due to the physical mass inertia and the regurgitation again depended only on the driving force between diastolic aortic and diastolic LV pressure, which was also not significantly altered by the changing SV.

Conclusion

Increased heart rate and SV resulted in a decreased RF of severe AR that was unaffected by the rigid Dacron prosthesis. These results affirm that bradycardic and negatively inotropic drugs such as beta-blockers or calcium channel blockers should be avoided in patients with advanced severe AR.

Limitations

The experiments presented are in vitro porcine aorta studies and not studies on patients. However, the model of the mock heart with ascending aorta used comes close to the human circulatory system in terms of design and performance due to the selected haemodynamic key stones and is often used in cardiac surgery to test biological heart valves.10 Therefore, the obtained results of heart rate, SV and vascular stiffness in severe AR can be applied to the clinic.

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