Cardiovascular Journal of Africa: Vol 28 No 2 (March/April 2017)

CARDIOVASCULAR JOURNAL OF AFRICA • Volume 28, No 2, March/April 2017

AFRICA

Discussion

Coronary collateral arteries have their origin from the same

embryonic precursor as the native coronary arteries during

embryogenesis; therefore the foundation of these collateral

arterial networks is laid down during embryonic life and is

present in the newborn.

9,25

The normal human heart contains

interconnecting channels,

hence, coronary collateral pathways

are present in both normal and diseased hearts.

These channels

exist as microvessels whose function is not clear and is not

demonstrable angiographically when coronary circulation is

normal or mildly obstructed.

11,26

Functional collaterals were suggested to have developed from

hypertrophic evolution of the vessels present in the normal heart.

This evolutionary process is triggered by myocardial ischaemia

and/or an increase in the pressure gradient in the collateral

network.

8,25,27

Due to this pressure gradient, there is an increase

in the volume of blood propelled through these channels. They

progressively dilate and are eventually angiographically visible as

coronary collateral channels.

The pressure gradient also results

in an increased fluid shear stress in the vessel.

This fluid shear

stress is a primary morphogenic physical factor that determines

the size of the developing collateral vessel.

In the present study, the best developed CACs were recorded

in those patients who had proximally located lesions (40.9%).

Excellent collaterals were found in 29.7 and 25% of middle

and distally located lesions, respectively. The more proximally

located the lesion, the higher the pressure gradient between the

normal (collateral-donating) coronary artery and the obstructed

(collateral-receiving) vessel. In addition, the more proximal the

lesion was situated, the greater the mass of ‘at risk’ ischaemic

myocardium.

Therefore, the highest prevalence of excellent

collaterals in patients with proximally located lesions in the

present study may have resulted from the combination of these

factors (increased pressure gradient and myocardial ischaemia).

Consequently, this results in an increased stimulus for collateral

vessel formation.

It is apparent from the literature reviewed that there are no

reports on the relationship between the situation of the lesion

and LV function. In the present study, the mean EF calculated

for the patients with proximally located lesions was the highest

(63.3%) compared to mean EF for the middle (57.8%) and

distally (57.5%) located lesions (Table 3). However, the current

study did not find any significant difference in the prevalence of

CACs with regard to the location of atherosclerotic lesion and

the resultant preservation of LV function.

There are conflicting reports with regard to the functional

importance of coronary collateral arteries. Sheehan

et al

examined

global left ventricular ejection fraction (LVEF) in patients with

acute myocardial infarction before treatment and at discharge.

They reported that global LVEF increased in patients with CACs

but was the same in patients without coronary collaterals.

Habib

et al

divided patients who failed to canalise at 90

minutes after administration of a thrombolytic agent, into two

groups (with and without collaterals) and reported that global

LVEF was significantly greater in patients with CACs at hospital

discharge. On the contrary, Wackers

et al

found no difference

in the global LVEF in patients with and without CACs.

There is yet another supposed negative effect of coronary

collaterals, namely coronary ‘steal’. This occurs either when the

pressure in the donor vessel is suddenly low or when there is

higher resistance in the collateral pathway.

Therefore, it results

in the flow of blood from the region of the collateral-receiving

vessel to the collateral-donating vessel. However, patients with

poorly developed CACs are more prone to coronary steal than

those with well-developed CACs.

To our knowledge, this study is the first attempt at establishing

a relationship between the different grades of CACs and LVEF

in the presence of total coronary arterial obstruction. There was

a significant difference (

0.001) in the mean EF calculated for

the different grades of CACs. In addition, a

post hoc

test showed

a significant difference in the mean EF between excellent and

absent collaterals (

0.004) and excellent and poor collaterals (

0.001). Therefore the development of excellent collaterals has

a significant supportive effect in the preservation of LV function

compared to patients with absent or poor collateralisation.

There was also a significant positive correlation between CAC

grades and mean EF calculated for the different CAC grades.

Our study corroborated the findings of Sheehan

et al

and

Habib

et al

that the presence of excellent and well-developed

CACs had a significant role in the preservation of LV function.

In addition, the present study showed that, as the grades of

the CACs increased, there was an improvement in the ability

of these collaterals to preserve LV function. Consequently, LV

myocardial perfusion was greater in patients with well-developed

CACs where the native artery was totally occluded, and resulted

in better preservation of LV function even in the face of an

acute coronary event.

To date, the significance of collateral

circulation in coronary bypass surgery has not yet been fully

investigated. However, it has been reported that the collateral

circulation is favourable for the successful construction of

coronary artery bypass grafts.

From the result of this study, it can therefore be seen that

the presence of well-developed CACs should be considered in

decision making in the management of patients with coronary

arterial obstruction. In the presence of an adequately preserved

LV function by coronary collaterals in asymptomatic patients,

a strong case can be made for no intervention. Anecdotally,

most cardiac practitioners would be aware of patients with

total coronary arterial obstruction who have been leading a

normal life, and even engaging in high-intensity sport without

symptoms. Therefore, the significance of the coronary collateral

arteries should not be underestimated, as identification of the

CACs is relevant in clinical decision making.

The limitations to the current study include the absence

of clinical records, which made it impossible to determine

the patients with risk factors and co-morbid conditions, such

as diabetes mellitus and hypertension, which may also have

influenced collateral vessel development. This would have

enhanced the study; however, the aim of this study was to

evaluate the functional importance of coronary collaterals on

LV function, which was achieved by analysing the angiographic

records.

Conclusion

The location of atherosclerotic lesion had no significant effect

on the prevalence of CAC grades and the resultant LV function.

However, with the development of well-functioning coronary

collaterals, there was a significant improvement in the ability of

these collaterals to preserve LV function.