Doctor of Philosophy(PhD) / The vast majority of children undergoing surgical repair of cardiac lesions do spectacularly well. However a significant proportion, ~ 25%, struggle to progress in the early postoperative period and require additional pharmacological and occasionally mechanical circulatory support. All children typically have some degree of postoperative myocardial dysfunction, with the severe spectrum termed the low cardiac output state (LCOS). LCOS is clinically defined as the requirement for new or escalated inotrope therapy, a widened arteriovenous oxygen difference, cardiac arrest or the need for reinstitution of mechanical circulatory support. LCOS is largely responsible for the morbidity and mortality involved in paediatric cardiac surgery. Despite the predictability of LCOS in the initial postoperative hours, the underlying pathophysiology remains unclear. The period of decline in cardiac function that typifies LCOS is temporally associated with the development of oedema in the tissues of the body, including the heart. This relationship between oedema and dysfunction has increasingly become blurred, with a tendency to elevate the temporal association to a causal link. We sought to explore the causes and contributions to myocardial dysfunction in this setting, including the roles of oedema and ischaemia within the heart. In focusing on oedema and ischaemia we also examined the effects of these insults on relevant myocardial membrane proteins, including those that permit rapid water transport – aquaporins (AQPs), and those involved in membrane mechanics – dystrophin, and membrane repair – dysferlin. Experimental settings which enabled the in vitro dissection of these insults and proteins of interest were combined with a clinically accurate in vivo model. This thesis describes a series of thematically linked experiments that examined LCOS, myocardial oedema and the role of various membrane proteins. We performed isolated cardiomyocyte studies, isolated heart studies as well as a clinically relevant large animal (lamb) cardiopulmonary bypass (CPB) model. Across these models we also explored the role of therapeutically protecting myocardial membranes with Poloxamer 188 (P188) and assessed any influence on myocardial function, oedema and membrane proteins. vi The results from these three models suggest that the clinically accepted dogma of a causative link between myocardial oedema and dysfunction overstates the contribution of myocardial oedema to LCOS. We found that ischaemia/reperfusion was of primary importance in causing myocardial dysfunction. Myocardial oedema without ischaemia had a mild and reversible contribution to myocardial dysfunction, but this was minor in comparison to the gross dysfunction attributable to ischaemia. Isolated cardiomyocytes, with induced oedema, functioned well. Whilst ischaemic cardiomyocytes, with less swelling still had severe contractile dysfunction. Isolated hearts, perfused with an oedema inducing crystalloid perfusate developed myocardial oedema and had minimal reversible systolic and diastolic dysfunction. Isolated hearts which experienced global ischaemia had comparable degrees of myocardial oedema, and significantly greater degrees of myocardial dysfunction that increased in severity with increasing duration of ischaemia. In the lamb CPB model, only those lambs which underwent aortic cross clamping and had a period of ischaemia had poor myocardial function. These lambs also had swollen hearts, raised myocardial AQP1 mRNA and reduced membrane dysferlin protein expression. Membrane dystrophin protein expression was not altered, somewhat unexpectedly with CPB with or without ischaemia. Lambs placed on CPB without ischaemia had good myocardial function, minimal oedema and unchanged membrane protein expression during the survival period. In a blinded lamb CPB trial of P188 there were improved haemodynamics and indicies of myocardial function associated with its use. This was also associated with preservation of dysferlin expression and reduced membrane injury. In parallel isolated heart trials of this therapy, there was a reduction in myocardial oedema associated with its use in non-ischaemic experiments. There was also a suggestion of improved diastolic function in ischaemic experiments, but no change in myocardial water content. In conclusion, we have highlighted the primacy of ischaemia/reperfusion over oedema in contributing to LCOS. We have refuted the accepted dogma that myocardial oedema causes significant dysfunction in itself, with important oedema likely to result from ischaemia. We have shown that AQP1 may be involved in the pathogenesis of the capillary leak syndrome. Finally we have hinted at a role for prophylactic P188 in the vii setting of LCOS, possibly highlighting the role of membrane repair in recovery after surgery. Isolated heart trials of P188 further support a non-rheological mechanism of action and also lend support to the causal separation of myocardial oedema and dysfunction. The integral membrane protein dysferlin, rather than dystrophin, is relevant in the setting of LCOS in the current era.
| Identifer | oai:union.ndltd.org:ADTP/269587 |
| Date | January 2009 |
| Creators | Egan, Jonathan Rogers |
| Publisher | Faculty of Medicine, University of Sydney. |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | The author retains copyright of this thesis., http://www.library.usyd.edu.au/copyright.html |
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