How much fluid must build up before frank tamponade will occur?

Frank cardiac tamponade occurs when excess fluid accumulates in the pericardial space, creating pressure on the heart that hinders its ability to pump effectively. The threshold for this condition is generally around 200 to 300 mL of fluid accumulation. At this volume, the pericardial cavity becomes significantly distended, leading to compromised diastolic filling of the heart, which ultimately results in reduced cardiac output and potential hemodynamic instability.

It's important to note that while smaller volumes of fluid (less than 100 mL) may not cause significant pressure changes due to the pericardium's ability to accommodate fluid, the mechanical properties and anatomic constraints of the heart and surrounding structures mean that as little as 200 mL can overwhelm the pericardial space leading to the life-threatening effects associated with tamponade.