Background

Microvascular Obstruction (MVO) is a complication of myocardial infarction and an independent predictor of mortality and left ventricular (LV) remodelling. Pathogenenesis of MVO is largely attributable to distal microembolisation. MVO is diagnosed by cardiovascular magnetic resonance imaging (CMR) 3-7 days following the index event. A large animal model of MVO for validation of new diagnostic and therapeutic methods has been missing. Here we report a reliable and reproducible novel large-animal model for post-ischaemic MVO that allows to measure changes in dynamic vascular resistance (CoFl™) as a potential diagnostic marker of MVO.

Methods

In five domestic pigs (80kg ± 5kg) autologous arterial thrombi were created by a carotid artery crush manoeuvre. Following echographic confirmation of thrombus formation after 60 min, the vessel was dissected and thrombotic material was manually cut to a size of approximately 200µm, thus creating microthrombi for subsequent injections. After 90 minutes of balloon LCx-occlusion (to create STEMI-conditions) the vessel was reperfused and 200 microthrombi (MT) were injected into the LCx distal from the occlusion site. TIMI flow and dynamic vascular resistance (CoFl™, assessed by occluding the vessel and infusing saline solution at 5, 10, 20 and 30 ml/min) were measured at base line, after STEMI reperfusion as well as 10 minutes and 4 hours after microthrombus injection. During the experiments, ECG, Troponin (T), Myoglobin (MG) and Creatine Kinase (CK) were assessed every 30 minutes. Six hours following MT injection, animals underwent CMR to assess cardiac function and infarction area as well as MVO by cine imaging and late gadolinium enhancement (LGE). Following in vivo CMR imaging animals were euthanized and high resolution LGE imaging was performed post mortem. The organs were harvested for further histological evaluation.

Results

Angiography following STEMI-induction, as well as directly after and 4h post-MT injection revealed no signs of coronary occlusions with TIMI flow 3 in 4/5 and TIMI flow 2 in 1/5 animals. Typical signs of STEMI were observed, with corresponding ST-segment elevation in ECG, significant increase in T (max = 9500 ng/L), MG (max=300 μg/L) and CK (max = 9100 U/L).  CoFI™ diagnostic sequence showed mild change (10%-15%) in microvascular resistance following STEMI-reperfusion as well as 10 min. after injection of MT. At 4h post-injection, it was found to be approx. 40% increased in comparison to base line. In all animals CMR revealed LV posterior wall motion abnormalities and oedema of the infarcted area. Infarct size and MVO were found to be of 22-25% and 5-7% of the LV wall, respectively. Presence of MT was confirmed by histology in CMR-indicated areas of MVO, whereas no MTs could be found in non-MVO areas of infarction nor in healthy tissue.

Conclusion

Post-STEMI cardiac microvascular obstruction can be reliably reproduced in a porcine large animal model by injection of autologous microthrombus material distal to the STEMI occlusion site. Such model closely mimics all clinically relevant aspects (such as T increase, ST elevation, MVO signal in LGE MRI) of MVO without overtly impeding blood flow in epicardial vessel as demonstrated by near-normal TIMI flows. Furthermore, increase in CoFI™ diagnostic sequence assessed microvascular resistance correlated well with presence of microthrombi in the affected heart region.