Background: Acute pericardial tamponade is a life-threatening complication of invasive cardiovascular procedures. Survival depends on early recognition and treatment. A diagnostic test to detect pericardial fluid accumulation before a significant fall in blood pressure and without contamination of the sterile field would be valuable. Objective: We tested the hypothesis that fluoroscopic excursion of the cardiac silhouette decreases early in the course of acute pericardial tamponade and precedes hemodynamic compromise. Methods: The pericardial space of seven pigs was accessed by a sub-xiphoid puncture. Tamponade was produced by intrapericardial saline infusion at 20-25 mL/minute until the pericardial pressure equalized with right ventricular end diastolic pressure or the systolic blood pressure reached 40 mmHg. Supine fluoroscopic images were obtained every 2 minutes in the left anterior oblique view with simultaneous echocardiography. The fluoroscopic heart silhouette was digitized, and the maximum excursion during the cardiac cycle was quantified by custom software. The qualitative excursion of the fluoroscopic heart silhouette on randomly selected video images was also graded by two independent observers who were blinded to the time course of the experiment and the hemodynamics. Results: During progressive pericardial fluid accumulation, the cardiac silhouette excursion quantified by the custom software (p < 0.001) and by video rating (p < 0.0001) was significantly reduced within 2 minutes. A statistically significant fall in blood pressure compared with baseline did not occur until 6 minutes (89 ± 21 vs. 121 ± 15 mmHg, p < 0.001). The interobserver agreement was very close, with a kappa statistic of 0.78. The reduction in cardiac silhouette excursion was apparent as soon as the effusion was detected by echocardiography. Conclusion: Cardiac silhouette excursion becomes reduced early in the course of acute pericardial fluid accumulation. This fluoroscopic observation can be used to detect impending pericardial tamponade before hemodynamic collapse. © 2005 Heart Rhythm Society. All rights reserved.