A historical review of experimental imaging of the beating heart coronary microcirculation in vivo

Following a myocardial infarction (MI), the prognosis of patients is highly dependent upon the re-establishment of perfusion not only in the occluded coronary artery, but also within the coronary microcirculation. However, our fundamental understanding of the pathophysiology of the tiniest blood vessels of the heart is limited primarily because no current clinical imaging tools can directly visualise them. Moreover, in vivo experimental studies of the beating heart using intravital imaging have also been hampered due to obvious difficulties related to significant inherent contractile motion, movement of the heart brought about by nearby lungs and its location in an anatomically challenging position for microscopy. However, recent advances in microscopy techniques, and the development of fluorescent reporter mice and fluorescently conjugated antibodies allowing visualisation of vascular structures, thromboinflammatory cells and blood flow, have allowed us to overcome some of these challenges and increase our basic understanding of cardiac microvascular pathophysiology. In this review, the elegant attempts of the pioneers in intravital imaging of the beating heart will be discussed, which focussed on providing new insights into the anatomy and physiology of the healthy heart microvessels. The reviews end with the more recent studies that focussed on disease pathology and increasing our understanding of myocardial thromboinflammatory cell recruitment and flow disturbances, particularly in the setting of diseases such as MI.

Imaging the injured beating heart intravitally and the vasculoprotection afforded by haematopoietic stem cells

Aims

Adequate microcirculatory perfusion, and not just opening of occluded arteries, is critical to salvage heart tissue following myocardial infarction. However, the degree of microvascular perfusion taking place is not known, limited primarily by an inability to directly image coronary microcirculation in a beating heart in vivo. Haematopoietic stem/progenitor cells (HSPCs) offer a potential therapy but little is known about their homing dynamics at a cellular level and whether they protect coronary microvessels. This study used intravital microscopy to image the anaesthetized mouse beating heart microcirculation following stabilization.

Methods and results

A 3D-printed stabilizer was attached to the ischaemia–reperfusion injured (IRI) beating heart. The kinetics of neutrophil, platelet and HSPC recruitment, as well as functional capillary density (FCD), was imaged post-reperfusion. Laser speckle contrast imaging (LSCI) was used for the first time to monitor ventricular blood flow in beating hearts. Sustained hyperaemic responses were measured throughout reperfusion, initially indicating adequate flow resumption. Intravital microscopy confirmed large vessel perfusion but demonstrated poor transmission of flow to downstream coronary microvessels. Significant neutrophil adhesion and microthrombus formation occurred within capillaries with the latter occluding them, resulting in patchy perfusion and reduced FCD. Interestingly, ‘patrolling’ neutrophils were also observed in capillaries. Haematopoietic stem/progenitor cells readily trafficked through the heart but local retention was poor. Despite this, remarkable anti-thromboinflammatory effects were observed, consequently improving microvascular perfusion.

Conclusion

We present a novel approach for imaging multiple microcirculatory perturbations in the beating heart with LSCI assessment of blood flow. Despite deceptive hyperaemic responses, increased microcirculatory flow heterogeneity was seen, with non-perfused areas interspersed with perfused areas. Microthrombi, rather than neutrophils, appeared to be the major causative factor. We further applied this technique to demonstrate local stem cell presence is not a pre-requisite to confer vasculoprotection. This is the first detailed in vivo characterization of coronary microcirculatory responses post-reperfusion injury.

First-ever ischemic stroke and increased risk of incident heart disease in older adults

Objective

Poststroke cardiac complications are common. It is unknown whether the reason is shared risk factors and preexisting heart disease or stroke-associated myocardial and coronary injury. We tested the hypothesis that first-ever ischemic stroke is associated with increased risk of incident cardiovascular complications in patients without known preexisting cardiac comorbid conditions.

Methods

This population-based cohort study included residents in Ontario between 2002 and 2012 who were ≥66 years of age without known cardiovascular disease. We compared the incident risk of major adverse cardiovascular events (MACE), defined as myocardial infarction, unstable angina, congestive heart failure, coronary artery disease, coronary artery revascularization, or cardiovascular death, at 1 year in patients with first-ever ischemic stroke vs propensity-matched individuals without stroke (4:1 matching using 31 variables). To estimate cause-specific hazard ratios (HRs), we used Cox regression models adjusted for variables with weighted standardized differences >0.10 or known to influence the risk of MACE.

Results

We included 21,931 patients with first-ever ischemic stroke and 71,696 propensity-matched individuals, well balanced on all variables used for propensity matching. First-ever ischemic stroke was associated with increased unadjusted incident MACE risk (HR 4.5, 95% confidence interval [CI] 4.3–4.8). MACE adjusted risk was highest in the first 30 days (HR 25.0, 95% CI 20.5–30.5) and declined both at 31 to 90 days (HR 4.8, 95% CI 4.1–5.7) and at 91 to 365 days (HR 2.2, 95% CI 2.0–2.4).

Conclusions

In this large population-based study, ischemic stroke was independently associated with increased risk of incident MACE. Whether this association is explained by stroke-associated cardiac injury, preexisting subclinical cardiovascular comorbid conditions, or both remains unknown.