Cerebral blood flow alteration following acute myocardial infarction in mice

Introduction

Appropriate tissue blood perfusion is necessary for normal organ functions and is dependent on several factors including cardiac output (CO), blood volume, systemic blood pressure (BP), and local vasculature autoregulation, all of which positively depend on metabolic demand [1,2]. Distinct amongst other organs, the brain requires a constant supply of oxygen and nutrients due to its limited substrate storage capacity and high metabolic activity [3]. Despite the fact that the brain contributes only 2% to total body weight, it receives approximately 12% of the CO [4].

Cerebral perfusion is supplied by two carotid arteries and two vertebral arteries. Each carotid artery supplies approximately 40% to the total cerebral perfusion, while the two vertebral arteries supply the remaining 20% [5]. Cerebral blood flow (CBF) regulation is an integrative and multifaceted process that combines the function of multiple processes including pulmonary gas exchange, cardiovascular function, and intracranial cerebrovascular regulators [3]. Although CO and mean arterial pressure (MAP) are interrelated hemodynamic parameters that could exert simultaneous effects on CBF [4], multiple studies have revealed that the strongest correlation with CBF belonged to CO, especially in conditions with low CO such as heart failure disease [4,6,7]. In addition, several studies reported that therapies targetted at improvement of CO, including pharmacological interventions, cardiac transplantation, and cardiac resynchronization therapy, exerted a positive effect on improvement of CBF [8–10].

Recent studies suggest that impaired CBF as a result of CO reduction in heart failure patients may be a critical risk factor for Alzheimer’s disease (AD) development and accelerated brain ageing processes [11]. To date, no treatment is available to stop or reverse AD progression and consequently, preventive therapy is highly warranted [11,12]. Linking MI-induced CO reduction and subsequent CBF reduction to potential AD development and brain ageing in later stages of life is rational and worth investigating. Of note, cumulative heart failure rates of development 1 year following acute coronary syndrome hospitalization were 23.4% in ST-elevation myocardial infarction, 25.4% in non-ST-elevation myocardial infarction, and 16% in unstable angina patients in spite of interventional and pharmacological modern therapy [13]. Early diagnosis and accurate medical treatment of risk factors such as low CO levels can reduce the prevalence of AD and delay disease progression. In fact, it was projected that if intervention delays disease onset by 1 year, there would be 9 million fewer cases of AD by 2050 [11,12,14,15].

At the onset of MI, cardiac function, measured by CO and other hemodynamic parameters, rapidly deteriorates and persists until progressive restoration through medical intervention [13,16,17]. Depending on the infarct size and time to intervention, left ventricular (LV) remodeling post-MI is characterized by LV chamber size, shape, and function alterations subsequent to genomic, molecular, cellular, and interstitial changes [18,19]. In spite of full cardiac function restoration following acute medical intervention, clinical manifestation of heart failure development and its ramifications can progressively occur and be fully established in years following the onset of MI in patients [20]. Changes in the CBF as a result of CO deterioration with acute MI may have strong implications on the brain function; however, these changes have not been thoroughly investigated and remain poorly understood. Thus, it is critical to determine the CBF status in early stages of MI-induced CO reduction in order to prevent unnoticed early brain injury development that could translate to brain ageing and AD in later stages of life.

In addition to direct alteration of CO through myocardial mechanical injury and LV remodeling [19,21,22], MI induces a systemic inflammation and an elevated renin–angiotensin–aldosterone system (RAAS) activation affecting the remote organs including kidneys and the brain, and have been directly linked to brain perfusion impairment [4,23–26]. Although a strong correlation between chronic heart failure and CBF alteration has been established, no existing data link CO to CBF post-MI through the main phases of MI-induced cardiac remodeling including the inflammatory, granulation, and wound-healing phases [27]. This is the first study to serially investigate the hemodynamic impact of acute MI on CBF as early as day 1 and up to 30 days post-MI.

Materials and methods

Results

Data presented in this section are summarized in Tables 1 and 2. In agreement with other groups and confirming our previous data [28,29,31], we observed a progressive LV remodeling over the course of 30 days after initiation of MI. LV remodeling was evidenced by consistently increasing LV volumes (R = 0.99, R² = 0.98, P<0.0012) and progressively deteriorating systolic function measured by EF (Figure 2, R = −0.73, R² = 0.53, P<0.0001) and CO (Figure 3). Unlike study group, sham operated mice showed preserved LV systolic function during 7 days post-op (Figure 9).

We then investigated the impact of impaired LV systolic function on brain perfusion and measured the RCCA FL via Doppler echocardiography. Similar to CO, the RCCA FL showed a moderate but significant decrease over the course of MI with similar fluctuation pattern observed on days 1, 7, and 30 post-MI when compared with baseline (Figure 3, R = −0.61, R² = 0.37, P<0.0001, R = −0.44, R² = 0.19, P<0.0082, respectively). In sham operated mice however, both RCCA FL and CO fluctuated during 7 days post-op while remaining within the baseline level (Figure 9). We observed a strong positive correlation between CO and RCCA FL at baseline and at day 30 (R = 0.71 and R = 0.72, Figure 4A,D), and a moderate positive correlation at days 1 and 7 post-MI (R = 0.51 and R = 0.56, Figure 4B,C). However, in sham operated mice, a strong and positive correlation between CO and RCCA FL was observed at baseline, days 1 and 7 post-op (R = 0.82, R = 0.82, and R = 0.84, respectively. Figure 10A–C). While not significant, the fractional RCCA FL (%) showed a steady trend to increase from the baseline and to day 30 post-MI (Figure 5). The correlation between fractional RCCA FL (%) and CO was negative and moderate with no linear change (Figure 6, R = −0.44, R² = 0.2, P=0.2).

Both CO and RCCA FL were inversely correlated with the infarct size (R = −0.81, R² = 0.65, P=0.0091, R = −0.57, R² = 0.33, P=0.10, respectively). However, the relationship of these changes was only significant for the CO (Figure 7A,B). Finally, MAP and flow in the right coronary artery assessment at day 30 post-MI revealed no correlation (Figure 8, R = −0.31, R² = 0.10, P=0.40).

Discussion

Following MI in all mammals, ischemic hearts undergo an intensive structural and functional remodeling in three major phases: inflammatory phase (1–3 days); granulation phase (1–7 days); and maturation/wound healing phase (weeks) [24,27]. Although LV remodeling starts at the onset of MI, clinical manifestation of chronic heart failure may not develop for years in humans given the progressive maladaptive remodeling that may continue for a lifetime [20]. Multiple studies revealed the association between CO and CBF in both healthy and cardiac impaired individuals linking heart failure mediated low CO to CBF reduction [4,6–10]. However, no studies assessed the impact of early (day 1) till late stages (30 days) of acute cardiac injury post MI, on CBF. The findings reported in this manuscript are novel, showing for the first time the CBF through RCCA as early as day 1 and up to 30 days post-MI.

The major finding of the present study is that CBF decreases at the onset of MI and remains lower than baseline for 30 days (Table 1/Figure 3). Our findings correlate well with others who reported reduction in CBF 4–6 weeks after MI and before any signs of congestive heart failure development [35]. Interestingly, although SV, CO, and RCCA FL significantly decreased at day 1 post-MI and remained lower than baseline for 30 days, fractional RCCA FL (%) mildly but steadily and non-significantly increased, reaching its highest level at day 30 post-MI (Table 1/Figure 5). This means that the percentage of blood derived to the brain from total circulating blood increased after MI, but the amount of blood perfusing the brain decreased in-line with cardiac dysfunction. These findings indicate that CBF is directly associated with cardiac remodeling, functional stages, and the associated systemic compensatory mechanisms following MI. However, CBF regulatory mechanisms were not able to restore complete cerebral blood supply under low cardiac function and known inflammatory conditions accompanying MI since CBF remained lower than the baseline 30 days following MI (Table 1). Unlike previous reports where CO and fractional CBF inversely and significantly correlated with healthy individuals, our data showed no significant correlation between these parameters suggestive of unknown MI-related systemic regulation blunting mechanisms (Figure 6) [36].

Systemic regulatory mechanisms following both cardiac dysfunction and decrease in major organ blood perfusion are well documented [37,38]. Decrease in SV at the onset of MI triggers immediate autonomic activity characterized by catecholamine discharge and subsequent increase in heart rate and vasoconstriction [25]. Persistent hypoperfusion elicits a long-term compensatory mechanism involving the RAAS that promotes vasoconstriction, renal salt and water retention, and overall increase in blood volume [25,39,40]. Within the context of MI, compensatory mechanisms can either fully or partially restore CO depending on the infarct size and the absence or presence of comorbidities [24]. In-line with previous reports, our findings showed a strong correlation between CO and CBF, 4 weeks post-MI [24,35] (Figure 4). The effects of regulatory mechanisms were noticed in our hemodynamic analyses. For instance, in comparison with day 1 post-MI, we observed improved CO and RCCA FL at day 7 of cardiac remodeling notwithstanding increased LV dilatation (Figure 3). Interestingly, unlike the correlation at BL and day 30 post-MI, the correlation between CO and RCCA FL at days 1 and 7 post-MI was moderate and not statistically significant (opposite to what was observed in sham operated mice) (Figures 4 and 9). These data suggest that in the first and second stages of cardiac remodeling post-MI, regulatory mechanisms limit the impact of CO on CBF. In fact, CBF is an integrative and complex process combining the function of multiple systems including pulmonary gas exchange, cardiovascular function, and intracranial cerebrovascular regulators [3,4]. MI impact on cerebral perfusion is multifaceted and not only mediated through CO alteration. Individuals with MI experience a high level of cardiac and systemic inflammation that alters CBF in spite of the acute and long-term compensatory mechanisms that improve cerebral vascular beds perfusion [4,21,23,25]. Unlike early stages of MI that involve a vicious systemic inflammatory response including circulating pro-inflammatory cytokines, chemokines, and cell adhesion molecules, as well as peripheral leukocytes and platelets activation [21], later phases of cardiac remodeling post-MI are more settled with different inflammatory and functional properties. The multi-remodeling phases could explain the variation in CO and RCCA FL correlation in different stages post-MI [19,21,22]. In-line with infarct size impact, unlike the strong inverse and significant correlation with CO, a weak, inverse, and non-significant correlation was noticed with RCCA FL at day 30 post-MI (Figure 7). These findings suggest that infarct size is a good predictor of CO but not of CBF. Correlation between infarct size and RCCA FL is most probably weakened by MI-mediated multiple effects such as LV remodeling and its impact on regulators of cerebral perfusion.

With respect to BP variation and its impact on CBF, patients with uncomplicated MI and intact autonomic nervous system are usually normotensive [39]. Our findings were similar to what has been previously reported [35] with normal BP at 30 days post-MI and hence no correlation between CBF and MAP was noticed (Figure 8).

In summary, intact compensatory and regulatory mechanisms were not able to restore normal CBF in mouse model of 30 days of MI. These findings emphasize the importance of an appropriate intervention at cerebral, cardiac, and/or systemic level including inflammatory resolution to immediately restore CBF post-MI and prevent cerebral hypoperfusion ramifications on potential acute or time-dependent cerebral injury.

Conclusion and future direction

Our novel findings report the association between CBF and CO in early stages of cardiac remodeling and up to 30 days following acute MI. We showed that CBF significantly decreased along with CO following acute MI; however, regulatory systems could not restore CBF completely following reduced CO. The present study highlights the importance of impaired CBF at early and advanced stages of acute MI and its potential impact on cerebrovascular prognosis. Early intervention to fully restore normal CBF following acute MI is critical and warrant future investigation.

Funding

This work was supported by the American University of Beirut Medical Center Seed Grant [grant number 11410 (to F.A.Z.)]; the Oklahoma Center for the Advancement of Science and Technology [grant number HR17-070 (to A.Y.)]; the College of Medicine Alumni Association (to A.Y.); the NIH-supported Oklahoma Shared Clinical and Translational Resources [grant number NIGMS U54GM104938 (to A.Y.)]; and the South-Eastern Norway Regional Health Authority (HSØ-RHF) [grant number #2016089 (to R.A.)].

Author contribution

All authors significantly contributed to this manuscript. A.K. and F.A.Z. designed, performed, and analyzed all the experiments in addition to the manuscript writing. A.Y., R.A., and G.W.B. performed all the statistical analyses, data editing, manuscript writing in addition to generous intellectual contribution. R.G. was the project manager, who assured the smooth flow of work, troubleshooting, animal care, killings, data collection, and blinded analyses.

Competing interests

The authors declare that there are no competing interests associated with the manuscript.