Early Effects of Sacubitril/Valsartan on Exercise Tolerance in Patients with Heart Failure with Reduced Ejection Fraction

Background. Sacubitril/valsartan in heart failure (HF) with reduced ejection fraction (HFrEF) was shown to be superior to enalapril in reducing the risk of death and hospitalization for HF. Our aim was to evaluate the cardiopulmonary effects of sacubitril/valsartan in patients with HFrEF. Methods. We conducted an observational study. Ninety-nine ambulatory patients with HFrEF underwent serial cardiopulmonary exercise tests (CPET) after initiation of sacubitril/valsartan in addition to recommended therapy. Results. At baseline, 37% of patients had New York Heart Association (NYHA) class III. After a median follow-up of 6.2 months (range 3–14.9 months) systolic blood pressure decreased from 117 ± 14 to 101 ± 12 mmHg (p < 0.0001), left ventricular ejection fraction (LVEF) increased from 27 ± 6 to 29.7 ± 7% (p < 0.0001), peak oxygen consumption (VO2) improved from 14.6 ± 3.3 (% of predicted = 53.8 ± 14.1) to 17.2 ± 4.7 mL/kg/min (% of predicted = 64.7 ± 17.8) (p < 0.0001), minute ventilation/carbon dioxide production relationship (VE/VCO2 Slope) decreased from 34.1 ± 6.3 to 31.7 ± 6.1 (p = 0.006), VO2 at anaerobic threshold increased from 11.3 ± 2.6 to 12.6 ± 3.5 mL/kg/min (p = 0.007), oxygen pulse increased from 11.5 ± 3.0 to 13.4 ± 4.3 mL/kg/min (p < 0.0001), and ∆VO2/∆Work increased from 9.2 ± 1.5 to 10.1 ± 1.8 mL/min/watt (p = 0.0002). Conclusion. Sacubitril/valsartan improved exercise tolerance, LVEF, peak VO2, and ventilatory efficiency at 6.2 months follow-up. Further studies are necessary to better clarify underlying mechanisms of this functional improvement.


Introduction
Combining renin-angiotensin-aldosterone system blockade with natriuretic peptide system enhancement may deliver functional benefits to patients with heart failure (HF) with reduced Linda, CA, USA). Heart rate, 12 lead ECG, and oximetry (with pulse oximeter) were monitored continuously. Patients were encouraged to exercise until they felt unable to continue because of dyspnea or fatigue. The respiratory exchange ratio (RER) is the ratio between the amount of CO 2 produced in metabolism and O 2 used, representing a measure of exercise effort with RER > 1.05-1.10 indicating maximal effort [10]. Anaerobic threshold (i.e., the point during exercise when a switch from aerobic to anaerobic metabolism occurs) was measured using the V-slope analysis from the plot of carbon dioxide production (VCO 2 ) versus VO 2 and confirmed using ventilatory equivalents and end-tidal pressures of CO 2 and O 2 . The rate at which VO 2 increased per watt of work (∆VO 2 /∆Work) was calculated for the progressively increasing exercise period, beginning 1 min after work rate started to increase. ∆VO 2 /∆Work slope and VO 2 at anaerobic threshold (AT-VO 2 ) were used as a measure of muscle efficiency. The relationship between minute ventilation and carbon dioxide production (VE/VCO 2 slope) was used as a measure of ventilatory efficiency and was calculated from 1 min after the beginning of loaded exercise up to the end of the isocapnic buffering period. Reported values of VO 2 , ventilation, and tidal volume at peak exercise are the averages over the 30 s in which the examined event occurred. Percent predicted VO 2 represents the achieved peak VO 2 adjusted for age, weight, and height and expressed as a percentage. We measured percent predicted VO 2 using the equations by Wasserman and Hansen [13].

Statistical Analyses
Statistical analysis was performed using SAS JMP 9 software package. Continuous variables are described as mean±standard deviation, or as median and interquartile (IQ) range, in case of non-normal distribution. Categorical variables are expressed as number (percentages). Baseline and follow-up CPET parameters were compared using a Mann-Whitney U test for continuous variables and Fisher exact test for categorical variables, respectively. Changes from baseline were tested using a paired t-test or McNemar test, as appropriate. A p-value <0.05 was considered statistically significant. Nominal logistic regression was conducted to assess correlations between exercise tolerance, VO 2 , and VE/VCO 2 . A 6% increase of VO 2 from baseline was used as a cut-off to individuate a significant improvement in VO 2 , according to current literature on this topic [10].

Patients Characteristics
At present, a total of 125 patients have been enrolled and have undergone at least basal CPET. The final population for this study consisted of 99 patients for whom at least 1 follow up CPET was available ( Figure 1). Baseline characteristics are listed in Table 1. Mean age was 58.7 ± 9.3 years, 86% were males, 51% had ischemic heart disease, 63% were on NYHA class II, 37% were on NYHA class III, and 17% were on atrial fibrillation. Mean left ventricular ejection fraction (LVEF) was 27 ± 6%. The starting dose of sacubitril/valsartan was 24/26 mg in 69% of patients.    At a median follow-up of 6.2 months (range 3-14.9 months), 28%, 38%, and 34% of the patients were on 24/26 mg, 49/51 mg, and 97/103 mg of sacubitril/valsartan, respectively.
Patients characteristics in the sacubitril/valsartan low and high doses cohorts are reported in Table 2.
At follow-up, mean LVEF increased from 27 ± 6 to 29.7 ± 7% (p < 0.0001) and left ventricular end-systolic volume decreased from 153 ± 56 to 145 ± 52 mL (p = 0.030).   Table 4). A statistically significant reduction in terms of VE/VCO 2 slope was observed at follow-up in the subgroup of patients on the highest dose of sacubitril/valsartan (p = 0.01; Table 4, Figure 2A). Of note, no statistically significant differences were observed among these subgroups in terms of mean follow-up duration and baseline Peak VO 2 ( Figure 2B).

Discussion
CPET is a valuable tool to guide clinical decision-making and to derive prognostic information in HF patients [10,[14][15][16].
In the PARADIGM-HF trial [1], sacubitril/valsartan reduced the risk of death and hospitalization for HF in patients with HFrEF, as compared to enalapril; however, little is known on how sacubitril/valsartan influences cardiopulmonary function.
To the best of our knowledge, this is the largest observational study prospectively assessing the early effects of sacubitril/valsartan on cardiopulmonary parameters in patients with HFrEF. After initiation of sacubitril/valsartan, we observed a significant improvement in the main prognostically relevant CPET parameters. To date, only one study by Palau et al. [5] showed an improvement in peak VO 2 and VE/VCO 2 slope in 33 HFrEF patients at 30 days follow-up after sacubitril/valsartan initiation, mostly at low doses. In our study (entailing a larger population, with a longer follow-up and including all available dosages of sacubitril/valsartan) we confirmed the significant improvement in peak VO 2 at follow-up (∆= +2.6 mL/kg/min; p < 0.0001); of note, VE/VCO 2 slope improvement started at 6 months from sacubitril/valsartan initiation and reached a statistical significant difference only at 12 months ( Table 5).
Notably, at follow-up, sicker patients (i.e., patients with baseline VE/VCO 2 ≥ 34) improved both oxygen consumption and ventilatory efficiency while healthier patients (i.e., patients with baseline VE/VCO 2 < 34) only improved oxygen consumption ( Figure 3). Moreover, patients on the highest doses of sacubitril/valsartan were found to be the ones with the best functional improvement ( Figure 2). These results are consistent with those reported in the PARADIGM-HF study [1].
A PARADIGM-HF post-hoc analysis by Vardeny et al. [25] demonstrates that lower doses of sacubitril/valsartan confer a similar treatment benefit over enalapril; however, patients taking low doses were associated with a higher risk of the primary events. In our study, patients taking low doses had less improvement of peak VO 2 as compared to patients taking the highest dose; this may reflect patient frailty; indeed, patients taking low doses of sacubitril/valsartan showed lower systolic blood pressure (both at baseline and at follow-up), higher levels of NT-proBNP, increased prevalence of NYHA class III, higher furosemide dose use, lower estimated glomerular filtration rate, and a higher VE/VCO 2 slope at baseline (details provided in Tables 2 and 4).
Of note, exercise tolerance (namely, 1-watt increase) was found to be an independent predictor of 6% improvement of VO 2 (OR = 1.06; 95% CI: 1.03-1.10; p < 0.0001) at follow-up, and a trend towards statistical significance was found with regard to VE/VCO 2 slope decrease (OR = 1.02; 95% CI: 0.99-1.04; p = 0.057). It is likely that the weaker correlation with VE/VCO 2 slope decrease might be due to the small sample size of the study population.
Sacubitril/valsartan combines the effects of angiotensin receptor blocker with neprilysin inhibition which amplify the system of natriuretic peptides and other vasoactive peptides [26,27]. However, little is known about the overall effect of vasoactive peptides on heart and lung function. In our study population, we observed an improvement of LVEF and a decrease of left ventricular end-systolic volume at follow-up. We speculate that sacubitril/valsartan might have a synergistically favorable effect on hemodynamics and muscle efficiency through reduced afterload and left ventricular filling pressure. This might result in a net improvement of exercise tolerance and performance. Of note, recent data (a longitudinal and a retrospective study) support an improvement in left ventricular ejection fraction and in left ventricular reverse remodeling after sacubitril/valsartan initiation [28,29].
We also observed an increase in peak ventilatory responses which may be secondary to the improvement of cardiac performance, allowing patients to increase ventilation without increasing the VE/VCO 2 slope, although, at the moment, this remains speculative.

Study Limitations
This study has a number of limitations. First, we had no control group. However, the patients enrolled were hemodynamically stable and on optimized medical therapy; we may therefore consider patients at first evaluation as their own controls (versus follow-up). Importantly, since the PARADIGM-HF study has already demonstrated a relevant benefit of sacubitril/valsartan over enalapril in this setting and it is now recommended by international guidelines [11,30], denying sacubitril/valsartan to eligible patients in order to have a control group would have raised ethical issues. Conversely, selecting patients not eligible for sacubitril/valsartan as the control group, might have individuated frailer patients (i.e., with systolic arterial hypotension and more advanced chronic renal failure). Secondly, an important limitation of this study is the small sample size; nonetheless, to the best of our knowledge our work currently represents the largest series of HFrEF patients treated with sacubitril/valsartan for whom follow-up CPET parameters have been tested. Unfortunately, no data on diffusing capacity to carbon monoxide are available.
Further studies are necessary to confirm our preliminary results and to understand sacubitril/valsartan influence on cardiopulmonary function. A clinical trial evaluating the effect of sacubitril/valsartan on 6-month Exercise Tolerance in Patients with Heart Failure (NEPRIExTol) is currently ongoing (NCT03190304).

Conclusions
In this prospective observational study, administration of sacubitril/valsartan was associated with a significative improvement in exercise tolerance, peak oxygen consumption, and ventilatory efficiency at 6.2 months follow-up. Further studies are necessary to better clarify underlying mechanisms of this functional improvement.
Author Contributions: F.M.S. and G.V. conceived the study, participated in data collection, analyzed the data, wrote the manuscript, and approved the final submission; G.C., C.N., L.A., S.S. (Salvo Storniolo), S.S. (Silvia Sarullo), V.A., G.R., and G.N. participated in data collection and analysis, revised the manuscript, and approved the final submission; A.D.F. critically revised data analysis and the manuscript and approved the final submission; F.M.S. and F.C. participated to study design, led the entire research group, critically revised data interpretation, revised the manuscript, and approved the final submission.