The major limitation of the previously used invasive and noninvasive approaches to study the cardiac function in small animal models is that the hemodynamic parameters measured were largely dependent on loading conditions. Pressure-volume (P-V) analysis is a useful approach for examining intact chamber function independently of loading conditions. This analysis has been widely used in large animal studies and in humans (22-24, 27, 31, 36). Recent advances in the development and validation of miniature P-V catheters has made it possible to use this approach for studies in small animals (16, 18, 19, 29, 32, 41). Although P-V loop analysis is widely used in mice, the combined P-V conductance catheter for rats has been introduced only recently, and very limited normative data are available.

Here, we aimed to characterize the baseline cardiac performance and systolic and diastolic function at different preloads in aging F344 male rats using the Millar P-V conductance catheter system.

We found that aging was associated with decreased +dP/dt and EF (Table 1). Although +dP/dt has been widely used as a cardiac contractile parameter, it is well recognized that it is load dependent, especially on changes in preload (22-24, 27). EF is also known to be influenced by both preload and afterload and, therefore, cannot reliably be used to assess the contractile function in the models where both preload and afterload are altered.

Historically, ESPVR (E_max) was proposed as a fairly load-insensitive index of contractility. ESPVR was decreased in aging rats. However, because this relation can be altered not only by changes in inotropic state but also by changes in chamber geometry and other diastolic factors, we also calculated other parameters (22, 23, 31).

PRSW, the linear relation between SW and EDV, may be sensitive to changes in systolic function. It has advantage of being stable in the hearts of different sizes. Although it integrates data from the entire cardiac cycle, it is influenced most of all by systole (22, 23). PRSW was also decreased in aging rats compared with young controls.

Previous investigations (24, 27) have demonstrated that +dP/dt-EDV, another P-V-derived index, represents a sensitive but less load-dependent parameter of chamber contractility. In previous studies, the slope of this relationship increased and shifted leftward with increased contractility, and it decreased and shifted rightward with depressed contractility. We found that this index was also depressed in aging animals compared with young controls.

Our baseline hemodynamic data obtained with the P-V system are in a good agreement with the results of Capasso et al. (9). In that study, F344 rats were anesthetized with chloral hydrate (300 mg/kg ip) followed by measurements of ventricular pressure-derived parameters and ascending aortic flow. From 20 mo of age, there was an elevation in LVEDP, a significant decrease in +dP/dt and −dP/dt, a reduction of SV and CO, and an increase in total peripheral resistance. At 29 mo of age, EF, +dP/dt, −dP/dt, SV, CO, and HR were reduced by 59, 38, 48, 27, 40, and 17%, respectively, whereas LVEDP and total peripheral resistance increased by 275 and 59%, respectively (9). Similarly, in our study in barbiturate-anesthetized F344 male rats using P-V analysis, at 26 mo of age, EF, +dP/dt, −dP/dt, SV, CI, and HR were reduced by 43, 36, 41, 29, 44, and 11%, respectively, whereas LVEDP and total peripheral resistance increased by 139 and 41%, respectively (Table 1). In addition to the baseline parameters of contractility, we showed that all load-independent indexes of LV contractility (see above and also Table 1 and Figs. 2 and ​and3)3) demonstrate decreased contractile pump function in aging rats. The indexes of LV failure were accompanied by alterations in ventricular size and shape consisting of increases in the transverse and longitudinal chamber diameters and an abnormal expansion in diastolic and systolic ventricular chamber volumes. Consistent with the expansion of the diastolic chamber volume in the previous report (9), we also observe an increase in EDV in aging F344 rats using the P-V system (Table 1). A similar age-associated decline in cardiac function was also reported in mice using the P-V technique (41).

In a more recent study, Boluyt et al. (5) confirmed both systolic and diastolic dysfunction in isoflurane-anesthetized 30-mo-old female F344 rats using echocardiographic assessment. However, in the latter study, the reduction of systolic performance was less pronounced (EF and fractional shortening were decreased by 9 and 25%, respectively) than it was demonstated by Capasso et al. (9) and by the present investigation (see above). The reason for the less depressed systolic function found by Boluyt et al. (5) can be the protection against development of the cardiac dysfunction and heart failure in female F344 rats compared with male rats used in the previous (9) and our present studies.

One can also argue that aged rats are more sensitive to the cardiovascular depressant effects of barbiturate anesthesia than younger animals, and this could have a significant impact on the magnitude of changes noted in the aged versus younger animals. Although there is no perfect pharmacological agent available for anesthesia in the elderly, dozens of clinical studies have arrived to the same conclusion that no significant difference in outcome can be attributed solely to use of any specific anesthetic agent (4). Barbiturates have been a standard element of anesthesia practice both in the young and elderly patients for more that 50 years. Individual variability in response to thiopental is well known. In elderly patients, an age-related decrease in the initial distribution volume or differences in clearance to the rapidly equilibrating compartments were found (20). Compared with younger patients, geriatric patients require a 20–30% lesser induction dose (12). On the basis of the latter study, we also used a reduced dose of the drug in our aging rats (see METHODS). The predominant cardiovascular effect of thiopental is venodilation, followed by pooling of blood in the periphery (14). Myocardial contractility may be depressed, but not to the extent seen after volatile anesthetics (17). Importantly, barbiturates do not sensitize the heart to cathecholamines. Systemic vascular resistance usually remains unchanged, and no arrhythmia occurs. We have successfully used barbiturates for anesthesia in various rat models of shock, myocardial infarction, and heart failure and reported comparable, if not better, hemodynamic parameters than using inhalation agents (15, 33, 34). In a previous study (35) using 22-mo-old aging Wistar rats, we reported impaired endothelial nitric oxide-mediated dilation of isolated aortic rings without any depression of systolic cardiac function using the same anesthesia protocol. Thus it is unlikely that the increased sensitivity of aging animals to thiopental could have a significant contribution to the hemodynamic changes observed in the present study.

Impaired ventricular relaxation and increased end-diastolic stiffness were also observed in aging animals, as reflected by the decreased −dP/dt, prolonged τ, and increased LVEDP and EDPVR. Relaxation, an active process, depends mostly on calcium uptake by the sarcoplasmic reticulum during diastole, and under normal conditions usually takes place during the first third of diastole. End-diastolic stiffness is predominantly affected by alterations in myocardial structural components; however, it may also be influenced by various other factors. We observed both delayed relaxation and increased stiffness in aging rats. Consistent with diastolic dysfunction, LVEDP was also increased in aging animals.