Can technetium 99m pyrophosphate be used to quantify myocardial injury in donor hearts?

BACKGROUND: There are no prospective methods available to quantify the myocyte injury in hearts prior to transplantation. The potential of the isotope labeled infarct marker 99m Technetium pyrophosphate (TcPPT) being used in this role was investigated. METHODS: Brain death was induced by creating an extradural space occupying lesion in young adult swine after which hemodynamic changes were monitored and myocyte injury was quantified by histochemistry. TcPPT was administered 5 hours after induction of intracranial hypertension, and after hearts were harvested myocardial uptake was measured. These latter measurements were related to the histochemical assessment of myocyte injury. RESULTS: Sham animals (n = 4) maintained cardiovascular stability and experienced minimal myocyte injury, grades 0 to 3. BD animals (n = 10) exhibited varying patterns of hemodynamic change and myocyte injury, the latter was significant in 6, graded 4 to 11, p less than 0.05. Uptake of TcPPT by BD hearts was greater than twice the 90th centile sham value in 6. The sensitivity and specificity of greater uptake indicating the presence of myocyte injury was 83.3% and 75% respectively. CONCLUSIONS: TcPPT has the potential to quantify myocardial injury induced by brain death and its potential utility merits further investigation.

Effect of donor heart damage on survival after transplantation.

Evaluation of the functional condition of the heart prior to its removal from the donor or after transport to the recipient is difficult. Biopsies of the myocardium allow serial assessments to be made throughout this period, but suffer from the disadvantage that the average analysis of biopsies has only a tenuous connection with physiological function. Quantitative birefringence measurements (QBM), on the other hand, assess the ability of myocardial fibres to respond to ATP and calcium and have been shown to correlate well with measurements of cardiac function (P less than 0.001). A prospective study of myocardial biopsies before excision, after transport and again after transplantation, using quantitative birefringence measurement of biopsies of the heart has recently been completed. These studies have shown evidence of impaired myocardial function in 73 (43%) of 172 donor hearts studied prior to excision, with a further 27% showing significant deterioration during storage and transport to the recipient. Biopsy assessments therefore indicated that at the moment of implantation, only 30% of the donor hearts were normal. Functional assessment of the biopsies by QBM correlated with early clinical outcome of transplantation (P less than 0.001). Longer term follow-up of the recipients (up to 5 years) has shown that the mortality of recipients of hearts with impaired function before transplantation is significantly increased (44% of 120) compared with that of recipients of undamaged hearts (6% of 52, P less than 0.001).

Biopsy assessment of fifty hearts during transplantation.

The aim of the study was to evaluate myocardial protection during the transportation of human donor hearts stored at 4 degrees C in crystalloid cardioplegic solution. Biopsies were performed on 50 donor hearts before excision and at four subsequent time intervals during transplantation. Quantitative birefringence measurements on cryostat sections of the left ventricular biopsy specimens were used to assess myocardial function. Fifteen donor hearts had poor birefringence assessments before excision, and 67% received inotropic support after implantation; 22 deteriorated during transportation, and 50% received inotropic support; 13 were unchanged throughout the procedure, and none required inotropic support. This study demonstrates the need for improved protection of donor hearts during transportation and recommends the use of rigorous criteria in the selection of these hearts.

Increased myosin orientation during muscle contraction: a measure of cardiac contractility.

Myosin form birefringence has been studied in cryostat sections of left ventricular myocardium from the dog and human. The muscle in such sections has been shown to demonstrate the sliding filament phenomenon. The sarcomere length of canine myocardium agreed with that found in comparable electron micrographs. Unexpectedly, it was found that glycerol, normally used as an inert and optically ideal mountant, caused profound change in myosin birefringence. This apparently invalidates results obtained with this mountant. The absolute birefringence found in these sections, whether mounted in glycerol or in an ATP-calcium buffer, corresponded to values found by other workers with skeletal muscle and isolated myosin. However, the birefringent properties (optical path difference: o.p.d.) of well functioning muscle was found to be low, the o.p.d. increasing when exposed to ATP and calcium. Poorly functioning muscle could be distinguished from well functioning muscle on the basis of its higher 'in air' o.p.d. This difference correlated well with physiological assessments of myocardial function or with clinical assessments of cardiac failure. Evidence is presented indicating that changes in apparent birefringence, caused by ATP-calcium or by anoxia, are due to altered orientation of the myosin micelles and can be inhibited by agents that inhibit myosin ATPase activity.

Myocardial catecholamines following cold cardioplegic arrest during open-heart surgery.

Preservation of both right and left ventricular subendocardial and subepicardial muscle was assessed using quantitative polarization microscopy (birefringence measurements) with preservation of myocardial catecholamines measured by fluorescence microscopy in biopsies from 20 consecutive patients who underwent open heart surgery with cold cardioplegic arrest (St. Thomas' Solution). Six of the 7 patients with clinical complications were predicted from the birefringence results. One developed left ventricular deterioration during bypass, two patients right ventricular deterioration, one patient both left and right ventricular deterioration and two patients had poor left ventricular function before bypass. Birefringence measurements were thus reliable in predicting post-operative cardiac outcome. There were no significant changes during the bypass period in the catecholamine scores, even in those patients who had clinical complications. Fluorescence microscopy showed that the "free" myocardial nerve net and the pericoronary nerve plexuses retained their catecholamine stores equally well. This indicated that St. Thomas' cardioplegia preserves myocardial catecholamine stores, depletion of which would remove a potentially important compensatory mechanism in cardiac pump failure. There may however be a temporary blockade in the release of endogenous cardiac catecholamine (noradrenaline) stores from the adrenergic nerve terminals following cold cardioplegic arrest despite myocardial pump failure.

Use of microscopic interferometry for measuring changes in water content of small samples of tissue.

Oedema following periods of ischaemic arrest and subsequent reperfusion has been shown experimentally and clinically to affect the functional state of the heart. Tissue water content has been measured in myocardial sections by microscopic interferometry and densitometry, and the results correlated with those obtained by wet and dry weight analysis (r = 0.87; p less than 0.001). Microscopic interferometry also revealed the distribution of the water in the tissue. Experimentally induced ischaemic arrest in isolated rat hearts resulted in predominantly intra-fibrillar oedema, whilst subsequent reperfusion resulted in interfibrillar oedema. Microscopic interferometry facilitates accurate measurement of water content in tissue samples as small as 2 mg wet weight and shows (as conventional wet/dry weight analysis cannot) the distribution of the water in the tissue.

Clinical and quantitative birefringence assessment of 100 patients with aortic clamping periods in excess of 120 minutes after hypothermic cardioplegic arrest.

At St. Thomas' Hospital the first 100 patients with prolonged aortic cross-clamp times in excess of 120 minutes have been analyzed clinically (low cardiac output and mortality) and 49 of these patients from which left and right ventricular biopsies were taken, have been analyzed by quantitative birefringence (biophysical measurement of myocardial deterioration). A total of 8 patients died (8%) and 11 had low cardiac output syndrome (11%). The patients were divided into those given only a single infusion (n = 18) and those given hourly infusions (n = 82) of hypothermic cardioplegic solution. These were then subdivided into those with single (n = 37) or multiple (n = 63) corrective surgical procedures. In the patients who had a single corrective surgical procedure there was no difference at all, but in those patients who had multiple corrective surgical procedures, hourly infusion reduced low cardiac output from 12.5% to 9.4% in multiple valve patients and from 50% to 19% in bypass graft combined with valve replacement patients. In this latter group mortality fell from 50% to 4.8% but there were only 2 patients given a single infusion. There was no statistically significant advantage in hourly infusions compared with single infusions, either clinically or cytochemically. Twenty-six patients had aortic cross-clamp periods in excess of 150 minutes. Mortality and low cardiac output increased compared with the 120 to 150 minute group, rising from 3% to 19% and from 7% to 23% respectively. As a result of these analyses, surgical practice has been changed to 30-minute reinfusion intervals with currently improved results.