Aging is known to be associated with deterioration in many structural and functional properties of aortas and large arteries, including dilated vessel diameter, increased wall thickness, diminished wall elasticity, and endothelial dysfunction (Lakatta and Yin, 1982; O'Rourke and Nichols, 2005). As age advances, the following histological alterations in the vasculature are observed: an increased rate of endothelial cell apoptosis, the degeneration of smooth muscles in the media, fragmentation of and decrease in the content of elastic fibers, and an increase in the number of irregularly arranged collagen fibers in the stroma (Lakatta, 1979; Yin, 1980; Lakatta and Yin, 1982; Chang et al., 1998). Collagen crosslinking by non-enzymatic glycation is also enhanced within the arterial wall (Sims et al., 1996; Schleicher et al., 1997). All these factors contribute to the age-related changes in the mechanical properties of the vasculature, including elevated arterial impedance, diminished aorta distensibility, and an accelerated return of pulse wave reflection (O'Rourke and Nichols, 2005). These changes in vascular mechanics are accelerated in the incidence of hypertension (Najjar et al., 2005; Franklin, 2006), coronary heart diseases (Mattace-Raso et al., 2006), congestive heart failure (Sutton-Tyrrell et al., 2005), and stroke (O'Leary et al., 1999) withadvancing age.

The physical properties of the arterial system are reflected in the aortic input impedance (Z_i), which is the aortic pressure-flow relation in the frequency domain (McDonald, 1974; O'Rourke, 1982; Milnor, 1989; Wang et al., 2014a). While the aortic characteristic impedance (Z_c) is known (Nichols and O'Rourke, 2011), the wave separation method can be derived in the time domain to resolve the measured aortic pressure wave into its forward (P_f) and backward (P_b) components (Westerhof et al., 1972). The arterial wave transit time (τ_w) can be computed using the impulse response function, which is the time-domain equivalent of its input impedance in the frequency domain (Laxminarayan et al., 1978; Sipkema et al., 1980). Thus, the accurate measurement of arterial wave properties, including arterial τ_w and wave reflection magnitude (RM) or wave reflection index (RI), requires the simultaneous recording of aortic pressure and flow signals.

In practice, it is feasible to estimate the τ_w and magnitudes of the forward and backward pressure waves by using aortic pressure signal alone. Westerhof et al. (2006) provided a novel method to calculate the pressure wave reflection by using only the measured aortic pressure. Replacing the unknown flow signal with a triangular wave shape (Q^tri), they successfully resolved the measured aortic pressure wave into its components, P_f and P_b, to calculate the RM or RI. Chang et al. (2017) elaborated this concept by determining the arterial τ_w through vascular impulse response analysis. They discovered that the aortic impulse response is an effective method for the estimation of arterial τ_w by using a single pressure pulse recording with an assumed Q^tri.

In this study, we determined the age-related changes in arterial wave properties on the basis of the aortic pressure alone in Long–Evans male rats. The pulsatile pressure wave was the only signal recorded in the ascending aorta by using a high-fidelity pressure sensor. The timing at the peak of the Q^tri was derived using the fourth-order derivative of the aortic pressure waveform (Westerhof et al., 2006; Chang et al., 2017). On the basis of the measured aortic pressure and an assumed Q^tri, we calculated the arterial τ_w, magnitudes of the P_f and P_b waves, and augmentation index (AI) to delineate the age-related changes in the pulsatile component of the left ventricular (LV) afterload. The novelty of the study is that Q^tri is constructed using the measured aortic pressure wave to approximate its corresponding flow signal, and that calibration of Q^tri is not essential in the analysis.

Table 1 presents the effect of age on BW, basal heart rate (HR), cardiac cycle length (CL), LV ejection time (LVET), and aortic pressure profile. In the rats, a significant increase in BW, decrease in HR, increase in CL, and prolongation in LVET were observed with an increase in age. However, the BW, HR, CL, and LVET did not significantly differ between the rats aged 12 and 18 months. The systolic blood pressure (P_s) did not change significantly as animals aged; however, diastolic (P_d), mean (P_m), mean systolic (P_ms), and mean diastolic (P_md) aortic pressures, and the P_ms/P_md ratio were significantly lower in 18-month-old rats than in the rats from other age groups.

Table 2 presents the pressure characteristics in relation to the pulse wave reflection derived from the aortic pressure signal in rats of different ages. Rats aged 18 months had markedly higher T_IP than did the rats aged 4 and 6 months. However, P_IP significantly decreased and ΔP increased in the 18-month-old rats compared with those in rats from other age groups. The PP values were markedly higher in the 18-month-old rats than were those values in the 4-, 6-, and 12-month-old rats. Moreover, rats aged 18 months had significantly higher ∣P_b∣, but not ∣P_f∣ than did the rats in other age groups. Although, the arterial τ_w did not change significantly as animals aged, the τ_w/CL ratio was significantly lower in the rats aged 18 months than in the rats aged 4 and 6 months but not in those aged 12 months. No correlation between arterial τ_w and T_IP was observed.

Figure 1 illustrates the construction of an uncalibrated Q^tri from the measured pressure waveform in one rat aged 4 months as an example. The pressure and Q^m signals were simultaneously recorded in the ascending aorta under the anesthetized, open-chest condition. Figure 2 depicts the similarity between the measured aortic impulse response obtained from the measured pressure and Q^m (A) and the predicted aortic impulse response obtained from the measured pressure and Q^tri (B) in the same rat. The measured P_f and P_b waves (C) and the predicted P_f and P_b waves (D) are also presented. Although, the Q^tri shape is an approximation that may differ from the actual Q^m shape, this approximation gave results close to those obtained with the measured Q^m.

Figures 3A,B illustrate the construction of an uncalibrated triangular flow from the measured pressure waveform in 4- and an 18-month old rats, respectively. The rat aged 4 months was the same one as shown in Figure 1. The pressure wave was the only signal recorded in the ascending aorta under the anesthetized, closed-chest condition. Figure 4 depicts the aortic impulse responses obtained from the measured aortic pressure and Q^tri in the same 4- and 18-month old rats (Figures 4A,B, respectively). The P_f and P_b waves for the corresponding rats aged 4 (Figure 4C) or 18 (Figure 4D) months are also depicted.

Figures 5, 6 illustrate the effect of aging on the arterial wave properties in terms of the AM, AI, RM, and RI as well as the τ_w/LVET ratio. The AM (Figure 5A) and AI (Figure 5B) increased markedly in the 18-month-old rats compared with those in the 4- and 6-month-old rats. Moreover, the rats aged 18 months had significantly higher RM (Figure 6A) and RI (Figure 6B) values than did the rats aged 4 and 6 months. Although, the arterial τ_w did not change significantly as the rats aged (Table 2), the τ_w/LVET ratio was markedly lower in the 18-month-old rats than in the 4- and 6-month-old rats (Figure 6C).

Figure 7A illustrates relation between AI and τ_w/LVET, and Figure 7C depicts that between AI and RI, which were calculated from the measured aortic pressure and an assumed Q^tri. Figure 7A exhibits a significant inverse regression line for AI: AI = −0.7424 − 0.9026 × (τ_w/LVET) (r = 0.4901; P < 0.0001). The regression equation of AI is given by AI = −0.4844 + 2.3634 × RI (r = 0.8423; P < 0.0001), and it is provided in Figure 7C. Figure 7B depicts relationship between PP and τ_w/LVET, and Figure 7D presents that between PP and RI. Figure 7B depicts an inverse regression line given by PP = 44.0011 − 59.6778 × (τ_w/LVET) (r = 0.5179; P < 0.0001). The regression equation of PP is PP = 1.8636 + 59.0396 × RI (r = 0.3363; P < 0.005), and it is provided in Figure 7D.

Figure 8 illustrates relation between P_ms/P_md and τ_w/LVET, which exhibits a significant inverse regression line for P_ms/P_md: (P_ms/P_md) = 1.1395 − 0.2030 × (τ_w/LVET) (r = 0.4807; P < 0.0001). However, the P_ms/P_md ratio has no correlation with the RI.

Earlier studies on the age-related changes of arterial mechanical properties in different species have shown that considerable changes occur in response to age. In humans, Gundel et al. (1981) measured the aortic pressure and flow signals to calculate Z_i and Z_c, demonstrating no relationship between age and Z_c. By contrast, Nichols et al. (1985) reported that aging process increased the steady and pulsatile components of the hydraulic load and the arterial wave reflections returned earlier with increasing age. However, Cox (1977) measured the carotid elastic modulus in rats of different ages, and suggested that the aging heart was somewhat compensated by a decrease in hydraulic load. In the present study, the older rats exhibited increased arterial stiffness and magnitude of the reflected pressure wave, which enhanced the systolic workload on the heart and contributed to a mismatch between the myocardial oxygen demand andsupply.

In this study, the older rats exhibited a decline in HR and a prolongation in LVET. The decline in HR with age has also been described in other studies on rats (Bunag et al., 1990; Bunag and Teravainen, 1991). In the current study, significant declines in P_d and P_m but not P_s were observed in the 18-month-old rats. Although not reaching statistical significance, the rats aged 18 months had lower P_s than did the rats in the other age groups. The age-induced decline in the aortic pressure profile in the rats was consistent with the results of a previous report by Bunag et al. (1990).

The pulse wave velocity and traveling distance of pressure waves to reflection sites are the determinants of arterial τ_w. In this study, we analyzed the aortic impulse response to calculate the arterial τ_w and found that the calculation of arterial τ_w was influenced by the CL (Figures 4A,B). Although the arterial τ_w did not change significantly as animals aged (Table 2), the τ_w/CL ratio was significantly lower in the rats aged 18 months than in the rats aged 4 and 6 months but not in those aged 12 months. The decreased τ_w/CL ratio indicated a decline in the distensibility of the aorta in rats with advancing age. In the absence of any significant change in the ∣P_f∣, the significant rise in the ∣P_b∣ in senescence (Table 2) was responsible for the increased RM (Figure 6A) and RI (Figure 6B). Both the arterial RM and RI augmented by age in turn increased the intensity of the wave reflection from the peripheral circulation in the older rats.

For the calculation of AM and AI, defining an inflection point on the aortic pressure waveform is imperative (Westerhof et al., 2006; London and Pannier, 2010). In this study, the inflection point was identified by the first zero-crossing curve from positive to negative on the fourth derivative of the pressure signal during ventricular ejection (Kelly et al., 1989; Westerhof et al., 2006; Chang et al., 2017). With the inflection point determined, we found that the rats aged 18 months had higher ΔP values than did those in other age groups. Although PP also increased as the rats aged, the increased ΔP dominated the increased PP, leading to an augmentation in aortic AM and AI.

From the definition, AI depends on the overlap between the P_f and the P_b, which is determined by both the timing and magnitude of the reflected pressure wave (Westerhof et al., 2006; London and Pannier, 2010). The overlap between the P_f and the P_b depends on both the arterial τ_w and the LVET duration (London and Pannier, 2010). With a shortened τ_w, the reflected waves return earlier, thereby affecting the central arteries during systole rather than diastole. With a lengthened LVET, it is favorable for the reflected wave to return during systole. Thus, the decreased τ_w/LVET ratio may increase the overlap between the P_f and the P_b, thereby increasing the systolic workload on the heart and reducing aortic pressure during diastole (London and Pannier, 2010).

In the present study, the τ_w/LVET ratio was markedly smaller in the 18-month-old rats than in the rats from other age groups (Figure 6C). We found that the aortic AI was inversely affected by the arterial τ_w/LVET ratio; thus, the lower the arterial τ_w/LVET ratio, the higher the aortic AI (Figure 7A), which was consistent with those of a previous report by London and Pannier (2010). Although aging did not affect ∣P_f∣, the older rats had increased ∣P_b∣ (Table 2). Using simple linear regression analysis, we found that the aortic AI augmented by age was associated with the increased ∣P_b∣: AI = 0.2423 + 0.0192 × ∣P_b∣ (r = 0.5044; P < 0.0001), and had strong positive correlation with the RI (Figure 7C). As the arterial τ_w/LVET ratio decreased and the arterial RI increased with age, the aortic AI increased, thereby augmenting the systolic workload on the heart.

Similarly to AI, PP was augmented as arterial τ_w/LVET ratio decreased (Figure 7B) and arterial RI was increased (Figure 7D) in older rats. However, AI exhibited closer correlation with RI than PP did. These findings suggest that the aortic AI might be a better index describing the arterial wave properties than the aortic PP.

In considering the ventricular/vascular coupling, O'Rourke et al. (1984) suggested that the ascending aortic pressure wave includes two components: P_ms, which is relevant to LV performance as a pump, and P_md, which is relevant to LV perfusion. Thus, the augmented P_ms/P_md ratio may cause a mismatch between the myocardial oxygen demand and supply. In this study, the rats aged 18 months had lower P_ms than did the rats in the other age groups (Table 1). Although P_md also decreased as the rats aged, the decreased P_md dominated the decreased P_ms, leading to an increase in P_ms/P_md ratio. The P_ms/P_md ratio augmented by age was associated with the diminished τ_w/LVET ratio (Figure 8), suggesting that as aging stiffened aortas, the older rats were accompanied with deterioration in the myocardial oxygen demand/supply ratio.

This study has several limitations. Because Z_i cannot be measured in conscious animals, evaluating the effects of pentobarbital-induced anesthesia on rats is impossible. The results reported here pertain only to the measurements made in anesthetized rats (Wu et al., 2012). This condition might have induced changes in the aortic pressure profiles and introduced reflex effects that are not observed under ordinary conditions (Wang et al., 2014a). The degree to which anesthesia influences the pulsatile hemodynamics in rats is not known with certainty. However, studies on other animals suggest that the effects are small in relation to the biological and experimental variability between animals (Cox, 1974). Moreover, the uncalibrated Q^tri was constructed using the measured aortic pressure wave to approximate the corresponding flow signal. Although the Q^tri is an approximation that may differ from the actual flow wave shape (Westerhof et al., 2006), the use of this concept to describe the arterial wave properties has been validated in studies by Westerhof et al. (2006) and Chang et al. (2017).

We determined the mechanical defects due to arterial aging on the basis of the measured aortic pressure and an assumed triangular flow. Because the ∣P_f∣ was unaltered, the increase in ∣P_b∣ enhanced the intensity of the wave reflection, thereby augmenting RI in the older rats. A reduction in the τ_w/CL ratio with age indicated a decline in the distensibility of the aorta, which resulted in arterial wave reflections returned earlier with increasing age. As the arterial τ_w/LVET ratio decreased and arterial RI increased with age, the aortic AI increased. With an increase in the P_ms/P_md ratio, the older rats were accompanied with deterioration in the myocardial oxygen demand/supply ratio. All these findings suggest that aging potentially impairs the pulsatile component of arterial mechanics, thereby increasing the systolic workload imposed on the heart.

Our contribution in this endeavor is to provide a path to consider the clinical application of the method estimating the arterial wave properties, based on the measured pressure alone. The advantage of the technique is that an assumed Q^tri is derived from the measured pressure and that the flow calibration is not essential in the analysis. Westerhof et al. (2006) suggested that the method can also be performed using the carotid pressure wave as a surrogate for the pressure measured in the ascending aorta. The carotid pressure can be obtained non-invasively by applanation tonometry (Van Bortel et al., 2001) or by using a transfer function on finger arterial pressure (Westerhof, 2005) or radial artery pressure (Chen et al., 1997). In large epidemiological studies, it is helpful to evaluate the arterial wave properties as a function of age by using a minimally non-invasive measurement on aortic pressure alone, because the construction of the unknown Q^tri, the separation of the aortic pressure waves, and the calculation of the arterial τ_w can be automated.

CC, RC, CW, and KC developed the concept of the study, designed the experiments, and wrote the manuscript. CC, RC, and SH performed the animal experiments, collected the data, and performed statistical analysis. MW, YC, HK, and LL provided advice on the surgical procedures used in the study. CW and KC interpreted the data, supervised this work, and critically revised the manuscript. All authors have read and approved the final manuscript.