Microvascular obstruction after successful fibrinolytic therapy in acute myocardial infarction. Comparison of reteplase vs reteplase+abciximab: A cardiovascular magnetic resonance study.

BACKGROUND.: About one third of patients with TIMI 3 after reperfusion have evidence of microvascular obstruction (MO) which represents an independent predictor of myocardial wall rupture. This explains all efforts made to prevent MO. Magnetic resonance imaging (MRI) has proved to be particularly useful in detecting MO. The aim of this study was to evaluate with MRI if different fibrinolytic regimens in acute myocardial infarction display different effects on left ventricle (LV) volumes and ejection fraction (EF), as well as on myocardial infarct size (MIsz) and MO. METHODS.: Twenty male patients, mean age 58 years, affected by acute myocardial infarction, ten anterior and ten inferior, were treated with: full dose reteplase in ten, and half dose reteplase plus full dose abciximab (R+Abcx) in the other ten patients. In the fourth day after hospital admission, MRI STIR T2 images were used to quantify MIsz, while 2dflash cineloops were used after the injection of gadolinium, to quantify LV volumes, EF and to detect MO. RESULTS.: LV EF was higher in R+Abcx 51±10 than in reteplase 41±8. MIsz was similar in both treatment groups: however a close relationship was present between MIsz and EF in the reteplase group indicating that the greater the MIsz the lower the EF. In R+Abcx this relationship was no longer present, suggesting a protective effect of the drug on microcirculation. In fact extensive MO was present in 25% of all cases, 80% of which in the reteplase group while only 20% in R+Abcx. CONCLUSION.: R+Abcx prevents MO: compared to traditional fibrinolytic therapy it allows better LV function and most likely improved long term survival.

INTRODUCTION

The structure and the role of microvasculature has been progressively clarified in this last decade (1, 2). We now have adequate information on the time course and percentage occurrence of microvascular obstruction (MO). We know that the venules are the site of leukocyte adhesion during inflammation and that their endothelial surfaces express a number of adhesion molecules, whose production is significantly up-regulated after the onset of tissue injury (3). Similarly we know that about one fifth to one third of patients with TIMI grade 3 flow after mechanical or pharmacological reperfusion show evidence of MO (4, 5) which, in turn, seems the basis of intracardiac hemorrhage (6, 7) and of the development of myocardial wall rupture (8).

In patients with acute coronary syndromes undergoing PCI, aspiration of the coronary artery has revealed thrombus with or without plaque components in 15–50% of the patients (9–11).

This means that in a large percentage of cases, 50–85%, MO is a dynamic phenomenon which begins gradually and progressively develops after the occluded vessel has been reopened (12,13). It then persists for at least 1 month after reopening of the epicardial coronary artery, predicting worse scar thinning, infarct expansion, poor survival, and ultimately annulling the effect of PCI (14–16).

This explains all efforts made to prevent MO with mechanical devices (17,18) or with pharmacological strategies (19–33).

The rational of focusing downstream the open artery hypothesis may be further emphasized by considering that all cardiac ruptures, likely the main cause of death in the in-hospital course of myocardial infarction, seem to lag behind MO which represents a significant, and in all probability the principal independent predictor of cardiac rupture (34, 35)

Magnetic Resonance Imaging (MRI) has proved to be of great value in detecting and monitoring MO after occlusion and reopening of the coronary artery, both in experimental and in vivo studies (36–46). It also provides evidence of its value in recognizing myocardial hemorrhage and impending wall rupture, thus allowing a window of interventional opportunity in this often catastrophic event (8, 38, 47–55).

The aim of this study was to evaluate, by means of MRI, in subacute myocardial infarction, if the different pharmacological strategies of GUSTO V reperfusion protocol i.e. full dose reteplase vs half dose reteplase plus full dose abciximab (R+Abcx) , display different effects on volumes and function of the left ventricle (LV), as well as on miocardial infarct size (MIsz) and MO (29).

SUBJECTS AND METHODS

From the list of consecutive patients randomized in the GUSTO V study at our Institution, 20 male patients, mean age 58 years range 37–75, were addressed, after written consent, to MRI study in the 4th day of myocardial infarction.

Although selected from a progressive list, in agreement with the criteria of GUSTO V (29) , the grid of MRI studies was created to fulfil the following criteria: age matched patients affected by anterior and posterior myocardial infarctions , age matched patients treated with reteplase or R+Abcx. The grid was thus composed of: 5 anterior treated with reteplase, 5 anterior treated with R+Abcx, 5 inferior treated with reteplase, 5 inferior treated with R+Abcx. All groups had an identical pain to fibrinolytic therapy time. All patients had clinically uncomplicated and apparently reperfused myocardial infarction.

Statistical evaluation was made with SPSS 13.0 software.

Magnetic resonance protocol

Examinations were performed on a Somatom Vision 1.5T scanner (Siemens Erlangen Germany) and analyzed with the built-in Numaris cardiac software.

After initial ECG triggered turbo-flash scouts in axial and in double oblique direction, STIR T2 breath hold 10mm thickness images, were obtained in four chambers and in consecutive contiguous short axis views (SAX), encompassing the whole LV from the base to the apex. A complete three dimensional (3D) STIR T2 study was then created by assembling all base to apex slices in a 3D package, from which LV end diastolic volume (EDV) and LV end diastolic mass could be calculated. STIR T2 hyperintense signal was then manually outlined in each SAX slice and subsequently 3D reconstructed to obtain myocardial infarct size (MIsz), representative of ischemic/infarcted myocardium. MIsz was expressed as a percent of the entire LV mass (Fig. 1). In order to quantify the degree of hypersignal level, a circular region of interest (ROI) of 0.5 cm diameter was positioned in the center of STIR T2 hyperintense area and the value was compared with a similar ROI positioned in a region remote from the site of infarction. A dimensionless value, STIR Intensity ratio, was obtained to represent the signal intensity on myocardial infarction.

Fig. 1

- From base (A) to apex (F) STIR T2 contiguous slices. Endocardial and epicardial contours define the myocardial slice area from which a 3D quantification of LV mass can be obtained. Similarly contour of hypersignal area (A) applied to all slices, creates the MIsz, expressed in % of LV mass.

A bolus of 0.2 mmol Gd-DTPA per kg body weight (Magnevist, Schering-AG, Berlin, Germany) was then injected. The same SAX positions of the STIR study were repeated with 2D-flash cine sequences (2d-fl). By assembling base to apex contiguous SAX 2d-fl cineloops, a new 3D study was created from which EDV and ejection fraction (EF) were calculated as habitually done in cardiac MRI procedure.

As all T1 sequences, also 2d-fl is fit to visualize myocardial signal void in the presence of MO with the limitation of less contrasted images compared to more dedicated T1 sequences, but with the advantage of being able to follow the MO dynamically during systole (8). In order to visualize only persistent MO, 2d-fl acquisition started five minutes after Gd injection. Since delineation and quantification of small MO was practically unfeasible, MO was visually estimated and classified as: not present (group 1), small subendocardial (group 2), and large or transmural (group 3).

Due to the limited time allowed for each patient, potentially unstable in day four after myocardial infarction, to the limited contrast gain in late enhancement imaging of our present scanner, to the main goal of this study i.e. to discover differences in volumes and EF, and also to the still incomplete standardization of this technique for quantification of infarct size, (56) late enhancement quantification of infarct size was not performed. Thus each study could be contained in 30 minutes, the time allowed by the Ethical Committee of our Hospital for the study of this type of patients.

The list of patients, treatment and measurement performed is reported in Table I.

TABLE I

- LIST OF PATIENTS WITH INDIVIDUAL MEASUREMENTS

Name	Age	MI	Drug	STIR3D MIsz %	2d-fl3D EF %	STIRIntensity ratio	2d-fl3D EDV mL	2d-flMO 1-2-3
G.P.	48	Ant.	R+Abcx	31	36	1.57	173	1
A.G.C.	71	Ant.	R+Abcx	73	56	1.27	131	1
O.E.	67	Ant.	R+Abcx	13	50	1.6	120	2
B.G.	62	Ant.	R+Abcx	11	67	1.87	115	1
L.C.A.	60	Ant.	R+Abcx	14	54	1.7	136	1
S.M.	53	Ant.	Reteplase	19	44	1.65	145	2
N.M.	75	Ant.	Reteplase	41	26	2.2	104	2
F.G.	45	Ant.	Reteplase	26	35	1.82	237	1
F.L.	59	Ant.	Reteplase	24	44	1.8	76	2
M.E.	72	Ant.	Reteplase	24	43	1.5	187	1
M.C.	57	Inf.	R+Abcx	36	34	1.98	225	1
P.D.	53	Inf.	R+Abcx	39	60	1.67	149	1
A.U.	73	Inf.	R+Abcx	17	42	1.96	132	3
G.G.	51	Inf.	R+Abcx	35	56	1.39	221	2
S.G.	66	Inf.	R+Abcx	20	53	1.94	93	2
G.F.	48	Inf.	Reteplase	22	43	1.64	212	3
C.A.	63	Inf.	Reteplase	22	45	1.75	160	3
P.S.	37	Inf.	Reteplase	19	52	1.62	140	3
S.M.	37	Inf.	Reteplase	17	49	1.93	142	1
S.S.	65	Inf.	Reteplase	36	31	1.5	128	3

RESULTS

Table I sets out the list of patients with individual measurements; in Table II the descriptive statistics in both types of treatment; and in Table III the descriptive statistics divided by drug regimen and site of myocardial infarction.

TABLE II

- DESCRIPTIVE STATISTICS IN BOTH TYPES OF TREATMENTS

Drug		Minimum	Maximum	Mean	STD
R+Abcx	age	48	73	60.8	8.5
	STIR 3D MIsz %	11	73	28.9	18.7
	2d-fl 3D EF	34	67	50.8	10.5
	STIR Intensity ratio	1.27	2	1.7	0.2
	2d-fl 3D EDVmL	93	225	149.5	44
Reteplase	age	37	75	55.4	13.6
	STIR 3D MIsz %	17	41	25	7.7
	2d-fl 3D EF	26	52	41.2	8.1
	STIR Intensity ratio	1.50	2.20	1.75	0.2
	2d-fl 3D EDVmL	76	237	153.1	48.3

TABLE III

- DESCRIPTIVE STATISTICS DIVIDED BY DRUG REGIMEN

Drug	MI		Age	STIR 3D MIsz %	2d-fl 3D EF	STIRIntensity ratio	2d-fl3D EDV mL
R+Abcx	ant	mean	61.6	28.4	52.6	1.6	135
		STD	8.7	26.2	11.2	0.2	22.8
	inf	mean	60	29.4	49	1.8	164
		STD	9.3	10.1	10.7	0.3	57.6
	Total	mean	60.8	28.9	50.8	1.7	149.5
		STD	8.5	18.7	10.5	0.2	44.
Reteplase	ant	mean	60.8	26.8	38.4	1.8	149.8
		STD	12.6	8.3	7.9	0.3	64.3
	inf	mean	50	23.2	44	1.7	156.4
		STD	13.6	7.5	8.1	0.2	33.1
	Total	mean	55.4	25	41.2	1.8	153.1
		STD	13.6	7.7	8.1	0.2	48.3

Tables from IV to VII show the results of paired t-test (2 tailed significance) for the two different regimens of therapy.

TABLE IV

- INFARCT SIZE (MIsz%) AS % OF THE WHOLE, THREE DIMENSIONALLY (3D) CALCULATED, MYOCARDIAL MASS

STIR3D MIsz %	Mean	Standard Deviation	Sig. (2-tailed)
R+Abcx	28.9	18.72	0.586
Reteplase	25.0	7.70

TABLE VII

- END DIASTOLIC VOLUME (EDV), EXPRESSED IN MILLILITERS(ML) OBTAINED FROM CINE 2D-FLASH (2d-fl) SEQUENCES, IN BOTH DRUG REGIMENS

2d-fl 3D EDVmL	Mean	Standard Deviation	Sig. (2-tailed)
R+Abcx	149.5	44.03	0.842
Reteplase	153.1	48.36

As evident, mean values of MIsz, EF, Intensity Ratio and EDV, were not statistically different in patients treated with reteplase and in those treated with R+Abcx. Neither a further grouping for site of myocardial infarction provided additional differences. In spite of this, mean value of EF in Abciximab was only slightly depressed but clearly higher than that of the reteplase group. So when a correlation was attempted with EF and the other measures evaluated, it became clear that the linear regression between EF and MIsz was strongly significant in the reteplase group. In this last, the larger the MIsz, the lower the EF. This clear and expected correlation was no longer present in R+Abcx group with EF completely unrelated to MIsz (Fig. 2).

Fig. 2

- Inverse linear regression between infarct size (MIsz%) ad ejection fraction (2d-fl 3D EF) in both drug regimens. Details in the text.

Similarly, MO also showed a powerful relation to EF, with the strongest correlation in group 3 of patients, those with the greatest MO areas (Fig. 3).

Figure 3

Relationship between microvascular obstruction (2d-fl MO) infarct size (MIsz%) ad ejection fraction (2d-fl 3D EF). 1,2 and 3 represent: no MO, small MO and large MO respectively. Details in the text.

Large MO (group 3) was present in 25% of patients: i.e 5 out of 20. Four of these patients were in the reteplase group whereas small subendocardial (group 2) MO, were uniformly distributed in R+Abcx groups: see Table I.

Of note, two patients of this study died: C.A. and P.S. both for wall rupture, both in the reteplase group and with severe (group 3) MO. The first patient who died in day five, was the object of a previous publication (8). The second suddenly died at home 7 days after uncomplicated myocardial infarction.

Finally, no significant correlations could be found when patients were grouped according to the site of myocardial infarction (Fig. 4).

Fig. 4

- Inverse linear regression between infarct size (MIsz%) ad ejection fraction (2d-fl 3D EF) according to the site of myocardial infarction: anterior (Ant.) or inferior (Inf.). Details in the text.

DISCUSSION

The first consideration regards EF. As already known, MRI 3D reconstruction of the LV carries a small variability so that also a limited population can be sufficient to disclose differences not revealed by echocardiography (57–62). This is also the case of this 20 patient study, in which mean EF in R+Abcx patients was clearly higher, though with weak significance, compared to the reteplase group. This occurred despite similar MIsz and intensity ratio, indicating that different degrees of cell damage may be represented by similar STIR T2 signal, which thus seems unable to distinguish irreversible necrotic from reversible viable myocardium. This latter is the prevalent condition in MIsz of patients treated with R+Abcx.

The traditional onion configuration of myocardial ischemia and infarction suggests that the outer layers are less damaged than the inner myocardium. Under these circumstances with increasing MIsz a progressive impairment of the LV with lowering EF should be expected. In the reteplase group in fact all this happened, as represented by the clear inverse relation between MIsz and EF. This was the result of epicardial fibrinolysis, which though successful, could not prevent the development of MO. This was related, in our study with equivalent pain to treatment time between groups (63) to the extent of MIsz more than the success of epicardial trombolysis.

The above considerations were furthermore confirmed by the opposite behaviour observed in R+Abcx patients where not only epicardial but also microvascular vessels were pharmacologically preserved. In these patients despite unchanged MIsz compared to reteplase, the inverse relationship of Figure 2 was completely lost and almost no MO occurred.

A second consideration arises from STIR T2- imaging of myocardial infarction.

Almost contemporaneously some years ago, two different works indicated that T2 weighted (64) and T1 weighted sequences (39) were able to define infarct areas with good correspondence with the infarct size expressed by Thallium irreversible defect. In experimental comparison however (65), infarct areas seemed more accurately represented by T1 compared to those, slightly overestimated, obtained with T2 sequences. Nonetheless the same images also indicated that T2-weighted could detect subendocardial void of signal not visible in T1 sequence, indicating that two different tissue were at the same time present in that infarct area, both well detected by T2 approach. This closely calls MRI experimental data which indicate that in the same infarct area, irreversibly damaged myocites with patent microcirculation and irreversibly damaged myocytes with occluded microcirculation may coexist (12).

Since then, both T2 (66–68) and T1 late enhancement imaging have been utilized to estimate infarct size, T1-weighted largely dominating the scene (36, 39, 69–92).

In spite of this, some limitations suggest that beyond the clear experimental and clinical evidence of late enhancement usefulness, the exact quantification of infarct size requires further technological, procedural and methodological steps to be completely defined (56, 68, 74).

In addition, void of signal in T2 images most likely indicates hemorrhagic tissue, currently a prevalent domain of T2 imaging (93–96).

A comprehensive illustration of the above considerations is reproduced in the cases: C.A. Figure 5 whose imaging of wall rupture as been object of a previous publication (8) and P.S. Figure 6 suddenly dead for wall rupture as previously reported.

Fig. 5

- Short axis STIR image of the left ventricle (A) and the corresponding macroscopic autoptic specimen (C). Subendocardial void of signal in A corresponds to hemorrhagic tissue in C, while hypersignal in A corresponds to edema in C. (D) small red cells dominate the center of hemorrhagic area (dark subendocardium in C). (B) in the area of edema (white area in C) proceeding from the inner (right-bottom) to the outer (left-top) plane: the number of unbroken myocites progressively increases while the interstitial space progressively reduces.

Fig. 6

- Long axis images of the left ventricle in cine 2d-flash (A) and in STIR T2 sequence (B).

Both indicate the presence of MO, STIR image also suggesting the hemorrhagic nature of MO. Complete description in the text.

Figure 5 shows C.A. STIR T2 short axis slice with clear evidence of transmural hyperenhancement of the inferior wall surrounding a subendocardial void of signal (A). This respectively corresponds to: oedema and subendocardial haemorrhage in the macroscopic autoptic specimen (B). The histological aspect of those layers indicate subendocardial red blood cells predominance (D) and in (C) the extensive loss of myocytes in the confining zone near the hemorrhagic endocardium, progressively replaced in the outer layers by unbroken myocytes and parallel reduction of "expanded" interstitium. This last happened without a clear modification of signal intensity in hyperenhanced area of A Thus similar or contiguous levels of T2 intensity, represents different degrees of cell injure as already outlined in previous MRI studies on ischemia necrosis hemorrhage and healing tissue (97)

In Figure 6 is represented the second case of wall rupture: P.S. A long axis 2d-fl after Gadolinium injection clearly demonstrate the presence of a long MO extended from the base to the subepicardial inferior distal wall. Both 2d-fl (A) and STIR T2 (B) images were able to identify the presence of severe MO, STIR T2 also suggesting the haemorrhagic characteristics of MO, and 2d-fl allowing to follow MO dynamically during systole, thus indicating its threatening extension towards the epicardium.

CONCLUSION

R+Abcx prevents MO: compared to traditional fibrinolytic therapy, this allow a better LV function and most likely an improved long term survival.

Extensive MO was present and well recognized in 25% of all cases, 80% of which fall in the Reteplase group of treatment.

The combination of T1 dynamic 2d-flash cine and STIR T2 sequences may allow recognition and tissue characterisation of MO also suggesting the presence of impending ruptures.

TABLE V

- FROM CONTIGUOUS BASE TO APEX CINE 2d-flash SLICES, THREE DIMENSIONAL (3D) EJECTION FRACTION (EF) IS CALCULATED IN BOTH DRUG REGIMENS

2d-fl3D EF	Mean	Standard Deviation	Sig. (2-tailed)
R+Abcx	50.80	10.51	0.035
Reteplase	41.20	8.08

TABLE VI

- RATIO BETWEEN INTENSITY VALUES OBTAINED WITH STIR IMAGES, IN INFARCT AREA AND IN NORMAL MYOCARDIUM, IN BOTH DRUG REGIMENS

STIRIntensity ratio	Mean	Standard Deviation	Sig. (2-tails)
R+Abcx	1.69	0.25	0.653
Reteplase	1.74	0.22