In patients with cutaneous graft versus host disease (GvHD) that is resistant to traditional steroid therapy, imatinib is a first-generation tyrosine kinase inhibitor that seems to be a viable option. However, its antifibrotic activity can be associated with serosal inflammation and fluid retention.We report a case of an adult patient who, after allogenic hematopoietic stem cell transplantation, developed a GvHD treated with imatinib at low dosage, followed by multiorgan failure. Clinical examination and cardiac ultrasound were unable to clearly recognize the low cardiac output state; laboratory analysis, filling pressure, and computed tomography examination clarified the correct diagnosis.Low cardiac output state, secondary to pericardial effusion, is a diagnostic challenge. However, the association of four elements can help in its early recognition: increase in lactate levels and central venous pressure, associated with a low central venous saturation and a low brain natriuretic peptide level.Pericardial effusion with cardiac tamponade is a difficult diagnosis even with ultrasound. Lactate levels, central venous pressure plus venous saturation, and brain natriuretic peptide could help in early detection.