OBJECTIVE: To determine the effects of positive-pressure ventilation and experimentally induced pericardial effusion and tamponade on transmitral flow velocities in dogs. DESIGN: Descriptive. SETTING: University laboratory. PARTICIPANTS: Eleven tracheally intubated and mechanically ventilated dogs. INTERVENTIONS: Experimental pericardial effusion and cardiac tamponade were created by pericardial injection of warm saline. MEASUREMENTS AND MAIN RESULTS: Hemodynamic parameters and pericardial pressures were monitored in the 11 dogs. Pulsed-wave Doppler tracings of mitral valve flow were obtained at the leaflet tips along with hemodynamic measurements at 4 stages: control, effusion (no decrease in mean arterial pressure), tamponade (>or=40% decrease in mean arterial pressure), and tamponade relief (after evacuation of pericardial fluid). Maximal variation (36%) in transmitral flow velocity over the respiratory cycle during positive-pressure ventilation was seen in the control stage. In the effusion and tamponade stages, variation in transmitral flow velocity decreased progressively to 29% (p = 0.1804, not significant) and 16% (p < 0.0001), respectively. CONCLUSION: Intrathoracic pressure and lung volume changes caused by positive-pressure ventilation influence transmitral flow velocity patterns. Respiratory variation in transvalvular flow is pronounced during standard positive-pressure mechanical ventilation, decreases in the presence of pericardial effusion, and becomes almost nonexistent when cardiac tamponade is present. These findings show that the echocardiographic criteria used to diagnose cardiac tamponade based on mitral valve inflow patterns are different during positive-pressure ventilation from spontaneously breathing subjects.