Brain death-induced alterations in myocardial workload and high-energy phosphates: a phosphorus 31 magnetic resonance spectroscopy study in the cat.

BACKGROUND

Hemodynamic deterioration resulting from brain death-induced myocardial left ventricular dysfunction may preclude heart donation. A reduced myocardial high-energy phosphate content, assessed by biopsy specimens, has been suggested to be responsible for this phenomenon. By applying phosphorus 31 magnetic resonance spectroscopy, in vivo myocardial high-energy phosphate metabolism can be studied continuously.

METHODS

Twelve cats were sedated, intubated, ventilated, and studied for 240 minutes. Heart rate, arterial blood pressure, and arterial blood gases were monitored. Central venous pressure was kept constant. Myocardial work was expressed as rate-pressure product (RPP=heart rate x systolic arterial blood pressure). After sternotomy a radio frequency surface coil was positioned onto the left ventricle. A parietal trephine hole was drilled, and an inflatable balloon was inserted. The animal was placed into a 4.7 T horizontal 40 cm bore magnet interfaced to a spectrometer. Brain death (n=6) was induced by rapid inflation of the balloon; the six other cats served as a sham-operated control group. 31P spectra were obtained in 30 seconds, with ventilation and arterial blood pressure curve triggering. The phosphocreatine/to/adenosine triphosphate ratio, as an estimator of energy metabolism, was calculated.

RESULTS

Brain death was established within 30 seconds after inflation of the balloon. Changes in RPP were characterized by a triphasic profile with a maximum increase from 19.3+/-1.4 x 10(3) to 87.5+/-8.1 x 10(3) mm Hg x min(-1) (p < .0001 vs control group) at 2 minutes after inflation of the balloon. Subsequently, RPP decreased and was normalized at 15 minutes after inflation. The third phase was characterized by hemodynamic deterioration, which became significant at 180 minutes and resulted in mean arterial pressure of 71+/-12 mm Hg (p < .05 vs control group) at the end of the experimental period. RPP deteriorated to 14.6+/-2.0 x 10(3) mm Hg x min(-1) (p < .05 vs control group) at 240 minutes. Because the heart rate remained constant during the third phase, the decrease in RPP was caused by a decrease in systolic arterial blood pressure. The initial phosphocreatine/adenosine triphosphate ratio of 1.65+/-0.16 varied to 1.52+/-0.06 at 2 minutes, and to 1.73 +/-0.17 (all values NS vs control group and vs initial ratio) at 240 minutes.

CONCLUSIONS

The energy status of the heart is not affected by brain death. Therefore brain death-induced hemodynamic deterioration is not caused by impaired myocardial high-energy phosphate metabolism.