A 9-year-old boy with neuromuscular scoliosis is undergoing posterior spinal fusion. During dissection, the patient becomes progressively tachycardic and hypotensive. There are no changes in ventilation volumes or pressures. The end-tidal carbon dioxide value decreases to 18 mmHg and the arterial waveform is dampened. The clinical situation deteriorates to cardiac arrest. What is the MOST LIKELY cause of cardiac arrest in this patient?